Just off the back end of this brief spell of nice weather things start rolling from a teaching perspective. Today a small advance party are driving up to the Lochranza field centre which is at the north end of the Isle of Arran. We have a bit of a convey going, well I say convey, there are two vehicles on the road. Sam and I are in a luton van, doing the ‘white vanman’ thing and James and Dan are in the NCAS ambulance which will be used as a base for a lot of the measurements and data recording whilst in Lochranza.
We will rendezvous with Barbara on the ferry in Ardrossan for the short trip across the water to Broddick, which was home to Lord Kelvin for some time I learnt recently. After a major pitstop at the Coop in Broddick we then have a 45 minute drive awaiting us up to the north end of Arran.
We will be setting up for the annual atmospheric field course which begins in earnest tomorrow afternoon when the rest of the Leeds and Reading staff arrive with the students. In fact, by my reckoning, this will be the eleventh year we have visited Lochranza. The course has matured over time and it is now a joint activity between Leeds and our colleagues at the University of Reading. A couple of dozen undergraduates and a smattering of masters students here to gem up their field skills for the week. The course was originally a purely Leeds affair until a passing remark at a meal in Treviso during a research project with the UK 146 aircraft. I had to return early and mentioned why to Ellie (Highwood) who is from the University of Reading, they had recently had discussion about doing for field courses for their own students. she liked the sound of our field course and came along the following year to see what it was all about and then in 2006 the first joint field course took place. It works very nicely with the students being divided up into mixed groups, the Reading students tend to be much more ‘pure’ meteorologists whereas most of the Leeds are on our environmental science programme. The Leeds cohort have more experience doing fieldwork which results in a range of skills across the group, bringing an extra dimension to the team management when it comes to who does what. We have a variety of different objectives for them over the next few days, and the days are fairly long, but at the end the students hand in their workbooks happy in the knowledge that they have completed a module (or #OneInTheBag) before the start of term (this is an additional selling point of the course!)
Towers and Balloons……
We will be putting up a 10 metre flux mast in the field behind the centre and equip it with a multitude of measurements from simple temperature and wind speed sensors which are placed up the tower logarithmically to assess the drag effect of the surface on the wind speed through to fast response sonic anemometers which measure the turbulence at the top of the tower. Also at the top of the mast will be an instrument which measures water vapour and carbon dioxide (using infrared spectroscopy). This instrument makes a measurement every 50 milliseconds or 20 Hz and when these measurements are coupled to the sonic anemometers they can tell if there is an upward or downward flux of these two species and the magnitude of them. As the sun comes up in the morning and the grass below starts to photosynthesise carbon dioxide so any air rising past the sensor is likely to have a lower concentration of carbon dioxide and from this we can derive a flux of carbon dioxide, to ascertain this we need to know the vertical wind velocity too hence these two instruments are co-located on the mast whilst being careful not to shield each other. We also install a smaller mast with radiometers to measure up and down welling solar radiation, these are used to measure what is known as the surface energy balance. These towers make measurements close to the surface but we also launch radiosondes which are fastened to a balloon and sent off daily up into the heavens. These provide measurements of temperature, relative humidity, pressure and GPS position every couple of seconds up to around 20 km. Whilst we can derive a reasonable measure of height from the pressure data, the GPS provides better number for altitude. We can also use the GPS position to indicate wind direction and speed by comparing consecutive positions we can derive the winds. We also launch smaller un-instrumented balloons and track them using a pair of theodolites. By applying a bunch of trigonometric functions we can use these to also measure the wind speed, this activity is usually quite a challenge for the groups reading the elevation and azimuth from a vernier scale whilst someone turns handles to keep the telescope pointing at the balloon this all requires good teamwork and communication skills. During the week we will launch one radiosonde which will be accompanied by a second sensor package to measure ozone. The ozonesonde adds extra weight to the launch payload so we need a bigger balloon (echoes of ‘needing a bigger boat’ always spring to mind, but maybe that’s just me). The balloon for the ozonesonde is around two metres is diameter at launch and I always challenge the students to guess the maximum altitude, which is more like 30 km, and how big it will be at that height. They all know that there is a decrease in air pressure as one ascends through the atmosphere, but are generally quite surprised to discover that the balloon actually approaches ten metres in diameter. At this sort of altitude the pressure is less that 0.5% of what it is at the surface, strangely a balloon in a field on a scottish island makes this easier to comprehend. As it happens the latex of the balloon has more capacity to stretch but the temperature makes the rubber more ‘brittle” and any clouds encountered on the ascent may result in the balloon building up a coating of rime ice which makes bursting much more likely.
As an intermediate measurement of the verticality (is that really a word??) of the atmosphere we also do a walk up to the summit of Goat Fell, the summit of which is nearly 900 metres above sea level. At the top the pressure decreases to a little over 90% of what it is at the surface, this is more than enough for the students to investigate the hydrostatic relationship which considers the rate of change of pressure as one ascends through the atmosphere. It is quite an eye opener for many students that this relatively short walk takes them through nearly a tenth of all the molecules in the atmospheric column. In fact it is quite amazing that during this ascent the fall the the number of molecules of gas per unit volume is the same as is contained in the atmosphere above the tropopause!