Resources & Guidance

Model areas should have a few cells at the model border where no building cells are digitized. Terrain, soil profiles, plants should still be digitized correctly. Building cells near the model edge can block or channel wind flow, which may lead to large instabilities and ultimately simulation crashes.

How much open space is needed at the edge depends on the building density and building heights. As a rule of thumb, distance between model border and the first building should be half the size of the building height. That commonly leads to around 4-8 cells of open space at each model area boundary. However, in the vertical dimension, the distance should be significantly larger (more about this in the next question).

Most ENVI-met studies analyze outdoor thermal comfort at pedestrian level, thus needing a high vertical resolution. In general, the model area height should at least be twice the height of the highest building. However, if very tall buildings (for example 100 m height) are included in the model area, 100 Z cells in a 2 m resolution would be needed to accurately represent the model area and still obtain high resolution results at the pedestrian level (approximately at 1 m above the surface). We now have several solutions to decrease the amount of Z cells and thus the simulation time:

Solution A:
Use the telescoping option to stretch cells that are not in scope, i.e., starting above highest building height, we will only stretch air cells. A stretching factor to be specified in percent is then applied to the size of each previous cell: cell sizes thus quickly increase. Thereby, the model area height is achieved with less Z cells. Using our example above: with a telescoping factor of 20 % and a starting height of 60 m, we now only need 45 cells instead of 100 cells to reach more than 200 m in model area height. However, we would start to stretch within the building height range to achieve this objective.

Solution B:
Use the splitting option to divide the grid cell closest to the ground into 5 cells. The cells of particular interest at pedestrian level are now available in high resolution, while the default vertical resolution can be modified to a rather coarse value such as 5 m. Back to our example: with splitting activated and a 5 m resolution, we now only need 41 cells to reach more than 200 m in model area height and we still have high resolution outputs at pedestrian level (i.e., at 1.5 m height).

Solution C:
Use a combination of telescoping (Solution A) and splitting (Solution B) with a 5 m vertical resolution and the telescoping settings described in Solution A. We now only need 22 cells to reach more than 200 m in model area height. However, since we saved that many Z cells now, we could try to improve the stability even further and thus add some more Z cells to provide more free air cells above the highest building. We could also consider increasing vertical resolution to 3 m or 4 m or choose a higher starting height for telescoping / use a lower telescoping factor.

We strongly recommend you check these settings before you start digitizing your model area. Digitized 3D information such as special facade elements can be lost if the vertical gridding has to be changed again at the end (and in the process is converted back from 3D to 2.5D mode). Use the “Model Inspector” located in the Tools tab in Spaces to find the perfect vertical resolution for your model area.

Yes, soil profiles are only visually covered by terrain in Spaces but are still correctly used in the simulation.

Please check the CSV file carefully. Time steps need to be in 30-minute intervals, should not appear more than once and no time step should be missing. All columns must exist as in the template image, even if they are empty or carry invalid data.

Make sure that you chose the correct value separator and decimal separator for the text file import. Check the units of the imported values. Date and time need to be in the correct structure (example: 08:00:00, not 8:00:00). Temperature needs to be given in kelvin instead of Celsius.

The standard values for these two parameters generally do not have to be adjusted. If specific humidity values become rather large due to high humidity near the ground (this can happen in both Simple and Full Forcing), you can lower the specific humidity value at 2500 m to around 8 g/kg to avoid instabilities in the simulation.

Everything is fine as long as the Task Manager in Windows still shows that the ENVI-core program is using the CPU. ENVI-met does not react to Windows messages such as “Redraw yourself” during the simulation to save processing time. It will update the simulation window from time to time.

Simulation time strongly depends on three aspects:

• Hardware: how many cores are there, what is the CPU speed, how much RAM is available. Also note only some versions allow parallel processing.
• Simulation settings: e.g., pollutant simulations with active chemistry take longer than standard simulations
• Model area: Even if the model area is rather small in its horizontal dimensions, many users choose a very high vertical resolution. This can massively increase the simulation time and is not necessary in most cases. See the question “How do I find the best vertical gridding for my model area without the need of too much Z cells?” in Spaces section for more details.

