-
Further Reading
Appenzeller C, Holton JR, and Rosenlof KH (1996)Seasonal
variation of mass transport across the tropo-sphere. Journal of
Geophysical Research 101:15071–15078.
Baumgartner S, Beer J, SuterM, et al. (1997)Chlorine 36 fall-out
in the Summit Greenland Ice Core Project ice core.Journal of
Geophysical Research 102: 26659–26662.
Bhandari N, Lal D, and Rama (1966a) Stratosphericcirculation
studies based on natural and artificial radio-active trace
elements. Tellus 18: 391–406.
BhandariN, Bhat SG,KharkarDP,Krishnaswamy S, andLalD (1966b)
Cosmic ray produced 28Mg, 31Si, 38S, 38Cl and34mCl and other short
lived isotopes in wet precipitation.Tellus 18: 504–515.
Brewer AW (1949) Evidence for a world circulation provid-ed by
the measurements of helium and water vapordistribution in the
stratosphere.Quarterly Journal of theRoyal Meteorological Society
75: 351–363.
Brost RA, Fleichter J, and Heimann (1991) Three dimen-sional
simulation of 7Be in a global climate model.Journal of Geophysical
Research 96: 22423–22445.
Danielsen EF, Hipskind RS, Gaines SE, et al. (1987)
Three-dimensional analysis of potential vorticity associatedwith
tropopause folds and observed variations of ozoneand carbon
monoxide. Journal of Geophysical Research92: 2103–2111.
Dentener F, Feichter J, and Jeuken AD (1999) Simulationof the
transport of 222Rn using on-line and off-lineglobalmodels at
different horizontal resolutions: Adetailedcomparison with
measurements. Tellus 51B: 573–602.
Eluszkiewicz J (1996) A three dimensional view of
thestratosphere-to-troposphere exchange in theGFDL SKYHImodel.
Journal of Geophysical Research 23: 2489–2492.
Heimann M and Keeling CD (1989) A Three DimensionalModel of
Atmospheric CO2 Transport Based on Ob-served Winds. 2. Model
Description and SimulatedTracer Experiments. American Geophysical
Union Geo-physical Monograph No. 55, pp. 237–275. WashingtonDC:
ACOU.
Holton JR (1995) Stratosphere–troposphere exchange.Reviews of
Geophysics 33: 403–439.
Jacob DJ, Prather MJ, Wofsky SC, and McElroy B (1987)Atmospheric
distribution of 85Kr simulated with a
general circulation model. Journal of GeophysicalResearch 92:
6614–6626.
Joseph AB, Gustafson PF, Russell IR, et al. (1971) Sources
ofradioactivity and their characteristics. In: The Radioac-tivity
in the Marine Environment, pp. 6–41, WashingtonDC: National Academy
of Sciences.
Junge CE (1963) Air Chemistry and Radioactivity. SanDiego:
Academic Press.
LalDandPeters B (1967)Cosmic ray produced radioactivityon the
earth.Handbuch der Physik 46/2: 551–612.
Lal D and Rama (1966) Characteristics of globaltropospheric
mixing based on man-made 14C, 3Hand 90Sr. Journal of Geophysical
Research 71:2865–2874.
Lal D and Suess HE (1968) The radioactivity of theatomsophere
and hydrosphere. Annual Review ofNuclear Science 18: 407–434.
Mahlman JD (1997) Dynamics of transport processes in theupper
troposphere. Science 276: 1079–1083.
Prather M, McElroy M, Wofsky S, Russell G, and Rind D(1987)
Chemistry of the global troposphere: Fluorocar-bons as tracers of
air motion. Journal of GeophysicalResearch 100: 26141–26161.
RehfeldSandHeimann(1995)Threedimensionalatmospherictransport
simulation of the radioactive tracers 210Pb, 7Be,10Be, and 90Sr.
Journal of Geophysical Research 71:2865–2874.
Reiter ER (1978) Atmospheric Transport Processes. Radio-active
Tracers. Washington DC: US Department ofEnergy (TID-27114).
Warneck P (1988) Chemistry of the Natural Atmosphere,San Diego:
Academic Press.
Wofsy SC, Cohen RC, and Schmeltekopf (1994) Overview:The
stratospheric photochemistry aerosols and dynamicexpedition (SPADE)
and airborne arctic stratosphereexpedition II (AASE-II). Journal
ofGeophysical ResearchLetters 21: 2535–2538.
Wogman NA, Thomas CW, Cooper JA, Engelmann RJ, andPerkins RW
(1968) Cosmic ray-produced radionuclidesas tracers of atmospheric
precipitation processes. Science159: 189–192.
Yiou F, Raisbeck GM, Baumgartner S, et al. (1997) Bery-llium 10
in the Greenland Ice Core Project ice core atSummit, Greenland.
Journal of Geophysical Research102: 26783–26794.
RADIOSONDES
WFDabberdt and R Shellhorn, Vaisala Inc., Boulder,CO, USA
H Cole, National Center for Atmospheric Research,Boulder, CO,
USA
A Paukkunen, J Hörhammer and V Antikainen,Vaisala Oyj,
Helsinki, Finland
Copyright 2003 Elsevier Science Ltd. All Rights Reserved.
Introduction
The radiosonde is an expendable, balloon-bornedevice that
measures the vertical profile of meteoro-logical variables and
transmits the data to a ground-based receiving and processing
station. These profilesare typically obtained twice each day and
are the coreof the global weather observing system that
providesinputs to numerical forecast models. The sensor
1900 RADIOSONDES
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package routinelymeasures the variationwith altitudeof
temperature, humidity, and pressure as the balloonascends from the
land or ocean surface to heights up toabout 30 km (a pressure
altitude of about 11 hecto-pascals, hPa).When the device
alsomeasures winds, itis more properly called a rawinsonde,
although theterm radiosonde is commonly applied to both. Theheight
profile of these meteorological variables con-stitutes an upper-air
sounding that is known as aradiosonde observation or RAOB. In some
cases, aballoon without a radiosonde is tracked by eitheroptical or
radar techniques in order to measure onlywinds. This type of
balloon is known as a pilot balloonor simply a pibal, but it is not
a radiosonde.
