Dynamic and thermodynamic structure of the lower troposphere above rain forest and wet savanna during the EXPRESSO campaign

Abstract

This paper focuses on the energy budget of atmospheric surface and boundary layers over the wet savanna and the rain forest during the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). It assesses dynamic features at the transition between savanna and forest at the beginning of the dry season. The experimental zone was still influenced by the monsoon flow with water vapor mixing ratio of about 16 g/kg. Thermodynamic characteristics of the savanna and forest ecosystems are similar. However, for an equivalent net radiation, evaporation is greater over the forest and leads to lower Bowen ratios (0.2 over the forest, 0.45 over the savanna). Low horizontal winds with no mean direction are observed, due to the proximity of the Intertropical Convergence Zone (ITCZ). However, in spite of low horizontal winds, the momentum flux is high, owing to high drag coefficients and roughness lengths. Some differences appear in the turbulent kinetic energy budget: dynamic and thermal production terms over the forest are nearly half those over the savanna. As a consequence, turbulent exchanges are more efficient over the savanna. Entrainment velocities are deduced from the latent heat flux values at the top of the atmospheric boundary layer (ABL) and from the jump in water vapor mixing ratio between the ABL and the free troposphere. Entrainment velocities are of the same order as subsidence velocities deduced from European Centre Medium‐Range Weather Forecasts statistical analyses. High pollution levels are observed in the forest ABL (CO concentration is around 280 ppb). Pollutants, produced by savanna fires, are transported from the free troposphere into the forest ABL by entrainment. Furthermore, horizontal exchanges between the savanna and the forest, induced by large eddies, may occur, increasing the pollution of the forest ABL. Through this study, the ITCZ appears to be an exchange zone rather than a barrier to the transfer of pollutants.