If this issue arises, it is most likely due to a numerical instability. There are billions of operations executed in ENVI-met each minute and yet every user (including us) wishes for even greater complexity.
Almost all the variables calculated within the simulation depend on other spatial and temporal factors, which may give rise to a rogue variable that contains unrealistic values. The software corrects this in most cases, but it may also be the case that the variable unexpectedly went to a value such as zero before it is used for a division in the next operation, which triggers an error.
This does not arise because of a programming error, but of the dataset being damaged during calculations. Checking the validity of the data before every operation is not possible as it would increase the calculation time dramatically. An immense number of “intelligent” routines have been introduced into ENVI-met to autocorrect the most common problems. Ultimately, it is a sophisticated numerical tool, and these routine errors are inherent in numerical modeling.
You might also want to check our support center, where many individual cases are answered.

There is no general cause for why a model may not run correctly. In most cases you have to try different things to get a stable simulation if errors are occurring. However, if your configuration does not work, here are a few things to check:

• Was Windows working properly when the simulation crashed? ENVI-met allocates huge amounts of memory for data storage. If a program crashes or Windows has serious issues either before or during the model run, stored data may be lost. Do not run ENVI-met if there is little remaining memory, and make sure that the simulation is run in your physical memory, NOT in the virtual memory.
• Does ENVI-met crash at the beginning? Check the simulation log output present on the screen. Use the “Check Model” option to generate a review of the output. Check to see if the input files and database files are ok and contain realistic values. Problems with the model area design often lead to simulation crashes.
• Check the meteorological boundary conditions (especially important in Full Forcing!): Do radiation values look unreasonable to you? Are wind speeds too low (<0.8 m/s) or too high (> 5 m/s)? Is the wind direction rapidly and strongly changing between timesteps (e.g., from 0° to 180° within one hour)? Are the relative humidity values too high for your high temperature values, resulting in a very high specific humidity?

A few additional tips to promote a successful simulation are:

• Move complex buildings away from the model border
• Increase the vertical extents of the model
• Decrease the time step if the model gets unstable in the normal calculation loop
• Simplify your model, complex geometries of certain buildings can be adjusted to a simpler configuration

This feature is not implemented at the moment. If a simulation is cancelled, it must be restarted from the beginning.

Unfortunately, this is not yet possible.

If the license originates from a version before ENVI-met 4.4.5, the BIO-met license will only work for the old BIO-met version 1.5, which is available separately from our homepage (https://envi-met.info/doku.php?id=files:downloadv4).

The new BIO-met 2.0, which is included in the recent ENVI-met versions from 4.4.5 onwards, works in parallel mode (faster). A new license will be required to use this version.

UTCI cannot be calculated for wind speeds below 0.5 m/s. See here for more information: https://envi-met.info/doku.php?id=apps:biomet_utci.

Pollutant simulations in ENVI-met are complex. There are many circumstances that could lead to only some or no pollutants being simulated.

Please read this document regarding air pollution simulations (http://www.envi-met.info/doku.php?id=kb:sources) to find out more.

If Terrain is used together with Splitting and the Leonardo map is extracted with the option “Follow Terrain”, the elevation in meters is only valid for cells with an elevation of 0 m. Let us take as an example a model with a vertical resolution of 2 m, which we want to be extracted in Leonardo with Follow Terrain for the height level 3 (since the index counting in Leonardo starts at 0, we extract the 4th cell above the ground):

Cell A: This cell has terrain height of 0 m. Height information extraction takes place at 1.4 m height.
Cell B: This cell has terrain height of 2 m. Height extraction takes place at 9 m height.

Leonardo always shows the elevation that would be extracted without terrain, and therefore (in this example) it shows 1.4 m above ground. This means if a 5 m high building is placed in the model, it would be only displayed in Cell A, but not in Cell B.

It is recommended to disable Splitting if Terrain is used in simulations to avoid these complications.

The Air Temperature variable is now called Potential Air Temperature. This is a more accurate name for the variable, since ENVI-met always simulates at standard air pressure. However, it is planned to add air pressure as a variable in the future and thus there will be a difference between potential and absolute air temperature.

More information about all output variables of the atmosphere folder can be found here: https://envi-met.info/doku.php?id=filereference:output:atmosphere. Output variable descriptions of the other output folders are also found on the https://envi-met.info website.