Abstract
Abstract. We assimilate stratospheric ozone profiles from MLS (Microwave Limb Sounder) into the MOCAGE Chemistry Transport Model (CTM) to study Stratosphere-Troposphere Exchange (STE). This study uses two horizontal grid resolutions of 2° and 0.2°. The combined impacts of MLS ozone assimilation and high horizontal resolution are illustrated in two case studies where STE events occurred (23 June 2009 and 17 July 2009). At high resolution the filamentary structures of stratospheric air which characterise STE events are captured by the model. To test the impact of the assimilation and the resolution, we compare model outputs from different experiments (high resolution and low resolution; MLS assimilation run and free run) with independent data (MOZAIC aircraft ozone data; WOUDC ozone sonde network data). MLS ozone analyses show a better description of the Upper Troposphere Lower Stratosphere (UTLS) region and the stratospheric intrusions than the free model run. In particular, at high horizontal resolution the MLS ozone analyses present realistic filamentary ozone structures in the UTLS and laminae structures in the ozone profile. Despite a low aspect ratio between horizontal resolution and vertical resolution in the UTLS at high horizontal resolution, MLS ozone analyses improve the vertical structures of the ozone fields. Results from backward trajectories and ozone forecasts show that assimilation at high horizontal resolution of MLS ozone profiles between 10 hPa and 215 hPa has an impact on tropospheric ozone.
Highlights
In the stratosphere, ozone (O3) is known to shield the surface from harmful ultraviolet radiation
Filamentary structures of potential vorticity (PV) and ozone at lower stratosphere have been simulated by Hauchecorne et al (2002), Marchand et al (2003) and Tripathi et al (2006); these papers show that high resolution simulations are needed for detailed investigation of Stratospheretroposphere exchange (STE)
A good example of tropospheric impact is shown on figure 10 where a tropospheric ozone maximum structure was observed by the WOUDC profile and was reasonably well represented by the HR Microwave Limb Sounder (MLS) ozone analyses
Summary
Ozone (O3) is known to shield the surface from harmful ultraviolet radiation. In the middle and high troposphere ozone is the third most important greenhouse gas after carbon dioxide (CO2) and methane (CH4). Stratospheretroposphere exchange (STE) events play a key role in controlling the ozone budget in the Upper Troposphere Lower Stratosphere (UTLS), which in turn affects the radiation budget (IPCC 1996). The stratosphere is characterised by high values of PV and ozone concentrations, so intrusion of stratospheric air is expected to bring PV and ozone rich air into the troposphere. These intrusions typically form filamentary structures (Holton et al, 1995), which appear as laminae in the ozone profiles (Stohl et al, 2003) and often exhibit mesoscale features. Filamentary structures of PV and ozone at lower stratosphere have been simulated by Hauchecorne et al (2002), Marchand et al (2003) and Tripathi et al (2006); these papers show that high resolution simulations are needed for detailed investigation of STE
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