In 1999, there were 100 operational or synopticradiosonde
stations in the United States; they made adaily average of 182
soundings. In the continental US,an average distance of 315 km
separates radiosondestations. In 1999, there were 992 radiosonde
stationsworldwide (Figure 1) that made an average of 1209soundings
each day in support of weather forecastactivities, while an
additional 65 pibal stations made576 wind soundings daily.
Additional soundings aremade for specialized purposes of which
defenseapplications are the most significant. The globalnumbers of
RAOB and pibal soundings are downconsiderably from their peak daily
values in 1988 of1660 and 964, respectively. The approximately half
amillion radiosondes used annually are manufacturedby less than 10
companies worldwide. Of these, theVaisala company headquartered in
Helsinki manufac-tures about 70% of the global supply of
radiosondes.Vaisala was founded in 1936 by Professor VilhoVaisala,
who in 1931 invented one of the world’s firstradiosondes (see
Appendix).
Since 1957 all stations havemade their soundings atthe same
times, 00.00 and 12.00 UTC, althoughmany
stations outside the US and Europe have reducedsoundings to one
per day because of budgetaryconstraints. Countries launching
operational radio-sondes are members of the World
MeteorologicalOrganization’s World Weather Watch program; assuch,
they freely share their sounding data with eachother. Shortly after
an operational upper-air soundingis completed, a standard data
message is prepared andmade available to all nations using the
GlobalTelecommunications System. These TEMP messagesare transmitted
in a universal format that reportsmeteorological conditions at
various standard or so-called mandatory (pressure) levels as well
as atsignificant levels, which represent levels where pre-scribed
changes in meteorological conditions occur.
There are two primary purposes of upper-airsoundings: to analyze
and describe current weatherpatterns, and to provide inputs to
short- and medium-range computer-based weather forecast models.
Onevery important, specialized use of atmospheric sound-ings is in
support of forecasting hurricane movement.Special radiosondes
called dropwindsondes arelaunched from weather reconnaissance
aircraft toobserve atmospheric structure in the core of
thehurricane as well as in the area downwind of thestorm itself.
These dropwindsonde measurementswere the single most important
factor in a 20%increase in hurricane forecast accuracy over
thedecade of the 1990s. Other uses of radiosonde datainclude
climate studies, air pollution investigations,aviation operations,
and defense applications. Theradiosonde continues to be the
backbone of an eclecticsuite of measurement technologies
(measurementsboth remote and in situ that are made from
ground-based, airborne, and satellite platforms) used toprovide
data for input to numerical weather forecastmodels.
80
60
40
20
0
− 20
− 40
− 60
− 80
− 150 − 100 − 50 100 150 500
Figure 1 Global radiosonde station network.
RADIOSONDES 1901
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Radiosonde Operations
The radiosonde is carried aloft by a balloon as part of aflight
train (Figure 2). The balloon itself may be madeof either natural
rubber (latex) or synthetic rubber(neoprene). The mass of the
flight train, the desiredascent rate, the type of gas used, and the
maximumheight of the sounding determine the size of
theballoon.Operational radiosonde systems typically useballoons
that weigh anywhere from 300 to 1200 g;they are filled to ensure an
ascent rate of 300mmin� 1.Hydrogen is the gasmost commonly used to
inflate theballoon and provide its lifting capacity, althoughhelium
and natural gas are sometimes used for specialapplications. The
flight train consists of five compo-nents: (1) the balloon; (2) a
parachute to bring theradiosonde safely back to Earth after the
balloon
bursts; (3) 20–60m of nylon separation line thatisolates the
radiosonde’s sensors fromwater vapor andthermal contamination by
the balloon; (4) adereeler tolet out the nylon line after launch;
and (5) theradiosonde itself. A few countries such as the US
andSwitzerland actively seek to recover and then reusetheir
radiosondes. In the US, it is estimated that about18%are reused
after extensive refurbishment,while inSwitzerland, more than 60%
are recovered andreused.
Components of the ModernRadiosonde
The radiosonde is an electronics unit that comprisesthree major
sections: a suite of sophisticated meteor-ological sensors;
signal-processing electronics; and aradio transmitter to relay the
measurements back to areceiver at the radiosonde launch station.
The meteo-rologicalmeasurements aremade at intervals that varyfrom
1 to 6 s, depending on the type andmanufacturerof the radiosonde.
Themeteorological community hasbeen assigned two radio frequency
bands for use intransmitting meteorological data: 400–406MHz
and1675–1700MHz. These bands are under continuingpressure from the
telecommunications industry,whichseeks to use them for commercial
purposes. All of theworld’s radiosondes are required to meet
certainperformance standards that have been established bythe WMO
(see Table 1). Figure 3 illustrates fourdifferent radiosondes
currently in use around theworld.
Overview of Thermodynamic Sensors
Thermodynamic sensor types vary widely amongradiosondes
currently in use throughout the world.Temperature sensors are of
four designs: capacitancesensors, thermistors, resistance wires,
and bimetallicelements. The two common humidity elements arecarbon
hygristors and planar thin-film capacitancesensors, although
gold-beater’s skin is still used inRussia and China. Pressure
measurements are typi-cally made with either an aneroid cell or a
piezore-sistance element. There are about a dozen
differentradiosonde designs presently in use. As radiosondeshave
become more advanced, their changes have alsocreated special
challenges to climatologists seeking topiece together a consistent
and homogeneous multi-decadal global database to analyze and
understandclimate change. As a result, climate researchers
mustaccount for biases in the historical records due tochanges in
instrumentation and observing methods,many of which have poor or no
documentation. In the
Balloon
Parachute
Hanger board
Unwinder
Radiosonde
60 m string
Figure 2 Typical radiosonde flight train, including
balloon,parachute and hanger board, unwinder mechanism,
separation
line, and radiosonde.
