The Derivation of Consistent Fields of Wind and Geopotential Height from Mesoscale Rawinsonde Data

Abstract

With an attainable height resolution of no better than ±10 m from individual soundings, direct analysis of geopotential height gradient from mesoscale rawinsonde ascents could not be expected to produce reliable results. A method is presented for deriving a height field which incorporates the carefully analyzed horizontal winds measured by an upper air network specifically designed to study severe thunderstorms. Three-dimensional distributions of wind, temperature and moisture content are obtained as a function of time using a combination of subjective and objective analysis techniques, which take account of departure from scheduled release time, differing ascent rates, and the horizontal drift of balloons during ascent. The vertical component of air motion is computed from the kinematic approach. Adjustments are applied to divergence estimates to achieve physically realistic results for vertical motion in the upper troposphere. Errors in the horizontal wind components are likewise altered for consistency by assuming that they affect only the divergent part of the flow. The three-dimensionally consistent array of velocity components is used to evaluate the complete horizontal divergence equation to obtain the geopotential Laplacian as a residual, which, when integrated numerically, yields the horizontal height perturbations associated with the mesoscale winds. Results obtained from application of these techniques to rawinsonde data collected in a squall line case appear to be in qualitative agreement with recognized thunderstorm airflow and pressure distributions. Comparative magnitudes of individual terms in the divergence equation demonstrate that the balance approximation is inadequate for diagnosing the dynamics of mesoscale convective motions.