The atmospheric physics group maintains a number of radar facilities at our two field stations; Birdlings Flat and Scott Base.
The Birdlings Flat research site is situated near the southern end of Banks Peninsula on Kaitorete Spit about 45km south of Christchurch. The department has had a presence there since 1961 when John Gregory’s partial reflection radar was moved from the Department’s site at Rolleston. It was temporarily used for atmospheric soundings using rockets and has subsequently been known to locals as the rocket launching pad. The site has had a progression of RADARs developed to study atmospheric and astronomical phenomenon. Currently on site there are 3 main RADAR that probe various regions of the atmosphere from near ground level to over 100km altitude.
The Stratosphere Troposphere RADAR measures frontal systems, clear air turbulence, tropopause height and wind velocities from approximately 2km to 20km altitude. Currently it has a single 128 element phased array which is used for both pulse transmission and echo reception so measures vertical wind speeds only. Spaced receiving arrays are being constructed which will enable the system to measure both vertical and horizontal wind fields.
ST RADAR array
The location of the ST RADAR means that it can measure the turbulent effects of Canterbury's famous "nor' westerly" Fohn wind downstream from the Southern Alps. Understanding those effects will enable better weather forecasting of regional weather and meshes with the MF RADAR which can track the vertical propagating gravitational waves generated along with the 'nor wester.
Medium Frequency (MF) RADAR
The Birdlings Flat site MF RADAR was pioneered by Grahame Fraser. Fifty years on, Grahame's technique, called "partial reflection winds" using spaced receiver arrays, is still a cornerstone of data retrieval from MF RADAR stations world wide. Time delayed correlated echoes on the various spaced receiving antenna are used to measure the horizontal wind speed between 60 and 100km altitude. That region of the atmosphere is very important for global scale air mass transport. The group's data is one of the worlds longest climate records in that region, now known to be a good indicator region for climate change, and is used for climate modelling, forecasting and tracking ozone destructive air masses. A similar RADAR is situated at our sister site near Scott Base in Antarctica.
MF RADAR transmitter built by electronics workshop staff
AMOR Meteor RADAR
Advanced Meteor Orbit Radar. Uses a 26.2 MHz radar to measure the orientation in the atmosphere, and ionisation decay, of meteor trails caused by meteoroid grains which enter the Earth's at high speed and vapourise around 90km altitude. The observations provide information about the source of the dust grains (e.g. whether they be from inside or outside the solar system) and also about atmospheric winds and diffusion processes in the atmosphere.
AMOR has been supported by external grants - the Marsden Fund and the European Space Operations Centre (ESOC). These grants enable us to study the geometry of the solar system dust cloud. This work enables us to probe the dynamics and evolutionary processes of the material that formed the solar system - and provides data for models that allow the risks to space craft from high speed meteoric dust impacts to be quantified. It is also important as the solar system meteoroid dust cloud forms the interplanetary background against which recently discovered interstellar dust (and as recovered by the Stardust spacecraft) is detected. The AMOR system is highly sensitive with a limiting resolution of +14 magnitude in astronomical brightness terms. That magnitude equates to ionisation caused by micron sized dust particles travelling at tens of kilometres per second burning up in the upper atmosphere and is invisible to the unaided eye.
The AMOR dataset containing over 500,000 individual orbit measurements is a mine of information for scientists studying the formation and evolution of our solar system. Jack Baggaley, a world leading meteor scientist, used this radar discover stellar sources of meteoroid dust feeding our solar system (Taylor, A.D., Baggaley, W.J. and Steel, D.I. Discovery of interstellar dust entering the Earth's atmosphere. Nature v380, 1996: 323-25.)
One of the 500m long AMOR transmitter antennas
As part of a study into the effect of local weather on radar performance an inexpensive weather station was installed on site. It has turned out to be interesting to a wide variety of users from locals wanting to see how cold it got last night to fire authorities modelling wind patterns for burn offs. The data is updated every hour and is usually available here. Due to high telecommunication charges this service has been interrupted until a more cost effective solution of retrieving the data can be found.
