Tests and improvements of GCM cloud parameterizations using the CCCMA SCM with the SHEBA data set

A GCM cloud microphysics parameterization is tested and improved using the CCCMA single-column model with cloud properties obtained at the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) during the period of November 1997 to September 1998. The ECMWF reanalysis water vapor profile is scaled with rawinsonde data so that the new relative humidity profiles are compatible with rawinsonde data for nudging purposes. This study demonstrates that the treatment of ice nucleation number concentration is the controlling factor of the overestimation of monthly mean ice water path originally produced by this model. The parameterizations of accretion processes are modified to consider the accumulation due to an increase of precipitation flux through a model layer related to accretion processes. The horizontal inhomogeneity effect of cloud liquid water is considered in parameterization of autoconversion process. A new method developed for mixed-phase clouds to determine the water vapor saturation and partitioning of the condensed water into different phases is also tested in this model.When using a nudging technique with the adjusted ECMWF water vapor profile the model can well simulate the monthly total cloud cover and daily precipitation rate for the SHEBA period. Using the modified cloud microphysics parameterizations including improved treatments for accretion processes, ice nucleation number concentration, and auto-conversion, the monthly mean cloud liquid water path and ice water path are suitably simulated and compare reasonably well to those derived from measurements.