EFFECT OF MARINE STRATOCUMULUS ON TOMS OZONE

The algorithm used to correct total O3 froM the total ozone mapping sp ectrometer (TOMS) for cloud effects is based on the measured reflectiv ity, a climatological cloud top height, and an assumed tropospheric O3 column amount below clouds. In regions of persistent subtropical mari ne stratocumulus it is assumed that this introduces a positive error i nto total O3 because these clouds are lower than the assumed mean clou d height used in the algorithm. This appears to be confirmed by high c orrelation between Nimbus 7 TOMS total O3 and reflectivity data for ty pical regimes of persistent stratus, as identified by the internationa l satellite cloud climatology project (ISCCP) observations. The TOMS t otal O3 overestimate has been computed using Nimbus 7/solar backscatte red ultraviolet total O3 derived using temperature humidity infrared r adiometer (THIR) data for years 1979-1984. A functional relationship b etween the THIR/non-THIR total O3 difference and reflectivity is used with TOMS reflectivity to modify Nimbus 7 TOMS O3 data for selected re gions and periods. The correction diminishes or eliminates a number of apparent O3 Maxima, with reductions of up to 20 Dobson units (DU) in total O3 on daily maps and approximately 5 DU on monthly mean O3 maps. Significant correlation between corrected TOMS O3 and reflectivity da ta remains because low-altitude O3 is retrieved more efficiently over a high-albedo surface. It is also possible that dynamical influences l eading to stratocumulus formation bring O3-enriched air into the area. These results imply that although good arguments can be made for the use of TOMS total O3 as a proxy for tropospheric O3 in the tropics, ca ution must be exercised in the use of daily and even monthly O3 maps i n the vicinity of clouds. Further research into the TOMS algorithm in cloudy regions is required to derive reliable estimates of tropospheri c O3.