Amazonian Boundary Layer: Revisiting its Characteristics Using New Measurements and Modeling Outputs

The Amazon region is well known as a source of convection and it has a strong influence on the world climate. Although this region is mainly covered by pristine tropical forest, there is a high rate of deforestation for cattle activities and crop production. For a microscale perspective, the presence of a clearing of different size/dimensions can alter the energy budget at the surface producing a low level convergence and an increase of upward movement, thus originating clouds and rainfall. The boundary layer (BL) processes is the link between the surface and the base of the clouds and its characteristics and growth are very important to understand. Nevertheless, the actual knowledge about the boundary layer is mostly associated with the use of rawinsoundings and/or tethered balloon. The CHUVA Project / Go Amazon 2014/15 field campaigns (during the wet (feb-mar IOP1) and dry seasons (sept-oct IOP2) provide new and original data set as remote sensing (ceilometer, minisodar, windprofiler, microwave radiometer), aircraft measurements (US DOE G1 airplane twin propeller) as well routine rawinsounding data (6 launchings per day) for a heterogeneity site (mixture of tropical forest and agricultural pasture) at central Amazonia. This unique observational data has been analyzed and provide new information about some of the characteristics of the BL. For instance, the entrainment fluxes were measured and computed from aircraft data (25 flights between 11 and 14 local time) at the top of the BL and, normalized by its surface sensible heat fluxes, results in a value close to the one find in the literature (range -0.1 up to -0.2). The erosion of the NBL by ceilometer, minisodar and microwave radiometer was studied using profiles each 10 min. The onset of the NBL occurs before the sunset (18 LT) and its height is very stable during the whole night (typical values around 180-230 m). The erosion of the NBL lasts for 3.5 hours for IOP1 and it is fully eroded at 08 LT for the dry season. Finally, a LES model was run with these dataset for both wet and dry periods. The TKE budget was computed and the thermal production term proved to be the most important as it was expected for a tropical Amazonian site. The mechanical production is very weak due to the low winds and shears. Physical processes at surface (turbulent transport) produced up to 3 times more mixing in the lower extension of the BL (below 40% of the BL depth) than the upper part. This paper is a contribution of the Brazilian National Institute of Science and Technology (INCT) for Climate Change funded by CNPq Grant Number 573797/2008-0 e FAPESP Grant Number 2008/57719-9.