ANOMALY IN THE ALPINE DIURNAL PRESSURE SIGNAL - OBSERVATIONS AND THEORY

An examination is undertaken of the diurnal surface-pressure pattern a cross the main east-west ridge of the European Alps. A case study for clear-sky, quiescent synoptic conditions reveals a pronounced across-r idge asymmetry-to the north the primary feature is an approximate 0.5 hPa semi-diurnal oscillation whilst on the southern side there is an a pproximate 2.5 hPa diurnal oscillation. A one-year statistical climato logy and a two-month synoptic climatology confirm the existence, and i ndicate the prevalence of this asymmetry. The across-ridge variation i n the amplitude of the solar-diurnal (i.e. S1) component is about 1 hP a and it has a lateral scale of around 200 km. From a theoretical stan dpoint physical arguments and dynamical considerations are advanced to suggest that the S1 anomaly is not readily attributable to purely in situ effects. In addition a heuristic model is deployed to examine the possible nature of the Alpine modification of the planetary diurnal t ide. The model, comprising a cylindrical island embedded in 'reduced-g ravity' shallow water, reveals a terrain-induced modification in the f orm of a mesoscale, terrain-trapped Kelvin wave that interferes destru ctively (constructively) with the global S1 signal to the north (south ) of the terrain. The pattern's amplitude, phase and spatial structure are consistent with the main features of the observed signal.