Lakes are sources of nitrous oxide (N2O). However, labor-intensive gas analysis has confined evaluations of temporal variability in lakes to the seasonal time scale. We used a state-of-the-art laser-based N2O analyzer combined with a floating chamber to examine sub-daily variations in N2O emissions from a shallow eutrophic lake in Japan. We conducted intense daytime field campaigns during different seasons to reveal differences in sub-daily variations in N2O emissions from the water surface in conjunction with the dynamics of dissolved N2O concentrations in the lake water. The study revealed that N2O fluxes varied within a day: emissions increased in response to increased wind speeds. Variations in surface dissolved N2O concentrations caused by water mixing were also important during the summer, when accumulation of N2O initially occurred in the lake bottom layer during stably stratified conditions. During winter, the lake water was well mixed and dissolved N2O concentrations were uniform throughout the water column; thus, wind speed was the only factor controlling the diurnal variations in N2O emissions. These findings demonstrate the need to consider diurnal variability when estimating cumulative N2O emissions accurately over long periods.

Boreal wildfires impact surface climates with consequences for plant physiology, permafrost thaw, and carbon fluxes. Post-fire temperatures vary over decades due to successional changes in vegetation structure and composition. Yet, the underlying biophysical drivers remain uncertain. Here, we quantify surface climate changes following fire disturbances in the North American boreal forest and identify its dominant biophysical drivers. To do so, we analyse multi-year land-atmosphere energy exchange and satellite observations from across Canada and Alaska. We find post-fire daytime land surface temperatures to be substantially warmer for about five decades while winter temperatures are slightly cooler. Post-fire decadal changes are characterised by a decrease in leaf area index during the first decade, a sharp increase in snow cover period surface albedo, and a decrease in the efficiency of heat transfer for about 2-3 decades. Evaporative fraction increases in the first three decades before returning to lower values again. We find that warming is mainly explained by a decrease in the efficiency of heat transfer while cooling is additionally explained by increasing surface albedo. We estimate that current daytime surface temperatures of the boreal biome of Canada are 0.18 °C warmer in the summer and 0.04 °C cooler during the winter due to fire. For a scenario with a strong increase in burned area, we estimate doubled warming from fire until 2050. Our study highlights the potential for accelerated surface warming in the boreal biome with increasing wildfire activity and disentangles the biophysical drivers of fire-related surface climate impacts.

The drag coefficient (CDN), Stanton number (CHN) and Dalton number (CEN) are of particular importance for the bulk estimation of the surface turbulent fluxes of momentum, heat and water vapor at water surfaces. Although these bulk transfer coefficients have been extensively studied over the past several decades mainly in marine and large-lake environments, there are no studies focusing on their synthesis for many lakes. Here, we evaluated these coefficients through directly measured surface fluxes using the eddy-covariance technique over more than 30 lakes and reservoirs of different sizes and depths. Our analysis showed that generally CDN, CHN, CEN (adjusted to neutral atmospheric stability) were within the range reported in previous studies for large lakes and oceans. CHN was found to be on average a factor of 1.4 higher than CEN for all wind speeds, therefore, likely affecting the Bowen ratio method used for lake evaporation measurements. All bulk transfer coefficients exhibit substantial increase at low wind speeds (< 3 m s-1), which could not be explained by any of the existing physical approaches. However, the wind gustiness could partially explain this increase. At high wind speeds CDN, CHN, CEN remained relatively constant at values of 2 10-3, 1.5 10-3, 1.1 10 -3, respectively. We found that the variability of the transfer coefficients among the lakes could be associated with lake surface area or wind fetch. The empirical formula C=b1[1+b2exp(b3 U10)] described the dependence of CDN, CHN, CEN on wind speed well and it could be beneficial for modeling when coupling atmosphere and lakes.

Environmental controls on methane (CH) emission from lakes are poorly understood at sub-daily time scales due to a lack of continuous data, especially for ebullition. We used a novel technique to partition eddy covariance CH flux observed in the littoral zone of a mid-latitude shallow lake in Japan and examined the environmental controls on diffusion and ebullitive CH flux separately at a sub-daily time scale during different seasons. Both diffusive and ebullitive flux were significantly higher in summer than winter. The contribution of ebullitive flux to total flux was 56% on average. Diffusive flux increased with increasing wind speed due to increased subsurface turbulence. For a given wind speed, diffusive flux was higher in summer than in winter due to the higher concentration of dissolved CH in the surface water during summer. The transfer of accumulated dissolved CH from the bottom layer to the surface in summer and the accumulation of dissolved CH under surface ice in winter were important for explaining the variability of diffusive flux. In summer, ebullition tended to occur following triggers such as a decrease in hydrostatic pressure or an increase in wind speed. In winter, on the other hand, the impact of triggers was not obvious, and ebullition tended to occur in the morning when the wind speed began to increase. The low CH production rate in winter slowed the replenishment of bubbles in the sediment, negating the effect of triggers on ebullition.