Forecasting tropical cyclone intensity remains a difficult task for the operational and research communities, in large part because tropical cyclone intensity is dependent on many factors, such as the magnitude and direction of vertical shear within the storm core, sea surface temperature and oceanic mixed layer depth underneath the storm, and the magnitude and distribution of latent heat release within the storm circulation. High-resolution (grid length » 1 km) numerical models have been used as a method for addressing these factors. Such high resolution obviates the need for the parameterization of deep convection, a traditional source of uncertainty in determining latent heating profiles. While convective parameterization is avoided using high resolution, the parameterization of microphysical processes such as hydrometeor production, conversion, and fallout, and their dependence on rainwater, ice and graupel distributions, assumes great importance in determining latent heating distributions and, ultimately, tropical cyclone intensity. As a result of this sensitivity, the success of numerical simulations of tropical cyclones is to some extent dependent on how these microphysical processes are parameterized in the model.

In this presentation a commonly-used microphysical parameterization scheme, the Goddard microphysics scheme, has been investigated to determine its applicability to a tropical cyclone environment. The Goddard scheme is a three-class ice scheme that contains prognostic equations for cloud water (ice), rainwater (snow), and hail/graupel, and it allows for the generation of supercooled water. This scheme includes the processes of condensation/evaporation, freezing/melting, sublimation/deposition, autoconversion (i.e., aggregration) of cloud water (ice, snow) to form rainwater (snow, hail/graupel), collection by rainwater (snow), and accretion. High-resolution simulations (grid length of 1.67 km) of Hurricanes Bonnie and Georges of 1998 and Floyd of 1999 using the Penn State/NCAR mesoscale model MM5 has been examined to assess the perfomance of the microphysical scheme. This assessment was performed by comparing the statistics of the distributions of the model-produced hydrometeor species to flight-level data collected by penetrations by the NOAA P-3's. Comparisons will be conducted only after matching temperature levels and location within the storm (e.g., eyewall, rainband, stratiform region, etc.) The data will be further sorted by updraft/downdraft magnitude. Additionally, CDF's of model-produced vertical motions and hydrometeor distributions will be compared with CDF's of flight-level derived vertical motions and hydrometeor distributions to obtained reliable information on ability of the simulations to capture the evolution of the system independent of errors in track and intensity. Finally, several improvements to the parameterization scheme have been hypothesized. These improvements will be implemented in the scheme and preliminary tests will be performed to determine the extent to which they may improve the scheme. These techniques will be applied to cases flown during the recently completed CAMEX-IV field program, to be presented in future work.