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