Representation of transport and scavenging of trace particlesin the Emanuel moist convection scheme

摘要

In the Tropics, cumulus convection has a major influence on precipitation and vertical transport of atmospheric particles, which are subject to scavenging by precipitation. A new parametrization of transport and scavenging of trace particles by convective clouds and precipitation has been developed and introduced in the Laboratoire de Meteorologie Dynamique general circulation model (LMDz). This model uses the deep convection scheme of Emanuel, which is particularly suited for the Tropics. Our parametrization of transport and scavenging is closely linked to this scheme and our developments follow step-by-step the building of this convection representation. The purpose of this study is to understand better the influence of convection on the tracer vertical distribution and to assess the role of the convection parametrization. Short-term and long-term simulations have been performed focusing on the concentrations of the natural radionuclide Be-7, which is produced mainly in the stratosphere and upper troposphere and attaches to available aerosols. Single-column simulations forced by data from the Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) show the high efficiency of in-cloud scavenging by convective and large-scale processes in the removal of the tracer. These simulations show that, in the LMDz model, convection does not affect radionuclide concentrations as much as stratiform clouds and associated precipitation. In the free troposphere and in the boundary layer, below-cloud evaporation of rain has a major effect on tracer distribution, unlike impaction, which has a negligible effect. Three-dimensional model simulation results are compared with surface data of a station belonging to the worldwide network of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). We show that this new parametrization is able to reproduce the observed yearly averaged concentrations of Be-7 at the surface and decrease by a third the overestimation of radionuclides formerly simulated without convective scavenging. LMDz simulations have been also performed over the year 2007 on a global scale using the terragenic Pb-210 and cosmogenic Be-7 radionuclides.