Horizontal and Vertical Turbulent Fluxes Forced by a Gravity Wave Event in the Nocturnal Atmospheric Surface Layer Over the Amazon Forest

Abstract

A nocturnal gravity wave was detected over a south-western Amazon forest during the Large-Scale Biosphere-Atmosphere experiment in Amazonia (LBA) in the course of the dry-to-wet season campaign on October 2002. The atmospheric surface layer was stably stratified and had low turbulence activity, based on friction velocity values. However, the passage of the wave, an event with a period of about 180-300 s, caused negative turbulent fluxes of carbon dioxide (CO 2 ) and positive sensible heat fluxes, as measured by the eddy-covariance system at 60 m (≈30 m above the tree tops). The evolution of vertical profiles of air temperature, specific humidity and wind speed during the wave movement revealed that cold and drier air occupied the sub-canopy space while high wind speeds were measured above the vegetation. The analysis of wind speed and scalars high frequency data was performed using the wavelet technique, which enables the decomposition of signals in several frequencies allowed by the data sampling conditions. The results showed that the time series of vertical velocity and air temperature were −90 • out of phase during the passage of the wave, implying no direct vertical transport of heat. Similarly, the time series of vertical velocity and CO 2 concentration were 90 • out of phase. The wave was not directly associated with vertical fluxes of this variable but the mixing induced by its passage resulted in significant exchanges in smaller scales as measured by the eddy-covariance system. The phase differences between horizontal velocity and both air temperature and CO 2 concentration were, respectively, zero and 180 • , implying phase and anti-phase relationships. As a result, the wave contributed to positive horizontal fluxes of heat and negative horizontal fluxes of carbon dioxide. Such results have to be considered in nocturnal boundary-layer surface-atmosphere exchange schemes for modelling purposes.

The Amazon Tall Tower Observatory (ATTO): overview of pilot measurements on ecosystem ecology, meteorology, trace gases, and aerosols

“…In these circumstances, vertical fluxes converge to shallow layers in which the scalars may accumulate intensely over short time periods. In the absence of convective turbulence, which is the main factor for daytime transport, other physical processes become relevant in the stable boundary layer (SBL), such as drainage flow (Sun et al, 2004;Xu et al, 2015), vertical divergence of radiation (Drüe and Heinemann, 2007;Hoch et al, 2007), global intermittency (Mahrt, 1999), atmosphere-surface interactions (Steeneveld et al, 2008), and gravity waves (Nappo, 1991;Brown and Wood, 2003;Zeri and Sa, 2011). In this section, we discuss the role of intermittent turbulent events of variable intensity and periodicity, which provide episodic connection between the canopy and the atmosphere and can induce oscillatory behavior in the nocturnal boundary layer ( Van de Wiel et al, 2002).…”

Characteristics of Gravity Waves over an Antarctic Ice Sheet during an Austral Summer

“…In the cases of rare linear and monochromatic waves in which a spectral gap or phase relation between variables is clearly defined, the discrimination between waves and turbulence becomes possible by using wave phase averaging or band-passed filters in Fourier space [22][23][24]. However, in the frequent cases of "dirty waves", these techniques are not effective and different methods based on multiresolution decomposition or wavelet analysis have been proposed for their localisation properties [17,[25][26][27][28].…”

Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

“…Advection (Lee, 2004;Aubinet et al, 2005;Kutsch et al, 2008), drainage flows (Belcher et al, 2008;Mahrt, 2010), gravity waves (Zilitinkevich et al, 2009;Vecenaj et al, 2011;Zeri and Sa, 2011;Durden et al, 2013), and low-level jets (LLJs) (Banta et al, 2002;Mathieu et al, 2005;Darby et al, 2006;Karipot et al, 2008;Sun et al, 2012;Huang and Bou-Zeid, 2013) are the most commonly identified processes in this context. slopes in rather homogeneous terrain (Mahrt, 1999;Aubinet, 2008), while gravity waves are usually caused by topographic changes or irregularities of the canopy top (Lee et al, 1997).…”