Since turbulent mixing often becomes discontinuous and intermittent in the stable stratified boundary layer, so do nighttime CO
2 fluxes observed over an ecosystem. The friction velocity correction (
u*-correction) is applied to nighttime CO
2 fluxes measured with the eddy covariance technique for evaluating the ecosystem respiration rate. However, the applicability of this procedure is uncertain in intermittently turbulent conditions as the friction velocity oscillates between extremely large and small values.
We used a simple numerical model to simulate intermittent mixing in the stable boundary layer and analysed the characteristics of intermittent nighttime CO
2 fluxes. We then examined the applicability of the
u*-correction in intermittently turbulent conditions. The simulation results showed alternations between highly turbulent periods with large positive CO
2 fluxes and quiescent periods with small positive and zero CO
2 fluxes. With increasing the geostrophic wind in the range of 5.0 m s
-1 to 8.0 m s
-1, the magnitude of upward intermittent CO
2 fluxes as well as the time intervals between intermittent turbulent periods became smaller. A non-linear dependency of the upward CO
2 fluxes on the surface temperature was not clear during intermittently turbulent periods. The fluxes observed in conjunction with high
u* values showed a better relationship with the surface temperature although use of such fluxes in the
u*-correction procedure overestimated the ecosystem respiration rate: the
u*-corrected estimates of the respiration rate were twice as high as the ecosystem respiration rate. During flux events, CO
2 storage fluxes became large and negative. In such cases, the CO
2 storage flux must be added to the CO
2 flux for evaluating the ecosystem respiration rate using the
u*-correction procedure.
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