When most people think about radar observations of the atmosphere, they think about weather radar. Most are familiar with radar images showing the development and movement of storm systems. These radars scan horizontally and are specially designed to be capable of detecting backscattered signals from precipitation particles. What fewer people know is that there is another type of radar that is also routinely used to monitor our atmosphere. The so-called wind profilers receive backscattered energy from subtle fluctuations in the atmospheric pressure and humidity. As an analogy, think about the scintillations that can be seen to occur over a hot paved surface. Wind profilers are used to monitor the wind field and turbulence intensity directly above the radar. The height coverage depends upon the atmospheric conditions and the design of the radar.
The National Oceanic and Atmospheric Administration (NOAA) operates a network of wind profilers located across the United States. The NOAA Profiler Network (NPN) (http://www.profiler.noaa.gov/npn/) consists of 35 installations, which are primarily located in the central portion of the country (See Figure 1). A description of the NPN can be found at http://www.profiler.noaa.gov/npn/aboutNpnProfilers.jsp and real-time and archived data are available at http://www.profiler.noaa.gov/npn/profiler.jsp.
Figure 1 - Diagram showing the locations of the wind profilers in the NPN. The radar located at KAEFS is indicated as PRCO2.
One of the NOAA profilers is located at KAEFS and is referred to as the Purcell site. It operates at a frequency of 404 MHz (wavelength of 67 cm) and includes Radio Acoustic Sounding System (RASS) capabilities. RASS is used to obtain height profiles of the atmospheric temperature. A photo of the radar is shown in Figure 2.
Figure 2 – Picture of the NPN radar located at KAEFS.
In addition to the NPN radar, the School of Meteorology at the University of Oklahoma operates a smaller wind profiler used to study the atmospheric boundary layer. The so-called Boundary Layer Radar (BLR) uses a somewhat higher frequency of 915 MHz (wavelength of 33 cm), has a smaller antenna, and less transmit power than the NPN radars. The KAEFS BLR can be seen in Figure 3.
Figure 3 – Installation of the KAEFS Boundary Layer Radar
Wind profilers are also used to study precipitation. Although not as sensitive to backscatter from precipitation as weather radars, they provide valuable information on the vertical structure of precipitation and its evolution with height. An example of precipitation data collected with the BLR can be found in Figure 4. It shows the signal-to-noise ratio (SNR) as a function of time and height. The SNR is a measure of the amount of backscattered power from the precipitation – the more intense the precipitation, the larger the SNR. Also shown is the vertical velocity measured with the BLR. The vertical velocity represents the combined effects of the precipitation fall speed and the vertical air motion. Here, positive values indicate motion towards the radar.
Figure 4 – Plots of the signal-to-noise ratio (SNR) and vertical velocity associated with a period of precipitation observed using a Boundary Layer Radar.