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Synoptic Circulation and Land Surface Influences on Convection in the Midwest U.S. “Corn Belt” during the Summers of 1999 and 2000. Part II: Role of Vegetation Boundaries ANDREW M. CARLETON Department of Geography, and Earth and Environmental Systems Institute (EESI), The Pennsylvania State University, University Park, Pennsylvania
DAVID J. TRAVIS Department of Geography and Geology, University of Wisconsin—Whitewater, Whitewater, Wisconsin
JIMMY O. ADEGOKE Department of Geosciences, University of Missouri, Kansas City, Missouri
DAVID L. ARNOLD Department of Geography, Frostburg State University, Frostburg, Maryland
STEVE CURRAN Department of Geography, The Pennsylvania State University, University Park, Pennsylvania (Manuscript received 11 July 2006, in final form 5 November 2007) ABSTRACT In Part I of this observational study inquiring into the relative influences of “top down” synoptic atmospheric conditions and “bottom up” land surface mesoscale conditions in deep convection for the humid lowlands of the Midwest U.S. Central Corn Belt (CCB), the composite atmospheric environments for afternoon and evening periods of convection (CV) versus no convection (NC) were determined for two recent summers (1999 and 2000) having contrasting precipitation patterns and amounts. A close spatial correspondence was noted between composite synoptic features representing baroclinity and upward vertical motion with the observed precipitation on CV days when the “background” (i.e., free atmosphere) wind speed exceeded approximately 10 m s⫺1 at 500 hPa (i.e., “stronger flow”). However, on CV days when wind speeds were ⬍⬃10 m s⫺1 (i.e., “weaker flow”), areas of increased precipitation can be associated with synoptic composites that are not so different from those for corresponding NC days. From these observations, the presence of a land surface mesoscale influence on deep convection and precipitation is inferred that is better expressed on weaker flow days. Climatically, a likely candidate for enhancing low-level moisture convergence to promote deep convection are the quasi-permanent vegetation boundaries (QPVBs) between the two major land use and land cover (LULC) types of crop and forest that characterize much of the CCB. Accordingly, in this paper the role of these boundaries on summer precipitation variations for the CCB is extracted in two complementary ways: 1) for contrasting flow day types in the summers 1999 and 2000, by determining the spatially and temporally aggregated land surface influence on deep convection from composites of thermodynamic variables [e.g., surface lifted index (SLI), level of free convection (LFC), and lifted condensation level (LCL)] that are obtained from mapped data of the 6-h NCEP–NCAR reanalyses (NNR), and 0000 UTC rawinsonde ascents; and 2) for summer seasons 1995– 2001, from the statistical associations of satellite-retrieved LULC boundary attributes (i.e., length and width) and precipitation at high spatial resolutions.
Corresponding author address: Andrew M. Carleton, Department of Geography, and Earth and Environmental Systems Institute (EESI), The Pennsylvania State University, University Park, PA 16802. E-mail:
[email protected] DOI: 10.1175/2007JCLI1584.1 © 2008 American Meteorological Society
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For the 1999 and 2000 summers (item 1 above), thermodynamic composites determined for V(500) categories having minimal differences in synoptic meteorological fields on CV minus NC (CV ⫺ NC) days (i.e., weaker flow), show statistically significant increases in atmospheric moisture (e.g., greater precipitable water; lower LCL and LFC) and static instability [e.g., positive convective available potential energy (CAPE)] compared to NC days. Moreover, CV days for both weaker and stronger background flow have associated subregional-scale thermodynamic patterns indicating free convection at the earth’s surface, supported by a synoptic pattern of at least weakly upward motion of air in the midtroposphere in contrast to NC days. The possibility that aerodynamic contrasts along QPVBs readily permit air to be lofted above the LFC when the lower atmosphere is moist, thereby assisting or enhancing deep convection on CV days, is supported by the multiyear analysis (item 2 above). In early summer when LULC boundaries are most evident, precipitation on weaker flow days is significantly greater within 20 km of boundaries than farther away, but there is no statistical difference on stronger flow days. Statistical relationships between boundary mean attributes and mean precipitation change sign between early summer (positive) and late summer (negative), in accord with shifts in the satellite-retrieved maximum radiances from forest to crop areas. These phenological changes appear related, primarily, to contrasting soil moisture and implied evapotranspiration differences. Incorporating LULC boundary locations and phenological status into reliable forecast fields of lower-to-midtropospheric humidity and wind speed should lead to improved short-term predictions of convective precipitation in the Corn Belt and also, potentially, better climate seasonal forecasts.
1. Introduction a. Context In interior continental locations, summertime convective activity results from scale interactions between the “top down” climate control of synoptic atmospheric circulation favoring upward vertical motion in the free atmosphere (e.g., negative omega), and enhanced moisture convergence associated with mesoscale density gradients in the planetary boundary layer (PBL), or “bottom up” climate control (e.g., Weckwerth and Parsons 2006). The enhanced density gradients are associated with thunderstorm outflow boundaries and land surface boundaries comprising vegetation, soil moisture, and topography; the latter group typically being interrelated at a given location (e.g., Teuling and Troch 2005). Atmospheric boundaries are important for convection on short (subdaily to daily) time scales, and also climatically where forced by the land surface. The latter “stationary” influence may be expressed as repeated precipitation events over similar areas, synoptic atmospheric conditions permitting (e.g., Taylor and Lebel 1998; Parker et al. 2005). On climatic time scales in continental midlatitude locations, convective precipitation is the net result of moisture evaporated in situ (i.e., “recycled”) and advected from distant sources (e.g., Trenberth 1999; Zangvil et al. 2001). For the central United States, the Mississippi River basin and its subbasins contribute significantly to summertime precipitation recycling, and the interannual variability in recycling seems related to the occurrence of wet versus dry summers (Brubaker et al. 2001; Dominguez et al. 2006). A first-order stratification of land surface evaporation sources for precipi-
tation comprises natural vegetation (e.g., forest, woodland, and grassland) and agriculture (e.g., rainfed and irrigated) (Adegoke et al. 2007). The role of agricultural lands in precipitation recycling potentially is large in the central United States because of the dominance of primary production (e.g., Hicke and Lobell 2004; Zangvil et al. 2004; McCabe et al. 2005) and for which the acreage under cultivation has almost doubled since 1950 (Sandstrom et al. 2004). The impact of moist land surfaces on convective precipitation and severe weather in flatland regions (e.g., Cheresnick and Basara 2005) is likely enhanced when mechanical processes along extensive land cover boundaries possessing large height differences, such as between forest and crops (e.g., Travis 1997; Freedman et al. 2001; Pielke 2001), assist uplift of the air. Part I of this study (Carleton et al. 2008, hereafter Part I) comprised the first step in an observational study to determine the relative influences of top-down (synoptic circulation) and bottom-up (land surface mesoscale) controls on deep convection (i.e., radarindicated observed precipitation) in the Midwest’s Central Corn Belt (CCB; Fig. 1), for contrasting summer seasons (15 June–15 September) of 1999 and 2000. We identified the synoptic-scale atmospheric patterns associated with the precipitation variations using a synoptic climatological (i.e., stratified composite) approach based on categories of daily background flow [V(500)] and convective activity [i.e., convection (CV) and no convection (NC)]; the latter determined for the afternoon and evening combined period. The summer 1999 and 2000 results were situated within the longer period of summers 1996–2001 and shown to be anomalous. Synoptic circulation composite anomalies for each
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FIG. 1. Map of the larger study region for which synoptic atmospheric composites were compiled (Part I) and within which a smaller grid is nested. The shaded (innermost) area approximates the mostly flat Central Corn Belt (CCB) dominated by agriculture and containing crop-forest boundaries. Locations of the three upper-air stations in the CCB are shown.
study summer (1999, 2000) were compiled for the 10 combinations of background flow and convective activity (i.e., 5 ⫻ 2) by averaging National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalysis (NNR) daily-averaged atmospheric data on freeatmosphere variables (e.g., geopotential height, meridional wind, omega, and specific humidity). The Part I analyses showed that associations (visual, statistical) between deep convection and circulation in summers 1999 and 2000 were greatest on days when V(500) exceeded about 10 m s⫺1 [i.e., “stronger flow,” or categories moderate flow (MF), moderate flow-jet (MF-J), and jet maximum (J)]. On those composite days, atmospheric features indicating enhanced baroclinity (e.g., location on the south side of a jet maximum, upward vertical motion in midtroposphere, intersection of a low-level wind speed maximum and moisture axis) were synchronized spatially (i.e., occurred in the same general location) with the areas of observed maximum precipitation and “organized” radar echoes, confirming the importance of dynamical forcing for deep convection. On days in the same stronger flow categories that lacked afternoon and evening deep convection (i.e., NC), the synoptic composites essentially were opposite those for CV in their respective summer
(location north of a jet, free-atmosphere subsidence, negative anomalies of moisture). However, on “weaker flow” days (i.e., WF, W-MF), or V(500) ⱗ 10 m s⫺1, radar-indicated deep convection (CV) tended to lack spatial organization: the accompanying precipitation composite field was heterogeneous and showed a reduced spatial association with the synoptic circulation anomalies representing or promoting strong upward vertical motion. Moreover, weaker flow days lacking convection (NC) were accompanied by atmospheric fields broadly resembling their CV-day counterparts (e.g., in near-surface vector winds; lower-troposphere specific humidity anomalies). The latter results suggested a dominant influence of the synoptic circulation on convection in, and its absence from, the CCB on stronger flow days, but a contributory role for stationary land surface influences on weaker flow days.
