1. TOMS Ozone Retrieval Sensitivity to Assumption of Lambertian Cloud Surface Part 2. In-cloud Multiple Scattering 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, Arizona, USA 4. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Abstract. In part I, we find the assumption of isotropic cloud scattering is fairly good in TOMS ozone retrieval. In this poster, we study the effect of the neglect of enhanced ozone absorption by in-cloud multiple scattering on TOMS ozone retrieval. Ozone absorption enhancement in cloud depends significantly on solar zenith angle and view zenith angle, ozone amount in clouds, ozone distribution in clouds, and cloud optical thickness. It also depends somewhat on different cloud types and cloud location. The neglect of ozone absorption enhancement in clouds by assuming optically thick clouds as Lambertian reflecting surfaces in the ozone retrieval introduces significant positive errors to the retrieved total ozone, and is the dominating source of errors, especially at smaller zenith angles. 43 21 www.nsstc.uah.edu//atmchem Methodology The retrieved ozone difference with and without ozone in clouds in the forward calculation gives the the enhanced ozone. We study how the enhanced ozone 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, geometrical thickness, and ozone distribution in clouds. We also study the vertical distribution of ozone enhancement. Divide a cloud into layers, the retrieved ozone difference without ozone only in one layer and with ozone in all the layers approximates the contribution of that layer to the overall ozone enhancement. To represent those tropical high-reflecting clouds, a typical homogeneous cloud is put between 2 - 12 km with an optical thickness of 40. Generally, the same amount of ozone as present in the TOMS standard profile is homogeneously distributed in clouds. The sensitivity of ozone enhancement to ozone amounts and ozone distribution is studied. Introduction The motivation and objectives of this study are shown in part 1. In part 1, we study the effect of assuming cloud scattering as isotropic on TOMS ozone retrieval. For most conditions, the non-isotropic effect is within ±4 DU for optical thickness  20, indicating the assumption of isotropic cloud scattering is fairly good for optically thick clouds. We study the effect of neglect of ozone absorption enhancement due to in-cloud multiple scattering on TOMS ozone retrieval in this poster. The radiative transfer models used in this study are shown in part 1. Ozone Absorption Enhancement vs. Viewing Geometry Figure 1 shows the enhanced ozone vs. viewing geometry for a water cloud of optical depth (OD) of 40 between 2-12 km. There is 20.8 DU ozone in cloud. The enhanced ozone decreases with the increase of SZA and VZA (Figure 1 and Figure 2a) and is azimuthally independent, ~19 DU at nadir and only 0.15 DU at SZA = 75° and VZA = 70 °. The exchange of SZA and VZA does not change the amount of enhanced ozone. The photon path length in clouds decreases with increasing SZA and VZA. Furthermore, TOMS algorithm automatically accounts for the geometrical path length ( 1/cos(SZA) + 1/cos(VZA) ). These two factors lead to the dramatic decrease of enhanced ozone vs. geometrical path length. Figure 1. Enhanced Ozone vs. Viewing Geometry. * indicates the SZA. Figure 2. Enhanced Ozone vs. Cloud type (a), Cloud Optical Depth (b) and Ozone Amount in Clouds (c). Enhanced Ozone vs. Cloud Type and Cloud Optical Depth The enhanced ozone vs. viewing geometry is similar for different types of clouds. The enhanced ozone differs slightly in magnitude among WC, WCHG, HEX, and POLY (Figure 2a). The enhanced ozone is smallest for POLY, largest for WC, due primarily to their difference in the asymmetry factor g. The smaller g, the less photons interact with clouds before they are scattered back to atmosphere. However, their difference is smaller, at most 2 DU between POLY and WC. The enhanced ozone decreases with the increase of cloud optical thickness (Figure 2b) because photons penetrate less into thicker clouds. At nadir, the enhanced ozone is 19.2 DU for OD of 10 and 10.9 for OD of 500. Ozone Absorption Enhancement vs. Ozone Amount and Cloud Location The enhanced ozone is almost linearly proportional to the amount of ozone in clouds. The enhancement (ratio of enhanced ozone to input ozone in clouds) actually decreases with the increase of ozone in clouds (Figure 2c). At nadir, the enhancement is 0.89 for 5.2 DU in cloud and 0.84 for 41.6 DU in cloud. The more absorber in clouds, the less photons penetrate into clouds. The enhanced ozone increases with the increase of geometrical thickness due primarily to the increase of the ozone in clouds (Figure 3). The relative enhancement increases with the height of cloud location. At nadir, the enhancement is 0.81 for a cloud at 2-3 km and 0.95 for a cloud at 11-12 km. Ozone Absorption Enhancement vs. Ozone Distribution in Clouds Figure 4a shows six different profiles of ozone extinction coefficients in clouds. The ozone distribution above and below clouds is the same for all. Profile 1 (original L275 profile), profile 2 (homogeneously distribution), 3, and 4 contains the same amount of ozone, i.e., 20.8 DU. Profile 5 and 6 are similar to profile 2 except that they contain ozone only in the upper 2 km and the lower 2 km, respectively. Figure 4b shows that the enhanced ozone varies greatly with the ozone distribution of ozone in clouds. The enhanced ozone is smaller for the original profile compared to profile 2 because less ozone is distributed in the upper portion. The highest enhancement is for profile 3. The enhanced ozone is almost zero for profile 6 because all the ozone is distributed only in the lower 2 km. Figure 3. Ozone enhancement vs. cloud location (water clouds, OD = 40). Figure 4. (a) Six ozone profiles. (b) Ozone enhancement vs. ozone distribution in clouds. Vertical Distribution of Ozone Enhancement Figure 5a shows the vertical distribution (20 0.5-km layers) of ozone enhancement for WC and OD = 40 at a few selected angles. The layer that contributes most is located in the upper 1 km. The weight decreases dramatically deeper into clouds. This vertical distribution explains the results shown in Figure 4. Figure 5b shows the depth below the cloud top above which 50% of the ozone enhancement is contributed vs. geometrical path length for several clouds. The penetration depth decreases with increasing optical thickness and geometrical path length, explaining the results Figure 2a and 2b. Among WC, HEX, and POLY, the penetration depth is largest for WC and smallest for POLY, consistent with the results in Figure 2a. Figure 5. Vertical weighting functions of ozone enhancement for WC and OD = 40 (a), and half-folding ozone enhancement depth (b). Summary and Conclusions We study the neglect of ozone absorption enhancement in clouds on TOMS ozone retrieval. The ozone absorption enhancement is due to the in-cloud multiple scattering, which interacts with and intensifies ozone absorption. The enhanced ozone depends significantly on solar zenith angle and view zenith angle, ozone amount in clouds, ozone distribution in clouds, and cloud optical thickness. It also depends somewhat on different cloud types and cloud location. Positive ozone retrieval errors occur without correcting the enhanced ozone. Compared to the non-isotropic effect, the ozone enhancement in clouds is the dominating sources of retrieval error in the assumption of optically thick clouds as Lambertian surfaces especially for small zenith angles. The treatment of clouds as Lambertian surfaces in the TOMS retrieval algorithm sufficiently explains the 4-9 DU excess of ozone over cloudy areas over tropical high-reflecting convective areas. However, more information than is available about ozone distribution in clouds is needed to accurately characterize individual ozone retrieval errors associated with clouds. Further study will be performed on ozone retrieval errors over partial cloudy areas.