Abstract
Abstract. The response of cloud processes to an aerosol perturbation is one of the largest uncertainties in the anthropogenic forcing of the climate. It occurs at a variety of timescales, from the near-instantaneous Twomey effect to the longer timescales required for cloud adjustments. Understanding the temporal evolution of cloud properties following an aerosol perturbation is necessary to interpret the results of so-called “natural experiments” from a known aerosol source such as a ship or industrial site. This work uses reanalysis wind fields and ship emission information matched to observations of ship tracks to measure the timescales of cloud responses to aerosol in instantaneous (or“snapshot”) images taken by polar-orbiting satellites. As in previous studies, the local meteorological environment is shown to have a strong impact on the occurrence and properties of ship tracks, but there is a strong time dependence in their properties. The largest droplet number concentration (Nd) responses are found within 3 h of emission, while cloud adjustments continue to evolve over periods of 10 h or more. Cloud fraction is increased within the early life of ship tracks, with the formation of ship tracks in otherwise clear skies indicating that around 5 %–10 % of clear-sky cases in this region may be aerosol-limited. The liquid water path (LWP) enhancement and the Nd–LWP sensitivity are also time dependent and strong functions of the background cloud and meteorological state. The near-instant response of the LWP within ship tracks may be evidence of a bias in estimates of the LWP response to aerosol derived from natural experiments. These results highlight the importance of temporal development and the background cloud field for quantifying the aerosol impact on clouds, even in situations where the aerosol perturbation is clear.
Highlights
The response of a cloud to an aerosol perturbation is fundamentally time sensitive
The results from this work are split into two sections; the first deals with the macrophysical properties of the ship track, whereas the second focuses on the microphysical properties and the liquid water path
The median ship track in this study is last observed at a distance of about 200 km from the source ship, with longer ship tracks more commonly observed behind ships with higher SOx emissions (Fig. 3a)
Summary
The response of a cloud to an aerosol perturbation is fundamentally time sensitive. Increasing the number of cloud condensation nuclei (CCN) increases the number of cloud droplets at cloud base (Twomey, 1974) almost immediately, resulting in a near-instantaneous change to the properties of an individual air parcel. The Nd and effective radius (re) in the rest of the cloud respond on a timescale related to the cloud geometrical depth and the in-cloud updraught (on the order of 10–20 min for a cloud thickness of 200 m and an updraught of 0.2 m s−1) This response is referred to as the Twomey effect, which leads to the radiative forcing from aerosol–cloud interactions (RFaci; Boucher et al, 2013). Further changes to the LWP and CF may come through aerosol-dependent entrainment and mixing processes (Ackerman et al, 2004; Xue and Feingold, 2006; Bretherton et al, 2007; Seifert et al, 2015). For a large-scale change in cloud amount, the timescales may be even longer, as it requires the switching of cloud regime from open to closed
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