Use of a Mesoscale Model to Forecast Severe Weather Associated

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Aug 1, 2002 - butions of various convective indices related to insta- bility and shear .... gradient that lingers behind the lee trough (i.e., the gra- dient over Iowa ...
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Use of a Mesoscale Model to Forecast Severe Weather Associated with a Cold Front Aloft STANLEY F. ROSE, PETER V. HOBBS, JOHN D. LOCATELLI,

AND

MARK T. STOELINGA

Department of Atmospheric Sciences, University of Washington, Seattle, Washington (Manuscript received 26 April 2001, in final form 11 February 2002) ABSTRACT A forecast of severe weather and the potential for tornadoes associated with a cyclone that developed in the lee of the Rocky Mountains on 19–21 June 2000 is evaluated. The forecasting methods used by the National Weather Service for this case, which focused on the position of a surface trough and the location of favorable quasigeostrophic jet dynamics, poorly predicted the extent and location of the severe weather. Application of a conceptual model for cyclones east of the Rockies, which highlights the importance of cold fronts aloft (CFA), shows that a CFA was an important trigger to convection in the 19–21 June 2000 cyclone. A simple forecasting method is demonstrated that emphasizes the importance of lifting for cases that involve CFA. This method is applied to the 19–21 June 2000 cyclone and is found to improve greatly the determination of where severe weather occurred.

1. Introduction Forecasts of thunderstorms and associated severe weather have traditionally focused on four essential criteria: sufficient conditional instability, adequate moisture for deep convection, a lifting mechanism, and sufficient shear to support long-lasting storms (Johns and Doswell 1992; Galway 1992). Moisture, shear, and lifting are generally identified through direct measurement and thermodynamic analysis. However, the spatial and temporal prediction of vertical motion is a particularly challenging problem, since it generally requires the integration of model forecasts of vertical velocity with dynamical and conceptual models of mesoscale processes. The traditional approach to identifying large-scale triggering mechanisms for convection is based on identification of fronts or drylines and an examination of these features in the broader context of synoptic-scale vertical motion associated with quasigeostrophic (QG) dynamics (Schaefer 1986). This approach can work well with some classical Norwegian-type cyclones, but it can completely misforecast the location and extent of lifting and convection in lee cyclones that have a cold front aloft (CFA). A conceptual model that places emphasis on the CFA was described by Hobbs et al. (1996), who defined a Corresponding author address: Peter V. Hobbs, Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. E-mail: [email protected]

q 2002 American Meteorological Society

CFA as ‘‘the leading edge of a transition zone above the surface that separates advancing cold air from warmer air.’’ We retain this definition here but refine it for the purposes of clarity. When a Pacific cold frontal surface is tilted forward in the lower troposphere, the CFA is the farthest forward extent of the Pacific cold air mass aloft (Fig. 1). As such, the CFA represents a line that is generally not confined to a horizontal surface. However, the CFA can be marked on a surface map as the projection of this line onto this surface (see Fig. 1). This is similar to the depiction of cold fronts aloft or troughs of warm air aloft (TROWALS) on surface charts within the context of classical warm-type occlusions. A CFA is produced by the interaction of a Pacific cold front with a Rocky Mountain lee trough that is often coincident with a dryline and can be a major factor in the development and maintenance of large-scale (;1000 km) squall lines that precede a surface trough (Locatelli et al. 1998). When a CFA is associated with lee cyclogenesis, the primary lifting mechanism for convection is more likely to be collocated with the CFA than with the location of surface frontal features. Therefore, in CFA-type cyclones the identification and tracking of the CFA can be a valuable aid in forecasting the timing and location of convection. In this paper, we illustrate how output from a mesoscale model can be used to diagnose the presence of a CFA and the potential for severe weather. A low pressure system of moderate intensity that developed on 19– 21 June 2000 is examined here as one scenario for severe weather east of the Rockies. For a 2-yr period (2000–

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FIG. 1. Diagram of a cold front aloft in three-dimensional space.

01), we identified 14 cases of severe weather in the central and eastern United States in which our analysis of cyclone structures revealed the presence of a CFA. The 19–21 June 2000 cyclone is fairly representative of these CFA-type cyclones. 2. Analysis of the 19–21 June 2000 cyclone In this section and in section 3, we will use the surface chart analyses produced operationally by the National Weather Service (NWS) for our discussion of the 19– 21 June 2000 cyclone. These charts accurately show the pressure fields and locations of surface highs and lows. In section 4, we present an alternate interpretation of the surface frontal analyses that provides a clearer picture of the lower-tropospheric frontal structure and the relevant dynamic features of this storm. The 19–21 June 2000 cyclone began when a 500-hPa trough moved toward the southeast from the Pacific Northwest and amplified over the northern Rockies. In advance of the trough, a lee cyclone formed over northeast Wyoming early on 19 June 2000. At the same time, a Pacific cold front associated with a surface low located in northwestern Canada moved rapidly to the southeast. As the cold front swept over the Rocky Mountains, it encountered both the low in Wyoming and a lee trough that extended from the surface low pressure center southward through eastern New Mexico. By 0000 UTC 20 June, the low in Wyoming had started moving to the northeast into the Dakotas along the advancing front

