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A Study of Thunderstorm Microphysics with Multiparameter Radar and Aircraft Observations

E. A. BrandesNational Center for Atmospheric Research, Boulder, Colorado

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J. VivekanandanNational Center for Atmospheric Research, Boulder, Colorado

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J. D. TuttleNational Center for Atmospheric Research, Boulder, Colorado

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C. J. KessingerNational Center for Atmospheric Research, Boulder, Colorado

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Abstract

Excellent agreement was found between multiparameter radar signatures of hail, raindrops, and mixed-phase precipitation and in situ precipitation particle measurements made by aircraft in a northeastern Colorado hail-storm. Radar reflectivity estimates determined by remote measurement and from observed particle distributions generally agreed within 5 dB. Maximum values of differential reflectivity (ZDR) and the fractional contribution of liquid water to total reflectivity (frain) differed by less than 0.8 dB and a factor of 2, respectively.

A positive ZDR column, which extended more than 2 km above the freezing level, was nearly coincident with the storm updraft. The column contained mixed-phase precipitation, but the ZDR measurement was dominated by a small number of very large raindrops (some exceeding 5 mm in diameter). Trajectories computed with a precipitation growth model suggest that many drops originated with partially or totally melted particles from a quasi-stationary feeder band within the inflow region of the storm. The terminal velocity of the drops composing the ZDR column exceeded updraft speeds, and therefore, they may have simply fallen from the storm. Although particle observations and radar measurements in the column at approximately 3 km AGL and a temperature of −2°C revealed that the fractional contribution of drops to radar reflectivity was roughly 0.5–0.8, the concentration of supercooled water represented by the drops (a maximum of 0.5 g m−3 and an average of 0.2 g m−3) was about half that associated with cloud water. Hence, the relative importance of the large drops and consequently that of the ZDR column as a source of hail embryos, and a factor in hail growth, may have been minor.

Abstract

Excellent agreement was found between multiparameter radar signatures of hail, raindrops, and mixed-phase precipitation and in situ precipitation particle measurements made by aircraft in a northeastern Colorado hail-storm. Radar reflectivity estimates determined by remote measurement and from observed particle distributions generally agreed within 5 dB. Maximum values of differential reflectivity (ZDR) and the fractional contribution of liquid water to total reflectivity (frain) differed by less than 0.8 dB and a factor of 2, respectively.

A positive ZDR column, which extended more than 2 km above the freezing level, was nearly coincident with the storm updraft. The column contained mixed-phase precipitation, but the ZDR measurement was dominated by a small number of very large raindrops (some exceeding 5 mm in diameter). Trajectories computed with a precipitation growth model suggest that many drops originated with partially or totally melted particles from a quasi-stationary feeder band within the inflow region of the storm. The terminal velocity of the drops composing the ZDR column exceeded updraft speeds, and therefore, they may have simply fallen from the storm. Although particle observations and radar measurements in the column at approximately 3 km AGL and a temperature of −2°C revealed that the fractional contribution of drops to radar reflectivity was roughly 0.5–0.8, the concentration of supercooled water represented by the drops (a maximum of 0.5 g m−3 and an average of 0.2 g m−3) was about half that associated with cloud water. Hence, the relative importance of the large drops and consequently that of the ZDR column as a source of hail embryos, and a factor in hail growth, may have been minor.

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