[A-AS13] Hyper-dense observation and forecast to elucidate micro-scale atmospheric phenomena

[AAS13-06] Nocturnal Temperature Distribution under Fine and Weak Wind Conditions Based on Spatially High Density Observation Data in the Tokyo Metropolitan Area: Features in Winter

★Invited papers

*Hideo Takahashi¹, Shogo Shimizu¹, Hiroaki Yamato², Yoshihito Seto¹, Hitoshi Yokoyama³, Takehiko Mikami⁴ (1.Department of Geography, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 2.Nihon University, 3.Tokyo Metropolitan Research Institute for Environmental Protection, 4.Teikyo University)

Keywords:urban heat island, nocturnal temperature distribution, spatially high density observation, Tokyo wards area, winter

Tokyo metropolitan area where even only the special wards area of Tokyo has the diameter of about 30 km, indicating large heat island intensity, is a suitable experiment field to detect detailed structures of urban heat island. Urban temperature distribution, which is attributed to surface geometry/materials and anthropogenic heat emissions, is also affected by interactions with local wind systems such as land and sea breezes, deformation of urban boundary layer due to vertical mixing, and so on. Typical features of detailed nocturnal temperature distribution and its seasonal differences were investigated for the Tokyo metropolitan-ward area using spatially high-density observation data from the Automated Meteorological Data Acquisition System (AMeDAS) of the Japan Meteorological Agency, air pollution monitoring system (APMS) of the Tokyo Metropolitan Government and adjacent prefectures, and Extended-METROS (Meteorological Environmental Temperature and Rainfall Observation System) from 2006 to 2010. As for instrumentation, it should be noted that observations with aspirated shelters and Stevenson screens were coexisted, therefore we have to consider errors of about 0.2-0.3 ℃. To figure out the detailed structures of nocturnal temperature distributions with large urban-suburban temperature differences, hourly temperature data at 124 stations were interpolated into grid points with 1 km intervals.
In winter (from November to February), the high temperatures in the metropolitan area were concentrated throughout the night near the southern area of Chuo ward. In particular, it is noted that several zones of steep horizontal temperature gradients (HTGZ) were detected in the metropolitan-ward area. The HTGZs appeared during the period several hours before and after sunset, and gradually became more noticeable near sunrise. The distribution of temperature-decrease rate around sunset also showed steep horizontal gradient zones, which corresponded to the HTGZs between the inner city and outer area. After midnight, though wind speed decreased in the suburban inland region corresponding to the development of a near surface stable layer, that observed inner area of HTGZs remained comparatively large. This is likely due to the less stable atmospheric condition in central Tokyo with its high temperatures and/or large surface roughness. It is possible that this wind prevents the development of a stable layer, thus maintaining high temperatures downwind from the central Tokyo.
Summertime (July and August) features will be presented separately.