APPLETON LECTURE

Remote sounding of atmosphere and oceanfor climate research

Prof. J.T. Houghton, F.R.S.

Indexing terms: Satellite links and space communication, Earth sciences, Instrumentation and measuringscience, Radiation

Abstract: Observation from space of radiation reflected, scattered or emitted in various regions of the electro-magnetic spectrum can yield information about the structure of the earth's atmosphere and oceans, which isof importance for climate research. The accuracy of remote sounding observations of atmospheric tempera-ture is reviewed, plans for improved radiation budget observations are mentioned and new applications ofactive microwave measurements, the scatterometer and the radar altimeter, are described.

1 Introduction

The technique of remote sounding enables us to investigatethe structure of the atmosphere, or the surface of the land orocean, by means of radiation emitted, scattered or reflectedfrom the region under investigation. There is a wide range ofthe electromagnetic spectrum available for such investigations,from the ultra-violet through the visible, infra-red and micro-wave regions to radio wavelengths. Early exploration of theupper atmosphere, the ozone layer and the ionosphere, wascarried out by remote sounding from instruments on theearth's surface. More recently, orbiting satellites have providedideal platforms on which to mount remote sounding instru-mentation because continuous measurements with near-global coverage are possible from a combination of satellitesin different orbits.

It is this almost complete coverage in space and timewhich is so valuable from the point of view of climateresearch, a subject in which there has been increasing interestin recent years. Part of this interest may have arisen becauseof our increased ability to observe the atmosphere and tomodel its behaviour theoretically. Other reasons for it are theextreme variations of climate during the past decade, whichhave hit various parts of the world, especially Africa and India,and the increased vulnerability of many countries to food andwater resources.

The climates of the past provide a good idea of the range ofnatural climatic variation. Fig. 1 illustrates some of thechanges which have occurred during the last half-million years,the last 1000 years and the last 100 years. Note that quitesmall changes in mean conditions, for instance a 1 K change inmean temperature, imply significant changes in the climate.

Considerable concern is being shown in the possibility ofclimate change arising from man's activities. In particular, isthere any danger that large changes will occur because of theincreasing carbon-dioxide content of the atmosphere due tothe burning of fossil fuels? Because carbon-dioxide absorbsstrongly in the infra-red, radiation from the earth's surfaceis prevented from leaving the earth-atmosphere system, and soincreased carbon dioxide results in an increase in mean surfacetemperature.

Fig. 2 shows that a change of temperature of around 2 Kat the earth's surface is predicted to occur for a doubling ofthe atmospheric carbon-dioxide concentration. Such a doub-ling would be reached during the early years of the 22ndcentury if fossil fuel consumption continued at the presentlevel, and by the middle of the 21st century if fossil-fuelconsumption continues to rise at 4% per annum.

Paper 1488A, the 16th Appleton Lecture, delivered 14th January1981 at Savoy PlaceProf. Houghton is Director Appleton at the Rutherford & AppletonLaboratories, Chilton, Didcot OX11 OQX, Oxon., England

The Manabe and Wetherald model [4], quoted in Fig. 2,although probably the most complete of its kind, is not,however, adequate to make quantitative climatic predictions.It does not take into account two of the most importantfeedback processes which occur in the atmosphere, namely thefeedback between radiation and cloudiness and the interactionbetween the atmosphere and the oceans; a more detaileddescription of these processes will be given later. We note nowthat, because such processes are not sufficiently understood tobe included in numerical models of the atmospheric Circu-

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Fig. 1A Climate of last half-million years deduced from measure-ments of oxygen isotope ratio in plankton shells which relate to globalice volume [1]

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Fig. 1C Climate of last 100 years as evidenced by changes in averageannual temperature of northern hemisphere [3]

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lation, quantitative climatic prediction with such modelscannot yet be carried out.

The attainment of a better understanding of climate andclimatic change, particularly on time scales in the rangefrom weeks to decades, is a goal of the World ClimateResearch Programme (WCRP) an enterprise organised jointlyby the International Council of Scientific Unions and theWorld Meteorological Organisation. Within this programme,plans are being made for the development of numerical modelsof the climate system and for the development of observationswith coverage in space and time appropriate to the climatesystem and the aims of the WCRP.