b. Background The land surface–climate physical relationship involves the available net radiation at the earth’s surface (Q*) partitioned into the convective fluxes of sensible heat (QH) and latent, or evaporative, heat (QE), as expressed by the Bowen ratio ( ⫽ QH /QE) (e.g., Kunkel 1989; Pielke et al. 2002). On daily-averaged and longer time scales, these quantities differ by land-use and land-
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FIG. 2. Image analysis of the Midwest land surface conditions: (a) NDVI biweekly composite image of part of the CB for the early summer periods 7–20 June 1996 and 6–19 June 1997. Green (yellow) tones correspond to high (low) NDVI, associated with forest (i.e., crop and urban) areas. (b) Sample Boundary-Seer analysis of 6–19 June 1997 image (TerraSeer Inc. 2000). (c) Composite of combined objective and subjective interpretation (GIS) of mid-to-late-June boundaries on NDVI imagery for 1995–2000, with three classes of pixel gradient intensity (i.e., weak, moderate, and strong).
cover (LULC) type (e.g., deciduous forest, dryland agriculture, irrigated agriculture), soil conditions (e.g., type, moisture), and vegetation phenology (Freedman et al. 2001; Twine et al. 2004; McPherson and Stensrud 2005; Santanello et al. 2005; Notaro et al. 2006). The energy fluxes help determine the thermodynamics of the PBL (Hong and Pan 2000; Barros and Hwu 2002), as indicated by a number of stability indices [e.g., lifted index (LI), “cap strength,” CAPE; see Peppler and Lamb 1989; Eltahir and Pal 1996; Schreiber et al. 1996; Doran and Zhong 2000; Donner and Phillips 2003]. Moreover, large height differences between crops and trees aerodynamically influence PBL characteristics (Banta and White 2003), potentially inducing mesoscale vertical circulations across LULC boundaries that promote convection, or nonclassical mesoscale circulations (NCMCs) (e.g., Segal et al. 1989; Travis 1997; Dalu et al. 2000; Carleton et al. 2001). Experiments with regional atmospheric models for the central United States suggest a preferred horizontal length scale of 10–20 km (Baidya Roy et al. 2003) over which the energy and moisture contrasts associated with land surface heterogeneities become organized into NCMCs (Segal and Arritt 1992), although the relatively few ob-
servational studies undertaken have not unambiguously detected these circulations (Hubbe et al. 1997). The LULC boundaries, and their associated discontinuities in soil moisture content and soil type, intensify and potentially become important climatically when they induce convective clouds that differentially shade the surface and also precipitate (Blythe et al. 1994; Clark and Arritt 1995; Pielke 2001; Weaver 2004a). As suggested for subtropical semiarid regions, such convection may reinforce NCMCs, promoting repeated deep convection in similar locations on time scales of several weeks (Taylor et al. 1997; Taylor and Lebel 1998). In the U.S. High Plains, hail damage swaths having widths around 10–12.5 km have been inferred to influence localized deep convection in subsequent weeks during summer (Parker et al. 2005). For most of the CCB (Fig. 1), the lack of orography means that a land surface climatic influence on deep convection and precipitation most likely involves LULC, including the boundaries between cropland and remnant forest that have resulted from ecotone removal by humans (e.g., Carleton et al. 1994, 2001; Travis 1997). Soil moisture and soil type differences (Brown and Arnold 1998; Adegoke and Carleton 2002;
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FIG. 2. (Continued)
Kochendorfer and Ramírez 2005) play a complementary role to the vegetation boundaries in this region. The boundaries between urban and rural areas (e.g., Indianapolis) are also significant climatically, although they cover much less area than the crop–forest boundaries. Particularly near southern Lake Michigan, both urban–rural and crop–forest boundaries may interact with the more transient, yet dynamically active, lake breezes that result from heating differences between land and water (Changnon 1966; Carleton et al. 1994; O’Neal 1996; Scott and Changnon 1996). Because the primary vegetation boundaries are evident as horizontal gradients of albedo and surface radiation budgets denoting a first-order association with climate (Carleton et al. 1994; Roy and Yuan 2007), they can be retrieved in satellite data from enhanced pixel gradients of surface “greenness,” such as represented in the normalized difference vegetation index (NDVI) biweekly composite imagery (e.g., Fig. 2a). Objective image analysis of the NDVI identifies LULC boundaries from these pixel tight spatial gradients (Fig. 2b) and reveals the complexity and spatial variability of Midwest land
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surfaces in early summer. Clearly, most of this heterogeneity is below the resolution of atmospheric analyses (cf. Oleson et al. 2004). The Midwest NDVI-depicted vegetation boundaries (e.g., Figs. 2a,b) intensify and weaken subseasonally in response to crop phenology and “surface wetness,” the latter being a function of antecedent precipitation and the soil moisture (Carleton and O’Neal 1995; Bonan 2001). On average during summer, soil moisture in the larger Corn Belt is depleted as evapotranspiration from vegetation increases and exceeds precipitation (e.g., Robock et al. 2000, their Fig. 5). However, a strong LULC dependence on soil moisture variability there (Adegoke and Carleton 2002; Mahmood and Hubbard 2004) results, at least partly, from water use and vegetation structure differences between crops and trees (e.g., Leavitt 1993, 2002; Prueger et al. 2004; Hatfield et al. 2007). For the period of early crop growth (i.e., second half of June), geographic information system (GIS) classifications of the most distinct quasi-permanent vegetation boundaries (QPVBs) appearing on NDVI images for a multisummer period (Fig. 2c), and specifically for 1997 (Fig. 3a), show two kinds of boundaries: those that vary little interannually, particularly in south-central Indiana, and those that are more transient (e.g., central and southern Illinois). The former boundary type is associated with the crop–forest ecotone, while the latter type recurs in generally the same areas most years and at the same time each summer, but its “intensity” (i.e., magnitude of pixel horizontal gradient) and other attributes vary according to environmental factors (e.g., antecedent precipitation). Given their considerable spatial extent, these QPVBs are a likely candidate to examine for a land surface contributory influence on deep convection in the CCB. In the absence of high-resolution (miso- to mesoscale) thermodynamic observations, it is not possible to identify the climatic influence of an individual LULC boundary or group of boundaries. However, it is possible to use these data to investigate the broader-scale thermodynamic influences that occur collectively from boundaries when thermodynamic mapped variables are composited spatially and temporally with respect to the background flow conditions (cf. Doran and Zhong 2000; Shaw and Doran 2001). The areas of implied upward motion of air at or very close to the earth’s surface can be compared spatially with those for the free atmosphere derived from compositing fields of dynamic meteorological variables (Part I). Determining the character of the land surface influence on deep convection can be sought in the convective precipitation statistical associations of QPVBs at resolutions comparable to their
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ated with the crop–forest QPVBs. The latter statistical relationship will vary intraseasonally in association with vegetation phenology and soil moisture.
FIG. 3. Summary of the GIS technique used to identify (a) quasi-permanent vegetation boundaries and (b) boundary– precipitation associations for 6–19 June 1997. Buffer zones are plotted at successive 20-km-wide increments and overlain with precipitation observations at cooperative stations (entire period).
mutual interactions (cf. Nachamkin 2004). These two approaches are undertaken here.
