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RSMC Tokyo-Typhoon Center

Technical Review No.4  (March 2001)

   Outline of the New Typhoon Prediction Models at JMA
   Hiroshi MINO
   Numerical Prediction Division, Japan Meteorological Agency
   Masashi NAGATA
   National Typhoon Center, Japan Meteorological Agency

  Aiming at improving tropical cyclone (TC) track and intensity forecasts as well as general weather forecasts, the Japan Meteorological Agency (JMA) started the operation of a new suite of numerical prediction models on 1 March 2001 on the occasion of the installation of an advanced computer system (Computer System for Meteorological Services: COSMETS) in place of the 5-year-old predecessor. Both horizontal and vertical resolutions have been enhanced by 1.67 times in the Typhoon Model (TYM) while only the vertical resolution has been by 1.33 times in the Global Spectral Model (GSM). The new TYM runs four times a day to produce 84-hour predictions for each of a maximum of two TCs in the western North Pacific while the new GSM runs twice a day to produce 90-hour predictions for all TCs of tropical storm intensity or higher in the same basin.
  Since TYM and a simple guidance scheme based on it have been shown to produce 48-hour TC intensity predictions with fairly high accuracy, JMA will start 48-hour TC intensity forecasts in June 2001. JMA has a plan to extend intensity forecasts up to 72h, soon after it confirms that the combination of the new TYM and the guidance scheme makes 72-hour intensity predictions with enough accuracy for TCs in the coming 2001 season.
  During the 5-year period of the new computer system, a further enhancement of the resolutions of TYM by 1.20 times, the introduction of variational data assimilation schemes into global analysis, and the coupling of TYM with an ocean mixed-layer model are scheduled. These are expected to further improve performance of the numerical models in TC track and intensity predictions.
   Text [PDF 1.3MB]

   Impact of a Recent Major Version-up of the Global Spectral Model (GSM) at JMA on Tropical Cyclone Predictions
   Ken-ichi KUMA, Hiroto KITAGAWA, Hiroshi MINO
   Numerical Prediction Division, Japan Meteorological Agency
   Masashi NAGATA
   National Typhoon Center, Japan Meteorological Agency

  The Global Spectral Model (GSM) at the Japan Meteorological Agency (JMA) underwent a major version-up in December 1999. The version-up included the incorporation of cloud water content as a prognostic variable, a modification of the prognostic Arakawa-Schubert cumulus parameterization, and the calculation of direct effect of aerosols on short-wave radiation. A verification against a best track data has been made of predictions of tropical cyclones (TCs) in the western North Pacific by GSM for all the cases in the year 2000 and has been compared with those for the previous four-year period (1996-1999). Results show that the version-up produced significant improvements both in track and intensity predictions of TCs: the mean track error relative to that of the persistency forecast has been reduced by about 20(10)% for 24(72) h predictions, while the bias and the root-mean-square error (RMSE) of maximum wind speed have been reduced drastically from 14(17) m/s to 8(6) m/s and from 15(19) m/s to 10(11) m/s for 24(72) h predictions, respectively.
   Text [PDF 0.4MB]

   A Simple Guidance Scheme for Tropical Cyclone Predictions
   Masashi NAGATA, Jun TONOSHIRO
   National Typhoon Center, Japan Meteorological Agency

  A simple guidance scheme has been developed for tropical cyclone (TC) track forecast. The scheme consists of bias correction, initial adjustment, and ensemble averaging of numerical model TC track predictions. The bias correction is based on a verification of numerical model track predictions for a four-year period of 1996-1999. The scheme is tested with independent data in the year 2000 as well as with dependent data in the four-year period of 1996-1999. Results show that initial adjustment works well to reduce track prediction errors while bias correction does not work well in 2000 presumably because of a significant change in systematic error between the four-year period and the year 2000. Ensemble averaging of two numerical models does not work well in 2000, which is probably due to a difference in performance between the two models: the Global Spectral Model (GSM) performed much better in 2000 than in the four-year while the Typhoon Model (TYM) performed only slightly better than in 2000.
  Another simple guidance scheme similar to that for track forecast has been developed for TC intensity forecast. It consists of bias correction and initial adjustment of numerical model TC intensity predictions but does not employ ensemble averaging of numerical model TC intensity predictions because of a large difference in performance between the two numerical models. The same kind of test as that for track forecast is performed. Results show that both initial adjustment and bias correction work well to reduce intensity prediction error in 2000.
  In consideration of the verifications above and probable changes in TC prediction performance expected to result from the version-ups of GSM and TYM in March 2001, forecasters at the RSMC Tokyo - Typhoon Center will use a guidance scheme which employs only initial adjustment both for track and intensity forecasts in the coming 2001 season. Further investigation is required at the Center before the forecasters at the Center make a decision on if they should adopt bias correction and/or ensemble averaging besides initial adjustment in the guidance scheme.
    Text [PDF 0.9MB]

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