Evaluation of Seasonal Pattern of Energy, Water and Carbon Dioxide Fluxes Over Tokyo Bay

Tokyo Bay, which is a closed water area, next to Tokyo Metropolitan area, would have serious effects on the atmospheric environmental issues of the metropolitan area, such as the heat island phenomenon, heavy rainfall, and air pollution. However, due to few observations, we have little information on the influence of Tokyo Bay on the atmospheric environment. Therefore, we have investigated energy, water (hereafter H₂O) and carbon dioxide (hereafter CO₂) fluxes between the water surface and the atmosphere over Tokyo Bay. The measurement has been conducted at the height of 12m above mean sea level using an existing tower (N35.6°, E140.0°) since December, 2004. In this study, we compared these fluxes with those at a suburban area (Kugahara) in Tokyo, Japan (N35.6°, E139.7°). We found the following results on the energy, H₂O and CO₂ fluxes over Tokyo Bay during winter season. In this season the wind direction was almost northward; wind was from an urban area. Here, we focused on data of the daytime which over 80 percents of the rate of daylight.

1)Short wave(S↓)/Long wave(L↓) radiation Short wave radiation was larger than that at Kugahara whereas long wave radiation was smaller. These results were probably due to less cloud and lower concentration of suspended particulate matter over Tokyo Bay.

2)Sensible heat flux (H) Sensible heat flux varied corresponding to a temperature difference between sea surface and atmosphere. The temperature difference was caused because the diurnal variation of sea surface temperature was small whereas that of air temperature was large. The change of air temperature over sea was provably due to the advection from the urban area with a wide range of temperature.

3)Latent heat flux (LE) Latent heat flux was larger than that in the suburban area. LE over Tokyo Bay varied corresponding to wind speed rather than to radiative forcing. These results suggest that the advection of dry air from the urban area increase the evaporation at Tokyo Bay.

4)Carbon dioxide flux (CO₂ flux) The CO₂ flux in the suburban area was positive (CO₂ source), whereas that in Tokyo Bay was negative (CO₂ sink) or almost zero. The CO₂ of high concentration in the urban area was probably transported to Tokyo Bay. This causes a difference of the CO₂ concentration between the atmosphere and the vicinity of water surface, and thus the CO₂ flux was downward over Tokyo Bay.

These preliminary results were largely different from the well-known facts derived from the data of the ocean. Therefore we need further successive monitoring for long-term evaluation.