| P.H. Austin Atmospheric Sciences Programme 217 Geography, 1984 W. Mall University of British Columbia Vancouver, BC V6T 1Z2 CANADA tel. 604-822-2175; fax 604-822-6150 Internet: phil@geog.ubc.ca |
Y. Wang Atmospheric Sciences Programme University of British Columbia Vancouver, BC V6T IZ2, CANADA |
Stratocumulus precipitation measurements made by aircraft and inferred by satellite during the First lSSCP Regional Experiment (FIRE) show cloud regions with rain rates in excess of 8 mm/day. These precipitating clouds can lose 30-50% of their liquid water mass in as little as 1-2 hours.
We have developed a one-dimensional model of layer cloud precipitation which uses a new parameterization of small droplet autoconversion and a non-local transilient-type turbulence scheme to calculate coalescence growth as a function of liquid water profile, droplet number concentration, and the variance and Lagrangian length scale of the vertical velocity. Model calculations show that rain-rate is very sensitive to the autoconversion, which observations indicate can vary by an order of magnitude between adjacent regions of high and low precipitation. Model-predicted precipitation fluxes agree with in-situ observations to within 20% in both heavy (10 mm/day) and light (1 mmlday) precipitation.
Comparisons between our results and those of an analytic, steady-state
precipitation model (Baker, 1993, submitted to Tellus) show that the analytic
model is able to represent the principal features of the dropsize
distribution/precipitation flux we observe in FIRE clouds.