Sensitivity estimations for cloud droplet formation in the vicinity of the high-alpine research station Jungfraujoch (3580 m a.s.l.)

Abstract

Abstract. Aerosol radiative forcing estimates suffer from large uncertainties as a result of insufficient understanding of aerosol-cloud interactions. The main source of these uncertainties is dynamical processes such as turbulence and entrainment but also key aerosol parameters such as aerosol number concentration and size distribution, and to a much lesser extent, the composition. From June to August 2011 a Cloud and Aerosol Characterization Experiment (CLACE2011) was performed at the high-alpine research station Jungfraujoch (Switzerland, 3580 m a.s.l.) focusing on the activation of aerosol to form liquid-phase clouds (in the cloud base temperature range of −8 to 5 • C). With a box model the sensitivity of the effective peak supersaturation (SS peak ), an important parameter for cloud activation, to key aerosol and dynamical parameters was investigated. The updraft velocity, which defines the cooling rate of an air parcel, was found to have the greatest influence on SS peak . Smallscale variations in the cooling rate with large amplitudes can significantly alter CCN activation. Thus, an accurate knowledge of the air parcel history is required to estimate SS peak . The results show that the cloud base updraft velocities estimated from the horizontal wind measurements made at the Jungfraujoch can be divided by a factor of approximately 4 to get the updraft velocity required for the model to reproduce the observed SS peak . The aerosol number concentration and hygroscopic properties were found to be less important than the aerosol size in determining SS peak . Furthermore turbulence is found to have a maximum influence when SS peak is between approximately 0.2 and 0.4 %. Simulating the smallscale fluctuations with several amplitudes, frequencies and phases, revealed that independently of the amplitude, the effect of the frequency on SS peak shows a maximum at 0.46 Hz (median over all phases) and at higher frequencies, the maximum SS peak decreases again.

Hygroscopicity of aerosol particles and CCN activity of nearly hydrophobic particles in the urban atmosphere over Japan during summer

“…The SS conditions of 0.5%–1.0% are suited for the observation of the CCN activation of low hygroscopic particles [ Kawana et al , ]. Although these SS conditions are high, they occur in the atmosphere because a peak SS in convective clouds can exceed 1.0% [ Hammer et al , ; Farmer et al , ]. These measurements were performed by maintaining the ratios of the setting diameters of DMA2 to those of DMA1 to 1.0 and 1.1, with varying setting diameters of DMA1.…”

Chemical and physical influences on aerosol activation in liquid clouds: a study based on observations from the Jungfraujoch, Switzerland

“…Peak supersaturation values, updraught velocity, aerosol hygroscopicity, and cloud droplet number concentration were studied by Hammer et al (2014), who found that all these quantities showed statistically significant differences between the two wind sectors. This work was extended by Hammer et al (2015), who quantified the influence of updraught velocity and particle composition and concentration on peak supersaturation.…”

“…The parametrization was also run for the overall median hygroscopicity value given by Jurányi et al (2011) of 0.2, as well as a value of 0.3, to test the sensitivity of the results to small changes in assumed hygroscopicity within the bounds of that which has been measured at the JFJ. Vertical velocity for the parametrization input was calculated using the method of Hammer et al (2014), multiplied by an estimated correction factor of 0.25, following the suggestions of Hammer et al (2015). Pressure and temperature at cloud base are also used, calculated in the same way as for the statistical model.…”

Atmospheric Science Research at Whiteface Mountain, NY: Site Description and History

“…As noted in the IPCC AR5 report, mountain regions are highly temperature sensitive, and increasingly vulnerable to high impact extreme events under a warming climate (Cramer et al, 2014). From an experimental scientific perspective, mountain sites are quite often the ideal locations for many types of cloud related research, in particular the connections leading from aerosols to clouds and their effects on radiation and climate (Friedman et al, 2013;Hammer et al, 2015).…”