The LCL Zone and Cumulus Onset

Nicholas P. Wilde Boundary Layer Research Team, Department of Meteorology, University of Wisconsin, Madison, 97 53706

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Roland B. Stull Boundary Layer Research Team, Department of Meteorology, University of Wisconsin, Madison, 97 53706

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Edwin W. Eloranta Boundary Layer Research Team, Department of Meteorology, University of Wisconsin, Madison, 97 53706

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Abstract

Variations of the lifting condensation level (LCL) of surface layer air are documented based on data from the BLX83 field experiment in Oklahoma. For example, within a 25 km long region near Chickasha, the local LCL height was found to vary by 15–30% of its average height. This zone of variation, centered on the mean LCL height, is identified as the “LCL zone”. It is analogous to the entrainment zone for the local mixed layer depth. Cumulus clouds first form when the top of the entrainment zone reaches the bottom of the LCL zone. As more of the entrainment zone overlaps and reaches above the LCL zone, the cloud cover increases. Two case studies are presented to demonstrate the diagnosis of cumulus onset time and cloud cover amount using this overlap technique. Combined radar, aircraft, rawinsonde, and surface observations indicate that some of the air observed at cloud base has the same low LCL as that of the mean surface layer air. This leads us to speculate that some surface layer air is rising up to cloud base with relatively little dilution, perhaps within the cores of thermals.

Abstract

Variations of the lifting condensation level (LCL) of surface layer air are documented based on data from the BLX83 field experiment in Oklahoma. For example, within a 25 km long region near Chickasha, the local LCL height was found to vary by 15–30% of its average height. This zone of variation, centered on the mean LCL height, is identified as the “LCL zone”. It is analogous to the entrainment zone for the local mixed layer depth. Cumulus clouds first form when the top of the entrainment zone reaches the bottom of the LCL zone. As more of the entrainment zone overlaps and reaches above the LCL zone, the cloud cover increases. Two case studies are presented to demonstrate the diagnosis of cumulus onset time and cloud cover amount using this overlap technique. Combined radar, aircraft, rawinsonde, and surface observations indicate that some of the air observed at cloud base has the same low LCL as that of the mean surface layer air. This leads us to speculate that some surface layer air is rising up to cloud base with relatively little dilution, perhaps within the cores of thermals.

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