Structure and Evolution of the 22 February 1993 TOGA COARE Squall Line: Aircraft Observations of Precipitation, Circulation, and Surface Energy Fluxes

Abstract

This study documents the precipitation and kinematic structure of a mature, eastward propagating, oceanic squall line system observed by instrumented aircraft during the Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE). Doppler radar and low-level in situ observations are used to show the evolution of the convection from an initially linear NNW?SSE-oriented convective line to a highly bow-shaped structure with an embedded low- to midlevel counterclockwise rotating vortex on its northern flank. In addition to previously documented features of squall lines such as highly upshear-tilted convection on its leading edge, a channel of strong front-to-rear flow that ascended with height over a ?rear-inflow? that descended toward the convective line, and a pronounced low-level cold pool apparently fed from convective and mesoscale downdrafts from the convective line; rearward, the observations of this system showed distinct multiple maxima in updraft strength with height and reflectivity bands extending rearward transverse to the principal convective line. Vertical motions within the active convective region of the squall line system were determined using a new approach that utilized near-simultaneous observations by the Doppler radars on two aircraft with up to four Doppler radial velocity estimates at echo top. Echo-top vertical motion can then be derived directly, which obviates the traditional dual-Doppler assumption of no vertical velocity at the top boundary and results in a more accurate estimate of tropospheric vertical velocity through downward integration of horizontal divergence. Low-level flight-level observations of temperature, wind speed, and dew point collected rearward of the squall line are used to estimate bulk fluxes of dry and moist static energy. The strong near-surface fluxes, due to the warm sea and high winds, combined with estimates of mesoscale advection, are used to estimate boundary layer recovery time; they indicate that the boundary layer could recover from the effects of the cold dome within about 3 h of first cold air injection if the observed near-surface winds were maintained. However, the injection and spreading of air from above leads to cooling at a fixed spot ?20 km rearward of the convective line (surface ?e minimum point), suggesting that the cold pool could be still intensifying at the time of observation. Recovery time at a point is probably similar to that measured in previous studies.