Occurrence of ozone anomalies over cloudy areas in TOMS version-7 level-2 data

[摘要] This study investigates anomalous ozone distributions over cloudy areas in Nimbus-7(N7) and Earth-Probe (EP) TOMS version-7 data and analyzes the causes for ozoneanomaly formation.A 5°-longitude by 5°-latitude region is defined to contain a PositiveOzone Anomaly (POA) or Negative Ozone Anomaly (NOA) if the correlation coefficientbetween total ozone and reflectivity is > 0.5 or <-0.5.The average fractions of ozone anomalies among all cloud fields are31.8 ± 7.7% and 35.8 ± 7.7% in the N7 and EP TOMS data, respectively.Some ozone anomalies are caused by ozone retrieval errors, andothers are caused by actual geophysical phenomena.Large cloud-height errors are foundin the TOMS version-7 algorithm in comparison to the Temperature Humidity InfraredRadiometer (THIR) cloud data.On average, cloud-top pressures are overestimated by~200 hPa (THIR cloud-top pressure < 200 hPa) for high-altitude cloudsandunderestimated by ~150 hPa for low-altitude clouds (THIR cloud-top pressure> 750 hPa). Most tropical NOAs result from negative errors induced by large cloud-height errors, andmost tropical POAs are caused by positive errors due to intra-cloud ozone absorptionenhancement.However, positive and negative errors offset each other, reducing theozone anomaly occurrence in TOMS data.Large ozone/reflectivity slopes for mid-latitudePOAs show seasonal variation consistent with total ozone fluctuation, indicatingthat they result mainly from synoptic and planetary wave disturbances.POAs with anoccurrence fraction of 30--60% occur in regions of marine stratocumulus off the westcoast of South Africa and off the west coast of South America.Both fractions andozone/reflectivity slopes of these POAs show seasonal variations consistent with that inthe tropospheric ozone.About half the ozone/reflectivity slope can be explained byozone retrieval errors over clear and cloudy areas.The remaining slope may result fromthere being more ozone production because of rich ozone precursors and higherphotolysis rates over high-frequency, low-altitude clouds than in clear areas.Ozoneanomalies due to ozone retrieval errors have important implications in TOMSapplications such as tropospheric ozone derivation and analysis of ozone seasonalvariation.

[学科分类] 大气科学