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 Meteorological Studies large eddy simulation model (CIMMS LES) with an explicit microphysics scheme is developed. It is shown that simulation results are very sensitive to the drop spectrum remapping technique used in condensation calculations; however, the results are almost insensitive to doubling of the spectrum resolution used in the CIMMS LES model. It is also shown that the 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, is 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. The first simulation is based on the U.K. Meteorological Research Flight flight 526 measurements collected over the North Sea on 22 July 1982; the second simulation corresponds to the Atlantic Stratocumulus Transition Experiment flight A209 on 12-13 June 1992. The model is able to reproduce reasonably well most of the observed boundary layer parameters, including turbulent fluxes and variances of various fields, the intensity and vertical distribution of the turbulent kinetic energy, the upward and downward radiative fluxes, and the cloud drop spectra. It is speculated that the most noticeable discrepancy, which is an underestimation of the concentration of drops smaller than 6 mu m near the cloud top, may be an indicator of the need to refine theoretical formulation of small-scale turbulent mixing.