Development of Models
for Urban Meteorology

Numerical simulation is one of the most common method to study urban meteorology. Kusaka Lab. is developing numerical models to reproduce the characteristic of urban climates.

Large Eddy Simulation (LES) model

Kusaka Lab. has been developing Large Eddy Simulation (LES) models for urban climate, weather, thermals, fog, and other meteorological phenomena. Most meteorological models use a turbulence model based on ensemble averaging, which is not suitable for reproducing sudden eddies and disturbances, since random eddy fluctuations are not included in the calculation. On the other hand, LES directly numerically analyzes eddy fluctuations that can be captured in the computational grid spacing and only models eddy fluctuations on smaller scales. Therefore, LES can be used to directly calculate complex turbulent structures that develop within the atmospheric boundary layer and urban boundary layer.

To calculate the thermal environment in the urban area using LES, it is necessary to resolve urban buildings as well as having a model to calculate the radiation process in the urban district. The radiation environment in a city block is very complex because of the dense buildings and trees along the streets. Kusaka Lab. is developing an in-town radiation model using the radiosity method as a model to calculate such a complex radiation process. Using the radiosity method, radiation calculations can be performed with high accuracy even for complex city block structures. Surface temperatures and sensible and latent heat fluxes can be calculated based on the shortwave and longwave radiation values obtained by the radiosity model, which can be combined with the LES model to reproduce detailed thermal environments in urban areas.

The LES model can reproduce the thermal environment of an urban area with high resolution and accuracy by taking into account complex radiative processes; thus, there is no significant difference between the LES modeled surface temperature distribution (left) and the observed surface temperature distribution (right) which indicate the high accuracy of the LES model.

Urban Canopy Model (UCM)

Urban canopy model is a model that physically includes the effects of urban buildings and has been incorporated into WRF and other climate models. In particular, the single-layer urban canopy model developed by Prof. Kusaka has been officially adopted in the domain climate model WRF and is widely used worldwide (Kusaka et al. 2001). This model divides the urban surface into three surfaces (road surface, wall surface, and roof surface) and solves the heat balance for each surface. In addition, the effects of shading due to buildings, wind speed reduction between buildings, and reflection of solar radiation and long-wave radiation between buildings are taken into consideration. Using this model, it is now possible to accurately reproduce the effect of lower temperatures at night in urban areas.

Weather Research and Forecasting (WRF) model

WRF is a numerical weather model for both academic research and weather forecasting developed by the National Center for Atmospheric Research (NCAR) and the National Centers for Environmental Prediction (NCEP). Originally developed as a mesoscale model, it has recently been improved to be applicable to phenomena at various scales, including climate simulations and LES. At Kusaka Lab., we use WRF to elucidate phenomena on a wide variety of scales with eagerness to maximize the potential of WRF.

Prof. Kusaka is the developer of the Urban Canopy Model, which was the first official model used in the WRF. For this reason, research on urban climate using WRF has been actively conducted at the Kusaka Lab. WRF is also used in a wide range of other fields, such as sea breeze, urban precipitation, and urban warming prediction. It is being improved to consider detailed population heat distribution and urban densities, and we are aiming to enhancing urban models and incorporating more accurate land use.

(Written by: Erika Yamada and Aoi Yagi; Edited by Chiho Numata)