ENVI-met Model Architecture

This page gives an overview over the technical aspects and modules used in ENVI-met Version 3

Introduction

ENVI-met is a three-dimensional non-hydrostatic model for the simulation of surface-plant-air interactions not only for but especially inside urban environments. It is designed for microscale with a typical horizontal resolution from 0.5 to 10 m and a typical time frame of 24 to 48 hours with a time step of 10 sec at maximum. This resolution allows to analyze small-scale interactions between individual buildings, surfaces and plants.

The model calculation includes:

• Shortwave and longwave radiation fluxes with respect to shading, reflection and re-radiation from building systems and the vegetation
• Transpiration, Evaporation and sensible heat flux from the vegetation into the air including full simulation of all plant physical parameters (e.g. photosynthesis rate)
• Surface and wall temperature for each grid point and wall
• Water- and heat exchange inside the soil system
• Calculation of biometeorological parameters like Mean Radiant Temperature or Fanger's Predicted Mean Vote (PMV) -Value
• Dispersion of inert gases and particles including sedimentation of particles at leafs and surfaces

 

Buildings, vegetation, soils/ surfaces and pollutant sources can be placed inside the model area. Besides of natural and artificial surfaces, the model is also able to handle water bodies.

ENVI-met runs on WINDOWS NT/2000/XP platforms with a minimum of 128MByte RAM. Multiple-Processors Systems are not supported (=do not run faster than single ones).The exact memory requirements depend on the number of grid points used.
A rough overview of the memory requirements is:

Wind Field

The three-dimensional Navier-Stokes equations are used in the Boussinesq approximated non-hydrostatic form including sink terms for drag forces at vegetation elements. The pressure perturbation is removed from the equations and an auxiliary velocity field is computed. Mass conversation is satisfied by correcting the auxiliary field by an iterative solution of the Poisson-Equation and correction at the outflow boundaries.
The 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.


Temperature and Humidity

Advection and diffusion of temperature and humidity is calculated using the previous calculated wind field. The ground surface and vegetation is incorporated using a source/sink term in both equations, building walls are only acting as a source/sink for temperature.


Turbulence and Turbulent Kinetic Energy (TKE)

The 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 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).
Improved troubleshooting has been added since version 2.5, which effectively reduces the number of problems with the turbulence model even under difficult atmospheric conditions.

Temperature and Water flow inside the soil

The vertical distribution of temperature and water is calculated for natural soils as well as for artificial seal materials. For each vertical grid box a different soil material can be chosen in order to simulate different urban soils. The flow of water inside natural soils is calculated using the formulae from Clapp and Hornberger. The hydraulic equations include a sink term for water uptake by plant roots. The thermodynamic properties of the soil are estimated by means of the actual water content.


Water Bodies

Water bodies are represented as a special type of soil. The calculated processes inside the water include the transmission and absorption of shortwave radiation inside the water.
No second energy balance is used for the ground surface of the water pool, so that heating of shallow systems is lower than under real conditions where the main source of energy is the convection from the water ground surface rather than the absorption of radiation.
In addition, no turbulent mixing is included in the model so that the use is restricted to still waters (e.g. lakes). The water parameterisation will be extended to turbulent mixing (oceans) later on. Special water usage (e.g. fountains) cannot be calculated with the model at the moment.

The Vegetation

Foliage Temperature

The average temperature of the leafs in one grid box is calculated by solving the energy balance of the leaf surface with respect to the actual meteorological and plant physiological conditions. Turbulent fluxes of heat and vapour are calculated from the given wind field and the geometry of the plant (see next section). The calculation of radiative fluxes include the shading, absorption and shielding of radiation as well as the re-radiation from other plant layers.


Heat, Water and Vapor exchange with in-canopy air

The gas and heat exchange between the vegetation and the atmosphere is controlled by the local energy balance steering the leaf temperature and by the stomata conductance controlling the gas exchange (vapour and CO2).
The actual stomata conductance of a plant is a complex function depending on external meteorological conditions (air temperature, available solar radiation PAR and many others) as well as on the plants physiological processes (Photosynthesis rate, CO2 demand, CO2 fixation,...). ENVI-met uses a sophisticated model to simulate the stomata behaviour of the vegetation.
To define the height and the shape of a plant, the model uses standard normalized functions (Leaf area density profile LAD, Root area density profile RAD) which can be applied for grassy surface as well as for huge trees.

Water interception and transport

Liquid water on the leafs influences highly the evaporation of the plant. The condensation of water of the leafs, the absorption of rain and the transport between different layers or the ground surface due to gravity is treated as an independent system inside the model.

Ground surface

The energy budget is calculated at the ground surface . The results are the surface temperature and humidity as well as the fluxes of sensible and latent heat. The ground surface and the walls are used as boundary conditions for the atmospheric model (ground surface and walls) and for the soil model (ground surface).

Wall/ Roof surface temperature and heat exchange

The temperature of the walls and the roofs is calculated for each grid point with respect to surface orientation, albedo and heat exchange with the temperature inside the building.
Heat exchange between the walls and the atmosphere are given by the pre-calculated flow and local turbulence.

Wall/ Roof surface temperature and heat exchange

 The temperature of the walls and the roofs is calculated for each grid point with respect to surface orientation, albedo and heat exchange with the temperature inside the building. The heat exchange between the building walls/roof and the atmosphere can be calculated using the wind field, the surface temperature and the local turbulence.

PMV-Value

The PMV (Predicted Mean Vote) was defined by Fanger (1972). It relates the (simple) energy balance of the human body to the thermal comfort of the person.
Normally the PMV value is used between -4 (very cold) and +4 (very hot), but as it is related to the energy balance it can also reach higher or lower values.
The PMV Model used in ENVI-met is a special adaptation to outdoor conditions made by Jendritzky/ DWD
(see http://www.dwd.de/services/gfmm/gfmm_kmm.html).

The climBOTs

Why not ask the people living in your model world what they think about the design and the local climate?
Impossible? No it's the climBOT model !
Go and meet the climBOTs on www.botworld.info

Behind the friendly WINDOWS user interface there are a lot of nasty numerical routines doing hard work to calculate the climate in your model area. Just a few keyword, on what is used in ENVI-met:The model equations are solved in three dimensions using the ADI (alternating direction implicit) method. Using a fully implicit scheme allows ENVI-met to use time steps up to 10 sec without getting numerically unstable. Of course, different modules have to be solved one after the other in order to manage the data flow. For example, first all the surface temperatures are calculated from the energy budget and then the 3D temperature in the atmosphere is calculated using these surface temperatures.

Some modules require smaller time steps such as the turbulence, the pollutant dispersion model, especially if sedimentation processes are involved. To calculate the wind flow, the pressure is removed from the Navier-Stokes equations and an auxiliary flow field is computed first (splitting method). After that, the Poisson equation is solved to calculate the corresponding pressure perturbation field. Here, the SOR algorithm is used. It is a little bit slower than a direct method, but in general more "friendly". Although it is possible to calculate the wind field continuously (each time step), recent computers are still to slow to do that because the calculation of the wind field requires very small time steps due to steep gradients e.g. at building walls. Therefore the usual way is to "update" the wind flow after a given time interval in order to match it with the stratification of the atmosphere. As a drawback of this method, slow thermal flows cannot be calculated with this approach.