CONVECTIVE AND SYMMETRICAL INSTABILITIES AND THEIR EFFECTS ON PRECIPITATION - SEASONAL-VARIATIONS IN CENTRAL ALBERTA DURING 1990 AND 1991

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 bal loon soundings covering the 2-yr period 1990-91. We also identified so undings that supported convective or moist symmetric instability and h ad precipitation within 12 h after the launch of the soundings. The re corded precipitation amounts were also correlated with the stability p roperties of the soundings. During summer, 97% of the soundings were s usceptible to convective instability. About a third of these resulted in rainfall events. Soundings with convectively unstable layers were a lso common during spring (70%) and fall (57%) but rare during winter. Much of the precipitation recorded in spring and summer could be assoc iated with convectively unstable cases. During winter, 44% of the obse rved soundings were convectively stable yet indicated the potential fo r moist symmetric instability, and about half of the total snowfall am ounts was associated with these symmetrically unstable soundings. Soun dings that were convectively stable yet potentially unstable for moist symmetric instability were encountered less frequently during spring (20%) and fall (26%) and were rare during summer (3%). Mixed-type unst able conditions (i.e., a convectively unstable layer together with a s ymmetrically unstable layer) were common during spring, summer, and fa ll and were often associated with significant precipitation amounts. T he primary finding was that both convective and moist symmetric instab ility contributed to the annual precipitation, but a seasonal dependen ce existed in their relative importance.