Analysis of a jet stream induced gravity wave associated with an observed ice cloud over Greenland
[摘要] A polar stratospheric ice cloud (PSC type II) was observed by airbornelidar above Greenland on 14 January 2000. It was the unique observationof an ice cloud over Greenland during the SOLVE/THESEO 2000 campaign.Mesoscale simulations with the hydrostatic HRM model are presented which,in contrast to global analyses, are capable to produce a verticallypropagating gravity wave that induces the low temperatures at thelevel of the PSC afforded for the ice formation. The simulatedminimum temperature is ~8 K below thedriving analysesand ~4.5 K below the frost point, exactly coinciding withthe location of the observed ice cloud. Despite the high elevationsof the Greenland orography the simulated gravity wave is nota mountain wave. Analyses of the horizontal wind divergence,of the background wind profiles, of backward gravity waveray-tracing trajectories, of HRM experiments with reducedGreenland topography and of several diagnosticsnear the tropopause level provide evidence that the waveis emitted from an intense, rapidly evolving, anticyclonicallycurved jet stream. The precise physical process responsible for the waveemission could not be identified definitely, but geostrophicadjustment and shear instability are likely candidates.
In order to evaluate the potential frequency of such non-orographicpolar stratospheric cloud events, the non-linear balance equationdiagnostic is performed for the winter 1999/2000. Itindicates that ice-PSCs are only occasionally generated by gravity wavesemanating from spontaneous adjustment.
