An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ l ) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model, and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling.

[1] An extraordinarily severe yellow sand event was observed in Korea for the period of 21-23 March 2002. This event has been simulated using the three-dimensional eulerian transport model with meteorological outputs of the Regional Data Assimilation and Prediction System (RDAPS) together with the statistically derived dust emission conditions from WMO 3 hourly synoptic reporting data for seven spring seasons (MarchMay) from 1996 to 2002 in East Asia. The statistically derived dust-rise conditions with the help of the Chinese soil map in the source regions are found to be the threshold wind speed of 9.5, 7.5, 6.0 and 9.2 m s À1 and the upper limit of relative humidity of 60, 35, 30% and 40% in the Gobi, Sand, Loess and Mixed soil regions, respectively. These dust-rise conditions are implemented to the eulerian long-range transport model to simulate the extraordinarily intensive yellow sand event observed in Korea in terms of temporal and spatial variations of dust concentrations, starting and ending times of the yellow sand event over the Korean peninsula. It is found that the model simulates quite well the observed concentrations of more than 1,000 mg m À3 within the same order of magnitude and the starting and ending times of yellow sand in Korea within an hour. The spatial distribution of vertically integrated dust concentration in the model is quite well coincided with that of aerosol index obtained by total ozone mapping spectrometer (TOMS). The simulated dust particle sizes indicate that most of mass concentrations of dust in the source regions are mainly contributed by dust particles larger than PM 10 whereas those at distant regions from the sources are largely attributed to the PM 10 particles, suggesting the usefulness of the particle size spectrum in identifying the origin of a dust storm. The presently derived emission conditions enable to forecast the yellow sand event in Korea more reasonably than previously used conditions of threshold friction velocities.

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Basic entrainment equations applicable to the sheared convective boundary layer (CBL) are derived by assuming an inversion layer with a finite depth, i.e., the first-order jump model. Large-eddy simulation data are used to determine the constants involved in the parameterizations of the entrainment equations. Based on the integrated turbulent kinetic energy budget from surface to the top of the CBL, the resulting entrainment heat flux normalized by surface heat flux is a function of the inversion layer depth, the velocity jumps across the inversion layer, the friction velocity, and the convection velocity. The developed first-order jump model is tested against large-eddy simulation data of two independent cases with different inversion strengths. In both cases, the model reproduces quite reasonably the evolution of the CBL height, virtual potential temperature, and velocity components in the mixed layer and in the inversion layer.

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