Sensitivity of the global circulation to the suppression of precipitation by anthropogenic aerosols

From recent satellite observations, it is evident that an increase in cloud condensation nuclei-for instance, due to biomass burning-can substantially reduce rain efficiency of convective clouds. This is potentially important for the global climate since the release of latent heat due to condensation of water vapour and fallout of rain from cumulus convection is the most important source for available potential energy in the free troposphere. Beyond this, cumulus convection is a key process in controlling the water vapour content of the atmosphere. The sensitivity of the global climate to modification in rain efficiency of convective clouds due to the suppression of drop coalescence by anthropogenic aerosols is studied by using the atmospheric general circulation model (A-GCM), ECHAM4. This paper presents results from a 15-year sensitivity study, which considers the aerosol effect on warm precipitation formation. Effects on ice processes are not included yet, and therefore the results likely underestimate the magnitude of the full effects due to suppression of precipitation. Instantaneous forcing is large locally (up to 100% reduction of precipitation and the related latent heat release) but confined to small areas, leading to small large-scale mean anomalies in the convective heating and therefore the vertical temperature gradient. We found a definite perturbation of the global circulation, showing distinct sensitivity to the impact of aerosols on suppressing rainfall. (C) 2003 Elsevier Science B.V. All rights reserved

A new convective cloud field model based on principles of self-organisation

A new cumulus convection parameterisation is presented in this paper. The parameterisation uses an explicit spectral approach and determines, unlike other convection schemes, for each convection event a new cloud distribution function regarding to the given vertical temperature and humidity profiles. This is done by using a one dimensional cloud model to create a spectrum of different clouds. The interaction between all non convective physical processes in the AGCM and all different clouds is taken into account to calculate a selfconsistent cloud spectrum. The model has been implemented in the ECHAM5 AGCM and tested against a large eddy simulation model. The representation of a shallow cumulus cloud field by the AGCM could be much improved. Diurnal cycle, cloud cover, liquid water path and the vertical structure of the mass flux, determined by the new convection scheme are close to the large eddy simulation, whereas the standard convection scheme failed in simulating this convection episode