TOTAL OZONE UVB MONITORING AND FORECASTING - IMPACT OF CLOUDS AND THEHORIZONTAL RESOLUTION OF SATELLITE RETRIEVALS

This study compares the horizontal resolution of solar backscatter ult raviolet 2 (SBUV2) total ozone (Omega) fields with those from the new NASA earth probe (EP) and Advanced Earth Observing Satellite (ADEOS) t otal ozone mapping spectrometer (TOMS) side-scanning photometers. The latter instruments provide high resolution, easily resolving the mediu m-scale waves (4-7 wavelengths around the Earth at a fixed latitude) t hat dominate day-to-day midlatitude Omega fluctuations. In contrast, S BUV2 instruments do not, since these devices measure only at nadir (st raight downward), yielding similar to 14 measurements daily at a given latitude. This method has consequences not only for global monitoring of Omega and ultraviolet B (UVB, 290-320 nm), but also for short-time scale Omega and UVB predictions in summer because timescales of a few days are coupled to medium horizontal scales (several thousand kilomet ers) by baroclinic waves that typically force the observed Omega varia tions. We use a simple Omega prediction model to test the use of Omega fields from TOMS and SBUV instruments and show that the higher zonal resolution from side-scanning TOMS instruments results in sizeable red uctions in Omega prediction errors, whereas predictions using SBUV2 Om ega are no better than persistence (where tomorrow's Omega is taken to be today's) in the biologically important summer months. Daily variab ilities (equivalent to errors in 24-hour persistence forecasting of Om ega) in high-resolution TOMS midlatitude ozone during summer are shown to sometimes exceed 50 Dobson units, producing daily changes of 20% o r greater in computed ground-level clear-sky UV index. This study demo nstrates that even these large daily changes in measured or predicted clear-sky UV are usually smaller than daily UV changes associated with transient clouds. While surface UVB variability is dominated by local cloudiness variations, Omega forecasts can enhance UVB prediction in relatively cloud free regions such as the U.S. desert southwest and in stagnant high-pressure regimes that can persist for 1-2 weeks in summ er. Furthermore, as weather forecast models increase in accuracy of fo recasted cloudiness, accurate predictions will allow more accurate UVB forecasts for cloud free regions, the locations where they are most n eeded. Results from the present paper show, however, that high-resolut ion TOMS-like side-scanning Omega measurements are required for ozone and UVB monitoring and prediction, rather than SBUV-type nadir observa tions.