Transport and chemical transformations influenced by shallow cumulus over land

[摘要] The distribution and evolution of reactive species in a boundarylayer characterized by the presence of shallow cumulus over landis studied by means of two large-eddy simulation models: the NCARand WUR codes. The study focuses on two physical processes that caninfluence the chemistry: the enhancement of the vertical transportby the buoyant convection associated with cloud formation and theperturbation of the photolysis rates below, in and above the clouds.It is shown that the dilution of the reactant mixing ratio causedby the deepening of the atmospheric boundary layer is an importantprocess and that it can decrease reactant mixing ratios by 10 to50 percent compared to very similar conditions but with no cloudformation.Additionally, clouds transport chemical species tohigher elevations in the boundary layer compared to the case withno clouds which influences the reactant mixing ratios of the nocturnalresidual layers following the collapse of the daytime boundarylayer.Estimates of the rate of reactant transport based on the calculationof the integrated flux divergence range from to −0.2 ppb hr^-1to −1 ppb hr^-1, indicating a net loss of sub-cloud layer airtransported into the cloud layer.A comparison of this flux to aparameterized mass flux shows good agreement in mid-cloud, but at cloudbase the parameterization underestimates the mass flux.Scattering ofradiation 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 havelittle effect on mixing ratios averaged over space and time.We find that the ultravioletradiance perturbation becomes more important for chemical transformationsthat react with a similar order time scale as the turbulenttransport in clouds.Finally, the detailed intercomparison of the LESresults shows very good agreement between the two codes whenconsidering the evolution of the reactant mean, flux and (co-)variancevertical profiles.

[学科分类] 大气科学