SHIP-PRODUCED CLOUD LINES OF 13 JULY 1991

On 13 July 1991, a well-defined cloud line produced by an unidentified steaming ship was detected in satellite imagery and was simultaneously photographed from the R/V EGABRAG III. The EGABRAG produced a much less well-defined cloud line. Measuremnts made from the EGABRAG revealed that the cloud lines formed in a shallow boundary layer which was nearly saturated, unstable, drizzling and nearly free of cloud condensation nuclei (CCN). The EGABRAG passed through the plume of the ship as indicated by elevated CCN concentrations coincident with the cloud line. Thereafter, both ships passed under a shallow stratus layer where background CCN concentrations increased significantly. Only the cloud line produced by the ship extended into the stratus layer; the EGABRAG did not affect the layer. The CCN and updraft from the ship were involved in the formation of the cloud line. In contrast, the CCN and updraft from the EGABRAG were insufficient to produce a well-defined cloud line. Production of the cloud lines appeared dependent on a combination of environmental conditions and ship-produced CCN and updrafts.