NCEP Model Output to Use Tv Corrected Parcel in CAPE/CIN/LI Computations

[thewxmann]

NOUS41 KWBC 121438

PNSWSH

Public Information Statement, Comment Request

National Weather Service Headquarters Washington, DC

938 AM EST Wed Jan 12 2011

To: Subscribers:

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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....kin/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

http://www.weather.g...s11virtcape.txt

For more on the virtual temperature correction, see: http://www.cimms.ou....al/virtual.html