Gao approach to Water Vapor Retrieval

1. Use of Shadows to Retrieve Water Vapor in Hazy AtmospheresDr. North Larsen, Lockheed Martin IS&SDr. Knut Stamnes, Stevens Institute Technology 1. Techniques aimed at retrieving water vapor from satellite data of reflected near-infrared solar radiation have progressed significantly in recent years. These techniques rely on observation of water vapor attenuation of near-infrared solar radiation reflected by the Earth's surface. Ratios of measured radiances at wavelengths inside and outside water vapor absorbing channels are used for retrieval purposes. These ratios partially remove the dependence of surface reflectance on wavelength and are used to retrieve the total column water vapor amount. Hazy atmospheric conditions, however, introduce errors into this widely used technique. A new method based on radiance differences between clear and nearby shadowed surfaces, combined with ratios between water vapor absorbing and window regions, is presented that improves water vapor retrievals under hazy atmospheric conditions. Radiative transfer simulations are used to demonstrate the advantage offered by this technique Simulations of the reflectance ratios for Gao’s method. Solid line is the clear sky case, while dashed lines are the visibilities for 10-2 km. Surface albedo= 0.5, nadir looking sensor, and sun at 45 degrees solar zenith angle. Where annual average total Precipitable Water vapor 0.25 – 5 g/cm2 (polar – tropics) • Gao approach to Water Vapor Retrieval • Approximations used ignores the Upwelled Radiance term, thus ignoring Haze • TOA Reflectance Ratios of 3 NIR absorbing bands to a nonabsorbing band generate LUTs to • retrieve water vapor. 3 channels used due to varying strength of water vapor absorption and need over nonuniform surface reflectances (soil) • Under low visibility conditions (<10 km) errors in total column water vapor > 10% • MODIS sensitivity studies have shown for every 0.01 error in transmittances led to a 2.5% error in retrieved water vapor. Producing errors of up to 10% for visibilities less than 10 km. • 4. The Shadow Method to water vapor retrieval • Includes the Upwelled Radiance term • By subtracting the shadowed radiance from the clear the upwelled radiance and downward diffuse radiance are both eliminated – leaving only directly transmitted radiance • Uses TOA (Clear-Shadow) Reflectance Ratios of 3 NIR absorbing bands to a nonabsorbing band generate LUTs to retrieve water vapor • Convergence of lines of varying visibilities illustrates methods improvement Simulations of the reflectance ratios for the Shadow method. Solid line is the clear sky case, while dashed lines are the visibilities for 10-2 km. Surface albedo= 0.5, nadir looking sensor, and sun at 45 degrees solar zenith angle. 5. End result is that water vapor amounts may be retrieved more accurately under hazy atmospheric conditions Incident irradiance TOA EAR Upwelled Radiance • Band Ratios of TOA Reflectances • allow for water vapor retrievals