A large eddy simulation model with explicit microphysics: Validation against aircraft observations of a stratocumulus-topped boundary layer

A new dynamical framework for the Cooperative Institute for Mesoscale Meteo rological Studies large eddy simulation model (CIMMS LES) with an explicit microphysics scheme is developed. It is shown that simulation results are v ery sensitive to the drop spectrum remapping technique used in condensation calculations; however, the results are almost insensitive to doubling of t he spectrum resolution used in the CIMMS LES model. It is also shown that t he drop coagulation procedure conserves the liquid water content as long as the predominant radius of the drop size spectrum, defined as the cube root of the ratio of the drop radar reflectivity to the liquid water content, i s below a threshold value of 250 mu m Finally, it is demonstrated that for typical maritime conditions this threshold radius is exceeded only in 0.1% of all cloudy points. Realism of the model is evaluated by a direct comparison of its predictions with the aircraft observations of a stratocumulus-topped boundary layer. T he first simulation is based on the U.K. Meteorological Research Flight fli ght 526 measurements collected over the North Sea on 22 July 1982; the seco nd simulation corresponds to the Atlantic Stratocumulus Transition Experime nt flight A209 on 12-13 June 1992. The model is able to reproduce reasonabl y well most of the observed boundary layer parameters, including turbulent fluxes and variances of various fields, the intensity and vertical distribu tion of the turbulent kinetic energy, the upward and downward radiative flu xes, and the cloud drop spectra. It is speculated that the most noticeable discrepancy, which is an underestimation of the concentration of drops smal ler than 6 mu m near the cloud top, may be an indicator of the need to refi ne theoretical formulation of small-scale turbulent mixing.