Pic. 1: Wind vector profiles during a persistent elevated Inversion at a site in Northern Germany
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Pic. 2: Temperature profiles during a persistent elevated inversion at a site in northern Germany.
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Pic. 1 and Pic. 2 show wind and temperature measurements for the duration of 24 hours after the end of a 3 day high pressure system period with high fog. During this time an elevated inversion had been developed, which remained due to the less incoming solar radiation over 3 days. The measurements were performed at the facilities of METEK in Elmshorn, Germany, with a mobile Doppler SODAR PCS.2000-24 and a 1290 MHz RASS.

The system was adjusted to 10 minutes averages and a height resolution of 20m.

The colour coding of picture 2 shows the temperature distribution with height and time. The noticeable jump in the colours indicates the inversion with a temperature increase of 8.4 Kelvin in a vertical distance of 60m. Until the end of the day the warm air of the higher altitudes (above the inversion) was replaced by colder air. On the ground the temperature increased in this time period of about 4 Kelvin as a result of mixing processes in lower altitudes. Also the strong vertical gradients disappeared.

The corresponding wind field has below the inversion weak wind velocities. Above the inversion are northerly and in comparison stronger wind velocities obvious. With the change of the air masses at the end of the day a northwest flow starts and decreases with height according to the descending inversion. During this process strong differences between the wind values of the higher and low altitudes occur.

This example shows, that even in regions with less orographie and relative strong wind velocities complicate vertical structures develop in the wind and temperature field with a duration of several days. To point this out, two sources of emission with different heights were drawn into the plot. Depending whether the height the emission is under and above the inversion different concentrations of the pollutants on the ground occur. On the basis of the usually used Gaussian model for the calculation of the dispersion of air pollutants these situations cannot be fully reproduced. At least the knowledge of the inversion height and thickness will deliver a measure for the quality of the results of the numerical modelling. Numerical models on the basis of the particle dispersion would bring an enormous improvement for the evaluation of dispersion situations.

During the whole described period, the parallel measurements with an Ultrasonic Anemometer brought for the turbulence parameters only small sensible heat fluxes. The maximum was recorded at 15 W/m². That leads to the insight, that there was no way to get rid of the inversion by thermodynamic mixing. Only the replacement of the air masses was able to change the atmospheric situation. Models based on the particle motion take also the turbulent fluxes into account and would significantly improve the prediction of dispersion processes.

The picture 3 shows for a three days period the diurnal change between a ground based inversion during night times and a well mixed layer in the early afternoon hours. The nightime inversion was caused by the radiative cooling of the surface and was mixed away after the incoming radiation triggered the mixing by warming the surface. The whole weather situation was characterised by a height pressure weather system with partly high fog episodes.

Within the early afternoon hours the situation 1 in picture 3 represents a well mixed layer. Vertical mixing is more or less triggered by mechanical induced turbulence – temperature layer did not support or limit the vertical exchange of air masses. Higher wind speeds on the ground can be expected due to the mixing between lower and higher altitudes. Because of the increased mechanical induced turbulence pollutants will be transported away from the source of emission and there is a well mixing of air pollutants and clean air. In consequence also higher concentrations of emissions will be quick diluted.

After sunset a gradual cooling of the surface and the near surface air masses starts. The temperature of the higher air remains nearly unchanged. This situation 2 is indicated in the picture 3, where the height of the emission is above the developing inversion. Air pollutants released in higher altitudes are limited in terms of downward mixing, which gives a positive effect for the concentrations of pollutants on the ground. But within the layers in the release height (above the surface layers), there will be an increased concentration due to the lower mixing in the near surface layer. Additionally the higher layer are decoupled from the surface roughness which leads to higher wind speeds in these levels. On the one side, there is a faster transportation of the pollutants, but on the other side in remote sites higher concentrations occur. In case of horizontal broadening of the plume, this effect might be less important.