(Fig. 2). The NWS analyzed a lee trough through central Nebraska southward to the Oklahoma and Texas Panhandles. This trough had little or no east–west gradient of surface temperature on its east side but a significant gradient on its west side. In terms of surface potential temperature (not shown), which Sanders and Doswell (1995) recommend for use in surface frontal analysis, the lee trough is characterized by a weak thermal maximum. Along part of its length, the trough was collocated with a strong dewpoint gradient, and it could have been analyzed as a dryline. Winds east of the trough were southeasterly while winds west of the trough had a westerly component. At 0300 UTC 20 June, the NWS surface analysis indicated a low pressure center over the Dakotas and a trailing cold front extending southwestward through the Rocky Mountains (Fig. 3). The cold front marked a shift to northwesterly winds north of the front. By 0600 UTC 20 June, the front had crossed the Rockies and had begun to merge with the lee trough. It was almost completely merged with the lee trough by 1200 UTC 20 June (Fig. 4). At this time, the trough exhibited all of the characteristics of a surface cold front: strong temperature and dewpoint gradients and a distinct wind shift. By the early afternoon of 20 June, the position of the front as analyzed by NWS had changed little to the southwest of the Nebraska–Missouri border; closer to the low pressure center it had moved slightly eastward. However, the temperature gradient across the front had

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FIG. 2. NWS surface analysis for 0000 UTC 20 Jun 2000. The NWS radar summary is shown by the shaded overlay.

weakened considerably, and the analyzed cold front was now coincident with the lee trough. By 0000 UTC 21 June, the trough was still indicated as a cold front by the NWS, despite the fact that temperatures at some stations behind the front were higher than those ahead of the front. This was true even for stations that were unaffected by the squall line (see Fig. 8a, described later). At 0300 UTC 21 June, the NWS analysis shows a squall line in the warm sector well ahead of the analyzed front (Fig. 5). The 19–21 June 2000 cyclone intensified over southern Manitoba and western Ontario, reaching a central low pressure of 982 hPa by 0600 UTC 21 June. By this time, a large area of stratiform clouds and precipitation was affecting central Canada while narrow convective elements were traveling through the Ohio River valley. The storm gradually weakened as it moved eastward across Canada. There were three distinct convective systems asso-

ciated with the 19–21 June 2000 cyclone. The first was a squall line that developed during the late afternoon of 19 June. It formed along the convergent zone of the lee trough from the Dakotas through central Nebraska. The convection associated with this system was strongest during the overnight hours and weakened during the day of 20 June. As this system weakened, a second line of convection was triggered by outflow and/or dynamics associated with a mesoscale convective vortex that accompanied the weakening system. Cells within this second line of convection moved through central Illinois and Indiana from 1900 UTC 20 June to 0030 UTC 21 June and spawned several tornadoes: three in central Illinois (Christian County, 1935 UTC; Montgomery County, 1950 UTC; and Shelby County, ;2000 UTC) and at least two in east-central Indiana (Henry County, 0000 and 0026 UTC). The mesoscale processes that produced the first two lines of convection represent a separate forecasting challenge, distinct from the syn-

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FIG. 3. NWS surface analysis for 0300 UTC 20 Jun 2000. The NWS radar summary is shown by the shaded overlay.

optic-scale frontal processes associated with the cyclone of 19–21 June. Therefore, the forecasting of these first two lines of convection is not explicitly addressed in this paper. However, one cannot preclude the possibility that remnant outflow from this system influenced a third line of convection that is the subject of this study and may have locally enhanced parameters that are conducive to the development of severe storms. The most extensive source of severe weather associated with the 19–21 June 2000 cyclone was a third line of convection that began in northeast Kansas on the afternoon of 20 June. By 0300 UTC 21 June it extended in an arc from the central border of Kansas and Oklahoma northeast to the southwest corner of Michigan (Fig. 6). Cells in this line produced tornadoes in Lafayette County, Missouri (2122 UTC 20 June); Logan County, Illinois (0000 UTC 21 June); Ralls County, Missouri (0000 UTC 21 June); Piatt County, Illinois (0208 UTC 21 June); and Champaign County, Illinois

(0227 UTC 21 June). The last two tornadoes caused property damage, including the destruction of a barn. In addition to the tornadoes, wind gusts of about 30 m s 21 (60–70 mi h 21 ) were reported from the Oklahoma Panhandle to Illinois, and hail of up to 4.5-cm diameter (1.75 in.) was reported in southeast Kansas. 3. The National Weather Service forecast of the 19–21 June 2000 cyclone The NWS Storm Prediction Center (SPC) issues convective outlooks several times per day to alert the public of the likelihood of severe weather for both the current day and next two days (‘‘day-1, -2, and -3’’ outlooks). These outlooks are accompanied by a forecast discussion explaining the reasoning behind the various risk areas identified in the graphical convective outlook map. The SPC’s day-1 convective outlook issued at 1630

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FIG. 4. NWS surface analysis for 1200 UTC 20 Jun 2000. The NWS radar summary is shown by the shaded overlay. OAX indicates the location of Omaha, NE, for which soundings are shown in Fig. 10.

