Steven M. Cavallo1 and Gregory J. Hakim2,
J. Atmos. Sci., 71, submitted.
1School of Meteorology, University of Oklahoma, Norman, OK
2Department of Atmospheric Sciences, University of Washington,Seattle, WA
Characterized by radii as large as 800 km and lifetimes up to months, cyclonic tropopause polar vortices (TPVs) are coherent circulation features over the Arctic that are important precursors for surface cyclogenesis in high and middle latitudes. TPVs have been shown to be maintained by radiative processes over the Arctic due to limited amounts of latent heating. This study explores the hypothesis that a downward extension of dry, stratospheric air associated with TPVs, results in an increase in longwave radiative cooling that intensifies the vortex.
Idealized numerical modeling experiments are performed to isolate physical interactions, beginning with radiative forcing in a dry atmosphere and culminating with multiple physical interactions between radiation and clouds that more accurately represent the observed environment of TPVs. % GH longwave cooling ... is ...responsible Results show that longwave radiative cooling associated with a rapid decrease in water vapor concentration near the tropopause is primarily responsible for observed TPV intensification. These enhanced water vapor gradients result from a lower tropopause within the vortex, which places dry stratospheric air above relatively moist tropospheric air. Cloud-top radiative cooling enhances this effect, and also promotes the maintenance of clouds by destabilizing the region near-cloud top. Shortwave radiation and latent heating offset the longwave intensification mechanism. Heating from shortwave radiation reduces cloud-water mixing ratio by preferentially warming levels above cloud-tops.