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Martin Charron
and
Gilbert Brunet

Abstract

The theory of empirical normal modes (ENMs) is adapted to diagnose gravity waves generated by a relatively high-resolution numerical model solving the primitive equations. The ENM approach is based on the principal component analysis (which consists of finding the most efficient basis explaining the variance of a time series), except that it takes advantage of wave-activity conservation laws. In the present work, the small-amplitude version of the pseudoenergy is used to extract from data quasi-monochromatic three-dimensional empirical modes that describe atmospheric wave activity. The spatial distributions of these quasi-monochromatic modes are identical to the normal modes of the linearized primitive equations when the underlying dynamics can be described with a stochastic linear and forced model, thus establishing a bridge between statistics and dynamics. This diagnostic method is used to study inertia–gravity wave generation, propagation, transience, and breaking over the Rockies, the North Pacific, and Central America in the troposphere–stratosphere–mesosphere Geophysical Fluid Dynamics Laboratory SKYHI general circulation model at a resolution of 1° of latitude by 1.2° of longitude. Besides the action of mountains in exciting orographic waves, inertia–gravity wave activity has been found to be generated at the jet stream level as a possible consequence of a sustained nonlinear and ageostrophic flow. In the tropical region of the model (Central America), the inertia–gravity wave source mechanism produced mainly waves with a westward vertical tilt. A significant proportion of these inertia–gravity waves was able to reach the model mesosphere without much dissipation and absorption.

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Martin Charron
and
Elisa Manzini

Abstract

Current parameterizations of the gravity wave processes that are relevant to middle atmosphere general circulation modeling need to have specified somewhere in the lower atmosphere a number of characteristics of the gravity wave spectrum that arise from different possible gravity wave sources (i.e., the so-called gravity wave source spectrum). The aim of this study is to take into account in the specification of the gravity wave source spectrum a space and time modulation of the gravity wave wind variance and propagation direction associated with the occurrence of frontal systems. Given that fronts are poorly resolved at the truncations commonly used in middle atmosphere models (typically T21–T42), first a method is devised to diagnose conditions that are considered to be the precursor of frontogenesis in a space and time-dependent low-resolution flow. This is achieved by evaluating horizontal isotherm compression due to flow deformation and convergence. Second, when particular conditions are satisfied, the precursor to frontogenesis is used as an indicator of subgrid-scale gravity wave emission in the model. Third, the wind variance and the propagation direction of the gravity waves at the source level are specified according to empirical evidences of frontal generation of gravity waves. The MAECHAM4 middle atmosphere response to this gravity wave forcing is presented. The study is restricted to fronts since they are thought to be one of the major nonstationary gravity wave sources in the extratropics, other gravity wave source mechanisms being left for later examination.

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