Wind tunnel investigations on riming of ice particles: Retention of water-soluble organics and the influence of turbulence on the growth of graupels.
Final Report Abstract
In recent years, gaseous and particulate organic species in the atmosphere have received increased attention. Organic compounds are found in cloud and rain water as well as in snow samples. However, the removal processes underlying their scavenging are not adequately enough understood such that these are amenable for integration in numerical models. One possibility via the ice phase could be the retention of organics during riming which is the deposition of super-cooled water droplets on ice particles. This process was simulated in wind tunnel experiments with freely floating ice particles and snowflakes exposed to clouds of supercooled droplets. Water-soluble organic substances of atmospheric significance, i.e. carboxylic acids and aldehydes were present in the liquid phase. The results confirm that the major parameter which decides retention in the ice phase is the effective Henry’s law constant which combines dissociation and solubility of a species. Minor factors are temperature and shape of the rime collector. They become more important at retentions lower than 100%. It was found that oxalic and malonic acid as well as formaldehyde remain in the ice phase by 100% while formic and acetic acid are still retained by 70 and 62%, respectively. This means that riming represents an important way to remove these organic species from the atmosphere. The retention coefficients determined in this project are applicable to cloud chemistry models to describe the partitioning of the investigated species between the phases. Another topic of great interest on the riming process is the influence of turbulence. The collisional growth of cloud droplets is significantly enhanced by turbulence; therefore, similar results could be expected in the case of riming as an analogous process in the ice phase. In wind tunnel experiments the growth rate of ice particles by riming was determined under laminar and turbulent air flow conditions. Turbulence was generated by an obstructing object placed upstream of the rime collector. The results showed, however, no significant differences between the growth rates during riming under laminar and turbulent air stream. The wavelength range of the generated turbulence spectrum with λ < 7 cm corresponds to the turbulence scale relevant for microphysical processes in clouds Therefore, one can conclude that the so far used collection kernels in cloud and precipitation models represent realistic situations in atmospheric clouds and do not require modification.
Publications
- Wind tunnel investigations on the retention of carboxylic acids during riming. EGU General Assembly, April 2015
Jost, A., K. Diehl, M. Szakáll, S.K. Mitra, and S. Borrmann
