an algorithm for measuring rain over oceans using the ... · conclusion •a statistical rain rate...
TRANSCRIPT
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An Algorithm for Measuring
Rain over Oceans using the
QuikSCAT Radiometer
Mladen Susanj
July 14, 2000
Thesis Defense for the Degree of M.S.E.E.
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Overview
1. SeaWinds on QuikSCAT
2. Effect of Precipitation on Ocean Tb
3. Rain Algorithm Definition
4. Rain Algorithm Validation and Results
5. Conclusion
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• Mission - to provide continuous
measurements of the Ocean surface winds.
• Problem - Wind vector products occasionally
degraded by the presence of rain.
• Idea - to develop the technique for
identification of contaminated wv cells.
Chapter - 1
SeaWinds on QuikSCAT
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Geometry of the Sea Winds scatterometer on QuikSCAT
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QuikSCAT swath projection
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QuikSCAT Radiometer’s (QRad) simplified block diagram
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Be
Bn
Frequency
Pow
er
Received Spectrum
Radar Echo
Echo Channel
Noise Channel
GBBTEEN nensysenxk )(
QuikSCAT Radiometer receiver spectrum
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Chapter - 2
Effect of Precipitation on Ocean Tb
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Theoretical QRad Brightness Temp. vs. Rain Rate
),,,,( RainsalanitywvwsSSTfTb • General case
• Theoretical QRad Tb salinitywvwsSSTRainfTb ,,,)(
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Typical Rain Signature in QRad Tb
QRad 3-Day Avg.
H-pol Brightness
SSM/I Monthly Avg.
Rain Rate
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Chapter - 3
Rain Algorithm Definition
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Surface Truth - TMI and SSMI Rain Rates
Advantage:
• utilization of multiple frequencies and
multiple polarization enable superior
accuracy of measurements.
Restrictions:
• variable spatial and time collocation with
QuikSCAT wind vector cells,
• measurements just in Tropics (TMI).
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QuikSCAT and TRMM Swath
21 rain events covered by both instruments within one
hour time difference used to develop the algorithm
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Rain Event #1493
QRad Tb
TMI
Rain Rate
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Collocation of QuikSCAT and SSMI Measurements
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Collocation of QuikSCAT and SSMI Measurements
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pbpbbp oceanTQRadTT )()(
Ocean Background Brightness and “Excess Brightness”
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Excess Brightness
Calculation
Rain Index
Calculation &
Quantization
Qrad Tb’sOcean
Tbackgroun
d
Flow Chart - 1st Part
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Rain Index
• At QSCAT wind vector cells, calculate rain indices for
both H & V-pol
0
,
,,
,,
n ji
p
ppjib
pji
TTRI
n i,j … # of measurements
p … standard deviation of Tb
i , j … bin indices
p … polarization
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Brightness Temp. Offset T0
Excess Brightness vs. TMI Rain Rate
’h=12K
Exce
ss T
b, K
TMI Rain Rate, mm/hr
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Initial Rain Mask
For SQRI 4
Excess Brightness
Calculation
Rain Index
Calculation &
Quantization
SQRI = QRIh+QRIv
Qrad Tb’sOcean
Tbackgroun
d
Flow Chart - 1st Part
To Spatial Filter
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Threshold Determination - I
Rain Index Probability Density Functions
Clear Sky and Ocean Moderate Rain
Mean QRI = - 1 Mean QRI = + 1
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T
c dRIRIpPFA )(
T
r dRIRIpPMc )(
FAMcE PPP
Threshold Determination - II
Threshold “T” is defined where PE has minimum !!
Clear Ocean and Sky pdf
Moderate Rain pdf
Threshold
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Rain Event #1493
QRad Tb
Rain Event Area
No-Rain Area
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Initial Rain Mask - Input to Spatial Filter
Random Rain False Alarms
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Flow Chart - 2nd part Spatial Filter
Center bin ‘No-
Rain’ Flag 0
Input Rain Mask
Center bin
‘Rain’
3
Neighbors
Flag 1
< 3
Neighbors
Comparison
with higher
threshold (6)
for SQRI
Flag
0/1
Revised Rain Mask
0/1
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Spatial Filter Performance
Output
Input
5 iterations
Spatial Filter
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Flow Chart - Part-3
Assign
Probabilities
Joint Probability
Calculation
Rain Rate
Calculation
Output of the Spatial Filter
Rain Flag
Calculation
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Rain Index Probability and Neighbor Probability
QRIh
Rain
In
dex
Pro
bab
ilit
y
H-pol Rain Index Pr Neighbor Pr
# Rain-Flagged Neighbors
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QRad Joint Probability
)(Pr)(Pr)Pr( QRIQRINbsJP vh
TMI Rain Rate QRad Joint Pr
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QRad Rain Rate
JPBJPARR 2
Regression TMI Rain Rates
with Joint Probabilities
gives the expression;
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QRad Rain Flag
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Chapter - 4
Rain Algorithm Validation and Results
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Rain Rate Comparison with TMI - Rain Event #1176
TMI QRad
Accumulative
Rain Rate, mmTMI QRad QRad/TMI
2,462 1,750 0.71
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Rain Rate Comparison with TMI - Rain Event #1493
TMI QRad
Accumulative
Rain Rate, mm
TMI QRad QRad/TMI
5,745 4,701 0.82
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Rain Rate Comparison with SSMI - QuikSCAT Rev #1439
QRad SSMI
Accumulative
Rain Rate, mmQRad SSMI QRad/SSMI
4473 4539 0.99
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Correct No-Rain
Correct Rain
Misclassification
False Alarm
Rain Flag Classification - Rain Event #1176
QRadTMI
Classification
(89.27%)
(70.00%)
(30.00%)
(10.73%)
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Rain Flag Classification - Rain Event #1493
Correct No-Rain
Correct Rain
Misclassification
False Alarms
QRadTMI
Classification
(88.86%)
(85.67%)
(14.33%)
(11.14%)
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Conclusion
•A statistical rain rate retrieval algorithm has been developed
using QRad Tb’s and rain rates derived from TRMM TMI.
•Also a binary rain flag has been produced using 1.4 mm/hr
as the threshold.
•Comparisons between QRad, TMI & SSMI rain rates show
excellent spatial correlation when properly collocated in
time.
•Some solutions and procedures proposed in this thesis have
been accepted by the sponsor of the project, JPL, and
implemented into operational SeaWinds rain algorithm.