Cloud slicing: A new technique to derive upper tropospheric ozone from satellite measurements

A new technique called cloud slicing has been developed for measuring upper tropospheric O-3. Cloud slicing takes advantage of the opaque property of water vapor clouds to ultraviolet wavelength radiation. Measurements of above-cloud column O-3 from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument are combined together with Nimbus 7 temperature-humidity and infrared radiometer (THIR) cloud-top pressure data to derive O-3 column amounts in the upper troposphere. In this study, tropical TOMS and THIR data for the period 1979-1984 are analyzed. By combining total tropospheric column ozone (referred to as TCO) measurements from the convective cloud differential (CCD) method with 100- to 400-hPa upper tropospheric column O-3 amounts from cloud slicing, it is possible to estimate 400- to 1000-hPa lower tropospheric column O-3 and evaluate its spatial and temporal variability. Results for both the upper and lower tropical troposphere show a year-round zonal wave number 1 pattern in column O-3 with the largest amounts in the Atlantic region (up to similar to 15 DU in the 100- to 400-hPa pressure band and similar to 25-30 DU in the 400- to 1000-hPa pressure band). Upper tropospheric O-3 derived from cloud slicing shows maximum column amounts in the Atlantic region in the June-August and September-November seasons which are similar to the seasonal variability of CCD-derived TCO in the region. For the lower troposphere, the largest column amounts occur in the September-November season over Brazil in South America and also southern Africa. Localized increases in the tropics in the lower tropospheric O-3 are found over the northern region of South America around August and off the west coast of equatorial Africa in the March-May season. Time series analysis for several regions in South America and Africa show an anomalous increase in O-3 in the lower troposphere around the month of March which is not observed in the upper troposphere. The eastern Pacific indicates weak seasonal variability of upper, lower, and total tropospheric O-3 compared with the western Pacific, which shows the largest TCO amounts in both hemispheres around spring months. O-3 variability in the western Pacific is expected to have greater variability caused by strong convection, pollution and biomass burning, land-sea contrast, and monsoon developments.