Assimilation of EOS MLS ozone observations in the Met Office data‐assimilation system

Abstract

ABSTRACT:In this paper the impact of Earth Observing System Microwave Limb Sounder (EOS MLS) observations on ozone analyses is investigated using the Met Office data-assimilation system. EOS MLS was launched in 2004, and produces high-quality ozone observations in the upper troposphere and stratosphere at high vertical and horizontal resolution. The experiments shown here are run using 3D-Var and a forecast model that has 50 levels -from the surface to approximately 63 km -and a horizontal resolution of 2.5°latitude by 3.75°longitude. Most of the experiments are run for the period January-February 2005.The chief impact of assimilating EOS MLS data is a reduction of the mean analysis error, and of the standard deviation, in the lower stratosphere. Compared with control simulations, mean errors drop by 5%-25% in the Southern-Hemisphere extratropics, by around 10% in the Northern-Hemisphere extratropics, and by around 50% (with respect to ozonesondes) in the region of the tropical upper troposphere and lower stratosphere.Further investigation shows that the improved ozone analyses in the Southern Hemisphere largely result from a much better representation of summertime low-ozone events. These events have been documented in the northern summer stratosphere, but never before in the southern summer stratosphere. In the Northern Hemisphere the addition of EOS MLS to the assimilation system is shown to lead to a better representation of winter polar ozone depletion. At low latitudes, it appears that errors in the background model transport fields often lead to errors in the ozone analysis, but that the addition of high-density, good-quality EOS MLS data alleviates much of this error. It is noted that the EOS MLS data have a more beneficial impact on ozone analyses in this region than do data from the Michelson Interferometer for Passive Atmospheric Sounding, which has a similar resolution and observational error to EOS MLS.  Crown

Estimation of Arctic ozone loss in winter 2004/05 based on assimilation of EOS MLS and SBUV/2 observations

“…Positive values at the vortex edge at 450 K are due to the fact that the collar of high ozone that is present around the vortex edge is better represented in the assimilation run than in the reference run. Transport errors in the dynamical fields in both the assimilation and reference runs act to smear out the high-ozone collar, but the assimilation of the EOS MLS data tends to mask such errors in the assimilation run (as also reported by Jackson, 2007). (Of course, by the same argument the ozone loss seen near the vortex edge in Figure 3 need not necessarily be entirely due to chemistry, but can be affected by transport errors, too).…”

“…EOS MLS and SBUV/2 ozone observations are assimilated, together with dynamical observations from satellites, aircraft, radiosondes and surface stations. The ozone assimilation scheme is described in Jackson and Saunders (2002) and Jackson (2004) and the impact of the assimilation of EOS MLS and SBUV/2 data on ozone analyses is described in Jackson (2007). Because the assimilation runs are computationally relatively expensive, they were limited to the 26 January-10 March period, rather than the entire winter.…”

“…The horizontal resolution is about 200 km, and no observations are made in the polar night. Jackson (2007) showed that the SBUV/2 data have a small positive impact on the quality of the ozone analyses, but the positive impact seen when adding EOS MLS data is much greater.…”

Four‐dimensional variational assimilation of ozone profiles from the Microwave Limb Sounder on the Aura satellite

“…[18] Like other experiments for assimilation of ozone observations [Struthers et al, 2002;Dethof, 2003;Jackson, 2007], we do not attempt to remove possible biases from ozone observations before assimilation, mainly because of the difficulty in knowing the 'truth'. As a result, there may be systematic differences existing between different observations.…”

“…In recent years, ozone observations have been assimilated into various chemical transport models (CTMs) driven by off-line winds to generate continuous global ozone fields [Levelt et al, 1998;Stajner et al, 2001;Errera and Fonteyn, 2001;Eskes et al, 2003]. In addition, many weather centers have also implemented ozone assimilation into their NWP models [e.g., Jackson and Saunders, 2002; Struthers et al, 2002;Geer et al, 2006;Jackson, 2007]. At the European Centre for Medium-range Weather Forecasts (ECMWF), assimilation of ozone layer observations from the Solar Backscatter Ultraviolet instruments (SBUV on NIMBUS-7 and SBUV/2 on several NOAA satellites) and total ozone columns from other nadir instruments were first implemented into the ECMWF 45-year reanalysis project ERA-40 before becoming operational in 2002 [Dethof and Hólm, 2004].…”