Comparison of AMSU-B Brightness Temperature with Simulated
Brightness Temperature using Global Radiosonde Data
iup
V.O. John, M. Kuvatov, S. A. Buehler, and A. Houshangpour
Institute of Environmental Physics, University of Bremen, Otto-Hahn-Alle 1 , 28359 Bremen, Germany, vojohn@uni-bremen.de
Introduction
Error Model
200
Sources of error for the comparison are:
NOAA-15
2002
NOAA-16
100
20
0
19
100
Number of Occurances
Upper tropospheric humidity (UTH) is a crucial parameter for meteorology and climate research. There
are two global and continuous data sets for this parameter, one from polar orbiting meteorological sensors,
the other from synoptic meteorological radiosondes. The basic idea of the study is to compare satellite
and radiosonde data. A radiative transfer (RT) model is used to generate simulated AMSU measurement
from the radiosonde data.
The aims of the study are, to develop a robust methodology for such a comparison and to pave the way for
a systematic comparison of all stations in the global radiosonde network to satellite data. This will allow
an intercomparison and quality control of the different radiosonde stations, assuming that the satellite
instrument’s properties are stable during a few orbits.
Lindenberg Radiosonde Data
• Radiometric noise of the AMSU measurement
• Sampling error, due to atmospheric
inhomogeneity
• Radiosonde measurement error in humidity and temperature
0
18
100
• RT model error (systematic)
• AMSU calibration error
0
σ(i) = C0 + σ50km(i)
17
100
0
The Meteorological Observatory Lindenberg (MOL)
- 52◦ 220 N, 14◦ 120 E.
C0 = 0.5 K
16
100
0
0
• High Resolution Not Corrected(HRNC)
1
2
3
σ50km [ K ]
• High Resolution Corrected(HRC)
4
σ50km - standard deviation of the pixel
brightness temperatures in the target
area of 50 km circle around the station
• Low Resolution Corrected(LRC)
Corrections for HRC / LRC:
Results
• Elimination of data affected
by sensor icing
UTH [500 hPa - 200 hPa]
40
Average Upper trpospheric
Water Vapor Content, 2002
[kg m−2]
• HRC - 0.721
∆ UTH [ % ]
Average Total Water Vapor
Content, 2002 [kg m−2]
• LRC - 16.712
10
<∆ UTHLH> = 2.632 ± 4.046
<∆ UTHHH> = -3.356 ± 2.684
AMSU TB [ K ]
-10
LRC
HRNC
-20
• LRC has wet bias
0
200
400
• HRNC has dry bias
600
Launches
800
1000
1200
ARTS - Atmospheric Radiative Transfer Simulator [1]
ARTS is used to simulate brightness temperature for AMSU-B channels. The clear-sky version of ARTS is
used in this study. A cloudy version, which can handle scattering due to cirrus clouds has been developed
and is being validated.
The table below shows the effect of cutting the profiles at 100 hPa. Simulations for the ECMWF 60-level
data set [2] were used to assess this. The difference in brightness temperature in mK is given in the table
(GN: Global profiles, nadir view, GON: Global profiles, most off-nadir view, MN: Midlatitude profiles,
nadir view, MON: Midlatitude profiles, most off-nadir view).
GON
32±74
32±29
130±57
46±16
38±14
MN
-30±34
-7±15
60±25
18±6
10±8
– H2O, N2, O2 - PWR-98, PWR-93, PWR-98
• Same viewing angle
• Surface emissivity = 0.95
No. of Occurances
120 NOAA-15
NOAA-16
NOAA-15
NOAA-16
2002
100
• AMSU radiance in a target area, a
circle of 50 km radius (mean of all pixels in the circle)
80
• Radiance is simulated with mean
looking angle of the circle
60
• At least 12 pixels should be in the
circle
40
• Displacement, D of air parcel ≤ 50 km
• D = Ave. wind speed x Time diff.
20
• Radiosonde time - half an hour before
synoptic hour
0
6
12
18
Time [ Hours, UTC ]
24
260
BIAS = -0.27 ± 0.06
CORR = 0.97
N = 220
BIAS = 2.75 ± 0.13
CORR = 0.94
N = 36
270
260
250
SLOPE = 0.94 ± 0.02
OFFSET = 15.70 ± 4.89
250
260
270
ARTS TB [ K ]
280
275
270
260
SLOPE = 0.99 ± 0.02
OFFSET = 1.74 ± 5.50
260
270
280
ARTS TB [ K ]
290
SLOPE = 1.05 ± 0.05
OFFSET = -9.00 ± 13.43
240
280
270
250
260
ARTS TB [ K ]
270
AMSU 20 [ 183.31 ± 7 GHz ]
BIAS = 0.10 ± 0.13
CORR = 0.93
N = 36
Offset
24.34±3.17
11.76±3.50
02.81±3.90
20.32±3.26
09.49±3.55
03.45±3.90
44.43±2.93
26.87±3.31
11.31±3.78
(i) −TBARTS (i) ) /σ
i ( TBAMSUP
−2
i σ (i)
2
Corr
0.95
0.95
0.95
0.94
0.94
0.95
0.93
0.94
0.95
(i)
• Small dry bias in HRC compared to
AMSU
• Dry bias in HRNC compared to HRC
• Wet bias in LRC compared to HRC
Figure shows comparison of two types of humidty sensors. Data is taken from BADC.
