Numerical Weather Prediction Center (NWPC), Beijing, China 100081 Assessing Hourly Precipitation Forecast Skill with the Fractions Skill Score Zhao Bin Verification Group Numerical Weather Prediction Center (NWPC), Beijing, China 100081 EMS Annual Meeting 2017 (Dublin, 6 Sep 2017)

Outline: Motivation Application of FSS on Precipitation Evaluation Assessing Hourly Skill with the Fractions Skill Score Comprehensive score Summary

Motivation It can tell: Which scale is predictable and skillful But How to use it as traditional skill score ?

Application of FSS on Precipitation Evaluation FCST: GRAPES_MESO v4.0 OBS: QPE Time：20160701-20160831 Initial：00UTC Variables：24h & 1h Resolution：0.1 deg Define: the skillful scale of 24h heavy rain is reference scale

TS can get a similar trend with FSS with small precipitation threshold and represents unstable with higher thresholds FSS has obvious advantages to distinguish precipitation differences during the time series, especially in the assessment of heavy rainfall

If the reference scale is acceptable ? It shows a similar trend with different scales and the difference is mainly shown in the magnitude

Some scores is less than 0. 5, but ACC less than 0 Some scores is less than 0.5, but ACC less than 0.6 still has analytical value.

South Part : 20︒N-30︒N，110︒E-125︒E North Part: 45︒N-55︒N，120︒E-135︒E FSS TS 0.1mm 0.84 0.61 0.73 0.47 10mm 0.65 0.26 0.21 25mm 0.44 0.15 0.27 0.09 50mm 0.18 0.07 South Part : 20︒N-30︒N，110︒E-125︒E North Part: 45︒N-55︒N，120︒E-135︒E

Assessing Hourly Skill with the Fractions Skill Score model spin-up period (6 h) should effect assessment stability the skillful scale of 1.0mm/h is reference scale (170km)

Decomposition of FSS

The systematic error play a insignificant role in FBS even can be ignored The coefficient between forecast and observation is similar to FSS, that is, FSS can be a useful index to indicate correlation The dispersion of observation exhibits a diurnal cycle and the standard deviation of forecast is similar to the pattern of reference maximal FBS

Best : 8h Worst : 24 h

Even in the poor forecast period still shows advantage and it is easy to make an effective statistical analysis of the comprehensive performance of daily precipitation by means of the spatial test method, even with heavy rainfall threshold

Comprehensive score The weight coefficient of the Gaussian distribution: if if

Assessing Hourly precipitation Skill score

Summary FSS represents better ability to describe the real precipitation forecast. With regard to the mean squared error of fraction (FBS), the role of systematic error is insignificant and ignored. The dispersion of observation exhibits a diurnal cycle and the standard deviation of forecast is similar to the pattern of reference maximal FBS FSS maintain a high degree of consistency with the correlation coefficient between forecast and observed fractions FSS has obvious advantages in precipitation assessment, especially for the heavy rainfall forecast Comprehensive score can synthesize the scores of each scales and it represents advantage clearly

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