20010226: MRF changes-

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: 20010226: MRF changes-
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Mon, 26 Feb 2001 09:10:38 -0700

All,

  This is a preliminary notice of a major change in some of the
physical process parameterization in the NCEP Medium Range Forecast
(MRF) model. The forecast suites that will be affected are the AVN, the
MRF, and the global ensemble suites. The target window of implementation
is April to June 2001. We have made extensive internal evaluation of the
changes and are in the process of making NCEP-wide evaluation. We plan
to finish the evaluation by the end of March and go to the NWS Committee
on Analysis and Forecast Technique Implementation (CAFTI) for approval
in April. We welcome comments from all users to help with the evaluation
process. The attached Notice of Intent to Change includes Web sites
where daily forecast maps and verification statistics can be found.

 The major changes are the following:

1. We are adding a prognostic cloud water/ice process parameterization
into the model. We currently use the model relative humidity to estimate
the cloud amount. The new procedure explicitly models the condensation
and precipitation processes. In addition, the cloud optical depth used
in the radiation calculation is now computed from the cloud water/ice
which should make the cloud radiation feedback process more
realistically modelled. The detainment of water/ice from the cumulus
clouds is also included in the parameterization. This is an important
source of cirrus in the tropics.

2. We are including a cumulus momentum mixing algorithm in the cumulus
parameterization scheme. The AVN/MRF model has been plagued with false
alarm storms in the tropics for several years and the addition of this
process greatly reduces the occurrence of false alarm storms. A crude
form of the parameterization of the pressure effect on cloud parcels is
included in this version.

3. We are modifying the cumulus cloud parameterization scheme to
simulate the effect of an ensemble of clouds.  This is accomplished by
randomly selecting cloud tops from a range of possible heights at each
time step. The current procedure led to excess drying of the mid and
upper troposphere and the new procedure improves the moisture climate of
the model forecasts.


  The major results of the new package are:

1. Significant improvement of the atmosphere circulation forecast skill
in the medium range (3-7 days) and the extended range (7-10 days). Most
of the improvement appears in the summer hemisphere. Our forecast skills
are generally lower in the summer than in the winter and this change
lowers the gap.

2. Significant reduction of the false alarm predictions of the tropical
storms. Analysis improvements have led to a reduction of false alarm
storms in the model initial states. The 3-5 day AVN/MRF forecasts
continue to generate many false alarm storms presently. In our extended
period of parallel runs and retrospective runs of the model, we noticed
very few false alarm storms in the forecast. The real storms are still
predicted by the model but with weaker intensity than the current model
predictions.

3. Improvement of tropical circulation forecast skills. The changes in
convection and cloud water/ice algorithms have led to a reduction of
noises in the forecasts in tropics. This has led to improved skill
scores.

 Please feel free to contact me if you have any question about this set
of changes.

Hua-Lu Pan
Chief, Global Modeling Branch, EMC, NCEP

--

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      Hua-Lu Pan
      W/NP23, RM 204, WWBG
      NOAA
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      Telephone and fax number:
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      fax: 301-763-8545

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