Upstream-Propagating Wave Modes in Moist and Dry Flow over Topography

Abstract

Previous studies have observed upstream-propagating modes in two-dimensional numerical simulations of idealized flow over topography with moist, nearly neutral conditions in the troposphere, topped by a stable stratosphere. The generation and propagation mechanisms for these modes were attributed to localized and dramatic changes in stability induced by the desaturation of the flow impinging on the mountain. In the present paper it is shown that these modes are transient upstream-propagating gravity waves, which are a fundamental feature of both moist and dry flow over topography of a two-layer troposphere-stratosphere atmospheric profile impulsively started from rest. The mode selection and propagation speeds of these transient waves are highly dependent on the tropospheric stability, as well as the wind speed and tropopause depth. In the moist case these modes appear to propagate according to an effective static stability that is intermediate to the normal dry stability and the lower moist stability. Comparisons with the linear, timedependent, hydrostatic analytic solution show that these modes are similar to the transients observed in flow of a constant wind and stability layer over topography with a rigid upper boundary.

Editorial

“…This feature is likely due to the tuning of the basic mountain wave between the ground and the tropopause (e.g. Baines, 1995; Keller et al , 2012).…”

Generation of mountain wave‐induced mean flows and turbulence near the tropopause

“…This feature is likely due to the tuning of the basic mountain wave between the ground and the tropopause (e.g. Baines, 1995;Keller et al, 2012).…”

Testing of a Cell-Integrated Semi-Lagrangian Semi-Implicit Nonhydrostatic Atmospheric Solver (CSLAM-NH) with Idealized Orography

“…Miglietta and Rotunno (2005) noticed in their simulations that for intermediate mountain heights (500 # h m # 1500 m) the unsaturated region downstream of the hill unexpectedly extends upstream as well. A later study by Keller et al (2012) showed that this upstream extent of the subsaturated air is due to local adiabatic descent and warming caused by a transient upstream-propagating gravity wave, a fundamental feature of a two-layer two-dimensional atmosphere with topography introduced impulsively. The purpose of performing the test case as prescribed in Miglietta and Rotunno (2005) with a mountain 700 m high is to ensure that CSLAM-NH can generate comparable results to models used in the literature, such as that in Miglietta and Rotunno (2005), who used the Weather Research and Forecasting (WRF) Model (version 1.3).…”