Influence of a priori profiles on trend calculations from Umkehr data

Abstract

Although the new (1992) ozone profile retrieval algorithm for Umkehr measurements provides much better agreement with ozone sounding results than the old (1964) algorithm, considerable discrepancies remain with respect to ozone trends at different levels in the atmosphere. These discrepancies have been found by the comparison of long‐term trends obtained from the Umkehr measurements at Arosa and the ozone balloon soundings at Payerne (Switzerland). It is investigated here whether these obvious discrepancies can be removed by using time‐dependent a priori profiles. This procedure is successful only in the lowest part of the atmosphere, below about 19 km. To further explore this problem, synthetic Umkehr observations are calculated from the ozonesonde profiles. Trends are calculated for both the synthetic and actual Umkehr observations. The difference pattern between these Umkehr observation trends is compared with the difference in ozone profile retrieval trends from the synthetic and actual observations. The distinctive difference patterns strongly indicate an inherent disagreement between the Umkehr observations and the ozonesonde profiles. The application of corrections for stratospheric aerosol effects to the Umkehr profiles reduces, but does not eliminate, a discrepancy above 32 km. It is concluded that the discrepancies are due to the constant mixing ratio assumption used in computing the residual ozone above balloon burst level and to the fair‐weather bias of Umkehr observations (there are Umkehr observations at Arosa on fewer than 20% of the sonde observation days at Payerne). This sampling difference influences the results for the lower stratosphere. The study furthermore indicates that the ozone trends derived from Umkehr measurements for altitudes above about 32 km are robust for time‐dependent changes in the a priori profiles at lower altitudes. Based on the results of this study, we conclude with revised recommendations as to which atmospheric layers should be used for Umkehr trend studies.

Calculation of daily tropospheric ozone residuals using TOMS and empirically improved SBUV measurements: Application to an ozone pollution episode over the eastern United States

“…A priori information is constructed from a library of 46 profiles: 23 for each hemisphere subdivided by total ozone and latitude. The profile inversion from measured radiances and a priori information is done for 12 layers that correspond to the traditional Umkehr layers [see, e.g., McPeters et al, 1984; Bhartia et al, 1996;Mateer and DeLuisi, 1992;Mateer et al, 1996]. Both SBUV and SBUV2 data have been processed using the same algorithm, described by Bhartia et al [1996].…”

Intercomparison of Stratospheric Aerosol and Gas Experiment (SAGE) with Umkehr[64] and Umkehr[92] ozone profiles and time series: 1979–1991

“…altitudinally, so that the Umkehr will actually capture almost all the layer trend as shown by Miller et al [1997]. This relationship between a priori trends and Umkehr-measured trends has been considered independently by other authors [Mateer et al, 1996].…”

Information content of Umkehr and solar backscattered ultraviolet (SBUV) 2 satellite data for ozone trends and solar responses in the stratosphere

“…Comparisons between the trends derived from the old and new algorithm show that the lower stratospheric trends are substantially less negative using the new algorithm, while the trends in the middle stratosphere are slightly more negative [Reinsel et al, 1994]. More recently, Mateer et al [1996] have examined the effect of the a priori profiles on the Umkehr algorithm in the lower stratosphere through comparison with balloonsonde observations. Through their results and consideration of the averaging kernels, they conclude that trend information at the top and bottom of the retrieved profiles is available in only broad altitude regions and recommend that the trend data be considered in the sum of layers 1-3, and layers 4, 5, 6, 7, and 8+.…”