Climatologies of subtropical mixing derived from 3D models
[摘要] Fingerlike structures reaching from lower into extra-tropical latitudes significantlycontribute to the tropical-extratropical exchange of air masses. This is also anexchange of upper tropospheric and stratospheric air. Those so called streamers can,on a horizontal plane, be detected in N2O or O3 since they are characterised by highN2O or low O3 values compared to undisturbed mid-latitude values. A climatology ofstreamer events has been established, employing the chemical-transport modelKASIMA, which is driven by ECMWF re-analyses (ERA) and operational analyses.For the first time, the seasonal and geographical distribution of streamer frequencies has been determined on the basis of 9 years ofmeteorological analyses.
For the current investigation, a meridional gradient criterion has been newlyformulated and applied to the N2O distributions calculated withKASIMA. A climatology has been derived by counting all streamer events between 21and 25 km for the years 1990 to 1998. The results have been compared with a streamer climatology which has been established in the sameway employing data of a multi-year simulation with the coupled chemistry-climate model ECHAM4.L39(DLR)/CHEM (E39/C). Bothclimatologies are qualitatively in agreement, in particular in the northern hemisphere, where much higher streamer frequencies are found inwinter than in summer. In the southern hemisphere, the KASIMA analyses indicatestrongest streamer activity in September. E39/C streamer frequencies clearlydisplays an offset from June to October, pointing to model deficiencies withrespect to tropospheric dynamics. KASIMA and E39/C results agree well fromNovember to May. Some of the findings give strong indications that the streamer events foundin the altitude region between 21 and 25 km are mainly forced from the troposphere and arenot directly related to the dynamics of the stratosphere, in particular not to the dynamicsof the polar vortex.
Sensitivity simulations with E39/C, which represent recent and possible future atmospheric conditions, have been employed to answer the question howclimate change would alter streamer frequencies. This shows that the seasonal cycle does not change but that significant changes occur in monthsof minimum and maximum streamer frequencies. This could have an impact on the mid-latitude distribution of chemical tracers and compounds.
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[效力级别] [学科分类] 大气科学
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