Convective bands are caused by low-level lines of convergence, which has been demonstrated by simulations using numerical models (Nagata et. al. 1986). They often develop in cold air outbreaks in which the coastal shape and sea-surface temperature pattern have a profound effect in establishing a low-level mesoscale circulation. Due to the large temperature difference between land and sea, offshore winds are generated. Convergence occurs when offshore winds, from opposite coasts meet each other.
The cold air stream becomes unstable due to turbulent transport of sensible and latent heat from the warm sea surface. Convergence Lines form downwind of major bends in the coastline. A bay with offshore winds is thus an ideal generation area. A good example is the development of lines over the Gulf of Finland (Andersson and Nilsson 1990). In general the lines are advected with the base flow and can bring large amounts of precipitation to adjacent areas. The warm sea water heats the continental air from below. The initial temperature profile is shown by the red line and has a stable stratification. The initial temperature at surface is about 1°C. The seawater temperature has a temperature of 9-10°C.
The initial continental air is heated by the warm surface, eventually reaching a temperature of around 10°C after a certain time. This new profile has an unstable character and allows convection uptil 500 hPa, which is shown by the yellow lines.
Continuation of this topographical effect results in the convection becoming organised in bands. A Convergence Line that is generated in the Skaggerak is advected to the southwest. In the XHiRLAM field, the line is seen as a long stretched area of vertical motion.