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All,This is a final 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 planned date of implementation is 15 May 2001 (1200 GMT). We have made extensive internal evaluation of the changes as well as NCEP-wide evaluation. The attached Notice of Intent to Change includes Web sites where daily forecast maps and verification statistics can be found. A good place to read about the changes is the draft Technical Procedure Bulletin at :
http://sgi62.wwb.noaa.gov:8080/tpb97/CLW/html/clw.html 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 and emissivity 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 water vapor climate of the model forecasts.
The major results of the new package are:1. Significant improvement in skill for forecasts of atmospheric circulation in the medium range (3-7 days) and the extended range (7-10 days). Some bias toward less-than-observed wind speeds in mid and upper levels in the extratropics and low levels in the tropics.
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.
4. The lower atmosphere is colder in general. The long-standing warm bias near the surface in the West has been replaced with a cool bias. We are working to reduce the biases in a follow-up implementation.
5. The wind speeds aloft in the forecasts have small bias in the first two days and develop a negative bias with time.
6. The precipitation forecasts in the U.S. show a slightly larger wet bias than the operational for small amounts (< 0.1"), but the overall totals are similar.
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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