466 Reducing the Systematic Wet Bias in the NAM

Tuesday, 24 January 2017
4E (Washington State Convention Center )
Yihua Wu, NCEP, College Park, MD; and M. B. Ek

It has been noticed that the NAM has a systematic wet bias in the 2-meter dew point temperatures.  To solve this problem, three modifications were made to the calculation of evapotranspiration in the LSM in the NAM system.  The first modification considers the frozen soil water content effect.  The total soil moisture includes three parts: solid (ice), liquid (water) and gas (water vapor).  The gas part is ignored in most LSMs.  The liquid soil moisture is the same as the total soil moisture in unfrozen soil (summer), but is the difference between the total soil moisture and soil ice in frozen soil (winter).  In most LSMs, the calculation of evapotranspiration is based on the total soil moisture, not liquid soil moisture, which is also the case in the NAM.  This not only causes an inconsistency between the evapotranspiration calculations in summer and winter, but also overestimates evapotranspiration in winter seasons.  The second modification is to scale the constant leave area index (LAI) by green vegetation fraction (GVF) in the NAM, where an unrealistic constant LAI of 4.0 is used for all plant types.  The third modification is made in the scheme for the light effect on canopy resistance. All together, these modifications not only remove the inconsistency between summer and winter seasons, but also improve NAM performance, where the systematic wet bias was clearly reduced.  The first modification has been implemented in the latest version of the operational NAM system.  The other two modifications will be tested and validated with more cases before the implementation into the operation NAN system.
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