ECPC's U.S. Forecasts

contributed by J. Roads, S. -C. Chen, J. Ritchie

Experimental Climate Prediction Center Scripps Institution of Oceanography UCSD La Jolla, CA

1. ECPC's Global to Regional Atmospheric Forecast System

The Scripps Experimental Climate Prediction Center's (ECPC's) atmospheric forecast system was previously described by Roads et al. (1998, 1999b). At the largest space (global 200 km resolution) and time (monthly to seasonal) scales, we use the National Centers for Environmental Prediction's (NCEP's) medium range forecast (MRF) model or global spectral model (GSM; Kalnay et al. 1996; Roads et al. 1999a) and start these forecasts from the NCEP operational global analysis. The GSM then forces a regional spectral model (RSM; Juang and Kanamitsu, 1994; Juang et al. 1997; Chen et al. 1999, Anderson et al. 1999, Roads and Chen 1999) in order to gain increased spatial resolution (50-25 km resolution) at shorter time scales (4xdaily-7 days and weekly to 4-weeks) for several selected regions (US, CA, SW,Brazil). At even smaller space (2-km resolution) and time scales (8xdaily to daily) either the NCEP analysis or GSM can force a corresponding nonhydrostatic mesoscale spectral model (MSM; Juang 1999) for the Hawaiian Islands (Stevens et al. 1999). All atmospheric models are based upon the same physics used in the GSM and can, in principle, be updated as the GSM is updated. Output products from the atmospheric models include a fire weather index (FWI, see Roads et al. 1997) and associated variables such as 2m-temperature, relative humidity and 10m windspeed as well as precipitation and soil moisture for the globe and for several selected regions (e.g. US). Since the global and regional atmospheric models are now forcing an ocean model (Auad et al. 1999), we are also beginning to display surface water and energy forcings as well as monthly ocean forecasts. Ocean forecasts, forcings and corresponding output from the ocean and land models will be presented in later ELLFB issues.

2. Preliminary Forecast Skill Evaluations

The background climatology used to calculate anomalies for the analysis and forecasts previously came from the NCEP reanalysis (Kalnay et al. 1996) and did not take into account significant biases that these models can produce. We have now obtained a year's worth of global forecasts and have used these forecasts to develop a new forecast climatology dependent upon season as well as lag. (Similar efforts are underway for the regional forecasts.) As discussed by Roads et al. (1998), still limited forecast samples suggest that the GSM provides more skillful forecasts of temperature, precipitation, soil moisture, and fire weather index than persistence, even at long forecast ranges. Although the greatest skill occurs initially and then decays toward zero, daily, weekly or monthly forecast skill does not ever reach zero and forecasts averaged into monthly and seasonal averages demonstrate small but significant skill, which may be comparable to other long-range forecast methodologies.

3. Dec. 1999 US GSM and RSM ECPC Forecasts

Beginning with this issue, we show the RSM as well as the GSM forecasts for the US region. In particular, Figs. 1-3 show the GSM monthly anomaly forecast for Dec. 1999 along with the corresponding RSM monthly anomaly forecast of FWI, 2-m surface temperature, and precipitation. Currently both the GSM and the RSM use the same GSM climatology to calculate the anomalies. However, we are trying to develop a more suitable RSM climatology for the RSM simulations.

As shown in Fig. 1, during Dec. 1999, both the GSM and the RSM are predicting lower than normal FWI just about everywhere. The major differences between the GSM and RSM occur in regions where the differences are small and insignificant. Developing a more suitable RSM climatology may reduce these differences further.

As shown in Fig. 2, the GSM and RSM temperature Dec. 1999 forecasts have more similar features. Slightly above normal temperatures are expected in the US West, especially in the RSM, and less than normal temperatures are expected in the Southeast. Note that the negative anomalies are somewhat larger in the RSM, which might be partially attributed to the different orographies used for the GSM and the RSM. Again, developing a more suitable climatology for the RSM may reduce this perceived bias.

As shown in Fig. 3, the GSM and RSM precipitation Dec. 1999 forecasts also have similar features as well as some small differences. Above normal precipitation is expected in the northwest in the GSM. However, this above normal precipitation is more intense over the coastal ranges in the RSM. In both models, dry anomalies are expected in the southern Gulf of Mexico. The RSM shows some minor shifts landward along the east coast in comparison to the GSM.

Other experimental GSM and RSM forecast fields (soil moisture, wind speed, relative humidity and additional forecast months) can be found at http://meteora.ucsd.edu/ecpc/projects/ellfb/.

References:

Anderson, B.T., J. O. Roads, S. -C. Chen, and H. -M. Huang, 1999: Regional Modeling of the Low-level Monsoon Winds Over the Gulf of California and Southwest United States: Simulation and Validation, (submitted).

Auad, G., J. Roads, A. Miller, D. Cayan, W. White, 1999: Comparison of wind stresses and surface heat fluxes from the COADS, FSU and NCEP data sets. (submitted)

Chen, S. -C., J. O. Roads, H. H. -M. Juang, and M. Kanamitsu, 1999 Global to Regional Simulations of California Wintertime Precipitation. J. Geophys. Res. (in press, special precipitation issue)

Juang, H. -M. H., and M. Kanamitsu, 1994: The NMC nested regional spectral model. Mon. Wea. Rev., 122, 3-26.

Juang, H.-M. H., S.-Y. Hong and M. Kanamitsu, 1997: The NCEP regional spectral model: an update. Bulletin Amer. Meteor. Soc., 78, 2125-2143.

Juang, H. -M. H., 1999: The EMC/NCEP mesoscale spectral model: A revised version of the nonhydrostatic regional spectral model. Mon. Wea. Rev., submitted.

Kalnay, E. et al., 1996: The NMC/NCAR reanalysis project, Bull. Am. Meteor. Soc., 77, 437- 471, 1996.

Roads, J.O., S. -C. Chen, F.M. Fujioka, H. Juang, and M. Kanamitsu. 1997. Global to Regional Fire Weather Forecasts. Int. Forest Fire News, 33-37.

Roads, J., Chen, S. -C., Ritchie, J., 1998: Evaluation of the Experimental Climate Prediction Center's global to regional and daily to seasonal prediction system. Proceedings of the 23rd Annual Climate Diagnostics Meeting. Miami, Florida

Roads, J. O., S. -C. Chen, M. Kanamitsu, H. Juang, 1999a: Surface Water Characteristics in NCEP's Reanalysis and Global Spectral Model. J. Geophys. Res.-Atmos. (in press, special GCIP Issue)

Roads, J., S. -C. Chen, J. Ritchie, 1999b: ECPC's Weekly to Seasonal U.S. Forecasts of FWI, Soil Moisture, and Precipitation. ELLFB bulletin, Sept. 1999

Roads, J. and S. Chen, 1999: Surface Water and Energy Budgets in the NCEP Regional Spectral Model. J. Geophys. Res. (submitted)

Stevens, D. D. Funayama, J. Roads, S. Chen, W. Smith, C. McCord, H. Juang, F. Fujioka, 1999: Experimental short-term weather forecasts for Hawaii. MHPCC application briefs 1999. (Available from MHPCC, Kihei, Maui, HI 96753), 19.

Fig. 1 FWI (dimensionless) Dec. 1999 forecast anomalies: (upper) GSM; (lower) RSM.

Fig. 2 Temperature (C) Dec. 1999 forecast anomalies: (upper) GSM; (lower) RSM.

Fig. 3 Precipitation (mm) Dec. 1999 forecast anomalies: (upper) GSM; (lower) RSM.