The Structure and Evolution of Extratropical Cyclones ... - AMS Journals

The Structure and Evolution of Extratropical Cyclones, Fronts, Jet Streams, and the Tropopause in the GEOS General Circulation Model A. L. Conaty,* * J. C. Jusem,* * L. Takacs,** D. Keyser,+ and R. Atlas* *

ABSTRACT The realism of extratropical cyclones, fronts, jet streams, and the tropopause in the Goddard Earth Observing System (GEOS) general circulation model (GCM), implemented in assimilation and simulation modes, is evaluated from climatological and case-study perspectives using the GEOS-1 reanalysis climatology and applicable conceptual models as benchmarks for comparison. The latitude-longitude grid spacing of the datasets derived from the GEOS GCM ranges from 2° x 2.5° to 0.5° x 0.5°. Frontal systems in the higher-resolution datasets are characterized by horizontal potential temperature gradients that are narrower in scale and larger in magnitude than their lower-resolution counterparts, and various structural features in the Shapiro-Keyser cyclone model are replicated with reasonable fidelity at 1° x 1° resolution. The remainder of the evaluation focuses on a 3-month Northern Hemisphere winter simulation of the GEOS GCM at 1° x 1° resolution. The simulation realistically reproduces various large-scale circulation features related to the North Pacific and Atlantic jet streams when compared with the GEOS-1 reanalysis climatology, and conforms closely to a conceptualization of the zonally averaged troposphere and stratosphere proposed originally by Napier Shaw and revised by Hoskins. An extratropical cyclone that developed over the North Atlantic Ocean in the simulation features surface and tropopause evolutions corresponding to the Norwegian cyclone model and to the LC2 life cycle proposed by Thorncroft et al., respectively. These evolutions are related to the position of the developing cyclone with respect to upper-level jets identified in the time-mean and instantaneous flow fields. This article concludes with the enumeration of several research opportunities that may be addressed through the use of state-of-the-art GCMs possessing sufficient resolution to represent mesoscale phenomena and processes explicitly.

1. Introduction Increasing the horizontal resolution of the Goddard Earth Observing System (GEOS) general circulation model (GCM) has been a high priority in the Data Assimilation Office at the National Aeronautics and Space Administration Goddard Space Flight Center.

*Data Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland. + Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York. Additional affiliation: General Sciences Corp., a subsidiary of Science Applications International Corporation, Beltsville, Maryland. Corresponding author address: Dr. Robert Atlas, Code 910.3, NASA/GSFC, Greenbelt, MD 20771. E-mail: [email protected] In final form 30 March 2001. ©2001 American Meteorological Society

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Within the last two years, substantial increases in horizontal resolution have been implemented in GEOS. These increases have led to a very significant improvement in the ability of the GEOS GCM to represent the structure and evolution of extratropical baroclinic systems. As an illustration of this improvement, Fig. 1 shows near-surface (975 hPa) winds over the North Atlantic Ocean produced by the GEOS GCM run in simulation mode for three progressively finer horizontal resolutions. The top panel corresponds to a grid spacing of 2° latitude x 2.5° longitude (hereafter 2° x 2.5°). At this resolution, fronts consist of broad transition zones and are barely discernible. The middle panel in Fig. 1 shows the same cyclone-frontal features at 1 ° x 1° resolution. At this resolution, the GEOS GCM is able to resolve a cyclone-frontal family that has formed along a boundary characterized by sharp turning of the wind. The bottom panel shows that when the grid spacing is even finer [0.5° x 0.5° in an ex18 57

wind, potential temperature, and horizontal potential temperature gradient at 950 hPa, for a North Pacific cyclone-frontal system at 1800UTC 12 December 1991 produced by the GEOS Data Assimilation System (DAS). Results for 2° x 2.5° resolution are shown in the top panel and for 1 ° x 1° resolution in the bottom panel. The cold front in the top panel is depicted as a broad, diffuse zone of weak horizontal potential temperature gradient and a gradual turning of the wind across the frontal zone. The cold front in the bottom panel consists of a long, narrow zone with large, localized horizontal potential temperature gradient and a more abrupt change in wind direction across the frontal zone than in the lower-resolution assimilation. At this particular stage of cyclone-frontal evolution, the warm front bends around the surface cyclone, suggestive of the bent-back warm front and frontal T-bone stage of the ShapiroKeyser cyclone model (Shapiro and Keyser 1990). Figure 3 shows an extratropical cyclone at 1800 UTC 26 October 1998, also over the North Pacific Ocean but at a late stage in its evoluFIG. 1. GEOS simulations for three progressively finer resolutions showing cyclones and fronts over the North Atlantic Ocean. Near-surface (975 hPa) winds are plotted at every grid tion, produced by the GEOS point; vector scale (40 m s_1) is displayed beneath each panel, (top) Results for a uniform 2° DAS at 1° x 1° resolution. In this x 2.5° latitude-longitude grid, (middle) results for a uniform 1° x 1° latitude-longitude grid, cyclone, the 900-hPa potential and (bottom) results for an experimental stretched latitude-longitude grid with maximum temperature pattern exhibits a resolution o f 0 . 5 ° x 0 . 5 ° . local maximum immediately to the west of the cyclone center, a perimental stretched-grid version (Fox-Rabinovitz signature of the warm-core seclusion stage of the et al. 1997)], the GEOS GCM produces features in the Shapiro-Keyser conceptual model. wind field that are reminiscent of those seen in highThe remainder of this article focuses on the subresolution scatterometer wind data (e.g., Atlas et al. jective evaluation of the realism of extratropical cy1999, their Fig. 9d). clones, fronts, jet streams, and the tropopause in a The improvement in the ability of the GEOS GCM 3-month Northern Hemisphere winter (Dec-Feb, DJF) to resolve cyclone-frontal features realistically for simulation of the GEOS GCM at 1° x 1° resolution. increasing horizontal resolution is also seen when the The characteristics of the GEOS GCM and the datasets GEOS system is run in data-assimilation mode. to be analyzed are described in section 2. Since the Figure 2 depicts sea level pressure, along with the structure and evolution of cyclones and fronts are in1854

