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Ronald E. Rinehart and Alan Borho

Abstract

Microbursts continue to pose a serious problem to the aviation industry. Fortunately, Doppler weather radars are capable of detecting microbursts quite successfully. This study gives the results of a comparison of 84 microbursts detected by a pair of C-band Doppler radars near Orlando during the summer of 1991. The study shows that microbursts were detectable at nearly the same locations (average positional difference of 1 km) and times (average time of detection differed by only 23 s) by both radars. The differential wind velocity detected by each of the radars was also quite similar (average difference of only 0.01 m s−1) as were the radar reflectivity factors (average difference was 1 dB). The conclusion from this is that a C-band radar located anywhere near an airport should be fully capable of detecting hazardous wet-microburst events. Attenuation of the C-band signals was never strong enough to make microbursts undetectable. Bemuse all events were wet microbursts (average reflectivity was 47 dBZ) and the maximum reflectivity difference seen for any microburst was only 10 dB, all events would have been much stronger than the minimum detectable reflectivity at the relatively short ranges used in this study. Attenuation might, however, be a problem for the detection of weak gust fronts.

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Erwin T. Prater and Alan A. Borho

Abstract

Freezing rain is commonly caused by overrunning. The typical atmospheric structure and precipitation patterns associated with overrunning and freezing rain suggest a general association between 1) a bright band and 2) a wind-shear layer between the colder air mass near the surface and an overrunning warmer air mass above the frontal boundary. These features were easily detected by a Doppler radar, and their associated thermodynamic structure was documented by an instrumented aircraft in two freezing-rain events near Kansas City, Missouri, in February 1990. The two cases suggest that perhaps freezing rain caused by overrunning has recognizable and easily parameterized Doppler radar signatures that could be incorporated into a freezing-rain detection algorithm. A preliminary algorithm is discussed.

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Ronald E. Rinehart, Alan Borho, and Charles Curtiss

Abstract

Microburst rotation can be determined by measuring the difference in azimuths between the maximum approaching and maximum receding velocity centers on a Doppler radar. Nonrotating microbursts would have these centers exactly along the same radial from the radar. Microbursts rotating clockwise would have the approaching center clockwise of the receding center, and vise versa. In the fist part of this study the authors develop the relationships between the uniform wind, source strength, and rotational strength using potential flow theory and apply this to simulating real microbursts. In the second part the authors give observations of microburst rotation based on measurements of 908 microbursts made near Orlando, Florida, during 1992. While most microbursts had little rotation, 55.4% rotated cyclonically. The average tangential velocity of the rotational component was 1.1 m s−1; 5% had rotations equal to or greater than 2.5 m s−1. This may have significant implications for aviation. Finally, microburst strength measurements are compared with velocity shear and F factors for the 908 microbursts.

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