Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 120

4.4.1 Momentum Flux

K- and X-band RADARs have been used first to obtain turbulent vertical fluxes of
horizontal momentum (Kropfli 1986 ). For instance, he used insects and seeds in a
summertime convective boundary layer as tracers of air motion. He similarly used
these RADARs to observe the momentum flux in a non-precipitating cloud.
Vertical profiles of turbulence and momentum fluxes have been derived from
ground-based remote sensing with high-resolution pulsed Doppler LIDAR instru-
ments for the first time by Eberhard et al. ( 1989 ). Ubiquitous small aerosol particles
serve as tracers for the air motion. The LIDAR performs conical scans to detect all
three components of the wind vector. The valuation is then done in three steps. First,
the mean wind components are determined. Then, the variance of the measured
radial velocities is determined as a function of azimuth. Finally, using partial Fourier
decomposition, turbulence parameters are extracted from the azimuth-dependent
variance under the assumption of horizontally homogeneous statistics of the turbu-
lence (Eberhard et al. 1989 ). Figure 4.28 shows an example with an exponentially
decreasing momentum flux with height in a convective boundary layer. The flux
nearly vanishes at the top of the CBL. The s-shape near the top of the CBL is not
reliable (see error bar). The vertical resolution of the shown flux profile is about
150 m.
A different method tries to derive momentum fluxes from SODAR measure-
ments. This method is based on the assumption that there is in the lower part of
the neutr ally stratified boundary layer a simple relationship between the momentum
flux u w and the variance of vertical wind component

w (Kouznetsov et al., 2004 ):

σ

2

u w =−

0.77 σ

w .

(4.15)

Fig. 4.28 One-hour mean
momentum flux profile
compared to aircraft
measurements (diamonds).
From Eberhard et al. ( 1989 )