PARAMETERIZATION OF BULK CONDENSATION IN NUMERICAL CLOUD MODELS

The accuracy of the moist saturation adjustment scheme has been evalua ted using a three-dimensional explicit microphysical cloud model. It w as found that the error in saturation adjustment depends strongly on t he CCN concentration in the ambient atmosphere. The scheme provides ra ther accurate results in the case where a sufficiently large number of CCN (on the order of several hundred per cubic centimeter) is availab le. However, under conditions typical of marine stratocumulus cloud la yers with low CCN concentration, the error in the amounts of condensed water vapor and released latent heat may be as large as 40%-50%. A re vision of the saturation adjustment scheme is devised that employs the CCN concentration, dynamical supersaturation, and cloud water content as additional variables in the calculation of the condensation rate. The revised condensation model reduced the error in maximum updraft an d cloud water content in the climatically significant case of marine s tratocumulus cloud layers by an order of magnitude.