A holistic microclimate model

Total Model Height

What is the Minimum Model Height?

Selecting the correct size of the model domain is a central aspect in successful numerical modelling.
Whereas the horizontal dimension is more or less given by the dimension of your subject of interest, the vertical height of the model is less obvious and can - if not selected properly - cause major problems.

The height of the 3D model is a result of the number of vertical grid points used as defined in the Area Input File and grid size plus the method of grid creation (see also Model Layout).

A simple Example: If you have a 30 m building in your model and you choose to have 10 grids with a grid size of 2 m each, the total model height will be 20 m, or, in other words, your building will look 10 m out of the model domain. This is, of course, not acceptable.

Having 20 grids with 2 m each will result in a model top in 40 m. On the first glance, that seems sufficient because even the highest building will fit in. But: The upper model boundary acts like barrier to the model processes. For example, no vertical flow through the upper border is allowed. That means, that numerically the wind flow has to squeeze in the 10 m gap between the roof of the 30 m building and the model top at 40 m building. This will result in jet stream effects that have no relation to the real nature, where no such interaction exists.

Another critical point often missed is plants on roofs: If you place a 10 m tree on a 30 m building, your tree will end in 40 m height!

Obviously, the model has to end somewhere, but this end must be far enough from the top of the highest building to minimise such unwanted artificial effects.

Rule of Thumb: The total height in meter should be at least twice the height of the tallest structure (especially if it is a building) and at least 30 m in total!

What is the Ideal Way to reach a Sufficient Height?

This question cannot be answered in a straight way.
Obviously there are two ways to reach a desired height: Increase the number of vertical grids or increase the vertical grid size of a grid.
Increasing the number of vertical grids will drastically increase the memory amount occupied and the time needed for the simulation. The benefit is that you don't need to decrease your spatial resolution of the model.
Increasing the vertical grid size will save you a certain amount of grid cells and time needed for calculation. The drawback is that your resolution will go down.

Like for the choice of the horizontal grid resolution and grid dimension, you will have to find a compromise between both, number of grids used and resolution provided. Also, you might consider using a telescoping grid instead of an equidistant one if your processes in the higher atmosphere are of less interest. Click here to hear more about different kinds of vertical grids in ENVI-met.

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