A MODEL FOR PARTICLE MICROPHYSICS, TURBULENT MIXING, AND RADIATIVE-TRANSFER IN THE STRATOCUMULUS-TOPPED MARINE BOUNDARY-LAYER AND COMPARISONS WITH MEASUREMENTS

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics m odule that resolves the size distributions of aerosols and cloud dropl ets, a turbulence module that treats vertical mixing between layers, a nd a multiple wavelength radiative transfer module that calculates rad iative heating rates and cloud optical properties. The results of a 12 -h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an similar to 500-m thick, summertime marine stratocumulus cloud layer by Nichol ls. However, in this case, the model predictions of turbulent fluxes b etween the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine str atus layer made under gale conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties a re generally reproduced by the model, although it underpredicts the en trainment of overlying air at cloud top under gale conditions. Sensiti vities of the model results are explored. The vertical profile of clou d droplet concentration is sensitive to the lower size cutoff of the d roplet size distribution due to the presence of unactivated haze parti cles in the lower region of the modeled cloud. Increases in total drop let concentrations do not always produce less drizzle and more cloud w ater in the model. The radius of the mean droplet volume does not corr elate consistently with drizzle, but the effective droplet radius does . The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input conden sation nuclei and by omitting the effect of cloud-top radiative coolin g on the condensational growth of cloud droplets. The omission of infr ared scattering produces noticeable changes in cloud properties. The c ollection efficiencies for droplets <30-mu m radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizont al wind also has a significant effect on a 12-h model simulation of cl oud structure. Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplificati on is that the model assumes horizontal homogeneity, and, therefore, d oes not resolve updrafts and downdrafts. Likely consequences of this s implification include overprediction of the growth of droplets by cond ensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and underpredi ction of peak supersaturations.