Comparing the stratosphere and boundary layer as sources of trapped lee wave decay

The process through which trapped lee waves decay is of utmost importance when considering the impact that such waves have on their environment, and thus on the larger-scale climate system. Previous studies have begun to describe and quantify the impact of the boundary layer on lee wave decay. However, investigations into the role of the stratosphere have been limited to several field campaigns - with no broader analysis.

We present linear solutions and full numerical model output of trapped waves in several multi-layer atmospheres, with surface friction included in the numerical model. For all atmospheres investigated, inviscid downstream decay rates resulting from leakage into the stratosphere agree well between the linear solution and full model.

Given a simple atmospheric structure as used in previous boundary layer studies - characterized by a deep lower stable layer, weakly stable free troposphere, and no wind shear - minimal decay is generated by the stratosphere, and decay due to surface friction is both effective and dominant.

If we instead simulate a more realistic atmosphere containing a near-neutral surface layer capped by a shallow elevated inversion - shown by the linear model to be conducive for rapid downstream decay through stratospheric leakage - the stratosphere becomes the dominant decay mechanism. A larger rate of decay is seen due to the stratosphere in the inviscid case than created by a rough surface with a non-leaky solution. Decay produced by the boundary layer is similar between simple and more realistic atmospheric profiles.

Simulations including both surface friction and a leaky stratosphere show that interactions between the decay sources can be important. In cases where the stratosphere is dominant, the addition of surface friction acts to weaken the total decay - with a roughness length equivalent to an urban area required to reach a rate of decay close to that of the stratosphere alone.