Validation of Satellite Rainfall Estimates over the Mid-latitudes

1. Validation of Satellite Rainfall Estimates over the Mid-latitudes Chris Kidd University of Birmingham, UK

2. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 The European Context Geographically diverse • Large extent of coastlines and interiors • Plains & mountains • Variable background – snow cover, sand etc. Meteorologically diverse – hence climatologically • Maritime and continental influences • Stratiform vs convective precipitation • Seasonal variations – frigid vs stifling temperatures Plenty of light rain intensities…

3. The European Region

4. Raw Data ECMWF IR PMW CPCMW CMORPH European radar data 3B40 3B41 3B42 Data processing Data Products Validation Data PMIR GPI FDA Remap data to PSG Statistics at 20km and 50km Production of web pages 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Data processing

5. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Results generation Visual analysis • Imagery of observations and estimates • Descriptive statistics • Contingency tables, conditional rain rates • Statistical analysis • Bias, ratio, RMSE, Correlation, Heidke score etc • In addition: • Cumulative distribution of accumulation • Analysis of occurrence of precipitation • Cumulative distribution of occurrence by intensities

6. IPWG European validation

7. Resampled/remapped imagery

8. Scatterplot

9. Rainfall intensity distribution

10. Occurrence of rainfall by intensity

11. Accumulation of rainfall by intensity

12. Statistics

15. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 21-day moving average

16. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Ratio of occurrence >0 (21-day)

17. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Ratio of occurrence >1 (21-day)

18. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Rainfall ratio (21-day)

19. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Heidke Score >=0 (21-day)

20. Heidke Score >=1 (21-day)

21. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Initial results • Satellite observations show significant seasonality • Rainfall occurrence is underestimated, except by the ECMWF model reanalysis (resolution?) • Model results suggest an element of inconsistency • Day-to-day variations in performance are large and…

22. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Future strategy Broaden range of algorithms/products • (more please!) Back-date study as far as possible • (radar/gauge and algorithm radar) Include other radar data where available • (Baltex, Spain, Italy?) Incorporate gauge data when available • (available <1999 for UK, European?) BUT….

23. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Light rainfall detection • An algorithm with a rain/no-rain boundary of 1mm/hr should underestimate the rainfall by the contribution of rainfall below 1mm/hr • Algorithms that cannot identify all the rain should underestimate rainfall totals • Algorithms that are bias-corrected must compensate the lack of light-rainfall contribution with rainfall at higher intensities – i.e. they will underestimate the low rainfall and overestimate high rainfall. • (In reality algorithms might detect some light rain, but not all)

24. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 “Ideal” algorithms All algorithms produce identical results to any validation data set… radar e40 3B40 3B41 3B42 Accumulation

25. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 … reality Algorithms tend to be tuned to minimise the longer-term biases – but are they ‘correct’? 3B40 radar e40 3B41 3B42 Accumulation

26. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Rainfall accumulation The make-up of the ‘intensities’ to the total is of critical importance: 3B40 radar e40 3B41 3B42 Accumulation

27. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Rainfall accumulation The make-up of the ‘intensities’ to the total is very important: 3B40 radar e40 3B41 3B42 Accumulation

28. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Accumulation of precipitation Jan 2004 Feb 2004 Mar 2004 Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42 Apr 2004 May 2004 Jun 2004

29. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Occurrence of precipitation Jan 2004 Feb 2004 Mar 2004 Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42 Apr 2004 May 2004 Jun 2004

30. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Accumulation of precipitation <1 mm/hr <2 mm/hr

31. 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Occurrence of precipitation <1 mm/hr <2 mm/hr

32. mean rainrates mean rainrates > thresholds Generate ‘global’ ratio Bias (ratio) correct mean rainrates Plot grid-sized ratios Rain/no-rain induced biases TRMM 2A25 data

33. -4.0 -2.0 -0.5 -1.0 2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004 Rain/no-rain induced biases • Differences in rain/no-rain boundaries reveal regional variations that do not exist in reality • Further complicated since rain/no-rain boundaries tend to differ over land/sea areas

34. Recommendations • There is a need to identify regions over which climate change can be observed with a high degree of confidence • Parameters need to be chosen that can be retrieved with a high degree of confidence – basic ones means that the causes of changes can be understood • Cross-talk between parameters needs to be reduced as much as possible • Long-term changes need to consider RFI contamination, particularly for coastal regions