1902 RADIOSONDES
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United States alone, these changes have been variedand
significant. Four distinctly different humiditysensors have been in
use since 1943. Temperaturemeasurements have undergonemajor
changes, includ-ing sensor type, size, and coating, exposure to the
airstream, and corrections to account for radiationbiases. At
present, the US National Weather Serviceuses radiosondes from two
different manufacturers,each having its own distinct set of
pressure, temper-ature, and humidity sensors. The Vaisala
companyproduces about 70% of the world’s radiosondes, andadded
emphasis is given below to aspects of the designof its radiosondes
and sensors.
Thermodynamic Sensors
Sensors used with Vaisala radiosondes are all of thecapacitance
type. Changes in pressure, temperature,and humidity result in
changes in the capacitanceinformation from each sensor, which in
turn ischanged to a frequency signal by using sensor trans-ducer
electronics. Sensor frequency measurements arecompared with the
frequencies of reference capaci-tance transducers, and these in
turn are converted tophysical measurements based on factory
calibrationmeasurements. In the case of pressure, the
distancebetween capacitance plates changes as atmospheric
Table 1 Accuracy requirements (expressed as standard error) for
upper-air measurements for synoptic meteorology
Variable Range Accuracy requirement
Pressure Surface to 5hPa 71 hPaTemperature Surface to 100hPa
70.5K
100 to 5 hPa 71KRelative humidity Troposphere 75% (RH)Wind
direction Surface to 100hPa 751 for wind speedo15m/s
72.51 for wind speed415m/s100 to 5 hPa 751
Wind speed Surface to 100hPa 71m/s100 to 5 hPa 72m/s
Geopotential height
of significant
levels
Surface to 100hPa 71% near the surface decreasing to70.5% at 100
hPa
Source:WorldMeteorologicalOrganization (1996)Guide
toMeteorological InstrumentsandMethodsofObservation, 6th
edn.Publication
No. 8. Geneva: WMO.
RRS100RRadiosonde((China)
Mesei RS-01GRadiosonde(Japan)
Vaisala RS80Radiosonde(Finland)
Sippican Mark IIMicrosonde(USA)
Figure 3 Examples of Radiosondes in Current Use Around the
World.
RADIOSONDES 1903
-
pressure changes, causing a change in the measuredcapacitance.
Older pressure sensors use an aneroid orbellows-type sensor that
responds mechanically topressure changes. Modern pressure
transducers arevery small silicon, micromechanical sensors.
Pressuresensors also have a temperature dependence that
iscompensated by factory calibration of the sensor. Thetemperature
change of capacitive sensors is measuredby the change in the
dielectric constant of the sensor.Older capacitive sensors
consisted of a sensor that washermetically packed inside glass.
Newer capacitivetemperature sensors are extremely small and
fast,owing to a special twin-wire construction. Essentialaspects of
modern temperature and humidity sensors(and their
supportingmembers, i.e. the sensor boomasseen in Figure 4) are
their different coatings andtreatments to minimize solar heating
and improvewater repellency. The approach used to measurehumidity
in all Vaisala radiosondes is also based onchanges in the
dielectric constant. The humidity-sensing technology is basedon
so-called thin films.Thedielectric material is a very thin layer of
a specialproprietary polymer that has an optimum combina-tion of
measurement properties, including stability,repeatability,
hysteresis, response time, and tempera-ture dependence. Thin-film
humidity sensors arecalibrated to provide output in terms of
percentrelative humidity with respect to water; the tempera-ture
dependence is compensated by use of tempera-ture-dependent
calibration coefficients determinedfrom factory calibration tests.
Some Vaisala humidityprobes incorporate two sensor elements that
includeheating of the sensor elements to minimize affects of
water condensing on the sensors as the radiosondemoves from warm
to cold layers during its ascent.The two sensors are alternately
heated in sequence,and the measurement is taken from the passive
sensor.The sensors are very small and designed for
fastresponse.
The accuracy of radiosonde data is a combination ofmultiple
factors: sensor performance; related trans-ducer electronics;
mechanical construction of thesonde and sensor housing; sensor and
sensor-boomcoatings and treatments; calibration technology;
andcalibration and correction algorithms. In addition toissues of
radiosonde performance, the uncertainty ofupper-air measurements
includes sampling considera-tions, such as the density of the
observation network,time interval between observations, and the
homoge-neity of the atmosphere. Together, these instrumentaland
environmental factors govern the accuracy andrepresentativity of
the observations.
Specialized Radiosonde Sensors
Some radiosonde manufacturers offer optional sen-sors to make
supplemental environmental measure-ments. Additional electronics
are used to interface thesupplemental sensors to the radiosonde.
Measure-ments of ozone concentration and radioactivity are thetwo
most common supplemental measurements.Radiosonde measurements of
ozone are made world-wide, although at fewer stations and typically
onlyonce per day or less often. The most commonradiosonde ozone
sensor is the electrochemical type,while radioactivity is typically
measured with Geiger–Müller tubes. Other supplemental measurements
inuse today include dew point, optical backscattering byfine
particles, electric field, and video imaging ofparticles and
hydrometeors. Most advanced radio-sonde ground systems effectively
support both synop-tic and research users, and offer options for
post-ascent data calculation and analysis of
supplementalmeasurements.
The Vaisala ozonesonde consists of an electrochem-ical ozone
sensor connected to an interface unit and amodified radiosonde.