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fluenced by the background flow, it is appropriate to evaluate the extent to which the 3-month simulation exhibits realistic climatological patterns. This evaluation is conducted in section 3. Because the simulation spans 3 months, comparison with actual observations would not prove meaningful. Instead, the simulated cyclone-frontal features are compared with well-established conceptual models. This comparison is performed in the context of a case study in section 4, which emphasizes not only fronts and cyclones but also jet streams and the tropopause. The main results are summarized and the article is concluded in section 5.

2. Model description and datasets a. Model hydrodynamics and physics The GEOS GCM is discretized in the horizontal using the staggered Arakawa C-grid and employs version 2 of the Aries/GEOS Dynamical Core for the finite-differencing algorithm. Version 2 of the Aries/GEOS Dynamical Core is a fourth-order version of the Sadourny enFIG. 2. Sea level pressure (black contours, interval 4 hPa) and near-surface ergy- and potential-enstrophy-conserv- (950 hPa) winds (vector scale, 50 m s_1), potential temperature (red contours, ining scheme described by Burridge and terval 2 K), and horizontal potential temperature gradient [K (100 km) -1 , shaded Haseler (1977), and has been generalized according to legend on right-hand side of panel] at 1800 UTC 12 Dec 1991 proto allow for a nonuniform latitude- duced by the GEOS DAS at two different resolutions, (top) Results for a 2° x 2.5° latitude-longitude grid and (bottom) results for a 1° x 1° latitude-longitude grid. longitude spherical grid (Suarez and Near-surface fields in the bottom panel are not displayed where the 950-hPa surTakacs 1995). This scheme conserves face is subterranean. total energy and potential enstrophy for the nondivergent component of the flow. Horizontal advection of potential temperature, Suarez (1992), coupled with a Kessler-type scheme for moisture, and passive tracers is performed using the the evaporation of falling rain (Sud and Molod 1988). fourth-order scheme developed at the University of The moisture scheme also predicts liquid water conCalifornia, Los Angeles (A. Arakawa 1996, personal tent and fractional cloud cover associated with shallow communication). In the vertical the Aries/GEOS Dy- cumulus and stratocumulus clouds. These cloud propnamical Core uses a Lorenz or unstaggered grid in erties are allowed to interact with the radiation paramgeneralized sigma coordinates following Arakawa and eterization of the GCM, which includes effects from Suarez (1983). both shortwave and longwave processes. Turbulent The GEOS GCM physics package includes a full eddy fluxes of momentum, heat, and moisture in the set of subgrid-scale parameterizations, as well as a surface layer are calculated using stability-dependent coupled land surface model. Penetrative and shallow bulk formulas based on Monin-Obukhov similarity cumulus convection are parameterized using the re- functions. Above the surface layer, turbulent fluxes of laxed Arakawa-Schubert scheme of Moorthi and momentum, heat, and moisture are calculated using the Bulletin of the American Meteorological Society

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GEOS system at other resolutions, the model was run for a few multiday segments at 2° x 2.5° horizontal resolution on a uniform grid, and for a 2-day segment on the 0.5° x 0.5° experimental stretched grid. These multiday segments are used to produce the output shown in Fig. 1. In addition to the 3-month simulation and the multiday segments, the GEOS system was run in data-assimilation mode at 2° x 2.5° and 1 ° x 1° horizontal resolution. Sample output from the DAS is shown in Figs. 2 and 3, which have been discussed in the introduction. In the climatological evaluation and the case study that follow, we make extensive use of the dynamic tropopause (DT). The DT is defined in terms of the Ertel potential vorticity (PV), which will be expressed in PV units [PVU, defined as 10~6 m2 s"1 K kg-1 following Hoskins et al. (1985, section 2a)]. Here we deFIG. 3. Sea level pressure (contour interval 4 hPa) and 900-hPa potential temperature (K, shaded according to legend at bottom fine the DT as the 2.5-PVU surface in the of panel; shading intervals differ for "cool" and "warm" values) climatological evaluation and as the 1.5-PVU surface at 1800 UTC 26 Oct 1998 produced by the GEOS DAS for a 1 ° x in the case study. These values lie within the range of 1° latitude-longitude grid. 1-3.5 PVU cited by Morgan and Nielsen-Gammon (1998, section 2b) as having been used in earlier studlevel 2.5 Mellor-Yamada-type closure scheme of ies to represent the DT. Helfand and Labraga (1988), which predicts turbulent The GEOS dataset used in the climatological kinetic energy and determines the eddy transfer coef- evaluation refers to the simulated 3-month (DJF) ficients used for a bulk formulation. This scheme has Northern Hemisphere winter season derived from the been modified recently to account for buoyancy gen- GCM run at 1° x 1° horizontal resolution with 48 erated by moist processes through inclusion of the ef- sigma levels. These time-averaged fields are compared fects of small-scale condensation and evaporation. to a 14-yr DJF climatology based on the earlier Finally, the GEOS GCM employs the gravity wave GEOS-1 reanalysis for the 1981-94 period, which is drag scheme of Zhou et al. (1996) to provide physi- an extended version of the 5-yr reanalysis from March cally based momentum dissipation in the stratosphere 1985 through February 1990 performed by Schubert and mesosphere due to orographic forcing. et al. (1993). The horizontal and vertical resolutions of the DAS used to produce the GEOS-1 reanalysis are 2° x 2.5° and 20 sigma levels, which are coarser than b. Datasets those of the current GEOS GCM. The case study covThe GEOS GCM was run at 1° x 1 ° resolution with 48 sigma levels in simulation mode to produce the ers a 36-h period in the life cycle of a cyclone-frontal 3-month (DJF) dataset used in this study. This data- system over the North Atlantic Ocean beginning in its set is interpolated from sigma to pressure coordinates developing stage and ending after the cyclone has and consists of 40 levels from 1000 to 0.2 hPa. deepened considerably and the frontal system has beAnalyses from 0600 UTC 1 December 1993 are used gun wrapping around the cyclone. This cycloneto initialize the simulation. Snow cover, ground wet- frontal system is selected because of its distinctive ness, sea ice, and sea surface temperature are speci- surface evolution. Output from 0000 UTC model day fied as boundary conditions in the simulation using 54 through 1200 UTC model day 55 is used for the climatological values.1 To compare the performance case study. of the GEOS GCM at 1° x 1° resolution with the 3. Climatological evaluation 1

