Case study of multiple squall lines during the WETAMC/LBA and TRMM/Brazil: 7 Feb 1999

The large scale situation over SW Amazon Basin, specifically over the state of Rondonia, in the early morning of 7 February 1999 indicated very weak forcing for large scale uplift. The winds in the low levels were from the NW responding to a mid latitude frontal system approaching from Southern Brazil, Paraguay and Southern Bolivia. Low level convergence was found in the central part of the Amazon Basin colocated with upper level difluence. Rondonia was at the edge of this pattern close to the no divergence line. A previous day Mesoscale Convective System (MCS) located in SW Rondonia decayed in the early morning hours. Around 13 UTC (8 local time) isolated convection started in the northern part of the area in response to the diurnal heating. A MCS located at 8N 65W that was developing since 9 UTC, reached maturity and its outflow organized a convective band to the south. This band moves towards SW at about 30 km. hr-1 and the NCAR S-Pol radar measures 50 dbz in some of the leading edge cells with cloud tops at 15 km. By 18 UTC other bands originated from isolated cells initiated around 14 UTC in Mato Grosso start entering the Rondonia region from the NE with an orientation NW-SE. Cloud tops reach 10 - 15 km, propagation speeds are of the order of 15-20 km.hr-1 and reflectivities in the leading edge are still about 50 dBz, convergence being found from the surface up to 8 km. Around 22 UTC convection in the line start to decay. The interest in this case lies in the fact that no large scale forcing is evident, convection starts randomly in response to diurnal heating. The MCS start as small convective cells that quickly grow and get organized in several convective bands that are about 300-400 km long travelling at low speeds and with a life time of several hours. The testing of convective parameterizations has an ideal case in this event. A run of the Regional Mesoscale Modelling System - RAMS shows that with 20 km resolution the MCS is started but dissipates quickly. Higher resolution (5 km) is needed and cloud microphysics necessary for the production of a longer lived MCS. The production of a several bands is also a problem since in nature the recovery from the stabilizing effect of the convective line takes place more quickly than in the numerical model. Surface data indicate a recovery time in this case of 30-45 minutes.