Radiosondes

1. Radiosondes • History of upper air measurements • Radiosondes (sensors, calibration, telemetry,multiplexing) • The Vaisala radiosondes • Special radiosondes (Ozone,atmospheric electricity, radioactivity)

2. EuropeanUpper Air stations • Operational stations launch 4 times per day

3. Requirements for upper air measurements: (1) To make accurate measurements of important atmospheric parameters (usually temperature, pressure and humidity) above the surface (2) To send this information back in as close to real-time as possible [(1) and (2) usually achieved by making a profile measurement, with a balloon carried instrument, but aircraft data is also used. (2) was once achieved using sondes which dropped something (the lizardsonde), or even exploded (the crackersonde) when a certain condition was fulfilled.]

4. Kite-carried sensors Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

5. Dines’ Kite Meteorograph Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

6. Dines’ balloon meteorograph (1907-1939) Silvered recording plate Hair humidity element Bimetallic strip (temperature) Aneroid capsule (pressure) Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

7. Radiosondes Small and compact radio transmitters allow the data obtained by a sensors carried on a balloon to be transmitted back to a receiving station. First successful radiosonde in the UK was the “Kew” Met Office sonde, in use from 1939. Improvements followed, and the “Mark2” was used from 1945 up until the 1960s Radiosondes require: Sensors (with an electrical output) Radiotelemetry (the data transfer system) Batteries (which will work at low temperatures) …balloons and parachutes…

8. Sensors The sensors must ultimately provide an electrical output, which can be turned into a frequency for the radio transmission. Mechanical sensors are coupled to transducers to achieve this. An example is a pressure sensor. Small mechanical variations in an aneroid capsule are used to move an iron core within an electrical inductor. The inductance changes, which leads to a change in an audio frequency, transmitted directly over the radio link. Other sensors used include Temperature: bimetallic strips (mechanical), resistance wire (electrical) Humidity: hair or gold beaters skin (mechanical), the “humicap” (electrical) Pressure: aneroid (mechanical or electrical)

9. Radio Telemetry A simple (carrier) radio wave requires a change (modulation) to be applied for information to be transmitted. This is usually either AM (amplitude modulation) or FM (frequency modulation) AM Transmitted signal “Information” FM

10. Multiplexing If more than one signal is required, and in a radiosonde, three different signals (humidity, temperature and pressure) are usually sent, the radio transmitter has to be switched between the three sensors in turn. This is called multiplexing. If the three signals are sufficiently different, or the order of switching is known, the individual signals can be recovered. Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961 Multiplexing switch

11. Mk2 MO radiosonde sensors Thermionic valves in radio transmitter receiver Multiplexing switch driven by wind mill Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

12. Calibration • Radiosonde sensors have to be calibrated if they are to produce accurate measurements over a range of conditions. • Calibration requires the sensor to be exposed to the full range of variation they will receive in service, but in a controlled environment. • The results of a calibration are used to construct a response function, which is an equation used to link the values found by a sensor to the magnitude of the parameter it is sensing. • The precise response function is unique to each sensor, and is used by the receiving computer to turn the data received into meaningful physical values. • The response functions are typically polynomial functions, with many coefficients to cover the range of values required. These coefficients are supplied with each radiosonde.

13. Calibration

14. Mk3 MO radiosonde

15. View of Mk3 sonde Thermometer (resistance wire) Polystyrene housing rotary multiplexing switch

16. Vaisala RS80 Radiosonde Temperature sensor Relative Humidity sensor (“humicap”) (Vaisala)

17. RS80 Specification (Vaisala)

18. “Windfinding” • If the location of a radiosonde is known, and recorded, its direction of motion can be determined from a set of the locations. • This allows the wind directions to be found, often referred to as “windfinding”. The profile of wind direction and strengths can therefore also be plotted. • The location of a radiosonde can be found by different methods: • Tracking it with radar • Using a Global Positioning System (GPS) receiver on the sonde to send back its location • Using the LORAN positioning system on the sonde to send back its location

19. GPS satellite system

20. RS90 radiosonde (Vaisala)

21. RS90 specification (Vaisala)

22. Special radiosondes Radiosondes can carry a variety of sensors, either instead of, or in addition to, the standard meteorological sensors for temperature, pressure, and humidity. Atmospheric properties which have been extensively with modified radiosondes include: Ozone Atmospheric electricity (the charges and electric fields within clouds and thunderstorms) Radioactivity Radiosondes for measuring the profile of ozone in the atmosphere are known as Ozonesondes.

23. “Kew-Oxford” Ozonesonde Contains an ozone cell in which an electrolytic reaction occurs, using potassium iodide. When ozone is passed through iodine is formed, which causes a small current to flow. From Brewer and Milford, Proc Roy Soc, 256 1960

24. Ozone profiles From Brewer and Milford, Proc Roy Soc, 256 1960

25. Atmospheric electricity radiosondes Electric field probe- measures change in voltage with height, “Potential Gradient” Haze layer Venkiteshawaran S.P. Measurement of the electrical potential gradient and conductivity by radiosonde at Poona, India, pp89-100 In Smith L.G. (1958) Recent advances in atmospheric electricity, Pergamon Press

26. Charge distribution in thunderclouds Stolzenburg et al. (1998)

27. In-cloud measurements • Balloon-carried disposable instruments have been designed at Reading to make in-cloud measuerements. These have: • Detected charged particles emitted in aircraft exhausts • Found thin and persistent, highly-charged layers Sensor responds to changes in charge Harrison R.G. Rev Sci Inst72, 6 pp2738-2741 (2001)

28. RS80 radioactivity sonde Carries Geiger tubes, sensitive to beta and gamma radioactivity, as well as standard temperature, pressure and humidity (Vaisala)