Specification of snow cover and ground wetness is required because the GEOS GCM used in this study is an earlier version in which the land surface model was not yet implemented.

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As a first step in assessing the realism of the 3-month GEOS simulation, in Figs. 4-6, we compare global Vol. 82, No. 9, September 200 7

FIG. 4. Global distributions of time-mean (DJF) sea level pressure (contour interval 8 hPa). (top) Results from the GEOS GCM for a 1° x 1° latitude-longitude grid; the time-mean refers to the simulated 1993-94 Northern Hemisphere winter season, (bottom) Results from the GEOS-1 reanalysis for a 2° x 2.5° latitudelongitude grid; the time-mean refers to a 14-yr climatology for the 1981-94 period.

distributions of the time-mean (DJF) sea level pressure, 500-hPa geopotential height, and 250-hPa wind speed with their counterparts derived from the longerterm (14 yr) GEOS-1 reanalysis dataset. Although global patterns are presented, we focus on the Northern (winter) Hemisphere, given our emphasis on extratropical baroclinic systems. Background material documenting the structure and behavior of the wintertime climatological fields serving as the basis for the present comparison can be found in standard textbooks (e.g., Palmen and Newton 1969, sections 3.1, 3.4, and 3.5; Holton 1992, sections 6.1 and 10.5.2; Bluestein 1993, section 1.4.4). The time-mean sea level pressure fields from the simulation (Fig. 4, top panel) and the reanalysis (Fig. 4, bottom panel) both capture the Aleutian and Icelandic lows. The GEOS simulation exhibits lower central pressures than the GEOS-1 reanalysis, and the Bulletin of the American Meteorological Society

FIG. 5. As in Fig. 4 except for 500-hPa geopotential height (contour interval 120 m).

cyclone centers in the simulation are located farther east than in the reanalysis. The Siberian high is apparent in both the simulation and reanalysis, and, as is the case for the climatological lows, this high is somewhat stronger in the simulation than in the reanalysis. The time-mean 500-hPa geopotential height fields from the simulation (Fig. 5, top panel) and the reanalysis (Fig. 5, bottom panel) exhibit prominent troughs over eastern Asia and eastern North America, and ridges over western North America and the eastern North Atlantic Ocean. The positions of the troughs and ridges in the simulation match well with their counterparts in the reanalysis, and, consistent with the relative magnitudes of the sea level pressure centers, the troughs and ridges are amplified in the simulation relative to the reanalysis. The time-mean 500-hPa geopotential height gradients are larger in the bases of the troughs over eastern Asia and eastern North America in the simulation (Fig. 5, top panel) compared with the reanalysis (Fig. 5, bottom panel). In agreement with this comparison, the maximum wind speeds in the Pacific and Atlantic jet streams at 250 hPa are 18 57

slightly stronger in the simulation (Fig. 6, top panel) than in the reanalysis (Fig. 6, bottom panel). The confluence and diffluence patterns at 500 hPa (Fig. 5) coincide well with the entrance and exit regions of the jet streams at 250 hPa (Fig. 6): the Pacific jet exhibits confluence over eastern Asia and diffluence over the eastern North Pacific Ocean upstream of the western North American ridge, whereas the Atlantic jet exhibits confluence over North America and diffluence over the central North Atlantic Ocean upstream of the eastern North Atlantic ridge. The foregoing comparison indicates reasonable agreement between the current GEOS simulation and the GEOS-1 reanalysis. The overall greater strength and sharper definition of circulation features in the simulation compared with the reanalysis appear to be related primarily to the finer horizontal and vertical resolutions of the former (1° x 1° and 48 sigma levels) compared with the latter (2° x 2.5° and 20 sigma levels). Additional factors contributing to the discrepancies noted between the simulation and the reanalysis are the representativeness of a single season with respect to a longer-term (14 yr) climatology; differ-

ences in the formulations of the respective modeling systems used in the simulation and in the reanalysis; and the inherent distinction between a simulation and a data assimilation, whereby the former is unconstrained by observations following the initialization of the model and the latter is continually influenced by observations of the evolving atmosphere. In anticipation of the use of the DT in the forthcoming case study to illustrate the relationship between the tropopause and jet streams, we present zonal averages of the simulated DJF time-mean potential temperature and wind speed2 for the Northern Hemisphere in Fig. 7. Also shown are the positions of the DT (2.5-PVU contour) and the "dynamic equator" (0-PVU contour). This display is motivated by the schematic diagrams shown by Hoskins (1991, his Fig. 1) and Holton et al. (1995, their Fig. 3), which subdivide the zonally averaged troposphere and stratosphere into the so-called underworld, middleworld, and overworld. As stated by Hoskins (1991), this subdivision extends the underworld-overworld separation

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Here we use the term zonally averaged (time-mean) wind speed to refer to the speed of the zonally averaged (time-mean) horizontal vector wind.