Consequently, humidity, pres-sure, temperature, and geopotential
height can bemeasured simultaneously with ozone sampling.Upper-air
winds are also measured. This lightweight,balloon-borne instrument
is capable of measuring thevertical distribution of atmospheric
ozone up to 3 hPa.The uncertainty of the ozone measurement is of
order5–10% of the local values. The electrochemicalconcentration
cell (ECC) ozone sensor detects ozoneon the basis of an
iodine–iodide oxidation-reductionor redox electrode reaction in
neutral buffered solu-tion. The sensor consists of an
electrochemical
150 mm
50 mm
90 mm
Figure 4 Vaisala RS90 Radiosonde with sensor boom.
1904 RADIOSONDES
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concentration cell that contains two platinum elec-trodes
immersed in separate potassium iodide solu-tions of different
concentrations, which are separateanode and cathode chambers. The
chambers arelinked with an ion bridge. As air containing ozoneflows
into the cathode solution, a chemical reactionoccurs and the
platinum electrodes carry electronsbetween the cells of the sensor.
An electrical current isgenerated in proportion to the rate at
which ozoneenters the cell. The ozone concentration is
determinedfrom the electric current measurement using anequation
that considers the airflow rate, air pressure,and pump temperature.
The interface can also be usedwith other sensor types, such as the
Brewer–Mastsensor. The Brewer–Mast sensor uses similar
ozonedetection reaction, but instead of a reference chamber,the
driving potential for the measurement circuit is anelectrical
circuit. The ECC-type sensor is more accu-rate and is more widely
used.
The Vaisala radioactivity sonde is a combination ofa
radioactivity sensor and a modified radiosonde. Theradioactivity
sonde can measure the vertical profile ofradioactivity in the
troposphere and in the lowerstratosphere, up to altitudes of 40 km.
The radioac-tivity sensor measures radiation with two
Geiger–Müller detectors – ionization chambers filled withspecial
gas mixtures. One detector is sensitive only togamma radiation,
while the other measures bothgamma and beta radiation. This way it
is possible tomake measurements of both gamma and beta radia-tion.
The detectors have pulse outputs; the count rateis proportional to
the radiation intensity and is read atfixed time intervals. The
measurement accuracy isabout 710%.
Overview of Windfinding
There are several techniques formeasuringwindswithonly a balloon
or with a combination balloon andradiosonde. When a radiosonde
measures winds it iscalled a radio-wind-sonde or rawinsonde.
Raw-insondewindfindingmethods vary widely. In all cases,the winds
are determined by observing the drift of theballoon. One class of
wind measurement techniquestracks the balloon externally using one
of threemethods: (1) optical systems use a theodolite tovisually
track the balloon’s azimuth and elevation;(2) radio theodolites
track a radio signal sent from atransmitter on the radiosonde,
again to obtain azi-muth and elevation information; and (3) radar
systemstrack a radar retroreflector suspended from theballoon to
obtain slant range, azimuth, and elevation.The second class
ofwindmeasurement techniques usesvarious navigation systems. Two
such systems cur-
rently in use employ the LORAN-C navigation systemand various
VLF systems, such as the Russian ALPHAsystem and the US Navy’s VLF
system. A newnavigation-based windfinding technique is now com-ing
into widespread usage. A receiver inside theradiosonde accurately
measures the horizontal andvertical Doppler velocity of the
radiosonde withrespect to those Global Positioning System
(GPS)satellites that can be observed at any given time(typically,
four to eight satellites). Other types of GPSreceivers also observe
the latitude, longitude, andaltitude of the radiosonde. In both
cases, the GPSreceiver measures directly the drift velocity of
theballoon and hence the wind. Twomajor advantages ofthe GPS-based
techniques are the high accuracy andprecision of the wind
measurements, and the world-wide coverage of GPS.
Tracking Techniques
Optical tracking methods One of the earliest meth-ods for
determining the winds aloft was to visually oroptically track small
balloons, called pilot balloons(pibals). This methodwas developed
in themid-1870susing a small expendable balloon tracked with a
smalltelescope. The small optical device, similar to asurveyor’s
transit, is called a theodolite and canaccurately measure elevation
and azimuth angles. Ifthe balloon’s height can be determined then
itsposition can be found by trigonometry. There arebasically two
pilot balloon techniques still in use: (1)single-theodolite and (2)
double-theodolite. In theformer, the elevation and azimuth angles
of theballoon are measured at regular intervals (typicallyonce per
minute). Balloon altitude is determined byassuming a constant
ascent rate that is determinedfrom the size and free lift of the
balloon. Balloonposition is then calculated from the height and
theazimuth and elevation measurements. Tracking theballoon during a
nighttime observation is accom-plished by attaching a light stick
or small battery-powered light. In the double-theodolite technique
twotheodolites are located a known distance apart (thebaseline) and
simultaneous observations taken ofthe balloon at given time
intervals. By measuring theazimuth and elevation angles to the
balloon from thetwo known positions, the three-dimensional
balloonposition can be determined by the law of sines.
Thedouble-theodolite method enables accurate measure-ments of the
balloon position without assuming aconstant rate of ascent for the
balloon, which can be asource of error. In this method the baseline
distanceneeds to be accurately measured and should be at
leastone-fifth of the maximum range to the balloon. Thebaseline
should also be perpendicular to the prevailing
RADIOSONDES 1905
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winds. The method is not routinely used because ofbaseline
restrictions and the cost and difficulty ofcoordinating two sets of
observers.
Radiotheodolite and radar methods Another track-ing technique
used for determining winds is calledradio direction finding or RDF.
During World War IIthe US Army Signal Corps developed the first
RDFsystem, called the SCR-658. This system operated at400MHz and
used two separate operators to steer alarge antenna array to
determine the direction of theradiosonde transmitter. A more modern
radio direc-tion finding antenna automatically tracks the1680MHz
telemetry signal transmitted from theradiosonde. The antenna
azimuth and elevation dataare sent to a computer at the ground
station alongwiththe pressure height data from the radiosonde
todetermine the change in radiosonde position (winds)during flight.
