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PICTURE of the MONTH Mesoscale Vortices Embedded Within a Lake

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PICTURE of the MONTH Mesoscale Vortices Embedded Within a Lake SEPTEMBER 2004 PICTURE OF THE MONTH 2269 PICTURE OF THE MONTH Mesoscale Vortices Embedded within a Lake-Effect Shoreline Band JOSEPH A. GRIM AND NEIL F. L AIRD* Department of Atmospheric Sciences, University of Illinois at Urbana±Champaign, Urbana, Illinois DAVID A. R. KRISTOVICH Atmospheric Environment Section, Illinois State Water Survey, Illinois Department of Natural Resources, Champaign, and Department of Atmospheric Sciences, University of Illinois at Urbana±Champaign, Urbana, Illinois 15 December 2003 and 23 March 2004 1. Introduction cyclonic low-level circulations with diameters of about 50±120 km. Similarly, Pease et al. (1988) reported a Mesoscale vortices are one of the basic types of win- vortex over the southern basin of Lake Michigan that tertime atmospheric circulations that occur in the Great was accompanied by a weak cyclonic circulation and Lakes region as a result of lake-to-atmosphere heat and light snow. moisture exchanges. Mesoscale vortices associated with Past observational and numerical studies (e.g., Hjelm- the Great Lakes occur over a range of diameters from felt 1990; Pease et al. 1988) have indicated that meso- approximately 10 to 1000 km. On the largest scale, ag- b-scale lake-effect vortices tend to occur with large gregate vortices with widths of 500±1000 km form over lake±air temperature differences (DT) and weak syn- the Great Lakes region and often appear as weak low optic-scale winds. However, Laird et al. (2001) exam- pressure centers or large-scale sea level pressure (SLP) ined a mesoscale vortex that developed along an existing and circulation perturbations (e.g., Sousounis 1997). convergence zone under conditions of low DT (68±98C). This article presents a lake-effect event in which vortices More recently, Laird et al. (2003a,b) used numerical developed with diameters of 15±35 km, which fall with- simulations for a large range of lake-effect environments in large meso-g and small meso-b scales (Orlanski to determine that mesoscale vortex formation is related 1975). to small values of the ratio of wind speed (U) to overlake Meso-b-scale vortices with size scales near that of a fetch (L) and is independent of DT when DT $ 58C. lake basin are often the most impressive and discernable Meso-g-scale vortices are typically embedded within in appearance on satellite and radar imagery. For ex- or develop in close proximity to lake-effect shoreline ample, Laird (1999) documented an unusual case with snowbands. A number of past studies have indicated four meso-b-scale vortices simultaneously occurring signi®cant variations in small-scale structure along over Lakes Huron, Michigan, and Superior. These vor- shoreline snowbands (e.g., Shef®eld 1964; Pitts et al. tices are generally associated with weak winds and light 1977; Braham 1983; Passarelli and Braham 1981; snowfall (e.g., Forbes and Merritt 1984), although some Forbes and Merritt 1984). However, the study of can result in localized intense snowfall (Laird et al. Schoenberger (1986) was the only one to provide de- 2001). Forbes and Merritt (1984) documented 14 vortex tailed documentation of meso-g-scale vortices along a events over Lakes Huron, Michigan, and Superior dur- lake-effect snowband. He used dual-Doppler radar mea- ing a 5-yr period. The vortices were accompanied by surements to describe the structure and evolution of sev- brief snow squalls, lower surface pressures, and weak eral cyclonic and anticyclonic 10-km-scale vortices. While meso-g-scale vortices can cause signi®cant in- *Current af®liation: Department of Geoscience, Hobart and Wil- homogeneties in the structural coherence and snowfall liam Smith Colleges, Geneva, New York. distribution of shoreline bands, the mechanisms leading to their development are the least understood. Obtaining an understanding of the development of these cold-sea- Corresponding author address: Neil F. Laird, Department of Geo- science, Hobart and William Smith Colleges, 300 Pulteney St., Ge- son vortices may also provide insight toward mecha- neva, NY 14456. nisms responsible for analogous warm-season nonsu- E-mail: [email protected] percell vortex formation. q 2004 American Meteorological Society 2270 MONTHLY WEATHER REVIEW VOLUME 132 In this article, we provide detailed observations of a 5±8 km and a west-northwest to east-southeast orien- line of small-scale lake-effect vortices. These vortices tation. Peak effective re¯ectivity factors (re¯ectivities) developed along an intense lake-effect snowband over within each band ranged from 10 to 27 dBZ. Radial the eastern portions of Lake Michigan on 14 January velocities showed that the environment was character- 2003 (Fig. 1), had lifetimes of 2±5 h, and resulted in ized by nearly unidirectional ¯ow from 3008 with an signi®cant variations of the winds and snowfall rates increase