Three new post-processed fields will be added to Meso Eta GRIB output beginning at 12Z 2 July 2002. The additional fields will not be sent to AWIPS users at this time but are available to anyone who ftp's the 00z, 06z, 12z or 18z Meso Eta grids from either NCEP or NWS servers or UNIDATA's CONDUIT.
New computations of convective available potential energy (CAPE), convective inhibition (CIN), and helicity (HLCY) will be available on all output grids that already contain the existing CAPE, CIN and HLCY fields. The current computations will not be eliminated but rather the three new fields will be additions to the output files, not replacements.
Two CAPE and CIN fields are currently available. One is a surface- based field (actually the parcel with the highest theta-e in the lowest 70 millibars). They show up in a GEMPAK GDINFO listing as
0 NONE CAPE
0 NONE CINS
The other is a most-unstable field in which the 30 mb layer in the lowest 180 mb with the highest theta-e is lifted. They show up in a GDINFO listing as
180 0 PDLY CAPE
180 0 PDLY CINS
The new fields are mixed-layer fields. Recent Storm Prediction Center research has indicated that a mean lowest 100 mb parcel may best represent an environment with a well-mixed boundary layer. The Meso Eta Model computes average parcels for six 30 mb layers above ground, so to take advantage of these existing computations, the mixed-layer for the new CAPE/CIN computations is constructed by averaging the thermodynamic properties of the 3 lowest 30 mb layers (giving a 90 mb mixed layer). The new fields show up in a GDINFO listing as
90 0 PDLY CAPE
90 0 PDLY CINS
The current helicity computation uses the Bunkers dynamic method of computing the storm motion vector and then determines the helicity over the lowest 3 km using the Davies-Jones et al method. It is found in a GDINFO listing as
3000 0 HGHT HLCY
While the 0-3 km storm-relative helicity is a good indicator of the potential for storm rotation and general tornadic threat, the 0-1 km value is a better parameter to assess the threat of significant tornadoes. The second helicity field will be listed as
1000 0 HGHT HLCY
The new fields will show up at or near the end of the GRIB files. Therefore, if you are grabbing 0-3 km helicity by searching the PDS for the first appearance of a helicity parameter, you should be fine and will continue to get the same field as before. If, however, you want the new field, your PDS search will have to look the helicity with the 1000 level indicator (octet 11). Similarly, if you are getting most unstable CAPE by extracting the first CAPE field with a Table 3 octet 10 value of 116, you will want to look for an octet 11 value of 90 to get the new mixed layer field.