Recently, an increasing variety of schemes to represent cloud microphysical processes have been incorporated into mesoscale models. These schemes, which are usually "bulk" approaches to the microphysics in order to reduce computational cost, range from the rather simple to relatively complex in terms of the processes represented and their formulation. The schemes are based upon various theoretical, laboratory, field measurement, and cloud modeling studies that have appeared in the literature over the past forty years, studies which have focused almost exclusively on mid-latitude and tropical areas.

While significant effort has been exercised to validate such microphysical schemes in mid-latitude and tropical environments, relatively little systematic work has been done to consider how such schemes would behave in high latitudes. This is particularly for sophisticated regional models such as the Penn State/NCAR MM5 where the microphysical scheme used must interact with other physical schemes in complex and nonlinear ways. This issue is an important one to consider from an aviation weather point of view. Given that mesoscale model output forms an important component of most aviation weather forecasts and many diagnostic algorithms, it is important to have a good understanding of the role and potential variability in the model output fields due to the choice of a given microphysical scheme.

In this paper we systematically examine the impacts of different cloud microphysical treatments on MM5 simulations of warm season high latitude cloud and precipitation systems. We examine the sensitivity of the simulation mesoscale cloud, precipitation and dynamic fields to not only the choice of the various microphysical schemes routinely available with the MM5 system, but also to modifications to key parameters (baseline ice nuclei concentrations, temperature thresholds and supersaturation thresholds) within individual parameterization schemes, guided by recent and ongoing research of other investigators into Arctic cloud properties.

Our experiments focus on a period during mid-June 1998 during the Surface Heat Budget of the Arctic (SHEBA). Through the period there is considerable cloud property data available over the Western Arctic from AVHRR retrievals, as well as supplemental in-situ field data collected as part of the SHEBA field phase. We evaluate the impacts of the various microphysical schemes and/or parameter choices via an intercomparison among the various simulations as well as with the available analysis, satellite and field data. While our prime foci will be on the three-dimensional cloud, precipitation, hydrometeor species, temperature and humidity distributions (and their implications for model-based in-flight icing guidance), aspects of the synoptic and mesoscale system dynamics will be evaluated as well.