Abstract
Abstract. The transportable ground based microwave radiometer MIAWARA-C monitored the upper stratospheric and lower mesospheric (USLM) water vapor distribution over Sodankylä, Finland (67.4° N, 26.6° E) from January to June 2010. At the end of January, approximately 2 weeks after MIAWARA-C's start of operation in Finland, a stratospheric sudden warming (SSW) disturbed the circulation of the middle atmosphere. Shortly after the onset of the SSW water vapor rapidly increased at pressures between 1 and 0.01 hPa. Backward trajectory calculations show that this strong increase is due to the breakdown of the polar vortex and meridional advection of subtropical air to the Arctic USLM region. In addition, mesospheric upwelling in the course of the SSW led to an increase in observed water vapor between 0.1 and 0.03 hPa. After the SSW MIAWARA-C observed a decrease in mesospheric water vapor volume mixing ratio (VMR) due to the subsidence of H2O poor air masses in the polar region. Backward trajectory analysis and the zonal mean water vapor distribution from the Microwave Limb Sounder on the Aura satellite (Aura/MLS) indicate the occurrence of two regimes of circulation from 50° N to the North Pole: (1) regime of enhanced meridional mixing throughout February and (2) regime of an eastward circulation in the USLM region reestablished between early March and the equinox. The polar descent rate determined from MIAWARA-C's 5.2 parts per million volume (ppmv) isopleth is 350 ± 40 m d−1 in the pressure range 0.6 to 0.06 hPa between early February and early March. For the same time interval the descent rate in the same pressure range was determined using Transformed Eulerian Mean (TEM) wind fields simulated by means of the Whole Atmosphere Community Climate Model with Specified Dynamics (SD-WACCM). The average value of the SD-WACCM TEM vertical wind is 325 m d−1 while the along trajectory vertical displacement is 335 m d−1. The similar descent rates found indicate good agreement between the model and MIAWARA-C's measurements.
Highlights
Water vapor enters the stratosphere from the equatorial troposphere through the tropical tropopause layer from where it is transported upward into the mesosphere following the Brewer-Dobson circulation (Andrews et al, 1987)
This paper presents two different trajectory computation methods with two different information contents: (1) Lagrangian backward trajectories started over Sodankylagiving information on horizontal and vertical origin of air parcels sampled by MIAWARA-C and (2) zonal mean trajectories started at 67◦ N following the transformed Eulerian mean (TEM) circulation giving information on large scale meridional and vertical advection of air masses due to the combined effects of zonally averaged winds and wave momentum transport
As mesospheric air within the vortex is dryer than outside of it the good agreement in the water vapor data sets indicates that the Lagranto/European Center of Medium-range Weather Forecast (ECMWF) mesospheric trajectories allow to distinguish whether the air comes from inside or outside of the polar region
Summary
Water vapor enters the stratosphere from the equatorial troposphere through the tropical tropopause layer from where it is transported upward into the mesosphere following the Brewer-Dobson circulation (Andrews et al, 1987). Manney et al (2008, 2009a,b) investigated transport of the trace gases CO, H2O and N2O during the warmings in 2006 and in 2009 using satellite data from Aura/MLS, ACE/FTS and TIMED/SABER and model outputs from SLIMCAT, GEOS5 and ECMWF These trace gases showed values characteristic of low and mid-latitudes extending to Polar Regions and extremely weak gradients throughout the hemisphere. In addition we present post SSW Arctic mesospheric descent rates determined from ground based and space borne water vapor measurements and TEM vertical wind and trajectory calculations. To our knowledge this is the first study investigating polar descent after a SSW using ground based data.
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