Satellite-Observed Characteristics of Midwest Severe Thunderstorm Anvils

Abstract

This paper presents analyses of the cloud top and anvil structure of severe thunderstorms as observed by GOES for five SESAME cases during 1979 and four non-SESAME cases during 1980?82. Emphasis was placed on comparison of these observations with previous models and hypotheses for the so-called V feature and thermal couplets in the infrared (IR) observations. Thermal couplets are comprised of a low temperature region generally at or upwind of the cloud summit (?old point? near the vertex of the V), and one or more higher temperature regions downshear of the cloud summit (?warm points? within the interior of the V). Statistics of the various cloud top and anvil features were compiled and the different cases were compared. All studied except one (8 June 1979) had the V feature. The temperature difference between the claw-in warm and cold points comprising the thermal couplet ranged from 7° to 17°C and the mean separation distance between these points was 21?44 km. The observations also indicated that along with the thermal couplet in the vicinity of the peak cloud top height, a second less frequent warm point occurs ?40?120 km downshear in the anvil (?distant warm area?). The characteristics of the different cases extracted from GOES visible and IR data are examined and related to the upper-level temperature and wind conditions. The combination of strong tropospheric shear, especially near the tropopause level. intense updrafts, and overshooting tops appear to be an important ingredient in the V development. Severe weather and the V feature are strongly correlated because both are associated with strong updrafts and large tropospheric shear, although the former has especially strong shear at low to mid-levels, while the latter is associated with large shear at the tropopause level. The cloud top IR observations are compared with conceptual and numerical model results. The warm points downwind of the cloud top are still suggested to be due to subsidence, however some of the new analysis suggests the presence of subsidence due to mountainlike waves. A conceptual model is presented in which the close-in warm point is produced by both internal cloud air motions and stratospheric flow around and over the cloud top. The distant warm point is suggested to be due to either a wave perturbation from air flowing over the cloud top, or air flowing horizontally around the elevated portion of the cloud top and anvil. The V feature still appears to be supported by the combined radiative transfer-kinematical model by Heymsfield et al. Calculations with a horizontal two-dimensional kinematic model are discussed to provide insight on anvil sizes and orientations. The observed and modeled anvil orientations were found to be parallel to the storm relative winds at upper-levels. The modeled anvil width was found to be related to the vertical mass flux in the updrafts, the anvil thickness, and the anvil-level relative wind speed.