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Evaluation of NOAA Reforecast V2 Rainfall over India

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Wednesday, 7 January 2015
Raghavendra Ashrit, National Centre for Medium Range Weather Forecasting, Noida, U.P, India; and A. Roy, G. Iyengar, A. K. Mitra, and E. N. Rajagopal

Evaluation of NOAA Reforecast V2 Rainfall over India

Raghavendra Ashrit, Arka Roy, Gopal Iyengar, A. K. Mitra, E. N. Rajagopal

raghu.ashrit@nic.in, raghu@ncmrwf.gov.in

NCMRWF, A-50, Institutional Area Phase-II, Sec-62, Noida (India)

ABSTRACT

The 2nd-generation Reforecast Project of NOAA has produced a data set of historical weather forecasts generated with a fixed numerical model, using the 2012 version of NCEP's Global Ensemble Forecasting System (GEFS) (Hamil et al. 2013). This Reforecast V2 data set consists of an 11-member ensemble of forecasts, produced every day from 00 UTC initial conditions from Dec 1984 to present. The horizontal resolution of GEFS is T254 (about 50 km) out to 8 days, and T190 (about 70 km) from 8-16 days. The reforecast data set are useful for a number of applications, such as statistical post-processing, diagnosis of how well the model may forecast a phenomenon, and the initialization of regional model reforecasts etc.

In this study the Reforecast V2 rainfall data is evaluated over the India. The evaluation is carried out using the 1̊ x 1̊ grid NCMRWF-IMD Satellite and Gauge merged rainfall data over India. The rainfall forecasts (Day1-Day7) from 11-member ensemble is evaluated from 1st June-30th Sept for 15 years (1998-2012). The spatial mean rainfall (averaged over 8-38N and 68-98E) daily climatology shown in Figure 1 indicated huge bias in the Day1, Day-2, and Day-3 rainfall forecasts. The biases in the Day-4, Day-5, Day-6 and Day-7 forecasts are remarkably lower. The study summarizes results from detailed investigation of the Reforecast V2 rainfall data over India using standard methods used for verification of ensemble forecasts like ROC and Reliability diagrams along with Brier Score (BS). Further the use of Reforecast V2 climatology alongside the real-time ensemble predictions based on the NCMRWF Global Ensemble Forecast System (NGEFS) is demonstrated for daily Probabilistic Quantitative Precipitation Forecasts (PQPF) over India during monsoon (JJAS) 2014.

References:

Hamill, T. M., J. S. Whitaker, G. T.Bates, D. R. Murray, M. Fiorino, T. J. Galarneau, Jr., Y. Zhu, and W. Lapenta, 2013: NOAA's second generation global medium range ensemble forecast data set. Bull. Amer. Meteor. Soc., Bull. Amer. Meteor. Soc., 94, 15531565.