ECPC's U.S. Forecasts

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

Experimental Climate Prediction Center Scripps Institution of Oceanography UCSD, 0224 La Jolla, CA 92093

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,c). 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) in order to gain increased spatial resolution (50-25 km resolution) at shorter time scales (4xdaily to 4-weekly) for several selected regions (US, CA, SW). At even smaller space (2-km resolution) and time (8xdaily to daily) scales either the GSM or the RSM can force a corresponding nonhydrostatic mesoscale spectral model (MSM; Juang 1999; Roads et al. 1999c) for the Hawaiian Islands (Roads et al. 1999c). All atmospheric models are based upon the same physics used in the GSM and can 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 ocean and land surface models, we are also beginning to display surface water and energy forcings in conjunction with ocean and land forecasts. Long-range forecasts with these higher resolution models, forcings and corresponding output from the ocean and land models will be presented in later ELLFB issues. In this issue, we again present the GSM forecasts and verifying NCEP global analysis.

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 the significant biases that these models produce. We have now obtained a year's worth of forecasts and have used these forecasts to develop a forecast climatology dependent upon season as well as lag. 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. US Aug. GSM Forecasts, Aug. Analysis, Sept. GSM Forecasts

Fig. 1 compares the GSM monthly forecast for the Aug. (bottom panel) FWI with the Aug. FWI calculated from 4xdaily analysis (middle panel). The increased danger in the west and north central region was forecast well, as was the reduced danger in the Southwest. The interior Northwest, including Nevada, continues to be of concern. Increased FWI are now forecast to occur along the Southeast coast.

Fig. 2 compares the GSM monthly forecast for Aug. (bottom panel) precipitation with the Aug. precipitation calculated from 4xdaily analysis (middle panel). Precipitation was not forecast well during Aug., except perhaps over the dry Northeast. The dryness in the northern tier of states (especially the New England states) is forecast to continue. By contrast, precipitation is forecast to be above normal along the Gulf coast, somewhat consistent with the above normal precipitation in the eastern tropical Pacific.

Fig. 3 compares the GSM monthly forecast for Aug. (bottom panel) surface water (snow plus total soil moisture) with the Aug. surface water calculated from 4xdaily analysis (middle panel). The forecast pattern is similar to the previous months forecast indicating the slow time scales of the surface water, which is below normal in the interior and eastern portion of the country. The forecast surface water is also similar to the forecast precipitation, indicating the strong influence of precipitation on surface water. It should be noted that the forecast anomalies are somewhat weaker than the analysis anomalies during August, suggesting that the September forecasts may also be weak.

Fig. 4 compares the GSM monthly forecast for Aug. (bottom panel) temperature with the Aug. temperature calculated from 4xdaily analysis (middle panel). The August forecast pattern was similar to the analysis pattern, albeit weak. This large positive anomaly is forecast to continue and shift a bit to the east. This temperature anomaly, is consistent with the dry soil moisture anomaly, indicating the potential influence of soil moisture on the temperature.

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).

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 23^rd 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, Jun. 1999

Roads, J., S. Chen, D. Stevens, C. McCord, H. Juang, F. Fujioka, 1999c: Weather and climate analyses and forecasts at MHPCC. Seventh International Conference on High Performance Computing and Networking. Amsterdam, The Netherlands, April 12-14, 1999.