Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-8-14717-2008
http://www.atmos-chem-phys-discuss.net/8/14717/2008/
http://www.atmos-chem-phys-discuss.net/8/14717/2008/acpd-8-14717-2008.html
http://www.atmos-chem-phys-discuss.net/8/14717/2008/acpd-8-14717-2008.pdf
14717
14763
2008-07-31
Diffusional and accretional growth of water drops in a rising adiabatic parcel: effects of the turbulent collision kernel
W. W. Grabowski
grabow@ucar.edu
L.-P. Wang
Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, CO 80307, USA
Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA
A large set of rising adiabatic parcel simulations is executed
to investigate the combined diffusional and accretional growth of
cloud droplets in maritime and continental conditions, and to assess
the impact of enhanced droplet collisions due to small-scale cloud
turbulence. The microphysical model applies the droplet number density
function to represent spectral evolution of cloud and rain/drizzle drops,
and various numbers of bins in the numerical implementation, ranging
from 40 to 320. Simulations are performed applying two traditional
gravitational collection kernels and two kernels representing collisions
of cloud droplets in the turbulent environment, with turbulent kinetic
energy dissipation rates of 100 and 400 cm<sup>2</sup> s<sup>−3</sup>. The overall
result is that the rain initiation time significantly depends on the
number of bins used, with earlier initiation of rain when the number of
bins is low. This is explained as a combination of the increase of the
width of activated droplet spectrum and enhanced numerical spreading
of the spectrum during diffusional and collisional growth when the
number of model bins is low. Simulations applying around 300 bins seem
to produce rain at times which no longer depend on the number of bins,
but the activation spectra are unrealistically narrow. These results call
for an improved representation of droplet activation in numerical models
of the type used in this study.
<br><br>
Despite the numerical effects that impact the rain initiation time
in different simulations, the turbulent speedup factor, the ratio
of the rain initiation time for the turbulent collection kernel and
the corresponding time for the gravitational kernel, is approximately
independent of aerosol characteristics, parcel vertical velocity, and the
number of bins used in the numerical model. The turbulent speedup factor
is in the range 0.75–0.85 and 0.60–0.75 for the turbulent kinetic
energy dissipation rates of 100 and 400 cm<sup>2</sup> s<sup>−3</sup>, respectively.
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