REPORT to the TOMS SCIENCE TEAM GSFC
7-9 April 1999

CONTENTS

• Background
• PART I: Analysis of Ozone Anomalies over Cloudy Areas from TOMS V7.0 L2 Data
• Part II: Determining Clear and Cloudy Regions for Retrieving Tropospheric Ozone with TOMS V7.0 L2 Data

BACKGROUND

• Convective-cloud Differential (CCD) [Ziemke et al., 1998].

• Assumption: there is not much difference in the vertical ozone distribution between cloudy areas and neighboring clear areas. We can obtain the tropospheric ozone column by subtracting the ozone column above cloudy areas from the total ozone column in clear areas.

• An ozone anomaly in North Dakota on June 29, 1989 [Fig 1] . The higher the reflectivity, the higher the ozone amount, the largest ozone difference is over 40 DU, which almost amounts to the total tropospheric ozone column.

• First examine when and where this assumption works, and analyze the formation of ozone anomalies.

Part I: Analysis of Ozone Anomalies over Cloudy Areas from TOMS V7.0 L2 Data

APPROACH

• Grid the global region [180 W, 20 S, 180 E, 20 N] to 5 X 5 degree cells.

• In each cell, select those measured points : reflectivity > 0%, and error flag = 0.

• Calculate the correlation coefficient between the total ozone column and reflectivity for those cells with more than 20 required points. If the absolute correlation coefficient is greater than 0.5, we call it an ozone anomaly (either negative or positive).

• Plot the frequency distribution of correlation coefficients and ozone anomalies.

RESULTS

• Frequency distribution of all correlation coefficients for 1/1-15/80 and 7/1-15/80 [Fig 2] . There are total 3457 ozone anomalies in these 4 weeks.

• Frequency distribution of ozone anomalies during 1/1-15/80 and 7/1-15/80 [Fig 3] . The occurrence of ozone anomalies is not negligible. The average occurrence in each 5 by 5 pixel is 20%.

• Do the scan angle and solar zenith angle account for the correlation coefficient distribution [Fig4] ?

• A negative ozone anomaly with correlation coefficient of -0.9 [Fig 5] . There is a strong correlation between ozone and cloud pressure difference.

• Ozone retrieval error brought about by the cloud pressure difference [Fig 6-7].

• Ozone as a function of actual (THIR) cloud pressure - TOMRAD assumed pressure [Fig 8] for those scenes with reflectivity greater than 80%. It is consistent with our calculated results.

• A positive ozone anomaly with correlation coefficient of 0.95 [Fig 9]. However, there is a negative correlation between ozone and cloud pressure difference. There are other factors that contribute to the formation of ozone anomalies.

• Most negative anomalies (designed in terms of cloud fraction instead of reflectivity) are positively correlated with cloud pressure difference, most positive anomalies are negatively correlated with cloud pressure difference [Fig 10] (two left panels).

• Cloud fraction and cloud pressure difference are two factors that affect ozone anomalies. 2-factor linear regression: ozone = A1 * @ P + A2 * f_C

• Plot A1 Vs. A2. Under most conditions, ozone are positively correlated with cloud pressure difference (y axis) [Fig 10] (two right panels).

• Tropical maximum - minimum ozone in 5 X 5 cells is 15 + 10 DU [Fig 11] (two left panels) , and is independent on the correlation coefficient.

• Clear -- cloudy ozone above clouds (tropospheric ozone) is a strong function of the sign of the anomaly [Fig 11] ( two right panels).

FUTURE DIRECTIONS

• Using TUV, a 2-stream and Discrete Ordinate radiative transfer code, we will investigate cloud-height-induced errors on TOMS ozone retrieval and other cloud-induced errors such as cloud optical thickness.

• The scattering characteristics of ice crystals, the effects of neglecting ice crystals on high cirrus cloud areas on ozone retrieval.

• Effective J-values for depleting ozone increase above or in clouds [Fig 12] . We will evaluate chemical effects induced by clouds.

PART II: Determining Clear and Cloudy Regions for Retrieving Tropospheric Ozone with TOMS Data

APPROACH

• For cloudy points in tropics [ 30s,30n] : Reflectivity >= 90; THIR cloud pressure <= 100 hPa
• Clear points : Reflectivity <= 0.15

RESULTS

• [Fig 13] shows the reflectivity distribution in tropical areas on July 25, 1980. The reflectivity is related to cloud height [Fig 14] , so we can use reflectivity to estimate the capability of Clear/Cloud Method to retrieve the tropospheric ozone. High reflectivity can not guarantee high cloud height. The error is about 10% in tropics [30s, 30n].

RESULTS

• [Fig 15] and [Fig 16] show the tropical distribution of areas with reflectivity > 0.9 colored by altitude, when green is corresponding to THIR pressure < 200 hPa and red corresponding to lower altitude clouds. [Fig 17] shows the 1980 annual high-cloud distribution.

• Relevant to looking at lower-altitude clouds. [Fig 18] indicates limited potential for cloud altitude below 500 hPa.

CONCLUSIONS

• Ozone anomalies are pervasive.

• The anomalies seem to be related to incorrect cloud heights and also to the cloud fraction, which probably encompasses radiative transfer errors and possibly chemical effects. Solar zenith angle and scan angle seem to be unimportant.

• The distribution of clouds is less than ideal for calculating tropospheric ozone.