RUC cape values

[Andrew Stoller]

Why is the RUC so out to lunch with CAPE, and therefore EHI values? Some days it's on par with the other models, but some days, it showing over twice the actual realized CAPE? People have been mentioning this for a while and I thought I'd ask why it does this?

Thanks!

 

[Jeff Snyder]

I'm not sure, but I've noticed on more than a few days that the RUC CAPE, compared to observed soundings, tends to be high, and the RUC CINH tends to be too high as well (remember, CINH is a negative quantity, so "too high" means something like a RUC value of -50 j/kg vs. an observed value of -125 j/kg). CAPE is an integrated quantity relating the thermal characteristics of a parcel to that of the environment, so I assume that gross errors in CAPE and CINH are mostly attributable to incorrect surface (or low-level, if looking at MLCAPE / MLCINH) temperatures and dewpoints. On some days, the RUC really seems to overforecast surface Tds, which would adversely affect the CAPE and CINH forecasts. Errors in dewpoint forecasts would seem to be caused by errors in surface physics or boundary layer parameterizations (evapotranspiration, vertical mixing, etc.), but I'm not terribly well-versed in this area.

Remember that the SPC Mesoanalysis uses, amongst other sources, RUC forecast data, so errors in the 1hr RUC forecast are likely to show up in the SPC mesoanalysis as well. This is purely anecdotal, but I've seen many more cases of an overestimate of CAPE and underestimate of capping than the opposite.