OBSERVATIONS OF STRATOSPERIC CLOUDS AND THEIR CONNECTION WITH CONDITIONS FOR VERTICAL PROPAGATION OF MOUNTAIN WAVES

Forty years of synoptic observations of polar stratospheric clouds (PSCs) over Iceland are analyzed. The PSC are most frequent in January, followed by December and February. Only a handful of observations have been made in November and March and none outside this period of the year. Observations of PSCs are much more frequent in N- and E- Iceland than in SW-Iceland and there is large interannual variability in the occurrence of these clouds. An investigation of the atmospheric flow when PSCs are observed reveal that in most cases, the conditions are very favourable for the generation of orographic gravity waves and propagation of these waves far upwards in the atmosphere. This suggests that localized cooling in the ascending part of a gravity wave may be crucial for the formation of most PSCs over Iceland. This corresponds with the relatively low frequency of PSCs over SW-Iceland: PSCs are only observed when there are no or only few tropospheric clouds and under such conditions the low-level winds are usually from the north in SW-Iceland. In northerly flow, there is often a reverse wind shear in the troposphere inhibiting upward propagation of wave energy