Stable boundary layer and katabatic flow inside a boreal forest canopy with sloping terrain

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Title: Stable boundary layer and katabatic flow inside a boreal forest canopy with sloping terrain
Author: Keränen, Tuukka
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2021
Thesis level: master's thesis
Abstract: A stably stratified layer is often observed to form near the surface during nighttime. If the terrain is not flat, cold air near the surface can start to flow down the slope despite the wind direction above the stable layer being different. This slope flow is called katabatic wind. Katabatic winds are challenging for eddy covariance measurements that are commonly used to calculate the fluxes of gases and energy between the soil and the atmosphere. If the measurement is done above the canopy, the katabatic wind may lead to significant advective transport of gas and energy not detected by the measurement. Therefore eddy covariance measurements might underestimate the fluxes by a significant amount. Therefore it is important to understand the mechanisms of katabatic flow so that the effects of it can be taken into account when eddy covariance measurements are used in sloping terrain. This thesis determines how the katabatic flow within a boreal forest canopy in Hyytiälä, Finland depends on the static stability and depth of the stable layer within the canopy. The measurements are also compared to a simplified theory to find out how well the existing formulations of katabatic flow within canopies describe the observed conditions. Wind measurements done with sonic anemometers and temperature measurements done with distributed temperature sensing system during June-October 2019 are analyzed in this thesis to form understanding of the vertical profiles of temperature and wind within the canopy layer at the measurement site. In addition to the wind and temperature measurements, solar radiation measurements are also used to find the dominant driver for the formation of the stably stratified canopy layer. The measurement site represents typical Finnish Scots pine forest and has sloping terrain with main slope of 2° in the north-south direction. This study found evidence of katabatic northerly flow forming at the measurement site during stable nights. However, this study could not find a relation between the depth of the stable layer and strength of the katabatic flow. The katabatic flow was observed to get stronger in the open trunk space with increasing static stability within the canopy layer. The results of this study suggest, that katabatic flow follows the simplified theory well within the upper part of the canopy, where the majority of the foliage is. At lower levels within the open trunk space, the simplified model greatly underestimates the flow speed.
Subject: Stable boundary layer
static stability
katabatic wind
boreal forest canopy

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