20020627: Model update

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: 20020627: Model update
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Thu, 27 Jun 2002 08:25:41 -0600

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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
LEVL1      LEVL2      VCORD      PARM
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
LEVL1      LEVL2      VCORD      PARM
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
LEVL1      LEVL2      VCORD      PARM
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
LEVL1      LEVL2      VCORD      PARM
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
LEVL1      LEVL2      VCORD      PARM
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.

Please contact Geoff Manikin if you have any questions.
---------------------------------------------------------------------
Geoff Manikin                    internet:  geoffrey.manikin@xxxxxxxx
Mesoscale Modeling Branch        phone:  301-763-8000  x7263
NCEP/EMC                         fax:    301-763-8545
NOAA Science Center, Room 204
Camp Springs, MD  20746