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

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… 2 nd IPWG working group workshop, Monterey, CA October 2004

The European Region

2 nd IPWG working group workshop, Monterey, CA October B403B413B42 CMORPH ECMWF CPCMW Validation Data European radar data Production of web pages Statistics at 20km and 50km Remap data to PSG Data Products Raw Data Data processing PMWIR GPIPMIRFDA Data processing

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

IPWG European validation

Resampled/remapped imagery

Scatterplot

Rainfall intensity distribution

Occurrence of rainfall by intensity

Accumulation of rainfall by intensity

Statistics

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

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

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

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

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

Heidke Score >=1 (21-day)

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… 2 nd IPWG working group workshop, Monterey, CA 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…. 2 nd IPWG working group workshop, Monterey, CA 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) 2 nd IPWG working group workshop, Monterey, CA October 2004

radare403B403B413B42 “Ideal” algorithms All algorithms produce identical results to any validation data set… 2 nd IPWG working group workshop, Monterey, CA October 2004 Accumulation

radar e40 3B40 3B413B42 … reality Algorithms tend to be tuned to minimise the longer-term biases – but are they ‘correct’? 2 nd IPWG working group workshop, Monterey, CA October 2004 Accumulation

radar e40 3B40 3B413B42 Rainfall accumulation The make-up of the ‘intensities’ to the total is of critical importance: 2 nd IPWG working group workshop, Monterey, CA October 2004 Accumulation

radar e40 3B40 3B413B42 Rainfall accumulation The make-up of the ‘intensities’ to the total is very important: 2 nd IPWG working group workshop, Monterey, CA October 2004 Accumulation

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

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

Accumulation of precipitation <1 mm/hr <2 mm/hr 2 nd IPWG working group workshop, Monterey, CA October 2004

Occurrence of precipitation <1 mm/hr <2 mm/hr 2 nd IPWG working group workshop, Monterey, CA October 2004

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

-0.5 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 2 nd IPWG working group workshop, Monterey, CA October 2004

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

3B403B413B42 Data acquisition Day-01 Day-02 Day-03 Day-04 Day-… Day-20 Radar Global-IRSSM/I ECMWF Processing steps… Global-IR UoB PMIR GPI SSM/IUoB FDA Data processing 2 nd IPWG working group workshop, Monterey, CA October 2004

Are satellite rainfall algorithms correct? In one word, no. Why? General assumption that long-term rainfall amounts should be ‘correct’ - biases between validation and algorithm can be (and are) removed through bias- correction or ‘adjustments’ However, algorithms have ‘minimum detectable’ thresholds – i.e. the rain/no-rain boundary 2 nd IPWG working group workshop, Monterey, CA October 2004

Implications i)Current hydrological models that rely upon satellites estimates will be incorrect. Moreover, hydrological models treat different rainfall intensities differently ii)Climate change scenarios are varied, but imply that there will be a change in the distribution of rainfall intensities. If satellite estimates are already biased can we honestly detect these change – yet alone quantify them? 2 nd IPWG working group workshop, Monterey, CA October 2004

Conclusions Care needs to be taken when producing ‘correct’ results: In terms of rain occurrence (or area): Algorithms underestimate occurrence/extent by about half. Most of this occurs at light rainfall < 2mm/hr In terms of rain accumulation Although the light rainfall contribution relatively small, it is critical in obtaining spatial variations in rainfall correct 2 nd IPWG working group workshop, Monterey, CA October 2004

Occurrence of precipitation by intensity

radar e403B40 3B413B42 Rainfall occurrences April nd IPWG working group workshop, Monterey, CA October 2004

European radar data: 5km polar stereographic projection (equal area) ECMWF e40 reanalysis: nominally 1.125*x1.125 degree resolution 3B40 combinedMicro: 0.5x0.5 degree 3B41 calibratedIR: 0.5x0.5 degree 3B42 mergeIRMicro: 0.5x0.5 degree CPC microwave product CMORPH combined IR/MW product Model and satellite data remapped to radar data (20km and 50km), and compared on a daily time scale. Data sets 2 nd IPWG working group workshop, Monterey, CA October 2004