Forecasting stratospheric clouds

At high latitudes, stratospheric clouds are sometimes observed during the coldest period of the year. In the winter twilight, such clouds are exceptionally bright and colourful and they are generally considered to be very beautiful. It is of some value to predict the occurrence of stratospheric clouds, but in order to do so, their presence must be linked to elements of the atmospheric flow which are resolved by the numerical models. For this purpose, observations of stratospheric clouds over Iceland have been collected for a period of 40 years. The atmospheric flow pattern associated with the occurrence of these clouds is investigated. Firstly, there are strong winds at low tropospheric levels. Secondly, the wind speed is increasing with height throughout the troposphere. Thirdly, there is very little wind veering and finally, the stratosphere is anomalously cold. These features of the flow permit the generation and vertical propagation of gravity waves and they will be used as predictors in planned forecasting of the statospheric clouds.