UTC 20 June 2000 and valid through 1200 UTC 21 June 2000 contained elements of a common approach to forecasting severe weather: a forecast of the distributions of various convective indices related to instability and shear (to provide an idea of the likelihood of convection and the type of convection expected), a forecast of surface boundaries that might initiate convection (such as fronts, drylines, or outflow from previous convection), and a forecast of upper-level features such as 500-hPa troughs or jet streaks (to identify synoptic-scale forcing of vertical motion that may enhance convection over particular regions). The day-1 convective outlook issued at 1630 UTC 20 June 2000 called for a moderate risk of severe thunderstorms over portions of western Missouri, southern and eastern Kansas, and northern Oklahoma (Fig. 7). A slight risk for severe weather was forecast for an area extending from the Texas Panhandle to the Great Lakes.

Portions of northern Iowa and Wisconsin were included in the slight risk area, in part because of expectations of ‘‘large-scale forcing for upward vertical motion . . . north of a cyclonically curved jet (with) moderate instability’’ (CAPE of 1000–2000 J kg 21 was forecast) and ‘‘deep-layer shear sufficient for supercells.’’ The ‘‘primary focusing mechanism’’ for severe weather in the moderate-risk area was identified as a ‘‘cold front over central Kansas . . . along with pre-frontal convective outflow.’’ The front was expected to move southeastward over the moderate-risk area by the afternoon of 20 June. Sufficient ‘‘deep-layer shear,’’ along with projected surface-based CAPE at or above 4000 J kg 21 , was the basis for a forecast of supercell thunderstorms, very large hail, and damaging winds. Tornadoes were not explicitly forecast, although a forecast of possible supercells often implies the potential for tornadic development.

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FIG. 5. NWS surface analysis for 0300 UTC 21 Jun 2000. The NWS radar summary is shown by the shaded overlay.

4. Diagnosis of the 19–21 June 2000 cyclone using a mesoscale model We used the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5; Dudhia 1993; Grell et al. 1994) to diagnose and forecast the cyclone of 19– 21 June 2000. Other mesoscale models could have been used (such as the Eta or Rapid Update Cycle), but the operational version of MM5 run 2 times per day by NCAR was used for this study because it is readily available and easily processed. Also, the operational MM5 has been found to show some skill at indicating likely regions for convective initiation in environments favorable for supercell development (J. Bresch, Mesoscale Prediction Group, NCAR, 2001, personal communication). MM5 is a nonhydrostatic full-physics model that uses a terrain-following vertical sigma coordinate and a rect-

angular grid on a conformal map projection. Specific physical parameterizations for the model run included the PBL scheme devised by Troen and Mahrt (1986), the convective parameterization developed by Grell (1993), and a mixed-phase ice scheme (Reisner et al. 1998). The model grid consisted of a 120 3 190 gridpoint domain with 30-km grid spacing and of 27 vertical sigma levels. Initial and lateral boundary conditions were generated by interpolation of the National Centers for Environmental Prediction Eta Model analysis and initial conditions to the MM5 grid. These fields were reanalyzed using multiquadratic interpolation of available surface and upperair data. The model was initialized at 1200 UTC 20 June 2000 and was run for a 48-h forecast period. a. Comparison of model outputs with observed data Figures 8 and 9 show observed and modeled fields, respectively, of temperature and heights for 0000 UTC

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FIG. 6. Weather Surveillance Radar-1988 Doppler (WSR-88D) radar reflectivity echoes for 0300 UTC 21 Jun 2000. The vertical cross sections in Figs. 14 and 15 are through line A–B in this figure. The locations of the NWS-analyzed fronts for this time are shown by the standard frontal symbols.

21 June 2000. Both the model 12-h forecast and the analyzed observations show a surface low with a central pressure of about 998 hPa situated near the Ontario– Manitoba border. The low center is nearly stacked in the vertical. At the surface, both the model and obser-

FIG. 7. The SPC convective outlook for day 1, valid from 1630 UTC 20 Jun until 1200 UTC 21 Jun 2000.

vation analysis show a pressure trough extending through central Wisconsin and eastern Iowa, which coincides with a ridge of maximum temperature (Figs. 8a,9a). A strong temperature gradient is seen behind the trough, particularly in Nebraska and northwest Iowa. A second strong gradient is located to the north through upper Michigan, perhaps in association with a warm front. The observed cold front at 850 hPa is slightly ahead of the surface trough, whereas the model forecast places it nearly coincident with the trough. However, both analyses are similar in appearance (Figs. 8b,9b). Significant features that appear in the model outputs, such as the second low pressure center in New Mexico, correspond closely to the observations. At 700 hPa the position of the cold front is virtually identical in both the model-derived fields and the observed data analysis (Figs. 8c,9c). The model forecast at 500 hPa also accurately depicts the observed conditions at 0000 UTC 21 June (Figs. 8d,9d). Significant features include a strong west-southwesterly jet centered over southeast Nebraska, a low center in southwest Ontario, and a front that crosses northern Missouri, northwest Illinois, and eastern Wisconsin. The front seen in Figs. 8 and 9 precedes a temperature gradient of 38–48C (100 km) 21 . The

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FIG. 8. Observations and hand-analyzed temperatures for 0000 UTC 21 Jun 2000: (a) surface and (b) 850, (c) 700, and (d) 500 hPa. The location of the cold front at upper levels is indicated by a wide gray line.

