Subseasonal Variability of Water Vapor in the Upper Stratosphere/ Lower Mesosphere Over Northern Europe in Winter 2009/2010 – Case Study

For the upper stratosphere and lower mesosphere (USLM) we used microwave spectrometer measurements to investigate the subseasonal variability over Northern Europe especially at the Arctic Lidar Observatory for Middle Atmosphere Research ( ALOMAR) (Andenes, 69.3° N, 16.1° E), Northern Norway, and at the Leibniz Institute of Atmospheric Physics, (Kühlungsborn, 54.2° N, 11.8° E), Northern Germany, for winter 2009/2010. The Modern Era Retrospective-analysis for Research Applications (MERRA) data set of NASA is applied to study the link between the local variability of water vapor and the dynamical evolution in the USLM. Beside a slow increase in January 2010 and a stronger decrease in February and March, episodes of significant increase and decrease of water vapor were found over Northern Germany. These structure changes are in well agreement with the MERRA reanalysis which show similar changes induced by conservative horizontal transport of water vapor. Due to strong negative polar gradient and mixing barrier higher values of water vapor have been observed outside the polar vortex and lower values inside, as expected. In addition we found that the complex polar vortex evolution over Northern Germany during the minor stratospheric sudden warming (mSSW) in the beginning of December 2009 and the major warming (MSSW) at the end of January 2010 as well as between fit well into this relationship. Furthermore an episode of strong increase of water vapor over ALOMAR at about 55-60 km altitude was observed during the MSSW event on 27 January 2010 resulting in a significant double peak maximum. Based on MERRA data we show that this dual peak was caused by a relatively strong regional northward propagation of wetter air in the lower mesosphere which was in phase with the vortex breakdown. In the LM the strong polar intrusion of warm and wetter air occurred over Northern Europe resulting in a well mixed large-scale polar anticyclone on 30 January 2010. In comparison with observations the local maxima of water vapor of the MERRA reanalysis are underestimated for the volume mixing ratio in the order of 1-2 ppmv. The vertical descent rate of MERRA analysis after the MSSW is half as much as the observed one over Kühlungsborn. Nevertheless the high coherence in the water vapor variability between local measurements over Northern Germany and Northern Norway, and water vapor extracted from the MERRA reanalysis support the extended use of the horizontal winds for transport and propagation studies in the USLM.