An Investigation of the Relation between Sea Breeze Circulation and Diurnal Variation of Methane at a Swedish Lake

Abstract: Methane measurements over lake Tämnaren show a pronounced diurnal variation with high values at night and low values during daytime. The atmosphere over the lake and its surroundings is simulated with two different settings and resolutions of the WRF model during a period of eight days in May 2011 to investigate if a lake/land breeze circulation could be the cause of the observed methane variation. A night time land breeze can give rise to convergence over Tämnaren of the natural methane emissions from the lake which possibly could explain the diurnal variation. Analysis show that although Tämnaren is large enough to initiate a fully closed circulation these events are likely going to be rare because of the strong dependence of the background wind speed and cannot therefore be the cause of the pronounced diurnal variation. A fairly moderate wind speed will dominate over the thermodynamical forcing necessary to create a lake breeze. Even so, it is possible that a closed or nearly closed circulation could enhance the diurnal pattern with an increase of methane concentration at night and a decrease during the day. The reason for the high night time methane concentration is more likely due to the accumulation in a shallow internal boundary layer that develops over the lake combined with high night time methane flux caused by waterside convection.