Introduction - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 1

Chapter 1
Introduction
The atmospheric boundary layer (ABL) is that layer of the atmosphere which has
direct influence on all living species on Earth. On the other hand, it is the layer
which links the Earth's atmosphere to the remaining compartments of the system
Earth. Therefore, a detailed knowledge on all features of this layer is mandatory for a
physical understanding of the energy and mass fluxes and cycles in the system Earth
and for the assessment of the living conditions in the biosphere. These energy and
mass cycles in the system Earth are often addressed as biogeochemical cycles today.
Thus, ABL research not only forms part of atmospheric sciences and meteorology
but also has become an essential part of Earth system research and Global Change
research.
The overall depth of the ABL can vary considerable from a few tens of metres
to several kilometres. So, in any case, surface observations and measurements are
only able to detect the state of the surface layer which forms the lowest part of the
ABL. The larger part of the ABL cannot be probed directly without bringing in situ
instruments into this part of the layer by mounting them on masts, balloons, or air-
craft. The only other choice is indirect measurements by remote-sensing methods,
either from the ground or from satellites. But it is only ground-based remote sensing
that offers sufficient vertical resolution and temporal coverage for detailed bound-
ary layer studies. Therefore, remote probing of the atmospheric boundary layer by
ground-based remote sensing is the subject of this topic.
1.1 The Beginnings of Boundary Layer Research
Specialized research on the atmospheric boundary layer started with surface and
tower observations of vertical wind profiles and the transfer of laboratory experience
on wall flows from fluid dynamics to meteorology in the first half of the twentieth
century.
An unprecedented opportunity for probing the ABL away from the very surface
became available about a hundred years ago, when wireless broadcasting emerged
as a new medium in the first two decades of the twentieth century. The necessary
infrastructure required the erection of large towers to disseminate the radio signals.
Scientists rapidly detected these towers as a good platform to perform wind profile