Thursday, August 13, 2015

HRRR Geography Data Issues

The High Resolution Rapid Refresh model has a few issues around the Great Salt Lake. One we have noticed is that the modeled lake is too large! This causes increased dew point temperatures around the lake's location than are actually observed. Increased dew point temperatures lead to increased CAPE values, or the potential for thunderstorm development.

On August 10, 2015 at 23:00z the HRRR modeled dew point temperature was at least six degrees higher at the surface than observed during the 00z sounding. This produced nearly 2000 Joules of CAPE in the HRRR when there was actually about 200 Joules of CAPE. This means that the HRRR was saying that thunderstorm development was very favorable when in reality there was marginal potential energy available to cause deep convection. This issue will be looked at further. We intend to modify the lake area to produce more accurate simulations around the Great Salt Lake.

Skew-T diagram of HRRR analysis at 23z on August 8th (thick red) and actual weather balloon observation (thin blue). Surface specific humidity values were less than 7 g/kg, but the HRRR model thinks there are over 10 g/kg. Imagine lifting a parcel with dew point temperatures of 5C rather than 13C, you would get much less CAPE. 
The outer line representing the Great Salt Lake is the lake/land mask used by the HRRR. The inner line is a more accurate representation of the's current size. The coloring represents the specific humidity. Specific humidity is much higher following closer to the large lake boarder. The white dot represents the location of the KSLC sounding site where weather balloons are launched. You can see that this location is in the pink (> 12 g/kg) when in reality the humidity at that location is much less.



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