The RDF technique (Figure 5) is theradio frequency equivalent of
the optical theodolitemethod, and the tracking system is called a
radiothe-odolite. There are different types of RDF
antennas,including 2–3m diameter dish antennas and phased-array
flat-plate antennas. RDF systems can resolve theazimuth and
elevation angles to within 0.051. If theupper-level winds are high
then the radiosonde will bea long distance away, resulting in the
antenna eleva-tion angle being near the horizon. At stations
thatexperience high winds, the radiosondes can beequipped with a
transponder to measure slant rangeor distance to the radiosonde.
Winds can then bedetermined using azimuth, elevation, slant range,
andheight of the radiosonde. A similar method fortracking the
radiosonde uses a radar reflector on theballoon flight train so
that it can be tracked by
windfinding radar. Slant range to the radiosonde ismeasured by
the radar aswell as azimuth and elevationangles. Radar windfinding
is a common method usedin many countries around the world; in 1998
about45% of the stations used radar, as tallied in
Oakley(1998).
In the Russian and Chinese upper-air networks acombination
RDF-transponder method is used, calledsecondary radar. Some 200
such systems are deployedworldwide. The parabolic or array-type RDF
antennatransmits a short pulse that is received by the radio-sonde.
The radiosonde then ‘wakes up’ and retrans-mits the pulse by
transmitting the temperature andhumidity data, which are received
by the ground-based RDF antenna. The RDF antenna azimuth
andelevation angles are measured and the slant range isdetermined
from the travel time of the pulse. Second-ary radar systems use
radiosondes that do not have apressure sensor; pressure is
calculated from thehydrostatic equation.
Navigation aids (NAVAIDS) The use of navigationaids for
obtaining upper-air winds from radiosondesbegan in the early 1960s.
The US Weather Bureau(now the National Weather Service of the
NationalOceanic and Atmospheric Administration, an agencyof the US
Department of Commerce) sought to find away to measure winds at sea
for the Ships ofOpportunity Program. At that time the only way
tomeasure winds aloft at sea was with a radar or RDFsystem; both
systems were costly and required amechanical stabilization system
for the trackingantenna. In 1964, the bureau awarded a contract
toBeuker’s Laboratory Inc. (BLI) of New York todevelop a
windfinding system using retransmittedLoran-C navigation signals to
track the radiosonde.The technique proved successful. Owing to the
limitedcoverage of Loran-C, two years later the worldwideOmega
navigation system was proposed as an alter-native for windfinding.
Radiosondes that use theseNAVAID signals to determine winds contain
a small,inexpensive radio receiver to receive the navigationsignals
from fixed ground stations. The radiosondethen retransmits (Figure
6, usually at 400MHz) thesignals to the data processing system at
the groundstation. There are at present three types of
NAVAIDsignals in use: (1) Loran-C, (2) very low-frequency(VLF)
systems, and the (3) Global Positioning System(GPS).
Loran-C coverage has increased since 1964, butbecause its
primary use is for coastal navigation it doesnot provide worldwide
coverage. Loran-C stationstransmit a unique series of pulses at 100
kHz thatidentify each station. If the radiosonde receives
andretransmits signals from at least three stations, then
Free-floatingsensor
Vertical
North
Azimuth
Slant rangeHeight
Elevation
Radar,radiotheodolite, etc.
Figure 5 Angle-dependent tracking system.
1906 RADIOSONDES
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the data processing system at the ground station candetermine
the time of arrival of those signals at theradiosonde and its
distance from each ground station.Winds are determined from the
change in position ofthe radiosonde.
The other class of ground-transmitting navigationsystems is
theVLF systems. These operate in the 10–30kHz frequency range and
their long-wavelengthsignals are characterized by low attenuation
and theability to propagate long distances; this allows world-wide
coverage with a minimum number of groundtransmitters. The Omega
navigation system was themost widely used VLF system for both
navigation andwindfinding until it was closed down on 30
September1997 because of cost considerations and the emer-gence of
more accurate GPS windfinding systems.However, other VLF stations
operated by US andRussian defense agencies continue to operate.
VLFwindfinding is similar to Loran-C, except for thedifference in
radio frequency and the correspondingdecreased windfinding accuracy
of VLF.
The third type of NAVAID windfinding system usessignals from the
so-called Global Positioning System(GPS) satellites. GPS was
conceived in the early 1970sfor the US Department of Defense (DOD),
and isoperated by the US Air Force. The GPS system becamefully
operational in late 1995. There are 24 satellites insix orbital
planes spaced 60 degrees apart. Thesatellites are in a 20 200 km
circular orbit, with aninclination angle of 551 and a periodicity
of 12 hours.At any time or place in theworld, there are 6 to
11GPSsatellites 51 or more above the horizon and henceusable for
GPS windfinding. There are two primaryGPS techniques for
determining winds from radio-sondes. The GPS signals cannot be
retransmitted from
the radiosonde back to the ground because thebandwidth of the
1575MHz (called the L1 band)GPS carrier signal is too wide
(B2.0MHz). Theworldwide civilian use ofGPShasbecome so great
thatmany manufacturers produce inexpensive, small GPSreceivers each
the size of a credit card that can decodethe navigation message
every second and produceaccurate three-dimensional position
coordinates, aswell as speed and heading. A second, less
expensivemethod uses a codeless receiver in the radiosonde
thatmeasures only the Doppler shift of the carrier
fre-quency.TheDoppler shift has two components: (1) theDoppler
shift due to the satellite motion (i.e., thelargest component), and
(2) the Doppler shift due toradiosonde movement. The radiosonde
receiver sendsthe Doppler information back to the ground
datasystem. The ground data systemmust have a local GPSreceiver
that can decode the GPS message and inde-pendently measure the
Doppler shift from each sate-llite. The satellite Doppler shift is
subtracted from theradiosonde Doppler shift and the difference
yields theradiosonde motion.
