An Urban Scheme for the ECMWF Integrated Forecasting System: Single-Column and Global Offline Application

The societal benefits of numerical weather prediction (NWP) forecasts are most evident in populated areas. An urban representation within NWP models should provide improved forecast accuracy. Here, we present the preliminary implementation of an urban scheme within the Integrated Forecasting System (IFS) using a simplified single-layer urban canopy model. The scheme makes assumptions of canyon geometry and considers fluxes from roads, walls, and roofs. Temperature observations were used to optimize single-column model (SCM) parameters using the Gauss-Newton method. Observation comparisons over six European cities, show a 2-m temperature root-mean-squared error reduction from 1.85 to 1.75 K with the urban scheme. Optimized parameters were used globally at kilometric scale in a land surface model. A sensitivity experiment assuming a 100% urban world showed spatially averaged northern hemisphere 2-m temperatures increased by 0.54 K (January) and 0.42 K (July) at night caused by changes in the albedo, emissivity, roughness, and thermal and hydrological properties. Global similar to 1-km resolution simulations using ancillary urban mapping information produce an urban heat island effect over major and minor conurbations. Only major conurbations were well represented at similar to 9-km resolution. Results from SCM simulations show a heightening of the planetary boundary layer over city sites, with the largest enhancements occurring at night in July (84 +/- 48 m) caused by an increased sensible heat flux. These initial developments show the importance of a high-resolution urban representation within NWP models. Improved parameterization and mapping will enable an online representation of energy, water, and trace gas fluxes over residential areas.

Automated summary

The study implemented an urban scheme within the Integrated Forecasting System to improve forecast accuracy in populated areas, showing a reduction in temperature root-mean-squared error and an increase in nighttime temperatures in a 100% urbanized world. Results indicate the importance of high-resolution urban representation in NWP models for better parameterization and mapping of energy, water, and trace gas fluxes over residential areas.