FIG. 6. As in Fig. 4 except for 250-hPa wind speed (contour interval 10 m s -1 ).

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FIG. 7. Meridional cross section of zonally averaged time-mean potential temperature (black contours, interval 5 K) and wind speed (greater than or equal to 35 m s_1 shaded in red and contoured at a 2 m s _l interval) for the Northern Hemisphere. Also shown are the 298- and 380-K isentropes (purple contours), and the 0- and 2.5-PVU surfaces (blue contours). Cross section is based on results from the GEOS GCM for a 1° x 1° latitudelongitude grid, the ordinate and abscissa correspond to pressure (hPa) and latitude (°N), and the time-mean refers to the simulated 1993-94 DJF winter season.


introduced by Napier Shaw (1930, 316-318) through the addition of the middleworld. In this subdivision, the overworld lies above the lowest isentropic surface that intersects the DT where it becomes vertical in the Tropics [380 K, the nominal value given by Holton et al. (1995), is highlighted in Fig. 7]. This particular isentropic surface, which may be taken to define the tropical tropopause, delineates the boundary above and below which the extratropical stratosphere is and is not in communication with the troposphere through mass and chemical constituent transport along isentropic surfaces. The region where such transport can occur (i.e., where isentropic surfaces FIG. 8. Three-dimensional time-mean perspective for the Northern Hemisphere of the cross the DT) is referred to as the dynamic tropopause (2.5-PVU surface) with tropopause potential temperature (K) shaded middleworld. The middleworld according to legend. Also shown are the 35 m s_1 isosurface of wind speed (red shading), lies beneath the overworld and is and the 298- and 380-K isentropic surfaces (yellow and blue shading, respectively). For bounded from below by the isen- purposes of orientation, the view is toward the southeast with the Atlantic jet appearing in tropic surface that is tangent to the foreground and the Pacific jet in the background; the ordinate corresponds to geometric height ranging between 0 and 25 km. Perspective is based on results from the GEOS GCM the earth's surface in the Trop- for a 1° x 1° latitude-longitude grid, and the time-mean refers to the simulated 1993-94 ics (298 K in Fig. 7). In addition DJF winter season. to being tangent to the earth's surface in the Tropics, this bounding isentropic surface approaches the DT near DT and of the respective isentropic surfaces boundthe North Pole (see Fig. 7). The region beneath this ing the middleworld varies relatively slowly in the isentropic surface is referred to as the underworld, where zonal direction. The Pacific (background) and Atlanisentropic surfaces come into contact with the earth's tic (foreground) jet streams, indicated by the 35 m s_l surface. isosurface (corresponding to the outermost isotach Although not shown in the above-cited schemat- bounding the jet core in Fig. 7), straddle the DT and ics, Fig. 7 includes the zonally averaged wind speed stand out dramatically in Fig. 8. maximum, the core of which straddles the DT where its slope attains a local maximum at the inflection located slightly poleward of 30°N. This latitudinal po- 4 . Case study sition reflects the dominant influence of the Pacific and Atlantic jet streams during the Northern Hemisphere Continuing with the evaluation of the 3-month winter season (Fig. 6). In view of the remarkable con- GEOS simulation, we focus on a 36-h period in the sistency between the time-mean fields in Fig. 7 and life cycle of an extratropical cyclone that developed the under-middle-overworld conceptualization of the off the east coast of North America, beginning at zonally averaged troposphere and stratosphere, we examine in Fig. 8 the zonal variability of selected surfaces shown in Fig. 7 through a three-dimensional vi- 3 Documentation of Vis5D is available from the Space Science sualization produced using Vis5D graphical display and Engineering Center, University of Wisconsin—-Madison, software.3 Figure 8 shows that the "topography" of the through their Web site at http://www.ssec.wisc.edu/~billh/vis.html. Bulletin of the American Meteorological Society

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0000 UTC model day 54 (00Z/54) and ending at 1200 UTC model day 55 (12Z/55). The sea level pressure and 900-hPa potential temperature fields are shown in Fig. 9 at 12-h intervals. At 00Z/54, the cyclone center, located over the western North Atlantic Ocean, coincides with the peak of the warm sector evident in the 900-hPa potential temperature field. The cold front stretches from the center of the cyclone, the central pressure of which is 992 hPa, southwestward toward the North Carolina coast. At 12Z/54, the cyclone has deepened to 984 hPa and the cold front has elongated meridionally. At this time, the warm front is oriented zonally, its length is shorter than that of the cold front, and its cross-frontal potential temperature contrast is weaker than that of the cold front. By 00Z/55, the central pressure has decreased to 968 hPa, and the cold front is approaching the warm front such that the warm sector is beginning to narrow. At 12Z/55, the central pressure has reached 960 hPa, the

cold front continues to approach the warm front, and the warm sector narrows further, reminiscent of the evolution in the Norwegian cyclone model (e.g., Schultz et al. 1998). Figure 10 shows the sea level pressure field and the distribution of precipitation rate calculated over a 3-h period ending at the output time for the same times shown in Fig. 9. At 00Z/54, the heaviest precipitation is concentrated to the north and east of the cyclone center, while a band of lighter precipitation extends along the trough of low pressure corresponding to the cold front. At 12Z/54, the most intense precipitation is occurring just east of the cyclone center along the warm front and southwest of the center along the cold front. At 00Z/55, a short band of intense precipitation to the north and east of the cyclone lies along the warm front and has begun to wrap cyclonically around the center. At this time, a long narrow band of precipitation stretches from east of the cyclone center along the

