Possible Effects of Collisional Breakup on Mixed-Phase Deep Convection Simulated by a Spectral (Bin) Cloud Model

Axel Seifert Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universität Karlsruhe, Karlsruhe, Germany

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Alexander Khain Institute of the Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel

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Ulrich Blahak Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universität Karlsruhe, Karlsruhe, Germany

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Klaus D. Beheng Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universität Karlsruhe, Karlsruhe, Germany

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Abstract

The effects of the collisional breakup of raindrops are investigated using the Hebrew University Cloud Model (HUCM). The parameterizations, which are combined in the new breakup scheme, are those of Low and List, Beard and Ochs, as well as Brown. A sensitivity study reveals strong effects of collisional breakup on the precipitation formation in mixed-phase deep convective clouds for strong as well as for weak precipitation events. Collisional breakup reduces the number of large raindrops, increases the number of small raindrops, and, as a consequence, decreases surface rain rates and considerably reduces the speed of rain formation. In addition, it was found that including breakup can lead to a more intense triggering of secondary convective cells. But a statistical comparison with observed raindrop size distributions shows that the parameterizations might systematically overestimate collisional breakup.

Corresponding author address: Dr. Axel Seifert, Deutscher Wetterdienst, GB Forschung und Entwicklung, Kaisorleistr. 42, Offenbach a. M., Germany. Email: axel.seifert@dwd.de

Abstract

The effects of the collisional breakup of raindrops are investigated using the Hebrew University Cloud Model (HUCM). The parameterizations, which are combined in the new breakup scheme, are those of Low and List, Beard and Ochs, as well as Brown. A sensitivity study reveals strong effects of collisional breakup on the precipitation formation in mixed-phase deep convective clouds for strong as well as for weak precipitation events. Collisional breakup reduces the number of large raindrops, increases the number of small raindrops, and, as a consequence, decreases surface rain rates and considerably reduces the speed of rain formation. In addition, it was found that including breakup can lead to a more intense triggering of secondary convective cells. But a statistical comparison with observed raindrop size distributions shows that the parameterizations might systematically overestimate collisional breakup.

Corresponding author address: Dr. Axel Seifert, Deutscher Wetterdienst, GB Forschung und Entwicklung, Kaisorleistr. 42, Offenbach a. M., Germany. Email: axel.seifert@dwd.de

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