Convectively coupled waves in a sheared environment.

Boualem Khouider
University of Victoria

Convection in the tropics is organized into a hierarchy (again hierarchy!) of scales ranging from the cloud scale of a few kilometers and a few hours to planetary scale disturbances that vary on the intraseasonal time scale such as the Madden-Julian oscillation. Synoptic scale disturbances that are intermediate between the planetary scale envelopes and the mesoscale cloud clusters of periods of a few (from 2 to 10) days are of central importance for weather and climate variability in the tropics. Detailed analysis of recent satellite data reveal that those synoptic scale cloud clusters are associated with the moist (or conveclively coupled) analogs of equatorial beta-plane shallow water waves, including the Kelvin, mixed Rossby-gravity, n=1 westward interio-gravity, and n=1 Rossby waves. In a series of papers K. and Majda introduced and analyzed an idealized multicloud model for convectively coupled waves that takes into account the physics of and interactions between three cloud types as well as moisture, that are all believed to play a central role in the dynamics of tropical organized convective systems. The multicloud model exhibits (linear) instabilities on spatial and temporal scales that correspond roughly to the spectral peaks, in the wavenumber and frequency domain, of observed out-going long-wave radiation (OLR) of the satellite observations. More importantly, the horizontal and vertical structure of the simulated waves resemble strongly those of the observed waves. However, in the real word, convectively coupled waves interact constantly with each other, with midlatitude waves (thus providing means for tropical--extra-tropical interactions), and with the background climatology. Here we are interested in the behaviour of such waves in the presence of a background zonal wind shear in the context of the multicloud model. We consider both the situations of a vertically varying zonal wind (abusively called vertical shear) and of a meridionally varying (barotropic) shear (abusively called meridional shear). Both cases induce interesting effects on the structure and dynamics of the model convectively coupled waves.

Presentation (PDF File)

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