SECTION 1. GLOBAL AND REGIONAL NUMERICAL WEATHER PREDICTION SYSTEMS

Many forecast agencies, such as ECMWF (European Centre for Medium-range Weather Forecasts), JMA (Japan Meteorological Agency), the U.S. National Weather Service, the U.S. Navy, etc., operate central weather facilities where operational global and regional NWP (numerical weather prediction) systems are run at least daily. These systems provide up to five to fifteen-day forecasts of meteorological parameters such as surface pressure, geopotential heights, humidity, temperature, relative vorticity and wind. TC forecasters routinely use the graphical and numerical output from these NWP systems (e.g., the constant pressure charts, cross sections, etc) to assist in determining how environmental processes, such as large-scale steering flows and vertical wind shear, may affect TC motions.

The following are brief descriptions of 1993 status of four NWP systems for forecasting TC motion; they are the U.S. Navy, the U.S. NMC, the ECMWF, and the JMA systems. All NWP models need initial tropical cyclone data (such as tropical cyclone location, surface pressure and strong wind distributions, etc.) that are either poorly or not routinely observed. A tropical cyclone bogus procedure is frequently required in the preparation of initial data for a NWP model. The bogus procedure usually adopts an idealized or composited data set representing a tropical cyclone circulation, and then combines it with other data from observation and NWP model for final data analysis. There is a lack of inter-model comparison regarding their long-term weather forecast skills. The novice weather forecaster may consult with the experienced one regarding mean model past performance on specific weather phenomena.

The U.S. Navy uses two operational NWP systems: NOGAPS (Naval Operational Global Atmospheric Prediction Systems) and NORAPS (Naval Operational Regional Atmospheric Prediction Systems).

(a) NOGAPS

The NOGAPS uses a global NWP model with triangular truncation wave number 79 (i.e., T79) and 18 vertical levels. This model has a nominal horizontal resolution of about 125 km at the equator. Since 1990, synthetic TC observations (Fiorino et al., 1993) were introduced into NOGAPS in an effort to better depict the TC signals (Fig. 5.1).

(b) NORAPS

The NORAPS uses a regional NWP model. This model is a finite-difference model which incorporates boundary layer physics for predicting complex circulation patterns (particularly those associated with complex terrain) with greater detail. The horizontal resolution is 40 km with 21 vertical levels. Seven of these vertical levels are below 850 hPa.

NORAPS runs in a limited number of regions. Future plans call for NORAPS to run in any region of the world on request and for the horizontal resolution to increase to between 10 and 40 km. This increase in resolution will require improvements in boundary conditions and terrain features as well as an increase in computational time.

The U.S. National Meteorological Center (NMC) system uses a global spectral model. The model horizontal resolution is T126, or approximately 80 km with 18 vertical levels. The output from the NMC model is disseminated to users via digital facsimile (DIFAX) directly from NMC, via the Navy and NOAA Oceanographic Data Distribution System (NODDS), and through various commercial companies. Lord (1991) reported a 'bogus system' for initial vortex circulations (i.e., tropical cyclone circulation) in the NMC global forecast model (or synthetic TC observations).

The ECMWF NWP model is a global spectral model that has T213 horizontal resolution (or approximately 47 km at the equator) with 31 vertical levels. This model represents a cooperative effort among eighteen member nations to provide global numerical weather predictions for its members. The eighteen countries are Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, Turkey, United Kingdom, and Yugoslavia.

The ECMWF uses bogus observations to improve the analysis and forecast of tropical cyclones in ECMWF global NWP system (ECMWF, 1989).

The Japan Meteorological Agency (JMA) is the designated regional TC forecast agency for the World Meteorological Organization (WMO), whose area of responsibility is the western North Pacific in the vicinity of Japan. JMA supports their active role in TC predictions by using the Global Spectral Model (GSM), and the Typhoon Model (which is briefly discussed in Appendix B) to develop TC motion forecast guidance. In addition to GSM, JMA has two operational regional models. They are the Asia Spectral Model and Japan Spectral Model.

The JMA is the Japanese civilian counterpart to U.S. FNMOC in the western North Pacific. The output from GSM is disseminated to users via weather facsimile broadcast through commercial companies.

The horizontal resolution of the GSM is T106, or approximately 94 km at the equator with 21 vertical levels.

Section 2

Chapter 5