Analytical Description and Vertical Structure of the Atmospheric Boundary Layer - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 10

albedo, moisture content, heat emissivity, and heat capacity determine the momen-
tum and energy exchange between the surface and the atmosphere. The vertical
extend of the ABL types is mainly determined by the generation of turbulent
kinetic energy at and the input of heat from the lower surface. The then follow-
ing subsections will present some of the most important formings of the ABL types
with respect to the surface characteristics like the urban boundary layer (UBL) or
the marine boundary layer (MBL). In pure specification, these formings 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.
2.2 ABL Over Flat Terrain
The simplest structure of the ABL is found over flat, horizontally homogeneous ter-
rain with uniform soil type and land use and a uniform distribution of roughness
elements. Its vertical stratification in roughness sublayer, constant-flux sublayer
(Prandtl layer), and Ekman layer is depicted in Fig. 2.1 . This flat-terrain ABL is
mainly coined by the diurnal variation of the energy balance of the Earth's surface.
During daytime, when the sun is heating the ground, a CBL is growing due to the
input of heat from below, which generates thermal convection. 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. 2.2 ). If clouds, wind, and precipitation override
Fig. 2.1 Schematic diagram of the atmospheric boundary layer over flat homogeneous terrain.
Heights are given in terms of the roughness length, z 0 , and the boundary layer height, z i