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 (SSpeak), 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 SSpeak. Small-scale 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 SSpeak. 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 SSpeak. The aerosol number concentration and hygroscopic properties were found to be less important than the aerosol size in determining SSpeak. Furthermore turbulence is found to have a maximum influence when SSpeak is between approximately 0.2 and 0.4 %. Simulating the small-scale fluctuations with several amplitudes, frequencies and phases, revealed that independently of the amplitude, the effect of the frequency on SSpeak shows a maximum at 0.46 Hz (median over all phases) and at higher frequencies, the maximum SSpeak decreases again.
Highlights
The interactions between aerosols and clouds are the largest contributors to uncertainty in the calculation of aerosol radiative forcing (Boucher et al, 2013)
Aerosols with a certain size, shape and chemical composition are able to form a cloud droplet, if they are exposed to air which is supersaturated with respect to water vapour
Compromises and assumptions that accurately address the most important aerosol effects within the constraints of application are required (Cherian et al, 2014; Dufresne et al, 2013; Levy et al, 2013). It has been pointed out by Boucher et al (2013) and Spichtinger and Cziczo (2008) that the main uncertainties in the aerosol radiative forcing are due to aerosol–cloud interaction dynamical factors such as turbulent strength and entrainment controlling the cloud condensation rate, and the key aerosol parameters such as aerosol number concentration and size distribution, and to a much lesser extent, the composition
Summary
The interactions between aerosols and clouds are the largest contributors to uncertainty in the calculation of aerosol radiative forcing (Boucher et al, 2013). Compromises and assumptions that accurately address the most important aerosol effects within the constraints of application are required (Cherian et al, 2014; Dufresne et al, 2013; Levy et al, 2013). It has been pointed out by Boucher et al (2013) and Spichtinger and Cziczo (2008) that the main uncertainties in the aerosol radiative forcing are due to aerosol–cloud interaction dynamical factors such as turbulent strength and entrainment controlling the cloud condensation rate, and the key aerosol parameters such as aerosol number concentration and size distribution, and to a much lesser extent, the composition. In this study the influence of the variation of the turbelent strength and the updraft velocity on the cloud activation is investigated using a cloud parcel model
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