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Titel |
Comparison between natural Rain drop size distributions and corresponding models near equilibrium state during warm rain |
VerfasserIn |
Laurent Barthes, Cécile Mallet |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250036202
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Zusammenfassung |
Keywords: Rain Drop Size Distribution, Breakup, coalescence, disdrometer
The study of the vertical evolution of raindrop size distributions (DSDs) during rainfall,
from the freezing level isotherm to ground level, is a key to improving our understanding of
the microphysics of rain. In numerous domains such as remote sensing, telecommunications,
soil erosion, and the study of the rain’s efficiency in “washing” the atmosphere, the DSD
plays an important role. Among the different processes affecting the evolution of DSD,
breakup and coalescence are two of the most significant. Models of coalescence
and breakup lead to equilibrium of the raindrop size distribution (DSD) after a fall
through sufficient vertical height. At equilibrium, the DSD no longer evolves, and
its shape is unique whatever the rain rate or LWC. This implies that the DSD is
known, to within a multiplication constant. These models based on experimental
measurements have been developed over the past 40 years. The Low and List (1982a,b)
parameterization (hereinafter LL82) and the Greg M. McFarquhar (2004) model are both
based on the same laboratory experiments, which lead to an equilibrium drop size
distribution (EDSD) with two or three peaks, and an exponential tail with a slope of
approximately Î=65 cm-1. Numerous measurements using disdrometer collected in
different climatic areas: Paris, France (Mars to October 2000), Iowa-City (April to
October 2002), and Djougou (Benin June to September 2006) corresponding to
537 hours of rain period have shown that for high rain rates, close to a state of
equilibrium, this slope lies between Î=20 - 22 cm-1. This latter value is corroborated
by others measurements found in the literature (Hu & Srivastava, 1995). Hu &
Srivastava suggested that the Low and List parameterization may overestimate
the effects of the breakup process. This hypothesis is in adequation with recent
laboratory experiments (A.P. Barros 2008) in which the authors conclude that the
number of fragments droplets produced when small drops and large drops collide is
overestimate.
As new parameterization of LL82 is not possible due to the lack of new sufficient large
experimental dataset, we have simply tried in the present study to “compensate” the problem
previously mentioned by replacing the coalescence/breakup model proposed in LL82 by
another one in which the breakup process is less dominant. In order to evaluate the relevance
of this modification, some of the DSD parameters such as slope, mean volume diameter, and
relation between moments are calculated, and comparisons with experimental DSD are made.
Simulations at equilibrium lead to a DSD tail with a slope of 23 cm-1 and a mean volume
diameter equal to 2.5 mm. These values are in good agreements with experimental data.
Similarly, the linear relationship between No* and the rainfall rate is also in good
agreement.
In the last part, the modified parameterization is then used to study the evolution of an
initially gamma-like DSD in a 1D vertical rain shaft.
References
Barros, A. P., O. P. Prat, P. Shrestha, F. Y. Testik, and L. F. Bliven, 2008. Revisiting Low
and List (1982): evaluation of raindrop collision using laboratory observations and modeling.
Journal of the Atmospheric Sciences. Vol. 65(9), pp. 2983-2993.
Hu, Z., and R. C. Srivastava: 1995: Evolution of raindrop size distribution by
coalescence, breakup, and evaporation: Theory and observations. J. Atmos. Sci., 52,
1761–1783.
Low, T. B. and R. List, 1982a: Collision, coalescence, and breakup of raindrops. Part I:
Experimentally established coalescence efficiencies and fragment size distributions in
breakup. J. Atmos. Sci., 39, 1591–1606.
____ 1982b: Collision, coalescence, and breakup of raindrops. Part II: Parameterization
of fragment size distributions. J. Atmos. Sci., 39, 1607–1618.
McFarquhar, G. M, 2004: A new representation of collision-induced breakup of raindrops
and its implications for the shapes of raindrop size distributions, J. Atmos. Sci., 61(7),
777–794. |
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