Physical controls on orographic cirrus inhomogeneity

J. E. Kay (1) , Mark D. Baker (2) , D. Hegg (2)

1 NCAR - National Center for Atmospheric Research [Boulder]
2 Department of Atmospheric Sciences [Seattle]

Optical depth distributions (P(?)) are a useful measure of radiatively important cirrus (Ci) inhomogeneity. Using a parcel model with binned ice microphysics and kinematic trajectories from a mesoscale weather model (MM5), we assess physical controls on Ci P(?) during an orographic Ci case study. On 19 April 2001, satellite imagery revealed Ci formation in the lee of the Southern Rocky Mountains and Ci advection along a broad upper level ridge. Above Lamont, Oklahoma (USA), lidar observations indicated a broad Ci P(?). Along MM5 trajectories associated with the observed Ci, homogeneous freezing and mesoscale variability in vertical velocities led to broad modeled P(?) and variability in modeled Ci cloud lifetimes. The addition of background ice nuclei concentrations (N_IN=0.03 cm^?3) to air parcels had little impact on modeled Ci ? variability. The presence of background N_IN did increase cloud cover by increasing the frequency of small ? Ci. These results highlight the importance of homogeneous freezing and mesoscale vertical velocity variability in controlling Ci P(?) shapes along realistic upper tropospheric trajectories.

Domaines: Océan, Atmosphère

J. E. Kay, Mark D. Baker, D. Hegg. Physical controls on orographic cirrus inhomogeneity. Atmospheric Chemistry and Physics Discussions, 2007, 7 (2), pp.4889-4923. ⟨hal-00302707⟩