Q9 90 90° 80° 70° 60'

Fig. 2 Changes in atmospheric temperature (K) which would beexpected to occur if the atmospheric CO2 content were doubled aspredicted from a model due to Manabe and Wetherald [4]

Note that the largest change occurs in polar regions

Studying the climate on time scales of a few weeks — theshorter end of the time range addressed by the WCRP — is anatural extension of the study of 'weather' on time scales ofa few days. Throughout the 1970s the problem of global'weather' was addressed by the Global Atmospheric ResearchProgramme, culminating in 1979 in the Global Weather Exper-iment, during which the atmosphere was observed as com-pletely as possible for the period of a year. Before going on,therefore, to considering observations particularly relevant tothe longer time scales, it is appropriate that we should look,in the following Section, at the observing system for theGlobal Weather Experiment, in particular the part remotesounding observations played in it.

2 Global Weather Experiment

The purpose of the Global Weather Experiment was to investi-gate to what extent the atmosphere's large-scale circulationcan be modelled and predicted by large numerical models.Large-scale circulation includes those motions which possessa characteristic horizontal dimension greater than about100 km, the smaller scale motions in such models having tobe parameterised. The scientific questions being addressed bythe experiment therefore are: how are the different scales ofmotion in the atmosphere organised, what interaction is therebetween the small-scale motion and the large-scale, and overwhat timescale is the large-scale flow predictable? The last ofthese questions has, of course, very large practical applicationin weather forecasting.

To carry out the experiment, observations of the wholeatmosphere with adequate coverage in space and time wererequired. Over many parts of the globe, especially the southernhemisphere, and much of the ocean areas, such coverage canonly be obtained by employing satellites, both as platformsfrom which remotely located instruments (e.g. on free-floatingbuoys) can be interrogated, and as platforms which can carryremote sounding instrumentation.

A particularly important remote sounding measurementfrom satellites is that of the temperature structure of theatmosphere below the satellite. This can be inferred bymeasuring the amount of radiation emitted at various wave-lengths from the infra-red bands of carbon dioxide locatednear 15/xm and 4.3 pan wavelength, or from the microwaveband of oxygen near 5 mm in wavelength.

Nimbus 3, launched in 1969, carried the first instrumentsfor remotely sounding atmospheric temperature structure[5, 6] . Further instruments were flown on later satellites inthe Nimbus series (see review by Houghton and Taylor [7]).In particular Oxford and Heriot Watt Universities in the UKpioneered selective chopping techniques for remote soundingof the temperature of the stratosphere and mesophere. Thewhole atmosphere up to an altitude of ~ 90km therebybecame accessible to remote sounding measurement (Fig. 3).Because of the very complete coverage in space and timewhich is possible from space, a vast increase in our knowledgeof the structure and circulation of the middle atmosphere hasresulted from satellite observations [10].

90

1000-80

south

Fig. 3 Temperature (K) cross-section of the atmosphere from 80°Nto 80°S as deduced from radiance measurements from selective chopperradiometer on Nimbus 5 and pressure modulator radiometer on Nimbus6 for 4th August 1975 [8]

Method for retrieval of the temperature from the radiances is due toRodgers [9]

The impact of satellite soundings on our knowledge of thelower atmosphere is more difficult to assess [11]. The value ofthe observations depends critically on their accuracy, whichneeds to be of the order of 1 K in the retrieved atmospherictemperature. With careful calibration, the radiances from aninfra-red instrument can be measured to a fraction of a percent (better than 0.2 K in equivalent temperature [12]). Theaccuracy of the basic radiance measurements can thereforebe more than adequate. The major difficulty with interpret-ation of infra-red radiances is the disturbing influence ofclouds. Although clouds are nothing like so troublesome inthe microwave region, the variation of the Planck functionwith temperature there is less, and so the radiance measure-ment needs to be considerably more accurate for the sameaccuracy in equivalent temperature; also the rather largevariation of surface albedo at millimetre wavelengths createsa problem. For these reasons the impact of satellite soundings

IEEPROC, Vol. 128, Pt. A, No. 6, SEPTEMBER 1981 443

on operational meteorology has not been as large as was atfirst expected.