FIG. 9. Sea level pressure (contour interval 4 hPa) and 900-hPa potential temperature (K, shaded according to legend) from the GEOS simulation for a 1° x 1° latitude-longitude grid. Meridional and zonal extent of the display domain is between 15° and 70°N, and between 100°W and 10°E, respectively, (top left) Results for 0000 UTC model day 54, (top right) for 1200 UTC model day 54, (bottom left) for 0000 UTC model day 55, and (bottom right) for 1200 UTC model day 55.

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cold front to the south and west. At 12Z/55, the short band of precipitation continues to wrap cyclonically to the north and east of the cyclone, while the long narrow band of precipitation continues to extend south and west along the cold front. Figure 11 shows the sea level pressure, 500-hPa geopotential height, and 300-hPa wind speed fields. At 00Z/54, the surface cyclone is positioned between the left-exit and right-entrance regions of upper-level jet streaks situated upstream and downstream of the cyclone, respectively, a configuration favoring focused ascent in the vicinity of the cyclone center and concomitant deepening (Uccellini and Kocin 1987). A short-wave trough, evident in the 500-hPa geopotential height field and accompanying the upstream jet streak, is approaching from the west at this time. At 12Z/54, the surface cyclone, which has deepened 8 hPa (to 984 hPa) during the previous 12 h, remains positioned between the two upper-level jet streaks. The 500-hPa short-wave trough has amplified, acquired a negative tilt, and undergone a shortening of the half

wavelength between the short-wave trough and the downstream ridge, all signatures of continuing cyclogenesis. At 00Z/55, the surface cyclone has deepened an additional 16 hPa (to 968 hPa) and is located beneath the inflection in the 500-hPa geopotential height field between the axes of the short-wave trough and downstream ridge. This location corresponds to the cyclonic-shear side of the downstream jet streak, as the cyclone and the trough-ridge couplet have begun to separate from the left-exit region of the upstream upperlevel jet streak. The trough-ridge couplet continues to amplify as the ridge builds to the north-northeast of the surface cyclone. At 12Z/55, the surface cyclone has deepened by 8 hPa (to 960 hPa) during the previous 12 h, but is showing signs of the cessation of deepening, including the increasing separation of the cyclone center from both upper-level jet streaks and the vertically stacked orientation of the 500-hPa trough relative to the surface cyclone. Having documented the development of the simulated North Atlantic cyclone from a conventional, iso-

FIG. 10. As in Fig. 9 except for sea level pressure (contour interval 4 hPa) and precipitation rate (mm day -1 , shaded according to legend).


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baric perspective, we reexamine this development from an isentropic PV perspective in the spirit of Mclntyre (1999, p. 350), who predicts that in the future one might be able to use "'virtual reality' techniques . . . to fly oneself very quickly, supermanlike, around some kind of 3D representation of significant features such as the tropopause and other regions of steep isentropic gradients of PV." In Figs. 12-15, we use Vis5D software to illustrate the three-dimensional structure of the DT, tropopause folds, upper-level fronts, and jet streaks. The top panels of Figs. 12 and 13 depict the evolution of the sea level pressure and tropopause potential temperature fields from a twodimensional perspective, whereas the bottom panels of these figures depict the topography of the DT above a two-dimensional sea level pressure map. The latter depiction is motivated by the displays of Bleck (1974, his Fig. 2) and Uccellini (1990, his Fig. 6.12). As in their displays, the DT shown in Figs. 12 and 13 features various downward-protruding vortexlike

structures, one of which (that highlighted by the red arrows) is associated with the development of the North Atlantic cyclone. This particular structure will be shown to be the three-dimensional manifestation of a tropopause fold represented in a two-dimensional vertical cross section at 00Z/55 (Fig. 14, bottom panel). Given this correspondence, we shall refer to this structure as a "vortical feature" and a "tropopause fold" interchangeably throughout the remainder of this section. At 00Z/54, the vortical feature corresponding to the tropopause fold associated with the development of the North Atlantic cyclone is located upstream of the surface center. From the depiction of the DT topography, we can follow the vortical feature in time and observe its interaction with the surface cyclone. At 12Z/54, the region of low values of tropopause potential temperature, the southern tip of which coincides with the tropopause fold, begins wrapping around the surface cyclone in a cyclonic sense (Fig. 12,

FIG. 11. As in Fig. 9 except for sea level pressure (black contours, interval 4 hPa), 500-hPa geopotential height (blue contours, interval 60 m), and 300-hPa wind speed (shaded for values greater than or equal to 50 m s_1 according to legend beneath each panel).

