Evaluation of Microphysical Parameterizations for Tropical Cyclone Prediction
The impacts of cloud microphysical processes on prediction of tropical cyclone environments, track, intensity, and structure are examined for two microphysical parameterizations using the Coupled Ocean / Atmosphere Mesoscale Prediction System -Tropical Cyclone model. The control microphysical parameterization is a relatively typical single-moment scheme with five hydrometeor species: cloud water and ice, rain, snow, and graupel. An alternative newer method uses a hybrid approach of double-moment in cloud ice and rain and single moment in the other three species. Basin-scale synoptic flow simulations point to important differences between these two schemes. The upper level cloud ice concentrations produced by the control scheme are up to two orders of magnitude greater than by the newer scheme, primarily due to differing assumptions concerning the ice nuclei concentration. Significant (1-2C) warm biases near the 300-hPa level in the control experiments were not present using the newer scheme. The warm bias in the control simulations is associated with the longwave radiative heating near the base of the cloud ice layer.
The two schemes produced different track and intensity forecasts for 15 Atlantic storms. Rightward cross-track bias and positive intensity bias in the control forecasts are significantly reduced using the newer scheme.
Synthetic satellite imagery of Hurricane Igor (2010) shows more realistic brightness temperatures from the simulations using the newer scheme, in which the inner core structure is clearly discernible. Applying the synthetic satellite imagery in both quantitative and qualitative analyses helped to pinpoint the issue of excessive upper-level cloud ice in the control scheme.