Abstract
A large eddy simulation (LES) model, with ice phase included, has been used to study the marine convective boundary layer filled with snow. Extensions to Moeng's LES model include the diagnosis of cloud ice mixing ratio, snow precipitation, and the parameterization of detailed microphysical processes.
Model simulations are compared with cold air outbreak field observations over Lake Michigan, as well as with the liquid phase LES results for the same atmospheric conditions. The buoyancy flux and vertical velocity variance profiles generated by the ice phase LES are found to be more consistent with the observations than those generated by the liquid phase LES results. The incorporation of the ice phase into the LES model has also improved the agreement of vertical velocity skewness (Sw) between observations and LES model results. It has also been found that the presence of precipitation, and the associated microphysical processes, has a significant effect on the structure of the convective boundary layer. The snow precipitation may reduce the size of updrafts and thus can contribute to an increase of Sw. in the vicinity of the cloud base. The latent heat due to condensation, sublimation, and freeing provides an additional buoyancy source for the large eddies. As a result, the large eddies become more active in the cloud layer. The convective geometry simulated shows that the liquid phase LES model attained “open cell” or “spoke” patterns, while the ice phase LES obtained a two-dimensional “band” circulation structure. The ice phase simulation was considered to be much more consistent with the Lake Michigan observations of cloud bands during wintertime cold air outbreaks.
Finally, the statistical analysis used in this study to determine the reliability of model statistics shows that the one-time area average is adequate for the first moments. A longer period of time, however, is required for the second- and third-moment statistics. This continues to illustrate the necessity of having LES model results that are not only of adequate resolution also of sufficiently large domain.
