How Profilers Work

From Dick Lind: 

Wind

These profilers are active low-power pulsed radars operating
at 915 MHz.  Packets of radar are sent vertically, and obliquely (two azimuth
directions, 90 deg apart).  Radar energy is backscattered from turbulent
eddies matching the Bragg scattering size (half the radar wavelength: for 915
MHz radars, this size is about 17 cm).  Since the scatterers are small, it is
assumed they are advected with the wind.  Doppler frequency shifts are
computed for each radar "beam" direction and converted to horizontal and
vertical winds.

Problems with winds

Variations in the coverage of these winds are caused by a
variety of things:  undesired radar reflections (clutter), lack of scatterers
(such as seen in laminar flow), deterioration of hardware, etc.

Radar is best backscattered from humidity fluctuations and water droplets, and
to a lesser degree by temperature fluctuations.  If the air is very dry, there
is less backscattered radar enery and the signal strength drops below the
internal noise level--making determinations of the Doppler shifts impossible.

Virtual temperature

As for the virtual temperature part of these profilers, artificial scatterers
are generated by sending acoustic noise (pressure waves) from sound sources.
These pressure waves induce an adiabatic compression/relation at the Bragg
scatter wavelength.  When air is compressed, it warms; when it decompresses
it cools.  This sound is strong enough to allow the vertically oriented beam
of the radar to receive enough signal strength to compute Doppler frequency
shifts--except that in this case, it is tracking the speed of sound as it
moves upward from the source(s).  Since the speed of sound allows computation
of the virtual temperature, this turns out to be a clever way of remotely
sensing temperatures aloft.  (Virtual temperatures are always within 3-4 deg
C of true temperature--with bone dry air, they are equal.)

Problems with virtual temperature

Variations in height coverage for Tv can be caused by:  strong winds (blowing
the sound out of the radar beam), degradation of sound sources over time,
or ambient temperature profiles that approach the adiabatic lapse rate (near neutral stability).  In
this latter case, the warming/cooling caused by the pressure waves are
hidden since the same slope (dT/dZ) exists in the atmosphere.  So, if you find
that Tv is cooling with height and the coverage decays, it is a reasonable
assumption that the cooling with height extends well above the highest
height reporting a valid Tv value.

The site at Point Arena is very windy--talk to Clive about that!  As such,
the equipment takes a beating and needs human intervention more frequently.
Also, Tv height coverage decreases quite noticably when winds
exceed 15-20 m/s.