Squall Lines and Convectively Coupled Gravity Waves in the Tropics: Why Do Most Cloud Systems Propagate Westward?

The coupling between tropical convection and zonally propagating gravity waves is assessed through Fourier analysis of high-resolution (3-hourly, 0.5°) satellite rainfall data. Results show the familiar enhancement in power along the dispersion curves of equatorially trapped inertia–gravity waves with implied equivalent depths in the range 15–40 m (i.e., pure gravity wave speeds in the range 12–20 m s−1). Here, such wave signals are seen to extend all the way down to zonal wavelengths of around 500 km and periods of around 8 h, suggesting that convection–wave coupling may be important even in the context of mesoscale squall lines. This idea is supported by an objective wave-tracking algorithm, which shows that many previously studied squall lines, in addition to “2-day waves,” can be classified as convectively coupled inertia–gravity waves with the dispersion properties of shallow-water gravity waves. Most of these disturbances propagate westward at speeds faster than the background flow. To understand why, the Weather Research and Forecast (WRF) Model is used to perform some near-cloud-resolving simulations of convection on an equatorial beta plane. Results indicate that low-level easterly shear of the background zonal flow, as opposed to steering by any mean flow, is essential for explaining the observed westward-propagation bias.