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)
On 6/1/04, added new precipitation assimilation algorthim; see change log for details.
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