Lecture 7 - National Taiwan University

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Lecture 7 - National Taiwan University
Graduate Course: Advanced Remote Sensing Data Analysis and Application
RETRIEVAL OF SURFACE AIR
HUMIDITY FROM SSM/I
Shu-Hsien Chou
Dept. of Atmospheric Sciences
National Taiwan University
Objective :
Retrieve surface air humidity from SSM/I over
global oceans
Chou, S.-H., R. M. Atlas, C.-L. Shie and J. Ardizzone, 1995:
Estimates of surface humidity and latent heat fluxes over
oceans from SSM/I data. Mon. Wea. Rev., 123, 2405-2425.
Chou, S.-H., C.-L. Shie, R. M. Atlas and J. Ardizzone,
1997: Air-sea fluxes retrieved from Special Sensor
Microwave Imager data. J. Geophys. Res., 102, 1270512726.
Outlines:
• Height Variation of Correlation of Surface Air
Humidity (Q) with Thick Layer Water Vapor
Content
• EOF Method for Deriving Q (Chou et al.
1995)
• FGGE2b Stations Used for 6 W-based
Climatic Regimes of Humidity Soundings
• Vertical Profiles of Mean, 1st and 2nd EOFs
of 6 W-based Climate Regimes
• Comparison of EOF Method with SSG, and
Liu86 Methods using FGGEF2b Humidity
Soundings
• Comparison of SSM/I Retrieved-Q among
EOF, SSG, and Liu86 Methods (Validated
with Radiosonde Obs)
• Further Improvement of EOF Method for
Retrieving Q from SSM/I (Chou et al. 1997)
• Validation of SSM/I Retrieved-Q based on
Improved EOF Method Against 9 field
experiments
Table 1. Characterics of SSM/I on board DMSP satellites
Center of freq (GHZ)
19.35 22.24 37.00
85.50
Polarization
V, H
V
V, H
V, H
3 dB footprint (kmxkm)
69x43
50x40
37x29
15x13
Swath width (km)
1394
1394
1394
1394
Spatial sampling (km)
25
25
25
12.5__
SSM/I --- Special Sensor Microwave/Imager
DMSP --- Defense Meteorological Satellite Program
Retrieval of surface air humidity:
Empirical orthogonal function (EOF) METHOD:
(Chou et al. 1995, 1997)
profile
q(s) = qm(s) + C1 F1(s) + C2 F2(s)
surface Q10m = Mo + M1 W + M2 WB
(1)
(2)
Q10m ---- surface air specific humidity
s
---- (p - 200 mb)/(ps - 200 mb)
q(s) ---- specific humidity vertical profile
qm, F1, F2 --- mean, 1st & 2nd EOFs humidity profiles of a
climate regime (FGGE IIb q's, 6 W-based samples)
C1, C2 ---- principal components of F1 & F2
W --- SSM/I total column water vapor (Wentz 1994, 1997)
WB --- SSM/I 500m bottom layer water vapor (Schulz et al. 1993)
Mo (g kg-1):
0.09- 5.91
(Regimes 1- 6)
M1 (g kg-2 m2): (-0.04) - 0.02
(Regimes 1- 6)
M2 (g kg-2 m2):
(Regimes 1- 6)
1.94 - 1.23
Retrieval of WB from SSM/I: (Schulz et al. 1993)
WB = b0 + b1 Tv19 + b2 Th19 + b3 Tv22 + b4 Tv37
b0 = - 5.93390
b1 = 0.03697
b2 = - 0.02390
b3 = 0.01559
b4 = - 0.00497
*Criterion for valid WB (no rain): (Goodberlet et al. 1990)
Th19 < 165 K
(Tv37 – Th37) > 50 K
EOF1
56-86%
EOF1&2
71-92%
EOF2
6-15%
Q = f (W, WB)
Q = f (WB)
Q = f (W)
25 km cell
<100km
<1.5 hr
25 km cell
<100km
<1.5 hr
25 km cell
<100km
<1.5 hr
*Excluding CATCH, FETCH, and SCOPE for 9 collocated experiments
*Excluding CATCH, FETCH, and SCOPE for 9 collocated experiments
GSSTF2 surface air specific humidity vs those of
nine field experiments conducted by NOAA
ETL research ships. C: COARE, F: FASTEX,
X: other experiments.
10-m specific humidity averaged over 199293 for (a) GSSTF2 and differences of (b)
HOAPS, (c) NCEP/NCAR reanalysis, and
(d) da Silva et al. (1994) from GSSTF2.
CONCLUSIONS:
• GSSTF2 daily surface air humidity
validated reasonably well with
those measured by research ships
of nine field experiments over
tropical and northern midlatitude
oceans during 1991-99.
• Comparisons with high quality
research ship data, radiosonde
measurements, and three
humidity data sets over global
oceans suggest that EOF method
for retrieving surface air humidity
improves upon those of Liu (1986),
Schulz et al. (1993) and HOAPS.

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