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.
Highlights
The Umkehr measurement technique, first described by Gotz et al (1934), enables low-resolution retrievals of ozone profiles. This is achieved by measuring the intensity ratio of zenith sky scattered sunlight at a pair of ultraviolet (UV) wavelengths, typically at solar zenith angles (SZAs) between 60◦ and 90◦
The Umkehr technique measures the ratio of downward scattered zenith sky radiation for each wavelength pair. This is performed over a range of SZAs to allow retrieval of vertically resolved ozone profiles
To emphasise the extra information content obtained when measurements at SZAs greater than 90◦ are included, we have provided two examples from 13 April 1970: one with the measurements limited to 90◦, and one with no limit
Summary
The Umkehr measurement technique, first described by Gotz et al (1934), enables low-resolution retrievals of ozone profiles This is achieved by measuring the intensity ratio of zenith sky scattered sunlight at a pair of ultraviolet (UV) wavelengths, typically at solar zenith angles (SZAs) between 60◦ and 90◦. The Umkehr technique measures the ratio of downward scattered zenith sky radiation for each wavelength pair This is performed over a range of SZAs to allow retrieval of vertically resolved ozone profiles. Due to the combination of low intensities and large intensity ratios received at the detector for high SZAs near the turn-around point, errors can be introduced due to stray light effects, which are currently not taken into account These errors have been shown to have minimal effect on C-pair Umkehr-derived, long-term ozone trends (Petropavlovskikh et al, 2011). Taking measurements beyond a SZA of 90◦ introduces ozone diurnal effects that have consequences above 50 km, but is expected to be negligible due to the low ozone concentration at these altitudes
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