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Message: revised CAPE/CIN in RUC2/40kmMAPS

Posted by Stan Benjamin on 27 May 98, 11:01 MT

As some of you are aware, the CAPE/CIN diagnostic in RUC2 is in need of some help, and help is on the way, thanks to work by John Brown and many discussions between him and Phillip Bothwell at SPC. A revised version, which can be conservatively described as a huge improvement, was implemented in the 40km MAPS running at FSL on Fri 16 May. This version has been submitted to the NCEP Computing Office as a change to RUC2 and will be implemented there within 1-2 weeks. Another post to this board will occur when the change is made.

The new version gives much smaller CIN, slightly smaller CAPE, and avoids unsightly and confusing "zero holes" in the middle of high CAPE areas. The old version performed fairly well in early spring but was found to have several problems, especially in the presence of deep mixed layers that have now become much more prevalent as the season has progressed. The difference in performance can be seen by comparing the real-time CAPE/CIN fields on the RUC/MAPS home page from MAPS40 (red button) and RUC2 (blue button).

Here is a summary of the new routine's steps:
1. Consider only the layer between the LCL (lifting condensation level) and the EL (equilibrium level, near cloud top)
2. CAPE = sum of all positive area segments in this interval.
3. CIN = sum of all negative area segments in this interval.
4. Thermodynamic constants (specific heat, latent heat of condensation, etc.) will be essentially those used by Eta.
5. Implied form of the first law to derive the moist adiabatic ascent of best parcel will be slightly different than Eta, but will be consistent with that used by the RUC convective parameterization scheme.
6. Best parcel in lowest 180mb is considered.
7. To determine the best parcel, properties of the lowest 5 RUC native-coordinate vertical levels (roughly the lowest 20-25mb) are averaged to form a candidate parcel. From level 6 on up to 180 mb AGL, properties of each level are considered individually without any vertical averaging.
8. CIN will be set to zero at horizontal grid points where CAPE is zero.

Again, another post will be made when the RUC2 change is actually made.


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