Retrieved Thermodynamic Structure of a Subtropical, Orographically Influenced, Quasi-Stationary Convective Line

Abstract

Airborne Doppler radar and flight-level measurements were used to document the three-dimensional thermodynamic structure of a convective line that developed off the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. During the period of aircraft observation the convective line appeared to be quasi-stationary in a region ?90 km away from the coast. Calculations of perturbation pressure and buoyancy retrieved from the Doppler-radar-derived wind fields show that a high pressure perturbation associated with negative buoyancy existed in the region of heavy precipitation in the lowest level. In low levels, an elongated zone of low pressure was documented along the line, which was closely related to the distributions of buoyancy. These retrieved features are basically consistent with those calculated from in situ observations. In mid- to upper levels, the high (low) pressure perturbation lay around the updraft cores on the upshear (downshear) of environmental shear. The quantitative diagnosis for the perturbation pressure indicates that the positive buoyancy associated with the in-cloud latent heat release as well as evaporative cooling occurring near cloud base and subcloud regions is crucial to determine the low-level perturbation pressure distributions. However, dynamic forcing contributing to the pressure perturbation cannot be ignored in middle levels, because of updraft?shear interaction due to the presence of pronounced cross-line wind shear and corresponding updrafts reaching maximum values near midtroposphere. In distinct contrast to squall lines, which are rapidly moving, the near-surface temperature gradient across this quasi-stationary line was very weak, and at low levels no gust front, rear inflow, or dynamically induced high pressure was evident near the leading edge of the line. Analyses presented here suggest that the persistent, stationary convergence, in association with the deceleration of low-level southeasterly flow as it approached the nearshore blocked flow, played an important role in maintaining and organizing the deep convection in this case.