jet core in the model forecast for 500 hPa is about 77 kt (40 m s 21 ), slightly stronger than the observed maximum of 65 kt. Also, the southern portion of the front at 500 hPa is located farther to the east in the actual observations. However, the MM5 12-h forecast is substantially accurate in its depiction of the synoptic-scale conditions at 0000 UTC 21 June. b. CFA analysis MM5 developed a large-scale (over 1000 km in length) band of convective precipitation between 2100 and 2400 UTC 20 June (forecast hours 9–12). From its

outset, the model-generated rainband is located ahead of the surface trough/dryline, and it moves progressively eastward. Northern portions of the band are initially located the farthest east from the position of the surface trough. A large-scale well-defined line of convection that develops ahead of a lee trough/dryline that has interacted with a Pacific cold front can indicate the presence of a CFA (Locatelli et al. 1998). Another possible indicator of the formation of a CFA is a weakening of the temperature gradient behind the remaining surface trough after the surface cold front occludes with the dryline/ lee trough. Although other processes can lead to a weak-

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FIG. 9. MM5 forecast of temperature, surface pressure, geopotential height, and winds for 0000 UTC 21 Jun 2000: (a) surface and (b) 850, (c) 700, and (d) 500 hPa. The location of the cold front at upper levels is indicated by a wide gray line.

ening of the gradient behind a conventional backwardtilted cold front in the central United States (such as differential surface heating or differential adiabatic warming), the development of a CFA typically leaves a weaker gradient behind the remaining surface trough, and the main zone of enhanced thermal gradient is aloft and ahead of the trough. In the MM5 forecast for the storm of 19–21 June, the strong temperature gradient that was behind the surface trough early in the morning of 20 June weakened over the next 24 h. For 2100 UTC 20 June–0900 UTC 21 June, this weakening can be seen by examining the passage of the baroclinic zone over Iowa (panel a in Figs. 11–13, described below). The important dynamic characteristic of a CFA is that it develops in an occlusion of a Pacific cold front with

a lee trough. As the cold front overtakes the lee trough, cold air behind the front overrides a relatively stable moist air mass to the east and proceeds as a strong gradient of advancing cold air in the middle troposphere, generally between 800 and 500 hPa. Convection can develop along a narrow band of lifting beneath the CFA (Hobbs et al. 1996; Locatelli et al. 1998). At 1200 UTC 20 June, the NWS surface analysis indicated that the cold front had just intersected the lee trough (Fig. 4). Prior to the severe weather outbreak there were indications in sounding data that are consistent with CFA development. For example, Omaha, Nebraska (marked OAX in Fig. 4), was east of the lee trough at 0000 UTC 20 June, and its sounding (Fig. 10a) reveals deep warm advection (veering of winds)

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FIG. 10. Soundings for OAX (see Fig. 5) at (a) 0000 and (b) 1200 UTC 20 Jun 2000.

up to about 500 hPa, which is expected ahead of a thermal ridge–pressure trough. At 1200 UTC 20 June, Omaha was situated just east of the surface trough, and its sounding (Fig. 10b) shows backing winds above 700 hPa. This indicates that the cold air behind the trough was advancing aloft. To identify the CFA, Figs. 11–13 show potential temperature fields from the MM5 forecast at the surface and at standard pressure levels of 850, 700, and 500 hPa at 2100 UTC 20 June and at 0300 and 0900 UTC 21 June 2000 (forecast hours 9, 15, and 21). The radar reflectivity images for these times show the squall line to be nearly coincident with the CFA, as indicated by the MM5 forecasts (cf. Figs. 6 and 12). Under operational time constraints, we recommend that forecasters examine the standard pressure levels of 850, 700, and 500 hPa for indications of a CFA. However, because on any vertical cross section normal to the Pacific cold frontal surface the CFA is located at the most forward position of the surface, the CFA could be located between two standard pressure levels. Therefore, if an enhanced temperature gradient is found at the standard pressure levels, additional levels should be examined to determine the location of the leading edge of the Pacific cold frontal surface at each level. For the 19–21 June cyclone, we examined multiple levels every 50 hPa above the surface. However, for brevity we show only the standard levels, which adequately reveal the location of the CFA in this case. The MM5 forecasts show no discernible surface cold front near the model-generated squall line at 2100 UTC 20 June and 0300 UTC 21 June (forecast hours 9 and 15; Figs. 11a, 12a). In fact, a well-defined warm ridge, indicating the position of the dryline/lee trough, occurs

