The Dynamics of an Upper-Level Trough in the Baroclinic Westerlies: Analysis Based upon Data from a Wind Profiler Network

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  • 1 School of Meteorology, University of Oklahoma, Norman, Oklahoma
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Abstract

Hourly wind data from the National Oceanic and Atmospheric Administration's demonstration network of 404-MHz profilers in the central United States and hourly wind data from the standard National Weather Service surface network are used to determine the validity of the geostrophic momentum (GM) approximation in the vicinity of a sharp trough in the baroclinic westerlies by using observations alone. The ageostrophic and geostrophic components of the wind are retrieved from time-filtered and objectively analyzed wind data and from kinematically computed vertical velocities using the inviscid form of the horizontal equations of motion in pressure coordinates. The success of the retrieval technique is discussed critically in terms of the difference between the horizontal divergence of the wind and the horizontal divergence of the retrieved ageostrophic wind and in terms of the retrieval of the geostrophic wind field from simulated wind data using a primitive equation numerical model of a growing baroclinic wave.

It is shown that for a system that moved through the network in Kansas and Oklahoma on 7–8 October 1992, the GM approximation breaks down, owing to strong curvature around the trough. In particular, the geostrophic advection of ageostrophic momentum, which is neglected in the GM formulation of the equations of motion, is comparable in magnitude to the ageostrophic advection of geostrophic momentum, which is retained in the GM formulation. However, along- and downstream from the trough axis, the effects of geostrophic advection of ageostrophic momentum are counteracted by the local time rate of change of ageostrophic momentum and the vertical advection of ageostrophic momentum, effects not included in the GM formulation.

Abstract

Hourly wind data from the National Oceanic and Atmospheric Administration's demonstration network of 404-MHz profilers in the central United States and hourly wind data from the standard National Weather Service surface network are used to determine the validity of the geostrophic momentum (GM) approximation in the vicinity of a sharp trough in the baroclinic westerlies by using observations alone. The ageostrophic and geostrophic components of the wind are retrieved from time-filtered and objectively analyzed wind data and from kinematically computed vertical velocities using the inviscid form of the horizontal equations of motion in pressure coordinates. The success of the retrieval technique is discussed critically in terms of the difference between the horizontal divergence of the wind and the horizontal divergence of the retrieved ageostrophic wind and in terms of the retrieval of the geostrophic wind field from simulated wind data using a primitive equation numerical model of a growing baroclinic wave.

It is shown that for a system that moved through the network in Kansas and Oklahoma on 7–8 October 1992, the GM approximation breaks down, owing to strong curvature around the trough. In particular, the geostrophic advection of ageostrophic momentum, which is neglected in the GM formulation of the equations of motion, is comparable in magnitude to the ageostrophic advection of geostrophic momentum, which is retained in the GM formulation. However, along- and downstream from the trough axis, the effects of geostrophic advection of ageostrophic momentum are counteracted by the local time rate of change of ageostrophic momentum and the vertical advection of ageostrophic momentum, effects not included in the GM formulation.

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