TOMS Ozone Retrieval Sensitivity to Assumption of Lambertian Cloud Surface Part 1. Scattering Phase Function Xiong Liu, 1 Mike Newchurch, 1,2 Robert Loughman 3, and Pawan K. Bhartia 4 1. Department of Atmospheric Science, University of Alabama in Huntsville, Huntsville, Alabama, USA 2. National Center for Atmospheric Research, Boulder, Colorado, USA 3. Cooperative Center for Atmospheric Science & Technology, University of Arizona, Tucson, AZ, USA 4. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Abstract. Cloud surfaces are not Lambertian; cloud reflectivity is angularly dependent and the penetration depth is greater than zero. We study the effect of assuming isotropic scattering in the TOMS ozone retrieval. The non-isotropic effect results from the difference in the ozone absorption enhancement above clouds due to rayleigh scattering and multiple cloud reflection between the simulated scattering clouds and assumed Lambertian clouds in the ozone retrieval. This effect varies with viewing geometry, cloud optical thickness, cloud type (different phase functions), cloud-top height, and ozone above cloud. However, for most conditions, the non-isotropic effect is within ±4 DU, indicating the assumption of cloud scattering as isotropic is fairly good for clouds with optical thickness  Figure 1. Possible sources of ozone retrieval errors. Radiative Transfer Models Treat clouds as scattering medium, calculate the backscattered radiance at the top of the atmosphere using Polarized Plane-parallel Gauss-Seidel Radiative Transfer Code (PPGSRAD) at N7 TOMS six channels (312, 317, 331, 339, 360, and 380 nm). Polarization is considered for clouds with optical depth  150. Retrieve ozone using TOMS version-7 algorithm (TOMSV7), from which the look-up table is calculated using TOMRAD code at 10 pressure levels from 1.0 to 0.1 atm to reduce radiation interpolation error. The radiance difference between TOMRAD and PPGSRAD is 0.2%, on average, for clear sky conditions. Use wavelength-weighted ozone absorption coefficients, rayleigh scattering coefficients and molecular depolarization factor at each channel (consistent in PPGSRAD and TOMRAD). TOMS standard low-latitude ozone profile and temperature profile L275 are used as example profiles. Optical properties of water clouds are computed by Bohren-Huffman’s Mie Code. Optical properties of polycrystals and hexagon column crystals are computed by Ray Tracing Code. Methodology Separate the effect of assuming cloud scattering as isotropic on TOMS ozone retrieval from the effect of the neglect of ozone absorption in clouds. The non-isotropic effect is shown in this poster. If there is no ozone in the cloud in the forward calculation, ozone absorption and its enhancement do not occur. The difference between the retrieved ozone and the input ozone characterizes the non-isotropic effect. We study how the non-isotropic effect varies with solar zenith angle (SZA) and view zenith angle (VZA) (SZA  75 °, VZA  70 °), cloud types including water clouds (WC), hexagonal column ice crystals (HEX), polycrystals (POLY), and water clouds with Henyey-Greenstein phase function (WCHG), optical thickness of clouds, cloud location, and thickness. To represent those tropical high-reflecting clouds, a typical homogeneous cloud is put between km with an optical thickness of 40 (corresponding to cloud reflectivity of ~80% for water clouds). Motivation and Objectives We see significant total-ozone-column excess of DU over tropical high-altitude, highly reflecting clouds compared to clear observations. After accounting for errors involving incorrect cloud height, tropospheric ozone climatology, and considering potential dynamical, photochemical, and NIMBUS-7(N7)/Earth Probe calibration errors, approximately 4~9 DU excesses over cloudy scenes remain unexplained. We speculate that the TOMS algorithm approximation of optically thick clouds as opaque Lambertian reflecting surfaces may account for a significant portion of these unexplained excesses. Cloud surfaces are not Lambertian; the reflection of clouds is angularly dependent. Furthermore, photons penetrate