VHF radars similar to that at Davis are also known as Mesosphere-Stratosphere-Troposphere (MST) radars. Using a variety of techniques, they are able to detect reflections from turbulent structures in the clear air in the three atmospheric regions listed above.
Measurements of wind speed and direction
The strongest radar reflections that the radar detects come from the lower part of the atmosphere (the troposphere and the lower stratosphere). As a result, the radar is able to measure the wind speed and direction in this part of the atmosphere all year round.
Wind measurements rely on turbulent patches that move with the atmosphere. The pattern that the reflections from these patches make on the ground can be tracked across a set of spaced antennas and the wind speed and direction can be derived using the “full correlation analysis”. However, with the sophisticated antenna array that has been installed at Davis, it is also possible to make wind measurements by forming a narrow radar beam. The so-called “Doppler shift” gives the speed with which the air is moving toward or away from you along the beam and can be used to build up a three dimensional picture of the wind pattern above the radar.
The presence of enhanced reflections from the mesosphere during the summertime (Polar Mesosphere Summer Echo or PMSE) extends the reach of the radar into the upper part of the middle atmosphere. Both the spaced-antenna method and the Doppler method can be used to measure the wind speed and direction with these reflections.
VHF radars are also capable of detecting meteor trails in the upper mesosphere and lower thermosphere. These trails are formed when small particles of meteoric dust fall to earth and create short-lived ionised trails. For a short time, these trails are very good VHF radar reflectors. Once again, the movement of the trail either toward or away from the radar, along with the direction from which the reflection is coming, allow the wind pattern to be determined. The result is a measurement of the wind an amazing one hundred kilometres away. A smaller group of antennas, known as “the meteor array” is used for measurements from meteor trails.
The rate at which the meteor trail decays is related to the temperature in the region of the trail and can be used to infer the temperature in the mesosphere and lower thermosphere. Because the meteors can be detected all year round, these temperature measurements can be carried out in the summertime. The long daylight hours that are present during the summer period make optical measurements of temperature difficult or impossible.
The amount of momentum that is being transported vertically through the atmosphere by atmospheric gravity waves is an important parameter in determining the global circulation of the atmosphere. The narrow radar beams that can be formed using this radar can be configured to measure this parameter at the heights where it is possible to measure winds.