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Tropical squall lines and convectively coupled gravity waves: Why do most cloud systems move westward?

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October 28, 2010
Stefan Tulich (NOAA)
Hosted by Dick Johnson

Abstract

The existence of tropical squall line systems has been recognized for quite some time, yet their governing dynamics still remains unclear. Here it is proposed that many tropical squall lines are part of a broad family of convectively coupled gravity waves with periods in the range 6-48 hrs. Spectral filtering of high-resolution satellite rainfall data provides evidence to support this idea. In particular, it is shown that many tropical squall lines have the dispersion properties of Matsuno's equatorially-trapped inertia-gravity waves with equivalent depths in the range 12-50 m (pure gravity wave speeds in the
range 11-22 m/s). This statement is especially true over Africa, where most disturbances propagate
westward at speeds of 18 m/s (and exhibit a weak 2-day periodicity, in spite of strong diurnal forcing). In order to understand why most waves move westward, explicit simulations of convection on an equatorial beta plane are described.