Occurrence of TOMS V7 Level-2 Ozone Anomalies over Cloudy Areas Xiong Liu, 1 Mike Newchurch, 1,2 and Jae Kim 1,3 1. Department of Atmospheric Science, University of Alabama, in Huntsville, USA 2. National Center for Atmospheric Research, Boulder, Colorado, USA 3. Department of Atmospheric Science, Pusan National University, Korea Motivation We found high positive or negative correlation between ozone and reflectivity from TOMS V7 L2 data ( Figure 1 ). How frequent? Why? Geophysical phenomena, or ozone retrieval errors? Data and Methodology N7( ), EP( ) TOMS V7 L2 data. Perform aerosol and sun glint correction to exclude their effects. Focus on the effects of clouds. Calculate the correlation coefficient (r) and slope between ozone and reflectivity in a 5º x 5º area between 60ºS and 60º N. Positive Ozone Anomaly: r> = 0.5 Negative Ozone Anomaly: r <= Analyze spatial distribution and seasonal variation and identify causes of ozone anomalies. Frequency Occurrence of Ozone Anomalies The annual average frequency of occurrence (Figure 2) is about 23% in both N7 and EP. Anomaly over land : Anomaly over ocean  1:3. Ozone Anomalies are not evenly distributed on globe, and some regions are dominated with high frequency of positive or negative anomalies. The spatial features of ozone anomalies in N7 is very similar to those in EP. Contrast between EP and N7: (1) more positive anomalies in N7, more negative anomalies in EP; (2) In EP, high frequency of negative anomalies instead of positive anomalies in tropical ITCZ regions. This is partly due to some non-linearity calibration errors most probably in N7. Mid-latitude Ozone Anomalies Ozone Anomalies in mid-latitudes ( Figure 7 ): frequency peaks in the late spring and summer time, and minimizes in the winter time. More cloudiness in the summer time leads to more ozone anomalies in the summer. Ozone/ reflectivity slope ( Figure 7 ) peaks in the winter time and minimizes in the summer. The seasonal variation of slope and the magnitude of mid-latitude ozone anomalies are consistent with the seasonal variation and magnitude of total ozone fluctuations, suggesting that they are mainly controlled by the planetary-scale and medium-scale (synoptic-scale) wave activities. Clouds and high ozone are usually associated with cyclones, coupling to form positive ozone anomalies. Conclusions The annual global average frequency of occurrence of ozone anomalies is about 23% in both N7 and EP. There are two EP/N7 contrasts probably due to some calibration errors in N7 TOMS data. There are large cloud-top height errors in the TOMS algorithm. Cloud-top pressure is overestimated by ~200mb for high clouds and is underestimated by about ~100mb for low clouds.  P correction eliminates most of the negative anomalies, suggesting negative anomalies are due mainly to incorrect cloud heights.  P correction increase positive anomalies by 20-40% in tropics and 5-10% in mid-latitudes. There is usually more ozone over cloudy areas after  P correction. High frequent occurrence of tropical positive anomalies is most probably due to non- linearity calibration errors in N7 and ozone retrieval errors associated with clouds. Ozone anomalies in mid-latitude regions are mainly controlled by planetary-scale and synoptic-scale wave activities. Positive anomalies off the west coast of South Africa and South America are mainly due to the enhanced tropospheric ozone during biomass burning seasons and persistent low marine stratus clouds Cloud Height Error There is large pressure difference (Figure 5 ) between THIR cloud height (temporally and spatially collocated) and ISCCP cloud height (actually assumed in TOMS algorithm). Cloud-top pressure is overestimated by ~200mb for high clouds and is underestimated by about ~100mb for low clouds. If the the assumed cloud-top height is lower than the actual, TOMS algorithm underestimated total ozone and vice versa. We corrected the cloud-height induced errors using  P correction (described in TOMS accuracy poster) based on TOMS standard ozone profiles and the spatial distribution of ozone anomalies are shown in Figure 6. Ozone Anomaly after  P correction After  P correction, most of the negative anomalies in tropics and mid-latitudes are disappeared, suggesting they are mainly due to underestimated cloud-heights. The enhanced frequency of negative anomalies in the eastern Pacific Ocean during El Ni  o periods is due to the larger cloud height errors in TOMS data than during non-El Ni  o periods. There is a dramatic increase in the frequency of positive anomalies by 20-40% in tropical high convective cloudy areas and 5~10% in mid-latitudes. The spatial distribution of positive anomalies after correction is very similar to the distribution of clouds. More ozone is usually observed over cloudy areas after  P correction. The more ozone over tropical convective cloudy areas is due mainly to some non-linearity calibration errors in N7 and the treatment of clouds as Lambertian Surfaces. The dynamical and chemical effects are estimated to be small (See TOMS accuracy poster) The ozone retrieval errors that cause tropical ozone anomalies cannot account for the much larger ozone/reflectivity slope for mid-latitude ozone anomalies. Ozone Anomalies in Marine Stratocumulus Regions Both the slope and frequency of positive anomalies off the west coast of South Africa and and South America peak from August to October and minimize from Feburary to April ( Figure 8 ), consistent with the biomass burning season. These regions are covered with persistent low marine stratocumulus clouds. Ozone or its precursors generated due to biomass burning in South Africa and South America can be transported to these regions. The tropospheric ozone is usually larger than that in the TOMS standard profile in the biomass burning season. Ozone/Reflectivity Slope of Ozone Anomalies The spatial distribution of annual average ozone/ reflectivity slope (Figure 3 ) is mainly a function of latitude. Doesn’t dependent much on the sign of anomaly, land or ocean, high or low frequency of occurrence, EP or N7. In tropics: 12~30 DU/100% In mid-latitudes: 40~84 DU/100% Long-term Temporal Variation The spatial distribution of ozone anomalies is consistent from year to year in both N7 and EP TOMS data except that there is increased frequency of negative anomalies in the central and eastern Pacific Ocean. Annual average frequency and slope of positive and negative anomalies doesn’t change much from year to year except the N7/EP bias (Figure 4 ). TOMS sensors is not sensitive to the enhanced tropospheric ozone in the lower tropospheric ozone, the existence of persistent stratocumulus increases the ozone retrieval efficiency to even greater than 1 [Hudson et al., 1995]. The contrast in ozone retrieval efficiency between clear and cloudy areas mainly leads to positive ozone anomalies in these regions. In addition, enhanced ozone absorption in clouds might also contribute to the cloudy ozone excess in these regions. Updated on January 12, 2001