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Paul L. Smith Jr.

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Paul L. Smith

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Gamma functions are widely used in an effort to represent characteristics of observed raindrop size distributions, especially at the small-particle end. However, available instruments do not agree about the character of the small-drop region, and for many purposes that part of the spectrum is unimportant. At the large-drop end, sampling limitations impede reliable measurements. Thus, when moment methods are used to determine parameters for the fitted functions, the experimental uncertainties tend to be greater than the differences in important bulk quantities, such as rainfall rate or radar reflectivity factor, between the resulting gamma distributions and corresponding, simpler exponential distribution functions. It consequently makes little practical difference whether exponential or gamma functions are employed, and the exponential model is appropriate for many purposes.

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Paul L. Smith

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Paul L. Smith

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Simulation of sampling from gamma-distributed raindrop populations demonstrates that significant biases and substantial errors can occur in estimates of polarimetric radar variables based on samples of raindrop populations obtained with disdrometers. Biases and RMS errors of 0.5 dB or more in estimates of differential reflectivity Z dr can occur with samples of even a few hundred drops; significant biases and errors also occur in estimates of reflectivity Z H or specific differential phase K dp. The results indicate that very large samples would be required to obtain adequate representation of the population characteristics for many radar applications. They also suggest that greater attention is needed to the sample sizes in the disdrometer data used in developing polarimetric rainfall-rate estimators or hydrometeor classification algorithms.

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Paul L. Smith

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Paul L. Smith

This paper presents an analysis of the effects of shortcomings in the reporting of severe storm events on some common measures of warning performance. Such deficiencies lead to an apparent false alarm ratio (FAR) higher than the true value, and ordinarily to an apparent probability of detection (POD) also higher than the true value. An improved warning system may generate additional storm reports through closer collaboration between forecasters and storm spotters; an enhanced warning verification program will also tend to collect additional storm reports. Independently of any changes in the warning system, such additional reports tend to drive the apparent FAR down (and thus closer to the true value). If the verification efforts emphasize situations when warnings are in effect, the additional reports will further inflate the apparent POD. When changes occur in both the warning system and the verification program, the contributions of each to changes in the performance measures become intermingled. Understanding of these effects of the reporting system can aid in interpreting trends in the performance measures.

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Paul L. Smith

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This paper discusses the subject of weather radar system sensitivity from a general point of view, with emphasis an the influence of wavelength. Expressions for the echo signal-to-noise ratio are examined using a detection theory approach to develop factors describing the effects of different signal processing techniques. Then the variation of the equivalent signal-to-noise ratio with wavelength under certain typical system design constraints is examined. The effects of both theoretical and technology system design considerations are assessed. The results vary with the design scenario and the signal processing method, but the main conclusion is that short-wavelength weather radars are not necessarily more sensitive than long-wavelength ones.

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Paul L. Smith

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This note argues that the proper symbol for the logarithmic unit of radar reflectivity factor is dBz. The basis for this contention lies in both customary engineering practice and the international standard for unit symbols.

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Paul L. Smith

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Comments are made on opportunity recognition, treatment, and evaluation aspects of the implementation and testing of seeding concepts. The main topics include experimental design, experimental units, delivery and dispersion of seeding agents, and statistical evaluation procedures.

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Paul L. Smith
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