Organization of Mesoscale Convection within Equatorial Waves

It is well-known that convective disturbances in the tropics occur over a very broad spectrum of scales, ranging from individual cells to planetary scale features such as the Madden-Julian Oscillation (MJO). It is also observed that the larger scale features are composed of smaller scale equatorial waves, so that for example the "envelope" of the MJO is often comprised of eastward propagating Kelvin, and westward mixed Rossby gravity waves. These in turn are comprised of a broad spectrum of mesoscale features, which are predominantly westward propagating. A majority of these move too quickly (greater than 20 m/s) to be explained solely by advection. We present evidence that some of these features are likely inertio-gravity waves. Overall, the observed spectrum of cloudiness suggests a dominance of both upscale and downscale interactions in the organization of tropical convection. It seems evident that the MJO modulates the occurrence of smaller scale, higher frequency disturbances, but the mechanisms responsible for this modulation are not yet fully understood. Understanding the precise role of these scale interactions appears to be a crucial step towards the improved simulation of equatorial disturbances in models. A potential aid to the understanding of these interactions is the fact that there is a certain degree of "scale-invariance" in observed gross features of the dynamical structures of organized tropical convection, from the mesoscale on up to the planetary scale structure of the MJO. Convectively coupled disturbances universally exhibit strong vertical tilts in their wind, temperature, moisture, vertical velocity and diabatic heating fields. In general these disturbances display a warm lower troposphere ahead of the wave, with cooling behind, and a warm mid-troposphere within the convective region. Low level moisture is high ahead of the waves, and drying occurs first at low levels while it is still moist aloft behind the wave. These dynamical signals are consistent with the observation that the waves show a progression from a dominance of shallow to deep convection, and then stratiform precipitation, regardless of scale or propagation direction.