1. Uses the solar radiation parameterization developed by Chou (1990, 1992, 1999, and later updates), which was implemented in the GFS model and described in NCEP Office Note 441 by Hou, Moorthi, and Campana. The following COMET web pages provide further descriptions of the clear-sky radiative transfer and the shortwave radiation processes. (Note that the 5th bullet on the shortwave radiation processes should read "Calculated over each of the eight UV and visible absorption bands for O3".) Scattering and absorption are calculated for eight UV (ultraviolet)/visible and for three NIR (near infrared) spectral bands. For purposes of computational efficiency in shorter-range forecasts, absorption of solar radiation by oxygen and carbon dioxide are omitted without significant loss of accuracy. Optical properties are calculated to be internally consistent with the microphysical characteristics of cloud droplets, rain, cloud ice, and snow assumed in the grid-scale microphysical scheme., while the optical properties of convection are parameterized as described in the recent Eta implementation.
2. The fixed aerosol profile has been replaced with the GFS monthly aerosol tables at 5-degree resolution in latitude and longitude. Effects of aerosol scattering within clouds were removed.
3. Changes in the solar flux calculations take into account partial cloudiness.
4. Longwave emissivities from ice were increased.
5. Grid-scale cloud cover now takes into account partial cloudiness assuming Gaussian probability density functions (PDFs) for the variation of total relative humidity (water vapor plus cloud condensate divided by saturation mixing ratio with respect to water or ice) within the grid. The method is an adaption of the approach proposed by Ek and Mahrt (1991), but adjusted to take into account a much broader range of thermodynamic conditions. The spectral width or variance of the assumed distributions are assumed to be broader in subsaturated conditions, and progressively narrow as the grid approaches and exceeds saturation.
6. Convective cloud cover also takes into account partial cloudiness using the same method as for grid-scale clouds described above, except that broader spectral widths are assumed for the PDFs of total relative humidity to account for much higher levels of turbulence in convective clouds.
7. The temperature at which ice is allowed to form by nucleation was raised from -5C to 0C based on tuning experiments from November 2003, and the assumed cloud droplet number concentration was reduced from 200 /cm**3 to 75 /cm**3. The latter effect is most important, resulting in less suspended cloud water and more rapid conversion to rain through warm-rain coalescence processes.
8. The cloud-top calculation for shallow convection was revised to take into a small fraction (5%) of ambient air mixed into the top of the cloud following the approach proposed by Betts and Miller (1993)
PRECIPITATION ASSIMILATION CHANGES
The precipitation assimilation algorthim in the EDAS has been modified to be less aggressive by eliminating the addition /creation of latent heat and moisture fields. Details can be found at http://www.emc.ncep.noaa.gov/mmb/ylin/newpptasm/
The Eta 3DVAR code has been modified to use the NEXRAD Level 2.5 radial wind data.
Surface temperature observation are assimilated into the EDAS via a 2dvar surface analysis for the first model layer above ground
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