Assessment of Slantwise Convection in ERICA Cyclones

Abstract

Atmospheric stability properties for cumulus and slantwise convection in oceanic midlatitude cyclones are analyzed using dropsonde observations from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). Vertical cross sections perpendicular to the low-level wind shear are selected in the frontal regions for four ERICA storms. To assess the stability properties for conditional symmetric instability (CSI), a sounding analysis is carried out along surfaces of constant absolute angular momentum M. The buoyancy of the parcel along the slanted M surface is determined, both with and without the water loading effect. Our analysis suggests that a systematic bias toward overestimation of slantwise instability occurs when the loading effect is neglected. The major finding of our analysis is that the lower-tropospheric air on the warm side of the warm-frontal zone is stable or neutral with respect to vertical cumulus convection but unstable for slantwise convection. Convective instability, however, is found in the warm sector near the surface low of explosive cyclones during their period of most rapid growth. Our analysis shows that conditional slantwise instability, throughout a deep layer, can occur even in a slowly developing cyclone. Observed precipitation events were consistent with the occurrence of slantwise and cumulus convection.