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Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR) Forum
Re: CAPE of 12,241 J/kg
Posted By: John Brown In Response To: CAPE of 12,241 J/kg (Scott Dennstaedt)
Date: 4-August-08 2025Z
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Scott, As always, thanks for keeping an eye on our stuff and drawing attention to these sorts of problems. There are 2 sources of difficulty. The first and main one we have seen from time to time during this year's warm season with _loaded gun_ soundings characterized by a moist shallow mixed layer surmounted by a near dry adiabatic residual layer, with the two layers separated by an inversion or marked stable layer. The problem appears to arise from GOES precipitable water observations that are more moist than the 1-h forecast guess--the analysis increments the precipitable water by increasing the mixing ratio of the air in the inversion to unrealistic values, as seen in your example. The RUC change package now scheduled for implementation this fall (see a forum post at http://ruc.noaa.gov/forum/f2/Welcome.cgi/read/1713) generally handles these situations much better. The second one, less significant in this case, is related to a chronic issue with the CAPE as calculated by the RUC. In summer, with typically high dew points at low levels, the CAPE calculations become very sensitive to the properties of the lifted parcel due to nonlinearity of the relationship between dew point and water-vapor mixing ratio (small change in dew point corresponds to a large change in mixing ratio when dew points are high). In addition, the assumptions made in the calculations of the properties of the saturated lifted parcel (particularly, the form of the first law of thermodynamics) by the RUC tend to favor larger CAPE than produced by other methods when dew points are high.
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