in wind speed from 3 m s21 at about 100 m along the band. above the surface to 13 m s21 near the tops of the bands (approximately 1800 m). The orientation and spacing of the bands, as well as the environmental conditions, 2. Synoptic and mesoscale environment were similar to those reported in past studies of bound- Environmental conditions on 14 January 2003 were ary layer roll convection (i.e., wind-parallel bands) over conducive to the development of a strong land breeze Lake Michigan (e.g., Kelly 1986; Kristovich 1993). over western lower Michigan, leading to the formation The land breeze that had developed over western low- of a snowband oriented nearly parallel to the long axis er Michigan was initially observed by KGRR at around of Lake Michigan (i.e., shoreline band). At 0000 UTC, 0200 UTC. At the leading edge of the land breeze a a low pressure center was positioned near Maine with narrow region of weak divergence at 1500-m altitude a cold front along the northeastern Atlantic coast and could be seen in the radial velocity ®eld stretching along extending westward through Virginia and Kentucky. As the shore from 40 km north-northwest of Muskegon to the center of low pressure moved eastward, a weak about 85 km southwest of Grand Rapids. North-north- trough swung southward through the Great Lakes, pro- easterly near-surface winds were observed within the viding a region of large-scale convergence over Lakes land breeze by KGRR, while northwesterly winds con- Huron and Michigan (Fig. 2). A weak pressure gradient sistent with the synoptic pressure ®eld were observed over the western Great Lakes resulted in northwest sur- at higher levels. face winds generally less than 8 m s21. The increasingly The land-breeze front became more distinct in the radar cold air combined with Lake Michigan water surface re¯ectivity and radial velocity ®elds and moved slowly temperatures of 08±58C to cause DT to increase to 148± westward over the lake during the next several hours (Fig. 228C. At 850 hPa, temperatures in the vicinity of Lake 1f), while the radar observations suggested the wind- Michigan ranged from about 216.08 to 220.08C, with parallel bands became weaker and less coherent. Re¯ec- little evidence of the shallow, westward-extending sur- tivities increased along and just to the east of the land- face trough (Fig. 3). The temperature difference between breeze front as a shoreline snowband developed and the lake surface and 850 hPa exceeded the 138C criterion strengthened. By 1200 UTC the 20±30-km-wide, north± often cited for the development of lake-effect snows south-oriented, shoreline band had become stationary (e.g., Rothrock 1969; Niziol 1987). over the eastern portion of the lake (Fig. 1c). The band As both surface wind speeds in the vicinity of Lake intersected the shoreline both north of Muskegon and Michigan and surface temperatures over central lower near the Indiana±Michigan border. Re¯ectivities within Michigan began to decrease around 0000 UTC on 14 the band were as high as 32 dBZ, while divergence near January, a land breeze formed inland of the eastern shore the upper levels of the snowband (calculated from the of Lake Michigan from Manistee, Michigan, southward radial velocities) was about 6.7 3 1024 s21. to Benton Harbor, Michigan. Surface and radar obser- vations showed that by 0200 UTC the land-breeze front b. Small-scale vortices had moved westward to the shoreline, with the most coherent portion positioned offshore from Muskegon, Shortly after the shoreline band became stationary, Michigan, southward. As detailed in section 3, it is along wavelike perturbations developed along the band with the land-breeze front that an intense lake-effect shore- spacings of about 20 km. By 1330 UTC the band began line band formed and subsequently a line of small-scale propagating eastward while several of the wavelike per- vortices developed within the band. turbations developed into cyclonic vortices. Weak an- ticyclonic vertical vorticity was observed in some lo- cations, but anticyclonic vortices were not evident. 3. Radar observations Three cyclonic vortices developed along the southern portion of the shoreline band (south of KGRR) with a. Shoreline band formation radar re¯ectivity showing diameters ranging from 15 to Prior to the development of the shoreline snowband, 25 km. Initial values of vertical vorticity1 were about the National Weather Service (NWS) Grand Rapids 2.0 3 1023 s21, estimated from radial velocities. Shortly Doppler radar (KGRR) observed an area of wind-par- after 1330 UTC, the vortices propagated nearly due allel bands over Lake Michigan. At 0000 UTC 14 Jan- uary 2003, an area of widespread snow with embedded 1 Vertical vorticity for each vortex was estimated using peak in- banded structure was located just west of Grand Rapids, bound and outbound radial velocities along a line oriented perpen- Michigan (Fig. 1b). The bands had a spacing of about dicular to the radar beam through the vortex center.
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