Microphysical and Dynamical Controls on Orographic Cirrus Inhomogeneity

Cirrus cloud inhomogeneity results from both microphysical and dynamical processes. Optical depth distributions, i.e., the fraction of optical depths occurring at a given optical depth, are one measure of cirrus cloud inhomogeneity important for climate model radiative transfer calculations. We assess physical controls on optical depth distributions for orographic cirrus using a Lagrangian parcel model and kinematic trajectories derived from the PSU/NCAR mesoscale model (MM5). On April 19, 2001, GOES infrared imagery revealed cirrus formation in the lee of the Southern Rocky Mountains. The cirrus were advected by a broad 300-mb ridge and observed by a raman lidar approximately 5 hours after formation at the Atmospheric Radiation Measurement Southern Great Plains site (ARM SGP) in Lamont, Oklahoma (USA). Using MM5 trajectories ending at ARM SGP, we attempt to reproduce the cirrus observations by adjusting physical parameters in our parcel model. We found mesoscale dynamics are critical for reproducing the observed cirrus formation and evolution. In the trajectories derived from MM5 domains with grid cell resolutions less than 12 km, orographic gravity waves (wavelengths 30-45 km, amplitudes > 1 m/sec) resulted as air traveled over the Southern Rockies. Large vertical velocities associated with the waves were required to reproduce many of the cirrus observations including: the high vapor supersaturations required for ice formation in the lee of the Rockies, the persistence of cirrus over many hours, and the broad cirrus optical depth distributions observed by the ARM SGP lidar. Our presentation will further examine microphysical and dynamical processes occurring along the MM5 trajectories, with particular emphasis on how physical processes affect the resulting optical depth distributions.