Relevance of mountain waves for the formation of polar stratospheric clouds over Scandinavia: A 20 year climatology

A climatology of meteorological conditions necessary for the existence of polar stratospheric clouds (PSCs) over Scandinavia is presented. The frequency of low enough temperatures for synoptic‐scale PSCs is compared with the frequency of additional events when mesoscale PSCs might be induced by the adiabatic cooling in mountain waves. The climatology is based on European Centre for Medium‐Range Weather Forecasts analyses of the 20 winter seasons 1979–1980 to 1998–1999 and a simple parameterization of mesoscale stratospheric mountain wave activity. The synoptic‐scale formation potential of stratospheric clouds is determined as the fraction of time when the stratospheric minimum temperature is less than the thresholds TNAT (PSC of type I) and Tfrost (PSC of type II). The potential of additional mesoscale formation is computed by reducing the synoptic‐scale temperature on dates when stratospheric mountain wave activity is predicted. This simple approach allows an estimate of the climatological relevance of mountain waves for the existence conditions of PSCs above Scandinavia. In the climatological mean, maximum stratospheric mountain wave activity is found in January, and the maximum seasonal winter average for various levels amounts to 11% of all analysis times (i.e., approximately 3 days per month). Based on the temperature analyses the existence conditions for PSCs of type I are dominated by synoptic‐scale processes (maximum 36% of all analysis times are conducive to their existence) whereas the additional mesoscale fraction is less than 6%. Suitable conditions of ice PSCs occur less frequently. It is found that episodes of temperatures low enough for the existence of PSC of type II are controlled by mesoscale temperature anomalies induced by stratospheric mountain waves; on the synoptic scale the stratosphere above Scandinavia is almost always warmer than the threshold Tfrost.