The situation, however, improved considerably with thelaunch of the first of the Tiros-N series in 1978 (Fig. 4).These satellites carry three temperature-sounding instruments[14], an infra-red filter radiometer haying 20 channels, astratospheric sounding unit built along the lines of the OxfordUniversity Nimbus 6 instrument and a four-channel micro-wave radiometer. Fig. 5 shows that the weighting functions(i.e. the functions describing the atmospheric regions fromwhich the radiation originates) appropriate to the differentchannels have a great deal of overlap in the lowest 10 km ofthe atmosphere. By combining observations from these threeinstruments, much better temperature retrievals than pre-viously have been obtained.

of improving substantially the operational methods by whichsatellite radiances are turned into atmospheric temperaturestructure is the major one remaining in satellite temperaturesounding.

Bengtsson and Kallberg's study [15] is the first which hasbeen carried out with the data sets available from the GlobalWeather Experiment. Among their conclusions they find thatthe variability on a synoptic scale in the tropics and theassociated exchange between the northern and southernhemispheres is larger than expected, and that the influenceof the tropics on high latitudes occurs to a larger extent andmore rapidly than previously assumed. Hence they emphasisethat observations on a truly global scale, which can only beprovided through satellite systems, are imperative for studiesof climate.

solar arraydrive motor

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instrumentmountingpbtformsunshadeinstrument

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Fig. 4 Tiros-N spacecraft [13]Notice three temperature-sounding instruments, high-resolution infra-red radiation sounder (HIRS), stratospheric sounding unit (SSU) andmicrowave sounding unit (MSU)

An assessment of the accuracy of space-based data com-pared with conventional data has been carried out byBengtsson and Kallberg [15]. The impact of satellite data onsouthern-hemisphere analyses has been large (Fig. 6). Coverageof the southern oceans by conventional means is extremelysparse, and analyses there are now heavily dependent onsatellite soundings. What about the northern hemispherewhich has much better coverage of conventional soundings?Here Bengtsson and Kallberg show that, by removing fromtheir analysis scheme all observations except surface pressureand satellite soundings, sufficient information about theinitial state is available to their numerical model for theprovision of acceptable forecasts for a few days ahead for thewhole globe. However, they also show that, in areas wheregood coverage of routine observations is available, the satellitesoundings do not give as accurate a picture as the radiosondes.One of the main reasons for this is that the operational pro-cedures for treating the satellite data do not make the bestuse of them. Although the satellite observations have poorresolution in the vertical (owing to the vertical spread of theweighting functions, see Fig. 5), they have very high horizontalresolution. This wealth of detailed information in the horizon-tal has been successfully employed in research studies and formesoscale forecasting [14], but has not been adequatelyexploited in the production of operational retrievals. This task

3 Cloud-radiation feedback

In the preceding Section we considered observations fromspace employed for continuously monitoring the structure ofthe global atmosphere — observations which are required formedium-range weather forecasting (i.e. up to ~ lOd) as wellas for keeping a watch on the climate. We turn now to whatfurther information is required when changes over longer timescales — weeks to decades — are discussed. Two particularlyimportant problems were mentioned in the introduction:the first of these is cloud-radiation feedback.

Simply posed, the problem is the following. Suppose theinput of radiant energy from the sun changes by 1%, if cloudi-ness and atmospheric water-vapour content remain unchanged,a simple calculation shows that the increase in temperature atthe surface would be ~ 0.7 K. However, a temperature rise atthe surface would lead to increased evaporation, henceincreased water-vapour content, and we might also expectincreased cloud cover. The increased cloud cover in turn wouldhave two competing effects: first, it would reflect more solarradiation out to space, hence decreasing the energy input atthe surface, and secondly, it would act as a blanket on theemission of infra-red radiation, thus tending to increase thenet radiation at the surface.

The amount and sign of the cloud-radiation feedback

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depends critically on cloud type and height, as well as on thedetailed mechanism of cloud formation. Satellite measure-ments of the radiation budget at the top of the atmosphere,together with associated measurements of cloud height, typeand amount, can help very substantially to test the variousempirical schemes by which the radiative effects of clouds areintroduced into numerical models.

