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  • Author or Editor: J. C. Fankhauser x
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J. C. Fankhauser

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.

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J. C. Fankhauser

Abstract

Guidelines followed in designing and operating a special mesoscale rawinsonde network are discussed. Objective data reduction and analysis techniques are developed and applied to the wind, temperature and moisture data measured during a selected thunderstorm case. The goal is to appraise the sounding system's limitations and reliability for resolving the mesoscale circulations associated with convective processes. A consistent four-dimensional synoptic portrayal of the variables is achieved by accounting for balloon drift, differing station-to-station and sounding-to-sounding ascent rates, and departures from scheduled release time.Temporal variations in the spatial distributions of computed divergence and kinematic vertical motion are in good qualitative agreement with the location and intensity of thunderstorm radar echoes, after further objective adjustments are applied to compensate for the assumed character of wind measurement and analysis errors. For the purpose of assessing data and analytical credibility, independent estimates of the energy related to diabatic heating are obtained by evaluating thermodynamic energy and moisture continuity equations. Comparative results indicate that the network upper-air data and the methods adopted for its synthesis are internally consistent. On the other hand, the distribution and magnitude of the resolved kimematic and dynamic features demonstrate that the results are clearly a function of observational spacing and do not necessarily pertain to individual convective processes.

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G. B. Foote
and
J. C. Fankhauser

Abstract

A case study is presented of a persistent thunderstorm of moderate intensity which occurred in northeast Colorado, and which produced a light hailfall at the ground. The storm was intensively monitored by aircraft, radar, dropsondes, and surface and upper air networks involved in the National Hail Research Experiment. The present study emphasizes the measurements obtained by instrumented aircraft in examining the properties of the subcloud airflow.

Large-scale analysis shows that the storm formed on a surface confluence line and subsequently moved toward the region of surface moisture. A brief radar analysis of the storm during the mature stage of its ∼5-hr lifetime is presented, and identifies the general features as belonging to the category termed “supercell” by previous workers. A precipitation analysis is carried out, and rain and hailfall are correlated with the track of the storm.

Liberal use of a time-space conversion technique results in detailed mesoscale pressure, temperature, moisture and wind field analyses from a network of 22 remote meteorological stations. Surface divergence of mass and moisture is computed. Surface features are related to the position and structure of the radar echo.

Wind, temperature and moisture data obtained by aircraft encircling the storm in the region below cloud base are presented. Emphasis is placed on the airflow with respect to the moving storm, and details of the subcloud circulation are examined. Analysis of relative streamlines, supported by the observed temperature and moisture structure, delineates distinct regions of inflow and outflow for the storm. Partitioning of the measured flux into inflow and outflow segments at the aircraft levels and at the surface results in estimates of the mass and moisture budgets for the storm. The ratio of rainout measured at the ground, 2 × 109 gm sec−1, to the computed moisture influx, 13 × 109 gm sec−1, results in a precipitation efficiency of only 15%. A physical basis (following a discussion presented by Marwitz) for correlating this low efficiency with the fairly high value of vertical wind shear which existed around the storm, ∼5 × 10−2 sec−1, is elaborated upon, and the present results are compared with those of previous investigators. The computed mass inflow, 2 × 1012 gm sec−1, is rather large, but is shown to be compatible with other measurements made of the storm. The computations indicate that, in this case, only about half of the upward flux took place in the vicinity of the “echo-free vault,” and attention is drawn to a secondary region of echo overhang where various measurements indicate that significant vertical transports also occurred. Based on the airflow and thermodynamic measurements, some ideas concerning the energy source that drives the inflow circulation are presented.

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P. W. Summers
,
J. C. Fankhauser
,
G. M. Morgan Jr.
,
G. B. Foote
, and
A. C. Modahl

Abstract

A detailed analysis is made of the environmental conditions existing on each of the declared hail days during the randomized seeding experiment. From the many soundings available each day, the one most representative of the near-storm environment is carefully selected. This sounding is then used to compute several parameters known to influence hailfall. It is found that two parameters, both indicative of the thermodynamic instability, have a more instable mean value on the seed days than on the control days that in one case is statistically significant at the 10% level. Correcting for this draw would result in reducing the actual ratios of seed to control hail mass found in the primary statistical evaluation of the experiment. However, the reduction would not be sufficient in relation to the very wide 90% confidence limits to affect the statistical conclusions that the ratios were not significantly different from 1.0.

An analysis of the sequences of declared hail days showed that, in spite of the careful experimental design, the random selection process produced an actual partitioning of sequence starts into seed or control such that a sequence this extreme, or more extreme, had a chance of only 3 in 100 of occurring. However, it is not likely that this unexpected draw affected the evaluation of the experiment in any significant way, since it is taken care of indirectly in the analyses of the environmental parameters.

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