LONG TERM GOALS

Our long term goals are to understand the dynamics of atmospheric motions on scales of order 10 m-10 km in sufficient detail to be able to provide a consistent subgrid scale turbulent closure for models across a range of scales, and to be able to utilize simulated variances as a measure of forecast predictability. OBJECTIVES Boundary layer entrainment is a critical, much studied, yet relatively poorly understood element of the dynamics of the atmospheric boundary layer. The entrainment fluxes of heat and moisture across the capping inversion strongly affect both the dynamics and cloud structure within the boundary layer. The chief objective of the present grant is to better understand the physical processes which control the rate of this entrainment, and to formulate a closure model for cloud-top entrainment that is consistent for a broad range of boundary layer conditions and forcings. Ideally, this closure model would allow one to produce a simulation at any desired level of resolution, with the results of lower resolution simulations being approximately similar to results obtained by appropriate spatial filtering of the higher resolution simulation. APPROACH This research involves the utilization of the high resolution turbulent transport codes developed under previous ONR support. Our principal approach is to employ large eddy simulations (LES) to conduct controlled numerical studies of the effects of different boundary layer forcings and conditions (initial temperature and moisture profiles, surface heat and moisture fluxes, cloud-top radiation, wind shear, etc.) on the boundary layer dynamics, cloud structures and, in particular, entrainment rates, which result. The understanding and quantitative correlations gained can be used to better incorporate these effects into the subgrid parameterizations utilized in lower resolution models