The structure of the atmospheric boundary layer coupled with the heterogeneity of the surface during the SAMBBA Experiment held in Amazonia

The South AMerican Biomass Burning Analysis (SAMBBA) project is an international partnership between British (UK Met Office and 7 Universities) and Brazil (INPE and University of São Paulo) which has the goals to investigate the properties of biomass burning pollution over South America and its interaction with the Amazonian biosphere, weather and climate. The field campaign of SAMBBA was held during September/October 2012 in Amazonia and a set of equipment were deployed, specially an aircraft from UK Met Office (the UK Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft). This time of the year is the end of the dry season in the Amazon. During the flights, dropsondes (RD94 from Vaisala Oy, Finland) were released in order to get the basic meteorological parameters (temperature, humidity and winds – windspeed and direction). At the ground, rawinsoundings (RS92-SVP from Vaisala Oy, Finland) were launched at the synoptic times (00:00 and 12:00 UTC – corresponding to an early morning sounding at 08 Local Time).

The objective of this work is to analyze the time evolution of the properties of the atmospheric boundary layer during a specific flight (Sept 26, 2012), in which the aircraft flown over different types of vegetation (heterogeneity), which can trigger mesoscale processes. From the take-off (13:00 UTC, Porto Velho (8°S, 64°W – in the southwest of Amazonia)) up to the landing time (16:30 UTC, Palmas (10° S, 49°W – in the southeast of Amazonia and at the central of South America)), seven dropsondes (13:17, 13:57, 14:20, 14:44, 15:07, 15:33, 15:48 UTC) were released, providing information about the structure of the mid-lower troposphere (mostly within the convective boundary layer - CBL). This behaviour is associated with the landscape which varies from tropical forest (Porto Velho area) to savannah biome (at Palmas). Infrared satellite imagery (provided by INPE and NOAA) showed the presence of convective clouds in the area of the flight. Mixing ratio and potential temperature profiles were used to characterize the thickness of the mixed layer, its structure and time evolution.

The main results show the remarkable evolution of the boundary layer along the way. From the departure point, the CBL is very wet (specific humidity around 15-16 g/kg) and cold (mean potential temperature around 300 K), presenting a height of 500-700 m. Moving to the landing point, the CBL heated up (due to the solar heating but also due to change of surface vegetation and consenquently to the sensible heat fluxes) to values of 305-306 K. The CBL is dry (specific humidities around 13 g/kg) and its height is deeper (around 1000-1200 m). During all the dropsonde releases, the winds showed a noticeable clockwise turn (veer) with height, from southeast (surface) to north-northwest (above the top of the convective boundary layer). Therefore, this work showed, by means of dropsondes and aircraft measurements, the influence and coupling of the surface land use heterogeneity with the boundary layer.