Large scale and mesoscale systems evolution during the first wet season campaign of LBA in Amazonia

The WETAMC/LBA field phase in SW Amazonia was conducted in January/February 1999 jointly with the TRMM/LBA field campaign. The campaign setup was the most extensive ever carried in this tropical region and easily a few results emerged as quite interesting to the comunity involved. The term "maritime continent" probably introduced by Dr. Earle Williams was quickly adopted within the whole scientific community in Rondonia . The reason for this quick acceptance is that several observations during early January 1999 fit into this brief description. The beginning of the observation period in January was dominated by widespread rainfall associated to a large scale forcing due to the South Atlantic Convergence Zone episode that lasted for about a week. The TOGA radar screened the associated clouds and measured cloud tops reaching 7-8 km, well above the 0 oC isotherm, but much below the tropopause (at about 17 km). At the same time, CCN counts indicated a very clean atmosphere, at a few tens of CCN.m-3. Disdrometers showed rainfall contained very large drops. These features are typical of maritime clouds. As the experiment progressed, the SACZ decayed and we had almost a month of rainfall dominated by local convective systems and by travelling mesoscale convective systems (MCS). CCN counts increased at that time. The S-POL, the radar profiler, from the ground and at the TRMM satellite and aboard the ER-2 aircraft together discriminated precipitation species, confirmed by in situ measurements with the Citation aircraft mycrophysical measurements in the trailing stratriform region of the MCS. These information identified very large snow particles above the melting layer, with small terminal velocities. Just below the melting layer, large terminal speeds were observed. The Citation aircraft encountered large vertical speeds of updrafts and downdrafts in seemingly not so threatening Cumulus Congestus and Cumulonimbus. The large scale situation evolved including a westward intrusion of an upper level trough that inhibited Amazonian rainfall in a large area for a day or two. The MCS in the form of long squall lived, sometimes originating in the northern coast, progressed throughout the region with some influence in the organization of clouds observed in Rondonia. The growth of MCS from initially small isolated Cb was observed several times. The surface outflow observed through the visible GOES images indicated the very effective auto sustainability of the convective system sometimes leading to long lived MCS.