Using Digital Cloud Photogrammetry to Characterize the Onset and Transition from Shallow to Deep Convection over Orography

Joseph A. Zehnder Global Institute of Sustainability, Arizona State University, Tempe, Arizona

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Liyan Zhang Research Center of CAD/CAM Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China

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Dianne Hansford Partnership for Research in Spatial Modeling, Arizona State University, Tempe, Arizona

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Anshuman Radzan Partnership for Research in Spatial Modeling, Arizona State University, Tempe, Arizona

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Nancy Selover Department of Geography, Arizona State University, Tempe, Arizona

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Constance M. Brown Atmospheric Science Program, Department of Geography, Indiana University, Bloomington, Indiana

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Abstract

An automated method for segmenting digital images of orographic cumulus and a simple metric for characterizing the transition from shallow to deep convection are presented. The analysis is motivated by the hypothesis that shallow convection conditions the atmosphere for further deep convection by moistening it and preventing the evaporation of convective turrets through the entrainment of dry air. Time series of convective development are compared with sounding and surface data for 6 days during the summer of 2003. The observations suggest the existence of a threshold for the initiation of shallow convection based on the surface equivalent potential temperature and the saturated equivalent potential temperature above the cloud base. This criterion is similar to that controlling deep convection over the tropical oceans. The subsequent evolution of the convection depends on details of the environment. Surface fluxes of sensible and latent heat, along with the transport of boundary layer air by upslope flow, increase the surface equivalent potential temperature and once the threshold value is exceeded, shallow convection begins. The duration of the shallow convection period and growth rate of the deep convection are determined by the kinematic and thermodynamic structure of the mid- and upper troposphere.

Corresponding author address: Joseph A. Zehnder, Global Institute of Sustainability, Arizona State University, P.O. Box 873211, Tempe, AZ 85287-3211. Email: zehnder@asu.edu

Abstract

An automated method for segmenting digital images of orographic cumulus and a simple metric for characterizing the transition from shallow to deep convection are presented. The analysis is motivated by the hypothesis that shallow convection conditions the atmosphere for further deep convection by moistening it and preventing the evaporation of convective turrets through the entrainment of dry air. Time series of convective development are compared with sounding and surface data for 6 days during the summer of 2003. The observations suggest the existence of a threshold for the initiation of shallow convection based on the surface equivalent potential temperature and the saturated equivalent potential temperature above the cloud base. This criterion is similar to that controlling deep convection over the tropical oceans. The subsequent evolution of the convection depends on details of the environment. Surface fluxes of sensible and latent heat, along with the transport of boundary layer air by upslope flow, increase the surface equivalent potential temperature and once the threshold value is exceeded, shallow convection begins. The duration of the shallow convection period and growth rate of the deep convection are determined by the kinematic and thermodynamic structure of the mid- and upper troposphere.

Corresponding author address: Joseph A. Zehnder, Global Institute of Sustainability, Arizona State University, P.O. Box 873211, Tempe, AZ 85287-3211. Email: zehnder@asu.edu

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