TOTAL OZONE/UV-B MONITORING AND FORECASTING: IMPACT OF CLOUDS AND THE HORIZONTAL RESOLUTION OF SATELLITE RETRIEVALS


J. R. Ziemke(1), J. R. Herman(1), J. L. Stanford(2) and P. K. Bhartia(1)

(1) Code 916, NASA Goddard Space Flight Center, Greenbelt, MD 20771
(2) Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011

J. Geophys. Res., 101, 3865-3871 (1998)


Abstract

This study compares the horizontal resolution of solar backscatter ultraviolet 2 (SBUV2) total ozone (Omega) fields with those from the new NASA earth probe (EP) and advanced earth observing satellite (ADEOS) total ozone mapping spectrometer (TOMS) side-scanning photometers. The latter instruments provide high resolution, easily resolving the medium-scale waves (4-7 wavelengths around the earth at a fixed latitude) that dominate day to day midlatitude Omega fluctuations. In contrast, SBUV2 instruments do not, since these devices measure only at nadir (straight downwards), yielding $\sim$ 14 measurements daily at a given latitude. This has consequences not only for global monitoring of Omega and ultraviolet-B (UV-B, 290-320 nm), but also for short timescale Omega and UV-B predictions in summer because timescales of a few days are coupled to medium horizontal scales (several thousand km) by baroclinic waves that typically force the observed Omega variations. 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 reductions in Omega prediction errors, whereas predictions using SBUV2 Omega are no better than persistence (where tomorrow's Omega is taken to be today's) in the biologically important summer months. Daily variabilities (equivalent to errors in 24-hour persistence forecasting of Omega) in high-resolution TOMS midlatitude ozone during summer are shown to sometimes exceed 50 Dobson units, producing daily changes of 20\% or greater in computed ground-level clear-sky UV-index. This study demonstrates 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 UV-B variability is dominated by local cloudiness variations, Omega forecasts can enhance UV-B prediction in relatively cloud-free regions such as the USA's desert southwest and in stagnant high pressure regimes that can persist for 1-2 weeks in summer. Furthermore, as weather forecast models increase in accuracy of forecasted cloudiness, accurate predictions will allow more accurate UV-B forecasts for cloud-free regions, the locations where they are most needed. Results from the present paper show, however, that TOMS-like side-scanning Omega measurements are required for ozone prediction and monitoring, rather than SBUV-type nadir observations.