A COMPARISON OF THE CCM3 MODEL CLIMATE USING DIAGNOSED AND PREDICTED CONDENSATE PARAMETERIZATIONS

A parameterization is introduced for the prediction of cloud water in the National Center for Atmospheric Research Community Climate Model v ersion 3 (CCM3). The new parameterization makes a much closer connecti on between the meteorological processes that determine condensate form ation and the condensate amount. The parameterization removes some con straints from the simulation by allowing a substantially wider range o f variation in condensate amount than in the standard CCM3 and tying t he condensate amount to local physical processes. The parameterization also allows cloud drops to form prior to the onset of grid-box satura tion and can require a significant length of time to convert condensat e to a precipitable form, or to remove the condensate. The free parame ters of the scheme were adjusted to provide reasonable agreement with top of atmosphere and surface fluxes of energy. The parameterization w as evaluated by a comparison with satellite and in situ measures of li quid and ice cloud amounts. The effect of the parameterization on the model simulation was then examined by comparing long model simulations to a similar run with the standard CCM and through comparison with cl imatologies based upon meteorological observations. Global ice and liq uid water burdens are higher in the revised model :han in the control simulation, with an accompanying increase in height of the center of m ass of cloud water. Zonal averages of cloud water contents were 20%-50 % lower near the surface and much higher above. The range of variation of cloud water contents is much broader in the new parameterization b ut was still not as large as measurements suggest. Differences in the simulation were generally small. The largest significant changes found to the simulation were seen in polar regions (winter in the Arctic an d all seasons in the Antarctic). The new parameterization significantl y changes the Northern Hemisphere winter distribution of cloud water a nd improves the simulation of temperature and cloud amount there. Smal l changes were introduced in the cloud fraction to improve consistency of the meteorological parameterizations and to attempt to alleviate p roblems in the model tin particular, in the marine stratocumulus regim e). The small changes did no; make any appreciable improvement to the model simulation. The new parameterization adds significantly to the f lexibility in the model and the scope of problems that can be addresse d. Such a scheme is needed for a reasonable treatment of scavenging of atmospheric trace constituents, and cloud aqueous or surface chemistr y. The addition of a more realistic condensate parameterization provid es opportunities for a closer connection between radiative properties of the clouds, and their formation and dissipation. These processes mu st be treated for many problems of interest today (e.g., anthropogenic aerosol-climate interactions).