Comparison of warm rain detection
and quantification from spaceborne
passive microwave and radar sensors
Matt Lebsock
Chris Kummerow
Graeme Stephens
Tristan L’Ecuyer
Questions
1. What does CloudSat tell us about warm rain?
2. How does this compare with AMSR/E and PR?
3. Can this inform our understanding of the
capabilities of GPM?
The Roll of Various Satellite
Rainfall Sensors
• Passive Microwave (e.g. AMSR/E)
– Long term climate record & Frequent global sampling
– Imprecise, Cloud/Rain separation
• TRMM-Precipitation Radar (PR)
– The standard
– Minimum detectable signal (0.5 – 1.0 mmh-1)
• CloudSat-Cloud Profiling Radar (CPR)
– Extreme sensitivity to light rain
– Signal saturates in heavy rain
Complementary role (light rain)
CloudSat Algorithm Sensitivity:
Reflectivity vs. Attenuation
Observations
• Challenges
1.
2.
3.
•
Attenuation
Multiple-scattering
Limited sensitivity at high
rates
Opportunities
1.
2.
Extreme sensitivity to
light/moderate rain
~1km Spatial resolution

Useful for quantifying rain
from shallow isolated moist
convection that other sensors
may miss
Rain Rates
Attenuation
Solution
Reflectivity
Solution
Lebsock & L’Ecuyer, 2011 JGR
Precipitation Occurrence
from CloudSat
Warm Rain Distribution
• Global annual average
intensity = 0.23 mmd-1
 ~7% of global precipitation
• Areas of largest
accumulation:
 East-Pacific ITCZ
 Subtropical cumulus
regimes (not Scu)
Liu & Zipser, 2009 J. Clim.
CloudSat-AMSR/E GPROF Comparison
1. AMSR-E subset to CloudSat
ground Track
2. Common Data screening:
– 1 degree boxes in which CloudSat
observes no clouds colder than
273 K retained.
– Warm rain near deep convection
or cirrus screened.
Large regional bias remains
Missed
Accumulation
(89%)
Missed
Accumulation
(11%)
GPROF database bias
• GPROF database is stratified in
terms of SST and CWV
• GPROF database is built from
TRMM-PR/TMI observations

Extended Database
Bias inherent in the PR will
manifest itself in AMSR/E
product.
Regime dependent
biases separate sharply
CloudSat-TRMM/PR Comparison
CloudSat
TRMMPR
Hit
Miss
Hit
429
133
Miss
4204
77139
Year: 2006, DOY: 227, 2oS, 95oE, CloudSat Granual: 01594
• PR Probability of warm rain detection = (11.8%)
–
(unadjusted = 9.3%)
• PR/CloudSat warm rain accumulation (46.6%)
–
–
Weighted by area
Oceanic TRMM region
Colocation mismatch:
Requires bias adjustment
PR probability of detection:
resolution vs. sensitivity
Resolution
limited
Sensitivity
limited
Implication for GPM:
Sensitivity
~88%
~42%
GPM
• These figures show global means integrated over all areas (land&ocean)
Implication for GPM:
Resolution
Occurrence
Accumulation
14%
Occurrence dominated
(60%) by events with
horizontal dimensions < 5
km
Rough estimate of
spatial resolution
effects on PR/DPR
accumulation
Summary
• CloudSat provides a unique view of warm rain that complements the
TRMM-PR and passive microwave sensors.
– Global mean warm rain rate ~ 0.23 mm/day (~7% of the global rainfall)
• AMSR/E captures ~89% of warm rain accumulation
– Huge improvement in new GPROF2010 product
– Significant regional biases remain
• TRMM-PR captures ~45% of warm rain accumulation.
• Outlook for GPM-DPR is positive
– Can reasonably expect this ‘observed accumulation’ to be greater than 88%
based on increased sensitivity and 86% based on resolution (>74%).
– 98-99% of total rain accumulation