Specialized Types of RadiosondeSystems
Dropsonde
The dropsonde is the airborne counterpart to theconventional
radiosonde (sometimes also called anupsonde). Dropsondes are
ejected from researchaircraft and float to earth on a special
balloon-likeparachute. Current state-of-the-art dropsonde
sensorsinclude capacitance fine-wire sensors to measuretemperature,
capacitance silicon pressure sensors,and GPS receivers to measure
winds. Humidity ismeasured with a pair of thin-film capacitance
sensorsthat are heated alternately to avoid condensation ondescent
from colder to warmer air. All measurementsare made twice every
second, while the 400 g drop-sonde falls at an initial rate of
about 25m s� 1 at 15 kmaltitude, decreasing to about 10m s� 1 at
sea level.Dropsonde data are transmitted by radio from thesonde to
a data system in the aircraft. Atmosphericsoundings from dropsondes
provide the ability tomeasure conditions over remote areas such as
theoceans, polar regions, and sparsely inhabited land-masses; they
also provide ameans to obtain soundingsin and around severe weather
systems, such ashurricanes. Atmospheric soundings obtained
fromdropsondes during hurricane reconnaissanceflights have improved
the accuracy of forecasts ofhurricane landfall by about 20% over
the decade ofthe 1990s.
Free-floatingsensor
NAVAIDgrid
Receivingequipment
Radio link
Figure 6 NAVAID retransmission system.
RADIOSONDES 1907
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Dropsondes were first developed in the 1960s forthe US Navy and
Air Force for hurricane reconnais-sance and were an adaptation of
radiosonde techno-logy. These early dropsondes were heavy –
about2.5 kg – and did not have inherent windfindingcapability;
windfinding at that time still used onlyradar or RDF. With the
development of OmegaNAVAID windfinding technique for the radiosonde
itbecame possible to incorporate that technology intothe dropsonde.
This occurred in 1974 when theNational Center for Atmospheric
Research (NCAR)developed anOmegaDropwinsonde (ODW) for use inthe
Global Atmospheric Research Program’s AtlanticTropical Experiment.
In 1982, the Air Force adoptedthe ODW system for hurricane
reconnaissance andthis system was used until the early 1990s.
In 1985 NCAR began development of a smart
(i.e.microprocessor-based), lightweight digital dropsondethat
incorporated Loran andOmegawindfinding. TheOmega version of this
dropwinsonde was adopted bythe US Air Force in the early 1990s for
its hurricanereconnaissance mission (Figure 7). The next
majorimprovement in dropsonde technology occurred in1995 when NCAR
completed development and test-ing of a new GPS dropsonde with
codeless GPSwindfinding capability and an advanced aircraft
datasystem (AVAPS). In 1996 NCAR licensed Vaisala Inc.of Woburn,
Massachusetts, to commercialize produ-ction and sales of the GPS
dropsonde (Figure 8) andAVAPS. In the relatively short time the GPS
dropsondehas been in use it has found research applications in
thedetermination of hurricane structure and motion, thestudy of
clear-air turbulence associated with upper-level jet stream
structure, and observing strategies formidlatitude weather
forecasting. Current adaptations
of the GPS dropsonde technology are focusing onlaunches at
higher altitudes – including the lowerstratosphere – as well as
autonomous launches thateliminate the need for operators to launch
the sondeand record the data, and offer the promise that it willbe
possible one day to obtain operational drop-winsonde profiles from
commercial aircraft. Figure 9shows a test dropsonde launcher
mounted on theunderside of an ER-2 high-altitude weather
researchaircraft.
Driftsonde System
Improvements in short- and medium-range synoptic-scale weather
forecasts will depend on improvedupper-air soundings over the
data-sparse regions ofthe Northern and Southern Hemispheres.
Progresstowards this objective will require the optimal use
ofexisting data sources, creative new observing meth-ods, and
improved numerical methods for dataassimilation. The driftsonde
system is being developedas a cost-effective sounding system that
could fill thesecritical gaps in data coverage over oceanic and
remotearctic and continental regions. The driftsonde conceptseeks
to obtain a large number of high-vertical-resolution GPS dropsonde
profiles through the lowerstratosphere and the entire troposphere
by autono-mous launchingof dropsondes from specially
designedballoon platforms. The driftsonde system includes
apolyethylene carrier balloonwith an attached gondola(Figure 10)
that carries a payload of up to 24 GPSdropsondes. The carrier
balloon ascends to between50 and 100 hPa (20 and 16 km) and then
drifts in theprevailing stratospheric westerlies for up to five
days,deploying dropsondes at prescribed and specialtimes over
data-sparse regions of interest. The first
Figure 7 USAF C-130 Hurricane Hunter launching a GPS
dropsonde.
1908 RADIOSONDES
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application of the driftsonde system will be in supportof The
Hemispheric Observing System Research andPredictability Experiment
(THORPEX), a five-to-ten-year international program of atmospheric
observing
system research and development, and experimenta-tion with
numerical forecast systems that will beconducted in the 2002–2010
time frame.
Automated Shipboard Aerological Program (ASAP)
The Automated Shipboard Aerological Program(ASAP) is a
multinational effort initiated by Canadain 1982 to obtain upper-air
soundings over the oceans.Omega NAVAID radiosondes are launched
fromcommercial ships of opportunity using a speciallydesigned
launch system (Figure 11) that permits flighttrains to be launched
in high-wind conditions. Theupper-air sounding data from the
radiosonde are sentback to the shipboardASAP systemwhere the data
areprocessed in near real time to create a TEMP SHIPmessage. This
message is the ocean equivalent of theTEMP message generated for
land-based RAOBsystems. The ASAP system sends the message to aGOES
geostationary satellite that relays the informa-tion to the Global
Telecommunications System (GTS),which then transmits it to the
numerical weatherprediction centers around the world.
