A holistic microclimate model

Instability/ Numerical Errors

The problem...

In complex Model Areas (and sometimes even in configurations you consider as being “simple”) you might experience numerical instabilities from time to time. These instabilities appear as “PANIC Dumps” (if ENVI-met has detected that something went wrong) or simply as error messages or crushes of the model. The most frequent error message are “Division by Zero” or “Floating Point Error”. These messages do not contain any useful information except of the general message, that something went wrong. In fact, “Division by Zero” is most of the times only the end point of a long chain of problems producing very high or very low (and very wrong) numbers.

Why is it like that...

First of all, remember that you are working with a very complex numerical model. There is a conceptual difference between error messages you might get from, say, a text-processing program, and those you get from ENVI-met. Error messages (and the numerical problems which cause these messages) are inseperably connected with the whole process of numerical modelling. If it would be possible to construct any Model Area and get guaranteed results, numerical modelling would no longer be an advanced technology ;). In other words: When you decide to use non-liner models like ENVI-met, you must be prepared that things are not always running as you would like them to run. Sometimes models run on the edge of numerical stability and a complex configuration might cause that they fall over this edge and send you an error message.

What can I do against it?

There are no general rules for solving these problems. If they would exists, they would have been included in the model. The first step to take is to figure out, what exactly went wrong, or in other words: which component did cause the problem. To answer this question, it is important to figure out when/where exactly the problem have occurred. A simulation in ENVI-met consists of several modules and it is important to identify in which module the error happened. To do so, look at the last outputs generated by ENVI-met and try to locate the problem using the overview of the model run. Then, click on the module of which you think it causes the problem to obtain more information and possible solutions.

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