Solar Wind Plasma Streams, High-Level Clouds and Extratropical Storms

Time series of a high-level cloud area index (HCAI) based on infrared cloud amounts (cloud top pressure < 440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset covering 22 years are constructed for various geographic sectors. The HCAI is defined as a percentage ratio of 280×280 km2 cells for which the high-level cloud amounts exceeded a given threshold; typical HCAI values range between 20-40% at mid latitudes. Time series of HCAI are used in superposed epoch analysis keyed by the arrival time of co-rotating solar wind streams from coronal holes. A statistically significant decrease of the mean HCAI around the arrival of stream-interaction regions at the leading edge of high-speed solar wind followed by an increase in HCAI to a maximum a few days later is found. The observed mean amplitude (< 3.0% in HCAI units) of the HCAI response to solar wind forcing depends on season, geographic latitude and longitude, and on the phase of the quasi-biennial oscillation (QBO) of the zonal winds in the tropical stratosphere. It is known that the QBO affects the global atmospheric circulation dynamics which in turn has an impact on propagation of atmospheric gravity waves (AGWs). It is suggested that solar- wind-generated auroral AGWs seeding convective instabilities in the mid-latitude troposphere contribute to formation of deep convective clouds and growth of extratropical cyclones and, if ducted to low latitudes, may influence the development of tropical cyclones. Although small in amplitude, the auroral gravity wave induced vertical lift combined with a horizontal or upward tilting flow in the warm frontal zone may release the moist symmetric instability, thus initiating slantwise convection. Mesoscale cloud/rain bands are observed in infrared satellite images of extratropical cyclones a few hours after the auroral AGWs are launched. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis. It is observed that major extratropical storms tend to occur within a few days of the arrival of the high-speed solar wind, which is a source of magneto-hydrodynamic waves that couple to the magnetosphere and generate auroral atmospheric gravity waves.