CityCLIM – From an operational city weather forecast to a suite of services addressing the urban environment
Authors: Horn Stefan , Zimmer Janek 
Published: February 2023

The aim of the project CityCLIM is the development of a suite of services for the urban environment interesting for citizens and city administrations. The core of the CityCLIM services consists of a model chain to provide an operational weather forecast with an outstanding cell size of 100m for an area covering a whole city for a time span of up to three days. Therefore, the already operational SuperHD model by Meteologix is used to drive the new UltraHD model, which is a fully compressible large eddy simulation model including a two-moment microphysics parameterization. The UltraHD model was designed to use GPUs as the computational platform and is implemented using OpenCL. Surface orography and mountains were implemented using an immersed boundary layer approach and the model uses three-dimensional boundary conditions from the 1km SuperHD model. During the CityCLIM project further extensions for the UltraHD will be the implementation of a soil model for heat and moisture fluxes, a canopy layer to better represent vegetation fluxes and urban surface characteristics and a three-dimensional radiative transport code using raytracing for the short and a longwave band.The developed CityCLIM services target at two different groups, the citizens within a city and the city administrations.Services focused on the citizens are the Heat Wave Information and Warning Service, the Climate Information Service, the Pollution Information Service and the Citizen Weather Sensation Service. Except the Climate Information Service, the services will be based on operational everyday forecasts using the SuperHD and the GPU based UltraHD LES model chain. The Climate Information Service will be an extensive collection of climate and weather related parameters from available reanalysis models and measurement data on the Meteologix web portal. The Pollution Information Service will include aerosol compounds (PM10, PM2.5) and atmospheric trace gases (NOx, O3). With the added equations the computational effort for the UltraHD is significantly higher.The administrative services consist of identification services for heat islands, city air flow and pollution areas within the city and simulation and mitigation strategies services. For the identification services a collection of many operational UltraHD model runs are analyzed for hot spots within a city and areas which are more sensitive to extreme weather conditions can be identified. The simulation and mitigation strategies services use manipulated input fields like orography and land use for recalculations for selected days. Those will be performed on demand and can be compared to previous simulations. This will provide insight in the effects of certain climate mitigation strategies and can be used as a testbed for large scale city planning.To verify the results and to improve the initial model fields, high resolution measurement data is necessary. Therefore, the CityCLIM consortium installs additional weather stations within the pilot city regions and additional citizen science data will be collected. Earth observation data is extensively used, to characterize the surface of the city area with respect to land use and vegetation state and to compare model results like land surface temperature and soil moisture.