A study of models and microphysical representations using WRF, MM5, and observational data for the December 13-14 IMPROVE-2 event

This research seeks to determine shortcomings of the microphysics schemes used in current mesoscale models. By varying model dynamical cores using the same microphysics parameterization, the influence of different cores on microphysical quantities are determined; conversely, using a single core, microphysics schemes are varied to determine their respective impact. The IMPROVE-2 field campaign provides a comprehensive dataset, suitable for testing the parameterizations used in mesoscale models. The 13-14 December IMPROVE-2 event was selected due to extensive prior study and the quality of observational data assets available during a vigorous frontal passage through Oregon's Cascade range. The current study builds upon the prior work to include analysis of forecasts using the latest microphysics schemes: (1) the Reisner-Thompson scheme (2004) which accounts for the relationship between snow size distribution and temperature, (2) Woods' (2006) modification of the Reisner-Thompson scheme in order to predict the habit composition of the snow field, including seven prognostic equations, each of which predicts the mixing ratio of snow of a particular crystal type, and (3) the most recent Thompson scheme, the only scheme that uses both ice water content and temperature to determine snow size distributions, which it represents as a sum of exponential and gamma distributions (Thompson et al., 2006).

Unlike previous studies, which have emphasized the period of heavy rainfall prior to passage of the baroclinic trough, this investigation extends attention to the more convective post-frontal phase producing locally intense high-elevation snowfall rates. Evolution of cross-barrier flow and precipitation structures observed by the SPol radar and more directly observed surface precipitation rates along a well-observed transect are also accessed to evaluate model performance over this extended period.

This presentation will also compare WRF-ARW with MM5 using Greg Thompson's latest microphysics scheme to demonstrate the impact of varying cores.