Wave‐cloud lines over the Arabian Sea

Abstract

Meteosat visible satellite images between 2006 and 2011 show wave-cloud lines over the Arabian Sea in all months outside the summer monsoon (June-September). These lines are most frequent between January and May (2-3 per month in a given year). All wave-cloud lines in the region propagate offshore. As these wave-cloud lines are associated with coherent convergence lines, the objective technique described by Berry and Reeder is applied to the ERA-Interim reanalysis and a climatology of convergence lines at 850 hPa developed. Despite the coarse resolution of the ERA-Interim reanalysis, the statistical properties of these lines are broadly constant with those deduced from the Meteosat visible satellite images. The generation mechanism is investigated in a simulation with the Met Office Unified Model of a particular wave-cloud line (12 March 2011). The process appears to be similar to that over northwestern Australia, which has been documented previously. During the day, a synoptic-scale northeasterly flow opposes the inland advection of the sea breeze on the west coast of India. However, as the daytime turbulence decays and the boundary layer stabilizes, the northeasterly flow accelerates, pushing offshore the leading edge of the sea breeze during the late evening and early hours of the morning. A wave is generated as the northeasterlies penetrate the marine boundary layer, and this wave propagates westward, producing cloud at its leading edge where there is strong ascent.

On the propagation of nonlinear waves in the atmosphere

“…Morning glory clouds occur most often in coastal regions but, occasionally, also over the land or the sea (see figures 1, 2 and the photographs in [11][12][13][14][15][16][17][18]). These spectacular formations have been reported as appearing over the English Channel, in Central USA, Germany, Eastern Russia, Canada, Mexico, Brazil, Uruguay and, most particularly, Australia.…”

“…To initiate this further development, we note that waves of the morning-glory type, for example, as seen in a number of locations around the world, can extend up to 3 km or so above the Earth’s surface, but rarely is the wave train as long as 30 km; indeed, individual rolls are typically only a few hundred metres in diameter. The length of the wavefront is usually well over 100 km and, on occasions, it is observed to be as much as 1000 km, with the whole wave group sometimes propagating over hundreds of kilometres [ 10 , 12 ]; on these length scales, it is not advisable to ignore the effects of spherical geometry. We proceed by using the thin-shell parameter, , to provide a scale that differentiates the structure in the propagation direction from that along the wavefront.…”

On the propagation of waves in the atmosphere

“…The above analysis demonstrates that we can reasonably expect to match what is observed. Of course, the identification of flow structures and processes from the various available satellite data is rather subjective [22], but such exercises nevertheless provide major sources of insight. In the case of the wave pattern visible in figure 2, it is observed, during May, that the surface temperature of the north Indian Ocean becomes the highest among the world’s ocean surface temperatures, often in excess of 28∘C over the Arabian Sea, where evaporation largely exceeds precipitation; see [23].…”

Using Megha‐Tropiques satellite data to constrain humidity in regional convective simulations: A northern Australian test case

“…MG is an undular bore often resulting from the collision of sea breeze and fronts (Goler and Reeder, 2004; Smith and Reeder, 2014). Recently, MG's have also been observed in northwest Australia (Birch and Reeder, 2013) and the Arabian Sea (Birch et al ., 2014b).…”