This file contains background information to help you use the Met Office Radiosonde data held at the BADC.
1.1 The Radiosonde
A radiosonde is a small, lightweight package of instruments, suspended from a helium filled balloon (see figure 1).
When the balloon is released, the apparatus rises through the atmosphere at approximately 5m/s. Measurements of pressure, temperature (and humidity for 1990-1998 European stations only) are taken at intervals during flight. Wind speed and direction can be determined from additional windfinding equipment. Information is transmitted to the surface as a radio signal. The ground station equipment receives the signal and processes the data.
The balloon bursts at a height of 20 to 30km and the radiosonde is returned safely to the surface by means of a small parachute.
1.2 The Radiosonde type used by the Met Office
The RS80 radiosonde, manufactured by the Finnish company Vaisala, has been routinely used in the UK since 1981. Powered by a water activated battery, the instrument takes measurements at approximately 1.3 second intervals during the ascent. Pressure, temperature (and humidity) are measured using three capacitative sensors. A schematic diagram of the layout of the RS80 is shown in figure 2.
(i) The Pressure Sensor
Atmospheric pressure is measured using the "Barocap" pressure sensor. This device is an aneroid capsule containing two capacitive transducer plates. The plates are secured to the steel alloy capsule membrane by supporting rods. The sensor is 35.5mm in diameter and weighs 5g.
The aneroid capsule, shown schematically in figure 3, expands or contracts as the air pressure changes. The flexible membrane moves the supporting rods, which in turn alter the separation of the capacitor plates. The capacitance across two parallel plates is inversely proportional to the plate separation. Thus a change in air pressure is detected by the variation in capacitance across the plates.
The sensor has the following measurement characteristics._________________________________ Measuring range 1060mb to 3mb Resolution 0.1mb Accuracy +/- 0.5mb _________________________________
(ii) The Temperature Sensor
Temperature is measured using the "Thermocap" temperature sensor. This device consists glass capsule, or radius 1.5mm and length 2.5mm, shown schematically in figure 4. The capsule contains a chip of ceramic dielectric, of dimensions 0.5 x 0.5 x 0.2mm, placed between two electrodes.
The capacitance across the electrodes is a function of temperature. Thus a change in air temperature is detected by the variation in capacitance across the electrodes.
The sensor has the following measurement characteristics.___________________________________________________ Measuring range +60C to -90C Resolution 0.1C Accuracy +/- 0.2C Lag < 2.5s (at 6ms-1 flow at 1000mb) ___________________________________________________
(iii) The Humidity Sensor
Humidity is measured using the "Humicap" humidity sensor. This device is a thin-film capacitor of dimensions 4 x 4 x 0.2mm. A polymer dielectric of 1 micron thickness is placed between two electrodes.
The capacitance across the electrodes is a function of the amount of water absorbed by the polymer dielectric. Thus a change in the humidity of the atmosphere is detected by the variation in capacitance across the electrodes.
The sensor has the following measurement characteristics._____________________________________________________ Measuring range 0% to 100% relative humidity (RH) Resolution 1% RH Accuracy +/- 2% RH Lag < 1s (at 6ms-1 flow at 1000mb) _____________________________________________________
The Vaisala RS80 Technical Specifications are summarized here .
1.3 Windfinding Equipment
Wind speed and direction are not directly measured by the radiosonde. These parameters are calculated from the position of the sonde at successive time intervals. The UK upper air network currently use two such methods of windfinding. Each method requires additional equipment.
(i) The LORAN-C Radio Navigation System
This system, in use at all but two UK stations, was originally established to aid navigation at sea. It uses a network of LOng RAnge Navigation beacons, which transmit radio signals at known frequencies. In addition to the sensors of the RS80 already described, the RS80L radiosonde carries a radio receiver to detect the LORAN signals.
