Studies about turbulent exchanges, momentum and mass transfer and energy balance on mountain slopes allow abetter comprehension of the interactions between soil and atmosphere in complex orography. In addition, if longperiods of observations are considered, the evolution of energy and mass fluxes can be derived. This is usefulfor model delicate ecosystems such as in the highlands. Furthermore, the study on carbon dioxide fluxes can berelated to the increase of greenhouse gas.The eddy-covariance technique has some critical points: one of the most important is related to the relativeuncertainty in the fluxes estimation when there are bad weather or low-wind and nocturnal conditions.Our aim is divided into two parts: in the first one, the meandering was explored. In the second part, wecompared two approaches, the planar fit and the double rotation techniques for the computation of turbulent fluxes.Because of the high number of low-wind speed conditions (LWS), we investigated the “meandering”: inLWS conditions, wind speed components and scalars such as temperature can show oscillations visible in theauto-correlation function of the signals. In these cases, turbulent fluxes estimation may be difficult. We analysed11 months of data collected at 10 Hz, considering a 1-hour time scale, with the identification of surface-layerparameters. Meandering phenomenon was explored following the works of Mortarini et al. (2013, 2015). Weevaluated also the impact of clear-sky conditions on our data. We observed the validity of the formula for spectralanalysis proposed in the aforementioned papers in most part of the analysed hours. Meandering conditions occurin 305 hours over more than 8000, especially during winter and night, although there are diurnal episodes. Mete-orological conditions seem to play some role on the local phenomena because, although no certain relationshipbetween stability and meandering parameters was found, the sky was cloudy in most part of meandering hours.In the second part, 30-minutes turbulent fluxes (sensible heat flux, latent heat flux and mass fluxes of water vapourand carbon dioxide) were determined using planar fit and double rotation techniques and the eddy-covariancetechnique use was tested for our site having a slope of about 26◦. Then, computation of the energy balance wasdone. We made comparisons between estimated and measured data and considerations on sensible and latent heatfluxes, then energy and mass fluxes and net radiation were computed also at the daily scale.We found that anemometer rotations improve robustness of computation and the difference between planar fit anddouble rotation is not so high in fluxes computations. Planar fit seems to give more reliable values. Consideringthe ground flux, G, we obtained a better approximation of energy balance. In particular, the computation of theenergy balance ratio (EBR) showed that in general the balance is better during the daytime, while the seasons inwhich the energy balance is nearer to closure are summer and autumn.

Evaluation of Turbulent Fluxes on a mountain slope

Davide Gisolo;Davide Canone;Stefano Ferraris
2018-01-01

Abstract

Studies about turbulent exchanges, momentum and mass transfer and energy balance on mountain slopes allow abetter comprehension of the interactions between soil and atmosphere in complex orography. In addition, if longperiods of observations are considered, the evolution of energy and mass fluxes can be derived. This is usefulfor model delicate ecosystems such as in the highlands. Furthermore, the study on carbon dioxide fluxes can berelated to the increase of greenhouse gas.The eddy-covariance technique has some critical points: one of the most important is related to the relativeuncertainty in the fluxes estimation when there are bad weather or low-wind and nocturnal conditions.Our aim is divided into two parts: in the first one, the meandering was explored. In the second part, wecompared two approaches, the planar fit and the double rotation techniques for the computation of turbulent fluxes.Because of the high number of low-wind speed conditions (LWS), we investigated the “meandering”: inLWS conditions, wind speed components and scalars such as temperature can show oscillations visible in theauto-correlation function of the signals. In these cases, turbulent fluxes estimation may be difficult. We analysed11 months of data collected at 10 Hz, considering a 1-hour time scale, with the identification of surface-layerparameters. Meandering phenomenon was explored following the works of Mortarini et al. (2013, 2015). Weevaluated also the impact of clear-sky conditions on our data. We observed the validity of the formula for spectralanalysis proposed in the aforementioned papers in most part of the analysed hours. Meandering conditions occurin 305 hours over more than 8000, especially during winter and night, although there are diurnal episodes. Mete-orological conditions seem to play some role on the local phenomena because, although no certain relationshipbetween stability and meandering parameters was found, the sky was cloudy in most part of meandering hours.In the second part, 30-minutes turbulent fluxes (sensible heat flux, latent heat flux and mass fluxes of water vapourand carbon dioxide) were determined using planar fit and double rotation techniques and the eddy-covariancetechnique use was tested for our site having a slope of about 26◦. Then, computation of the energy balance wasdone. We made comparisons between estimated and measured data and considerations on sensible and latent heatfluxes, then energy and mass fluxes and net radiation were computed also at the daily scale.We found that anemometer rotations improve robustness of computation and the difference between planar fit anddouble rotation is not so high in fluxes computations. Planar fit seems to give more reliable values. Consideringthe ground flux, G, we obtained a better approximation of energy balance. In particular, the computation of theenergy balance ratio (EBR) showed that in general the balance is better during the daytime, while the seasons inwhich the energy balance is nearer to closure are summer and autumn.
EGU General Assembly 2018
Vienna
08-13 Aprile 2018
Geophysical Research Abstracts
European Geosciences Union
20
16515
16515
https://meetingorganizer.copernicus.org/EGU2018/EGU2018-16515.pdf
Davide Gisolo, Davide Canone, Stefano Ferraris
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/1725592
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