Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR) Forum
Re: CAPE Calculation
Posted By: John Brown In Response To: CAPE Calculation (Scott Dennstaedt)
Date: 10-October-07 0004Z
In Response To: CAPE Calculation (Scott Dennstaedt)
This is related to a chronic issue with the CAPE as calculated by the RUC. Below is how I answered a similar query of 5 years ago, asking why RUC CAPEs tended to be higher than those from the old Eta model (now replaced by the NMM in the NAM):
"When such abundant values of moisture exist, the CAPE calculations become very sensitive to the mixing ratio of the lifted parcel (nonlinearity of the Clausius-Clapeyron eqn) and the assumptions made in the calculations (particularly, the form of the first law of thermo, and the form of the Clausius-Clapeyron eq for saturation vapor pressure as function of the temperature). The Eta and RUC differ in these assumptions, with the RUC's assumptions tending to favor larger CAPE when dew points are high."
The CAPE calculations differ between that used in the RUC postprocessing ("CAPE" in your linked Skew-T image) and in Bill Moninger's sounding page, again, with the higher values originating with the RUC postprocessing. For consistency between the RUC model and its postprocessing, we chose to use the same procedure to compute CAPE in the postprocessing as is used within the convective scheme in the model itself.
Hope this helps...
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