2. Research hypothesis, objectives, and proposed conceptual model Given the results of Part I, we posit an additional hypothesis 2, as follows: The land surface mesoscale influence on deep convection (CV) in the CCB will be evident on weaker flow days from composite thermodynamic parameters representative of the land surface interaction with the lower atmosphere (e.g., LCL and SLI)—and which manifest air rising from the earth’s surface—and local (10–20-km length) increases in precipitation associ-
As in Part I we adopt a composite method to evaluate hypothesis 2. We express the hypothesized climatic role of Midwest QPVBs in deep convection by the following conceptual model. The vegetation boundaries encourage convection on days (CV) when the lower-atmosphere moisture is greater (from synoptic-scale advection and local evapotranspiration) but not when it is reduced (on NC days). In an environment of ascending air in the free atmosphere [i.e., 700 negative anomaly] solar heating of the dominantly cropped areas promotes air rising from the surface. The greater lower-atmosphere moisture on CV days should permit the marked aerodynamic contrasts across forest–crop boundaries to loft air above the lowered LFC, thereby promoting moisture convergence and enhancing the development of convective clouds and precipitation (cf. Freedman et al. 2001). Although heating of the land surface on NC days also will result in rising air at low altitudes, the reduced humidity in the lower atmosphere will give higher condensation levels, meaning that air is less likely to move above the LFC; especially in the presence of positive anomalies of 700 (i.e., subsidence) on those days. In the latter regard, we note the study of Midwest extreme dewpoint events on multiday time scales by Bentley and Stallins (2008). These authors found high moisture in the PBL attending strong subsidence, apparently related to evapotranspiration from crops and moist soil beneath a strong cap. While broadly similar conditions might occur on certain NC days in our study (e.g., WF), these will not be heavily represented in the composites for which the data on nonadjacent days and with one day per run of days, are averaged for a given background wind speed category (Part I). We evaluate hypothesis 2 via objectives 1 and 2, below (cf. objectives 1–3 in Part I): 1) For V(500) categories showing the fewest statistically significant differences in synoptic fields for “convection” minus “no convection” (CV ⫺ NC) days of summers 1999 and 2000, composite the values of thermodynamic variables retrieved at rawinsonde stations and also represented as mapped fields of NNR. A contribution of LULC to deep convection (i.e., presence of a land surface influence) for a given flow type will be inferred from spatial differences in the locations of strongest composite dynamical forcing [e.g., maxima of negative 700, and observed precipitation, and a closer association of the latter with maxima in variables repre-
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senting the thermal and mechanical land surface aggregated influence on the atmosphere. To evaluate whether such a land surface influence on deep convection is evident on days of stronger background flow (cf. Weaver and Avissar 2001; Weaver 2004a,b), composite thermodynamic variables for the V(500) category showing greatest differences (CV ⫺ NC) in synoptic fields of each summer; 2) If a land surface influence is inferred to be present (objective 1), determine the summer averaged (climatic) statistical associations between CCB vegetation boundary attributes (from satellite NDVI) and precipitation at the higher spatial density afforded by “cooperative” station precipitation observations, for summer seasons 1995–2001. Consider explicitly the role of vegetation phenology by deriving these associations for three adjacent subseasonal periods associated with crop green up and maturity (Schwartz 1998; Schwartz et al. 2002), as follows: early summer (15 June–15 July), midsummer (16 July–15 August), and late summer (15 August–15 September). Seek to explain precipitation– vegetation boundary associations so identified from analysis of the soil moisture.
3. Data and their analysis As in Part I, fulfillment of the research objectives utilizes datasets having a range of spatial and temporal resolutions, as follows: atmospheric reanalyses at 6-h time resolution, used to determine if a land surface climatic influence is detectable in synoptic- and subsynoptic-scale data; vertical temperature and moisture once-daily (early evening) soundings acquired at “upper-air” stations within the CCB, used to derive thermodynamic and stability indices for the spatially integrated atmospheric environment of the CCB on composite CV and NC days; and precipitation once-daily (early morning) reports at relatively high spatial resolution, used to compare with the composite reanalyses and also with satellite NDVI-retrieved locations and dominant attributes (length, width) of QPVBs.
a. NCEP–NCAR reanalyses (NNR) The thermodynamic variables of SLI, “best lifted index” (BLI), and “surface omega” [(sfc)] depicted on NNR 6-h maps (i.e., 0000, 0600, 1200, and 1800 UTC), combine observed and modeled values (i.e., Category B variables in Kalnay et al. 1996). The surface lifted index (SLI) and “SLI minus BLI” (SLI ⫺ BLI) express the thermal influence of the earth’s surface and PBL on deep convection at midlevels (e.g., 500 hPa), and (sfc)
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is a function of hydrodynamic forcing based on the continuity equation. We derive the difference map (SLI ⫺ BLI) because it reveals whether convection truly is surface based (i.e., SLI ⫺ BLI ⬍ 0) or whether it is initiated at slightly above the earth’s surface (i.e., SLI ⫺ BLI ⬎ 0) (cf. Craven et al. 2002). Because the lifted indices (SLI, BLI) show a strong diurnal variation that would be damped by forming composites using the NNR 24-h averages, we compile averages of the two 6-h reanalyses spanning the time of maximum surface heating (i.e., 1800 and 0000 UTC or 1200 and 1800 CST) and for which a land surface thermal influence typically is most evident (e.g., Segal and Arritt 1992). We develop composite fields of SLI, BLI, (SLI ⫺ BLI), and (sfc) by digitizing each day’s map and averaging the gridpoint values in a spreadsheet imported into a GIS spatial interpolation routine (Environmental Systems Research Institute 2004). Similar to the analysis of synoptic circulation influences on precipitation (Part I), the map domain for variables SLI and BLI comprises the region 30°–50°N, 100°–77°W; however, for (sfc) the daily fields are digitized for the smaller domain 36°–46°N, 96°–82°W (Fig. 1), and the values resampled at a spatial resolution of 2.5° ⫻ 2.5° latitude– longitude. The latter is undertaken because (sfc) shows considerable variability on subsynoptic scales (cf. SLI, BLI). Although the composite fields so derived are not as detailed as those of the new “regional reanalysis” (Kistler et al. 2001; Roads 2004; Mesinger et al. 2006), they are appropriate for the present study in which we seek to determine whether a land surface “stationary” influence on deep convection is detectable from generalizing (compositing) across multiple days (objective 1). Moreover, we supplement these mapped analyses with rawinsonde-retrieved data on thermodynamic parameters.
b. Atmospheric soundings Additional thermodynamic parameters, and also stability indices for summers 1999 and 2000, are retrieved from the 0000 UTC rawinsonde ascents at the three upper-air stations located within the CCB [i.e., Davenport, Iowa (DVN, station number 74455); Lincoln, Illinois (ILX, 74560); Wilmington, Ohio (ILN, 72426]; see Fig. 1. Raw sounding data are imported into the upperair software package RAOB (Environmental Research Services, Inc. 1994). The thermodynamic parameters considered in this study are as follows: positive CAPE (units of J kg⫺1), the PBL cap strength (“cap,” °C); altitude of the lifting condensation level (LCL, m), the LFC (m), the LI (°C), the severe weather threat (SWEAT) index (dimensionless), and precipitable water (PW, cm). Although each of these parameters de-
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pends on the synoptic-scale environment [vertical lapse rates are a primary variable in their calculation— smaller-scale variability in surface conditions being particularly important when considering LCL and LFC] these are highly sensitive to surface temperature and humidity. Peppler and Lamb (1989) found that most of the aforementioned indices, especially LCL and SWEAT, correlate significantly with summer afternoon precipitation in the central and eastern United States. The statistical significance of thermodynamic parameter composite differences (i.e., CV ⫺ NC) by V(500) category, is determined using the Student’s t test, which is well suited to smaller samples (e.g., Brown and Hall 1999). Although its use also assumes normality of the sampling distribution, in practice there is usually little difference between a result of statistical significance obtained using a t test and that derived using nonparametric tests having lower power.
c. Station precipitation As in Part I we use the early morning (i.e., 0700 LT ⫾ 2 h) observations of precipitation from approximately 1300 stations in the Midwest CB [(National Climatic Data Center) NCDC 2003], composited according to background flow category and convection/noconvection day type. Due to the localized nature of convective precipitation, composites based upon small sample sizes are more vulnerable to the influence of a single anomalously large event. Accordingly (section 4b; Table 1) we do not study in detail V(500) categories represented by only a few days in either summer 1999 or 2000, especially for CV (e.g., MF-J), and where possible emphasize those comprised of a larger sample of days [e.g., MF(2000)]. To determine station precipitation–vegetation boundary statistical associations for summer seasons in the period 1995–2001, it was first necessary to select boundaries, as summarized in the following steps: 1) identify LULC boundaries on NDVI biweekly composite images [available on CD-ROM from the Earth Resources Observation and Science (EROS) Data Center] using a combination of objective analysis (Fig. 2b) and manual (visual) inspection of processed imagery (available online at http://www.nass.usda.gov/research/avhrr/ history/htm; e.g., Figs. 2a,c and 3a); 2) visually determine boundaries having the most distinct pixel gradients across the boundary region; and 3) both visually and objectively determine boundaries having the most extensive lengths and widths of all boundaries identified on each image. As used in steps 1 and 3, the Boundary-Seer software (TerraSeer Inc. 2000) delineates boundaries through a “wombling” technique that quantifies the spatial rate of change of pixel gradients
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TABLE 1. Summary of t-test results for composite differences (CV ⫺ NC) of NNR variables, stratified by V(500) category. The sign of the difference (CV ⫺ NC) for each significant variable is given by ⫹/⫺. Values in parentheses beside each V(500) category abbreviation are the number of nonadjacent days used in each composite (i.e., CV/NC). The 公 symbol is statistically significant at 0.05 ⬎ p ⬎ 0.01 (two-tailed test) and the 嘸 symbol is statistically significant at 0.01 ⬎ p ⬎ 0.001 (two-tailed test).
