Observations of wind streaklines over the red sea from the ERTS-1 imagery

Abstract

Observations are reported of differences in radiance levels over the Red Sea in the scanner imagery from the Earth Resources Technology Satellite, now called Landsat-I. Two types of effects are observed: narrow streaks of lower radiance extending for up to 100 km leeward from islands or mountains near the shore (referred to as lee-lines); and striations of alternate darker and lighter bands, each up to I km broad. Two interpretations are discussed, of local sea-state differences and of local differences in the dust levels in the atmosphere. In the sea-state differences interpretation, the darker regions correspond to a lower sea-state linked to lower wind velocity. The lee-lines would then be interpreted as an imprint of surface wind differences caused by the sheltering effects of islands or mountains, and the striations as a modulation of the surface winds by atmospheric wave motions. However, objections can be raised to the sea-state differences interpretation, and it is suggested that the dust level differences in the atmosphere offer in our case the correct interpretation. The lee-lines are attributed to deposition of dust in the flow around high islands and mountains, a phenomenon that possibly was observed on Mars, leaving a leeward streakline with a reduced dust level. The striations are altributed also to local dust level differences, linked to the Kelvin-Helmholtz instabilities at the edges of the dust clouds.