RADIATION AND CLOUD MICROPHYSICS CHANGES (Removed at 12Z 10/26/2004; see change log for details)
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)
SHALLOW CONVECTION PARAMETERIZATION CHANGES (Removed at 12Z 9/1/2004, see change log for details)
Increased the depth of shallow (nonprecipitating) convection from 200 hPa to 400 hPa while keeping the same lower limit for deep (precipitating) convection at 200 hPa. Currently convection greater (less) than 200 hPa in depth is assumed to be deep (shallow). This change allows convection between 200 hPa and 400 hPa to be deep if it passes the enthalpy adjustment and entropy check, otherwise it is determined if is can support shallow convection.
The cloud-top level is associated with the highest level where the parcel temperature is warmer than the environment. The parcel temperature is calculated assuming a small fraction (5%) of ambient air is mixed at cloud top following the procedure of Betts and Miller (1993). Shallow convection is considered only if the level of free convection (LFC) is within 100 hPa of the lifting condensation level (LCL) of the most unstable parcel. Currently the top of shallow convection is associated with the maximum increase in relative humidity with pressure (similar to where the decrease in relative humidity with height is greatest).
Currently shallow convection is aborted if a number of criteria are not met (see Janjic, 1994), including the creation of supersaturated conditions within a layer. This particular criterion was eliminated, allowing shallow convection to continue to transport heat and moisture between different levels while at the same time also allowing grid-scale condensation to occur. These conditions are consistent with stratocumulus clouds where convective overturning is active within cloud layers.
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/
LAND-SURFACE PHYSICS CHANGES
A number of changes have been made to the Noah land-surface model (LSM) used in the operational mesoscale Eta model, from the previous version (2.3.2) to the current version (2.7). These involve changes to Noah LSM physics, model formulation parameters, and some additional numerical refinements. Also, removing the vegetation greenness factor from the snow albedo formulation leads to an increase in albedo under snow-covered conditions. The Eta model cloud microphysics now passes the fraction of frozen precipitation to the Noah LSM, eliminating the crude determination of frozen precipitation by the Noah LSM based on lowest (atmospheric) model level air temperature. Separate snow sublimation and non-snow-covered evaporation is now considered for patchy snow cover conditions when snowpack is shallow, reducing snow sublimation and snowpack depletion. Changes to parameters in the patchy snow cover formulation decrease the snow depth for 100 percent snow cover. A reduction in vegetation-dependent soil moisture threshold values will increase transpiration. The depth at which the lower boundary condition on soil temperature is applied is increased from 3 meters to 8 meters. The thermal heat capacity of mineral soil has been changed to a more standard value. A change to the coefficient in the thermal-roughness length calculation will decrease the surface skin-atmosphere temperature gradient. The sea-ice albedo is changed from 0.60 to 0.65. Including a diagnostic soil heat flux calculation at the end of the Noah LSM code leads to better closure of the surface energy budget.
Reduced parameter CZIL from 0.2 to 0.1. This change will reduce aerodyamnic resistance (i.e. surface turbulent exchange coefficients are too low during mid-day)
Parameter SMHIGH_DATA reduced from 6.0 to 3.0; this will raise the value of the reference soil moisture value below which vegetation becomes stressed (SMCREF), which (at first order) should reduce the transpiration (surface moisture flux).
Use hi-res soil and vegetation type classifications:
The Eta 3DVAR code has been modified to use the NEXRAD Level 2.5 radial wind data.
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