Variable
WF (12/10)
W-MF (6/5)
MF (12/7)
MF-J* (3/5)
J (9/9)
(a) Summer 1999 OLR T(sfc) T(850) T(700) T(500) SLP Z(1000) Z(850) Z(700) Z(500) (700) u(500) U(300) V(1000) V(850) V(700) q(850) PW (700)
公⫹
公⫹
公⫺
公⫺ 公⫹ 公⫹
公⫹
公⫺ 公⫺ 公⫺
公⫺
公⫺
公⫹
公 公 公 公 公
公⫹ 公⫹ 公⫹
⫹ ⫹ ⫹ ⫹ ⫹
公⫹ 公⫹ 公⫺
公⫹ 公 公 公 公 公 嘸
公⫹ 公⫺
⫹ ⫹ ⫹ ⫹ ⫹ ⫺
(b) Summer 2000 WF (9/6) OLR T(sfc) T(850) T(700) T(500) SLP Z(1000) Z(850) Z(700) Z(500) (700) u(500) U(300) V(1000) V(850) V(700) Q(850) PW (700)
公 公 公 公
⫺ ⫹ ⫹ ⫹
公⫹ 公⫹ 公⫹ 公⫹ 公⫺
W-MF (10/4) 公 公 公 公 公 公 公
⫺ ⫹ ⫹ ⫹ ⫹ ⫺ ⫺
MF (15/8)
MF-J (7/5)
J (11/9)
公 公 公 公 公 公 公
公⫺
公⫺
公⫺
公⫺ 公⫺ 公⫺
⫺ ⫹ ⫹ ⫹ ⫹ ⫺ ⫺
公⫹
公⫹ 公⫹ 公⫹
公 公 公 嘸 嘸 公
嘸 嘸 公 嘸 嘸 嘸
⫹ ⫹ ⫹ ⫹ ⫹ ⫺
⫹ ⫹ ⫹ ⫹ ⫹ ⫺
公⫹ 公⫹ 嘸 嘸 嘸 公 嘸 嘸
⫹ ⫹ ⫹ ⫹ ⫹ ⫺
公⫹ 公⫹ 公⫹ 公⫹ 嘸⫺
* Insufficient sample size (refer to text).
(Womble 1951). This approach has been used successfully in other ecological applications (e.g., Fortin 1997). Because the software is sensitive to even the smallest boundaries, including stream channels (e.g., Fig. 2b),
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we used the output as guidance to “confirm” our visual inspection of the NDVI imagery and generate the most distinct boundaries (Fig. 3a). Boundary-Seer also generates mean and maximum values of two boundary morphological attributes (step 3): length is determined as the number of lengthwise pixels measured continuously along a boundary, and width is the number of pixels measured perpendicularly across a boundary. For the CCB, a median of six boundaries was identified on each biweekly NDVI composite image. Figure 3b illustrates the GIS mesoscale analysis of vegetation boundary–precipitation climatic associations (objective 2). For each biweekly map of QPVBs (e.g., Fig. 3a), buffer zones were created on both sides of boundaries and precipitation stations were overlain based on their geographic location with respect to a given boundary. Although the software permits buffer widths of 10–50 km, we decided on 20-km-wide buffers to optimize spatial resolution with the number of precipitation stations. Precipitation mean values across summers were calculated by buffer zone surrounding each boundary and by subseasonal period (e.g., early summer). Implicit in this experiment design is the assumption that NCMCs are organized more-or-less symmetrically with respect to a given boundary; this is more likely for the weaker background flow categories (i.e., WF and W-MF) than for stronger flow (i.e., MF, MF-J, and J). Because near-surface wind speeds differ little across categories of background flow, at least on average for the 1999 and 2000 summers (see Table 1 in Part I), vertical tilting of NCMCs may be expected as wind speeds increase rapidly with height under stronger flow conditions. This phenomenon may help explain some of our results [section 4c(1)]. When accurate georeferencing of boundaries was required (e.g., Fig. 3), the analysis period involved a subset of years (e.g., 1996–99), owing to uncertainties generated by variables such as multiple overlapping boundaries that complicated the assignment of precipitation stations to a particular boundary (e.g., summer 1995) or satellite orbital drift in 2000 and 2001 (Gleason et al. 2001). However, the data on boundary attributes length and width retrieved for all summer seasons were used because determination of these values was not influenced by the number of boundaries or by satellite drift.
4. Results a. Composite differences of convection minus no convection (CV ⫺ NC) synoptic fields The number and sign (⫹/⫺) of statistically significant differences (CV ⫺ NC) in NNR composite synoptic
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fields by V(500) category, determined separately for summers 1999 and 2000 (Table 1), provide the basis for identifying candidate flow days suitable for detecting a land surface influence on deep convection (objective 1). For this purpose, a difference field is deemed significant if the critical threshold (two significance levels: p ⫽ 0.05–0.01; p ⫽ 0.01–0.001) is exceeded continuously over at least 20% of the CCB. The 20% criterion accommodates the spatial autocorrelation of many meteorological variables, which artificially increases the significance of differences at a given grid point. Also, because most meteorological variables are significantly autocorrelated at adjacent atmospheric standard levels (e.g., geopotential heights at 500 and 700 hPa), and some variables are intercorrelated ([e.g., PW and q(850): Ross and Elliott 2001], the occurrence of only one “significant” difference for such variables in a given V(500) category may result more from chance than representing a real difference in the (CV ⫺ NC) composite fields. Thus, when a significant difference occurs in a variable at one atmospheric level only [e.g., u(500) for J(1999) and MF-J(2000), Table 1], the possibility that this is a chance outcome increases. The number of atmospheric variables that are statistically significantly different (Table 1) differ markedly between the two summers, being fewer (greater) in 1999 (2000), which coincides with the reduced (greater) precipitation amounts in the CCB (see Fig. 2 in Part I). We infer from this result that synoptic circulation controls on precipitation generally were reduced (enhanced) in summer 1999 (2000). The suggested different roles of synoptic circulation conditions across most V(500) categories in the two summers are supported by the differences (CV ⫺ NC) of outgoing longwave radiation (OLR). For summer 1999, OLR in the CCB was significantly reduced on CV days only for the MF and J categories (Table 1a); however, for summer 2000 all flow-strength categories show statistically significant reductions in OLR on CV days (Table 1b). Moreover, Table 1 further confirms the manual classification of daily V(500) undertaken in Part I, from the lack of significant differences for both u(500) and u(300) in the two summers. A marked discrepancy in the summer 1999 results (Table 1a) involves adjacent flow categories J and MF-J having, respectively, the most and fewest significant differences for composite (CV ⫺ NC) days. The result for J is reasonable physically, given marked differences in SLP, height, and vector winds; however, that for MFJ/CV is explained by the very small sample of days (3) so classified. Examination of the rawinsonde data for Lincoln (ILX) and Wilmington (ILN) shows that 2 of
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FIG. 4. Composite difference map of the total precipitation (mm) for the convection minus no-convection (CV ⫺ NC) days classified as W-MF in summer 1999.
these 3 days were characterized by isentropic uplift of rich e air in a potentially unstable midtroposphere, thereby biasing the composites (cf. Moore et al. 2003). Because the results for MF-J(1999) cannot be considered representative of this V(500) category in this summer, they are not examined further.
b. Land surface aggregated contribution to convection for contrasting V(500) categories The weaker flow days (i.e., WF and W-MF) tend to show the fewest significant differences in composite synoptic fields (CV ⫺ NC) in both summers (Table 1), implying a greater relative influence of land surface processes on deep convection for these days contrasted with stronger flow days (hypothesis 2). Accordingly, we selected the W-MF day type in 1999 (comprising 10% of all days) and WF in 2000 (16% of days) for detailed analysis (objective 1). We anticipated (hypothesis 2 and objective 1) that dynamical forcing of convection on stronger flow days either would inhibit a land surface role in convection or mask it in our analyses (cf. Doran and Zhong 2000). To test this, we compare the results obtained for the weaker flow day types with those of composite J days in 1999 (23% of all days) and MF days in 2000 (24% of all days); those V(500) categories having the greatest number of composite difference fields (CV ⫺ NC) that are statistically significant in their respective summers (Table 1). Although it makes physical sense that the jet maximum category (1999, Table 1a) exhibits the largest number of significant differences in synoptic variables between CV and NC days,
such a physical link is less obvious for MF(2000) (Table 1b). An argument could be made for selecting MFJ(2000) instead of MF(2000): the same number of variables exhibit highly significant differences (at p ⬍ 0.01), and the significant differences at p ⬍ 0.05 associated with temperature and height on the 850-, 700-, and 500hPa surfaces [MF(2000)] could result from intercorrelation. However, because MF(2000) included a comparable number of days to that of J(1999), we decided to analyze this day type further. Moreover, analysis of both J(1999) and MF(2000) permits us to comment on whether a land surface influence on deep convection is detectable across a range of V(500) categories.
