Numerical modeling of ship tracks produced by injections of cloud condensation nuclei into marine stratiform clouds

Abstract

Ship tracks are long-lived, linear regions of enhanced reflectivity in low-lying marine clouds that appear in satellite imagery downwind of ships. Ship tracks were first observed as cloud lines in visible satellite imagery (type 1). A second (and more common) type of ship track (type 2), which is masked at visible wavelengths by natural variability in cloud reflectivity, is seen at near-infrared wavelengths in satellite imagery. A one-dimensional numerical model is used to simulate measurements of both types of ship tracks and to investigate interactions between aerosol and cloud microphysics, radiative transfer, and turbulent mixing in the cloud-topped marine boundary layer that lead to the formation and provide for the persistence of ship tracks. We find that cloud condensation nuclei (CCN) injections can account for many of the observed properties of ship tracks. Higher CCN concentrations produce increased droplet concentrations, which enhance cloud reflectivity by reducing droplet radius and increasing droplet cross-sectional area. The smaller droplets also reduce the drizzle rate, which can allow cloud water to increase under some conditions, thereby leading to higher cloud reflectivity. However, smaller droplets also evaporate more readily below cloud base. Increased evaporation reduces mixing between the cloud and the subcloud layers during daytime, which causes a decrease in cloud water. The distinction between the two types of ship tracks is suggested to be due to differences in ambient concentrations of CCN that cause variations in turbulent mixing in the boundary layer, through the effect of cloud droplet concentrations on cloud-top longwave radiative cooling. The model predicts lifetimes of > 1 day and > 2 days for the simulated type 1 and type 2 ship tracks, respectively. In the atmosphere, processes not treated in the model, such as horizontal dispersion and changes in large-scale atmospheric conditions, may limit ship track lifetimes. 1. Introduction Ship tracks are long-lived, linear regions of enhanced solar reflectivity in marine stratiform clouds that appear in satellite imagery downwind of ships. They were first reported as "anomalous cloud lines" in visible satellite imagery [Conover, 1966]. The cloud cover in the regions in which the features were observed ranged from apparently cloud-free to overcast, where the lines became indiscernible. Conover concluded that the cloud lines were low-lying water clouds, resulting from the addition of CCN present in ship exhausts under special meteorological conditions; Bowley [1967] observed that these cloud lines were typically associated with fog in a shallow boundary layer. Conover suggested that the ambient air in which ship tracks form must have very low concentrations of CCN; Twomey et al. [1968] reported that very low CCN concentrations (< 5 cm '3) are sometimes measured in areas where ship tracks occur. Using multiple Paper number 95JD00026. 0148-0227/95/95JD-00026505.00 wavelength detectors on modern satellites, Coakley et al. [1987] expanded the...

Modeling of clouds and radiation for developing parameterizations for general circulation models. Annual report, 1995

“…ai., 1994). We have found that aerosol concentrations in the cloud-topped marine boundary layer depend strongly on their formation rate, but the relationship is not bistable as suggested by other researchers (Ackerman et al, 1995a). We h,ave also found that clouds can deplete aerosol concentrations in the marine boundary layer until the clouds themselves dissipate and the boundary layer collapses.…”

Sensitivity of Twentieth-Century Sahel Rainfall to Sulfate Aerosol and CO2 Forcing

“…Therefore, the lower the initial concentrations of aerosol present, the more susceptible a cloud is to being influenced by an increase in aerosol concentration and the stronger its influence on climate. The effects of cloud susceptibility can be seen in ''ship track'' observations by Ackerman et al (1995).…”

Ship impacts on the marine atmosphere: insights into the contribution of shipping emissions to the properties of marine aerosol and clouds

“…Because many shipping lanes are located along coastal regions where marine stratocumuli persist, ship emissions have the potential to directly affect cloud microphysics. Studies of cargo ship exhaust/cloud interactions have shown distinct cloud responses to these perturbations (e.g., Coakley et al, 1987;Ackerman et al, 1995;Frick and Hoppel, 2000;Durkee et al, 2000;Hudson et al, 2000;Noone et al, 2000;Lu et al, 2009;Christensen and Stephens, 2011). Satellite imagery shows that ship tracks can exist hours after initial emission (Coakley et al, 1987;Durkee et al, 2000;Christensen and Stephens, 2011) and therefore have the potential to exert a significant effect on local radiative fluxes.…”