How low can you go? Retrieval of light precipitation in mid-latitudes
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How low can you go?Retrieval of light precipitation in mid-latitudes
Chris Kidd
School of Geography, Earth and Environmental Science
The University of Birmingham
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Truth? 8 Gauge / 4 MRR comparison
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Zonal distribution of light precipitation
• Light rainfall becomes increasingly important towards the polar regions
• COADS data shows light precipitation occurrence >80%; ~50% in mid-latitudes
• European radar suggests ~85% of precipitation <1 mmh-1 (35%<0.1 mmh-1)
• Accumulation of light precipitation is small however… particularly in the Tropics
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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-0.5
Rain/no-rain induced biases
-1.0
• 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
Problem:
• if a satellite estimate cannot retrieve light rain, it should underestimate rain totals
• corrections are usually made to correct this bias, so that it matches the 'real' rainfall
• this can lead to unintended regional biases
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Rainfall occurrence (Bham)
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Rain intensity (mm/hr)
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1 minute rain rates derived from Doppler radar (10 months data)
1 mmh-1
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Rainfall occurrence
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Rain intensity (mm/hr)
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1 minute rain rates derived from 4 Doppler radars (4x10 months data)
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Rainfall accumulation
1 minute rain rates derived from 4 Doppler radars (4x10 months data)
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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How well are we doing?
Occurrence of rainfall:Cumulative percentage of the occurrence of rainfall by daily totals- significant occurrence < 2 mm/day
Accumulation of rainfall:Mean daily rainfall with contributionby daily rainfall totals- accumulations > 4 mm/day important
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Rainfall occurrence and accumulation
Occurrence plots:
Radar ~ 50-60% rain occurrence
ECMWF model ~ 75-85% occurrence
NOGAPS model ~ 65-75% occurrence
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Radar ~ 40-50% rain occurrence
Satellite algorithms:
3B42RT ~15-25% occurrence(MPA-RT)
NRLgeo ~25-35% occurrence
CMORPH ~25-60% occurrence
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Radar ~ 40-50% rain occurrence
Satellite algorithms:
3B40-RT ~10-25% occurrence
NRLpmw ~10-25% occurrence
CMORPH ~15-40% occurrence
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Low rainfall identification
How well to algorithms identify light rainfall?
- Difficult to assess since the IPWG products are daily estimates, not instantaneous…
However, the radar data, available every 15 minutes, can be used:
Radar sensitivity to light rain rates is reduced by setting rain rates below a certain threshold to zero – thus matching the algorithms insensitivity.
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Effect of removing light rain rates
<0.0 → 0.0 <0.5 → 0.0 <1.0 → 0.0 <2.0 → 0.0
Con
vect
ive
Fro
ntal
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Rain area
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radar rain threshold
% o
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In terms of rain area: algorithms underestimate the area, e.g. CMORPH has a threshold of about 0.5 mm/hr; AMSU ~1.5 mm/hr for stratiform and > 2 mm/hr for convective.
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Rain totals ratio
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Radar rain threshold
% o
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In terms of rain amount the problem is less acute: some over-estimate rain amounts (while underestimating the rain areas); NRLgeo has a threshold ~1.2 and 0.6 mm/hr.
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3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Algorithm performance
Algorithms:• generally underestimate the rain area (0.6-0.9)• are equally split on rain amount (0.6-1.4)
Consequently algorithms overestimate the conditional rainfall
In addition, different rain situations create different problems:
CMORPH ~ same for rain area for both frontal and convective rainfall, but very different for total rainfall
AMSU which gets the rain totals about right, fails to get the rain areas right.
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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Conclusions
• Most products underestimate the accumulation of rainfall over the European region
• All products underestimate the occurrence of rainfall
- Statistics often confuse the issue (CC/RMSE/bias): more light rain tends to produce poorer statistics
• Instrument noise can be problematic (e.g. AMSR)• Surface screening - potential problems with 'false alarms'
over cold/snow surfaces – plus RFI issues
• Satellite observations are capable of retrieving light rainfall – at least statistically
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006
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RFI over Europe
Blue = 6.925 GHz Green = 10.65 GHz Red = 18.7 GHz
3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006