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Measurements of the Atmospheric Mass, Energy, and Momentum Budgets Over a 500-Kilometer Square of Tropical Ocean

JOSHUA Z. HOLLANDCenter for Experiment Design and Data Analysis, Environmental Data Service, NOAA, Washington, D.C.

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EUGENE M. RASMUSSONCenter for Experiment Design and Data Analysis, Environmental Data Service, NOAA, Washington, D.C.

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Abstract

The atmospheric budgets of mass, water vapor, heat, momentum, and mechanical energy have been analyzed for a 5-day undisturbed period (June 22–26, 1969) during the Barbados Oceanographic and Meteorological Experiment (BOMEX). Data were obtained from rawinsondes launched every 1½ hr from the four corner ships of the BOMEX array. Computations were limited to the lowest 500 mb within a 500-km square centered at 15°N, 56°30′W. The mass budget is characterized by mean, low-level divergence and downward motion. Maximum divergence (5 × 10−6 s−1) is found approximately 80 mb above the surface and maximum subsidence near the base of the trade-wind inversion. The computed average evaporation is 6.0 mm/day. The computations show an upward flux of moisture through the trade inversion by subgrid-scale eddies, but the mean downward flux dominates, leading to a net downward transfer of moisture through the trade inversion. A surface stress of 0.065 Newtons (N)·m−2 is computed from the momentum balance with the top of the friction layer approximately 130 mb above the surface. Computed frictional loss of grid-scale kinetic energy within the boundary layer is 0.8 W·m−2. Heat balance computations indicate a Bowen ratio of approximately 0.1. Comparison of profiles of subgrid-scale moisture flux and nonradiative diabatic heating suggests that condensation and evaporation processes associated with the development and dissipation of trade-wind cumulus can make a significant contribution to the heat balance of the cloud and trade inversion layers.

Abstract

The atmospheric budgets of mass, water vapor, heat, momentum, and mechanical energy have been analyzed for a 5-day undisturbed period (June 22–26, 1969) during the Barbados Oceanographic and Meteorological Experiment (BOMEX). Data were obtained from rawinsondes launched every 1½ hr from the four corner ships of the BOMEX array. Computations were limited to the lowest 500 mb within a 500-km square centered at 15°N, 56°30′W. The mass budget is characterized by mean, low-level divergence and downward motion. Maximum divergence (5 × 10−6 s−1) is found approximately 80 mb above the surface and maximum subsidence near the base of the trade-wind inversion. The computed average evaporation is 6.0 mm/day. The computations show an upward flux of moisture through the trade inversion by subgrid-scale eddies, but the mean downward flux dominates, leading to a net downward transfer of moisture through the trade inversion. A surface stress of 0.065 Newtons (N)·m−2 is computed from the momentum balance with the top of the friction layer approximately 130 mb above the surface. Computed frictional loss of grid-scale kinetic energy within the boundary layer is 0.8 W·m−2. Heat balance computations indicate a Bowen ratio of approximately 0.1. Comparison of profiles of subgrid-scale moisture flux and nonradiative diabatic heating suggests that condensation and evaporation processes associated with the development and dissipation of trade-wind cumulus can make a significant contribution to the heat balance of the cloud and trade inversion layers.

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