behind the model-generated convective rainband at these times. By 0900 UTC 21 June (Fig. 13a), a strong surface potential temperature gradient develops at the leading edge of the convective precipitation band, but this baroclinic zone is not the Pacific cold front—it is generated by convective outflow. The synoptic-scale gradient that lingers behind the lee trough (i.e., the gradient over Iowa and Wisconsin in Figs. 12a and 13a) weakens, decreasing from about 28C (100 km) 21 to 18C (100 km) 21 during the period of 2100 UTC 20 June– 0900 UTC 21 June. The baroclinic zone at 850 hPa is stronger than that behind the surface trough, with a consistent temperature gradient of about 28–38C (100 km) 21 (Figs. 11b, 12b, and 13b). The leading edge starts out well behind the model squall line but advances relative to it and catches up to it along its southern flank by 0900 UTC 21 June (Fig. 13b). The temperature gradients at 500 and 700 hPa are also about 38C (100 km) 21 , but they essentially track the model squall line from 0300 to 0900 UTC 21 June. The leading edge of cold air is initially more advanced to the north at 500 hPa, where the Pacific cold front occluded with the dryline/ lee trough at an earlier time. However, the leading edge of the advancing cold air is nearly coincident with the back edge of the advancing squall line during the 6-h period from 0300 to 0900 21 June. An examination of the MM5 forecasts of the stormrelative winds, vertical velocity, potential temperature u, and equivalent potential temperature u e in vertical cross sections perpendicular to the line of convection elucidates the nature of the CFA. A cross section through south-central Illinois at 0300 UTC 21 June indicates that the CFA is at about 700 hPa (Fig. 14, at approximately the 500-km mark on the distance axis).

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FIG. 11. MM5 forecasts of potential temperature and precipitation (convective and explicit; 1 h accumulated) for 2100 UTC 20 Jun 2000 [forecast hour 9; temperature is given in 18C (K) intervals]: (a) surface (or lowest sigma level) and (b) 850, (c) 700, and (d) 500 hPa.

Farther to the north, a cross section (not shown) through central Michigan reveals the CFA to be closer to the 500-hPa level. In both cases the vertical velocities are strongest just east of the leading edge of the temperature gradient that marks the CFA. Strong upward vertical velocities are present at the leading edge of the strong u e gradient located at about 700 hPa in Fig. 14. The surface trough is evident at this time as a comparable u e gradient located at the surface about 200 km to the west of the CFA. Storm-relative surface winds are easterly to the east of the trough, and the trough indicates a convergence zone and a marked shift to westerly winds. The surface trough marks a sharp transition from high- to low-u e air at the surface, and the CFA is seen as an intrusion of air aloft that is lower both in potential and equivalent potential temperature. By 1200 UTC 21 June (forecast hour 24), the vertical structure of u e in the MM5 forecast (Fig. 15) is deceptively similar to a standard cold front. The model indicates a pocket of relatively low u e air to the west of the main zone of ascent, which is produced by the strong region of descent west of the CFA. The descending motions, which are associated with the model’s convective parameterization, advect air with lower u e from aloft. Nevertheless, the CFA is marked by the strongest cold

advection and gradient of potential temperature aloft at about 700–750 hPa while the transition from westerly to easterly surface winds 200 km west of the CFA marks the position of the dryline/lee trough. c. Vertical velocity analysis The SPC’s convective outlook for 20 and 21 June 2000 called for a slight risk of severe weather in the middle to upper Mississippi River valley, which was the region in which most of the severe weather associated with the cyclone occurred. However, the potential for severe weather was considered conditional because of the possibility that ‘‘extensive cloud cover and ongoing precipitation (would) delay air mass destabilization.’’ The forecast predicted the potential for ‘‘scattered strong/severe thunderstorms,’’ due to large-scale forcing that was anticipated to be strongest north of a ‘‘cyclonically curved jet from (portions) of northern Iowa into Wisconsin’’ (SPC day-1 convective outlook; 1630 UTC 20 June 2000). Synoptic-scale vertical velocity fields are typically examined for indications of enhanced potential for convection. We will now compare the utility of three different methods of model-based forecasting of synoptic-scale vertical velocity patterns for the 19–

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FIG. 12. Same as Fig. 11 but for 0300 UTC 21 Jun 2000 (forecast hour 15).

21 June 2000 cyclone. These methods are 1) application of qualitative QG jet dynamics to model-depicted jet streaks, 2) use of model fields to calculate quantitatively the QG vertical velocity pattern, and 3) examination of vertical velocity fields calculated from the full physical equations. The MM5 forecast of winds at 0000 UTC 20 (Fig. 16) shows a cyclonically curved upper-level jet streak situated over the central and northern Great Plains and a southwesterly-to-southerly low-level jet extending from the southern Great Plains to south-central Canada. The two jets intersected over the northern Great Lakes region. Quasigeostrophic analysis of simplified jets shows that upward (downward) vertical velocities are enhanced beneath the right (left) entrance and left (right) exit regions of a straight jet streak. Furthermore, for a cyclonically curved jet streak, gradient wind balance implies enhanced upward motion beneath the exit region and downward motion beneath the entrance region. Coupling of upper and lower jets may enhance these circulations (Uccellini and Johnson 1979; Keyser and Shapiro 1986; Moore and VanKnowe 1992). These qualitative rules do not correspond well to the actual 700hPa vertical velocity forecast by the MM5 for 0000 UTC 21 June (Fig. 17). The vertical velocity pattern exhibits a long, narrow, arc-shaped band of upward motion that extends across the entire jet exit region and then curves

along the anticyclonic shear side of the jet all the way to the base of the trough. There is subsidence or nearzero vertical velocity along much of the jet axis, on both sides of the trough axis. It is difficult to reconstruct this pattern of vertical velocity with any linear combination of the straight jet streak quadrapole and the curved jet dipole patterns of vertical velocity. One might question how much (if any) of this narrow, arc-shaped vertical velocity pattern is associated with dynamics external to the convection (i.e., synoptic- and frontal-scale dynamics), and how much is a product of the convection itself (which is already developing in the model at this time). Although it is difficult to partition the vertical velocity in such a manner, one approach is to examine the QG-diagnosed vertical velocity, which in principle should not show convectively forced vertical velocity. Figure 18 depicts the field of vertical velocity calculated by means of a 3D inversion of the Qvector formulation of the QG v equation, using the MM5 geopotential and temperature fields at 0000 UTC 21 June. The domain-averaged Coriolis parameter and dry static-stability parameter were used in this calculation. Prior to calculating the Q vector, the data were filtered by a smoothing function, which acted as a lowpass filter with a 50% reduction in amplitude occurring at a wavelength of 720 km. This filtering smoothed out mesoscale effects, so that the fields more accurately rep-

