Advances in Methods for Urban Climate Research: Observations and Numerical Simulations

Urban climate research can be broadly divided into studies at the city scale (mesoscale) and the city-block scale (microscale). Each requires methods appropriate to the spatial scale of interest.

Mesoscale numerical weather prediction models are well suited to studies of weather and climate at the city scale. The Weather Research and Forecasting (WRF) model, developed by the National Center for Atmospheric Research (NCAR) in the United States, is now widely used worldwide. By calculating atmospheric conditions from the surface to altitudes exceeding 10,000 m, with horizontal resolutions of tens to hundreds of kilometers and vertical resolutions of tens to hundreds of meters, WRF can predict and reproduce atmospheric phenomena across a wide range of spatial and temporal scales, including cumulonimbus clouds, sea breezes, typhoons, and monsoons. Incorporating an urban canopy model (UCM) option also enables accurate simulations of urban weather and climate, including urban heat islands.

Satellite observations, radar measurements, and surface meteorological observations by meteorological agencies are also essential. These observations are indispensable both for understanding actual atmospheric conditions and for validating numerical weather prediction models such as WRF. Observations and numerical models are complementary, like the two wheels of a vehicle.

As examples of research at the city scale, this lecture presents findings on urban impacts on sea breezes and convective precipitation in the Tokyo metropolitan area. It also reports recent findings from studies of Greater Kuala Lumpur on urban impacts on cloud occurrence and interactions between heat waves and urban heat islands.

At the city-block scale, computational fluid dynamics (CFD) models are effective tools for predicting and reproducing air temperature, humidity, wind, radiation, and heat stress at spatial resolutions of a few meters. Because existing surface meteorological observations alone cannot adequately capture atmospheric conditions at this scale, dedicated field measurements and drone-based observations are needed. We have developed our own City-LES model as a method for high-resolution simulations at the city-block scale.

This lecture demonstrates how accurately City-LES can reproduce fine-scale spatial variations in urban surface temperature, airflow, and heat stress. It also explores the potential of heat mitigation measures, such as street trees, green walls, and mist spraying, to support urban adaptation to a warming climate.