A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

CHARACTERISTICS OF THE CONVECTIVE CLOUD SYSTEM ORGANIZATIONDURING WETAMC/LBA – COMPARISON WITHWEST AFRICAN CONVECTIVE SYSTEMSHenri Laurent and Luiz Augusto T. MachadoCentro Técnico Aeroespacial–Instituto de Aeronáutica e Espaço–Divisão de Ciências AtmosféricasSão José dos Campos/SP - 12228-904 , BrasilandIRD, LTHE, Grenoble, Francehlaurent@iae.cta.br - machado@iae.cta.brTropical convection tends to cluster into mesoscale convective systems that are responsible for themain vertical exchanges of energy in the tropical troposphere, and that account for most of the totalrainfall. An automatic method, based in infrared images from geostationary satellites, has been usedfor tracking cloud clusters during their life cycle. This objective tracking has been applied overAmazonia during the WETAMC/LBA (Wet season Atmospheric Mesoscale Campaign), in January-February 1999. The results allow for analyzing the MCS organization and propagation and tocompare them to the rain cells observed from meteorological surface radar. The convective activityshowed two different patterns named Easterly and Westerly regimes according to the wind flow inthe middle-to-low troposphere. During Easterly regime, MCS and rain cells have closely relatedpropagations, mostly associated to the mean flow at 700 hPa. During Westerly regime, thepropagations of both MCS and rain cells are much more disorganized, there is no clear relationshipwith the mean atmospheric flow at any level, and the low-level rain cells propagate quiteindependently from the high-level cloud cover.The same tracking methodology was originally employed over West Africa during the rainy season;therefore a limited comparison between these two continental tropical regions can be addressed.The convective systems are mostly driven by the diurnal cycle in the Southwestern Amazon duringthe rainy season, in contrast to the Sahel where squall lines and other long-lived MCS are the mostimportant convective systems. Another finding is that the behaviors of Amazonian and Sahelianconvective systems are quite similar during the Easterly regime, whereas they are very differentduring the Westerly regime. This is consistent with the presence of a mid-level “jet” favouring awind shear essential for the organization of the convection.