This paper examines the reliability of the TOVS ozone retrievals. The quality of the new TOVS total ozone soundings are compared visually and statistically to the corresponding TOMS values and to values reported by the ground-based DOBSON network. The three-way TOMS, TOVS, and DOBSON comparison concentrates on observations during the period of the great Antarctic "ozone hole" late in 1987.
Because of the complexity of the circulation patterns in the total ozone on a wide range of space-time scales, the results of this inter-comparison are best presented using an animated videotape to show three side-by-side versions of the development and decay of many different ozone features over the Earth (available from the authors).
The TOMS satellite-based instrument uses differential absorption of reflected solar ultraviolet light to estimate total ozone (Fleig et al., 1982). The TOMS data used in this study are version 5 of the globally gridded database with approximately 100 km resolution, designated "TOMS(5)", and have been converted to a convenient video format for browsing and comparison to other datasets (Chesters and Krueger, 1989).
The TOVS physical retrieval algorithm uses the difference between NOAA's pre-processed clear-column infrared 11 µm and 9.7 µm brightness temperatures to determine total ozone. The TOVS physical algorithm adopted by NOAA (Neuendorffer, manuscript in progress) is similar to prior algorithms used to retrieve total ozone from thermal infrared data (Prabhakara et al., 1976; Muller and Cayle, 1983; Shukorov and Shukorova, 1986; Lienesch, 1988). The non-ozone channels are used to determine a "foreground" temperature for the ozone and a "background" temperature for the surface. Then the brightness temperature from the semi-transparent 9.7 µm ozone band channel is used in a linear algorithm to determine the ozone opacity. Finally, a small set of ground-based DOBSON stations in 1984 are used to determine a monotonic relationship between TOVS-determined ozone opacity and the total column ozone. The relationship is otherwise uncoupled from other ozone measurements or time-dependent corrections. Daily synoptic maps are generated from the thousands of individual TOVS physical retrievals from the day- and night-time overpasses from two different satellites, using standard operational polar stereographic mapping routines at approximately 330 km resolution. Regions of missing TOVS ozone data values due to lost orbits are not marked, but are replaced by average values for the latitude band.
The TOMS and TOVS algorithms have both been designed and tuned to perform well at mid-latitudes. Indeed, TOMS and TOVS agree well at mid-latitudes, but each DOBSON ground station has its own individual pattern of disagreement with the two satellites. For example, Fig. 3 presents the ozone measurements by the NASA-operated instrument at Wallops Island, VA, USA. In this case, the DOBSON observations are all larger than the two satellites by 10 to 40 DU, a large disagreement between instruments thought to have ±5 DU accuracy and to be useful for detecting long-term trends to ±1% per year. Other DOBSON sites have other error patterns, such as biases lasting a few weeks to months, occasional excursions, no sensitivity to satellite-observed excursions, etc.. The discrepancies between the TOMS, TOVS, and DOBSON time-series at most mid-latitude sites raises questions about using the DOBSON network as "the standard" for measuring total ozone. Rather, one concludes that several different satellite and ground-based systems must be employed to discover and eliminate erroneous values from each, in order to arrive at a reliable consensus estimate for ozone amounts and trends. Each method for measuring ozone should be independently calibrated with respect to an accurate standard, and not with respect to each other.
Figures 4 and 5 present time-series plots of TOMS, TOVS, and DOBSON observations of total ozone before and after the onset of polar night near the North and South poles, respectively. The TOMS, TOVS, and DOBSON measurements inside the Arctic Circle presented in Fig. 4 indicate poor agreement among all three sensors before polar night, with TOMS appearing to significantly under estimate ozone on the edge of polar night, a strong impression also provided by the animated series of daily ozone images. TOVS provides ozone estimates in polar winter, but their quality is untested. The TOMS, TOVS, and DOBSON measurements at the South Pole presented in Fig. 5 indicate good agreement among all three sensors during the "ozone hole" just after the end after polar night. TOVS provides ozone estimates during the initial formation of the "ozone hole" early in polar winter night, but their quality is untested. However, during the breakdown of the polar vortex and dissolution of the "ozone hole" at the beginning of December, TOVS estimates of total ozone are in error, perhaps due to the poor sensitivity of the infrared channels to the changes that occur in the upper stratosphere at this time.
At mid-latitudes, ozone concentrations are definitely correlated with low pressure systems (Schoeberl and Krueger, 1983), and move with the prevailing westerlies rather than with the seasonal reverses in the stratospheric winds. On the synoptic scale, ozone concentrations have been noted near individual storms (Sechrist et al., 1986), possibly concentrated near the tropopause by folding and/or subsidence. Chesters et al. (1990) have shown that local ozone maxima near baroclinic waves actually occur eastward of the tropopause folds associated with mid-latitude troughs and jet streaks. In the tropics, weak ozone concentrations have been linked to upper-level troughs near hurricanes (Rodgers et al., 1986). However, the mechanisms linking total ozone to the weather are still poorly understood. Some of the TOVS ozone errors are due to the background emissivity fluctuations created by cirrus cloudtops. Likewise, polar cirrus may be causing some of the high latitude errors noted in TOMS ozone estimates (A.J. Krueger, private communication).
The new infrared-based TOVS ozone estimates are generally in good agreement with the venerable ultraviolet-based TOMS measurements. However: TOVS resolution is lower, Reststrahlen causes over estimates above deserts, cirrus clouds cause under estimates near the equator, and temperature-profile and ozone-profile abnormalities cause significant errors during large scale stratospheric disturbances such as the breakdown of the Antarctic ozone hole. TOVS errors in estimating ozone over the deserts would be minimized by the addition of an 9 µm channel to the system.
The TOMS ozone estimates appear generally reliable. However, TOMS estimates at the edge of polar night, especially in the northern hemisphere, are much lower than the corresponding TOVS and DOBSON observations, and are also significantly affected by polar stratospheric clouds. TOMS detects arctic "mini-holes", but appears to over estimate their "depth".
Each site in the DOBSON network has a highly individual pattern of differences with respect to the two satellites. It is difficult to generalize, but satellite biases with respect to mid-latitude stations are often significant (approximately ±30 DU, when ±5 DU is expected), and systematic errors in the "ground truth" are indicated. Some DOBSON sites are completely inconsistent with both satellites and with nearby DOBSON sites.
The TOMS, TOVS, and DOBSON datasets are periodically reprocessed to improve data processing techniques. CAE's DOBSON network database sporadically receives additional and corrected values of the "ground truth" observations from previous years. By the end of 1990, NASA's NIMBUS-7 data processing team will issue "version 6" of the TOMS gridded database, with improved calibration and extended time-coverage. In the latter half of 1990, NOAA's Climate Analysis Center (CAC) will implement the TOVS physical algorithm and provide ozone observations operationally. In 1991, CAC may refine the TOVS algorithm after the ozone data characteristics are studied during the 1990 "ozone hole" episode.
Later in the 1990's, several other satellite systems (METEOR, GOES, UARS and EOS) will be launched carrying ozone sensors to monitor the Earth. Eventually, a consensus among the many observations of total ozone could become a daily operational meteorological and climatological parameter available on demand over the nation's digital communications networks.
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