Eva Podgrajsek

Diurnal cycle of lake methane flux

Air-lake methane flux (FCH4) and partial pressure of methane in the atmosphere (pCH4a) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both FCH4 and pCH4a with high values during nighttime (FCH4 ≈ 300 nmol m−2 s−1, pCH4a ≈ 2.5 µatm) and low values during day (FCH4 ≈ 0 nmol m−2 s−1, pCH4a ≈ 2.0 µatm) for a large part of the data set. This diurnal cycle persist in all open water season; however, the magnitude of the diurnal cycle is largest in the spring months. Estimations of buoyancy in the water show that high nighttime fluxes coincide with convective periods. Our interpretation of these results is that the convective mixing enhances the diffusive flux, in analogy to previous studies. We also suggest that the convection may bring methane-rich water from the bottom to the surface and trigger bubble release from the sediment. A diurnal cycle is not observed for all convective occasions, indicating that the presence of convection is not sufficient for enhanced nighttime flux; other factors are also necessary. The observed diurnal cycle of pCH4a is explained with the variation of FCH4 and a changing internal boundary layer above the lake. The presence of a diurnal cycle of FCH4 stresses the importance of making long-term continuous flux measurements. A lack of FCH4 measurements during night may significantly bias estimations of total CH4 emissions from lakes to the atmosphere.

Diel cycle of lake‐air CO2 flux from a shallow lake and the impact of waterside convection on the transfer velocity

Two years of eddy covariance measurements of lake carbon dioxide (CO2) fluxes reveal a diel cycle with higher fluxes during night. Measurements of partial pressure in the air (pCO2a) and in the water (pCO2w), during 4 months, show that the high nighttime fluxes are not explained by changes in the difference between pCO2a and pCO2w. Analyzing the transfer velocity (k600,meas), which is a measure of the efficiency of the gas transfer, with respect to wind speed, shows that variations in wind speed do not explain the diel cycle. During nighttime, when the fluxes are high, the wind is normally low. Thus, a solely wind-based parameterization of the transfer velocity (ku,CC) results in large errors compared to k600,meas, especially for wind speeds lower than 6 m s−1. The mean absolute percentage error between ku,CC and k600,meas is 79%. By subtracting ku,CC from k600,meas, we investigate how waterside convection influence k600,meas. Our results show that the difference (k600,meas − ku,CC) increases with increasing waterside convection. Separating the transfer velocity parameterization in two parts, one depending on the wind speed and one depending on waterside convection, the mean absolute percentage error compared to the measurements reduces to 22%. The results in this paper show that the high nighttime CO2 fluxes can, to a large extent, be explained by waterside convection and that a transfer velocity parameterization based on both wind speed and waterside convection better fits the measurements compared to a parameterization based solely on wind speed.

Influence from Surrounding Land on the Turbulence Measurements Above a Lake

Turbulence measurements taken at a Swedish lake are analyzed. Although the measurements took place over a relatively large lake with several km of undisturbed fetch, the turbulence structure was found to be highly influenced by the surrounding land during daytime. Variance spectra of both horizontal velocity and scalars during both unstable and stable stratification displayed a low frequency peak. The energy at lower frequencies showed a daily variation, increasing in the morning and decreasing in the afternoon. This behaviour is explained by spectral lag, where the low frequency energy due to large eddies that originate from the convective boundary layer above the surrounding land. When the air is advected over the lake the small eddies rapidly equilibrate with the new surface forcing. However, the large eddies remain for an appreciable distance and influence the turbulence in the developing lake boundary layer. The variances of the horizontal velocity and scalars are increased by these large eddies, while the turbulent fluxes are mainly unaffected. The drag coefficient, Stanton number and Dalton number used to parametrize the momentum flux, heat flux and latent heat flux respectively all compare well with current parametrizations developed for open sea conditions. The diurnal cycle of the partial pressure of methane,

Evaluating Humidity and Sea Salt Disturbances on CO2 Flux Measurements

p\mathrm{CH}_{4}

Comparison of floating chamber and eddy covariance measurements of lake greenhouse gas fluxes

pCH4, observed at this site is closely related to the diurnal cycle of the lake-air methane flux. An idealized two-dimensional model simulation of the boundary layer at a lake site indicates that the strong response of

Enhanced nighttime gas emissions from a lake

p\mathrm{CH}_{4}