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FIG. 13. Same as Fig. 11 but for 0900 UTC 21 Jun 2000 (forecast hour 21).

resent synoptic scales. Although the QG vertical velocity field (Fig. 18) shows larger-scale patterns and much weaker overall magnitudes, the distribution of QG vertical velocity is qualitatively similar to the model vertical velocity field calculated from the full equations (Fig. 17). It is apparent that the QG and full-model vertical velocity fields resemble each other much more than either one resembles the pattern expected by qualitatively applying simple jet dynamic schemes to the 300-hPa jet. Therefore, even if a crude balance such as geostrophy is assumed, the complexity of the full 3D atmosphere can produce (via the QG omega equation) a different vertical velocity pattern than is qualitatively inferred from applying models of jet streak dynamics to the jets seen at one or two levels. Also, although the QG vertical velocities are significantly smaller in magnitude than the full-model vertical velocities, the similarity in pattern suggests a direct connection between the synoptic-scale frontal structure and the arc-shaped vertical velocity pattern shown in Fig. 17. In the current case, the specific synoptic-scale frontal feature of interest is the CFA, which is closely collocated with the narrow band of upward motion in Fig. 17. There are several reasons for the underestimation of the magnitude of the vertical velocity by the QG inversion: it cannot resolve the narrow cross-frontal scale associated with the CFA, it uses dry static stability, and the full-model

vertical velocity field contains significant enhancement from developing convection. Although the QG vertical velocity calculation improves upon the qualitative application of simple jet dynamics and helps to establish the connection between the synoptic-scale CFA and the band of upward motion and convection, it ultimately falls short as a tool for forecasting the convective rainband associated with a CFA. The reason for this is that the spatial scale at which QG is valid (several hundred kilometers at best) is as large as or larger than the typical horizontal distance between the surface trough and the CFA, which can be anywhere from 0 to 300 km. In other words, QG cannot resolve the distinction between a standard backwardtipped cold front and a forward-tipped cold front. Distinguishing between these two situations is an important forecasting determination, because it is essential to know where the primary focusing mechanism for severe weather will be located when a Pacific cold front crosses the central United States. We recommend that forecasters first determine whether a CFA is developing and whether mesoscale forecast models show the development of an extensive, organized convective band along the CFA. If a CFA is developing, it should be considered as a potential primary focusing mechanism for organized convection. Standard analysis of the environmental instability and shear parameters will be required to

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FIG. 14. Vertical cross section along line A–B shown in Fig. 6 at 0300 UTC 21 Jun 2000 showing MM5 forecasts of equivalent potential temperature (shading) and potential temperature (solid lines). Storm-relative horizontal winds and vertical velocities are shown as vectors. Storm velocity was determined from the approximate speed of the model-generated squall line. Horizontal storm-relative winds are therefore the component of motion within (parallel to) the plane of the cross section.

ascertain whether convection is possible along the CFA and, if so, what type of convection. This approach is illustrated below for the 19–21 June 2000 cyclone. d. Combining CFA forecasting with severe-weather parameters In this section we will demonstrate how three commonly used parameters (CAPE, helicity, and bulk Richardson number shear) were utilized in conjunction with MM5 predictions of the CFA location to forecast severe weather associated with the 19–21 June 2000 cyclone. For this purpose, we used a relatively simple forecasting scheme to explore how current forecasting techniques for the identification of severe-weather potential might be integrated with CFA identification. Because tornadoes constituted the most significant severe weather produced by the CFA-related squall line, we concentrate on a determination of those areas for which the potential for tornadic supercell development was greatest. Most of the other severe-weather reports (large hail and damaging wind) occurred in proximity to the tornadoes. Convective available potential energy (CAPE) is defined as CAPE 5 g

E

EL

LFC

u (z) 2 u (z) dz, u (z)

(1)

where g is the gravitational acceleration, LFC is the level of free convection of a lifted air parcel, EL is the

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FIG. 15. Same as Fig. 14 but at 1200 UTC 21 Jun 2000.

