Potential impacts from biological aerosols on ensembles of continental clouds simulated numerically

An aerosol-cloud modeling framework is described to simulate the activation of ice particles and droplets by biological aerosol particles, such as airborne ice-nucleation active (INA) bacteria. It includes the empirical parameterisation of heterogeneous ice nucleation and a semi-prognostic aerosol component, which have been incorporated into a cloud-system resolving model (CSRM) with double-moment bulk microphysics. The formation of cloud liquid by soluble material coated on these partially insoluble organic aerosols is represented. It determines their partial removal from deep convective clouds by accretion onto precipitation in the cloud model. This "aerosol-cloud model" is validated for diverse cases of deep convection with contrasting aerosol conditions, against satellite, ground-based and aircraft observations. Simulations are performed with the aerosol-cloud model for a month-long period of summertime convective activity over Oklahoma. It includes three cases of continental deep convection simulated previously by Phillips and Donner (2006). Elevated concentrations of insoluble organic aerosol, boosted by a factor of 100 beyond their usual values for this continental region, are found to influence significantly the following quantities: (1) the average numbers and sizes of ice crystals and droplets in the clouds; (2) the horizontal cloud coverage in the free troposphere; (3) precipitation at the ground; and (4) incident solar insolation at the surface. This factor of 100 is plausible for natural fluctuations of the concentration of insoluble organic aerosol, in view of variability of cell concentrations for airborne bacteria seen by Lindemann et al. (1982). In nature, such boosting of the insoluble organic aerosol loading could arise from enhanced emissions of biological aerosol particles from a land surface. Surface wetness and solar insolation at the ground are meteorological quantities known to influence rates of growth of certain biological particles (e.g. bacteria). Their rates of emission into the atmosphere must depend on these same quantities, in addition to surface wind speed, turbulence and convection. Finally, the present study is the first attempt at evaluating the impacts from biological aerosols on mesoscale cloud ensembles in the literature.