Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 65

Fig. 3.16 Scintillometer with
emitting device in the
background and receiving
device in the foreground
(Courtesy of Scintec AG,
Germany)

e.g. Kleissl et al. ( 2008 ) for a LAS intercomparison study). The first operate with
a thin laser beam, the latter with a wide light beam with a diameter of 10-20 cm,
which is either produced by optical widening of a normal light beam with a diffu-
sor or by a larger array of light-emitting diodes (LED). The more modern method
with the LEDs makes it easier to produce a uniform light intensity over the whole
beam width. The advantage of LAS compared to SAS is that LAS are not so much
affected by a saturation of the signal along the path (for saturation effects see, e.g.
Kohsiek et al. 2006 ). On the other hand, due to the large beam diameter, LAS in con-
trast to SAS cannot be used to determine the inner turbulence length scale, which
is in the order of 10 mm and is a direct measure for the dissipation rate turbulent
kinetic energy. While SAS and LAS are most sensitive to temperature fluctuations,
microwave scintillometer (MWS) also register scintillation from water vapour fluc-
tuations so that the combination of MWS and LAS can be used to derive latent heat
fluxes in the surface layer (see, e.g. Beyrich et al. 2005 ).
The path length can be chosen from between several hundreds of metres to sev-
eral kilometres, depending on the power of the emitter. The measurement yields the
structural parameter C n 2 ( n

refractive index) related to the turbulence intensity of
the air. This parameter may be used to estimate the magnitude (but not the direction)
of vertical turbulent fluxes (see, e.g. Thiermann and Grassl ( 1992 ), or Beyrich et al.
( 2002 )). A scintillometer must be placed in the atmospheric surface layer where
Monin-Obukhov similarity theory applies. The scintillometer beam height and set-
up distance have to be carefully chosen, such that the instrument operates in the
weak scattering regime, where the structure parameter of the refractive index C n 2
is small enough that it can be derived from the variance of signal intensity I from
first-order scattering theory (Clifford et al. 1974 ).
Some scintillometers, called crosswind scintillometers, operate with two parallel
beams in the horizontal (Lawrence et al. 1972 ). These instruments can estimate
the cross-wind speed from the time-lagged cross-variance between the signals of

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