Graduate Course: Advanced Remote Sensing Data Analysis and ApplicationGraduate Course: Advanced Remote Sensing Data Analysis and Application
RETRIEVAL OF SURFACE AIRHUMIDITY FROM SSM/I
Shu-Hsien Chou
Dept. of Atmospheric Sciences
National Taiwan University
Objective :
Retrieve surface air humidity from SSM/I overglobal oceans
Chou, S.-H., R. M. Atlas, C.-L. Shie and J. Ardizzone, 1995:Estimates of surface humidity and latent heat fluxes overoceans 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 SensorMicrowave Imager data. J. Geophys. Res., 102, 12705-12726.
Outlines:
•Height Variation of Correlation of Surface AirHumidity (Q) with Thick Layer Water VaporContent
•EOF Method for Deriving Q (Chou et al.1995)
•FGGE2b Stations Used for 6 W-basedClimatic Regimes of Humidity Soundings
•Vertical Profiles of Mean, 1st and 2nd EOFsof 6 W-based Climate Regimes
•Comparison of EOF Method with SSG, andLiu86 Methods using FGGEF2b HumiditySoundings
•Comparison of SSM/I Retrieved-Q amongEOF, SSG, and Liu86 Methods (Validatedwith Radiosonde Obs)
•Further Improvement of EOF Method forRetrieving Q from SSM/I (Chou et al. 1997)
•Validation of SSM/I Retrieved-Q based onImproved EOF Method Against 9 fieldexperiments
Table 1. Characterics of SSM/I on board DMSP satellites
Center of freq (GHZ)19.3522.2437.0085.50
PolarizationV, H V V, H V, H
3 dB footprint (kmxkm)69x4350x4037x2915x13
Swath width (km)13941394 1394 1394
Spatial sampling (km)25252512.5__
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() = qm() + C1 F1() + C2 F2() (1)
surface Q10m = Mo + M1 W + M2 WB (2)
Q10m ---- surface air specific humidity
---- (p - 200 mb)/(ps - 200 mb)
q() ---- specific humidity vertical profile
qm, F1, F2 --- mean, 1st & 2nd EOFs humidity profiles of aclimate 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): 1.94 - 1.23 (Regimes 1- 6)
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 ofnine field experiments conducted by NOAAETL research ships. C: COARE, F: FASTEX,X: other experiments.
10-m specific humidity averaged over 1992-93 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 humidityvalidated reasonably well withthose measured by research shipsof nine field experiments overtropical and northern midlatitudeoceans during 1991-99.
•Comparisons with high qualityresearch ship data, radiosondemeasurements, and threehumidity data sets over globaloceans suggest that EOF methodfor retrieving surface air humidityimproves upon those of Liu (1986),Schulz et al. (1993) and HOAPS.