Distributions of Surface-Layer Buoyancy Versus Lifting Condensation Level over a Heterogeneous Land Surface

Kelly Schrieber Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madisom Madison Wisconsin

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Roland Stull Atmospheric Science Programme, Department of Geography, University of British Columbia Vancouver, Canada

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Qing Zhang Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison Madison Wisconsin

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Abstract

Onset and coverage of small cumulus clouds depend on the relative abundance of surface-layer air parcels possessing favourable buoyancy and moisture—two variables that are coupled through the surface energy budget. This abundance is described using a joint frequency distribution (JFD) as a function of virtual potential temperature θv. and height of lifting condensation level zLCL. It is shown analytically that the shape and spread of this JFD depends on die ranges of Bowen ratios and solar forcings (albedoes, cloud shading, etc.) that exist within a domain of heterogeneous land use.

To sample the character of such JFDs in the real atmosphere, a case study is presented using turbulence data gathered by aircraft flying in the surface layer of southwest France. This case study includes 4 days of clear skies during the Hydrologic Atmospheric Pilot Experiment (HAPEX) of 1986. The full flight track during HAPEX overflew a wide range of land use including evergreen forest, corn, vineyards, pastures, and irrigated fields over varied topography. The JFDs from these full tracks are found to he quite complex, being frequently multimodal with a convoluted perimeter. However. when a full track is broken into segments, each over a subdomain of quasi-homogeneous land use, the resulting segment JFDs are mono-modal with simpler topology.

Such a characterization of IFDs provides guidance toward eventual subgrid cumulus parameterization in large-scale forecast models, with associated impacts in aviation forecasting, pollutant venting and chemical reactions, vertical dispersion and turbulence modulation, and radiation balance in climate-change models.

Abstract

Onset and coverage of small cumulus clouds depend on the relative abundance of surface-layer air parcels possessing favourable buoyancy and moisture—two variables that are coupled through the surface energy budget. This abundance is described using a joint frequency distribution (JFD) as a function of virtual potential temperature θv. and height of lifting condensation level zLCL. It is shown analytically that the shape and spread of this JFD depends on die ranges of Bowen ratios and solar forcings (albedoes, cloud shading, etc.) that exist within a domain of heterogeneous land use.

To sample the character of such JFDs in the real atmosphere, a case study is presented using turbulence data gathered by aircraft flying in the surface layer of southwest France. This case study includes 4 days of clear skies during the Hydrologic Atmospheric Pilot Experiment (HAPEX) of 1986. The full flight track during HAPEX overflew a wide range of land use including evergreen forest, corn, vineyards, pastures, and irrigated fields over varied topography. The JFDs from these full tracks are found to he quite complex, being frequently multimodal with a convoluted perimeter. However. when a full track is broken into segments, each over a subdomain of quasi-homogeneous land use, the resulting segment JFDs are mono-modal with simpler topology.

Such a characterization of IFDs provides guidance toward eventual subgrid cumulus parameterization in large-scale forecast models, with associated impacts in aviation forecasting, pollutant venting and chemical reactions, vertical dispersion and turbulence modulation, and radiation balance in climate-change models.

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