Assimilation of street observation data using building-resolved large-eddy simulation model

Development of micrometeorological digital twin that can suggest mitigation measures for urban heat environments is crucial for reducing the health risks due to summer heat. To estimate the urban heat environment, we perform large-eddy simulation (LES) of micrometeorology using Multi-Scale Simulator for the Geoenvironment (MSSG). MSSG is capable of running atmospheric simulations covering global, meso- and urban scales. At the urban scales, it works as the LES model coupled with three-dimensional radiative transfer model, resolving building shapes and tree crowns. The initial and side boundary data for the LES are given by mesoscale downscaling from the mesoscale reanalysis data of Japan Meteorological Agency. To reduce the discrepancy between the micrometeorology simulation data and the real situation, we have developed a data assimilation tool which fuses measurement data at street observation points. The present data assimilation particularly considers the hydrostatic equilibrium in the vertical profiles based on the three-dimensional variational method (3D-VAR), aiming to improve the mean temperature and humidity rather than fluctuations of those quantities. The 3D-VAR tool is tested to obtain the analysis data from the micrometeorological simulation for actual urban area giving street level observations. The results of the simulation starting from the analysis show that the consideration of the hydrostatic equilibrium improves the duration of the effect of the analysis increment, while the effect decays due to the advection and diffusion. These results suggest the importance of considering the vertical profiles in the 3D-VAR. We will present the detail of the LES model and the data assimilation and also discuss the possibility of using the Green function method.