The diurnal cycle of surface heat flux in summer fair weather conditions at mid- or low latitudes is known to affect wind-driven flow. The present study investigates diurnal cycle effects in various wind and weather conditions, with variable heating time, heating rate and cooling rate over a period of 1 day, using large-eddy simulations (LESs). Emphasis is given to the speed factor and deflection angle of the wind-driven flow at the surface, which have been observed to vary seasonally in the Tsushima Strait. The LES results show that diurnal cycle effects on the surface speed factor and deflection angle are considerable, even in non-fair weather conditions. The effects are particularly significant when the sea surface is heated during the daytime, while the daily mean heating rate is large and negative (strong cooling), which is typical in winter seasons. For example, the LES results show that the winter diurnal cycle in the Tsushima Strait doubles the speed factor relative to that without this cycle, even with the same imposed daily mean surface heat flux. The LES results also show that observed seasonal variations in speed factor and deflection angle can be explained by these diurnal cycle effects. Analytical solutions of the wind-driven flow are also sought under the diurnal cycle of surface heat flux in order to clarify the mechanism of the wind-driven flows under diurnal heat flux cycles. These results demonstrate the importance of the diurnal cycle of surface heat flux for wind-driven flow under a wide range of weather conditions.
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