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Local aerodynamic roughness parameters (zero-plane displacement, z d , and aerodynamic roughness length, z 0 ) are determined for an urban park and a suburban neighbourhood with a new morphometric parameterisation that includes vegetation. Inter-seasonal analysis at the urban park demonstrates z d determined with two anemometric methods is responsive to vegetation state and is 1–4 m greater during leaf-on periods. The seasonal change and directional variability in the magnitude of z d is reproduced by the morphometric methods, which also indicate z 0 can be more than halved during leaf-on periods. In the suburban neighbourhood during leaf-on, the anemometric and morphometric methods have similar directional variability for both z d and z 0 . Wind speeds at approximately 3 times the average roughness-element height are estimated most accurately when using a morphometric method which considers roughness-element height variability. Inclusion of vegetation in the morphometric parameterisation improves wind-speed estimation in all cases. Results indicate that the influence of both vegetation and roughness-element height variability are important for accurate determination of local aerodynamic parameters and the associated wind-speed estimates.

Using the eddy covariance method, this study reports the one-year turbulent fluxes of momentum, energy, and CO 2 , 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 CO 2 emission rate is approximately 13.1 kg CO 2 m −2 Áyear −1 with traffic, which is the major source of CO 2 (+2.3 μmolÁm −2 Ás −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 CO 2 budget. Further analysis indicates a unique coupling of urban surface energy partitioning and CO 2 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) CO 2 emission rates show seasonal variations with the progress of the summer monsoon.

The dilution effect caused by boundary-layer evolution over land has strong influences on air quality. Accurate and continuous measurements of the boundary-layer height over urban areas are therefore needed for complete air-quality assessments. Commercial ceilometers, in combination with reliable and simple methodologies, can be used to retrieve the mixed-layer height, and represent a means of obtaining information on vertical mixing and atmospheric structure above cities. Here, we evaluate various retrieval algorithms based on the gradient method against high-temporal-resolution radiosonde observations. Based on the results, we propose a simple algorithm by using the gradient method, the correction of background noise and the moving averages, with the minimum number of parameters that need to be adjusted to the local properties and the instrument itself. The algorithm is adjusted for Seoul, Korea, and improves the retrieval performance by reducing highfrequency noise. The algorithm is used to investigate the relationship between the evolution of the daytime mixed-layer height and air pollution under a two-layer mixing model where changes in concentration depend only on the urban boundary-layer growth and air entrainment from the free atmosphere. Using 2 months of ceilometer retrievals of mixedlayer height and air-quality data from across the city, we find strong negative correlations for primary emitted pollutants such as NO 2 , CO, SO 2 , and particulate matter smaller than 10 µm, and a modest positive correlation for O 3. The results provide insight into the significant influence of urban boundary-layer evolution on Seoul's air quality.

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