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Peter G. Black, John R. Proni, John C. Wilkerson, and Christopher E. Samsury

Abstract

Measurements of the underwater sound produced by rain were made at three U.S. coastal sites in a study to determine the feasibility and limitations of the acoustic detection and classification of rainfall over water. In the analysis of the rain sound spectra, concurrent radar reflectivity observations were used to identify convective and stratiform regions of the precipitating clouds overhead. It was found that acoustic classifications of rainfall as to type, based on information in the 4–30-kHz frequency band, were in general agreement with radar-derived classifications. The classification technique is based on use of an acoustic discriminant, DR, defined as the difference in average spectral levels between the 10–30- and 4–10-kHz bands. A high correlation was found between sound spectrum levels (in decibels) in the 4–10-kHz frequency band and radar reflectivity, dBZ, suggesting the possible use of the 4–10-kHz band sound spectral level as a classification tool using spatially distributed hydrophones in the same way that radar reflectivity is used in classifying precipitation. The results demonstrate the feasibility of the acoustic method for detecting and classifying rainfall at sea.

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Jeffrey A. Nystuen, John R. Proni, Peter G. Black, and John C. Wilkerson

Abstract

Automatic rain gauge systems are required to collect rainfall data at remote locations, especially oceanic sites where logistics prevent regular visits. Rainfall data from six different types of automatic rain gauge systems have been collected for a set of summertime subtropical rain events and for a set of wintertime rain events at Miami, Florida. The rain gauge systems include three types of collection gauges: weighing, capacitance, and tipping bucket; two gauges that inherently measure rainfall rate: optical scintillation and underwater acoustical inversion; and one gauge that detects individual raindrops: the disdrometer. All of these measurement techniques perform well; that is, they produce rainfall estimates that are highly correlated to one another. However, each method has limitations. The collection gauges are affected by flow irregularities between the catchment basin and the measurement chambers. This affects the accuracy of rainfall-rate measurements from these instruments, especially at low rainfall rates. In the case of the capacitance gauge, errors in 1-min rainfall rates can exceed +10 mm h−1. The rainfall rate gauges showed more scatter than the collection gauges for rainfall rates over 5 mm h−1, and the scatter was relatively independent of rainfall rate. Changes in drop size distribution within an event could not be used to explain the scatter observed in the optical rain gauge data. The acoustical inversion method can be used to measure the drop size distribution, allowing rainfall classification and estimation of other rain parameters—for example, reflectivity or liquid water content—in addition to rainfall rate. The acoustical inversion method has the advantage of an extremely large catchment area, resulting in very high time resolution. The disdrometer showed a large scatter relative to the other rain gauge systems for low rainfall rates. This is consistent with the small catchment area for the disdrometer system.

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