Eleventh ARM Science Team Meeting Proceedings, Atlanta, Georgia, March 19-23, 2001 Observed and Simulated Cirrus Cloud Properties at the SGP CART Site A. D. Del Genio and A. B. Wolf National Aeronautics and Space Administration Goddard Institute for Space Studies New York, New York G. G. Mace University of Utah Salt Lake City, Utah Introduction Despite their potential importance in a long-term climate change, less is known about cirrus clouds than most other cloud types, for a variety of reasons (Del Genio 2001) including: (1) the difficulty of remotely sensing ice water content (IWC), (2) uncertainty in the identities of ice nuclei and the relative importance of different nucleation processes, (3) significant variations in depth and optical thickness caused by formation and sedimentation of large particles, and (4) our relatively poor documentation of the dynamics of the upper troposphere. Aircraft observations suggest that the sub-grid variability of cirrus microphysical properties might be modeled using an approach similar to that for marine stratus clouds (Smith and Del Genio 2001a). Those data represent a very limited sample, however, and leave open the question of how cirrus radiative properties might be predicted given knowledge of their microphysical characteristics. Recently, though, the advent of the millimeter cloud radar (MMCR) has made it feasible to develop long-term surface-based climatologies of cirrus properties. Mace et al. (1998) have developed techniques for retrieving cirrus cloud boundaries, IWC, effective radius (r e ), and number concentration from MMCR reflectivities combined with atmospheric emitted radiance interferometer (AERI) downwelling radiances and have applied the algorithm to almost two years of MMCR data (11/96-5/98) at the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site. Three-minute mean retrievals are available at 8-minute intervals for isolated cirrus (i.e., no simultaneous low- or mid-level clouds) at temperatures < -20°C. In this paper we document some of the statistical properties of the observed cirrus and their sensitivity to environmental conditions and determine relationships between cirrus radiative and microphysical properties. We also compare the results for convectively generated summer cirrus to synoptically forced winter cirrus, and we test the ability of the Goddard Institute for Space Studies (GISS) stratiform cloud parameterization to reproduce some of the observed relationships by conducting single-column model (SCM) tests. Considine et al. (1997) presented a simple conceptual model to predict the cloud cover and probability density function (PDF) of liquid water path (LWP) for marine stratus clouds. The model was based on an assumed Gaussian distribution of cloud depths controlled by turbulent vertical velocities and 1