Model simulation of gravity waves triggered by a density current

Abstract

In this study, starting from an observational case of internal gravity waves (IGWs) generated at the top of a drainage flow during the SABLES2006 field campaign, we aim to reproduce the IGWs and their origin through mesoscale meteorological modelling. We used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km), testing the model capabilities to simulate the IGWs through a WRF fixed physics package option but two different planetary boundary layer schemes, the Mellor-Yamada-Janjić (MYJ) and the Yonsei University (YSU). The comparison between model simulations and measurements from a 100 m meteorological tower reveals that the MYJ scheme simulation gives much better results, as it better represents the main features of the density current measured by the tower instruments, although the event is predicted to occur sooner than it is observed to occur. The study has also shown the capacity of this scheme to detect the oscillations in temperature and specific humidity generated by the arrival of the density current. In contrast, the YSU scheme captures the arrival of the current on time but it fails to correctly track its properties and therefore it does not reproduce the gravity waves with the current arrival. In addition, wave parameters calculated from model outputs (MYJ) using the wavelet method reveal waves with longer periods and longer wavelengths (T = 20-22 min and λ = 8-10 km) than those calculated from measurements (T = 9.2 min and λ = 3.

Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study

“…A theoretical understanding of stable boundary layers (SBLs) is still an important and unachieved challenge (Mahrt, 2014), especially for numerical weather prediction (NWP) purposes (Van de Wiel et al, 2003;Baklanov et al, 2011;Holtslag et al, 2013;Davy and Esau, 2014;Fernando et al, 2015). NWP models have problems representing SBLs Steeneveld, 2014), which are related, for example, to the planetary boundary layer (PBL) evening transitions (Lapworth, 2015), minimum temperatures, low-level winds (Cuxart, 2008) and fog (Van der Velde et al, 2010;Román-Cascón et al, 2012) or air-quality (Andrén, 1990;Baklanov et al, 2009) forecasts.…”

“…Some small-scale gravity waves (GWs) and drainage flows can be included in the submeso motions; they can significantly change the stable and typical conditions of calm and clear nights through the generation of intermittent turbulence in the SBL (Nappo, 1991;Sun et al, 2002Sun et al, , 2004Sun et al, , 2012Van de Wiel et al, 2003;Mahrt, 2011Mahrt, , 2014Vindel and Yagüe, 2011). They can also change the vertical and horizontal gradients of scalars and consequently the turbulent fluxes observed near surface.…”

“…They are also typical SBL motions and manifest as sudden changes in wind direction, a temperature drop (due to the cooler current) or increasing winds at certain heights, among other effects (Yagüe et al, 2006;Viana et al, 2010;Udina et al, 2013). Several field campaigns have recently increased interest in these thermal circulations at different scales: e.g.…”

“…the Alps, Rotach et al, 2004;Nadeau et al, 2013, or Salt Lake Valley, Doran et al, 2002Monti et al, 2002). However, shallow drainage flows (SDFs) or density currents have been less studied (Mahrt et al, 2001;Soler et al, 2002;Udina et al, 2013;Oldroyd et al, 2014;Lehner et al, 2015a), in part because of their smaller scale, that often makes them more difficult to detect. Their proximity to the surface and their ability to change the surface conditions make them important and interesting phenomena worthy of analysis in SBL studies.…”

Aramis un modelo regional para la gestión de la calidad del aire. Evaluación y validación

“…Meteorological predictions over complex terrain require high horizontal and vertical resolution for resolving complex mesoscale circulation patterns, especially at night where turbulent mixing is greatly reduced or even completely suppressed, or becomes intermittent (Cuxart et al, 2000;Mahrt and Vickers, 2002;Poulos et al, 2002). In addition, this stable stratification in a non-uniform terrain induces local circulations, such as drainage flows (Soler et al, 2003), and leads to several phenomena such as gravity waves, density currents (Terradellas et al, 2005;Udina et al, 2013;Ferreres et al, 2013;Soler et al, 2014), intrusions and meandering, with the frequent presence of low-level jets (Conangla and Cuxart, 2006). The misrepresentation of these effects and the local physiographical features can lead to incorrect estimations of several degrees in the temperature, in the moisture levels and errors in the wind speed and wind direction (Bravo et al, 2008).…”