Resolving both entrainment-mixing and number of activated CCN in deep convective clouds 1The Department of Atmospheric Sciences, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
20 Dec 2011
2Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune 411008, India
Received: 24 February 2011 – Published in Atmos. Chem. Phys. Discuss.: 22 March 2011 Abstract. The number concentration of activated CCN (Na) is the most
fundamental microphysical property of a convective cloud. It determines the
rate of droplet growth with cloud depth and conversion into
precipitation-sized particles and affects the radiative properties of the
clouds. However, measuring Na is not always possible, even in the
cores of the convective clouds, because entrainment of sub-saturated ambient
air deeper into the cloud lowers the concentrations by dilution and may cause
partial or total droplet evaporation, depending on whether the mixing is
homogeneous or extreme inhomogeneous, respectively.
Revised: 27 November 2011 – Accepted: 01 December 2011 – Published: 20 December 2011
Here we describe a methodology to derive Na based on the rate of
cloud droplet effective radius (Re) growth with cloud depth and with
respect to the cloud mixing with the entrained ambient air. We use the slope
of the tight linear relationship between the adiabatic liquid water mixing
ratio and Re3 (or Rv3) to derive an upper limit for
Na assuming extreme inhomogeneous mixing. Then we tune Na
down to find the theoretical relative humidity that the entrained ambient air
would have for each horizontal cloud penetration, in case of homogeneous
mixing. This allows us to evaluate both the entrainment and mixing process in
the vertical dimension in addition to getting a better estimation for Na.
We found that the derived Na from the entire profile data is highly
correlated with the independent CCN measurements from below cloud base.
Moreover, it was found that mixing of sub-saturated ambient air into the
cloud at scales of ~100 m and above is inclined towards the extreme
inhomogeneous limit, i.e. that the time scale of droplet evaporation is
significantly smaller than that for turbulent mixing. This means that ambient
air that entrains the cloud is pre-moistened by total evaporation of cloud
droplets before it mixes deeper into the clouds where it can hardly change
the droplet size distribution, hence Re remains close to its
adiabatic value at any given cloud depth. However, the tendency towards the
extreme inhomogeneous mixing appeared to slightly decrease with altitude,
possibly due to enhanced turbulence and larger cloud drops aloft.
Quantifying these effects, based on more examples from other projects and
high resolution cloud models is essential for improving our understanding of
the interactions between the cloud and its environment. These interactions
may play an important role in cloud dynamics and microphysics, by affecting
cloud depth and droplet size spectra, for example, and may therefore
influence the cloud precipitation formation processes.
Citation: Freud, E., Rosenfeld, D., and Kulkarni, J. R.: Resolving both entrainment-mixing and number of activated CCN in deep convective clouds, Atmos. Chem. Phys., 11, 12887-12900, doi:10.5194/acp-11-12887-2011, 2011.