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AN APPROXIMATE METHOD OF DIVERGENCE MEASUREMENT

George P. CressmanAir Weather Service

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Abstract

With the vorticity equation for horizontal motion as a beginning, and with the aid of several assumptions, the equation ∇H·ν = νT·∇H In η is derived, giving the horizontal divergence as the product of (νT) the vector wind-difference between the level in question and 600 mb, and the logarithm of the absolute vorticity (ν) at the level where the divergence is desired. The divergence at 850 mb is computed for a variety of situations with the above equation and is compared with vertical velocities, with the divergence computed from the wind field, and with observed weather distributions, the comparisons yielding favorable results. Finally, suggestions are made for the use of the method for divergence determination in the prognostic routine.

Abstract

With the vorticity equation for horizontal motion as a beginning, and with the aid of several assumptions, the equation ∇H·ν = νT·∇H In η is derived, giving the horizontal divergence as the product of (νT) the vector wind-difference between the level in question and 600 mb, and the logarithm of the absolute vorticity (ν) at the level where the divergence is desired. The divergence at 850 mb is computed for a variety of situations with the above equation and is compared with vertical velocities, with the divergence computed from the wind field, and with observed weather distributions, the comparisons yielding favorable results. Finally, suggestions are made for the use of the method for divergence determination in the prognostic routine.

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