1) WEAK-MODERATE [W-MF(1999)]
FLOW, SUMMER
1999
The composite difference map (CV ⫺ NC) of observed precipitation for the W-MF(1999) day type (Fig. 4) shows a swath of positive departures across the southern Midwest. The lack of strong synoptic forcing of deep convection in the CCB is indicated by the schematic of circulation anomaly features composited for CV days (Fig. 5a), which looks broadly similar to that for NC (Fig. 5b); specifically, there is southerly flow at low levels, ridging occurs in midtroposphere, and major moisture axes at 850 hPa lie outside the CCB on both CV and NC days (refer Fig. 5c). However, the two day types (CV and NC) are most different synoptically in the field of (700) (cf. Portis and Lamb 1988; Birmingham and Lamb 1994): a relatively weak center of negative , denoting rising air, is located over Illinois on CV
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FIG. 5. Schematic synoptic features map for the W-MF day type in summer 1999: (a) CV days and (b) NC days. The legend is the same as that used to depict circulation features of the primary V(500) categories in Part I, where different colors pertain to different variables: positive (negative) anomalies are designated as solid (dashed) isolines, multiple light blue contours denote large anomalies of (700), and a thickening of the vector wind axis at 850 hPa indicates where the composite wind speed exceeds 9 m s⫺1.
days (dashed blue ellipse), but is displaced to the northwest—outside the CCB—on NC days. Therefore, much of the CCB is located beneath an area of quite small difference in (700) (Fig. 6a), with weakly upward (downward) motion occurring on CV (NC) days. Because the strong mesoscale increases in precipitation over much of the CCB on CV days (Fig. 4) do not seem to be explained by the composite synoptic conditions, we investigate the possibility of a land surface contribution. A land surface thermal influence on convection in the CCB for W-MF(1999) is suggested by negative values in the SLI composite difference (CV ⫺ NC) map
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FIG. 6. For the W-MF day type in summer 1999, the composite difference patterns (CV ⫺ NC) of (a) (700) anomaly (Pa s⫺1) and (b) SLI (°C).
for the afternoon and evening combined hours (Fig. 6b). Moreover, on CV days (Fig. 7a), strongly negative SLI coincides with the negative center (sfc), implying enhanced ascent of air from the earth’s surface (i.e., free convection). On NC days (Fig. 7b), the area of negative (sfc) over Illinois is associated with SLI values of around zero, reducing the likelihood of surfacebased moist convection there; especially in the eastern CCB. Spatial patterns of the composite difference (SLI ⫺ BLI) confirm the contrast between the W-MF day types (CV and NC) in terms of the implied influence of surface-based thermodynamic processes on deep convection (not shown): on W-MF/CV days, negative values of
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FIG. 7. For the W-MF day type in summer 1999, the composite patterns of (a) SLI (°C, thinner lines) and (sfc) (Pa s⫺1, thicker lines) on CV days, (b) SLI and (sfc) on NC days, (c) (700) anomaly (Pa s⫺1, thinner lines) and (sfc) (thicker lines) on CV days, and (d) (700) anomaly and (sfc) on NC days. In all maps, positive (negative) values are denoted by solid (dashed) isolines.
(SLI ⫺ BLI) cover much of the CCB (i.e., surfacebased initiation of convection), but the weakly positive values on NC days indicate stability near the earth’s surface. Taken together, the map composites of these thermodynamic variables for W-MF(1999) suggest that surface-based ascent of air due to both thermal and mechanical processes was enhanced (reduced) on CV (NC) days. The contrasts in SLI and (SLI ⫺ BLI) between CV and NC days in the CCB occur within a larger synoptic environment that favors ascent of air in the midtroposphere on both sets of days, but especially on CV (Figs. 7c,d). To help yield further insights into the differences between CV and NC day types associated with
W-MF(1999)—specifically moisture—Table 2a gives composite values of the rawinsonde-retrieved thermodynamic and stability parameters, and their statistical significance (i.e., CV ⫺ NC). Parameters LFC, LCL, PW, surface convection temperature (Tc), and SWEAT indicate greater moisture in the lower atmosphere on CV than NC days, although only PW is statistically significant ( p ⬍ 0.05). Accordingly (Table 2a), condensation levels were lower and the atmosphere more unstable (i.e., greater positive CAPE, more negative LI) on CV days contrasted with NC days (cf. Market et al. 2003), providing at least tentative support for the conceptual model proposed in section 2. The increased moisture on CV days likely was a combination of water
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TABLE 2. Thermodynamic parameters composited by V(500) category. Values are averages for the three rawinsonde stations in the CCB (DVN, ILN, and ILX); 0000 UTC soundings. Alpha (␣) values in bold indicate statistically significant differences (CV ⫺ NC) for that flow type, given the degrees of freedom (df) and y values. LFC (m)
LCL (m)
PW (cm)
Tc (°C)
cap (°C)
⫹CAPE
LI (°C)
SWEAT
0.9 1.2 ⫺0.51 13 0.58
2886.1 1992.7 1.10 21 0.29
⫺4.3 ⫺1.4 ⫺1.60 26 0.13
211.5 179.4 0.91 24 0.37
1.4 1.8 ⫺0.44 13 0.66
1825.3 1434.5 0.81 23 0.42
⫺4.1 0.2 ⫺3.09 34 0.00
234.5 140.1 3.91 34 0.00
1.5 1.5 0.00 26 1.00
1297.7 382.3 2.17 19 0.04
⫺0.3 3.5 ⫺4.14 39 0.00
180.5 112.9 3.38 47 0.00
1.1 2.4 ⫺3.17 14 0.01
1867.3 698.8 3.83 16 0.00
⫺3.5 1.4 ⫺3.92 13 0.00
243.0 146.7 5.04 18 0.00
(a) W-MF in 1999 CV NC t df ␣
4707.4 5857.0 ⫺0.83 9 0.42
4000.4 5251.8 ⫺1.47 20 0.16
1.4 1.1 2.17 28 0.04
35.1 36.5 ⫺0.72 26 0.47 (b) WF in 2000
CV NC t df ␣
1761.0 2358.9 ⫺0.79 16 0.44
1004.4 1405.7 ⫺2.35 39 0.02
1.4 1.0 4.31 34 0.00
32.2 32.5 ⫺0.20 28 0.84 (c) J in 1999
CV NC t df ␣
2035.2 2712.9 ⫺0.94 21 0.35
1147.0 1577.1 ⫺3.18 50 0.00
1.1 0.7 4.58 36 0.00
32.4 33.4 ⫺0.54 46 0.59 (d) MF in 2000
CV NC t df ␣
1185.3 2334.2 ⫺3.26 5 0.01
719.1 1044.4 ⫺2.91 14 0.01
1.5 1.0 5.64 17 0.00
31.3 34.4 ⫺1.47 10 0.18
vapor advected into the CCB, and that evapotranspired from croplands within the region (e.g., Sandstrom et al. 2004; Cheresnick and Basara 2005; Notaro et al. 2006; Bentley and Stallins 2008).