Studying with the group can involve field work in breath taking Antarctica. The Antarctic region has always held an imposing awe about it and is a challenging environment. It has unique atmospheric properties e.g. the polar vortex, injection region of high energy solar particles along the Earth's field lines into the mesosphere, so is a scientifically dynamic and important area.
The department has had a presence at Scott Base since the IGY in 1958-59. Staff from the department have been involved in a variety of Ionospheric and Mesospheric studies. Our first research project was started by Dr John Gregory when he installed a medium-frequency radar at Scott Base in the 1958/1959 summer. It ran for five years, observing ionisation and turbulence in the upper atmosphere at altitudes of 40 -100 km. The particular ionisation of interest was that produced by energetic solar particles at high latitudes emitted by the active sun. The turbulence was associated with the strong winds of the polar vortex. There was also evidence of the re-distribution of ionisation due to the global atmospheric circulation.
Our next venture began in 1978/9 when Dr Andre vonBiel installed a polarimeter radar, initially to measure the ionisation from energetic particles. Grahame Fraser installed a wind measuring radar sharing the same transmitter during early summer 1982. That programme continues as K055, measuring ionisation and winds in the Antarctic mesosphere at altitudes of 40 -100 km. We also run a similar MF radar at Birdlings Flat. Because it is a specialist system the majority of the components were designed and built by the electronics workshop staff in the physics department.
For the layman here is a simple explanation of the full correlation technique used to determine atmospheric winds. Our system uses a high power pulse transmitter to launch a series of radio wave pulses vertically upwards into the atmosphere. Those pulses will travel, essentially unimpeded, until they reach regions where there is ionisation of the neutral atmosphere. Interactions with the charged particles (e.g. ions and free electrons) through a variety of processes may cause the pulse train to be absorbed, refracted or scattered back to the ground. The return "signal" amplitude is tiny in comparison with the transmitted pulses because absorption processes dominate.
Essentially, the region of the atmosphere causing the return echoes can be thought of as a poor reflector. Areas of localised ionisation, wave structure and motion of the "reflectors" blur and move the reflected image so what is returned to the ground is a travelling radio wave diffraction pattern. In visual terms it is much like seeing the shadow of a cloud, pushed by the wind, racing across the ground.
Our receiver site is at Arrival Heights, a few kilometres from the transmitter at Scott Base. Highly sensitive receivers are connected to an array of receiving antennas. A computer system monitors the signals on each of the receivers. As the diffraction pattern travels across the site it will be detected at different times on the various receivers. Wind velocities are calculated from these time differences and the scattering height from transmit to receive timing. To complicate things the pattern may change as it moves from one receiver to another. This is due to changes in the reflecting medium as it moves over the site. For this reason "full correlation analysis" is used to track the time varying signals.
The data set from the Antarctic MF radar is one of the longest climate records for the Antarctic region. Recent developments include storing all raw data for re-analysis and a near real-time data display on the atmospheric group pages. The plot below shows an example of the zonal (east-west) wind. The colour scale runs from 50m/s (red) to over -50m/s (blue). Up until day 27 there is a diurnal tide shown by the regular, near vertical, banding followed by a few days of relative calm finishing with a period (middle of day 30) where the wind changes direction from easterly to westerly and back again in just a few hours.
The atmosphere is always changing and modelling it is often difficult. Many of the global scale climate models used for current climate change studies do not model the Antarctic region well. The MF dataset is being used to ground truth models and provide boundary conditions for forecasting. If you are interested in collaborative research contact Adrian.
We also maintain an IPS 42 at Scott Base. This instrument, inherited from the DSIR break up in the late 1980's, continues the longest single scientific data record from the McMurdo region. Data from the ionosonde is used to predict radio wave propagation, provide communication forecasts and ionospheric studies. The data is freely available from the Ionospheric Prediction Service for a network of ionosondes around the globe.