Trapping of Low-Level Internal Gravity Waves

Abstract

The characteristics of internal gravity waves propagating on a layer of high stratification near the ground with a deeper, weakly stratified layer above are examined with the aid of a nonhydrostatic numerical model. Simulations are performed of a density current propagating into an environment with a typically observed thermodynamic structure and with no shear. These simulations indicate that the amplitude of the disturbance that forms ahead of the density current is limited considerably by the upward propagation of energy in the upper layer. To explain the large amplitude of observed gravity waves there must exist some additional mechanism, besides the weak stratification in the upper layer, to trap energy at low levels. A thorough examination of several observed gravity wave events suggested three commonly occurring mechanisms. The first mechanism, explored in a previous paper, occurs when winds in the upper layer oppose the wave motion. This reduces the Scorer parameter l2 = N2/(U ? c)2 ? U?/(U ? c) in the upper layer and causes waves to evanesce in that region. The second mechanism, which also depends on a reduction in the Scorer parameter, occurs when a jet exists in the lower layer that opposes the wave motion. It is shown that the curvature in the velocity profile above this jet can produce a layer of negative Scorer parameter. Numerical simulations indicate that a considerable amount of energy can be trapped below this region of curvature. The third mechanism involves an inversion at a certain height above the lower stable layer. In this system the Scorer parameter is actually increased, however for certain inversion hieghts energy can be reflected off the inversion and lead to an enhancement of the wave amplitude at the ground. Observations of low-level internal gravity waves are then examined in an attempt to determine the relative importance of the three trapping mechanisms in the real atmosphere. This examination suggests that the low-level opposing flow is the most prevalent mechanism for trapping energy at low levels.