Transport and chemical transformations influenced by shallow cumulus over land J. Vilà-Guerau de Arellano1, S.-W. Kim2, M. C. Barth2, and E. G. Patton2 1Meteorology and Air Quality Section, Wageningen University, The Netherlands 2Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, USA
Abstract. The distribution and evolution of reactive species in a boundary
layer characterized by the presence of shallow cumulus over land
is studied by means of two large-eddy simulation models: the NCAR
and WUR codes. The study focuses on two physical processes that can
influence the chemistry: the enhancement of the vertical transport
by the buoyant convection associated with cloud formation and the
perturbation of the photolysis rates below, in and above the clouds.
It is shown that the dilution of the reactant mixing ratio caused
by the deepening of the atmospheric boundary layer is an important
process and that it can decrease reactant mixing ratios by 10 to
50 percent compared to very similar conditions but with no cloud
formation. Additionally, clouds transport chemical species to
higher elevations in the boundary layer compared to the case with
no clouds which influences the reactant mixing ratios of the nocturnal
residual layers following the collapse of the daytime boundary
layer. Estimates of the rate of reactant transport based on the calculation
of the integrated flux divergence range from to −0.2 ppb hr-1
to −1 ppb hr-1, indicating a net loss of sub-cloud layer air
transported into the cloud layer. A comparison of this flux to a
parameterized mass flux shows good agreement in mid-cloud, but at cloud
base the parameterization underestimates the mass flux. Scattering of
radiation by cloud drops perturbs photolysis rates.
It is found that these perturbed photolysis rates substantially (10–40%)
affect mixing ratios locally (spatially and temporally), but have
little effect on mixing ratios averaged over space and time.
We find that the ultraviolet
radiance perturbation becomes more important for chemical transformations
that react with a similar order time scale as the turbulent
transport in clouds. Finally, the detailed intercomparison of the LES
results shows very good agreement between the two codes when
considering the evolution of the reactant mean, flux and (co-)variance
Citation: Vilà-Guerau de Arellano, J., Kim, S.-W., Barth, M. C., and Patton, E. G.: Transport and chemical transformations influenced by shallow cumulus over land, Atmos. Chem. Phys., 5, 3219-3231, doi:10.5194/acp-5-3219-2005, 2005.