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top-right panel). At 12Z/54 (Fig. 12, bottom-right panel) and 00Z/55 (Fig. 13, bottom-left panel), the fold has penetrated into the lower troposphere. By 00Z/55, a hook-shaped region of low-tropopause potential temperature has wrapped cyclonically around the surface cyclone (Fig. 13, top-left panel), which has deepened 16 hPa since the onset of cyclonic wrapping 12 h earlier (Fig. 12, top-right panel). At 12Z/55, coinciding with the cessation of surface deepening, the cyclonic wrapup of the region of low-tropopause potential temperature has progressed to the point where the tip of the hook overlies the surface cyclone (Fig. 13, top-right panel), and the vortical feature corresponding to the tropopause fold is retracting upward (i.e., "unfolding") (Fig. 13, bottom-right panel). It is apparent that the downward penetration of the tropopause fold and the associated cyclonic wrapup of the tropopause potential temperature minimum are playing an important role in the deepening of the cyclone. In this regard, Browning (1999, p. 282) comments that, "The effect of the high-PV air [associated with a descending tropopause fold], especially where it overruns a low-level baroclinic zone, is to trigger (or enhance) cyclogenesis and ascent." The cyclonic wrapup evident in the tropopause potential temperature field documented in Figs. 12 and 13 is suggestive of the LC2 life cycle proposed by Thorncroft et al. (1993). The LC2 life cycle is favored on the cyclonic-shear side of the largescale jet stream providing the environmental flow for the cy-

FIG. 12. (top) Sea level pressure (contour interval 4 hPa) and tropopause potential temperature (K, shaded according to legend); (bottom) three-dimensional perspective from a southern viewpoint of the dynamic tropopause (1.5 PVU) with tropopause potential temperature (K) shaded according to legend, and sea level pressure contoured (interval 4 hPa) on lower horizontal plane. Geographical region displayed is the same as in Figs. 9-11; the ordinate in the bottom panels corresponds to geometric height ranging between 0.46 and 16.67 km. Panels show results from the GEOS simulation for a 1° x 1° latitude-longitude grid: (left) for 0000 UTC model day 54 and (right) for 1200 UTC model day 54. Red arrows in bottom panels indicate tropopause fold discussed in text.

FIG. 13. As in Fig. 12 except for model day 55.


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clone evolution. 4 The cyclone in the present case evolves in the left-exit region of both the simulated DJF time-mean North Atlantic jet (Fig. 6, top panel) and the instantaneous upstream upper-level jet streak (Fig. 11); this region is characterized by cyclonic shear and diffluence. Although neither the simulated DJF time-mean flow nor the instantaneous flow might be

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The environmental flow for an extratropical cyclone may be defined as one in which the cyclone has been extracted from the total flow, which may be accomplished through PV inversion (e.g., Hakim et al. 1996) or digital filtering (e.g., Lackmann et al. 1997; Schultz et al. 1998).

FIG. 14. (top) Sea level pressure (contour interval 4 hPa), tropopause potential temperature (K, shaded according to legend), and projection of the 60 m s~' isosurface of wind speed onto the horizontal plane (red shading); (bottom) meridional cross section along 48°W of potential temperature (blue contours, interval 5 K), wind speed (greater than or equal to 60 m s_1 shaded in red and contoured at a 2 m s_1 interval), and the 1.5-PVU surface (black contour). Geographical region displayed in the top panel is the same as in Figs. 9-13; the ordinate and abscissa in the bottom panel correspond to pressure (hPa) and latitude (°N). The vertical line in the top panel indicates the location of the cross section shown in the bottom panel. Panels show results from the GEOS simulation for a 1° x 1° latitude-longitude grid at 0000 UTC model day 55.

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expected to correspond closely to the environmental flow for the cyclone evolution in the present case (because the former is an average applying to an entire season, and the latter is unaveraged and thus contains the cyclone as well as its environment), the location of the developing cyclone on the cyclonic-shear side of the time-mean and instantaneous jets is consistent with the cyclonic wrapup in the tropopause potential temperature field (Thorncroft et al. 1993; Shapiro et al. 2001). Similarly, the location of the developing cyclone in the diffluent exit region of these respective jets is consistent with the evolution of the 900-hPa potential temperature field in accord with the Norwegian cyclone model documented in Fig. 9, including the meridionally extensive cold front, the zonally compact warm front, and the narrowing warm sector (Schultz et al. 1998). Further documentation of the interaction between the tropopause fold and the developing cyclone at 00Z/55, coinciding with the end of the 12-h period of most rapid surface deepening, is provided in Figs. 14 and 15. The top panel of Fig. 14 reproduces the sea level pressure and tropopause potential temperature patterns that are shown in the top-left panel of Fig. 13, with the added depiction of the upstream upper-level jet streak. The bottom panel of Fig. 14, consisting of a meridionally oriented cross section along 48°W, reveals a deep tropopause fold extending below the 700-hPa level along a strong frontal zone. This frontal zone, which extends from the depressed tropopause to the earth's surface, corresponds to the cold front shown in the bottom-left panel of Fig. 9. The bisection of the jet core by the DT recalls the relationship between the time-mean DT and jet stream shown in Fig. 7, where the time-mean jet core straddles the DT. In contrast to the approximately zonally symmetric structure of the time-mean DT in the latitudinal vicinity of the Pacific and Atlantic jet streams evident in Fig. 8, the tropopause fold is highly localized in space, consistent with its prior characterization as a vortical feature (Fig. 13, bottom-left panel). The threedimensional nature of the tropopause fold is illustrated further in Fig. 15, which is a rotated version of the bottom-left panel of Fig. 13 that includes depictions of the jet streak associated with the tropopause fold and the meridional cross section along 48°W (see Fig. 14). From this perspective, it is evident that the jet streak straddles the DT where the latter becomes nearly vertical in the form of a "PV wall," which constitutes the equatorward boundary of the stratospheric polar vortex. Also apparent is the vortical nature of the Vol. 82, No. 9, September 200 7

