Ground-based Measurements of Surface Reflectivity and Emissivity over Desert Regions

1. Ground-based Measurements of Surface Reflectivity and Emissivity over Desert Regions Si-Chee Tsayand the GSFC Team* NASA Goddard Space Flight Center *Q. Jack Ji, Richard A. Hansell, N. Christina Hsu, M-J. Jeong

2. Energetics: relating limited measurements of angular reflectance(or skin temperature) to flux albedo (or flux emittance), a crucial input parameter to Global Climate Model or General Circulation Model (e.g., the GSFC fvGCM). Remote sensing: characterizing surface anisotropy to interpret off-nadir radiances acquired by air- and/or space-borne sensors (e.g., the Deep-Blue). Goals: Accurate Boundary Conditions for Physical Models

3. Time scale: short enough, but sufficient long, for obtaining measurements at multiple viewing angle (stable skin temperature) to minimize the effects of changing source and target conditions. Consistency: simultaneously measuring the downwelling radiance from the source and upwelling radiance from the target. Requirements

4. measurement +  = reflector measurement +  = 4 skin • Reality: at all-scale inhomogeneity tree, grass, dirt, etc. vegetation & water gravel & sand • Theory:

5. Goniometer-based D. K. Perovich 1994 • Spectral Range: • 0.4-1.0 µm, 256 channels • 8 nm spectral resolution • Field-Of-View: 1° • Time: 15 min. for 3/12 zenith/azimuth angles • Diameter: ~1 m

6. PARABOLA(Portable Apparatus for Rapid Acquisitions of Bidirectional Observations of Land and Atmosphere) • 2-Axis Scanning Radiometer: • 3 channel, 0.66, 0.83 & 1.65 µm • Field-Of-View: 15° • 2 m2 nadir, 6 m2 45°-off-nadir • Time: 11 sec. for 2 hemispheres • Reference: Spectralon-white reflector D. W. Deering & P. Leone, 1986 Arctic Tundra T. F. Eck 1995

7. Bidirectional Reflectance Measurements: CAR • Roll: ~20° • Time: ~2 min • Speed: ~80 m s-1 • Height: ~600 m • Diameter: ~3 km • Resolution: • 10 m (nadir) • 270 m ( = 80°) • Channel: • 8 continuously sampled (0.34-1.25 µm) • 2 filter wheel channels used for BRDF measurements (1.64 & 2.20 µm) M. D. King et al., 1986 Tsay, S. C., M. D. King, G. T. Arnold and J. Y. Li, 1998: Airborne Spectral Measurements of Surface Anisotropy during SCAR-B, J. Geophys. Res., 103, 31,943-31,953.

8. Light Through a Camel’s Eyes … Field deployment, not only a SCIENCE project but also an ART work … … Encompassed by sands and dunes in a sea of deserts, it is all but irremediable for ordinary humans to maintain their sense of direction. Thus, as it is said in scriptures, the Queen of Sheba came with a caravan of camels when she sought the Wisdom of Solomon…

9. coastal regions O3 3 2 Radiance (Wm-2sr-1nm-1) 1 H2O O2-A 0 0.35 0.85 1.35 1.85 2.35 Wavelength (µm) Inland regions ASD spectroradiometer • Spectral Range: • 0.35 - 2.35 µm • Field-Of-View: 18° • 0.3 m2 nadir • 1 m2 45°-off-nadir • Time: 2 min. • Height: 1 m Spectralon white reflector Al Ain, UAE

10. Opposition effect “hot spot” Bidirectional Reflectance Wavelength (nm) Specular reflection “blind spot” True Azimuthal Angle sun=27.8°, sun=173°, =30°, t=64 sec.

11. Bidirectional Reflectance - DesertSaudi Arabia(q0= 48°)in 1991 l= 1.22 µm l= 0.67 µm 0.0 0.2 0.4 0.6 0.8 1.0

12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al Ain, UAE q0 Opposition 2.13 µm 1.64 µm 1.24 µm Bidirectional Reflectance 0.87 µm 0.67 µm 0.47 µm 0.41 µm 0.38 µm Backward Scattering Angle Forward Scattering

13. Bidirectional Reflectance - DesertSaudi Arabia(q0= 48°)in 1991 Saudi Arabian Desert (Landsat Image)

14. l Band Terra Aqua 1 0.65 µm Deep-Blue Deep-Blue Deep-Blue 2 0.86 µm 3 0.47 µm 4 0.56 µm 5 1.24 µm 6 1.64 µm 7 2.13 µm  8 0.41 µm Al Ain, UAE 2.13 µm Surface Reflectance (MODIS) Surface Reflectance (ASD)

15. A lively touch… …for skin temperature The Creation of Adam, Sistine Chapel, by Michelangelo 1511.

16. Scanning Microwave Radiometer Surface Temperature (oC) Air (30cm) footprint 23GHz 36GHz 4 minutes cycle -75o to zenith to +135o thermistors Time (h:m:s) Temperature Difference (oC) ~ C = 1/1400 sec-1 for entire UAE2 T = Teq(1 e-ct) Elapse Time (min.) Elapse Time (sec.)

17. Temperature (oC) Date

18. _ _ + + T~ 5K T~ 5K Satellite Retrievals vs. In-situ MODIS/Terra&Aqua Al Ain MODIS band emissivity: prescribed by land-cover type

19. AERI Atmospheric Emitted Radiance Interferometer Scanning Microwave Radiometer Instrument Characteristics • Scan: -45º to +165º from horizon • Channels: 23.0, 23.8, 36.5 GHz • Beam width: 6º • Calibration: tipping curve • ~30 temperature monitoring SMiR

20. _ _ + + Emissivity ~ 5% ~ 4% 220 230 240 250 260 270 280 220 230 240 250 260 270 280 Day of Year Day of Year Inferred Microwave Emissivity Special Sensor Microwave/Imager Advanced Microwave Sounding Unit Studies for

21. Simple, yet effective, devices developed for quantifying boundary conditions over desert regions to refine and to acquire extended measurements in the near future Mild zenith/azimuthal anisotropy for UAE2 deserts at UV & blue, but increasing dependency for longer wavelengths compared reasonably well, over spectral & angular features, with other comprehensive measurements (e.g., CAR) in nearby deserts and collocated satellite measurements (e.g., MODIS/Terra&Aqua, Deep-Blue data base) Large diurnal variation in surface skin temperature (25-30oC) and less variation over air temperature (10-15oC) In-situ measurements and MODIS retrievals of surface skin temperature, bias ranged Or, surface emittance bias of , severely impair the accuracy of retrieving atmospheric temperature and humidity profiles from SSMI/AMSU microwave sensors _ _ + + ~ 4% T~ 5K Summary