THE NERC MST RADAR FACILITY AT ABERYSTWYTH
Historical Background
The NERC MST Radar at Aberystwyth is the most powerful and versatile
wind-profiling instrument within the UK. It covers the nominal
altitude range of 2 - 20 km in the ST-mode and has the unique ability
to be operated in the M-mode, i.e. covering the 60 - 96 km altitude
range. Nevertheless the radar returns from the mesospheric altitude
region tend to be weak and only sporadically occurring and so for the
most part the radar has been run in the ST-mode. It began operations
in 1990 and was initially run for limited periods, of between a
few hours and a few days at a time, in one of a number of experimental
beam-pointing sequences.
The first regular observations began in 1993. ST-mode wind-profile data were collected on a 6-hourly basis on behalf of the Met Office; the sampling frequency increased to 3-hourly in late 1994. These runs typically made use of observations made in the vertical direction and at 6° off-vertical in two orthogonal azimuths which allowed for no redundancy in the derivation of the wind-profile data. In between the radar was either run in one of the experimental sequences or not at all.
Since late October 1997 the radar has been run on a nominally continuous basis. This has been almost exclusively in the ST-mode, at 300 m range resolution, and with only minor variations in the beam-pointing sequence. The latter typically includes observations made in the vertical direction, at 6° off-vertical in all four available azimuths, and at 4.2° off-vertical in one or two of the available azimuths; this allows for redundancy in the derivation of wind-profile data and has become a de facto standard-mode. The cycle time, and hence the time resolution of derived wind-profile data, has generally been less than 3 minutes. The resulting high-resolution dataset is extensive, nearly-continuous, quasi-uniform and of potential use to a wide variety of scientists; the Met Office wind-profile data are now derived from these observations rather than from specially scheduled runs.
The radar is still occasionally operated in "Experimental-Modes", e.g. covering the ST region at 150 m range resolution or covering the mesospheric altitude region. However, these observations tend to be interlaced with those of the Standard-Mode, rather than being made exclusively, resulting in a total cycle time of 4 - 5 minutes.
Scientific Considerations
The horizontal wind field does not, in general, change significantly
over time scales of less than half an hour. Although vertical
velocities can be expected to fluctuate over time scales down to the
order of the Brunt-Vaisala period, i.e. approximately 10 minutes in
the troposphere and 5 minutes in the lower stratosphere, in the case
of mountain lee wave activity reversals of vertical velocity direction
are more typically, but not exclusively, on time scales of half an
hour or more. In the case of convective activity, on the other hand,
reversals can take place over time scales of a few
minutes. Nevertheless, convective activity is comparatively rarely
apparent in the radar returns and so cycle times of up to 6 minutes
will suffice for most purposes.
Guidelines
The members of the NERC Atmospheric Radar Facilities Steering
Committee (NARFSC) have agreed on the following guidelines for future
operation of the NERC MST Radar taking the above factors into
consideration:
- Except when the NARFSC has given prior approval for the radar to
be operated in an Experimental-Mode, the radar should continue
to be operated continuously, and exclusively, in the
Standard-Mode.
- The Standard-Mode observations should cover the altitude range
2-20 km at 300 m range resolution and consist of sufficient beam
pointing directions to allow the three-dimensional wind vector
to be derived. At a minimum this should therefore consist of
observations made in the vertical direction and at 6°
off-vertical in two orthogonal azimuths. However, where possible
it should consist of observations made in the vertical direction
and at 6° off-vertical in all 4 available azimuths and,
ideally, additional observations should be made at 4.2°
off-vertical in two orthogonal azimuths; the latter allows for
the nominal beam pointing zenith angles to be corrected for the
effects of aspect sensitivity.
- For the purposes of improving the service, the NERC MST Radar
Facility Project Scientist, Principal Investigator and Site
Manager may make any changes that they see fit to the
Standard-Mode observation sequence, without seeking prior
approval from the NARFSC, so long as the total cycle time is
kept to within 6 minutes. They may also interrupt radar
operations at any time they see fit, for as long as is
necessary, and without prior permission from the NARFSC, in
order to carry out repairs or maintenance.
- If a user scientist requires the radar to be operated in an
Experimental-Mode, the appropriate Supporting Scientific Case
should be completed and returned to the NERC MST Radar Facility
Project Scientist for approval by the NARFSC. The form requires
detailed information on:
- the scientific/technical background to the project
- the aim of the project
- the relevance of, and justification for, use of the radar
- the observational requirements
- the period for which Experimental-Mode observations are required
- Whenever possible, the Experimental-Mode observations should be
interlaced with those of the Standard-Mode and the combined
cycle time should be kept to within 6 minutes. User scientists
should submit their application a MINIMUM of three months before
the start date of their proposed experiment.
- Applications for the radar to be operated in an
Experimental-Mode will be judged on the scientific merits
relevant to the project in question rather than on the potential
usefulness of the data to other users.
- So as to ensure that unexpected and potentially valuable opportunities are not missed, e.g. the flight of an instrumented aircraft over the radar site, the radar can be operated in an Experimental-Mode without prior approval from the NARFSC so long as it has the unanimous approval of the NERC MST Radar Project Scientist, Principal Investigator and Site Manager. Under such exceptional circumstances, the Experimental-Mode observations should not be extended for longer than is necessary and a scientific justification for the action should be submitted to the NARFSC as soon as is possible after the event.