1 LaRC Evapotranspiration flux estimations using combined satellite measurements Bing Lin 1, Qilong Min 2, and Wenbo Sun 3 1 NASA Langley Research Center, USA 2 State University of New York at Albany, USA 3 Hampton University, USA The LandFlux Workshop Toulouse, France May 28 ~ June 1, 2007 Title

2 LaRC  Background oTurbulent flux oLand surface emissivity oCombined satellite technique  MW-VIS-IR For Land Surface o EDVI, NDVI, & ET flux o Vegetation development: growth season  Current Issues & Potential Approaches oPhysical relation: EDVI  leaf properties oSW, LW, storage, & turbulent fluxes  Summary Outline

3 LaRC Land surface fluxes  Vegetation Canopies & Atmosphere oET & SH fluxes: turbulent, statistical oUnderstanding water & energy cycle large biases in annual means oSite measurements (no large-scale obs.)  Indirect Satellite Approaches oNDVI: PAR, limited by clouds & aerosols high spatial resolution, daytime only oMW emissivity  VWC, cover, T, structure day & night, multiple sensors low spatial resolution

4 LaRC Forests canopy emissivity  C, crown layer (  ) emission from soil & trunk (  s )

5 LaRC Land surface emissivity   C =1–(1–  s )exp(-2  )  (1  exp(-  ))[1+(1–  s )exp(-  )]  EDVI p = 2(  19 p –  37 p )/(  19 p +  37 p )  N EDVI =(EDVI – EDVI onset )/ (EDVI max – EDVI onset )

6 LaRC Simplified scheme (VWC vs EDVI) GHz 19GHz Emissivity V VWC (kg/m 2 ) EDVI V E19 = E19 = E19 = 0.975

7 LaRC Retrieval technique full MW RT atmos. abs. correction LWP & WV: VIS-IR iteration  surface site: SW, LW, PAR vege. status turbulent obs. collocation

8 LaRC Harvard Forest (1990) EF EDVI V EDVI H

9 LaRC NDVI vs EDVI EF NDVI EDVI V R=0.52 R=0.86

10 LaRC ET flux ET(W/m 2 ) EDVI V *SW R=0.95 solid: clear sky daytime indicating some saturation at high end

11 LaRC Growth Season ( ) N EDVI & leaf amount EDVI day of year (1999) day of year (2000) bars: onset & end-of-season days of sfc obs. vertical lines: N EDVI decided days solid: leaf amount

12 LaRC Issues in this estimation  Empirical technique oPhysical relation between EDVI & EF oEmissivity vs vegetation physical properties VWC, scattering albedo, phase function oEDVI*SW vs ET fluxes  Land surface energy balance oSW, LW, Net, LH, SH, and storage oTs, Tv and Ta  Spatial/temporal resolution: NDVI/EDVI No RT theoretical solutions Heat storage, SH & other parameters?

13 LaRC Potential Approaches satellite data MW, VIS, IR, etc EDVI/NDVI surface rad. assimilation heat storage weather status partitioning LH & SH balancing heat budget evaluating vegetation status theoretical cal.  vs VWC etc.

14 LaRC Surface Net Radiation SRB

15 LaRC Current energy balance To estimate SH, storage terms are also needed even with net radiation LH SH

16 LaRC Calculation: FDTD  3D Finite Difference Time Domain oUniaxial perfectly matched layer (absorbing boundary conditions)  Scattering of elliptic disc (simulated leaf) o5, 2.5, 0.10 cm; half horizontal size oRandom orientation  Vegetation property oVegetation water content (VWC): 0% ~ 70 % oDry vegetation density (0.33 g/cm 3 ) oDielectric constant: dual-dispersion model oAt 19 and 37 SSM/I frequencies

17 LaRC Calculation: geometry E ● H ●● E

18 LaRC Single scattering albedo VWC: vegetation water content (%)

19 LaRC Effective efficiencies (full size)

20 LaRC phase function (37G) phase function scattering angle (°) half size

21 LaRC phase function (diff size 37G) phase function scattering angle (°) vwc = 58%

22 LaRC phase function (19G vs 37G) phase function scattering angle (°) vwc = 58% similar size parameter

23 LaRC  Combining MW, VIS and IR, EDVI & NDVI values can be estimated.  Sampling rates will be increased with a combination of EDVI & NDVI.  EDVI is empirically related to ET fluxes and canopy changes during growth seasons.  Assimilation data of surface parameters are needed for LH and SH partitions.  Scattering calculations potentially provide critical links between vegetation properties and microwave land surface emissivity. Summary

24 LaRC Acknowledgement  This study is supported by the NASA NEWS and Radiation programs.  We would like to thank Y. Hu and G. Gibson of Langley for their many helps.

25 LaRC Potential Approaches  Theoretical calculation oLeaf scattering properties: single scattering VWC vs scattering albedo, phase function oVegetation emissivity: multiple scattering  vs VWC (size, shape, type & other para.) oSatellite estimation: SW + LW fluxes EDVI-like vs ET fluxes?  Assimilation data oHeat storage oTs, Tv, Ta, and other parameters oPartitioning between LH & SH

26 LaRC Approximate: VIE and MoM Koh and Sarabandi (2005, IEEE Trans. AP) 3cm square 0.2mm thickness 10GHz vertical incident (90deg)

27 LaRC Asymmetry factor

28 LaRC Effective efficiencies (half size)

29 LaRC phase function (19G) phase function scattering angle (°) size: 5cm, 2.5cm and 0.1cm