A. There are four evaporation components of the total latent heat flux in the NLDAS-2 model datasets.
They are as follows (with PDS ID number, variable names, full description, and units):
199:EVBSsfc:Direct evaporation from bare soil [W/m^2]
200:EVCWsfc:Canopy water evaporation [W/m^2]
198:SBSNOsfc:Sublimation (evaporation from snow) [W/m^2]
These variables correspond to the following evaporation components of the ALMA standard:
NLDAS names ALMA names ALMA description/units
----------- ---------- ----------------------
EVBSsfc ESoil Evaporation from bare soil [kg/m2s]
EVCWsfc ECanop Evaporation from canopy interception [kg/m2s]
SBSNOsfc SubSnow Total sublimation from the ground snow pack [kg/m2s]
TRANSsfc TVeg Transpiration from canopy [kg/m2s]
Additionally, the total latent heat flux is:
121:LHTFLsfc:Latent heat flux [W/m^2]
For NLDAS-2 Mosaic, these components are defined as positive in the downward direction, which is the opposite direction as the traditional ALMA standard for these variables. However, the latent heat flux for NLDAS-2 Mosaic is defined as positive in the upward direction, which means that the latent heat flux is roughly equal to the negative of the sum of these four components for NLDAS-2 Mosaic:
LHTFLsfc ~= -(EVBSsfc + EVCWsfc + SBSNOsfc + TRANSsfc)
As an additional complication, the sign of the SBSNOsfc variable in NLDAS-2 Mosaic switched directions on and after 9 March 2008. Before this date, NLDAS-2 Mosaic SBSNOsfc was defined as positive in the downward direction, but after this date, SBSNOsfc is defined as positive in the upward direction. Thus, on and after 9 March 2008, the evaporation balance equation for NLDAS-2 Mosaic becomes:
LHTFLsfc ~= -(EVBSsfc + EVCWsfc - SBSNOsfc + TRANSsfc)
For NLDAS-2 Noah and NLDAS-2 VIC, these components and the latent heat flux are all defined as positive in the upward direction. The latent heat flux is roughly equal to the sum of the four evaporation components:
LHTFLsfc ~= EVBSsfc + EVCWsfc + SBSNOsfc + TRANSsfc
There is another energy term that describes snow phase-change heat flux:
229:SNOHFsfc:Snow phase-change heat flux [W/m^2]
The SNOHFsfc can be thought of as roughly identical to the sum of the Qsm and Qfz terms (albeit, in different units) from the ALMA standard.
When comparing these four evaporation components and the SNOHFsfc variable between NLDAS-2 Mosaic and Noah/VIC, please reverse the sign in the NLDAS-2 Mosaic data. The exception to this is for SBSNOsfc, but only on or after 9 March 2008.
Additionally, another term for the total evapotranspiration (in mass units) can be found here:
057:EVPsfc:Total evapotranspiration [kg/m^2]
The EVPsfc variable is in mass units and the LHTFLsfc variable is in energy units, but both represent the total upwards water flux. A unit conversion between the two variables (assuming a constant latent heat of vaporization value) shows that the values of these variables should be roughly equal.