Surface pressure features and precipitation structure of PRE-STORM mesoscale convective systems

Fall 1992.Also issued as author's thesis (M.S.) -- Colorado State University, 1992.Includes bibliographical references.The surface pressure features accompanying 16 mesoscale convective systems (MCSs) have been documented using data from the May-June 1985 Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM). The general synoptic-scale environmental conditions as well as the detailed mesoscale aspects of the systems are examined. Radar data are examined to show the reflectivity structure of each MCS. Also, the upper-air data are examined to show the system-relative airflow structure associated with these systems. The general synoptic-scale conditions were very similar to those shown by Maddox (1983) found in conjunction with the genesis region of mesoscale convective complexes (MCCs). There was a generally weak surface front , most often quasi-stationary in nature , and low-level warm advection by a low-level southerly jet which also advected in very moist air. Also, a weak 500 mb short wave feature was often found in conjunction with these systems. In the mature-to-dissipating stages of 12 of the 16 cases. the radar reflectivity and surface pressure structures were found to be very similar. Composite depictions of the pressure features are developed based on these similarities. At some time during the mature-to-dissipating stage for each case, the radar reflectivity structure became asymmetric (Houze et al. 1990) in nature with a leading bow-shaped convective line with a region of enhanced stratiform precipitation found to the rear of the far northern portion of the convective line. This structure is quite similar to that shown by Pedgley (1962) for MCS cases in England. Two of the four other MCSs that did not develop these particular structures were cold-frontal systems positioned directly along a cold front. Atmospheric conditions in these regions did not allow the development of these common structures. One system was too short-lived to have developed into the common structure of the 12 systems. There were four paths that the 12 systems took to the development of this asymmetric structure. First, there were systems which initially had a disorganized pattern of convection, but towards the end of their existence developed a small convective line on their southern end. Any convection on the northern end became stratiform, leading to the asymmetric structure (2 cases). Second, there were convective lines which were initially symmetric in nature but slowly developed a region of enhanced stratiform precipitation on their northern ends (4 cases). Third, there were cases where a back-building convective line led to a region at the southern end of the system lacking stratiform precipitation (3 cases). Finally, there were cases consisting initially of intersecting convective lines, one oriented east-west and the other oriented northeast-southwest extending to its south. An enhanced stratiform area developed to the northwest of the apex of the two lines and then the east-west line dissipated (3 cases). The surface pressure structure shows a fairly weak pre-squall low ahead of the convective line. A mesohigh was typically associated with the convective line as well as much of the stratiform precipitation region. A wake low was found at the back edge of the enhanced stratiform precipitation region. An intense pressure gradient was typically found along the back edge of and extending into the enhanced stratiform precipitation region. For the cases containing special soundings, the system-relative upper-air flow structure showed a rapidly descending, strong rear inflow jet in the region of the wake low. In summary, a remarkable and unexpected result from this study is that although MCSs observed over the mesonetwork during PRE-STORM originated in a variety of ways, they tended to develop a common cloud and precipitation structure during their mature-to-dissipating stages, a pattern characterized here by the term "asymmetric" after Houze et al. (1990). This repeatable structure also had a repeatable surface pressure pattern associated with it (as described above). These findings have led to the development of an updated model for the surface pressure pattern accompanying squall lines, one containing both symmetric and asymmetric structures. This model is consistent with but extends past models based on a small number of case (Fujita 1955, Pedgley 962, Johnson and Hamilton 1988)