Evaporation Duct Height

Description

The evaporation duct height (EDH) is associated with the sharp drop in moisture immediately above a water surface and is given by the height in the surface layer where the gradient in modified refractivity (M) changes from negative to positive.  EDH values can vary from a maximum value of 40 m to very low heights near zero depending upon the stability, sea surface temperature, wind speed, and relative humidity.  COAMPS computes the EDH on a special high-resolution vertical grid (1-m spacing) using surface similarity theory.   These expressions can be problematic for both light winds and stable surface fluxes.  Under those conditions, the near surface M-gradient can be quite weak and a specific EDH value difficult to determine.    

Results

We have evaluated the time series of COAMPS EDH from the Wallops-2000 experiment against that computed from the NPS Flux buoy measurements located ~12 km offshore of Wallops Island, VA during April/May 2000 (Fig. 1).  Statistical computations yield Bias, RMS and standard deviation error values from this model-obs comparison.  Values in column 1 are for the entire series, in column 2 are for EDH less than 40 m, and in column 3 for EDH less than 30 m.  The agreement between modeled and measured EDH improves (from 11 m to 5 m) by removing values at the upper limit.  Figure 2 is an example of the spatial distribution of EDH forecast by COAMPS (green shading) off the east coast of the U.S overlaid with wind barbs and contours of surface pressure (mb).  Note the higher EDH to the north and west of the low pressure cell generated by the cold, dry post-fontal air mass advecting over the warm gulf stream current thereby creating a sharp surface moisture gradient.