Effects of Aerosols on Cloud Albedo: Evaluation of Twomey’s Parameterization of Cloud Susceptibility Using Measurements of Ship Tracks

Abstract

Airborne measurements from the Meteorological Research Flight's Hercules C-130 and the University of Washington's Convair C-131A during the Monterey Area Ship Track field project are used to evaluate Twomey's analytic expression for cloud susceptibility, which describes the sensitivity of cloud albedo to changes in droplet concentrations. This expression incorporates assumptions about cloud physics, such as the independence of the cloud liquid water content and the width of the droplet size distribution on droplet concentrations. Averaged over all 69 ship track penetrations, cloud liquid water content decreased slightly and the droplet size distributions broadened from the ambient values. For the 17 cases for which albedos were measured during overflights, Twomey's parameterization represents the trend of albedo changes with droplet concentrations remarkably well, passing through the midpoints of the considerable spread in the data. The fortuitous agreement results from compensating changes in cloud properties. Together with the albedo changes, the changes in cloud liquid water content and droplet size distributions imply that cloud thickness usually increased in the ship tracks. Such an increase was observed on the occasions that changes in cloud thickness were recorded (in the Sanko Peace ship track during very clean ambient conditions). Unfortunately systematic measurements of cloud thickness were not made for most of the ship tracks observed. The greatest outlier in the data corresponds to measurements made under horizontally inhomogeneous ambient conditions; possible explanations for its divergence include an increase in cloud thickness or an error in matching above-cloud albedo measurements with in-cloud microphysics measurements.

Simultaneous observations of aerosol–cloud–albedo interactions with three stacked unmanned aerial vehicles

“…1). Studies of shiptracks (27) have shown satisfactory agreement with ⌬␣ c and report a weak correlation between lwc and n D which removes the 2⌬lwc/lwc Ϫ1 and 3⌬h/h term in Eq. 3.…”

Climate forcing from the transport sectors

“…In addition to the RF mechanisms considered in this study, other indirect processes have been proposed. These include reduction of snow and ice albedo by BC deposition (38)(39)(40), the so-called semidirect effect of BC on clouds (41), and indirect effects on cirrus clouds by particles from aviation (8). These RF mechanisms are generally given a low or very low level of scientific understanding by the IPCC (34).…”

Precipitating Condensation Clouds in Substellar Atmospheres

“…For example, for a case study over the North Sea, where f rain D 0.2 N \ 100 cm~3 (Nicholls 1984), while in California stratocumulus it increased from 0.3 to 0.5 as N decreased from 40 cm~3 in clouds contaminated by ship exhaust to 10 cm~3 in clean ambient air (from Fig. 3b of Ackerman et al 2000b). We note that in clean marine stratocumulus, reduced rain is observed (e.g., Taylor & Ackerman 1999) and predicted (e.g., Ackerman, Toon, & Hobbs 1993 ;Stevens et al 1998) to result in deeper cloud layers, which is consistent with our simple model.…”