Radon-222 as a test of convective transport in a general circulation model

Abstract

The distribution of ²²²Rn over North America is simulated with a 3-d chemical tracer model (CTM) based on the meteorology of the GISS general circulation model (GCM). The GISS GCM has been used extensively for studies of climate change and global transport of chemical tracers. Simulation of ²²²Rn (e-folding lifetime 5.5 days) tests the ability of the model to describe the transport of pollutants in the boundary layer and the exchange of mass between the boundary layer and the free troposphere. Model results are compared to surface observations from 5 sites in the United States. It is found that the ²²²Rn concentrations are regulated primarily by dry convection. At night, the model underpredicts observations because it does not resolve the sharp ²²²Rn concentration gradient which forms near the surface. In daytime, the predicted and observed concentrations are usually in good agreement, indicating that vertical mixing of surface air is reasonably simulated. Inspection of seasonal trends reveals, however, several significant discrepancies which are traced to anomalies in the GCM meteorology. In particular, the simulated ²²²Rn concentrations over the northeastern United States are too high in the spring, because of excessive rainfall which suppresses dry convection, and too low in the fall, because of a severe drought which allows intense dry convection. Ventilation of ²²²Rn to the free troposphere is most efficient in the western half of the North American continent, due to intense dry convection, and is followed by rapid eastward advection of ²²²Rn in the upper westerlies. This transport mechanism produces a layer of high ²²²Rn concentrations in the upper troposphere over the eastern United States and over the western Atlantic Ocean in summer.