Preface to the special issue on “Theoretical Developments in Tropical Meteorology”

Abstract

During the 13th Atmosphere–Ocean Fluid Dynamics Conference of the American Meteorological Society in Breckenridge in 2001, I was invited to participate as an external consultant in the NSF-Focused Research Group (FRG) on “The weak temperature gradient equations for tropical atmosphere dynamics”, which several of the authors in this special issue were planning to propose at the time. The FRG, which was designed to forge cooperations amongst theoretical meteorologists and applied mathematicians, was granted and during the subsequent 4 years I witnessed a number of intriguing new theoretical developments in tropical meteorology pursued within the FRG and its associated research groups. A range of different facets of the ubiquitous moisture-related scale interactions within the tropical atmosphere were addressed successfully, and it seemed only natural to propose a special issue on “Theoretical Developments in Tropical Meteorology” of Theoretical and Computational Fluid Dynamics to the editor in chief, Prof. Yussuf Hussaini, who judged it to be an important and timely endeavour. Tropical moist processes originate mostly in the atmospheric boundary layer near the ocean surface. Convective, well-mixed, and cloud-topped boundary layers play a particularly important role in driving and conditioning bulk tropospheric motions. The papers by Stevens, Neggers et al., Sobel and Neelin, and Khouider and Majda specifically consider the parameterization of moist boundary layer processes and their influence on the large scale, bulk tropospheric dynamics. A fruitful approach to constructing approximate reduced models for bulk tropospheric flows is vertical mode decomposition. The idea being that only very few dominant vertical modes need to be accounted for in order to capture the essence of the large scale motions. In the tropics, nonlinear moist processes associated with a number of essentially different cloud regimes participate strongly in setting up these large scale flows, making such vertical mode decompositions a highly nontrivial task. The papers by Sobel and Neelin, Khouider and Majda, Stechman and Majda, Biello and Majda, Bretherton et al., Burns et al., and Zhou and Sobel address various facets of this overall problem by considering vertical mode interactions, analyzing the importance of various approximations introduced often in addition to the vertical mode-related order reduction, and by analyzing in detail the structure of solutions supported by such low-order models. Deep moist convection, which covers vertical scales comparable with the height of the troposphere, is of central importance for a wide range of phenomena in the near-equatorial atmosphere. It arises when the atmosphere is unstable under the added buoyancy from latent heat release and leads to deep vertical motions occurring on horizontal scales of about 1 km. As a consequence, the net effect of deep convection on meso(∼200 km) to planetary scales is of an inherently multiscale nature. The papers by Pauluis et al., Peters and Bretherton, and Klein and Majda address these effects through techniques of direct computational modelling and of systematic multiscale asymptotics.