PhD Thesis, 2007 - University College Cork

Chapter 3
Materials and Methods
accelerates upwards creating turbulence in a process called positive buoyancy.
When an air parcel has a higher density than the surrounding air (due to lower
temperature or humidity), it accelerates downwards, suppressing turbulence in a
process called negative buoyancy.
3.3.4 Potential temperature and atmospheric conditions
Potential temperature is the temperature that an air parcel would have if brought
adiabatically to the pressure at the 1000 mb level (Kaimal & Finnigan, 1994). The
vertical gradient of temperature, also called the temperature profile, defines
buoyancy and plays a fundamental role in describing the motion of air parcels,
through the definition of atmospheric conditions: neutral, unstable and stable
atmospheric conditions (Stull, 1988).
- In neutral conditions the vertical gradient of the potential temperature is zero, thus
there is no variation of potential temperature with height. Parcels of air displaced
up and down adiabatically maintain exactly the same density as the surrounding air,
experiencing no net buoyancy effects. Wind speed increases according to the
logarithmic wind profile. Neutral stability conditions are often transitory.
- Unstable conditions generally occur during the day when the soil surface is heated
by solar radiation and the layer of air just above it is then heated by the ground
through irradiance, inducing a thermal mixing. In these conditions, the potential
temperature decreases with height; the air temperature thus decreases vertically
faster than the adiabatic lapse rate. Parcels of air displaced up will be warmer than
surrounding air and continue to rise, generating a condition of instability or mixed
convection.
- Stable conditions generally occur during the night when air immediately above
the soil cools due to proximity of the cold soil surface. In these conditions, the
potential temperature increases with height because air close to the surface is cooler
than above; the air temperature thus decreases vertically slower than the adiabatic
lapse rate. Parcels of air displaced up will be cooler than surroundings and will
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