The authors use large-eddy simulation (LES) to investigate entrainment and structure of the inversion layer of a clear convectively driven planetary boundary layer (PBL) over a range of bulk Richardson numbers, Ri. The LES code uses a nested grid technique to achieve fine resolution in all three directions in the inversion layer. Extensive flow visualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion. It is found that coherent structures in the convective PBL, that is, thermal plumes, are primary instigators of entrainment in the Ri range 13.6 Յ Ri Յ 43.8. At Ri ϭ 13.6, strong horizontal and downward velocities are generated near the inversion layer because of the plume-interface interaction. This leads to folding of the interface and hence entrainment of warm inversion air at the plume's edge. At Ri ϭ 34.5, the inversion's strong stability prevents folding of the interface but strong horizontal and downward motions near the plume's edge pull down pockets of warm air below the nominal inversion height. These pockets of warm air are then scoured off by turbulent motions and entrained into the PBL. The structure of the inversion interface from LES is in good visual agreement with lidar measurements in the PBL obtained during the Lidars in Flat Terrain field experiment. A quadrant analysis of the buoyancy flux shows that net entrainment flux (or average minimum buoyancy flux w min) is identified with quadrant IV w Ϫ ϩ Ͻ 0 motions, that is, warm air moving downward. Plumes generate both large negative quadrant II w ϩ Ϫ Ͻ 0 and positive quadrant III w Ϫ Ϫ Ͼ 0 buoyancy fluxes that tend to cancel. The maximum vertical gradient in potential temperature at every (x, y) grid point is used to define a local PBL height, z i (x, y). A statistical analysis of z i shows that skewness of z i depends on the inversion strength. Spectra of z i exhibit a sensitivity to grid resolution. The normalized entrainment rate w e /w * , where w e and w * are entrainment and convective velocities, varies as ARi Ϫ1 with A ഠ 0.2 in the range 13.6 Յ Ri Յ 43.8 and is in good agreement with convection tank measurements. For a clear convective PBL, the authors found that the finite thickness of the inversion layer needs to be considered in an entrainment rate parameterization derived from a jump condition.

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Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.

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Measurements in marine stratocumulus over the northeast Pacific help scientists unravel the mysteries of this important cloud regime.T he stratocumulus-topped boundary layer (hereafter the STBL), which prevails in the subtropics in regions where the underlying ocean is much colder than the overlying atmosphere, is thought to be an important component of the climate system. Perhaps most striking is its impact on the radiative balance at the top of the atmosphere. The seasonally averaged net cloud radiative forcing from the STBL has been estimated to be as large as 70 W nr 2 (Stephens and Greenwald 1991), more than an order of magnitude larger than the radiative forcing associated with a doubling of atmospheric C0 2 . This means that even rather subtle sensitivities of the STBL to changes in the properties of the atmospheric aero-

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The development and improvement of cloud microphysical and radiative parameterizations for use in cloud and numerical weather prediction models. OBJECTIVES Detailed study of marine stratocumulus cloud microphysical and radiative processes using a high-resolution large eddy simulation (LES) model with explicit microphysics. Better understanding of interactions between microphysical, radiative and boundary layer thermodynamical processes in order to improve prediction of drizzle, marine stratocumulus cloud base height and visibility. Towards this goal, we investigate: 1) The dependence of drizzle on marine stratocumulus cloud microstructure 2) The effects of aerosol and moisture fluxes on cloud base height, drizzle, and visibility 3) Methods to characterize and formulate variability of cloud parameters for use in numerical forecast models APPROACH The research is based on a high-resolution LES model of marine boundary layer stratocumulus clouds with explicit formulation of aerosol and drop size-resolving microphysics. The LES simulations, as well as observations from ASTEX filed project were used to: 1) develop a drizzle parameterization for marine stratocumulus clouds, and 2) study the effects of aerosol and moisture fluxes on cloud base height and visibility. Measurements obtained by Millimeter Wave Cloud Radar (MMCR) have been used to study the variability of radar reflectivity in boundary layer stratocumulus and low altitude stratiform clouds. WORK COMPLETED The following tasks have been completed this year: 1. The development of a one-term drizzle parameterization for stratocumulus clouds in the range of drop concentration from 10 to 60 cm-3. 2. Analysis of over 50 LES simulations investigating the response of cloud base height, drizzle, and visibility range to the strength of CCN and moisture sources. 3. Analysis of the variability of boundary layer stratocumulus and low altitude stratiform clouds based on radar data collected over two years of observations.