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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.