20010810: model changes

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: 20010810: model changes
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Fri, 10 Aug 2001 08:22:10 -0600

All,

   We plan to make a set of very minor changes to the MRF/AVN system
starting at the 12Z run on 14 August 2001 (barring critical weather day
delays). The changes are corrections to the major implementation we made
in May and minor tuning of cloud parameters.  The notice of intent to
change is listed below:

Notice of Intent to Change the NCEP AVN/MRF Forecast System

Brief description of change(s):
  1) Minor corrections are made to gravity wave drag, random cloud types
in the convection, and evaporation with trace snow on the ground.
  2) Small modification to auto-conversion rate for ice is done to
reduce light precipitation. Effective radius for ice crystals is
calculated empirically based on observations of Heymsfield and
McFarquhar (1996). Evaporation of falling precipitation is also slightly
increased. Critical relative humidity for condensation is now made to be
a function of resolution.

Reason for Change(s):
During testing of the package we implemented in May 2001, several minor
coding bugs were discovered. These bugs were not fixed in the package to
ensure that clean tests were done for extended period of time. Tests
were done after the implementation and we found no major impact to these
changes.  We are now ready to fix the minor errors in the model. In
addition, slight tuning of the cloud and radiation parameters has also
been done to correct biases we found with the new cloud parameterization
scheme.

Schedule for change:  1200 GMT 14 August, 2001

Final testing completed:  completed July 2001.
CAFTI approval :  not required
Expected implementation : 14 August 2001.

Description of testing:

Statistical summaries :
http://sgi62.wwb.noaa.gov:8080/STATS/STATS.html

Forecast examples :
http://sgi62.wwb.noaa.gov:8080/STATS/MAPS.html

Anticipated impact on forecasts:  Improvement of the spin up
characteristics in the tropics. Stronger tropical storm are maintained
in the forecasts. Slightly less precipitation bias over mid-latitude
land area.

Human point of contact for further information: Pete Caplan
(301-763-8000 ext 7229, peter.caplan@xxxxxxxx)

Field Evaluation: We have been working with the NCEP HPC and TPC
forecasters and the forecaster responses have generally been positive.

Approvals: EMC Director:    Stephen J. Lord
Date: 7 August 2001