A Statistical Evaluation of Lid Strength on Deep Convection

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  • 1 Department of Meteorology, The Pennsylvania State University, University Park, PA 16802
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Abstract

A statistical analysis of the effects of lower-tropospheric inversions (called lids) on deep convection was made using a “logistic” model and reports of deep convection for a six month period over the central two-thirds of the United States during 1982. Observations consist of radar echo intensity, radar echo height and direct reports of severe storm sightings or damage. We find that both the lid strength and buoyancy (lifted index) effects need to be considered jointly. Lid strength appears to exhibit an effective cutoff value, above which deep convection becomes relatively unlikely, even for unstable values of the buoyancy term. On the other hand, deep convection is unlikely for stable buoyancy values. Further, for a given value of buoyancy the probability of severe convection increases with increasing lid strength, although the total probability of deep convection diminished with increasing lid strength. This result seems to support the idea that the lid allows high values of θw to form near the surface and that a favorable location for the release of the latent instability is along the lateral boundary of the lid. The primary value in examining both buoyancy and lid strength separately is that the method helps to eliminate large areas from consideration by the forecaster where the air near the surface is latently unstable but unlikely to produce deep convection due to the presence of a lid.

Abstract

A statistical analysis of the effects of lower-tropospheric inversions (called lids) on deep convection was made using a “logistic” model and reports of deep convection for a six month period over the central two-thirds of the United States during 1982. Observations consist of radar echo intensity, radar echo height and direct reports of severe storm sightings or damage. We find that both the lid strength and buoyancy (lifted index) effects need to be considered jointly. Lid strength appears to exhibit an effective cutoff value, above which deep convection becomes relatively unlikely, even for unstable values of the buoyancy term. On the other hand, deep convection is unlikely for stable buoyancy values. Further, for a given value of buoyancy the probability of severe convection increases with increasing lid strength, although the total probability of deep convection diminished with increasing lid strength. This result seems to support the idea that the lid allows high values of θw to form near the surface and that a favorable location for the release of the latent instability is along the lateral boundary of the lid. The primary value in examining both buoyancy and lid strength separately is that the method helps to eliminate large areas from consideration by the forecaster where the air near the surface is latently unstable but unlikely to produce deep convection due to the presence of a lid.

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