JP7.6
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1 Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, WI ; 2 Czech Hydrometeorological Institute, Praha 4, Czech Republic;
3 NOAA/National Severe Storms Laboratory, Norman, OK., 4 Space Science and Engineering Center, University of Wisconsin-Madison, Madison, WI
Satellite observations of severe thunderstorms reveal many intriguing features such as the enhanced-
V, warm-cold couplet, distant warm area, cold ring and various complex wave structures at the storm top in both visible and infrared bands. Understanding the physical mechanisms responsible for these features not only is of fundamental importance in atmospheric science but helps the detection, monitoring and nowcasting of severe storms and the alleviation of storm damages.
In this study, we use a cloud model WISCDYMM (3-D, quasi-compressible, nonhydrostatic) to simulate the CCOPE supercell occurred in Montana, Aug. 2, 1981 with and without wind shear to show the mechanisms responsible for these features.
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Enhanced-V – a chevron-shaped feature usually appears in the upwind edge of the storm anvil. Depending on the apex angle of the chevron, it may appear to be either a V or U-shape. It has also been called “cold-V” because the IR brightness temperature is colder than the majority of the anvil.
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Cold Area (CA) – a compact area usually located slightly downstream of but near the apex of V. The temperature is often the coldest in the storm top.
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Close-in Warm Area (CWA) – a compact warm area downstream of but near the cold area. The temperature is often the warmest.
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Warm-Cold Couplet – CA and CWA together form such a couplet
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Distant Warm Area (DWA) – further downstream of the CWA there is usually a general wider are with warm temperatures
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Cold Ring (CR) – some storms appear to have a relatively round anvil and the rim of the anvil is colder than the inner part
A example of enhanced-V in MODIS IR image http://cimss.ssec.wisc.edu/goes/blog/archives/424
CWA
You can get a sense that such a “trench” can surround an overshooting top by examining astronaut photography of thunderstorms taken from the space shuttle (image courtesy of
Earth Sciences and Image Analysis Laboratory, NASA Johnson
Space Center).
CA
DWA
Enhanced-V
The blocking of the ambient flow may also cause the occasional appearance of ship wave signatures because the flow past a stationary obstacle is equivalent to a ship moving relative to still water.
Severe thunderstorms developed over northwestern Kansas on 19 June 2007 . An AWIPS image of the MODIS 11.0µm InfraRed
(IR) channel (above) revealed an “enhanced-v” and a “warm trench” IR signature on adjacent storm tops. The coldest cloud top brightness temperature values were -77º C on both signatures; the warmest IR temperature associated with the “enhancedv” was -58º C , while the warmest IR temperature in the “warm trench” was 61º C .
The cloud model simulation results show that:
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Essentially all storm top major IR features are simulated well
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The main mechanism responsible for these IR features is the interaction of the ambient flow with the strong updraft of the deep convection.
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Near the cloud top, the updraft behaves as a block to the ambient flow so that the flow goes around the updraft core (usually indicated by the overshooting top) to form a chevron shape in the upstream. The chevron is raised due to the buildup of pressure by winds against an obstacle (a stagnation region) and hence is colder. The overshooting top is usually the highest point and also the coldest.
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Passing the overshooting top, the flow subsidizes rapidly , causing deep pockets which form the close-in warm area. This is similar to the case of lee waves.
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The downstream part of the anvil is warmer due to the generally lower altitudes.
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In the case of no or weak wind shear, a cold ring may appear due to the cloud top oscillation.
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Because of the cloud top blocking, ship wave signature may also appear.
ACKNOWLEDGEMENTS We would like to acknowledge the support of the National Science
Foundation grant ATM-0729898.
*Corresponding author: Pao K. Wang • Address: 1225 W. Dayton Street • Madison, WI 53706 • Phone: 608-263-6479 • Email: pao@windy.aos.wisc.edu