Abstract

Storms on 20 May 1977 generated a vast cirrus deck. Disturbed areas at storm top had equivalent black-body temperatures (T _BB) much lower than the tropopause temperature, indicative of overshooting tops.

The area of T _BB ≤ −71°C represents the area of convective activity penetrating ∼2 km above the tropopause. This area was relatively large after cloud tops and radar reflectivities reached their maximum heights. It became much smaller during tornadoes when reflectivities were decreasing. T _BB was at a minimum at the time of mesocyclone formation.

The Del City storm had two periods of growth, as indicated both by reflectivities and the T _BB areas. The mesocyclone was first detected during the second less intense period of growth; the tornado occurred during decreasing reflectivities.

The maintenance of large areas of relatively low T _BB after tornado dissipation is ascribed to continued convection on the flanks of the storm and to residual updrafts in a thick anvil cloud.

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.

Abstract

Radar data collected by a network of ships, shore stations and aircraft over the eastern Pacific from mid-February to the end of June 1965, have been studied. Analyses of these radar data and concurrent TIROSIX photographs were made. The data sample included deep cyclones with extensive radar-detected precipitation, weaker cyclones with localized rainfall, cold anticyclones with extensive air mass showers, and blocking anticyclones with no precipitation. In the latter case it has been found that the appearance of the cloud cover is a good indicator of areas of anomalous radio propagation. Models have been prepared that illustrate the varying patterned association of cloud and rainfall characteristic of such synoptic situations. Such associations range from comparable cloud-precipitation areas through scattered showers where only a portion of the clouds contain precipitation, to complete absence of precipitation within large areas of low stratiform clouds or fog as in a blocking anticyclone.

Abstract

This paper discusses the observational characteristics of the upper level structure of severe tornadic storms in Oklahoma on 2 May 1979 during SESAME. The data analyzed consist of limited-scan GOES-East and West visible, infrared (11 μm), and stereo satellite data, dual-Doppler radar observations, and special storm scale soundings. The time-histories of stereo cloud top height, minimum equivalent blackbody temperature (T_BB ) and radar reflectivity are followed for three severe storms over a several hour period; two of the storms are tornadic. Cloud top IR growth rates and vertical velocities of the storms are computed and found to have maxima which fall into Adler and Fenn's severe storm classification. For one of the storms there is an interesting coupling between cloud top parameters and low-level radar echoes; the other tornadic storm showed no unique relationship. Hail damage began shortly after tropopause penetration by thee storms. Two major IR cold areas associated with the leading downwind storm (i.e., Lahoma storm), are both about 10°C lower than the minimum (tropopause) temperature in an upwind sounding. One is displaced upwind about 15 km from the visible cloud top and the inferred updraft position from radar; the other is located about 15 km to the south of the visible cloud top. A “V” pattern of lower T_BB with embedded higher temperature (warm areas) developed after tropopause penetration by the Lahoma storm. Composites of stereo height contours on IR images indicated that T_BB is not uniquely related to height.

The warm areas are deduced to be of two types: one called the “close-in” warm am is located about 10–20 km downwind of the cloud top of the Lahoma storm, and the other called the “distant” warm area is about 50–75 km downwind. The close-in warm area has a motion similar to that of the storms and appears to be dynamically linked to the leading storm. A model is proposed to explain this warm area based on mixing processes and subsidence near cloud top. The distant warm area advects with a direction similar to the 9–14 km upper level winds but with a speed 10–20 m s⁻¹ lower. This appears to be anvil cirrus material. However, the T_BB in this area are several degrees warmer the stratospheric environmental temperatures at the anvil top. Stratospheric above-anvil cirrus (Fujita) explains neither the “V” shape nor the internal warm areas. Doppler radar derived winds are presented to add insight into the development of the upper level structure of the storms.

Abstract

An analysis of the GOES measurements of a severe thunderstorm anvil on 2 May 1979 presented in Part I (Heymsfield et al.) showed a “V” shaped region of low infrared temperatures (T_BB ) and an internal region of high T_BB . Several hypotheses have been given in the literature (e.g., dynamical and above-anvil cirrus) concerning the formation of the “V” pattern. In this paper, the radiative characteristics of the cirrus are examined as a partial explanation for the IR observations. Calculations are made relevant to the radiative properties using a plane parallel radiative transfer model which shows the sensitivity of T_BB to ice water content (IWC), and an ice particle trajectory model which simulates the horizontal ice particle distribution. A variation in the horizontal distribution of IWC is postulated as an explanation for the “V” shaped area and internal warm region. The radiative model calculations support the hypothesis that the higher T_BB values in the internal warm region may result from the radiometer seeing down into the anvil layer. The ice particle trajectory model results indicate that the “V” shape can be produced by the ice particle number distribution, where a higher concentration of particles is found in the arms of the “V". Asymmetry of the “V” results from the inclusion of storm motion in the trajectory calculations. Further, the calculated anvil is oriented along the storm relative wind vector in good agreement with the observations. Based on the results of the models, a variation in horizontal distribution of IWC is postulated as a partial explanation for the “V” shaped area and internal warm region. That is, the lower T_BB values in the “V” arms are suggested to result in part from the IWC being higher there than that in the internal warm region in the top few kilometers of the anvil. The proposed mechanism may act together with other plausible mechanisms to produce the observed IR pattern. The relative importance of the proposed mechanism cannot be assessed however, given the uncertainties in the observations and models.