TDC: Wind Profiler Database: General Description of Wind Profilers
A profiler is a Doppler radar with radar beams directed vertically and a few degrees off zenith. The fundamental difference between profilers and Doppler scanning radars is how the radar pulses are processed to produce the Doppler quantities. A profiler transmits thousands of low power pulses during a rather long dwell time (of the order 30 seconds) to produce a spectrum of Doppler velocities. In contrast, a Doppler scanning radar transmits only a few high power pulses with a short dwell time (of the order of milliseconds) and produces moments of the resolved volume. Detailed descriptions of profiler technology are in Carter et al. , and only a brief description and outline is included in this document.
The profilers operated by the Aeronomy Laboratory of NOAA transmit in one of 3 frequency bands, UHF (915 MHz), S-Band (2835 MHz), and VHF (50 MHz). The radar transmits energy and detects back-scattered energy from the refractivity turbulence (Bragg scattering) and from hard targets (Rayleigh scattering). At these operating frequencies, inhomogeneities of moisture and temperature in turbulent regions are the primary mechanisms for Bragg scattering. Biological targets (insects, birds, and bats) and hydrometeors are the primary distributed targets responsible for Rayleigh scattering. While precipitation is often defined to be the accumulation of hydrometeors reaching the surface, the profilers observe hydrometeors in the atmosphere. Clearly, not all hydrometeors observed aloft reach the surface as precipitation. The term hydrometeors is used primarily in this document, but the use of the term precipitation used here is tantamount to falling hydrometeors at any altitude.
Doppler Beam Swinging and RASS Modes
The profilers described here operate in two different modes: a Doppler Beam Swinging (DBS) mode, and a Radio Acoustic Sounding System (RASS) mode. Doppler Beam Swinging mode consists of alternating the radar beam direction and measuring the Doppler spectra in each of 3 or 5 radial directions. Doppler radars measure the motion along the radial direction, thus, they measure the radial component of the horizontal wind. From the different beam directions the horizontal wind is determined. The RASS mode consists of simultaneous transmission of acoustic and radio frequency energy to deduce the speed of sound as a function of altitude. The virtual temperature as a function of altitude is estimated from the measured speed of sound and vertical air motion. The profiler can not operate in the DBS and RASS modes simultaneously. The time multiplexing usually consists of 5 minutes of RASS and 25 minutes of DBS every 30 minutes. Not all profiler sites have RASS capabilities.
"Low" and "High" Modes
During the DBS mode, the profiler usually is programmed to sample the atmosphere with two different sensitivities. These modes are referred to in this document as the "low" and "high" modes, based on the length of the respective transmitted radar pulse lengths. The low mode pulse length is shorter than the high mode pulse length. Compared to the high mode, the low mode has a finer height resolution and a lower maximum height of useable data.