The use of modeling as a support tool for crisis management and decision planning requires fast simulations in complex built-up areas. The Parallel Micro SWIFT SPRAY (PMSS) modeling system offers a tradeoff between accuracy and fast calculations, while retaining the capability to model buildings at high resolution in three dimensions. PMSS has been applied to actual areas of responsibilities of emergency teams during the EMERGENCIES (very high rEsolution eMERGEncy simulatioN for citIES) and EMED (Emergencies for the MEDiterranean sea) projects: these areas cover several thousands of square kilometers. Usage of metric meshes on such large areas requires domain decomposition parallel algorithms within PMSS. Sensitivity and performance of the domain decomposition has been evaluated both for the flow and dispersion models, using from 341 up to 8052 computing cores. Efficiency of the Parallel SWIFT (PSWIFT) flow model on the EMED domain remains above 50% for up to 4700 cores. Influence of domain decomposition on the Parallel SPRAY (PSPRAY) Lagrangian dispersion model is less straightforward to evaluate due to the complex load balancing process. Due to load balancing, better performance is achieved with the finest domain decomposition. PMSS is able to simulate accidental or malevolent airborne release at high resolution on very large areas, consistent with emergency team responsibility constrains, and with computation time compatible with operational use. This demonstrates that PMSS is an important asset for emergency response applications.Atmosphere 2019, 10, 404 2 of 17 Gaussian models can provide simulation results in a short time and partly account for the effects of buildings. Nonetheless, they cannot take into account the unsteady flow in and above the urban canopy and the complex dispersion pattern between the buildings.Contrary to simplified models, computational fluid dynamics (CFD) models solve the Navier-Stokes equations. CFD models are categorized, according to the scale of the range of the spatio-temporal scales that are resolved, in Reynolds averaged Navier-Stokes (RANS) [11,12] and large eddy simulation (LES) [13][14][15]. While they provide reference solutions for complex flows in a built-up configuration, they are most often limited to academic configurations and/or small built-up domains: they indeed require very long computational times by default, making them unsuitable for live operational use.As a trade-off between computational time and accuracy of the solution, Röckle [16] first suggested an original approach. This approach uses mass consistency with observations to solve for the mean flow. Vortex flow structures are analytically defined around and between buildings and within street canyons, and then a mass consistency scheme is applied. Kaplan and Dinar [17] improved the approach by coupling the flow with a Lagrangian particle dispersion model to compute dispersion in an urban area. The PMSS modeling system uses this approach and integrates subsequent modifications using more recent urban tra...