Cloud optical thickness and liquid water path – does the k coefficient vary with droplet concentration? 1CNRM/GAME, Météo-France/CNRS, URA1357, Toulouse, France
21 Sep 2011
2Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
Received: 03 December 2010 – Published in Atmos. Chem. Phys. Discuss.: 11 February 2011 Abstract. Cloud radiative transfer calculations in general circulation models involve
a link between cloud microphysical and optical properties. Indeed, the
liquid water content expresses as a function of the mean volume droplet
radius, while the light extinction is a function of their mean surface
radius. There is a small difference between these two parameters because of
the droplet spectrum width. This issue has been addressed by introducing an
empirical multiplying correction factor to the droplet concentration.
Analysis of in situ sampled data, however, revealed that the correction
factor decreases when the concentration increases, hence partially
mitigating the aerosol indirect effect.
Revised: 26 August 2011 – Accepted: 29 August 2011 – Published: 21 September 2011
Five field experiments are reanalyzed here, in which standard and upgraded
versions of the droplet spectrometer were used to document shallow cumulus
and stratocumulus topped boundary layers. They suggest that the standard
probe noticeably underestimates the correction factor compared to the
upgraded versions. The analysis is further refined to demonstrate that the
value of the correction factor derived by averaging values calculated
locally along the flight path overestimates the value derived from liquid
water path and optical thickness of a cloudy column, and that there is no
detectable relationship between the correction factor and the droplet
concentration. It is also shown that the droplet concentration dilution by
entrainment-mixing after CCN activation is significantly stronger in shallow
cumuli than in stratocumulus layers. These various effects are finally
combined to produce the today best estimate of the correction factor to use
in general circulation models.
Citation: Brenguier, J.-L., Burnet, F., and Geoffroy, O.: Cloud optical thickness and liquid water path – does the k coefficient vary with droplet concentration?, Atmos. Chem. Phys., 11, 9771-9786, doi:10.5194/acp-11-9771-2011, 2011.