Radiation-budget measurements have been carried out fromspace for some ten years or more. Those made so far are,however, insufficiently accurate. Not only do the radianceobservations themselves need to be made to rather better than1%, but the sampling in space and time needs to be adequateto remove bias due to, for instance, the variation in cloudcover with local time [11]. A major step forward in theseobservations will occur with the realisation of the EarthRadiation Budget Satellite System of the USA around 1984.In this system three satellites in different orbits will carryradiation-budget instruments, so overcoming much of theproblem of sampling. Associated with these observations ofradiation budget, it will be necessary to devise means ofdescribing the amount and type of cloud cover and preferablyalso the cloud height.

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Fig. 5 Weighting functions (functions describing region of atmos-phere from which radiaton received by different channels originates)for various channels of radiometers on Tiros-N (adapted from Smithetal. [14]

Fig. 4 shows the different radiometers. Weighting functions:HIRS long-wave channelsHIRS short-wave channelsMSU channelsSSU channels

4 Importance of oceans

That the oceans are a very important component of theclimate system is readily demonstrated by Fig. 7, which showsestimates of the flux of heat across various lines of latitude inthe oceans of the world as deduced by Stommel [16], largelyfrom oceanographic data. Other estimates of the transport ofheat within the ocean have been made by Oort and VonderHaar[17], as a residual from an energy-budget calculationbeginning with satellite measurements of the net energybudget at the top of the atmosphere and using the best esti-mates available of the transport by the atmosphere. Becausethe data on which they are based are not really sufficientlyaccurate, a great deal of confidence cannot be placed in theaccuracy of these estimates of ocean transport. They agree,however, in the general conclusion that the transport of heatfrom equatorial to polar regions through the ocean circulation,

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Fig. 6 Southern hemisphere analyses of height (dm) of the 500 mbarsurface on 9th Jan 1979 for complete meteorological data set and forspace-based/surface pressure data only [15/

Note that most of the information is contained in the analysis fromthe space-based system/surface pressure above.a Space-based data plus surface pressure and aircraftb Complete data set

IEEPROC, Vol. 128, Pt. A, No. 6, SEPTEMBER 1981 445

at an average rate of about 1015 W, is roughly equal to thetransport by the atmosphere. It is clear, therefore, that weshall not advance much in our understanding of climateuntil we know a great deal more about the circulation of theoceans.

New possibilities for observing the oceans from space werepioneered with the US satellite Seasat. Launched in 1978,although it only had a 3-month life, enough data were returnedto prove the importance of several new active microwaveinstruments (Fig. 8). Two of these, namely the scatterometer-and the radar altimeter, are of particular importance forinvestigations of the ocean circulation.

The Seasat-A scatterometer system (SASS) is a sideways-looking radar at 14.6 GHz, the return signal from whichdepends largely on Bragg scattering from the small capillarywaves of a few centimetres wavelength on the ocean surface.Four antennas are mounted on the satellite; comparing signalsfrom these antennas gives information on wave direction aswell as wave amplitude.

Algorithms, largely developed empirically, relate theSASS signals to wind speed and direction. Results fromcomparisons of SASS observations with direct measurementsfrom buoys show that, under most conditions, the wind speedand direction can be determined as accurately from the

90 ' 90 180°

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Fig. 7 Estimates of flux of heat in oceans [16]

Numbers are values of flux across lines of latitude in different oceansin units of 1013 W

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Fig. 8 Instrument complement on US satellite Seasat

Five sensors were carried; total mass was 2300kg, and length of syn-thetic aperture radar (SAR) antenna was 10.7 m. Seasat was flown ina sun-synchronous near-polar orbit at an altitude of 800 km [18].

satellite data as is possible using in situ instruments (Fig. 9).There remains a problem of directional ambiguity in thecomparison of data from the four satellite antennas; this caneasily be removed by ensuring consistency of the data insimple atmospheric models.

360r

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Fig. 9 SASS wind vector comparisons with buoy reports duringstudy in Gulf of Alaska [19]

Observations of the surface wind are, of course, very valu-able to the meteorologist. They are also vital to any study ofocean circulation because it is the surface wind (closelyrelated to the surface stress) which is the basic force drivingthe ocean circulation. Any model of the ocean circulationneeds the surface stress as its basic input.

