scope:

This standard discusses measurement-based calculation methods for atmospheric parameters (turbulence, wind velocity, wind direction, temperature, mixed layer height) which must be available as input data for various types of dispersion models (e.g. VDI 3945 Part 1 and Part 3).

The following simplifications are assumed:

• steady-state conditions

• even and homogeneous terrain

• description of surface texture through a mean roughness length and where relevant through a zero displacement height

• characterisation of the stratification stability using the Obukhov length

In addition to the topography, the mean meteorological situation in the applicable region should also be as homogeneous as possible. Especially in the presence of mesoscale circulation forms, e.g. mountain and valley winds, land and sea breeze circulations or fronts, systematic deviations may occur from the standard profiles defined here. Where necessary, the suitability of the profile methods can be checked through comparison with a three-dimensional flow model.

The quoted profile functions for wind and turbulence were derived from assumptions and measurements in the near-ground atmospheric boundary layer. Within the canopy layer and for greater heights, pragmatic methods are provided which ensure that when using the profiles, the dispersion in the whole vertical dispersion space can be described.

The parameterisations contain an explicit dependence on the averaging period of the readings. This period is at least one minute and at most one hour.

In general, it should be noted that the result of a dispersion calculation based on these parameterisations describes not a real discrete situation but a statistical mean over the real events that may occur during the averaging period (ensemble mean). Therefore, it is impossible e.g. to draw from the results direct conclusions about the concentration fluctuations within the averaging period. In the event of short-term substance release, for example abnormal incident calculations, it should similarly be noted that the resulting immission values do not describe a real discrete situation, but only an ensemble mean by reference to the given meteorological conditions.