Further Analysis of the Composite Wind and Thermodynamic Structure of the 12 September GATE Squall Line

Abstract

An objective analysis technique is applied to the time-composite wind and thermodynamic fields of the 12 September GATE tropical squall line. Previous subjective analyses described by Gamache and Houze are confirmed and several new results are obtained. In the previous analyses, mesoscale upward motion was found in the upper troposphere of the stratiform precipitation region immediately trailing the squall line. Mesoscale downward motion was found in the lower troposphere of the stratiform region. The convective clouds were found to be the source of condensate for more than half of the stratiform precipitation, but mesoscale-updraft condensation was also found to be substantial. In these previous studies, thermodynamic structure was not analyzed, the wind analyses were limited by the number of levels included and vorticity was not analyzed. By employing an objective analysis method in the present study, we have refined and extended the previous work by including more levels, computing vorticity and analyzing the thermodynamic fields. In the stratiform region, the level of zero vertical motion separating the mesoscale updraft in the upper troposphere from the mesoscale downdraft below is found to be at the 520 mb level (a higher altitude than was indicated by the previous subjective analyses). Maximum convergence in the stratiform region occurred near this level (at 500 mb), but maximum positive vorticity is found to have been at a somewhat lower altitude (650 mb). The thermodynamic structure of the mesoscale updraft in the stratiform region is indicated by the objective analysis to have been more complex than previously estimated. In its central layer the mesoscale updraft contained a warm anomaly with a humidity that was saturated with respect to ice. Cool anomalies are indicated to have existed near the top of the stratiform cloud deck and (possibly) at the base of the mesoscale updraft. The structure of the squall system was apparently strongly affected by interaction with the wake of an earlier squall line and with a convective line existing immediately ahead of the squall and intersecting it at nearly right angles. The portion of the squall line feeding on the stabilized wake air associated with these two convective lines was characterized by systematically lower cell tops, as determined by radar, than the remainder of the line. The portion of the stratiform region trailing this part of the line exhibited a distinctly different thermodynamic stratification than was observed to the rear of the deeper-cell section of the squall line. This difference is attributed to the lower altitudes at which condensate and water vapor were determined from this portion of the line are inferred to have advected into the stratiform region.