Until recently, theoretical studies of stratified flow past topography have largely focused on understanding topographic circulations at one particular scale of motion in isolation from all other scales. Large-scale effects have thus been studied independently of smaller-scale phenomena, while intermediate-scale effects and scale interactions have in large part remained neglected. Recent numerical modeling studies and observational programs increasingly suggest that this traditional separation between large and small scales in topographic flows is no longer adequate. The research aims to start bridging this gap in scales by considering interactions between scales in two simplified, but representative contexts: i) orographic wake formation and the interaction of wakes with larger scales; and ii) the nonlinear generation of gravity waves in quasi-balanced topographic flows at small Rossby number (Ro).

Orographic wake formation has long been known to have important consequences for local and regional-scale circulations in mountainous regions. Recent work suggests that orographic wakes may also have an important upscale effect by enhancing synoptic-scale cyclogenesis in the lee of topography. The Principal Investigator will study aspects of orographic wakes spanning the full range of scales important to wake formation and dynamics -from the small-scale processes that generate wakes to the influence of wakes on larger-scale flows. The key questions to be addressed include: What are the dynamical and physical processes that produce orographic wakes? b) How are the formation and subsequent dynamics of wakes modified by background rotation in intermediate-Ro flow? c) What effects do orographic wakes have on cyclone growth in developing baroclinic wave? The Principal Investigator will approach these issues using idealized numerical simulations combined with sophisticated diagnostic tools designed to address the dynamics of both vorticity and potential vorticity (PV) in stratified flows.

The generation of mesoscale gravity waves in small-Ro flows is topic of considerable interest in the general dynamic meteorology community. Flow past a ridge at small Ro provides an intriguing context in which to examine such phenomena due to the inherent simplicity of the quasi-balanced topographic flow that produces the mesoscale waves. The Principal Investigator will study wave generation in small-Ro topographic flows using newly developed semi-analytic framework that allows fully finite-amplitude two-dimensional solutions for flows over broad ridges. An attempt will then be made to extend these results to three dimensions using weakly nonlinear analysis combined with idealized numerical simulations. It is hoped that this analysis of wave generation in the topographic case will provide insights into the mechanisms of wave generation in other small-Ro, non-topographic flows as well.

If successful, the work on orographic wakes should provide insights into multiscale effects and scale interactions in stratified flow past topography. The consideration of such multiscale phenomena is important to further understanding of orographic effects on both weather and climate. The study of wave generation at small Ro is expected to provide insights applicable to wave generation in more general small-Ro flows as well. As such waves have significant impact on weather and clear-air turbulence, better understanding of their generation mechanisms could have important implications for forecasting.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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Stephan P. Nelson
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Texas A&M Research Foundation
College Station
United States
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