equilibrium level of the parcel (at which its temperature is equal to the environmental temperature), u (z) is the potential temperature of the parcel as it ascends moist adiabatically, and u (z) is the potential temperature of the environment. We examine predicted fields of CAPE first to determine which regions have environments more favorable for deep convection. Also, tornadoes are more likely to occur in conjunction with higher values of CAPE (in excess of about 1000 J kg 21 ), although the only essential requirement is the presence of positive CAPE (Stensrud et al. 1997; Johns and Doswell 1992). Vertical cross sections of CAPE from the MM5 forecasts (not shown) in the vicinity of the nascent squall line indicate the highest values were associated with parcels lifted from the surface. A map of predicted CAPE values from MM5 shows a narrow band of instability with values as high as 3500 J kg 21 by 0300 UTC 20 June (Fig. 19). The highest CAPE values are in portions of Kansas. Through eastern Iowa and southern Wisconsin CAPE values of nearly 1600 J kg 21 are present while in portions of Illinois there are values from 1000 to 1500 J kg 21 . The axis of high instability roughly corresponds to the zone of strong dewpoint gradient at the surface and reflects a juxtaposition of warm moist surface air with cold air aloft. The MM5 forecast suggests that relatively high values of CAPE (.1000 J kg 21 ) existed throughout much of the area in which convective precipitation occurred, indicating a favorable environment for thunderstorm development. Therefore, next we examine shear parameters to identify specific regions that had environments conducive to long-lasting storms. Storm-relative environmental helicity (SREH) is defined as

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FIG. 16. MM5 forecasts of isotachs (m s 21 ) at 300 (shaded) and 850 (lines) hPa at 0000 UTC 21 Jun 2000.

SREH 5

E

0

h

k · (V 2 c) 3

]V dz, ]z

(2)

where k is a unit vertical vector, c is the storm motion vector, V is the environmental wind velocity vector, and h is an inflow depth (we assumed a typical value of 3000 m). Storm motion for this study was estimated by calculating the mean wind from 3 to 10 km above ground and then assuming that the storm speed was 75% of the mean wind speed and was at a direction 308 to the right of the mean wind direction (Maddox 1976). SREH values in excess of 100 m 2 s 22 can indicate an environment favorable for supercell development (Brooks et al. 1993). Larger values of SREH may indicate a greater likelihood for supercells. However, SREH values are highly sensitive to mesoscale processes that may not be predicted well by the model. Therefore, any specific value of SREH has limited value as a forecast tool if it is considered alone (Markowski et al. 1998). The MM5 forecast shows a broad band of SREH

values in excess of 100 m 2 s 22 , whereas the highest values of SREH (as viewed on a surface plot) are just ahead of the CFA at 700–650 hPa (Fig. 20). The SREH values drop off sharply behind the upper front. A secondary small maximum in SREH is located at the surface trough. However, SREH values above 100 m 2 s 22 are collocated with significant convection and strong midlevel shear (i.e., favorable values of BRNSHR; see below) along the CFA but not at the surface boundary. Having determined those areas in which there is a greater potential for supercell development, we next examine the bulk Richardson number shear (BRNSHR) as one possible means of discriminating between those regions favorable or unfavorable to tornadic development. BRNSHR is defined as 0.5(U 6000 2 U 500 ) 2 ,

(3)

where U 6000 and U 500 represent the difference in the pressure-weighted mean vector winds between the lowest 6000 m and lowest 500 m of the troposphere, respectively. BRNSHR has been found to correlate with

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FIG. 17. MM5 forecasts of isotachs at 300 hPa (shaded; 7 m s 21 contour interval) and vertical velocity at 700 hPa (solid lines are upward velocities, dashed lines are downward velocities; contour interval is 2 dPa s 21 , starting at 2 and 22 dPa s 21 ) at 0000 UTC 21 Jun 2000.

the maximum vertical vorticity in cloud-modeling studies of thunderstorms (Drogemeier et al. 1993), and in a recent study a range of forecast BRNSHR values were found to be associated with the occurrence of tornadic supercells (Stensrud et al. 1997). Stensrud et al. (1997) hypothesize that midlevel winds are important to the development of low-level rotation, with a medium value of storm-relative flow being necessary to support storm outflow sufficient to sustain a low-level mesocyclone. Weak storm-relative winds at middle levels allow for the generation of an excessive rain-cooled outflow, which can undercut the low-level mesocyclone and disrupt low-level circulation (Brooks et al. 1994). Strong storm-relative winds at middle levels can transport sufficient precipitation away from the core of the storm to disrupt the production of outflow along the rear flank of the supercell and to decrease generation of low-level baroclinic vorticity. By demonstrating that BRNSHR can be used as a surrogate for storm-relative midlevel winds, Stensrud et al. found that tornadic supercells

were more likely to develop when forecast values of BRNSHR were between 40 and 100 m 2 s 22 , provided that sufficient storm-relative helicity and CAPE are present. The MM5 forecast for 20–21 June 2000 shows a broad band of BRNSHR values of between 40 and 100 m 2 s 22 covering portions of the central Midwest and Great Lakes region. From 0000 to 0300 UTC 21 June, the maximum values of BRNSHR increase from 85 to over 140 m 2 s 22 in a band extending from northern Kansas through central Iowa. By 0300 UTC 21 June, values of BRNSHR in the range 40–100 m 2 s 22 are confined to a narrower band extending from southeast Kansas through Illinois and north to Lake Michigan (Fig. 21). The model forecast shows convection at this time intersecting the band of favorable BRNSHR, most noticeably in northeast Missouri and west-central Illinois and to a lesser extent in Kansas and central Michigan (Figs. 12 and 21). According to Stensrud et al. (1997), the environment

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FIG. 18. MM5 forecasts of isotachs at 300 hPa (shaded; 5 m s 21 contour interval) and QG v at 700 hPa (solid lines are upward vertical velocities; dashed lines are downward vertical velocities; contour interval is 2 dPa s 21 starting at 2 and 22 dPa s 21 ) at 0000 UTC 21 Jun 2000.

