Application of remote sensing techniques to the PBL has started to realize some
of its promise, although rigorous comparisons with in-situ data are still
needed.
Even the fair-weather boundary layer responds well to remote
sensing because radars ``see'' refractive-index fluctuations and lidars
``see'' tiny particles in the air. Momentum fields and statistics have been
obtained in several ways. Gal-Chen et al.
[1992] used Doppler lidar to obtain
spectra and turbulence statistics that compared fairly well with those
from aircraft and tower data. Data gathered by single Doppler radars doing
360
scans were used to estimate
mean and turbulence statistics
( Xu and Gal-Chen 1993) and the mesoscale flow field
( Xu et al. 1994). The momentum
fluxes from radar were obtained from
fluctuations of the radial velocity
around circles centered at
the radar (the so-called velocity-azimuth display or VAD
technique); fluxes by motions too small to
be resolved by the radar were estimated from pulse-volume (grid-volume)
Doppler spectra. The
mesoscale fields were found by following the motion of
reflectivity features using simple adjoint techniques. Ground-based 915-MHz
wind-profilers employing three or five stationary beams
looking at angles close to vertical have
for several years proved useful in
obtaining mean winds, as illustrated by Martner et al. [1993];
more recently they have been used for estimations of
momentum fluxes (e.g. Angevine et al. 1993).
However, ground-clutter contamination and scatter can
present problems for both mean and fluctuating winds.
Atmospheric thermodynamic structure can be determined through exploiting the refractive index dependence on temperature and humidity, and through use of the radio-acoustic sounding (RASS) technique, in which the virtual temperature is estimated from the speed of sound waves, tracked using a collocated wind profiler. Fairall [1991] showed that tropical or mid-latitude marine boundary layers produced radar reflectivity mainly through humidity fluctuations, while the reflectivity over warm land was more from temperature fluctuations. White et al. [1991] used this information to determine the humidity structure in and above a marine boundary layer. Mean temperatures from RASS agree reasonably well with radiosonde data ( Martner et al. 1993), and virtual temperature fluxes have been successfully obtained from combining collocated profiler and RASS measurements ( Angevine et al. 1993), albeit with considerable scatter. Parsons et al. [1994] used RASS to show how the virtual temperature profile evolved as a marine boundary layer was reestablished after precipitation.