The ASAP program had its beginning in June
of1981,whenCanadadecided todiscontinue itsweathership program owing
to the high costs of operating andmaintaining the ocean weather
ship PAPA located inthe Gulf of Alaska at 501N, 1451W. The
originalintent was to replace the weather ship data withsatellite
observations; however, persistent cloudinessin areas such as the
Gulf of Alaska and the NorthAtlantic, coupled with the lack of
surface weatherdata, made this goal impossible to attain. To
remedythis problem, the Atmospheric Environment Service(AES) of
Environment Canada, the National Weather
Figure 8 GPS dropsonde descending on its parachute.
Figure 9 Remote-controlled GPS dropsonde launcher system
installed on the NASA ER-2 high-altitude weather research
aircraft.
RADIOSONDES 1909
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Service (NWS) of NOAA, and NCAR established ajoint ASAP project
to develop a modular, mobile,moderately priced, upper-air sounding
system. Thissystem, when placed on commercial vessels
(ships-of-opportunity) routinely crossing the Pacific and Atla-ntic
oceans, provides real-time upper-air soundingsthat complement those
of the global land-based upper-air network.
The ASAP program operated by AES Canadastarted in the spring of
1982 with one commercialship (a Japanese automobile carrier) that
operatedfromVancouver, British Columbia, to Japan. By 2001,it had
evolved into an international program with 11countries operating 22
ASAP units – see Table 2.Since 1994, the ASAP program has made
about 5300upper-air soundings per year. In 2001 a lengthySouthern
Hemisphere route was inaugurated withport calls in Germany, the UK,
South Africa, Australia(both east and west coasts), New Zealand,
and SouthAmerica.
Rocketsonde
The rocketsonde is similar to adropsonde except that arocket is
used to carry the sonde to the desireddeployment altitude where the
sonde is ejected andfloats to Earth on a small parachute. Two types
ofrocketsondes are in use today, and are classifiedaccording to
their maximum altitude. High-altituderocketsondes are used
primarily by the military anduse a large rocket to carry the sensor
package toaltitudes in excess of 70 km. The Super Loki
solid-fuelrocket motor is typically the launch vehicle for
high-altitude rocketsonde deployments. Twometers long,
itaccelerates to 1500m per second, and delivers itsmeteorological
payload above the stratosphere into
the mesosphere. The typical payload package, called adart, is
approximately 1.1m long with an inside tubediameter of less than 5
cm, and contains the meteor-ological sonde. After the rocket motor
burns out, thedart continues to coast to an altitude ranging from
70to 110 km. At apogee, a timed detonation of a smallexplosive
charge located in the tail of the dart ejects themeteorological
payload, which then begins its para-chute-aided descent. The
payload consists of either ameteorological sensor package – the
rocketsonde – oran inflatable sphere. The high-altitude
rocketsondesoften contain a transponder, a miniature
receiver–transmitter that can be tracked by a radio
directionfinding and ranging system to determine winds andaltitude.
The inflatable sphere provides atmosphericdensity data, obtained
from its fall velocity as deter-mined by a precision tracking
radar.
The second type of rocketsonde is smaller and lessexpensive, and
is used to measure only thermodynam-ic variables in the lower 1–3
km of the atmosphereabove earth. The Vaisala RK91 low-altitude
rocket-sonde (Figure 12) is primarily designed for navalshipboard
operations that require observations of therefractive index profile
near the ocean surface, but canalso be used over land where only
thermodynamicdata are required. The RK91 can be prepared forlaunch
in less than 10min; it reaches apogee in lessthan 20 s, and
provides a detailed thermodynamicprofile with 1 s resolution. After
ejection of the sondepayload, the sonde drifts on a parachute to
the surfacefrom an altitude of 1 km in less than 6min.
Verticalresolution is dependent on the rate of descent (typi-cally
3m s� 1), rate of data transmission (1Hz) andsensor response time.
At temperatures above freezing,vertical resolution is about 3m.
ATLANTIC OCEANEUROPE
NORTHAMERICA
~58,000 ft50−100 mb
OrbCommLEO satellite
Zero-pressureballoon
Gondola(24 sonde capacity)
6 hoursbetweendrops
Figure 10 The driftsonde system concept.
1910 RADIOSONDES
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Appendix
Historical milestones leading to the development ofthe modern
meteorological radiosonde
Year Milestone
1643 Evangelista Torricelli invents the barome-ter in Florence,
Italy.
1648 French mathematician Blaise Pascal ob-serves the decrease
of atmospheric pressurewith altitude.
1749 Alexander Wilson, Glasgow, Scotland,uses kites to study the
variation of temper-ature with altitude.
1783 The French Montgolfier brothers, Joseph-Michel, and
Jacques-Étienne invent thehot-air balloon.
1783 Jacques Alexandre Césare Charles, Paris,France, uses a
manned balloon to makethe first measurements of variationsof
pressure and temperature withaltitude.
1784 Englishman John Jeffries, London, andFrenchman Jean-Pierre
Blanchard beginthe systematic study of the atmosphereusing manned
balloons.
1804 French physicists Louis Gay-Lussac andJean Baptiste Biot
ascend to 7 km in a
Figure 11 The ASAP system is housed in either a standard 3m
(shown above) or 6m sea container with a specially designed hatch
toenable routine radiosonde launches in sustained winds up to 25ms�
1 (gusts up to 35ms� 1). (Bottom part) Worldwide ASAP
radiosoundings for November 2001.
RADIOSONDES 1911
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balloon and discover that water vapordecreases with
altitude.