FIG. 15. Three-dimensional perspective from a southwestern viewpoint of the dynamic tropopause (1.5 PVU surface) with tropopause potential temperature (K) shaded according to legend. Also shown are 60 m s_1 isosurface of wind speed (red shading), meridional cross section along 48°W of potential temperature (purple contours, interval 5 K), and sea level pressure (contour interval 4 hPa) on lower horizontal plane. Geographical region displayed is the same as in Figs. 9-14; the ordinate corresponds to geometric height ranging between 0.46 and 16.67 km. Perspective is based on results from the GEOS GCM for a 1° x 1° latitude-longitude grid at 0000 UTC model day 55.

tropopause fold, which tilts downward along the region of strong horizontal potential temperature gradient corresponding to the cold front.

5 . Summary and conclusions Increasing the horizontal resolution of the GEOS GCM through a reduction of the latitude-longitude grid spacing from 2° x 2.5° to 1° x 1° has resulted in a substantial improvement in the representation of extratropical baroclinic systems. These systems include cyclones, fronts, jet streams, and the tropopause—the structure and evolution of which are evaluated subjectively from climatological and case-study perspectives. The former evaluation is conducted using climatological fields for the Northern Hemisphere winter season (Dec-Feb) derived from the GEOS-1 reanalysis for the 1981-94 period, and the undermiddle-overworld conceptualization of the zonally averaged troposphere and stratosphere described by Bulletin of the American Meteorological Society

Hoskins (1991), as benchmarks for comparison. Various large-scale circulation features related to the Pacific and Atlantic jet streams are reproduced realistically in the time-mean fields simulated by the GEOS GCM for a single 3-month winter season. A case study of an extratropical cyclone that developed off the east coast of North America revealed an evolution of the low-level potential temperature field suggestive of the Norwegian cyclone model, featuring a meridionally extensive cold front, a short warm front, and a narrowing warm sector. Analysis of the dynamic tropopause indicates that the cyclogenesis involves the interaction between a deep tropopause fold and the surface cyclone; three-dimensional visualization of the tropopause fold using Vis5D graphical display software shows this feature to be highly localized, exhibiting a vortical structure. The evolution of the potential temperature field on the dynamic tropopause features a cyclonic wrapup, reminiscent of the LC2 life cycle proposed by Thorncroft et al. (1993). The surface cyclone intensifies in the left-exit region 18 57

of both the time-mean North Atlantic jet stream and an upper-level jet streak depicted in the instantaneous flow field. The cyclonic wrapup in the tropopause potential temperature field may be related to the cyclonically sheared environment of the left-exit region, whereas the evolution of the low-level potential temperature field in accord with the Norwegian cyclone model may be related to the diffluent character of the jet-exit region. This evaluation indicates that high-resolution stateof-the-art GCMs are on the verge of resolving mesoscale phenomena and processes explicitly. As such, they offer the unprecedented opportunity to investigate the structure and dynamics of extratropical baroclinic systems from a multiscale perspective over intraseasonal timescales. When run in assimilation mode, GCMs possessing mesoscale resolution will be able to take advantage of the fine structure presently available in satellite data such as scatterometer winds. Specialized observations collected during intensive field programs also may be assimilated into GCMs, yielding unique high-resolution experimental datasets for analysis and diagnosis. Finally, high-resolution GCMs are well suited to addressing issues related to climate change. As an example, it should be possible to simulate regional weather and climate anomalies with increasing certainty because of the improved resolution of individual weather systems in GCMs applied to various climate change scenarios. Acknowledgments. The authors thank the three anonymous reviewers for their advice and suggestions for improving the presentation, and Professor Chris Thorncroft for sharing his insights on the relationship between extratropical cyclone life cycles and the large-scale flow. Support for D. Keyser's participation in the preparation of this article was provided by NASA through Grant NAG5-7469, awarded to the University at Albany, State University of New York.

References Arakawa, A., and M. J. Suarez, 1983: Vertical differencing of the primitive equations in sigma coordinates. Mon. Wea. Rev., Ill, 34-45. Atlas, R., S. Bloom, R. N. Hoffman, E. Brin, J. Ardizzone, J. Terry, D. Bungato, and J. C. Jusem, 1999: Geophysical validation of NSCAT winds using atmospheric data and analyses. J. Geophys. Res., 104 (C5), 11 405-11 424. Bleck, R., 1974: Short-range prediction in isentropic coordinates with filtered and unfiltered numerical models. Mon. Wea. Rev., 102, 813-829. Bluestein, H. B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes, Observations and Theory of Weather Systems, Vol. II, Oxford University Press, 594 pp.

