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Abstract
A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This line is analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of the cells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes. Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms; 2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) mass and moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropical and Midwest squall line cases. The 2 May case does not exhibit a “trailing stratiform” anvil during the period mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.
The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergence line found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shear vector, which is directed ∼45° clockwise from the line orientation; then the cells turn to the right (nearly normal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1) mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.
Triple Doppler analysis of a cell along the line indicates maximum updrafts of ∼35 m s−1, and strongest downdrafts at middle to upper levels located between cells along the line. The structure of the squall line is somewhat different from that in the case presented by Newton and other documented squall line studies in that there are not well-organized downdrafts on the rear side at low to midlevels. In addition, low-to midlevel inflow on the rear side of the squall line is apparently absent.
Man and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previous squall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 25–40%, which is somewhat lower than other reported values (>50%). The low precipitation efficiency is suggested to be due in part to large moisture losses at upper levels.
Abstract
A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This line is analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of the cells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes. Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms; 2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) mass and moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropical and Midwest squall line cases. The 2 May case does not exhibit a “trailing stratiform” anvil during the period mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.
The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergence line found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shear vector, which is directed ∼45° clockwise from the line orientation; then the cells turn to the right (nearly normal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1) mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.
Triple Doppler analysis of a cell along the line indicates maximum updrafts of ∼35 m s−1, and strongest downdrafts at middle to upper levels located between cells along the line. The structure of the squall line is somewhat different from that in the case presented by Newton and other documented squall line studies in that there are not well-organized downdrafts on the rear side at low to midlevels. In addition, low-to midlevel inflow on the rear side of the squall line is apparently absent.
Man and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previous squall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 25–40%, which is somewhat lower than other reported values (>50%). The low precipitation efficiency is suggested to be due in part to large moisture losses at upper levels.
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 (TBB ) 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 TBB with embedded higher temperature (warm areas) developed after tropopause penetration by the Lahoma storm. Composites of stereo height contours on IR images indicated that TBB 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 TBB 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
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 (TBB ) 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 TBB with embedded higher temperature (warm areas) developed after tropopause penetration by the Lahoma storm. Composites of stereo height contours on IR images indicated that TBB 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 TBB 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 (TBB ) and an internal region of high TBB . 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 TBB 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 TBB 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 TBB 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.
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 (TBB ) and an internal region of high TBB . 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 TBB 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 TBB 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 TBB 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.
