Analysis of Gravity Waves Generated at the Top of a Drainage Flow

Abstract

Drainage or katabatic flows are common mesoscale circulations established as a result of differential radiative cooling of near-surface air masses in sloping terrain. The initial irruption of these flows, with sudden shifts in wind speed and direction, may result in vertical displacements of air parcels from their equilibrium position, which prove to be a common source of internal gravity waves. This paper illustrates this mechanism and describes the main features of the oscillations following the study of observational data gathered throughout one night during the Stable Atmospheric Boundary Layer Experiment in Spain 2006 (SABLES2006) field campaign. Pressure differences, measured by microbarometers set at different levels of a tower, help to interpret the evolution of other atmospheric variables, provide a detailed picture of the irruption of a drainage current, and reveal the formation of gravity waves at its top. The main parameters of the waves are derived from wavelet cross correlation of pressure time series, recorded by a surface array of microbarometers. The analysis yields, among other parameters, the horizontal component of the phase and group velocities of the gravity waves, which compare well with the velocity of irruption of the drainage current. Wavelet and other multiresolution techniques are also applied to sonic anemometer records to study the interaction between turbulence and larger-scale motions. The analysis shows evidence of heat flux divergence induced by the gravity waves, which may constitute a key factor for the vertical thermal profile in the nocturnal boundary layer (NBL) in situations of weak turbulence and important wave activity. FIG. 10. Vertical kinematic heat flux density per scale unit (K m s 22 ) at large time scales evaluated at z 5 96.6 m at different moments of the night showing wave activity. DECEMBER 2010 V I A N A E T A L .

The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence

“…These high-precision digital instruments can detect very small pressure perturbations, of the order of 0.1 Pa, at 2Hz sampling frequency. The objective was to study the small-scale static pressure fluctuations produced in the atmospheric boundary layer due to turbulent motions or the propagation of waves of different types (Viana et al, 2009(Viana et al, , 2010Sastre et al, 2012;Román et al, 2014). …”

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

“…It represents a simple orthogonal decomposition whose spectra satisfy Reynolds averaging at every scale. It has been shown to be a powerful tool for turbulence studies, since it allows the separation of turbulent eddies from possible non-turbulent motions of larger scales when a spec- tral gap (or minimum of energy of the spectrum) is well defined (van den Kroonenberg and Bange, 2007;Viana et al, 2009Viana et al, , 2010. In Sect.…”

“…They have been observationally analysed using different approaches (Ralph et al, 1997;Doyle and Durran, 2002;Viana et al, 2009Viana et al, , 2010Viana et al, , 2012Sun et al, 2012;Román-Cascón et al, 2015a). All these studies illustrate the difficulties in determining the origin and formation mechanisms of GWs, their importance as sources of momentum and heat transport (Sukoriansky et al, 2009;Fernando and Weil, 2010) and the necessity of their accurate parameterization in NWP models (Fritts, 2003;Kim and Hong, 2009;Belušić and Mahrt, 2012;Nappo, 2012;Sun et al, 2015b).…”

“…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.…”

Vertical structure of the stable boundary layer detected by RASS-SODAR and in-situ measurements in SABLES 2006 field campaign

“…During the IOP, wind and temperature vertical profiles (reaching a maximum height of 300-900 m) were also obtained by a tethered balloon system with hourly soundings whenever winds were lower than 8 m s -1 (for security reasons). Detailed characteristics of the instrumentation used in SABLES 2006 can be found in Viana et al (2009Viana et al ( , 2010. Figure 1a, b shows the evolution of temperature and the wind velocity modulus obtained from the tower measurements for the nights of the IOP.…”