FIG. 19. MM5 forecasts of CAPE (surface based) at 0300 UTC 21 Jun 2000.

is conducive to the development of tornadic supercells where regions of positive CAPE, SREH values greater than 100 m 2 s 22 , and BRNSHR values from 40 to 100 m 2 s 22 coincide. Figure 22 shows the regions that satisfied these three criteria according to the MM5 forecasts of the storm of 19–21 June at times when tornadoes occurred. Although appropriate values of shear and instability are present in Ohio and Wisconsin, severe weather was not reported in these regions. Because the strongest upward vertical velocity associated with the Pacific cold front generally occurs at the farthest forward extent of the Pacific cold-frontal surface, the ultimate goal of examining multiple pressure-level maps and vertical cross sections (as described in section 4b) is to be able to show the location of a CFA on a single map. Such a map shows the projection on a plan-view chart (such as a surface weather map) of the CFA. This was shown schematically in Fig. 1. Such an analysis was performed for the 19–21 June

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FIG. 20. MM5 forecasts of SREH at 0300 UTC 21 Jun 2000.

2000 cyclone using the MM5 forecast. The location of the CFA is shown in Fig. 22. As depicted in Figs. 22a– c, the CFA represents a location for potential triggering of convection, which can be compared with areas that are forecast to have other favorable conditions for severe weather. The locations of the reported tornadoes correlate very closely to the intersection of the CFA with zones of appropriate shear and instability. Regions of intersection for which tornadoes were not reported (viz., central Wisconsin in Fig. 22a, portions of Missouri and Kansas in Fig. 22b, and central Michigan in Fig. 22c) are areas of relatively low CAPE (,500 J kg 21 ). We have not determined what effect, if any, the CFA had on the severe-weather parameters utilized in the forecast. Also, it is not known to what extent the effects of prior convection, such as remnant outflow, had on the severe-weather parameters. Even when environmental conditions are favorable for the development of severe weather, such weather often does not occur. Thus, the success rate for determining the occurrence of severe weather based on a combination of parameters is generally low (Rasmussen and Blanchard 1998). However, in the case discussed here, the severe weather was clear-

FIG. 22. Shading shows regions with positive CAPE, SREH . 100 m 2 s 22 , and BRNSHR from 40 to 100 m 2 s 22 at (a) 2100 UTC 20 Jun, (b) 0000 UTC 21 Jun, and (c) 0300 UTC 21 Jun 2000. The location of the CFA is indicated by the heavy solid line. The locations of reported tornadoes at or near (61 h) the three times is indicated by a T.

ly associated with a squall line that was triggered and maintained by a CFA. 5. Conclusions

FIG. 21. MM5 forecasts of BRNSHR at 0300 UTC 21 Jun 2000.

For the 19–21 June 2000 cyclone, we have shown that a cold front aloft contributed significantly to the development of a squall line, which in turn produced tornadoes and other severe weather. The role of the CFA was to provide lifting that helped to initiate convection and to maintain a squall line. The presence and location of the CFA were determined from an examination of a mesoscale-model forecast of the temperature gradient at

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multiple levels from the surface to about 500 hPa, and vertical cross sections of potential and equivalent potential temperature normal to the Pacific cold front. Tornadoes occurred at locations at which the model-produced CFA and squall line intersected regions that were forecast to have favorable environments for tornadic supercell development (i.e., positive CAPE, BRNSHR values from 40 to 100 m 2 s 22 , and SREH values greater than 100 m 2 s 22 ). These results suggest that the identification and tracking of a CFA can be a useful aid in forecasting the possible location of convection that may result in severe weather given an environment conducive to storm development. Further research is needed to determine the exact relationship between CFAs and severe weather. Acknowledgments. This work was supported by Grant ATM-9632580 from the Mesoscale Dynamic Meteorology Program, Atmospheric Sciences Division, National Science Foundation. REFERENCES Brooks, H. E., C. A. Doswell III, and R. Davies-Jones, 1993: Environmental helicity and the maintenance and evolution of lowlevel mesocyclones. The Tornado: Its Structure, Dynamics, Prediction, and Hazards, Geophys. Monogr., No. 79, Amer. Geophys. Union, 97–104. ——, ——, and R. B. Wilhemson, 1994: The role of midtropospheric winds in the evolution and maintenance of low-level mesocyclones. Mon. Wea. Rev., 122, 126–136. Drogemeier, K. K., S. M. Lazarus, and R. Davies-Jones, 1993: The influence of helicity on numerically simulated convective storms. Mon. Wea. Rev., 121, 2005–2029. Dudhia, J., 1993: A nonhydrostatic version of the Penn State–NCAR Mesoscale Model: Validation tests and simulation of an Atlantic cyclone and cold front. Mon. Wea. Rev., 121, 1493–1513. Galway, J. G., 1992: Early severe thunderstorm forecasting and research by the United States Weather Bureau. Wea. Forecasting, 7, 564–587.

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