Twelve large-eddy simulations, with a wide range of microphysical representations, are compared to each other and to independent measurements. The measurements and the initial and forcing data for the simulations are taken from the undisturbed period of the Rain in Cumulus over the Ocean (RICO) field study. A regional downscaling of meteorological analyses is performed so as to provide forcing data consistent with the measurements. The ensemble average of the simulations plausibly reproduces many features of the observed clouds, including the vertical structure of cloud fraction, profiles of cloud and rain water, and to a lesser degree the population density of rain drops. The simulations do show considerable departures from one another in the representation of the cloud microphysical structure and the ensuant surface precipitation rates, increasingly so for the more simplified microphysical models. There is a robust tendency for simulations that develop rain to produce a shallower, somewhat more stable cloud layer. Relations between cloud cover and precipitation are ambiguous.

Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

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.

No abstract

The second research flight of the Second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is analyzed. This case attracted attention because it combined the presence of high drizzle rates with the occurrence of clearings in the cloud cover, which previous work has suggested could be due to a drizzle-induced change in cloud structure. Recent work has named the configuration of these open-cell-like features pocket of open cells (POC). A division of the data, based on the difference of brightness temperature of the 11- and 4-μm channels of the Geostationary Operational Environmental Satellite-10 (GOES-10), is used to condition averages over POC and non-POC sections of the data. Based on this division, significant precipitation is observed almost entirely within the POC. Overall, the observed PBL was markedly energetic and well mixed, commensurate with observations of similarly forced nonprecipitating boundary layers. Regions of elevated equivalent potential temperature, Θe, are encountered in association with the POC and are particularly pronounced below cloud in regions of active precipitation. In the same areas, evaporative cooling and moistening of the subcloud layer air and a marked reduction of the vertical velocity variance below cloud is noted. The POC, and in particular the drizzling areas in the POC, have a mean upward velocity, while the non-POC is associated with descending air. In addition to exhibiting more mesoscale variability the POC differs microphysically from the non-POC: in the POC liquid water amount is higher, cloud droplet number concentration lower, and the effective radii larger. Factors maintaining regions of elevated Θe are discussed, and targets for future modeling or observational studies are identified.

No abstract

No abstract