ENVI-Met

ENVI-met [138] is a three-dimensional, non-hydrostatic model for simulating a microclimate, especially within the urban canyon, taking into account the physical interactions among solid surfaces (e.g., ground and building surfaces), vegetation, and air. It is based on the theoretical background of Computational Fluid Dynamics. It applies the FDM discretization scheme, and it makes use of advanced numerical algorithms for solving the airflow-governing equations, i.e., conservation of mass, momentum, thermal energy, chemical–species' concentration, and turbulence parameters, as well as particle dispersion.

The main input of the model includes, among others, the properties of the incoming wind of the urban domain (wind speed, direction, temperature, relative humidity), a simplified geometry of the urban domain (since only structured grids and cartesian geometries are supported), thermo-physical properties of ground and building materials and of vegetation, and personal parameters of pedestrians (such as metabolic rates and clothing insulation) when the BIO-met is employed. The simulator then executes an iterative solution procedure and produces Distribution of temperature, relative humidity, pollutant concentration, turbulence parameters, wind speed, and thermal comfort indicators (e.g., mean radiant temperature and PMV modified for outdoor conditions), at different heights throughout the urban area of interest.

The background of the ENVI-met system includes sub-models solving for the following special physical mechanisms:


ENVI-met is a useful micro-scale model for the prediction of UHI effects within the urban canopy with acceptable accuracy provided that the model settings are correctly defined. In the case of complex geometries, radical simplifications may be required (such as building merging) in order to comply with grid-mesh restrictions. In addition, mesh possibilities are limited to structured grids with large grid cells (typical spatial resolution: 0.5–10 m). Hence, the effect of viscous sublayers (near solid surfaces) may be seriously underestimated. Another drawback is that only the Standard k-ε model is available

for turbulence modelling. Due to the large number of computational nodes, it presents normally very high CPU time until full convergence.

Wania et al. [139] used the ENVI-met system to study the influence of different vertical and horizontal densities of street vegetation on particle dispersion. It was demonstrated that vegetation reduces wind speed, which limits a canyon's ventilation and, therefore, leads to an increase in particle concentration. Vegetation was also found to reduce wind speed at crown height and to disrupt the flow field in close vicinity of the canopy. Szucs [140] highlighted that comfortable and healthy public open spaces encourage people to spend more time outdoors, socialize, exercise, and participate in re-creational events. In this framework, Szucs (2013) used ENVI-met to examine whether climatic conditions in Dublin boost long-term outdoor activities during summer and investigated the extent to which urban planning and the resulting urban morphology of the built environment influence the microclimate created by means of the wind profile. It was confirmed that areas of limited long-term outdoor activities are subjected to high wind speeds, often at the windward sections and around corners of buildings. Compared to the UCM tools SOLWEIG and Rayman, it presents a much better accuracy in comparison to actual measured data regarding radiation parameters [132]. Wai et al. [141] developed an integrated methodology including both ENVI-met and the Weather Research and Forecast (WRF) to explore the cooling performance of a water-spraying system in a sub-tropical compact and high-rise cityscape in a future-climate summer (2050) condition. It was indicated that the spraying system may provide cooling of 2–3 ◦C for ambient air temperature at the pedestrian level, improving significantly the thermal comfort conditions. In general, it has been widely used for urban planning purposes combating microclimate extremes worldwide; for example, in MDPI one can find 69 research articles with the keyword "envi-met" in their abstract. It presents good compatibility with BES tools; for example, its interconnection with EnergyPlus is now a well-established method [55,142] towards assessments of local climate impacts on building energy performance, especially when building-envelope measures are tested (green roofs, cool materials, insulation materials, PCMs, etc.).
