Additionally to the synoptic weather patterns, the meteorology of the PBL is strongly influenced
by the surface characteristics and turbulence structure. Therefore, we will in this introductory
section shortly summarise qualitative aspects the different processes influencing the PBL
conditions additionally to a more detailed discussion of the statistical methods used in general
and which we shall use throughout the text to describe the characteristics of the PBL. Both with
respect to mean characteristics, variability and fluxes the PBL is dominated by turbulent motion.
Therefore it is appropriate firstly to consider what we should understand with turbulence, a
subject that has filled many pages in the scientific literature.
Here we just notice that motions of systems that can be described by the nonlinear fluid
equations tend to show strongly varying stochastic components, the turbulence, as well as more
smooth and predictable characteristics. Turbulence can occur on many scales of motion and be
described by as either two-dimensional motion or three-dimensional motion.
In the PBL, the wind speed as well as temperature and humidity, and indeed all atmospheric
variables, show this stochastic behaviour on all spatial and temporal scales of variation. In
figures 2.1, this is illustrated by a measured time series of the wind speed observed through
different time windows. The following figures 2.2-26 all illustrate different processes and scales
of variability within the PBL.
While the motion in the PBL can vary on virtually all scales, the processes within the PBL that
create so called PBL turbulence occur most on time scales of the order of and less than one
hour, with associated spatial scales. This PBL turbulence is three dimensional and therefore
can carry most of the vertical fluxes that is essential for the coupling between the atmosphere
and the surface.
On these time scales the main mechanism for producing turbulence is the vertical gradient of
the mean wind. In figure 2.2 we show typical vertical variations of wind speed, humidity, and
temperature between their surface values and values at the top and above the PBL.
Temperature and humidity can both increase and decrease with height, depending on whether
their surface values or values in the free atmosphere are the larger.
However, the wind speed will always increase with height from zero at the ground to its value in
the free atmosphere just above the PBL. The vertical wind shear gives rise to overturning of the
air, producing the turbulence (Tennekes and Lumley, 1982). This provides a formidable
mechanism for carrying the vertical fluxes compared to the molecular transport mechanism that
would have been an alternative. For example, a temperature gradient of 2 K across the lowest
10 meter height with a wind speed at 5 m/s give rise to a heat flux of about 0.5 mK/s (or 600
W/m2). If the flux had to be carried by molecular diffusion only, the result would be 4•10-6 mK/s
(or 5 mW/m2) only.
The temperature structure of the PBL strongly influences the turbulence production through its
influence on the density of the air. If the air is warmer and thereby lighter close to the ground, it
will enhance the production; if it is cooler at the ground the production will be reduced. To a
lesser extent the humidity has similar, although smaller effect because also admixture of water
vapour changes the density of the air.