The receiver measures the difference in time taken for the signals from two beacons of known position to reach the sonde. Such points of equal time difference form the loci of a set of rectangular hyperbolae. Signals are received from three pairs of beacons. The difference in the time of signal reception from each pair identifies a hyperbola, as shown in figure 5. The radiosonde is thus located at the intersection of these hyperbolae, a known distance from the fixed LORAN beacons. The wind speed and direction can then be calculated from the difference between successive positions of the sonde. These calculations are performed by the ground station equipment.
The LORAN-C method calculates the position of the radisonde with an accuracy of approximately +/- 300m. Wind speeds are calculated with an accuracy of +/- 1 to 2ms-1.
(ii) Tracking with Radar
The second method, in use at Larkhill and Aberporth, is to track the radiosonde with a windfinding radar. The RS80 must carry a radar reflection shield, as shown in figure 1, in order to reflect the beam emitted by the radar. The shield consists of three perpendicular intersecting planes made of metallised mesh.
The time for the beam to be returned to the radar provides the distance of the sonde from the radar. The radar also measures the sonde's angle of elevation and the bearing from true north. The sonde is thus located at a known distance, height and direction from the fixed radar. The wind speed and direction can then be calculated from the difference between successive positions of the sonde. These calculations are performed by the ground station equipment.
The windfinding radar method enables wind speeds to be calculated with an accuracy of approximately +/- 1ms-1 at a height of 7km. Height is accurate to within +/-40m.
1.4 Ground Station Equipment
The ground equipment of a radiosonde station varies with the type of sonde that is used but it must at least comprise a radio receiver and a means of interpreting the signals in terms of the meteorological quantities observed.
The Vaisala PC-Cora ground station is used by all of the UK upper air stations to recieve, record and process the measurements of pressure, temperature and humidity made by the radiosonde. It can be used to calculate wind speed and direction from either of the above windfinding methods.
(i) Storing and processing the data
(ii) Calculating dew-point temperature
(iii) Calculating humidity mixing ratio
(iv) Calculating height
(v) Calculating wind speed and direction
The data consists of vertical profiles of temperature, dew-point temperature, wind speed and wind direction, from the surface to pressures of approximately 20mb.
2.2 Spatial Coverage
- Horizontal Coverage
The BADC have data from stations worldwide. Currently, the global radiosonde network includes about 900 upper-air stations, and about two-thirds make observations twice daily (at 0000 and 1200 UTC). The network is predominantly land-based and favours the Northern Hemisphere.
- Vertical Coverage
A radiosonde is released from the surface and rises to between 20 and 30km before the balloon bursts. The flight of the instrument varies according to the strength of the wind and thus may not ascend strictly vertically. Nevertheless, when analysing weather systems on the synoptic scale, the profiles can be assumed to be vertical without significant error.
2.3 Temporal Coverage
Upper air data is reported up to four times per day at the synoptic hours of 00, 06, 12 and 18 GMT. The number of accents varies widely between countries and stations.
Standard practice states that all measurements in the profile are attributed to the nominal hour of the ascent. This is the hour at which the sonde reaches 100mb. It takes approximately an hour for the balloon to rise to this level and thus the sondes are released one hour before the synoptic hours.
The radiosonde takes measurements at intervals of approximately 2 seconds. The high resolution data files contain all such data. The standard resolution data files contain measurements taken at particular levels of the atmosphere.
Upper air stations are required to submit standard resolution data to the Met Office. Measurements are reported at standard and significant pressure levels.
(i) Standard Pressure Levels
The standard pressure levels are 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, 50, 30, 20 and 10 mb.
(ii) Significant Pressure Levels
These are levels added to give a clearer picture of the measured profile. For example at the top and bottom of an inversion, large changes in wind, temperature or humidity may well be missed by the standard levels. The surface and the highest level of the sounding are mandatory significant levels. Additional levels are also added to ensure that linear interpolation yields values of temperature which do not differ from the observed value by 1 degree C below 300 hPa and 2 degrees C after that.