Effects from Bacteria and other Biological Particles on Mesoscale Convective Systems Simulated Numerically

Aircraft observations from the Ice in Clouds Experiment - Layer-clouds (ICE-L) that took place near Wyoming in 2007 have shown that biological particles are present in the residual material from heterogeneously nucleated ice crystals at a frequency (< 30% of residual particles) that varies greatly (Pratt et al. 2008). It is likely that some participated in nucleating the crystals in which they were found. Also, in freshly fallen snow, biogenic nucleators of ice have been seen at concentrations of up to about 500 particles per litre of melted snow at about -10 degC. Many of them were of bacterial origin (Christner et al. 2008). The relatively warm activity of these IN imply they have a phenotype (an observable characteristic of an organism) that boosts their ice nucleating capabilities. In order to simulate such nucleating effects of bacteria and other biological particle on cloud systems, 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. 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”, with double-moment bulk microphysics and a semi-prognostic aerosol component, is validated for diverse cases of deep convection with contrasting aerosol conditions, against satellite, ground-based and aircraft observations. Simulations with the aerosol-cloud model have been performed for a month-long period of summertime convective activity over Oklahoma. 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.