CHANGES TO ETA MODEL

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: CHANGES TO ETA MODEL
• From: General Support <support@xxxxxxxxxxxxxxxx>
• Date: Thu, 21 Apr 1994 18:43:14 -0600

NATIONAL TECHNICAL INFORMATION MESSAGE 94-10
NATIONAL WEATHER SERVICE HEADQUARTERS
200 PM EDT WED APR 20 1994

TO:         USERS OF NWS' Eta MODEL

FROM:       MARY C. NEWTON
            ACTING CHIEF, SERVICES DEVELOPMENT BRANCH

SUBJECT:    CHANGES TO THE ETA MODEL

(THE FOLLOWING INFORMATION WAS PROVIDED BY MIKE BALDWIN, NMC
DEVELOPMENT DIVISION)

At 1200 UTC on April 19, 1994, several changes were made to the
Eta Model code.  These changes are intended to improve the
shallow (non-precipitating) convection scheme and the diurnal
cycle of short wave radiation at the surface.  The formulation of
shallow convection in the previous operational code contained
several tests which restricted the occurrence of shallow
convection a considerable amount of the time.  Modifications were
made to select a cloud top based upon the equilibrium level
rather than considering vertical humidity changes.  Several
restrictive tests were relaxed to allow more shallow convection
without violating the entropy and stability change requirements
of the previous code.  Results from tests including these changes
showed the number of points experiencing shallow convection
nearly doubled, with minor changes to the forecast fields
overall.  Areal coverage of light precipitation amounts decreased
slightly, while areal coverage of heavier amounts (greater than
0.75 inches per day) increased, resulting in slightly improved
precipitation skill scores overall.  Since shallow convection
acts to transport moisture up out of the boundary layer, relative
humidities will generally be higher above the top of the boundary
layer and lower below the boundary layer top in regions of
shallow convection.

In the previous version of the Eta Model, short wave radiation at
the surface was computed every two hours and held fixed during
the period between updates.  This was changed to allow the short
wave radiation at the surface to change at every time step,
resulting in a more realistic diurnal cycle of radiation at the
ground.  Overall, the impact upon the resulting forecasts is
minimal, besides a more reasonable diurnal cycle of surface
temperature and fluxes of sensible and latent heat.

Please call Geoff DiMego at (301) 763-8056 if you have any
questions.

END