Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR) Forum
Re: Low iCAPE
Posted By: Stan Benjamin In Response To: Low iCAPE (Scott Dennstaedt)
Date: 5-May-07 2126Z
In Response To: Low iCAPE (Scott Dennstaedt)
*** Scott, on your questions:
Looking at the KSAT 12-h forecast from Op20, it appears that the iCAPE value is very low (based on a surface-based lifted parcel) as compared to the Best CAPE value included in the top right of the diagram. Why is the iCAPE so low when there is obviously very high amount of CAPE above the upper LFC (at 708 mb)? See http://chesavtraining.com/images/Low-iCAPE.gif
***** I plotted the same sounding you showed and got 5320 J/kg for the surface parcel. Be careful in how you click on the surface level, since you the sounding app forces in the temperature that you click on. Your sounding shows that you caught a very thin positive buoyant layer from 900-810 hPa, a fluke for the value you clicked on for your sounding.
*** I think it is a bit overstated compared to more traditional CAPE calculations (as in the RUC sounding program for iCAPE/iCIN). The CAPE routine used in the RUC does indeed seem to exaggerate values at the high end (say, above 3000 J/kg). The RUC CAPE routine uses the same cloud work function from the Grell/Devenyi scheme ( http://ruc.noaa.gov/RUC.pubs.html and http://www-frd.fsl.noaa.gov/pub/papers/Grell2001a/p.pdf). There is a little more CAPE/CIN info at http://ruc.noaa.gov/vartxt.html#CAPE-def.
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