The role of background cloud microphysics in the radiative formation of ship tracks

The authors investigate the extent to which the contrast brightness of ship tracks, that is, the relative change in observed solar reflectance, in vis ible and near-infrared imagery can be explained by the microphysics of the background cloud in which they form. The sensitivity of visible and near-in frared wavelengths for detecting reflectance changes in ship tracks is disc ussed, including the use of a modified cloud susceptibility parameter, term ed the "contrast susceptibility," for assessing the sensitivity of backgrou nd cloud microphysics on potential track development. It is shown that the relative change in cloud reflectance for ship tracks is expected to be larg er in the near-infrared than in the visible and that 3.7-mu m channels, wid ely known to be useful for detecting tracks, have the greatest sensitivity. The usefulness of contrast susceptibility as a predictor of ship track con trast is tested with airborne and satellite remote sensing retrievals of ba ckground cloud parameters and track contrast. Retrievals are made with the high spatial resolution Moderate Resolution Imaging Spectroradiometer Airbo rne Simulator flown on the National Aeronautics and Space Administration's high-altitude ER-2 aircraft, and with the larger-scale perspective of the a dvanced very high resolution radiometer. Observed modifications in cloud dr oplet effective radius, optical thickness, liquid water path, contrast susc eptibility, and reflectance contrast are presented for several ship tracks formed in background clouds with both small and large droplet sizes. The re mote sensing results are augmented with in situ measurements of cloud micro physics that provide data at the smaller spatial scales.