A new Dobson Umkehr ozone profile retrieval method optimising information content and resolution

Abstract

Abstract. The standard Dobson Umkehr methodology to retrieve coarse-resolution ozone profiles used by the National Oceanographic and Atmospheric Administration uses designated solar zenith angles (SZAs). However, some information may be lost if measurements lie outside the designated SZA range (between 60 • and 90 • ), or do not conform to the fitting technique. Also, while Umkehr measurements can be taken using multiple wavelength pairs (A, C and D), past retrieval methods have focused on a single pair (C). Here we present an Umkehr inversion method that uses measurements at all SZAs (termed operational) and all wavelength pairs. (Although, we caution direct comparison to other algorithms.)Information content for a Melbourne, Australia (38 • S, 145 • E) Umkehr measurement case study from 28 January 1994, with SZA range similar to that designated in previous algorithms is shown. When comparing the typical single wavelength pair with designated SZAs to the operational measurements, the total degrees of freedom (independent pieces of information) increases from 3.1 to 3.4, with the majority of the information gain originating from Umkehr layers 2 + 3 and 4 (10-20 km and 25-30 km respectively). In addition to this, using all available wavelength pairs increases the total degrees of freedom to 5.2, with the most significant increases in Umkehr layers 2 + 3 to 7 and 9+ (10-40 and 45-80 km). Investigating a case from 13 April 1970 where the measurements extend beyond the 90 • SZA range gives further information gain, with total degrees of freedom extending to 6.5. Similar increases are seen in the information content. Comparing the retrieved Melbourne Umkehr time series with ozonesondes shows excellent agreement in layers 2 + 3 and 4 (10-20 and 25-30 km) for both C and A + C + D-pairs. Retrievals in layers 5 and 6 (25-30 and 30-35 km) consistently show lower ozone partial column compared to ozonesondes. This is likely due to stray light effects that are not accounted for in the forward model, and under represented stratospheric aerosol.

Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties

“…Umkehr measurements rely on the fact that the scattering intensity of solar UV (effective scattering height) changes with solar zenith angle (SZA). Since most of the ozone in the atmospheric column is in the stratosphere, the information content of the tropospheric part of the retrieval is limited, and only ~5 0% or less of the information in layer 1 (1000-250 hPa in the standard retrieval) comes from the troposphere (Stone et al, 2015; Text S-9), while the rest of the information comes from the adjacent stratospheric layers.…”

Retrieving vertical ozone profiles from measurements of global spectral irradiance

“…Zenith-sky Umkehr retrievals from Dobson instruments that have historically been processed with the algorithm developed by Mateer and DeLuisi (1992) used TOCdependent a priori profiles to constrain the retrieval. While this practice can lead to artefacts when calculating trends (Petropavlovskikh et al, 2005;Stone et al, 2015), the approach may be the best choice if a profile with the smallest uncertainty possible is sought for a specific purpose.…”

Response to comments of Referee #2

“…A value of d s = 3.0 may seem low, but it is consistent with values of d s resulting from the standard zenith-sky Umkehr technique. For example, Stone et al (2015) reported a value of d s = 3.1 for Dobson zenithsky Umkehr retrievals using the Dobson C wavelength pair (311.4 and 332.4 nm) and the standard Dobson SZAs ranging from 60 to 90 • .…”