Simulating the impacts of soil dust with ice-active organic compounds on cloud on regional scales

Simulating the impacts of soil dust with ice-active organic compounds on cloud on regional scales

M. Hummel, C. Hoose, C. Schaupp, I. Steinke, O. Möhler

Institute for Meteorology and Climate Research, Karlsruhe Institute for Technology, Karlsruhe, Germany

The initial formation of ice crystals in mixed phase clouds is presumed to be catalyzed by ice nuclei (IN), which are ice nucleating active particles in the atmosphere. Heterogeneous ice nucleation is therefore triggered by IN at certain temperatures and relative humidity, which mainly depends on the properties of the aerosol particle acting as IN (Hoose und Möhler, 2012). Only a small proportion of atmospheric aerosol particles can be activated to IN (Murray et al., 2012). Soil dust with organic compounds and some primary biological aerosol particles (PBAP) are found to be ice nucleating active in immersion freezing mode. Their onset freezing temperature is larger than for mineral dust, one of the most important IN in Earth's atmosphere. Some soil dusts and PBAP are therefore among the most efficient IN regarding onset freezing temperature (Després et al., 2012; Hoose und Möhler, 2012; Murray et al., 2012; Morris et al., 2013).

The influence of organic soil dust or equivalently ice nucleating active PBAP on cloud properties is analyzed with a regional scale atmospheric model (COSMO-ART, Vogel et al., 2009) for Europe as a proxy. In the simulation, soil dust is emitted into atmosphere from nutrient-rich fallow agricultural areas by wind erosion. In addition to commonly used dust emission schemes (e.g. Alfaro und Gomes, 2001), only fields without vegetation cover contribute to this emission parameterization. Monthly mean tile fraction of fallow croplands are characterized by means of a normalized differential vegetation index (NDVI) of croplands from satellite images (Champeaux et al., 2000).

Parameterization for heterogeneous ice nucleation of soil dust is developed from AIDA cloud simulation chamber results. In the model, a temperature-dependent density of ice nucleating active surface sites (INAS) of soil dust is applied to a variable soil dust diameter, calculated from simulated number and mass density. For further analyzing the influences of biological compounds on cloud ice, heterogeneous ice nucleation of pseudomonas syringae bacteria as well as fungal spore species, which are frequently found in soils, are taken into account. In order to limit the parameterizations to immersion freezing, its calculations are only done at model grid boxes which also include liquid droplets.

Preliminary results indicate that soil dust and PBAP contribute to a lesser degree to cloud ice than mineral dust, because their IN concentrations are ~0.1 L^-1 at -15°C. Besides this direct influence, the ice nucleating efficiency of organic compounds allows them to be ice nucleating active at T < -15°C. Thus, an altering of cloud properties in low altitudes (~2 km) can change the development of the cloud. This potential indirect influence may be a further contribution to cloud impacts of soil dust and PBAP.

References

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