1. 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
EMS Annual Meeting 2017
(Dublin, 6 Sep 2017)

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

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

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

5. 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

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

8. 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.

9. 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

10. 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)

11. Decomposition of FSS

12. 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

13. Best : 8h Worst : 24 h

14. 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

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

17. Assessing Hourly precipitation
Skill score

18. 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

19. Thanks for your attention