The Dynamical Structure of the Planetary Boundary Layer over the Arabian Sea, as Deduced from Constant-Level Balloon Trajectories

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  • 1 Laboratoire de Météorologie Dynamique, Paris, France
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Abstract

During Summer 1979, 88 super-pressure constant-level balloons were launched at the nominal flight level of 900 mb from the Seychelles Islands and from the northern tip fo the Malagasy Republic as part of the French participation in the MONEX experiment. Balloons tracked by the Argos system on board TIROSN and NOAA-6 satellites provide good estimates of the wind velocity and of the Lagrangian acceleration of air masses within the planetary boundary layer (PBL). With additional wind and pressure data, a study of the balance of forces and of the vertical wind structure in the planetary boundary layer has been performed, including the computation of the vertical transport of horizontal momentum by turbulent mixing. The accuracy of this estimate is marginal in equatorial regions, but sufficient in the Northern Hemisphere to derive a friction coefficient. The meridional variation of the wind veering within the planetary boundary layer is interpreted using a simple one-dimensional model. Veering is weak in the vicinity of the equator and appears to be of the Ekman type more than 10° away. If one follows a cross-equatorial trajectory, the transition in the direction of the veering occurs ∼5° north of the equator. This provides a measure of the relative importance of advective terms compared to local forcing terms in the dynamics of the tropical boundary layer.

Abstract

During Summer 1979, 88 super-pressure constant-level balloons were launched at the nominal flight level of 900 mb from the Seychelles Islands and from the northern tip fo the Malagasy Republic as part of the French participation in the MONEX experiment. Balloons tracked by the Argos system on board TIROSN and NOAA-6 satellites provide good estimates of the wind velocity and of the Lagrangian acceleration of air masses within the planetary boundary layer (PBL). With additional wind and pressure data, a study of the balance of forces and of the vertical wind structure in the planetary boundary layer has been performed, including the computation of the vertical transport of horizontal momentum by turbulent mixing. The accuracy of this estimate is marginal in equatorial regions, but sufficient in the Northern Hemisphere to derive a friction coefficient. The meridional variation of the wind veering within the planetary boundary layer is interpreted using a simple one-dimensional model. Veering is weak in the vicinity of the equator and appears to be of the Ekman type more than 10° away. If one follows a cross-equatorial trajectory, the transition in the direction of the veering occurs ∼5° north of the equator. This provides a measure of the relative importance of advective terms compared to local forcing terms in the dynamics of the tropical boundary layer.

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