Intersatellite Calibrated HIRS Upper
Tropospheric Water Vapor
Lei Shi and John J. Bates
NOAA National Climatic Data Center
Asheville, North Carolina, U.S.A.
1 NOAA National Climatic Data Center
ITSC-XVI
7-13 May 2008
Outline
• Motivation
– Time series discontinuity from satellite to satellite, particularly
from TOVS to ATOVS
– Upper tropospheric water vapor (UTWV) is an important
fundamental climate data record (CDR)
– UTWV is key to water vapor feedback
• Approach
– New and improved intersatellite calibration based on two
datasets
• Overlaps of zonal means
• Simultaneous nadir overpass (SNO)
• Result
– Extended time series of the fundamental CDR to present
2 NOAA National Climatic Data Center
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Motivation – Uncorrected Intersatellite
Differences of UTWV (Channel 12)
30S - 30N
248
246
244
N06
N07
N08
N09
N10
N11
N12
N14
N15
N16
N17
242
240
238
1980
1984
Start of HIRS/3
1988
1992
1996
2000
2004
Year
•
•
Due to the independence of individual HIRS instrument’s
calibration, biases exist from satellite to satellite.
These intersatellite biases have become a common source of
uncertainty faced by long-term studies.
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Spectral Filter Functions
H IR S /2
H IR S /3
1
N 06
N 07
0 .8
N 08
N 09
N 10
0 .6
N 11
N 12
0 .4
N 14
N 15
N 16
0 .2
N 17
Tra n
0
1 3 50
1 4 00
1 4 50
1 5 00
1 5 50
1 6 00
W ave N u m be r (pe r cm )
•
•
•
Differences between HIRS/2 and HIRS/3 are expected due to
different filter functions.
In-orbit performance still has biases unexplained by filter functions.
Thus empirical approach is considered.
4 NOAA National Climatic Data Center
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Approach – Two Datasets for Intersatellite
Calibration to Cover Diverse Atmospheres
• Overlaps of zonal means for tropical and midlatitude atmosphere cases
– Zonal means of channel 12 (UTWV) are computed for
every 10-degree latitude belt.
– Differences are derived from overlapping satellites.
• Simultaneous nadir overpass (SNO)
observations for polar cases
– SNOs occur when two satellites cross each other
– Data taken at the same location at the satellite nadir
within a few seconds
– In the regions 70-80N and 70-80S
5 NOAA National Climatic Data Center
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Temperature-dependent Intersatellite
Differences from Zonal Mean Approach
1.0
0.5
0.0
−1.0
2
−0.5
4
Bias (K)
6
8
T_N−N06
N06−N07
N07−N08
N09−N10
N10−N11
N11−N12
N12−N14
N14−N15
N15−N16
N16−N17
N17−M02
−1.5
0
Bias (K)
1.5
Tropical and Mid-latitude Cases
220
225
230
235
240
T (K)
245
250
255
220
225
230
235
240
245
250
T (K)
• More than half of satellites have bias variations larger
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thanNational
0.5 K.
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255
Temperature-dependent Intersatellite
Differences from SNO Approach
Polar Cases
0.8
5
4
3
N6-N7
N7-N8
N9-N10
N10-N11
N11-N12
N12-N14
N14-N15
N15-N16
N16-N17
0.6
0.4
0.2
N6-N7
N7-N8
N9-N10
N10-N11
N11-N12
N12-N14
N14-N15
N15-N16
N16-N17
0
2
-0.2
1
-0.4
0
-0.6
CH12
-1
205 210 215 220 225 230 235 240 245
T(K)
CH12
-0.8
205 210 215 220 225 230 235 240 245
T(K)
• Biases are also temperature dependent.
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Extension of Time Series
30S - 30N
• Orbital, daily,
and monthly,
1979-present
• At both pixel
resolution and
2.5x2.5 lat/lon
grids
• Intersatellitecalibrated
249
248.5
248
247.5
247
246.5
V1
V2
246
1980
1984
1988
1992
1996
Year
2000
2004
* V1 is derived based on methods discussed in Bates, J. J., X. Wu, and D. L. Jackson, (1996) Interannual variability of
upper-troposphere water vapor band brightness temperatures. J. Climate, 9, 428-438.
8 NOAA National Climatic Data Center
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Intersatellite Calibrated to N-12
(showing 30S – 30N)
2 49
• Biases
minimized.
• Temperature
dependent
biases
accounted for.
• Similar overall
variances
between
HIRS/2 and
HIRS/3.
• Time series can
be extended as
variance
preserved.
2 48.5
2 48
2 47.5
2 47
2 46.5
N 06
N 07
N 08
N 09
N 10
N 11
N 12
N 14
N 15
N 16
N 17
2 46
19 80
1 984
19 88
1 992
19 96
2 000
2 004
1980
1984
1988
1992
1996
2000
2004
0.4
0.2
0
-0.2
-0.4
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Year
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Climatologic Mean for Jan, Apr, Jul, and Oct
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Seasonal Variation
• Within tropics, drier atmosphere produces a
higher value of UTWV (channel 12 “sees” lower
atmosphere).
• Minima in the tropics indicate moist upper
troposphere caused by convection.
• Maxima in northern tropics in January move to
southern tropics in July, indicating the move of
dry regions in upper troposphere from north to
south.
• Zones of low values indicate positions of ITCZ.
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Difference of UTWV between 2002-2003
El Niño and 1999-2000 La Niña
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UTWV Time Series (showing 30S – 30N)
2 4 8.8
2 4 8.4
248
2 4 7.6
2 4 7.2
2 4 6.8
2 4 6.4
1 9 80
1 9 84
1 9 88
1 9 92
Year
1 9 96
2 0 00
2 0 04
UTWV anomaly
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Long-term UTWV Anomaly Dataset
7.5S-7.5N
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UTWV Has Good Coverage for Tropical Convections
OLR
15 NOAA National Climatic Data Center
HIRS Upper Tropospheric Water Vapor
Å
Obtained from
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http://www.cdc.noaa.gov/map/clim/olr_modes/hovEa.html
7-13 May 2008
Conclusions
• Improved intersatellite calibration.
– Temperature dependent inter-calibration
• Extension of time series to current.
– HIRS/2 and HIRS/3 series connected
• The HIRS UTWV anomaly data are
useful in monitoring Madden-Julian
oscillation and various equatorial
waves.
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