2) WEAK
FLOW, SUMMER
2000 [WF(2000)]
For WF(2000), the OLR differences (Fig. 8a) are statistically significant for the CCB (Table 1b), and precipitation is substantially greater on CV contrasted with NC days (Fig. 8b). Schematic maps of the synoptic circulation anomaly features on WF/CV and WF/NC days (see Figs. 8a, b in Part I) confirm relatively little difference in most fields for the CCB, except again that of (700), whereby negative values occurred on CV days and strongly positive values (i.e., strong subsidence) on NC days. The heterogeneous pattern to the precipitation difference map (Fig. 8b) again suggests a lack of synoptic forcing, and the operation of additional factors in the elevated amounts occurring at mesoscales. On WF/CV days (Fig. 8c), an area of maximum difference (CV ⫺ NC) in the SLI field for the combined afternoon and evening period covers the CCB and is accompanied
by negative differences in (sfc); similar to WMF(1999). Composites of (SLI ⫺ BLI) on CV days (not shown) confirm surface-based convection—negative differences—on CV days, but positive differences on NC days. As also evident for W-MF(1999), the composite fields of (sfc) differ in their relationship to SLI according to CV versus NC day types for WF(2000); supporting a possible land surface–deep convection influence for this weaker flow day type. On CV days (Fig. 9a), both (sfc) and SLI are directed strongly upward throughout the CCB. On NC days (Fig. 9b), negative values of (sfc) indicate air rising from the earth’s surface; however, this is accompanied by positive SLI or a reduced magnitude of vertical motion necessary for moist convection. There is also a contrast between CV and NC day types in the relationship of (sfc) to the free-atmosphere vertical motion represented by (700); both are directed upward on CV days (Fig. 9c), potentially enhancing deep convection, but the negative (sfc) on NC days is opposed by sinking air aloft (Fig. 9d), suppressing deep convection. It is interesting that the tight horizontal gradients in (sfc) over south-
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FIG. 8. Composite difference maps (CV ⫺ NC) for days classified as WF in summer 2000: (a) OLR anomaly (W m⫺2), (b) observed precipitation (mm), and (c) SLI (°C, thinner lines) and (sfc) (Pa s⫺1, thicker lines) where positive (negative) values are denoted by solid (dashed) isolines.
ern Indiana (Figs. 8c and 9a) more or less coincide with prominent transition zones of vegetation and orography (e.g., Figs. 2a,c). The WF(2000) day type shows greater (reduced) moisture in the lower troposphere on CV (NC) composite days (Table 2b), similar to the pattern noted for W-MF(1999); only the differences are statistically significant for a greater number of variables (i.e., LCL, PW, LI, SWEAT). Thus, consistent with the hypothesized conceptual model (section 2), deep convection in the CCB may have been enhanced on CV days by aerodynamic contrasts between trees and crops, occurring within unstable atmospheric conditions and a synoptic-
scale environment that favored upward vertical motion in the midtroposphere, that is, negative (700). Conversely on WF/NC days, condensation levels were higher, the lower atmosphere was more stable, and the sign of midtroposphere vertical motion (positive) was detrimental to air ascending through an appreciable depth (e.g., slightly greater cap strength; Table 2b). Under these conditions, the LULC contrasts were evidently not sufficient to generate deep convection.
3) JET
MAXIMUM, SUMMER
1999 [J(1999)]
To contrast with WF(1999), the statistically significant differences in tropospheric composite fields on
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FIG. 9. For the WF day type in summer 2000, the composite patterns of (a) SLI (thinner lines) and (sfc) (thicker lines) on CV days, (b) SLI and (sfc) on NC days, (c) (700) anomaly (thinner lines) and (sfc) (thicker lines) on CV days, and (d) (700) anomaly and (sfc) on NC days. In all maps, positive (negative) values are denoted by solid (dashed) isolines.
J(1999) days (see Figs. 7e, f in Part I) include large differences in OLR for the Indiana–Ohio–Michigan area (this paper, Fig. 10a) and in observed precipitation (Fig. 10b). The latter are greatest and spatially most coherent over Ohio, corresponding to the composite jet exit region. On CV days, coherent positive departures of precipitation over Iowa (Fig. 10b) are associated with uplift on the south side of the jet, and along a moisture axis intersected by a southerly wind maximum at 850 hPa (see Fig. 7e in Part I). The anomaly center in the difference pattern of (700) (this paper, Fig. 10c) is spatially synchronous with those of OLR (Fig. 10a) and PW (not shown), confirming ascent and moistening of air over the Corn Belt on J/CV days. On NC days, sinking air in the midtroposphere accompanies PW negative anomalies. Even on this stronger flow day type, the spotty pre-
cipitation increases evident on J/CV days versus J/NC (Fig. 10b) between western and eastern portions of the CCB, and their spatial separation from the anomaly centers of (700), PW, and OLR, suggest the operation of mesoscale processes potentially connected with the land surface. This possibility is supported by the SLI difference pattern (CV ⫺ NC, Fig. 10d), which resembles those of the two weaker flow day types studied for 1999 and 2000 (Figs. 6b and 8c). Moreover, the composite (SLI ⫺ BLI) map for J/CV days (not shown) indicates slightly negative values (i.e., surface-based ascent of air) over much of the CCB, but positive values on NC days. Negative differences (CV ⫺ NC) of (sfc) for most of the CCB again coincide with negative differences of SLI, although the east–west contrasts are quite small (Fig. 10d). The composites of (sfc) and (700) for J/CV days of 1999 (Fig. 11a) imply that air
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FIG. 10. Composite difference maps (CV ⫺ NC) for days classified as jet maximum in summer 1999, for the following variables: (a) OLR anomaly (W m⫺2), (b) total precipitation (mm), (c) (700) anomaly (Pa s⫺1), and (d) SLI (°C, thinner lines) and (sfc) (Pa s⫺1, thicker lines) where positive (negative) values are denoted by solid (dashed) isolines.
rising from the earth’s surface was supported by strong upward motion in the midtroposphere [cf. W-MF(1999) and WF(2000)]. Farther west in the CCB, sinking air at the earth’s surface opposed rising air in the free atmosphere. Conversely, on J/NC days (Fig. 11b) the entire region lay beneath sinking air in midtroposphere, while slightly negative (positive) values of (sfc) occurred over eastern (western) parts of the CCB. Thus, despite the strong synoptic forcing of deep convection evident on J/CV(1999) in the CCB, the land surface evidently still played a role in contrast to NC days. Further support for this possibility appears in the composite statistics of CCB rawinsonde-retrieved thermodynamic parameters on J(1999) days, which are broadly similar to
those for W-MF(1999) and WF(2000); instability and atmospheric moisture were significantly greater on CV than NC days (Table 2c).
4) MODERATE
FLOW, SUMMER
2000 [MF(2000)]
For MF(2000), selected as the stronger flow day type to be analyzed in summer 2000, the OLR difference (CV ⫺ NC) map (Fig. 12a) shows coherent negative values over the entire Corn Belt maximizing (⬎⫺45 W m⫺2) in Iowa, much of Illinois, and southern Wisconsin. The precipitation difference pattern (Fig. 12c) exhibits even less correspondence with the difference patterns of OLR (Fig. 12a), (700) (Fig. 12b), and PW (not shown) than for J(1999), especially for southern areas
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lyzed: the lower atmosphere is moister (i.e., greater PW, lower LFC and LCL) and more unstable (greater positive CAPE and SWEAT, more negative LI, and a reduced cap) on days classified as CV rather than NC. For MF(2000), all thermodynamic variables were significantly different between CV and NC days, except the Tc. Thus, these results again suggest a contributory influence of the CCB land surface on deep convection, even in the presence of strong synoptic forcing. This possibility is consistent with the studies by Weaver and Avissar (2001) and Weaver (2004a,b), which provided evidence for surface-induced mesoscale circulations in the southern Great Plains over a range of background wind speeds. As expressed in the hypothesized conceptual model (section 2), the determining factors on CV days for both weaker flow and stronger flow day types appear to be the presence of at least weakly upward vertical motion in midtroposphere negative (700); upward vertical motion of air near the earth’s surface [i.e., negative SLI, negative (SLI ⫺ BLI), negative (sfc))]; and increased moisture in the lower atmosphere, in contrast to NC days. Moreover, a spatial coincidence of the negative maxima in mapped fields of SLI, (SLI ⫺ BLI), and (sfc) occurs for composite CV days but not NC days.
c. LULC boundaries as the potential mechanism for enhancing Midwest CCB convection
FIG. 11. For the jet maximum day type in summer 1999, the composite patterns of (a) (700) anomaly (Pa s⫺1, thinner lines) and (sfc) (thicker lines) on CV days and (b) (700) anomaly and (sfc) on NC days. In all maps, positive (negative) values are denoted by solid (dashed) isolines.
of Illinois and Indiana. Moreover, the lack of spatial association between the composite maxima of freeatmosphere variables depicting or representing strong upward vertical motion and those areas experiencing some of the highest precipitation departures on MF/CV days (Fig. 13a) again suggests a contributory influence of the land surface on deep convection. This possibility is supported by the much closer spatial association of these high precipitation areas with the centers of maximum negative difference (CV ⫺ NC) of SLI and (sfc) (cf. Figs. 13a,b). Composite values of rawinsonde-retrieved thermodynamic parameters for MF(2000) days (Table 2d) are consistent with results for the other flow categories ana-
The foregoing results, based on relatively coarseresolution data, support the presence of a land surface influence on summertime deep convection (CV) for the CCB, across a range of background flow categories in the contrasting (precipitation) summers of 1999 and 2000. To evaluate more directly the hypothesized climatic role of the quasi-permanent vegetation boundaries in enhancing deep convection on days identified as CV from the afternoon and evening Next Generation Weather Radar (NEXRAD) data, we determine statistical relationships between precipitation amounts and the boundaries at similar spatial resolutions for summer seasons in the longer period 1995–2001 (objective 2).
