In the past few decades, VHF (Very High Frequency: 30–300 MHz) radars became potential tools not only for studying the dynamics of the atmosphere, but also for meteorological applications. They operate continuously and provide vertical profiles of the three components (U, V and W) of wind, in the troposphere and lower stratosphere, with high vertical range resolution. These radars detect echoes from irregularities of radio refractive index caused by fluctuations in temperature and humidity, and fluctuations in electron density (through Bragg scattering) and also from precipitating particles (through Rayleigh scattering). Small-scale motions with dimensions less than a few kilometres, in particular turbulence, can be studied extensively by VHF radar.
A network of windprofiling radars are established within Ontario and Quebec, with the intention to demonstrate the usefulness of such a network for weather forecasting and atmospheric science in the Canadian context. Below is a view of McGill VHF radar, located at J. S. Marshall Radar Observatory, during its construction.
The McGill VHF radar is operating at a frequency of 52 MHz with a peak power of 40kW and a 2-way beam halfwidth of 1.6°. Two techniques, namely, spaced antenna technique (for low level winds – below 1.5 km) and Doppler beam swinging technique (for high level winds), are used to compute the three components of the wind. A typical example of wind vectors observed with VHF radar for a period of three days i.e., from 01 March 2005 to 03 March 2005 is shown in the figure below.
The VHF band frequencies in the range 40 to 55 MHz will help avoid the problems related to signal contamination due to birds and insects, and will also reduce the contamination which can occur due to precipitation (during light to moderate precipitation). In addition, tropopause heights are reported on an hourly basis, and this has proved useful in studies of ozone exchange between the stratosphere and troposphere, and indeed for general studies of STE (stratosphere troposphere exchange). Turbulence strengths are also reported as functions of height and time, and are useful for aircraft safety as well as for studies of stratospheric-tropospheric exchanges.
The vertical profiles of DSD (drop size distribution) are very much crucial to know the dynamics and microphysics of precipitating systems. Since, vertical air motion affects the Doppler spectra of precipitating particles, information on the background vertical air motion is highly essential in retrieving the DSD accurately. The capability of VHF radars is to detect echoes, simultaneously, from both the radio refractive index irregularities through Bragg scattering and precipitation particles through Rayleigh scattering during moderate to heavy precipitation, will help in getting the vertical wind field during precipitating systems.
The antenna layout and associated polar diagram of McGill VHF radar is shown below. The antenna layout uses a cross-like structure for the antenna pattern, and requires more land, but produces a narrower main lobe of the polar diagram, and higher gain. It also produces stronger sidelobes, but these are not a serious contaminant as long as suitable software spectral processing is used. Despite the higher sidelobes, the higher gain of this system allows measurements to greater altitudes.
Campos, E.F. and W. Hocking, "Vertical motions observed with the new McGill VHF radar and associated dynamical characteristics",MST10, the 10th International Workshop on Technical and Scientific aspects of Mesosphere-Stratosphere-Troposphere Radars, 2004.
Hocking, W.K., P.A. Taylor, P.S. Argall, I. Zawadzki, F. Fabry, G. Mcbean, R. Sica, H. Hangan, G. Klaassen, J. Barron and R. Mercen, " A VHF wind profiler network in Ontario and Quebec, Canada: design details and capabilities", AMS conference, 2009.
Hocking, W.K., "Comparison of IDI and FCA analyses for Boundary Layer VHF wind measurements", Paper I.3-27, MST11, the 11th International Workshop on Technical and Scientific aspects of Mesosphere-Stratosphere-Troposphere Radars, Gadanki/Tirupati, India, Dec 11-15, 2006.