MICROPHYSICAL MODELING OF CIRRUS .1. COMPARISON WITH 1986 FIRE IFO MEASUREMENTS

We have used a one-dimensional model of cirrus formation to study the development of cirrus clouds during the 1986 First ISCCP (Internationa l Satellite Cloud Climatology Project) Regional Experiment (FIRE) inte nsive field observations (IFO). The cirrus model includes microphysica l, dynamical, and radiative processes. Sulfate aerosols, solution drop s, ice crystals, and water vapor are all treated as interactive elemen ts in the model. Ice crystal size distributions are fully resolved bas ed on calculations of homogeneous freezing nucleation, growth by water vapor deposition, evaporation, coagulation, and vertical transport. W e have focused on the cirrus observed on November 1, 1986. Vertical wi nd speed for the one-dimensional simulation is taken from a mesoscale model simulation for the appropriate time period. The mesoscale model simulation suggested that strong upward motions over Wyoming and subse quent horizontal transport of upper level moisture were responsible fo r the cirrus observed over Wisconsin on this date. We assumed that our one-dimensional model could be used to represent a vertical column mo ving from Wyoming to Wisconsin over a period of several hours. Ice cry stal nucleation occurs in our model in the 8 to 10-km region as a resu lt of the strong updrafts (and cooling) early in the simulation. Growt h, coagulation, and sedimentation of these ice crystals result in a br oad cloud region (5-10 km thick) with an optical depth of 1-2 after a few hours, in agreement with the FIRE measurements. Comparison with ai rcraft microphysical measurements made over Wisconsin indicates that t he simulation generated reasonable ice water content, but the predicte d ice number densities are too low, especially for radii less than abo ut 50 mum. Sensitivity tests suggest, that better agreement between si mulated and observed microphysical properties is achieved if the nucle ation rate is higher or stronger vertical mixing (perhaps associated w ith multidimensional motions) is present.