1) BOUNDARY–PRECIPITATION V(500) CATEGORIES
ASSOCIATIONS BY
The LULC boundaries are most distinct in early summer (e.g., Fig. 2a). Accordingly, for the four adjacent early summer seasons in the middle of our study period (1996–99) that showed a representative range of precipitation conditions and for which accurate geolocation of boundaries was necessary (section 3c), we averaged the precipitation data according to station perpen-
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FIG. 12. Composite difference maps (CV ⫺ NC) for days classified as MF in summer 2000, for the following variables: (a) OLR anomaly (W m⫺2), (b) observed precipitation (mm), and (c) (700) anomaly (Pa s⫺1).
dicular distance from the nearest boundary and for two broad groups of V(500); weaker flow (combined WF and W-MF) and stronger flow (combined MF, MF-J, and J). Combining V(500) categories into one of two larger groups is permissible, given our earlier results (section 4b) that showed marked differences in the implied synoptic contribution to deep convection and precipitation, for example, (700) of the weaker flow versus stronger flow categories. For weaker flow CV days (Fig. 14a), the mean precipitation is 288 mm within 20 km of a quasi-permanent boundary, but only 241 mm for stations ⬎20 km away; a statistically significant difference. In contrast, the corresponding mean precipitation amounts on stronger flow CV days (Fig. 14b) are 339 and 356 mm, not significantly different. Experimenting with different-sized buffers for the stronger
flow days did not appreciably change the results shown in Fig. 14b. Thus, a climatic role for the Midwest forest– crop boundaries in precipitation is suggested on days of weaker background flow, at least, in early summer. Although the results do not support a similar role on stronger flow days [cf. the results in sections 4b(3) and 4b(4)], any NCMCs present probably would be tilted with height or even advected away from the boundary location, thereby masking a boundary–precipitation association. To evaluate further a possible QPVB influence on precipitation it is appropriate to correlate boundary attributes of length and width (section 3c) with precipitation amounts by subseasonal periods. Such an analysis can be undertaken using data for all study years (1995–2001) because accurate georeferencing of boundary locations is not required.
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FIG. 13. For the MF day type in summer of 2000, the composite patterns of (a) observed total precipitation (mm) on convection days and (b) difference map (CV ⫺ NC) of SLI (°C, dashed lines) and (sfc) (Pa s⫺1, solid lines).
2) BOUNDARY
ATTRIBUTES IN RELATION TO
OBSERVED PRECIPITATION
Scatterplots of the association between mean values of CCB boundary attributes and mean precipitation amounts show a reversal in sign between early summer (Figs. 15a,b), when the relationship is positive, and late summer (Figs. 15c,d), when it is negative. There is no relationship between boundary attributes and precipitation in midsummer (not shown). Although the correlation coefficients of the plots shown in Fig. 15 are not significant statistically, due to the small sample size, the fact that the associations are positive (negative) for both attributes in the early (late) summer suggests that they may be meaningful physically. With the analysis of additional years of data—increasing the sample size—it is likely that the mean precipitation–NDVI attribute relationships shown here would become statistically significant. Support for this assertion comes from a related analysis we undertook of maximum values of boundary parameters length and width for a combined mid-tolate-summer period 16 July–31 August of 1995–2001 (not shown), which increases the sample size from 14 to 21. The precipitation–boundary relationship is confirmed to be negative for both length (r ⫽ ⫺0.46) and width (r ⫽ ⫺0.56), and also is statistically significant ( p ⬍ 0.05). In combination with the apparent increased sensitivity of the most dominant boundaries for each biweekly period—defined by their maximum length and width—the increased sample size supports our contention that boundary attributes are both statistically and physically important for influencing precipitation distribution in the CCB. The precipitation–boundary relationships (Fig. 15)
are consistent with phenological changes in the forest and crop areas between early and late summer, which manifest a reversal in their relative NDVI: from dense forest vegetation that is actively photosynthesizing (i.e.,
FIG. 14. Histograms of total precipitation (CV days) for earlysummer periods (15 Jun–15 Jul) of 1996–99 (refer to text), stratified by distance class (20-km increments) from quasi-permanent LULC boundaries on (a) weaker flow days (i.e., WF ⫹ W-MF) and (b) stronger flow days (MF, MF-J, and J).
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FIG. 15. Scatterplots of NDVI biweekly mean values of vegetation boundary parameters (length, width) and observed precipitation (mm) for summer subseasonal periods of 1995–2001: (a) early-summer length, precipitation; (b) early-summer width, precipitation; (c) late-summer length, precipitation; and (d) late-summer width, precipitation. The 14 data points in each graph include the two NDVI values for each of the seven years (i.e., 2 ⫻ 7).
high NDVI), located adjacent to low density of immature crops with some bare soil exposed (i.e., lower NDVI; cf. Figs. 2a and 16a) to a high density of maturing crops (i.e., increased NDVI) adjacent to trees for which the NDVI changes little after green-up in April and May (Fig. 16a). The seasonal reversal in relative NDVI of croplands and forest areas of the Midwest CB is strongly related to differences in vegetation rooting depth and the soil moisture (Carleton et al. 1994; Adegoke and Carleton 2002; Leavitt 2002, 2007). Neutronprobe measurements of moisture in the upper 30 cm of soil were acquired for two sites [Carbondale, Illinois (SIU) and Bondville, Illinois (BVL)] where the dominant land cover (⬎70%) over about a 3.5-km radius centered on the site is, respectively, forest and cropland. Figure 16b, confirms results for the earlier period 1990–94 (Adegoke and Carleton 2002, their Fig. 3): for summer as a whole, the depletion of soil moisture is sustained under forests (trees have deeper roots), but water usage for croplands is greater in the June–August period when corn is rapidly growing and maturing (e.g., Prueger et al. 2004; Hatfield et al. 2007). For these cropped areas, the rapid depletion of soil moisture and the accompanying strong increases in NDVI imply large evaporative losses to the atmosphere. In contrast, the broadleaf trees characteristic of the Corn Belt tend to reduce evapotranspiration through increased stomatal resistance as daytime temperatures reach their seasonal maxima in mid- to late summer (e.g., Bonan 2001; Leavitt 2007). Because the sign change in the relationship between
FIG. 16. Mean (1995–2001) subannual “cycle” of (a) NDVI and (b) water depth by volume (mm) in the top 30 cm of soil at sites representative of areas where the dominant land cover is forest (SIU) and cropland (BVL).
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precipitation mean amounts and vegetation boundary attributes (Fig. 15) between early summer (positive) and late summer (negative) coincides with distinct changes in the NDVI and soil moisture attributes of conjoint forest and cropped areas (Fig. 16), we can speculate on the physical mechanisms at work. We propose that lifting of the air by convergence along the vegetation boundaries, or “moisture pooling” (e.g., Johns 1993), is favored in early summer when boundaries are distinct, but not favored in late summer when boundaries are less distinct. In early summer, given a synoptic environment of negative (700), the trees may enhance deep convection by their large evapotranspiration values (high NDVI, strong depletion of soil moisture) and by the aerodynamic effect of their large height relative to adjacent bare ground or immature crops. Thus, wider and longer boundaries could enhance moisture pooling, increasing the precipitation (Figs. 15a,b). Conversely, in late summer, precipitation in the vicinity of longer and wider boundaries is reduced (Figs. 15c,d), possibly because the trees are conserving water, and the greater evapotranspiration rates shift to the crops that have reduced aerodynamic effect relative to trees (they grow closer to the ground). This potential mechanism also would explain the lack of association noted between boundary attributes and precipitation mean amounts for the midsummer period (mid-July to mid-August): the seasonal trend of soil moisture for cropped areas reverses at that time in contrast to forest areas (Fig. 16b), thereby offsetting the precipitation–boundary positive relationship of early summer with the negative relationship of late summer. Although confirmation of the physical processes proposed here awaits further study (e.g., using combinations of field analyses and mesoscale modeling), our present results indicate that the CCB vegetation boundary–precipitation relationship changes intraseasonally in association with vegetation phenology and soil moisture differential patterns in conjoint crop and forest areas.
