A numerical study of mountain waves in the upper troposphere and lower stratosphere

Abstract

Abstract.A numerical study of mountain waves in the Upper Troposphere and Lower Stratosphere (UTLS) is presented for two Intensive Observational Periods (IOPs) of the Terrain-induced Rotor Experiment (T-REX). The simulations use the Weather Research and Forecasting (WRF) model and a microscale model that is driven by the finest WRF nest. During IOP8, the simulation results reveal presence of perturbations with short wavelengths in zones of strong vertical wind shear in the UTLS that cause a reversal of momentum fluxes. The spectral properties of these perturbations and the attendant vertical profiles of heat and momentum fluxes show strong divergence near the tropopause indicating that they are generated by shear instability along shear lines locally induced by the primary mountain wave originating from the lower troposphere. This is further confirmed by results of an idealized simulation initialized with the temperature and wind profiles obtained from the microscale model. For IOP6, we analyze distributions of O 3 and CO observed in aircraft measurements. They show small scale fluctuations with amplitudes and phases that vary along the path of the flight. Detailed comparisons between these fluctuations and the observed vertical velocity show that the behavior of these short fluctuations is due not only to the vertical motion, but also to the local mean vertical gradients where the waves evolve, which are modulated by larger variations. The microscale model simulation results show favorable agreement with in situ radiosonde and aircraft observations. The high vertical resolution offered by the microscale model is found to be critical for resolution of smaller scale processes such as formation of inversion layer associated with trapped lee waves in the troposphere, and propagating mountain waves in the lower stratosphere.

Detailed Dual-Doppler Structure of Kelvin–Helmholtz Waves from an Airborne Profiling Radar over Complex Terrain. Part I: Dynamic Structure

“…Other studies have used numerical models to describe KH waves using either large-eddy simulations (e.g., Sauer et al 2016) or high-resolution numerical weather prediction models such as WRF (e.g., Mahalov et al 2011;Conrick et al 2018). However, in this study, we do not attempt to resolve the KH waves themselves (900-m grid spacing is insufficient for the small wavelengths we observe), only the environment in which they form.…”

“…Individual case studies in the literature suggest numerous environments in which KH instability can be released: KH waves (or ''billows'' as they are commonly referred to) have been observed in or near cumulonimbus anvils (Petre and Verlinde 2004;Trier 2016), near the tropopause (Mahalov et al 2011;Trier et al 2012), at the base of low-level jets (e.g., Nakanishi et al 2014;Lin et al 2019), near the top of density currents associated with a sea breeze (Sha et al 1991), behind a cold front (Geerts et al 2006;Friedrich et al 2008;Samelson and Skyllingstad 2016), at the upper boundaries of other cold-air intrusions (Zhou and Chow 2013), in midlatitude cyclones (Wakimoto et al 1992), and above mountainous terrain (Geerts and Miao 2010;Medina and Houze 2016).…”

Airborne measurements and large-eddy simulations of small-scale gravity waves at the tropopause inversion layer over Scandinavia

“…Measurements on HALO are conducted by the Basic HALO Measurement and Sensor System (BAHAMAS). Recent method and calibration details can be found in Mallaun et al (2015) and Giez et al (2017). For the horizontal wind, the measurement uncertainties are smaller than 0.5 m s −1 for HALO and 0.9 m s −1 for Falcon, and smaller than 0.3 m s −1 for the vertical wind (Heller et al, 2017;Bramberger et al, 2018).…”

Mountain waves impact wind power generation

“…Taking advantage of the rich data set of the Second Wind Forecast Improvement Project (WFIP2; Shaw et al 2019 and and mesoscale simulations from the Weather Research and Forecasting (WRF) model, this paper focuses on one phenomenon only-the impact of mountain waves on wind farms . Many studies have analysed mountain wakes for the last decades (e.g., Lindsay 1962;Bourgeault et al 1993;Klemp and Lilly 1997;Doyle and Durran 2002;Durran 2003;Smith 2003;Smith and Broad 2003;Grubišić and Billings 2007;Smith et al 2007;Mahalov et al 2011;Nappo 2012;Vosper et al 2012;Miglietta et al 2013;Durran 2015;and Fritts 2015). However, even though one article (Rasheed et al 2014) mentions that mountain waves result in horizontal and vertical wind shear, which can significantly impact wind power production, none quantified that impact.…”