into clouds and the path length is enhanced due to in-cloud multiple scattering, resulting in enhanced ozone absorption. In addition, clouds might be ice (not water) clouds. Possible sources of ozone retrieval errors are illustrated in Figure 1. We use radiative transfer codes to address the effects of these aspects on TOMS ozone retrieval for thick clouds. Effect of Assuming isotropic cloud scattering Figure 2 (left) shows the retrieved total ozone as a function of viewing geometry for a water cloud between 2-12 km with optical depth (OD) of 40. The non-isotropic effect varies with viewing geometry, but is ±4.5 DU for the L275 DU ozone profile illustrated here. The non-isotropic effect decreases with increasing cloud OD as shown in Figure 2 and Table 1. When OD  30, the patterns of the non-isotropic effect vs. viewing geometry are very similar, but differs significantly when OD  20 (Figure 2). When OD increases, clouds act more like Lambertian surfaces; therefore the non-isotropic decreases. In addition, at smaller cloud OD, the partial cloud model is used in the ozone retrieval, introducing additional ozone retrieval errors. The non-isotropic effect shows different patterns vs. viewing geometry for WC (Figure 2 left), HEX (Figure 3 left), POLY (Figure 3 right), and WCHG. For POLY, the total ozone is ±3.5 DU of the original 275 DU. For HEX, the total ozone is within ±4.5 DU of 275 DU except at a few angles where the the error could be -7.5 or DU. For WCHG, the total ozone is similar to WC except at anti-solar side and SZA  70, where the error ranges from -10 to -7 DU (Table 1). The non-isotropic effect varies with the ozone profiles. The more ozone above cloud, the larger the non-isotropic effect (Table 1). The non-isotropic effect varies a great deal with cloud-top height. The three clouds (2-12 km, km, km) show almost the same pattern, but the three clouds (2-4 km, 2-8 km, 2-12 km) shows very different patterns of non-isotropic effect (Figure 4). Figure 2. Non-isotropic effect for water clouds at OD=40 (left) and OD=10 (right). * indicates the solar zenith angle. Figure 3. Non-isotropic effect for ice clouds. Hexagon column crystals (left). Polycrystals (right). Non-isotropic effect The non-isotropic effect is mainly due to the difference in the ozone absorption enhancement resulting from Rayleigh scattering and cloud reflection between simulated scattering clouds in PPGSRAD and assumed Lambertian clouds in TOMRAD. That is why it varies with cloud-top height, cloud optical thickness, ozone profiles, and phase function. The non-isotropic effect is within ±4 DU for most cloudy conditions (Table 1) with OD  20, approximately within the accuracy of TOMS ozone retrieval, indicating assuming cloud scattering as isotropic is fairly good. Figure 4. Non-isotropic effect for water clouds between km (left) and km (right). Table 1. The range of non-isotropic effect, the average and standard deviation over viewing geometry (SZA: 0°, 15°, 30°, 45°, 60°, 70°, and 75°; VZA from 0° to 70° every 5°; AZA from 0° to 180° every 30°) for different conditions. Summary and Conclusions Motivated by the desire to explain the excess observed ozone over cloudy areas from our previous studies, we use radiative transfer models to study the ozone retrieval errors due to the treatment of optically thick clouds as opaque Lambertian reflecting surfaces. We separate ozone retrieval errors due to the assumption of isotropic cloud scattering from those errors due to the neglect of enhanced ozone absorption in clouds. The non-isotropic effect results from the difference in the ozone absorption enhancement above cloud due to the Rayleigh scattering and multiple cloud reflection between the simulated scattered clouds and Lambertian clouds. The non-isotropic effect varies with viewing geometry, cloud optical thickness, different types of clouds (different phase function), cloud-top height, and ozone above cloud. However, for most conditions, the non-isotropic effect is within ±4 DU, indicating the assumption of isotropic cloud scattering is fairly good for clouds with optical thickness  20.