AFOSR Project : Prediction of Global Cloud

We have successfully implemented an explicit cloud scheme within the Florida State University Global Spectral Model. This includes the liquid water mixing ratio and cloud fractions as two additional dependant variables. The main purpose of this extension is two fold: we wished to improve our global cloud forecasting capability (low, medium and high clouds) and to have a better definition of the cloud radiative effects. A band model is being used for the short and long wave radiative transfer. A major component of this study is the initialization of clouds. For this purpose we have utilized the U.S. Airforce Real-Time Nephanalysis product called RTNEPH. The microwave radiances from the U.S. Airforce fleet of DMSP satellites is another source of data. These are the special sensor microwave instruments carried by these satellites. This information provides measures of vertically integrated liquid water mixing ratios. The liquid water mixing ratios are vertically partitioned using weights from the RTNEPH; this provides an initial definition of clouds and cloud fractions. These were further initialized using the procedure of physical initialization. The impact studies of this cloud specification and initialization appear very promising. CO CO CO o GO o to o CO w Introduction: The objectives of this research supported by AFOSR were to develop a physically based cloud scheme and to improve cloud forecasts with a large-scale model. Towards this goal a prognostic cloud scheme has been developed and incorporated into the Florida State University Global Spectral Model (FSUGSM) via the introduction of a cloud parameterization which includes cloud water/ice content and cloud fraction. The time evolution of clouds is defined through the large-scale budget equations for cloud water content and fractional cloud cover. The scheme considers the formation of clouds in connection with large-scale ascent, diabatic cooling, boundarylayer turbulence, and vertical transport of cloud water from convective updrafts. Clouds dissipate through adiabatic and diabatic heating, turbulent mixing of cloud air with unsaturated environment air, and depletion of cloud water by precipitation. Unlike conventional schemes, the scheme is fully prognostic and model consistent. Furthermore, the formation of anvil and cirrus clouds originating by cumulus updrafts and boundary-layer clouds is included. Details of Analysis: Operational analyses from the National Center for Environmental Prediction (NCEP) provide initial conditions for model integrations. The NCEP analyses consist of geopotential height, temperature, zonal and meridional wind on 12 mandatory pressure levels (1000, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, and 50 mb), and relative humidity on the six lowest levels. The gridded analyses are vertically interpolated to the model's c surfaces (for RH a constant value is used above 300 mb), and expanded in the spherical harmonic basis functions of the global spectral model. A diabatic nonlinear normal mode initialization is used. Cloud data used for development and verification of cloud schemes were the Real-Time Nephanalysis (RTNEPH) obtained from Air Force Globla Weather Control via the NCEP data facility. The RTNEPH data set includes a global analysis of cloud amount, cloud type, cloud bases and tops based on satellite and conventional observations. This provides us with unique data source for the cloud initialization. The RTNEPH analysis is done with respect to polar-stereographic grids of the northern and southern hemispheres with a horizontal resolution 47.625 Km true at 60 degree north and south latitude. Data sources for the analysis are primarily the infrared (IR) and visible (VIS) channels on two Defense Meteorological Satellite Program (DMSP) satellites, with some information obtained from polar-orbiting satellites(e.g. NOAA 11/12) and surface observations, when available. A manual bogus is also employed during each analysis cycle. The RTNEPH data set consists of total cloud and up to 4 distinct layered clouds. Each pixel point contains cloud coverage, geopotential height of the layered cloud bases and tops, cloud type, time of observation, and other diagnostic information. Details are described in Hamill et. al. (1992). Three-dimensional cloud fraction Heights of cloud tops and bases, and cloud amounts are the primary source of global and vertical distribution of cloud fraction. Three-dimensional cloud fraction is derived on the sigma coordinate at the gaussian grid for use in the model initialization and verification. The coordinate transformation from the polar-stereographic grid to the gaussian grid is carried out in both hemispheres. The gridded field of geopotential height on 12 mandatory pressure levels is horizontally interpolated to the gaussian grid. The heights of cloud bases and tops and the cloud amounts of each layered cloud are decoded from the compressed RTNEPH data set in both hemispheres. Since RTNEPH cloud height is measured from the mean sea level, terrain height should be added to obtain the ground-based cloud height at each grid point. In-cloud pressures are obtained by interpolating geopotential heights of the mandatory pressure levels to the height of the grid clouds. Given surface pressure, we can directly calculate corresponding sigma values of cloud bases and tops for each cloud. Once we have sigma values of cloud base and top, we assign corresponding cloud amount onto sigma levels throughout each layered cloud. Total clouds are computed by using the method of random overlap through the column as