Vertical Profiles Over Flat Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 23

heat and moisture concentrations than in the free atmosphere above. The detection
of the vertical profiles of the just mentioned atmospheric variables can thus help to
identify the vertical structure and extent of the ABL.
Three principal types of the ABL can be distinguished: (1) if heat input from
below dominates we find a convective boundary layer (CBL), (2) if the atmosphere
is cooled from below we find a stable boundary layer (SBL), and (3) if the heat flux
at the lower surface is vanishing and dynamical shear forces are dominating, we
find a neutral or dynamical boundary layer. We will start with the principal
description of the vertical structure of these three ABL types in the section below.
The vertical structure of these three ABL types additionally depends to a large
extent on the type and texture of the underlying surface. Its shape, roughness, albedo,
moisture content, heat emissivity, and heat capacity determine the momentum and
energy exchange between the surface and the atmosphere. The vertical extent of the
ABL is mainly determined by the generation of turbulent kinetic energy at and the
input of heat from the lower surface. The following chapters and sections will
present some of the most important characteristics of the ABL with respect to the
surface features as found, e.g., within the urban boundary layer (UBL, Sect. 3.7 )or
the marine boundary layer (MBL, Chap. 5 ) . In theory, these characteristics will only
appear if the flow is in equilibrium with the underlying surface. Each time when the
horizontal atmospheric flow crosses a boundary from one surface type or subtype to
the next a new internal boundary layer forms which will eventually—if no further
change in surface conditions takes place—reach a new equilibrium. Wind profiles
within internal boundary layers are presented in Sect. 3.5 .
The simplest structure of the ABL is found over flat, horizontally homogeneous
terrain with uniform soil type and land use and a uniform distribution of roughness
elements. Its vertical stratification in the roughness sublayer, constant-flux sub-
layer (Prandtl layer) and Ekman layer is depicted in Fig. 3.1 . The evolution of the
flat-terrain ABL is mainly determined by the diurnal variation of the energy bal-
ance of the Earth's surface. During daytime when the sun is heating the ground, a
convective boundary layer (CBL) is growing due to the input of heat from below
which generates thermal convection. The CBL is dominated by intense vertical
mixing and thus small vertical gradients. During night-time when the ground cools
due to the emission of long-wave radiation, a new nocturnal SBL forms near the
ground (see Fig. 3.2 ). The SBL is characterized by low turbulence intensity and
large vertical gradients. If clouds, wind, and precipitation override the influence of
short-wave and long-wave radiation, the ABL is even simpler and a neutral
boundary layer with nearly no diurnal variation forms. Its depth is then mainly
determined by the magnitude of the wind shear within it and by the advection of
warmer or colder air masses aloft with their own prescribed thermal stratification.
Apart from a viscous or laminar sublayer directly above the surface that is only
a few millimetres deep (too shallow in order to be shown in Fig. 3.1 ), we have two
main compartments of the ABL which must be distinguished by the balance of
forces within them: (1) the surface (Prandtl) layer or constant-flux layer and (2) the
Ekman layer. We will start with the well-known relations for the surface (Prandtl)
layer.