Energy balance closure and advective fluxes at ADVEX sites
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ORIGINAL PAPER
Energy balance closure and advective fluxes at ADVEX sites Uta Moderow 1
&
Thomas Grünwald 1 & Ronald Queck 1 & Uwe Spank 1 & Christian Bernhofer 1
Received: 7 January 2019 / Accepted: 25 September 2020 # The Author(s) 2020
Abstract When measuring the energy balance at the earth’s surface using the Eddy covariance technique, the obtained budgets seldom produce a closed energy balance. The measurements often miss some of the energy fluxes. A possible reason is the neglect of non-turbulent surface fluxes of latent heat and sensible heat, i.e. advective fluxes of these quantities. We present estimates of advective latent and sensible heat fluxes for three different sites across Europe based on the ADVEX dataset. The obtained horizontal and vertical advective fluxes were site-specific and characterized by large scatter. In relative terms, the data indicated that the sensible heat budget was less affected by advection than the latent heat budget during nighttime; this is because vertical turbulent latent heat fluxes were very small or close to zero during the night. The results further showed that the additional energy gain by sensible heat advection might have triggered enhanced evaporation for two sites during nighttime. Accounting for advective fluxes improved the energy balance closure for one of the three ADVEX sites. However, the energy balance closure of the other two sites did not improve overall. A comparison with energy balance residuals (energy missed by the measurements without accounting for advection) indicated a large influence of systematic errors. An inspection of the energy balance for the sloped site of the ADVEX dataset underlined the necessity of slope-parallel measurement of radiation.
1 Introduction
Rn −G− J ¼ H þ LE;
The energy balance is fundamental in the interconnected earth-atmosphere system (Odum 1983; Oke 1987). Equation 1 denotes the energy balance at the earth’s surface. Rn −G ¼ H þ LE;
ð1Þ
where Rn denotes net radiation, G ground heat flux, H sensible heat flux, and LE latent heat flux. The left hand side of Eq. 1 is termed available energy (AE). AE is partitioned between the sensible heat flux (H) and the latent heat flux (LE), both of which redistribute the available energy back into the atmosphere. All terms in Eq. 1 are in W m−2. The energy balance is a formulation of the first law of thermodynamics (energy conservation). Not all components of Eq. 1 can be directly measured at the earth’s surface; therefore, we have to account for possible heat storage changes (J) between the earth’s surface and a chosen reference height above the earth’s surface. Equation 1 becomes to:
* Uta Moderow [email protected] 1
Faculty of Environmental Sciences, Institute of Hydrology and Meteorology, Technische Universität Dresden, Dresden, Germany
ð2Þ
where the available energy now includes J (storage changes) in W m−2. It is not always possible to close the energy balance based on the measurements obtained (e.g. Tsvang et al. 1991; Kanemasu et al. 1992; Wilson et al. 2
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