Introduction - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 2

measurements (Hellmann 1915 , Peppler 1921 ) and mounted cup anemometers in
several heights at these towers. The new database made scientists think about the
shape of the vertical wind profile. Hellmann and Peppler empirically derived from
their measurements that the wind speed is proportional to the fourth root of the
height above ground near the surface and to fifth root in higher levels between 16
and 124 m. Apart from this, both researchers noted that the diurnal variation of the
wind speed differs between lower heights (having a daytime maximum in layers
lower than about 50 m) and upper heights (exhibiting a nocturnal maximum).
Seminal theoretical papers, which originated shortly afterwards, were those of
Prandtl ( 1925 , 1932 ) where he introduced the mixing length concept for the descrip-
tion of boundary layer turbulence and for the first time formulated the logarithmic
law for the vertical wind profile near the surface. Wind observations to heights of
about 500 m were made at about the same time by Mildner ( 1932 ) in 1931, near
Leipzig. He observed the ascents of 28 pilot balloons with two theodolites. His
measurements became later famous as the “Leipzig wind profile” due to the evalua-
tions of Lettau ( 1950 ) who re-examined this data set, and due to the formulation of
the maximum mixing length in the ABL by Blackadar ( 1962 ), which is again based
on Lettau's re-examination.
Similarly important were the papers of Obukhov ( 1946 ), Monin and Obukhov
( 1954 ), and Kazanski and Monin ( 1960 ), who introduced a similarity theory which
is now known as the Monin Obukhov similarity theory (MOST). MOST describes
the vertical profiles of wind and turbulent fluxes in the surface layer of the
atmosphere, which is often called Prandtl layer. The validity of the MOST was
investigated and tested in several field experiments in the 1960s and 1970s. The
most famous experiments were the Wangara experiment in July and August 1967
over flat terrain mainly covered with sparse grass in Australia (Clarke et al. 1971 )
and the Kansas experiment in a wheat-farming country in summer 1968 in the
United States (Izumi 1971 ). The evaluations of wind and flux data from mast mea-
surements during these two experiments (16 m with the Wangara experiment and
32 m with the Kansas experiment) laid the foundations for the Prandtl-layer pro-
file laws described in Chapter 2 . Today, these profiles are commonly addressed as
Dyer-Businger profiles (Businger et al. 1971 , Dyer 1974 ).
The part of boundary layer meteorology that concentrates on flux measurements
near the surface has become a special subject within boundary layer meteorology
called micrometeorology (Foken 2008 ). Micrometeorology will be no subject in
this topic, because this topic focuses on the overall vertical structure of the boundary
layer. Therefore, we will address measurement techniques in the remaining sections
of this introduction, which are suited to investigate and monitor the whole depth of
the boundary layer.
1.2 Short History of Upper-Air Measurements
Until the beginning of the renaissance, people were just able to sense what was in
their direct neighbourhood at the Earth surface. The only possibility to learn some-
thing about higher layers of the atmosphere would have been to climb up in the