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A new parameterisation for the sedimentation of drop ensembles using a finite maximum particle diameter

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Citation:
Ziemer, C. and Wacker, U. (2012): A new parameterisation for the sedimentation of drop ensembles using a finite maximum particle diameter , 16th International Conference on Clouds and Precipitation, Leipzig, 30 July 2012 - 3 August 2012 .
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Abstract:

The parameterised treatment of the sedimentation of a hydrometeor ensemble still is a demanding task. Recently, several studies have pointed out problems in the representation of physically relevant quantities when using the parameterisation of sedimentation as is. They circumvented these problems with empirical correction methods for the moment's mean fall speeds. This contribution presents a new parameterisation with two prognostic moments, based on a more physical definition of the integral which constitutes the moments: while conventional methods employ an upper diameter limit of infinity for the sake of easy calculation, here a finite maximum particle diameter is introduced. This corresponds to the non-existence of huge particles in nature. Hereby the mathematical properties of the equations are changed. A mirroring technique is required when the mean particle mass of the ensemble is large. Using the example of raindrops, results of the new parameterisation are presented and compared with the solution of the spectral budget equation and with results of other parameterisation methods. It turns out that a finite maximum drop diameter leads to improved results, especially when assuming a Marshall-Palmer spectrum. Concerning the value of the maximum diameter, the presented sensitivity studies suggest choosing it in the range of the observed value of 5 to 10 mm.

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