265
260
255
250
P
Slope
0.90±0.01
0.95±0.01
0.99±0.01
0.91±0.01
0.96±0.01
0.98±0.01
0.82±0.01
0.89±0.01
0.95±0.01
Tcorrect
BARTS = Offset + Slope × TBARTS
250
280
BIAS = -0.86 ± 0.06
CORR = 0.96
N = 220
Bias =
260
240
AMSU 20 [ 183.31 ± 7 GHz ]
4
2
0
-2
-4
-6
-8
-10
0
5
0
SLOPE = 1.04 ± 0.06
OFFSET = -10.79 ± 15.11
245
245 250 255 260 265 270 275 280
ARTS TB [ K ]
• Lindenberg - RS80 - Vaisala
• Kem - goldbeater’s skin
• Wettest - 43.08
Wettest
• Medium - 16.67
• Driest - 1.91
10
20
30
40
50
Medium
-10
-15
0
10
5
10
20
30
40
50
Driest
• Make collocated data set for available stations in the BADC radiosonde archive
-5
-15
-20
0
In very dry cases channel 20 shows opposite
behavior compared to channel 18
Future plans:
0
-10
This figure can be used to translate back the
bias in brightness temperature to bias in relative humidity in the Upper Troposphere.
• A bias of 7 K for Kem compared to Lindenberg implies that there can be about
12 - 15 % wet bias in RH [3]
-5
-20
• Profiles were cut at 100 hPa [see table]
Selecting Matches
240
250
ARTS TB [ K ]
Bias
-1.36±06
-0.93±06
-1.20±06
-2.06±06
-1.21±06
-1.36±06
-0.85±06
-0.76±06
-1.16±06
Total Water Vapor Content [kg m−2]
• Absorption Models:
MON
-33±57
0±26
90±47
31±11
23±10
230
Type Ch.
18
HRC
19
20
18
HRNC 19
20
18
LRC
19
20
Sensitivity
∆ TB [ K ]
GN
01±42
12±19
81±31
27±9
20±9
220
220
270
SLOPE = 0.95 ± 0.06
OFFSET = 18.99 ± 15.01
AMSU 19 [ 183.31 ± 3 GHz ]
250
250
∆ TB [ K ]
Ch.
16
17
18
19
20
240
230
SLOPE = 0.92 ± 0.02
OFFSET = 18.99 ± 4.62
240
250
260
ARTS TB [ K ]
250
AMSU 19 [ 183.31 ± 3 GHz ]
270
290
∆ TB [ K ]
• HRNC - 16.476
240
BIAS = 6.66 ± 0.14
CORR = 0.86
N = 36
260
250
240
240
0
• HRC - 16.660
260
280
20
• HRNC - 0.698
BIAS = -0.59 ± 0.06
CORR = 0.95
N = 220
2002
AMSU TB [ K ]
• LRC - 0.745
AMSU 18 [ 183.31 ± 1 GHz ]
230
230
<UTHLRC> = 37.921
<UTHHRC> = 35.289
<UTHHRNC> = 31.933
30
AMSU 18 [ 183.31 ± 1 GHz ]
AMSU TB [ K ]
LRC - only standard and significant levels
of HRC
Kem [Russia]
AMSU TB [ K ]
• Time lag correction
AMSU TB [ K ]
270
Lindenberg
AMSU TB [ K ]
• Dry bias correction
Ch-16
Ch-17
Ch-18
Ch-19
Ch-20
10
20
30
∆ RH [ % ]
40
50
• Analyze quality of the humidity data
References
[1] S. A. Buehler, P. Eriksson, T. Kuhn, A. von Engeln, and C. Verdes. ARTS, the Atmospheric Radiative
Transfer Simulator. submitted 2003. Preprint at: http://www.sat.uni-bremen.de.
[2] F. Chevallier. Sampled databases of 60-level atmospheric profiles from the ECMWF analysis. Research
report no. 4, EUMETSAF / ECMWF, 2001.
[3] B. J. Soden and J. R. Lanzante. An Assessment of Satellite and Radiosonde Climatologies of UpperTropospheric Water Vapor. J. Climate., 9:1235–1250, 1995.