5. Summary and concluding remarks Using an observational approach, we infer the presence of LULC mesoscale (i.e., “bottom up”) climate conditions to deep convection in the CCB for two summers of contrasting precipitation amounts (1999 dry; 2000 moist) and highlight explicitly the possible role of QPVBs in precipitation for summer seasons during the longer period 1995–2001. A spatially aggregated influence of the land surface on convection in summers 1999 and 2000 is inferred from composite thermodynamic parameters represented in the coarse-resolution NNR
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fields and station rawinsonde data, for both “weaker flow” and “stronger flow” day types that exhibited the fewest and most, respectively, significant differences (i.e., CV ⫺ NC) in synoptic fields for their respective summer seasons [W-MF(1999) and WF(2000); J(1999) and MF(2000)]. Specifically, days classified from the NEXRAD data as CV (both weaker flow and stronger flow) showed a spatial coincidence of composite maxima in NNR thermodynamic variables in the CCB, as follows: negative SLI, negative (SLI ⫺ BLI), and negative (sfc). The implied influence of the land surface on deep convection took place in an environment of negative anomalies of (700). On NC days, despite the presence of negative (sfc), the observed associations discouraged moist convection, as follows: positive or near zero values of SLI, positive (SLI ⫺ BLI), and near-zero or positive anomalies of (700). The critical synoptic forcing factor differentiating CV from NC days seems to be the vertical motion in the midtroposphere (cf. Portis and Lamb 1988; Birmingham and Lamb 1994). On the weaker flow and stronger flow day types analyzed in detail for summers 1999 and 2000, CV days exhibited greater atmospheric moisture and instability compared to NC days of the same V(500) category. The increased moisture occurred consistently for PW, likely a combination of moisture advected into the region and that derived locally from evapotranspiration over extensive croplands (cf. Bentley and Stallins 2008). The greater moisture and instability similarly were indicated by increased values of SWEAT and positive CAPE, more negative LI, and lower altitudes of condensation levels and the LFC. That this effect was evident on weaker flow and stronger flow days alike in the contrasting summers 1999 and 2000 may indicate a climatically significant land surface influence on Midwest convection, similar to that identified for the southern Great Plains (Weaver 2004b). The inferential results supporting a CCB land surface influence on deep convection are consistent with our hypothesized conceptual model involving explicitly the LULC boundaries between forest and crop areas; that is, aerodynamic discontinuities associated with the QPVBs (cf. Sakai et al. 2001) may help loft air above the LFC on CV days—when the lower atmosphere is moister and less stable—contrasted with NC days. Therefore, we consider this part of hypothesis 2 essentially supported, although the presence of a land surface influence on convection for stronger flow days in 1999 and 2000 was unexpected and tantalizing. Accordingly, we analyzed statistical associations between physical attributes of the QPVBs (length, width), retrieved using a combination of objective and manual
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analysis methods applied to biweekly satellite NDVI images and precipitation at the high spatial resolutions yielded by the “cooperative” station network, for summer seasons in the period 1995–2001. For the subseasonal period when vegetation boundaries are most distinct (mid-June–mid-July), statistically significant associations between QPVBs and precipitation in the CCB were evident on weaker flow days; precipitation was greater at stations within 20 km of a boundary, contrasted with amounts farther away. This effect was not evident on stronger flow days—at least, for early summer, possibly because the greater wind speeds above the PBL vertically tilt NCMCs associated with the boundaries. Correlating boundary mean length and width values with mean precipitation for the early (late) summer periods of 1995–2001 reveals positive (negative) associations that are evidently related to phenology and soil moisture climatic differences between conjoint forest and crop areas. We argue that “moisture pooling” along the boundaries changes from being enhanced in early summer to not being favored in the late summer, as trees restrict their water loss and the adjacent crops mature. Future work utilizing field analyses and mesoscale modeling should test this hypothesized mechanism for the LULC boundary–convection feedback. This study did not address the relative contributions of local versus remote moisture sources on CV days. Such an undertaking might utilize a backtrajectory method to separate evapotranspiration from vegetation and moisture advected from the Gulf of Mexico (e.g., Cheresnick and Basara 2005). Also, future empirical work for the Midwest CB should seek to explicitly separate the influences of vegetation and soil moisture at subgrid scales (cf. Findell and Eltahir 1997; Koster et al. 2003; Notaro et al. 2006). An observational analysis of this type for case days or time periods, run in tandem with regional-scale model simulations of surface– atmosphere interactions (cf. Pielke et al. 1999; Weaver and Avissar 2001; Weaver 2004a,b; Miguez-Macho et al. 2005; Adegoke et al. 2006), would benefit from the higher resolution and improved moisture budget of the new NCEP “regional reanalysis” (Roads 2004; Mesinger et al. 2006; Bentley and Stallins 2008). Finally, our results have implications for efforts to improve short-term predictions of convective precipitation in this agriculturally crucial region (e.g., Westcott et al. 2005), specifically, incorporating location knowledge of the Midwest’s dominant LULC boundaries into forecast fields of lower-to-mid-tropospheric winds and humidity (cf. Fritsch and Carbone 2004; Holt et al. 2006). Ultimately, it is hoped that such an improved spatial resolution of precipitation forecasts may occur
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on lead times of several months (cf. Koster and Suarez 2004). Acknowledgments. This research was supported by NSF ATM Grants 9876753, 9876823, and 9876828. We are grateful to the following student research assistants: Gordon Strassberg (PSU and Ball State University: BSU); Leia Jamieson, Chris Ruelle, and Mieko YokoiShelton (UWW); and Angela Burkhardt (BSU). The comments of the anonymous reviewers on the first submission of this paper were helpful when revising the manuscript, which was undertaken while the first author was the EESI Scholar-in-Residence (January–July 2007). REFERENCES Adegoke, J. O., and A. M. Carleton, 2002: Relations between soil moisture and satellite vegetation indices in the U.S. Corn Belt. J. Hydrometeor., 3, 395–405. ——, S. Vezhapparambu, C. L. Castro, R. A. Pielke Sr., and A. M. Carleton, 2006: Influence of variations in low-level moisture and soil moisture on the organization of summer convective systems in the U.S. Midwest. Preprints, 18th Conf. on Climate Variability and Change/20th Conf. on Hydrology, Atlanta, GA, Amer. Meteor. Soc., J5.12. [Available online at http:// ams.confex.com/ams/pdfpapers/104829.pdf.] ——, R. A. Pielke Sr., and A. M. Carleton, 2007: Observational and modeling studies of the impacts of agriculture-related land use change on planetary boundary layer processes in the central U.S. Agric. For. Meteor., 142, 203–215. Baidya Roy, S. B., C. P. Weaver, D. S. Nolan, and R. Avissar, 2003: A preferred scale for landscape forced mesoscale circulations. J. Geophys. Res., 108, 8854, doi:10.1029/2002JD003097. Banta, R. M., and A. B. White, 2003: Mixing-height differences between land use types: Dependence on wind speed. J. Geophys. Res., 108, 4321, doi:10.1029/2002JD002748. Barros, A. P., and W. Hwu, 2002: A study of land-atmosphere interactions during summertime rainfall using a mesoscale model. J. Geophys. Res., 107, 4227, doi:10.1029/2000JD000254. Bentley, M. L., and J. A. Stallins, 2008: Synoptic evolution of Midwestern U.S. extreme dew point events. Int. J. Climatol., in press, doi:10.1002/joc.1626. Birmingham, D. L., and P. J. Lamb, 1994: Diagnostics of growing season rainfall fluctuations in central North America. Proc. 18th Annual Climate Diagnostics Workshop, Washington, D.C., U.S. Dept. of Commerce, 283–286. Blythe, E. M., A. J. Dolman, and J. Noilhan, 1994: The effect of forest on mesoscale rainfall: An example from HAPEXMOBILHY. J. Appl. Meteor., 33, 445–454. Bonan, G. B., 2001: Observational evidence for reduction of daily maximum temperature by croplands in the Midwest United States. J. Climate, 14, 2430–2442. Brown, M. E., and D. L. Arnold, 1998: Land surface-atmosphere interactions associated with deep convection in Illinois. Int. J. Climatol., 18, 1637–1653. Brown, T. J., and B. L. Hall, 1999:The use of t values in climatological composite analyses. J. Climate, 12, 2941–2944. Brubaker, K. L., P. A. Dirmeyer, A. Sudradjet, B. S. Levy, and F. Bernal, 2001: A 36-yr climatological description of the evapo-
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