87th AMS Annual Meeting

Wednesday, 17 January 2007: 1:30 PM
Self Similarity and Scale Interactions in Organized Tropical Convection (Invited)
206B (Henry B. Gonzalez Convention Center)
George N. Kiladis, NOAA Earth System Research Laboratory, Boulder, CO
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 Kelvin, westward-inertio gravity, and mixed Rossby gravity waves, and these in turn are comprised of a broad spectrum of mesoscale features not necessarily organized into "waves". This 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. While processes such as upscale convective momentum transport have been reasonably well elucidated, the importance of other aspects of the upscale interaction provided by the higher frequency are not obvious, such as those due to heterogeneous fields of diabatic heating on the large scale. Since two different MJOs that behave in similar ways can be composed of an entirely different suite of equatorial waves and mesoscale features, evidently the upscale interactions can be enabled by a wide variety of disturbances. 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 scale interactions is the fact that there is a certain degree of "self-similarity" 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 and thus CAPE and moist static energy is high ahead of the waves, and drying occurs first at low levels while it is still moist aloft behind the wave. Low level diabatic heating precedes deep convective heating, followed by a signal of upper tropospheric heating over cooling. 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. It is a remarkable fact that the temporal and spatial evolution of mesoscale convective complexes, which can be traced back to microphysical arguments, also exists at a certain level on the scale of the MJO.

These observations have implications for the simulation of convectively coupled waves. Some General Circulation Models appear to have peaks in their rainfall spectra corresponding to the observed spectra of tropical cloudiness. However, all of the waves identified in models thus far have corresponding equivalent depths that are universally too deep and therefore phase speeds that are too fast. These waves all scale to around the same equivalent depth, a fact which perhaps provides clues to the deficiencies of physical parameterizations involved. Simple modeling and cloud resolving studies are beginning to provide some realistic results, and will no doubt provide useful testbeds for the development of improved parameterizations in next generation GCMs.

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