RADIATION AND CLOUD CHANGES
1. Modified version of operational Lacis-Hansen scheme with modified water & ice absorption coefficients that are more consistent with those in the GFS radiation scheme
2. In calculating solar absorption, the optical depths for cloud water and for ice (cloud ice and snow) are obtained using the relationships described by eqs. (5.2), (5.3), and Table 9 from Hou et al. (2002), assuming a constant effective radius of 10 microns for cloud water and 75 microns for ice. For cloud water optical depths, equal weighting is assumed for absorption of UV-VIS and near IR radiation.
3. A minimum optical depth is assumed for grid-scale liquid water clouds consistent with a minimum mixing ratio of 0.1 g/kg. This lower limit has been removed, and this change has been found to have the biggest impact.
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 NAM 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 NAM 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).
Minimum stomatal conductance increased from 40s/m to 70s/m for cropland, pasture, and grassland vegetation types in order to decrease surface evaporation.
The hi-res soil and vegetation type classifications:
The NAM 3DVAR code has been modified to use the NEXRAD Level 2.5 radial wind data, and a 2dvar module is now run to analyze surface temperature data
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