Observations of Slantwise Convective instability in Winter Cyclones

Abstract

Seven precipitation bands observed during the Canadian Atlantic Storms Program (CASP) are studied to assess the importance of slantwise convective instability. Using three-hourly radiosonde data from regular and special stations, vertical cross sections of ?e*(saturated equivalent potential temperature) and M,(absolute angular momentum) and ?e(equivalent potential temperature) and M surfaces are constructed. A comparison of the slopes of constant M,?e and ?e* surfaces indicates the presence of potential and conditional slantwise instability. This study focuses on the time evolution of the stability field and on the adjustment to its neutral state. Consistent results were found in seven cases analyzed. The atmosphere is shown to contain shallow layers of air that are slightly unstable for conditional slantwise convection, particularly in regions having pronounced windshear. In the upper levels, the potential for instability usually remains only a potential because the lack of moisture precludes the actual release of energy. On the other hand, in the lower part of the atmosphere saturation is often realized and the instability is released leading to heavy precipitation which is sometimes organized in multiple bands. Our results also demonstrate that the atmosphere is undergoing an adjustment toward a state of conditional neutrality with respect to slantwise convection in saturated regions. The adjustment time of less than three hours is consistent with Enianuel's hypothesis of rapid adjustment.