Convective and Symmetric Instabilities and Their Effects on Precipitation: Seasonal Variations in Central Alberta during 1990 and 1991

Abstract

Sounding data from central Alberta were analyzed to assess whether the atmosphere indicated deep layers that were susceptible to convective or moist symmetric instability. The observations consisted of 1460 balloon soundings covering the 2-yr period 1990?91. We also identified soundings that supported convective or moist symmetric instability and had precipitation within 12 h after the launch of the soundings. The recorded precipitation amounts were also correlated with the stability properties of the soundings. During summer, 97% of the soundings were susceptible to convective instability. About a third of these resulted in rainfall events. Soundings with convectively unstable layers were also common during spring (70%) and fall (57%) but rare during winter. Much of the precipitation recorded in spring and summer could be associated with convectively unstable cases. During winter, 44% of the observed soundings were convectively stable yet indicated the potential for moist symmetric instability, and about half of the total snowfall amounts was associated with these symmetrically unstable soundings. Soundings that were convectively stable yet potentially unstable for moist symmetric instability were encountered less frequently during spring (20%) and fall (26%) and were ram during summer (3%). Mixed-type unstable conditions (i.e., a convectively unstable layer together with a symmetrically unstable layer) were common during spring, summer, and fall and were often associated with significant precipitation amounts. The primary finding was that both convective and moist symmetric instability contributed to the annual precipitation, but a seasonal dependence existed in their relative importance.