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-------- Original Message --------
792
NOUS41 KWBC 121438
PNSWSH
Public Information Statement, Comment Request
National Weather Service Headquarters Washington, DC
938 AM EST Wed Jan 12 2011
To: Subscribers:
-Family of Services
-NOAA Weather Wire Service
-Emergency Managers Weather Information Network
-NOAAPORT
Other NWS Partners, Users and Employees
From: Geoff DiMego
Chief, Mesoscale Modeling Branch
NCEP/Environmental Modeling Center
Subject: Soliciting Public Comments through February 28, 2011
on Modification of the Computation of Instability
Parameters in the NCEP Model Suite
The National Centers for Environmental Prediction (NCEP) is
proposing to modify the computations of convective available
potential energy (CAPE), convective inhibition (CIN), and lifted
index (LI) in its modeling systems during 2011. The change is
based on the need to account for moisture in computations
involving lifted parcels and has only a modest impact on the
values. The NWS is seeking comments on this proposed change
through February 28, 2011.
The proposed changes involve using the virtual temperature
instead of temperature for all parameters involving the
stability of a lifted air parcel. All of the CAPE, CIN and LI
parameters (best, surfaced-based, mixed layer) will be computed
using virtual temperature to properly compute the density of the
air parcel being lifted (Doswell and Rasmussen, Weather and
Forecasting, December 1994).
NCEPs Storm Prediction Center (SPC) has been using the virtual
correction in its internal sounding analysis programs for
15 years, since computations incorporating the virtual
temperature are more physically correct, and they strongly
support the plan to change the computations in the NCEP models.
The incorporation of the virtual temperature correction into the
NCEP model instability fields will also be more consistent with
forecast tools on the SPC web page including displays of
observed sounding parameters and hourly Mesoscale Analysis
fields, which are used extensively throughout the operational
forecast community.
http://www.spc.noaa.gov/exper/
The impact to the instability parameters is generally minimal,
with the magnitude of the impact proportional to the water vapor
content of the atmosphere. For those with algorithms based on
particular threshold values of cape or lifted index, the largest
changes occur when values of those parameters are already
extreme and do not affect the overall interpretation of the
field.
A full description of the changes with examples of the impact
can be found at
http://www.emc.ncep.noaa.gov/mmb/gmanikin/virtual.ppt
If this proposal is approved, NCEP will change to the North
American Mesoscale (NAM) model as part of the upgrade scheduled
for this summer. The revised computation will not be added to
the Rapid Update Cycle (RUC); it will instead be included in the
Rapid Refresh model, which will replace the RUC late this
summer. This change must also be made to the Global Forecast
System (GFS) and Short-Range Ensemble Forecast (SREF). It is
possible there will be one major implementation to unify the
cape/cin/LI computations across all NCEP models this summer.
Details regarding this plan will be sent in another TIN after
the process of vetting the proposed changes is completed.
NWS will evaluate all comments to determine whether to proceed
with this change. If approved, a TIN will be issued containing
implementation dates.
Send comments on this proposal and requests for test files to:
Geoff Manikin
NWS/NCEP Environmental Modeling Center
Camp Springs, MD
301-763-8000 x7263
geoffrey.manikin@xxxxxxxx
National Public Information Statements are online at:
http://www.weather.gov/os/notif.htm
$$
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