A three-dimensional large-scale cloud model: testing the role of radiative heating and ice phase processes

Abstract

A time-dependent, three-dimensional, large-scale cloud model has been developed for the prediction of cloud cover, cloud liquid/ice water content (LWC/IWC), precipitation, specific humidity and temperature. Partial cloudiness is allowed to form when large-scale relative.humidity is less than 100%. Both liquid and ice phases are included in the model. The liquid phase processes consist of evaporation, condensation, autoconversion and precipitation. The ice phase processes include heterogeneous nucleation to generate ice crystals, depositional growth to simulate Bergeron-Findeisen's process, sublimation to deplete ice crystals, and gravitational settling of ice crystals. The radiative transfer parameterization scheme is based on a broadband method and involves the transfer of infrared and solar radiation in clear and cloudy regions. The broadband infrared emissivity, reflectivity, and transmissivity for cirrus clouds, as well as the broadband solar absorption, reflection, and transmission values for low, middle and high clouds are computed based on the cloud LWC and IWC interactively generated by the cloud model. Large amounts of satellite data, including cloud cover climatology derived from the US Air Force three-dimensional nephanalysis (3DNEPH) and earth radiation budget (ERB), have been processed into formats appropriate for verification. The 96-h model simulations of cloud cover, outgoing long-wave radiation (OLR), and cloud LWC have been verified against data analyzed from 3DNEPH, ERB and the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) satellite observations, respectively. The predicted cloud IWC is compared to in situ observations as well as to results from other studies. The predicted cloud and radiation results compare well with those analyzed from satellite data. Numerical experiments are carried out with and without radiative heating and ice phase processes in the cloud formation scheme. The inclusion of radiative heating produces a significant change in temperature, cloud cover and total cloud water content, while the inclusion of ice phase processes generates a substantial change only in total cloud water content. If the cloud LWC is sufficiently large to initiate effective Bergeron-Findeisen's processes, the total cloud water content decreases, indicating that gravitational settling is an efficient mechanism in reducing the cloud IWC.