Seasonal variations in the surface energy and CO₂ flux over a high-rise, high-population, residential urban area in the East Asian monsoon region

Using the eddy covariance method, this study reports the one-year turbulent fluxes of momentum, energy, and CO₂, and their seasonal variations over a recently redeveloped high-rise, high-population, residential area in the metropolitan city of Seoul, Korea. The study area is affected by the Asian monsoon, which is accompanied by long rain spells and a related mid-season depression of solar radiation in the summer. Our analysis shows that the urban surface energy balance and turbulence characteristics demonstrate typical urban properties. Unstable conditions dominate all day, and the storage heat flux (night-time and morning) and sensible heat flux (afternoon) significantly affect the diurnal variations in the urban surface energy balance. Owing to the rough urban surface, the turbulence intensities are higher than those reported previously in other cities. The annual CO₂ emission rate is approximately 13.1 kg CO₂ m⁻²⋅year⁻¹ with traffic, which is the major source of CO₂ (+2.3 μmol⋅m⁻²⋅s⁻¹ per 100 vehicles). Ecosystem respiration, including that by vegetation, soil, and humans, becomes dominant in the night-time (00:00–05:00), thus contributing significantly to the annual CO₂ budget. Further analysis indicates a unique coupling of urban surface energy partitioning and CO₂ emission rates with the seasonal progression of the Asian monsoon: (a) surface albedo has annual minima in late summer when the sun elevation angle, is relatively higher and the urban surface condition is wetter than in other seasons; (b) the Bowen ratio ranges from 1.7 (summer) to 7.0 (winter); and (c) CO₂ emission rates show seasonal variations with the progress of the summer monsoon.