validation of satellite rainfall estimates over the mid-latitudes
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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…
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
The European
Region
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
3B40 3B41 3B42
CMORPH
ECMWF
CPCMW
Validation Data
Europeanradar data
Production of web pages
Statistics at 20km and 50km
Remap data to PSG
Data Products
Raw Data
Data processing
PMWIR
GPIPMIR FDA
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
2nd IPWG working group workshop, Monterey, CA. 25-28 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
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Ratio of occurrence >0 (21-day)
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Ratio of occurrence >1 (21-day)
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Rainfall ratio (21-day)
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Heidke Score >=0 (21-day)
2nd IPWG working group workshop, Monterey, CA. 25-28 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…
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….
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)
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
radar e40 3B40 3B41 3B42
“Ideal” algorithms
All algorithms produce identical results to any validation data set…
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Acc
umul
atio
n
radare40
3B40
3B41 3B42
… reality
Algorithms tend to be tuned to minimise the longer-term biases – but are they ‘correct’?
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Acc
umul
atio
n
radare40
3B40
3B41 3B42
Rainfall accumulation
The make-up of the ‘intensities’ to the total is of critical importance:
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Acc
umul
atio
n
radare40
3B40
3B41 3B42
Rainfall accumulation
The make-up of the ‘intensities’ to the total is very important:
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Acc
umul
atio
n
Jan2004
Feb2004
Mar2004
Apr2004
May2004
Jun2004
Accumulation of precipitation
Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Jan 2004 Feb 2004 Mar 2004
Apr 2004 May 2004 Jun 2004
Occurrence of precipitation
Radar e40 3B40 3B41 3B42 Radar e40 3B40 3B41 3B42
Radar e40 3B40 3B41 3B42
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Accumulation of precipitation
<1 mm/hr
<2 mm/hr
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Occurrence of precipitation
<1 mm/hr
<2 mm/hr
2nd IPWG working group workshop, Monterey, CA. 25-28 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
-1.0-2.0-4.0
• 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
2nd IPWG working group workshop, Monterey, CA. 25-28 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
3B40 3B41 3B42
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/I UoB FDA
Data processing
2nd IPWG working group workshop, Monterey, CA. 25-28 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
2nd IPWG working group workshop, Monterey, CA. 25-28 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?
2nd IPWG working group workshop, Monterey, CA. 25-28 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
http://kermit.bham.ac.uk/~kidd/ipwg_eu/ipwg_eu.html
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
Occurrence of precipitation by intensity
0.0-0.50.5-11-22-44-88-1616-3232-6464-128128+
radar e40 3B40 3B41 3B42
Rainfall occurrences
April2004
2nd IPWG working group workshop, Monterey, CA. 25-28 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
2nd IPWG working group workshop, Monterey, CA. 25-28 October 2004
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