L. Ricci

The Cloud Ice Mountain Experiment (CIME) 1998: experiment overview and modelling of the microphysical processes during the seeding by isentropic gas expansion

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dome in the centre of France. The content of residual aerosol particles, of H2O2 and NH3 in cloud droplets was evaluated by evaporating the drops larger than 5 μm in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 μm diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud. In the present paper, an overview is given of the CIME 98 experiment and the instrumentation deployed. The meteorological situation during the experiment was analysed with the help of a cloud scale model. The microphysics processes and the behaviour of the scavenged aerosol particles before and during seeding are analysed with the detailed microphysical model ExMix. The simulation results agreed well with the observations and confirmed the assumption that the Bergeron–Findeisen process was dominating during seeding and was influencing the partitioning of aerosol particles between drops and ice crystals. The results of the CIME 98 experiment give an insight on microphysical changes, redistribution of aerosol particles and cloud chemistry during the Bergeron–Findeisen process when acting also in natural clouds.

Behaviour of H2O2, NH3, and black carbon in mixed-phase clouds during CIME

Abstract We investigated the partitioning of trace substances during the phase transition from supercooled to mixed-phase cloud induced by artificial seeding. Simultaneous determination of the concentrations of H 2 O 2 , NH 3 and black carbon (BC) in both condensed and interstitial phases with high time resolution showed that the three species undergo different behaviour in the presence of a mixture of ice crystals and supercooled droplets. Both H 2 O 2 and NH 3 are efficiently scavenged by growing ice crystals, whereas BC stayed predominantly in the interstitial phase. In addition, the scavenging of H 2 O 2 is driven by co-condensation with water vapour onto ice crystals while NH 3 uptake into the ice phase is more efficient than co-condensation alone. The high solubility of NH 4 + in the ice could explain this result. Finally, it appears that the H 2 O 2 –SO 2 reaction is very slow in the ice phase with respect to the liquid phase. Our results are directly applicable for clouds undergoing limited riming.

Subproject PROCLOUD Phase Partitioning of Chemical Species in Mixed Clouds during CIME

Due to the lack of in-situ measurements of multiphase processes in mixed clouds, the fate of the scavenged pollutant material in mixed phase clouds is poorly understood. In the few numerical models which treat the ice phase in clouds, and the associated chemical processes, it is assumed either that chemical species are completely incorporated into the ice phase upon freezing of droplets or that the incorporation process follows a Henry-like equilibrium. However, it is not known whether this is true or whether part of the material is expelled, redistributed or changed in any way during the transfer process to the ice phase. The scavenging of chemical species in mixed clouds was studied during the EU project CIME (Cloud Ice Mountain Experiment) at the Puy de Dome, central France (1465 m a.s.l.).