Cloud Trails past Bermuda: A Five-Year Climatology from 2012 to 2016

Abstract

Abstract Cloud trails are primarily thermally forced bands of cloud that extend downwind of small islands. A novel algorithm to classify conventional geostationary visible-channel satellite images as cloud trail (CT), nontrail (NT), or obscured (OB) is defined. The algorithm is then applied to the warm season months of five years at Bermuda comprising 16 400 images. Bermuda’s low elevation and location make this island ideal for isolating the role of the island thermal contrast on CT formation. CTs are found to occur at Bermuda with an annual cycle, peaking in July, and a diurnal cycle that peaks in midafternoon. Composites of radiosonde observations and ERA-Interim data suggest that a warm and humid low-level environment is conducive for CT development. From a Lagrangian perspective, wind direction modulates CT formation by maximizing low-level heating on local scales when winds are parallel to the long axis of the island. On larger scales, low-level wind direction also controls low-level humidity through advection.