12th Conference on Middle Atmosphere

Wednesday, 6 November 2002
Non-orographic generation of arctic PSCs during December 1999
Matthew H. Hitchman, University of Wisconsin, Madison, WI; and M. L. Buker
During December 1999 polar stratospheric clouds (PSCs) were observed in the absence of conditions conducive to generation by topographic gravity waves. The possibility is explored that PSCs can be generated by inertia-gravity waves (IGW) radiating from breaking synoptic scale Rossby waves on the polar front jet. The aerosol features on December 7 and 12 are selected for comparison with theory and with simulations using the University of Wisconsin Non-hydrostatic Modeling System (UWNMS). Consistent with Rossby adjustment theory, a common feature in the UWNMS simulations is radiation of IGW from the tropopause polar front jet, especially from sectors which are evolving rapidly in the Rossby wave breaking process. Packets of gravity wave energy radiate upward and poleward into the cold pool, while individual wave crests propagate poleward and downward, causing mesoscale variations in vertical motion and temperature. On December 12 the eastbound DC8 lidar observations exhibited a fairly uniform field of six waves in aerosol enhancement in the 14-20 km layer, consistent with vertical displacement by a field of IGW propagating antiparallel to the flow, with characteristic horizontal and vertical wavelengths of 300 km and 10 km. UWNMS simulations show emanation of a field of IGW upward and southwestward from a northward incursion of the polar front jet. The orientation and evolution of the aerosol features on December 7 are consistent with a single PSC induced by an IGW packet propagating from a breaking Rossby wave over western Russia toward the northeast into the coldest part of the base of the polar vortex, with characteristic period 9 hr, vertical wavelength 12 km, and horizontal wavelength 1000 km. Linear theory shows that for both of these cases, IGW energy propagates upward at 1 km/hr and horizontally at 100 km/hr, with characteristic trace speed 30 m/s. The spatial orientation of the PSC along IGW phase lines is contrasted with the nearly-horizontal filamentary structures in the PSC, which are indicative of flow streamlines. It is suggested that vertical displacement is a crucial factor in determining whether a PSC will form and that most PSCs are relatable to specific synoptic and mesoscale motions.

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