Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 33

Chapter 3
Basic Principles of Surface-Based Remote
Sensing

3.1 Introduction

In situ measurements at the ground or from masts, tethered balloons, radiosondes,
or aircraft offer only limited access to the atmospheric boundary layer. They are
restricted either to certain sites or to short time intervals or both. The wish to over-
come these limitations has led to the development of remote-sensing techniques that
allow the continuous probing of the whole depth of the atmospheric boundary layer
with sufficient vertical resolution. Two main lines of development have been fol-
lowed in the past 60 years for this purpose: active and passive ground-based remote
sensing. Observations from satellites do not offer a sufficient vertical resolution for
retrievals from the atmospheric boundary layer.
Most of the ground-based remote-sensing techniques presently used in boundary
layer meteorology are active techniques, which are based on the controlled emis-
sion of some radiation and the analysis of a small backscattered fraction of this
radiation. We will concentrate on these techniques here. A general overview of all
meteorological measurement techniques can be found in Emeis ( 2010 ).
There are three relevant wavelength bands within which active remote sensing
of the atmosphere based on backscatter is possible: centimetre to metre waves
backscattered from refractive index gradients due to atmospheric turbulence, mil-
limetre to centimetre waves backscattered from falling or soaring water droplets, and
micrometer waves backscattered from aerosol particles and gas molecules (Fig. 3.1 ).
In any case optimal backscatter is received if the scattering objects in the atmosphere
have about half the size of the wavelength

of the emitted optical, electromagnetic,
or acoustic radiation (Bragg condition). The Bragg condition that is essential for an
optimal atmospheric remote sensing has been described for the first time in 1913
by the British physicists William Henry Bragg (1862-1942) and William Lawrence
Bragg (1890-1971), father and son, a few years after returning from a more than 20
year stay in Adelaide, Australia. They referred to the scattering of x-rays at crystals
(Bragg and Bragg 1913 ). If the Bragg condition is met, backscatters from different
centres of action have a maximum positive interference.
Further we have to differentiate between elastic (Mie and Rayleigh) and inelas-
tic (Raman) scattering. Mie scattering occurs if the size of the scattering particles
(molecules, aerosol particles, droplets, insects) has about the same wavelength as

λ