Heat and Moisture Budgets and Circulation Characteristics of a Frontal Squall Line

Abstract

Heat and moisture budgets and mesoscale circulation features for the developing, mature, and dissipating stages of an intense frontal squall line that occurred in the central United States are investigated. The slow propagating behavior of the squall line made the dataset unique since observations covered a large fraction of the squall line life cycle. Budgets have been performed at six different times at intervals of 90 minutes using 1985 OK PRE-STORM rawinsonde data. The squall line was followed by a low-level cold front. The flow pattern normal to the squall line was generally similar to previous squall line studies except that a low-level rear inflow associated with the cold front was superimposed upon expected squall line FTR/RTF (front to rear/rear to front) flows. The midlevel RTF flow was quite weak well behind the squall line during the developing and mature stages and significantly strengthened during the dissipating stage as the stratiform region developed, suggesting that internal processes within the expanding stratiform region played an important role in RTF flow development. A convergence band resulting from system RTF and FTR flows extended upward and rearward from low levels near the leading edge of the system. During the developing and mature stages, peak convergence was located at low levels around the leading edge. At the dissipating stage, midlevel convergence behind the convective region intensified as the stratiform region developed, while low-level convergence near the leading edge gradually weakened. Both the apparent heat source Q1, and apparent moisture sink Q2 showed an increasing upshear tilt when the stratiform region developed, as did the vertical velocity field. The system-averaged heating peak Q1 was located at middle levels between 500 and 550 hPa throughout the evolution. The moisture sink Q2 exhibited a single drying peak, which resulted from the convective region, at low levels around 700 hPa through most of the developing and early mature stages. During the late mature and dissipating stages, a double-peak structure in Q2 become very pronounced. The coexistence of convective and stratiform drying appears to be the causal mechanism for the double peak in Q2 at these stages. At later stages, a single drying peak resulting from the stratiform region was present at middle levels around 475 pHa.