The ENVI-met atmospheric model is the core engine of the software, responsible for simulating the dynamic interactions between the air, surfaces, and plants. It is a three-dimensional, non-hydrostatic model that calculates the physical processes occurring in the urban micro-environment with high resolution. This allows it to capture fine-scale phenomena that are often missed by larger-scale models.
The atmospheric model in ENVI-met is a 3D non-hydrostatic model that uses a finite difference scheme to solve the governing equations of fluid dynamics, heat, and moisture transport. It operates on a grid covering the simulation domain, where each grid cell represents a small parcel of air, and the model computes the exchange of momentum, heat, and water vapor between cells.
The atmospheric model is fully coupled with the soil, vegetation, and building modules. It receives inputs such as surface temperatures and evapotranspiration from the soil and vegetation models, as well as building geometry and thermal properties from the building model. In turn, it provides wind, air temperature, humidity, and turbulence conditions that affect these other components, enabling a holistic and dynamic simulation of the urban microclimate.
Key Processes Handled by the Atmospheric Model
Wind field ENVI-met includes a full 3D Computational Fluid Dynamics (CFD) model. It solves the Reynolds-averaged non-hydrostatic Navier-Stokes equations for each grid in space and for each time step. The effects of vegetation are included as drag forces in the wind field. For detailed building physics simulation, the wind flow close to each facade and roof segment is calculated. With the new Single Wall feature, wind patterns inside complex or semi-open structures can be simulated as well.
The wind flow is updated at given time intervals. ENVI-met also supports a real-time flow calculation which means that the flow field is treated as a normal prognostic variable and calculated each step. Due to the very small time steps needed here, this way of calculation need very powerful computers.
Air temperature and humidity Air temperature and specific humidity of the air are determined by the different sources and sinks of sensible heat and vapour inside the model domain. Based on the calculated three-dimensional wind field, advection and diffusion in the air is simulated. The ground surface and vegetation leafs act as sources or sinks for both temperature and humidity in the atmosphere model. Building walls and roofs mainly act as surfaces interchanging heat with the atmosphere, but can also act as humidity sources if facade or rooftop greening is applied.
Turbulence Turbulence is calculated using the E-epsilon 1.5order closure (“E-epsilon” or “k-epsilon” model). Two prognostic equations for turbulent energy production (E) and its dissipation (epsilon) are used to simulate the distribution of turbulent energy. Exchange coefficients (K) in the air are calculated using the Prandtl-Kolmogorov relation. For low wind situations, the 1st order mixing length model can be used instead of the E-epsilon model (which often fails in this situations).
Radiative fluxes ENVI-met contains newly developed analysis modules to model the fluxes of shortwave and longwave radiation inside of complex environments. The scheme takes into account shading by complex geometries, reflections by different surface and building materials and the effect of vegetation on all radiative fluxes. The full versions of ENVI-met include the new IVS method, in which each urban element is considered using its actual state (sun reflection, thermal radiation) instead of averaged fluxes.
Pollutant dispersion The pollutant dispersion model of ENVI-met allows the synchronous release, dispersion and deposition of up to 6 different pollutants including particles, passive gases and reactive gases. Sedimentation and deposition at surfaces and vegetation is taken into account as well as the photochemical reaction between NO, NO2 and Ozone (O3).