Download Note 110: Temperature inversions within ADMS

Note 110: Temperature inversions within ADMS
Keywords: ADMS, ADMS-Urban, ADMS-Roads,
inversions, meteorology
ADMS-Airport,
temperature
This note describes how the ADMS models take account of temperature inversions. Within the
atmosphere, the temperature typically decreases with height. When instead the temperature increases
with height, this is called a temperature inversion; in such an inversion, the air is stable, i.e. it inhibits
vertical motion. In this helpdesk note, we discuss the case of an inversion near the ground, which
typically occurs when there is strong surface cooling, e.g. on a clear night. Another case, sometimes
referred to as a capping inversion, occurs at the top of the boundary layer, this case is briefly
discussed at the end of the note.
Stable conditions
An important measure for describing the state of the atmosphere is the potential temperature, a
measure of temperature adjusted to remove the effect that changing the pressure has on the
temperature. Increasing potential temperature with height signifies a decrease in density with height,
whilst a decrease in potential temperature with height an increase in density with height. Figure 1
shows plots from the ADMS meteorological processor of the variation of temperature and potential
temperature with height in the bottom 100 m of the atmosphere, for a range of different atmospheric
stabilities.
Figure 1 – Temperature (solid line) and potential temperature (dashed line) varying with height for two
stable (left), one neutral (centre) and one convective (right) atmospheric conditions, as calculated by
the ADMS meteorological processor
As we can see from the plot, in stable conditions the potential temperature increases with height
(density decreases), whereas the temperature initially increases but then decreases with height well
away from the ground. In neutral conditions, the potential temperature is constant with height, with the
temperature decreasing with height. In convective conditions, both the potential temperature and
temperature decrease with height.
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Note 110: Temperature inversions within ADMS
In the cases where the potential temperature increases with height, the air is statically stable and thus
vertical motion within the atmosphere is reduced. This can result in pollutants being trapped near
ground level due to reduced vertical mixing. The ADMS models always predict an increase in
potential temperature with height when there is surface cooling. Further details of the profiles used by
the ADMS models are given in the Boundary Layer Structure Technical Specification document
(P09/01).
Both the temperature and potential temperature vertical profiles, along with those of other boundary
layer properties, can be output using the Boundary Layer Profile output option. This option outputs
various boundary layer properties, including wind speed and temperature, at a variety of heights.
Please refer to the appropriate model User Guide for more details on using this option.
Complex terrain
The stability of the atmosphere near the ground also plays an important role in hilly terrain. If the
atmosphere is stable then flow is more likely to be around hills than over them. The FLOWSTAR
model, which is used within the ADMS models to calculate the flow field due to complex terrain, has
specific algorithms to calculate these effects. More details of this are given in the Complex Terrain
Module Technical Specification (P14/01).
During very low wind stable conditions in hilly terrain, horizontal gradients in density can cause
katabatic (downslope) winds, which may influence the background flow in deep valleys. These effects
are not specifically accounted for in ADMS; please contact CERC to discuss specific cases.
Capping inversions
Another form of temperature inversion which can play an important role within dispersion calculations
is when a temperature inversion occurs at the top of the boundary layer; this is sometimes called a
capping inversion. In these conditions, the inversion acts as a cap to the boundary layer and makes it
difficult for material to pass out of the boundary layer.
ADMS models these effects by including extra terms in the plume concentration algorithms to allow for
the reflection of material below the boundary layer top. Within the ADMS models, such inversions are
taken to be present at the top of the boundary layer in all convective and neutral conditions. In stable
conditions, an inversion may or may not be present. To determine whether or not an inversion is
present in stable conditions, you can look in the processed section of the .MOP file at the
DELTATHETA column; if this value is greater than 0, then the model has calculated a temperature
jump at the top of the boundary layer and therefore an inversion.
Plumes can penetrate the boundary layer if they have sufficient plume rise, in which case the inversion
may trap this material above the boundary layer. The fraction of the plume penetrating the inversion
layer can be obtained from the line plotting output.
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