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Browning, K. A., 1999: Mesoscale aspects of extratropical cyclones: An observational perspective. The Life Cycles of Extratropical Cyclones, M. A. Shapiro and S. Gr0nas, Eds., Amer. Meteor. Soc., 265-283. Burridge, D. M., and J. Haseler, 1977: A model for medium-range weather forecasting—Adiabatic formulation. ECMWF Tech. Rep. 4, Bracknell, Berkshire, United Kingdom, 46 pp. Fox-Rabinovitz, M. S., G. L. Stenchikov, M. J. Suarez, and L. L. Takacs, 1997: A finite-difference GCM dynamical core with a variable-resolution stretched grid. Mon. Wea. Rev., 125, 2943-2968. Hakim, G. J., D. Keyser, and L. F. Bosart, 1996: The Ohio Valley wave-merger cyclogenesis event of 25-26 January 1978. Part II: Diagnosis using quasigeostrophic potential vorticity inversion. Mon. Wea. Rev., 124, 2176-2205. Helfand, H. M., and J. C. Labraga, 1988: Design of a nonsingular level 2.5 second-order closure model for the prediction of atmospheric turbulence. J. Atmos. Sci., 45, 113-132. Holton, J. R., 1992: An Introduction to Dynamic Meteorology. 3d ed. Academic Press, 511 pp. , P. H. Haynes, M. E. Mclntyre, A. R. Douglass, R. B. Rood, and L. Pfister, 1995: Stratosphere-troposphere exchange. Rev. Geophys., 33, 4 0 3 ^ 3 9 . Hoskins, B. J., 1991: Towards a PV-0 view of the general circulation. Tellus, 43AB, 27-35. , M. E. Mclntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., I l l , 877-946. Lackmann, G. M., D. Keyser, and L. F. Bosart, 1997: A characteristic life cycle of upper-tropospheric cyclogenetic precursors during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). Mon. Wea. Rev., 125, 2729-2758. Mclntyre, M. E., 1999: Numerical weather prediction: A vision of the future, updated still further. The Life Cycles of Extratropical Cyclones, M. A. Shapiro and S. Gr0nas, Eds., Amer. Meteor. Soc., 337-355. Moorthi, S., and M. J. Suarez, 1992: Relaxed Arakawa-Schubert: A parameterization of moist convection for general circulation models. Mon. Wea. Rev., 120, 978-1002. Morgan, M. C., and J. W. Nielsen-Gammon, 1998: Using tropopause maps to diagnose midlatitude weather systems. Mon. Wea. Rev., 126, 2555-2579. Napier Shaw, W., Sir, 1930: Manual of Meteorology, The Physical Processes of Weather. Vol. Ill, Cambridge University Press, 445 pp. Palmen, E., and C. W. Newton, 1969: Atmospheric Circulation Systems: Their Structure and Physical Interpretation. International Geophysics Series, Vol. 13, Academic Press, 603 pp. Schubert, S. D., R. B. Rood, and J. Pfaendtner, 1993: An assimilated dataset for earth science applications. Bull. Amer. Meteor. Soc., 74, 2331-2342. Schultz, D. M., D. Keyser, and L. F. Bosart, 1998: The effect of large-scale flow on low-level frontal structure and evolution in midlatitude cyclones. Mon. Wea. Rev., 126, 1767-1791. Shapiro, M. A., and D. Keyser, 1990: Fronts, jet streams and the tropopause. Extratropical Cyclones, The Erik Palmen Memorial Volume, C. W. Newton and E. O. Holopainen, Eds., Amer. Meteor. Soc., 167-191. , H. Wernli, N. A. Bond, and R. Langland, 2001: The influence of the 1997-99 El Nino-Southern Oscillation on extrat-


ropical baroclinic life cycles over the eastern North Pacific. Quart. J. Roy. Meteor. Soc., 127, 331-342. Suarez, M. J., and L. L. Takacs, 1995: Documentation of the Aries/GEOS dynamical core: Version 2. N A S A Tech. Memo. 1 0 4 6 0 6 , V o l . 5, N A S A G o d d a r d S p a c e F l i g h t C e n t e r , Greenbelt, MD, 45 pp. Sud, Y., and A. Molod, 1988: The roles of dry convection, cloudradiation feedback processes and the influence of recent improvements in the parameterization of convection in the GLA GCM. Mon. Wea. Rev., 116, 2366-2387. Thorncroft, C. D., B. J. Hoskins, and M. E. Mclntyre, 1993: Two paradigms of baroclinic-wave life-cycle behaviour. Quart. J. Roy Meteor. Soc., 119, 17-55.

Uccellini, L. W., 1990: Processes contributing to the rapid development of extratropical cyclones. Extratropical Cyclones, The Erik Palmen Memorial Volume, C. W. Newton and E. O. Holopainen, Eds., Amer. Meteor. Soc., 81-105. , and P. J. Kocin, 1987: The interaction of jet streak circulations during heavy snow events along the east coast of the United States. Wea. Forecasting, 2, 289-308. Zhou, J., Y. C. Sud, and K.-M. Lau, 1996: Impact of orographically induced gravity-wave drag in the GLA GCM. Quart. J. Roy. Meteor. Soc., 122, 903-927.

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W E A T H E R I N G THE STORMS

SVERRE PETTERSSEN? THE D-DAY FORECAST, AND THE RISE OF MODERN METEOROLOGY mum

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Meteorology today is the beneficiary of the fundamental work in weather analysis and forecasting of Sverre Petterssen (1898-1974), a giant in the field and an international leader in meteorology during its formative era. In this livelyJ and insightful autobiographical memoir, written just before his death, Petterssen shares intimate memories from his childhood in Norway, his education and service with the famous Bergen school of meteorology, and his extensive experiences in polar forecasting and as head of the meteorology department at MIT. The crisis of World War II comes alive in his passionate recounting of how forecasts were made for bombing° raids and special R

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operations, including the contentious ' HB; I I BHBHBBBSffiI forecasts for D-Day. Sverre Petterssen's complete autobiographical memoir, published here for the first time in English, offers a fascinating view of a man, an era, and a science. Anyone interested in weather, World War II, the history of science, or Norwegian history will enjoy this book.

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