INITIALIZATION AND VALIDATION OF A SIMULATION OF CIRRUS USING FIRE-IIDATA

Observations from a wide variety of instruments and platforms are used to validate many different aspects of a three-dimensional mesoscale s imulation of the dynamics, cloud microphysics, and radiative transfer of a cirrus cloud system observed on 26 November 1991 during the secon d cirrus field program of the First International Satellite Cloud Clim atology Program (ISCCP) Regional Experiment (FIRE-II) located in south eastern Kansas. The simulation was made with a mesoscale dynamical mod el utilizing a simplified bulk water cloud scheme and a spectral model of radiative transfer. Expressions for cirrus optical properties for solar and infrared wavelength intervals as functions of ice water cont ent and effective particle radius are modified for the midlatitude cir rus observed during FIRE II and are shown to compare favorably with ex plicit size-resolving calculations of the optical properties. Rawinson de. Raman lidar, and satellite data are evaluated and combined to prod uce a time-height cross section of humidity at the central FIRE-II sit e for model verification. Due to the wide spacing of rawinsondes and t heir infrequent release, important moisture features go undetected and are absent in the con ventional analyses. The upper-tropospheric humi dities used for the initial conditions were generally less than 50% of those inferred from satellite data, yet over the course of a 24-h sim ulation the model produced a distribution that closely resembles the l arge-scale features of the satellite analysis. The simulated distribut ion and concentration of ice compares favorably wi th data from radar, lidar, satellite, and aircraft. Direct comparison is made between the radiative transfer simulation and data from broadband and spectral se nsors and inferred quantities such as cloud albedo, optical depth, and top-of-the atmosphere 11-mu m brightness temperature, and the 6.7-mu m brightness temperature. Comparison is also made with theoretical hea ting rates calculated using the rawinsonde data and measured ice water size distributions hear the central site. For this case study, and pe rhaps for most other mesoscale applications, the differences between t he observed and simulated radiative quantities are due more to errors in the prediction of ice water content, than to errors in the optical properties or the radiative transfer solution technique.