1822 Englishmen Sir Edward Parry and the Rev.George Fisher use
kites with recordingthermometers to study the Arctic
atmo-sphere.
1847 WilliamRadcliff Birt is the first to measurewinds aloft
(and temperature) with a kiteflown from Kew Observatory,
London.
1892 Frenchmen H. Hermite and G. Besançonlaunch the first
free-flying weather balloonwith mechanical recording system
(the‘meteorograph’).
1893 Lawrence Hargrave, Sydney, Australia,invents the box kite;
by end of decade,many major observatories are using boxkites
routinely to measure the atmosphere;they include: Blue Hill (near
Boston, Mas-sachusetts), the Central Physical Observa-tory
(Moscow), Trappes (near Paris), Kew(London), Lindenberg (Germany),
andIlmala (Helsinki).
B1900 British scientist W. H. Dines invents themechanical
meteorograph design that iswidely used until 1939.
1901 Richard Assmann, Germany, is first to use‘extensible’
rubber balloons for free-flyingsoundings with meteorographs.
1917 Germans F. Herath and M. Robizsch usethe ‘telemeteorograph’
to transmit mete-orological data from a kite using the steelkite
cable as the signal cable.
1920 US Weather Bureau and Army Air Corpsestablish a program of
daily upper-airsoundings using airplanes at 20
locationsnationwide.
1921 US Weather Bureau establishes a kitenetwork for routine
upper-air observa-tions; this remains in operation until 1933.
1927 M. R. Bureau andM. Idrac (France) inventthe ‘shortwave’
(RF) tube-type transmitterand publish a paper describing the flight
oftheir first balloon-borne sonde (although itis unclear whether
any meteorologicalvariables were actualy measured). Theirpaper is
the first documented use of the
18 in845.7 cm7
227 in688.6 cm
67 cm
Figure 12 Low-altitude rocketsonde unit with detached sonde
descending on parachute (insert).
Table 2 Number of ASAP units operating in 1989 and 2001
Country Number of ASAP units
Year 1989 2001
Australia/UK/USA (Southern
Hemisphere)
1
Canada 5a
Denmark 2 2
EUMETNET 2
Finland 1
France 4 4
Germany 4 3
Iceland/Sweden 1
Japan 5
Russia 1
Spain 1 1
United Kingdom 2 1
United States of America 5a 1
aJointly supported by Canada and US.
1912 RADIOSONDES
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term ‘radiosonde,’ which they attribute toH.Hergesell (president
of the internationalaerological commission).
1929 January 17:M. Idrac andM.R. Bureau testthe first
free-flying radiosonde, called the‘Thermoradio’, with a bi-metallic
temper-ature sensing element to transmit temper-ature data to a
ground station.
1930 January 30: P.A.Moltchanov (Russia) usesa radiosonde to
measure temperature andpressure to a height of 10,000m fromSlutzk.
From 1930 to 1936 several thou-sand soundings were made in the
USSRwith the Moltchanov radiosonde.May 8: M.R. Bureau launches a
radio-sondemeasuring temperature and pressurefrom Trappes, France,
reaching an altitudeof 14,400m.May22: P.Duckert (Germany) flies the
firstradiosonde measuring pressure, tempera-ture and humidity to a
height of 15 000mfrom the Aerological Observatory at
Lin-denberg.
1931 December 30: Prof.VilhoVäisälä (Finland)flies a
radiosonde fromHelsinki telemeter-ing temperature to the ground up
to aheight of 7 km; like Duckert, Väisäläused the measuring
elements to controlthe capacitance of the radio
oscillatorcircuit.
1936 July 30: Prof. Väisälä establishes theVäisälä Company
and delivers the firstcommercial order for 20 radiosondes,delivered
to Prof. Carl Gustav Rossbyat the Massachusetts Institute
ofTechnology.
1974 The National Center for AtmosphericResearch (Boulder,
Colorado) developsthe dropsonde, a special radiosonde thatis
launched from research aircraft andmeasures winds, pressure,
temperature,and humidity while descending on a para-ċhute.
1976 The Vaisala Oy company (Helsinki) intro-duces the first
computer-controlled upper-air sounding systems.
1982 The US National Oceanographic and At-mospheric
Administration begins routineuse of dropsondes for hurricane
research;one year later, the US Air Force initiates itshurricane
reconnaissance program.
1995 The first commercial radiosonde systemsusing the satellite
Global PositioningSystem to measure winds are introducedby the
Atmospheric InstrumentationResearch company (Boulder, Colorado)and
the Vaisala Oy company (Helsinki).
See also
Observation Platforms: Kites; Rockets. ObservationsforChemistry
(RemoteSensing):Microwave.SatelliteRemote Sensing: GPS
Meteorology.
Further Reading
Beelitz P (1954) Radiosonden, VEB Verlag Technik,
Berlin,Germany.
Federal Meteorological Handbook No. 3 (1997) Raw-insonde and
Pibal Observations, FCM-H3-1997. Wash-ington, DC: Office of the
Federal Coordinator ofMeteorology.
Hock TR and Franklin JL (1999) The NCAR GPS drop-windsonde.
Bulletin of the American MeteorologicalAssociation 80(3):
407–420.
Oakley T (1998) Instruments and Observing Methods.World
Meteorological Organization Report No. 72.Geneva: WMO.
Shea DJ, Worley SJ, Stern IR and Hoar TJ (1994) AnIntroduction
to Atmospheric and Oceanographic Data.Report TN-40411A, Boulder,
CO: National Center forAtmospheric Research.
World Meteorological Organization (1996)Guide to Mete-orological
Instruments andMethods of Observation, 6thedn. Publication No. 8.
Geneva: WMO.
RAINBOWS
See OPTICS, ATMOSPHERIC: Optical Phenomena
RADIOSONDES 1913