Atmospheric turbulent flux observation on the tidal zone over the Ariake Sea, Japan

The Ariake Sea is a closed bay on the west of Kyushu Island, Japan, with a dynamic tidal range over 5 m in the closed-off section of bay in the spring tide. The Ariake Sea also has a vast tidal flat: the exposed area becomes wider than 200 km² in the spring tide, occupying about 40 % of the total exposed area in Japan.

The energy budget on the tidal flat surface has been thought to play an important role in controlling the thermodynamic properties of seawater and the biophysical chemical processes of the tidal bottom surface. However, the land sea surface-atmosphere interaction on the tidal flat is complicated because of including not only the diurnal cycle mainly driven by solar radiation but also the tidal cycle controlling the seawater coverage in the tidal bottom surface. Therefore, the objective of this study is to observe the surface energy fluxes on the tidal flat using the direct method of turbulent eddy correlation.

The observation system was constructed on the tidal zone near the Kumamoto Port (32°46´5" N, 130°35´40" E). The turbulent flux observation system of the AWS is composed of the sonic anemo-thermometer (81000, YONG) and the infrared H₂O/CO₂ gas analyzer (LI-7500, Li-COR). The data are sampled at 10 Hz and stored on the data logger (CR-5000, Campbell). The surface around the system exposes offward about 1.5 km at lowest water level. Around site, the bottom surface exposes as long as 3.5 hours in a half-day tidal cycle in the spring tide. In the neap tide, however, the bottom surface is always submerged.

While the bottom surface was submerged in the daytime, the sensible heat flux and latent heat flux was as low as 100 Wm^-2, although the net radiation flux became higher than 700 Wm^-2. As soon as the bottom surface was exposed, both the sensible heat flux and the latent heat flux increased because the bottom soil surface was heated directly by the net radiation. Under the exposed condition, the bottom soil was still wet and the latent heat flux was dominant. During several hours from sunset, the latent heat flux became higher than 150 Wm^-2. The diurnal variation of the water temperature, skin surface temperature, and atmospheric temperature showed that the atmospheric stratification was still unstable during the night with the bottom surface covered by seawater, which released the heat energy both into atmosphere and into the bottom soil.

A scale analysis of seawater temperature equation showed that vertically integrated advection term became an order of 100-500 Wm^-2, by substituting the typical value in the Ariake Sea. This value was comparable to other components of the surface energy fluxes.