Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes 1Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
2CNRM/GAME (Météo-France, CNRS), Toulouse, France
Received: 16 July 2009 – Published in Atmos. Chem. Phys. Discuss.: 25 August 2009 Abstract. Parametric functions are currently used to represent droplet spectra in
clouds and to develop bulk parameterizations of the microphysical processes
and of their interactions with radiation. The most frequently used
parametric functions are the Lognormal and the Generalized Gamma which have
three and four independent parameters, respectively. In a bulk
parameterization, two parameters are constrained by the total droplet number
concentration and the liquid water content. In the Generalized Gamma
function, one parameter is specified a priori, and the fourth one, like the
third parameter of the Lognormal function, shall be tuned, for the
parametric function to statistically best fit observed droplet spectra.
Revised: 01 February 2010 – Accepted: 03 May 2010 – Published: 26 May 2010
These parametric functions are evaluated here using droplet spectra
collected in non-or slightly precipitating stratocumulus and shallow
cumulus. Optimum values of the tuning parameters are derived by minimizing
either the absolute or the relative error for successively the first,
second, fifth, and sixth moments of the droplet size distribution. A
trade-off value is also proposed that minimizes both absolute and relative
errors for the four moments concomitantly. Finally, a parameterization is
proposed in which the tuning parameter depends on the liquid water content.
This approach significantly improves the fit for the smallest and largest
values of the moments.
Citation: Geoffroy, O., Brenguier, J.-L., and Burnet, F.: Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes, Atmos. Chem. Phys., 10, 4835-4848, doi:10.5194/acp-10-4835-2010, 2010.