Recorded presentation6B.4
CALIPSO Global Ocean Surface Roughness and Wind Speed Measurements and Potential Application
Yongxiang Hu, NASA/LaRC, Hampton, VA; and B. Lin
The parallel polarization component of CALIPSO lidar backscatter from ocean surface is primarily a result of specular reflection. When a laser beam hits a water surface at near normal incidence, about 2% of the laser energy is reflected and the rest of the energy goes into the water. For weak winds, the water surface is smooth and specular reflection is a narrow beam with little divergence. As wind speed increases, the surface roughens and the divergence increases while the reflected energy remains at 2%. Thus, lidar backscatter intensity reduces as wind increases. Simple relations [Hu et al., 2008] between wind speed and ocean surface roughness (wave slope variance) can be applied to estimate ocean surface wind speed from 1064nm lidar clear sky ocean measurements.
To drive accurate sea surface wind speed during day-time, sea surface skin temperature and air temperature from AIRS, are also required in order to account for the surface roughness dependence of sea state.
With in situ eddy correlation measurements, recent studies suggest turbulence exchange (k) can be greatly improved with short-term/steady wind speed: k=cV^3(660/Sc)^0.5. This suggests that global high resolution wind statistics can help reduce uncertainties in air-sea exchange. QuikScat is not sufficient since averaged wind at 10-30 km does not resolve sub-kilometer scale eddy processes: a^3 + b^3 > [ 0.5* (a+b) ]^3. Higher spatial resolution CALIPSO wind (70m) statistics helps improving the gas exchange at the air-sea interface. With turbulence exchange derived from CALIPSO wind (70m) speed, the estimate of global carbon uptake by ocean can be improved. Carbon Uptake = cV^3(660/Sc)^0.5 s D(Pco2). Here Sc is the Schmidt number, s is the solubility of CO2, and D(Pco2) is partial pressure difference between water and air.
The high resolution CALIPSO wind speed may also help improve wind power estimation at various off-shore sites, as wind power is also proportional to the cubic of instantaneous wind speed.
Averaging along-track to the AMSR-E footprint, the CALIPSO wind speed agrees well with AMSR-E wind speed. This experimental CALIPSO wind speed is available to the public.
Hu, Y., K. Stamnes, M. Vaughan, J. Pelon, C. Weimer, D. Wu, M. Cisewski, W. Sun, P. Yang, B. Lin, A. Omar, D. Flittner, C. Hostetler, C. Trepte, D. Winker, G. Gibson, and M. Santa-Maria, 2008: “Sea surface wind speed estimation from space-based lidar measurements”, Atmos. Chem. Phys., 8, 3593-3601.
Session 6B, Development and use of global air-sea data sets
Tuesday, 13 January 2009, 1:30 PM-2:45 PM, Room 128B
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