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Article Type:
Research Article

Continuous Time Series of Catchment-Averaged Sensible Heat Flux from a Large Aperture Scintillometer: Efficient Estimation of Stability Conditions and Importance of Fluxes under Stable Conditions

Bruno Samain Laboratory of Hydrology and Water Management, Ghent University, Ghent, Belgium

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Willem Defloor Flemish Environmental Agency–Department Operational Water Management, Brussels, Belgium

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Valentijn R. N. Pauwels Laboratory of Hydrology and Water Management, Ghent University, Ghent, Belgium

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Print Publication:
01 Apr 2012
DOI:
https://doi.org/10.1175/JHM-D-11-030.1
Page(s):
423–442
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Abstract

A large aperture scintillometer (LAS) observes the intensity of the atmospheric turbulence across large distances, which is related to the path-averaged sensible heat flux H. In this paper, two problems in the derivation of continuous series of H from LAS data are investigated and the importance of nighttime H fluxes is assessed. First, as an LAS is unable to determine the sign of H, the transition from unstable to stable conditions is evaluated in order to make continuous H series. Therefore, different algorithms to judge the atmospheric stability for an LAS installed over a distance of 9.5 km have been tested. The diurnal cycle of the refractive index structure parameter, , results in the best suitable, operational algorithm. A second issue is the humidity correction for LAS data, which is performed by using the Bowen ratio (β). As β is taken from ground-based measurements with data gaps, the number of resulting H values is reduced. Not including this humidity correction results in a marginal error in H, but increases the completeness of the H series. Applying these conclusions to the 2-yr time series of the LAS results in an almost-continuous H time series. As the majority of the time steps has been found to be under stable conditions, there is a clear impact of Hstable on H24h—the 24-h average of H. For stable conditions, Hstable values are mostly negative, and hence lower than the H = 0 W m−2 assumption as mostly adopted. For months where stable conditions prevail (winter), H24h is overestimated using this assumption, and calculation of Hstable is recommended.

Corresponding author address: Bruno Samain, Laboratory of Hydrology and Water Management, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium. E-mail: bruno.samain@ugent.be

Abstract

A large aperture scintillometer (LAS) observes the intensity of the atmospheric turbulence across large distances, which is related to the path-averaged sensible heat flux H. In this paper, two problems in the derivation of continuous series of H from LAS data are investigated and the importance of nighttime H fluxes is assessed. First, as an LAS is unable to determine the sign of H, the transition from unstable to stable conditions is evaluated in order to make continuous H series. Therefore, different algorithms to judge the atmospheric stability for an LAS installed over a distance of 9.5 km have been tested. The diurnal cycle of the refractive index structure parameter, , results in the best suitable, operational algorithm. A second issue is the humidity correction for LAS data, which is performed by using the Bowen ratio (β). As β is taken from ground-based measurements with data gaps, the number of resulting H values is reduced. Not including this humidity correction results in a marginal error in H, but increases the completeness of the H series. Applying these conclusions to the 2-yr time series of the LAS results in an almost-continuous H time series. As the majority of the time steps has been found to be under stable conditions, there is a clear impact of Hstable on H24h—the 24-h average of H. For stable conditions, Hstable values are mostly negative, and hence lower than the H = 0 W m−2 assumption as mostly adopted. For months where stable conditions prevail (winter), H24h is overestimated using this assumption, and calculation of Hstable is recommended.

Corresponding author address: Bruno Samain, Laboratory of Hydrology and Water Management, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium. E-mail: bruno.samain@ugent.be
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