Changes to the 2002 NCEP Operational MRF/AVN
Global Analysis/Forecast System:
Mark Iredell, Hua-Lu Pan and Peter Caplan
National Centers for Environmental Prediction
Global Modeling Branch
W/NP23, World Weather Building,
Washington DC 20233, USA
1. Introduction. On 29 Oct 2002, the followingchanges will be implemented in the MRF/AVN analysis /forecast system (now called the Global Forecast System):
2. Resolution changes.
3. Arrangement of layers.
The distribution of layers in the vertical is shown in Fig. 1 and in the table below:
4. Other changes
5.Impact of the changes on model behavior - subjective evaluations
from the field.
6. Model performance statistics vs. analysis - mass and wind fields In evaluations of forecast mass and wind fields against verifying analyses, the new package showed consistent improvement through a period of testing extending from September 2001 through April 2002 The 500-hPa anomaly correlation dieoff curves for the period 10 October 14 April 14, 2002 given in Fig. 2a for the Northern Hemisphere and Fig. 2b for the Southern Hemisphere, show the same general picture. The new model (MRFX, blue)gets better scores on average for all forecast lengths out to 7-8 days, with slightly less skill for days 8-9. This is true in both hemispheres, for all wave number groups shown (an additional one-month test of the "further" analysis changes alluded to above produced comparable results in the Northern Hemisphere and further improvement in the Southern Hemisphere). It is interesting to note that in spite of the decline after day 7, the score for the week-2 average forecast is noticeably better in the new than in the operational(current), as can be seen from Fig. 3 The consistency of performance over time is shown in Fig. 4 in the form of month-by-month averages, new vs current for 5-day forecasts. Another measure of performance, the frequency distribution of scores is shown in Fig. 5 , which indicates that the new model had a lower frequency of poor scores for 5-day forecasts at 500 hPa. The daily 5-day scores (with a 1-2-1 time smoother) show very similar behavior in the two models (Fig. 6). , In the tropics (20S-20N), Fig. 7 shows that there was no significant difference between the old and new models for 3-day wind forecasts at 850 hPa and a slight increase in error at 200 hPa, as verified against analyses.
7. Model performance statistics vs. analysis - temperature The current model has a cold bias over land in the cold season. The new model shows similar behavior but with a somewhat reduced magnitude of the bias at forecast day 5, as shown in Fig. 8 The reduction in bias continues growing with time out to 16 days.
8. Verifications against rawinsondes Over a period of five months, the verifications against rawinsonde observations for two-day wind forecasts at 200 hPa smoothed with one-week running averages for the Northern Hemisphere indicate little difference between the models (Fig. 9a) but when broken down by area, the results did reveal small differences over Asia, where the current model was better (Fig. 9b) and over North America, where the new model won. (Fig. 9c)
8. Precipitation verifications Based on observations over the Continental U.S., the new model yielded much the same threat scores as the current for low and moderate amounts, but somewhat higher for heavy amounts (over 1.5 inches = 38 mm)for forecasts for the 24 hours ending at hour 36. Results only for the month of April are shown (Fig. 10a), but similar results were found in the preceding months. The bias scores in Fig. 10b show that the new model is comparable to the current for small amounts, but wetter at moderate and heavy amounts. In the wettest categories it seems to be considerably closer to a bias of 1.0 than the current model.
9. Tropical storm tracks Judging from the limited number of cases available, the two models performed at comparable levels in both the Atlantic Fig. 11a, and the Pacific, Fig. 11b, with some questions to be resolved about longer time ranges in the Atlantic.
10. Summary and future work
Our conclusions are that the increase of horizontal and vertical resolution in the
new model has improved forecasts slightly at almost all time ranges. Objective
evaluations indicate slightly better mass and wind fields but show little effect on
the accuracy of tropical storm tracks and precipitation forecasts
over the limited time/area where these were evaluated.
Subjective evaluations from various users mostly indicate little difference in model
behavior, but an overall preference for the perceived better definition of
synoptic features in the new model.