Wednesday, 17 January 2007
Numerical simulations of the electrification and microphysics of the 22nd February 1993 TOGA COARE tropical squall line case
Exhibit Hall C (Henry B. Gonzalez Convention Center)
In the recent years, little attention has been given to the microphysical and electrical properties of mature squall lines, particularly in the tropics. Indeed, the last comprehensive study of such events was carried out about 10 years ago. In February 1993, several oceanic tropical squall lines were well sampled by the TOGA COARE field program. One of these, namely the 22nd February 1993 maritime tropical squall line case, was particularly well studied and simulated by Trier et al (1996). Their simulations and observations, however, did not include lightning. In the current study, we reproduced the same squall line case using a finer horizontal and vertical grid spacing along with a far more sophisticated microphysical package (10-ICE instead of 3-ICE) and a 3-D lightning module. We will also include the effect of the Coriolis force and evaluate how the latter affects the final result. The main goal of this study was to determine if one could get a better insight on the simulated microphysical fields and particularly electrical properties of these systems. Particular focus will be directed towards the reflectivity profiles evolution, the depth of the convection, the wind profiles and the precipitation structure and intensity. Also we will determine how the latter parameters are related to the simulated intra-cloud and cloud-to-ground flash rate. The simulated lightning will be compared with a more recent squall line observational study by Petersen et al (1999). Overall, the simulation results were consistent with the Trier et al. (1996) observations and simulations: the updraft speeds across the line seldom exceeded 8 m s-1, consistent with relatively shallow 30 dBZ echo tops which rarely exceeded the top of the mixed phase layer (-20°C isotherm). Similar to earlier studies, warm rain processes were found to be particularly efficient ahead of the line. This accounted for an overall small amount of supercooled water and graupel pellets above the melting level. Consistent with this, the total lightning activity across the line was generally weak. Most of the charges present in the line were generated within some storm cells in the transition zone (between the stratiform region at the rear of the line and the leading edge of the gust front) which had sufficiently strong updraft speeds near the melting level to carry graupel in the mixed phase region. The cells exhibiting the largest total flash rate were characterized by deeper 30 dBZ echo tops reaching the -10°C level. As these cells aged and became largely glaciated, they formed a trailing stratiform region. The majority of the charges there were acquired while the storm cell was in the transition zone. An additional set of four simulations was carried out with grid spacings ranging from 300 m to 2 km, and showed some notable differences in the overall squall line behavior, despite the fact that the basic flow structure, reflectivity profile and lightning characteristics remained identical in all four cases.