The other instrument on Seasat of particular importancefor ocean observations is the radar altimeter. This is a radarat 13.5 GHz which measures the shape of the return pulsefrom the ocean surface beneath the satellite and the timetaken for the pulse to be returned to the satellite receiver.From the shape of the pulse information about the averageheight of the waves below the satellite (known as the signifi-cant wave height) is obtained [20]. From the time for returnof the pulse, an accurate measurement is obtained of theheight of the ocean surface relative to the height of the satel-lite Qia in Fig. 10). If the satellite's orbit is known, and the

satellite orbit

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Fig. 10 Principle of radar altimeter

geoid (i.e. the surface of constant geopotential) is also known,radar-altimeter measurements of ha (Fig. 10) enable theheight of the sea surface, relative to the geoid, to be deter-mined. Were the ocean perfectly still relative to the .earth thesurfaces of the ocean and the geoid would coincide. Theocean, however, is moving; ocean currents are driven bypressure gradients in much the same way as are winds in theatmosphere. The pressure gradients show up as variations inheight of the ocean surface. Under the geostrophic approxi-mation, the geostrophic velocity Vg at the ocean surface isgiven by

e ~ fdn

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Table 1: Supporting information re-quired for interpretation of radaraltimeter data in terms of ocean

circulation

Knowledge of geoidKnowledge of satellite positionCorrection for atmospheric refractionCorrection for character of sea surfaceCorrection for tidal variations

where g is the acceleration due to gravity, / i s the geostrophicparameter (7.3 x 1CT5 at latitude 30°) and dh/dn is themagnitude of the slope of the ocean surface. At 30° latitude,Vg = 0.5 ms"1 (~ 1 knot) for a slope of 35 cm in 100 km. Theaccuracy of the measurement needs therefore to be very high.Furthermore, for interpretation of the altimeter measure-ments, other information needs to be acquired; a list of themain requirements for these additional data is given in Table 1.Further details can be found in the report of a study on radaraltimetry carried out for the European Space Agency by theRutherford & Appleton Laboratories and the Institute forOceanographic Sciences in the UK [21]. Considering theextreme accuracy required in the final result, and the compli-cations of the measurements involved, it might seem at firstsight rather unlikely that useful measurements of the slope ofthe ocean surface can be achieved. However, for some part ofits mission, Seasat was in an orbit which repeated the sametracks with respect to the earth's surface every 3 d. By com-paring measurements over the same tracks at 3 d intervals, andlooking for changes, variations in the geoid or systematicvariations in the satellite orbit could be eliminated. Thatinformation about the ocean circulation is forthcoming isillustrated in Fig. 11. The Gulf Stream is clearly marked; thesurface elevation of the ocean surface drops by about 1 mas the Gulf Stream is crossed. A cold eddy having a depressionof ~40cm at the centre is also clearly visible in Fig. 11.About six such eddies are formed per year as a result of themeandering of the Gulf Stream [23]. Similar warm eddiesare formed on the northern boundary of the Gulf Stream.Their formation and decay results in a significant heat trans-

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Fig. 11 Collinear passes of Seasat Radar Altimeter across the GulfStream at 3d intervals from 17th September 1978 to 8th October1978Note cold eddy which passed westwards out of satellite's track [22]

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port. Measurements of these features from Seasat clearlyindicate the potential of the technique.

5 Discussion and Conclusions

This paper has rather briefly described different types ofsatellite-borne remote sounding techniques of importancefor the World Gimate Research Programme. I have tried toindicate the developments which are needed so far as thesetechniques are concerned. With remote temperature sounding,it is the improvement of operational algorithms and pro-cedures which make more complete use of the data which isurgently required. For radiation-budget measurementsimproved sampling is necessary, and methods need to bedeveloped to describe consistently the amount of cloudcover to relate to the radiation-budget measurements. Methodsof monitoring cloud height also need to be developed ifprogress in the understanding of the cloud-radiation feedbackproblem is to be forthcoming.

For observations of the ocean, Seasat, despite its shortlife, has pioneered several important developments. What ofthe future in this area?

Plans are now being made by the European Space Agencyfor mounting a scatterometer and radar altimeter on the firstEuropean remote sensing satellite ERS-1, due for launcharound 1987. The Japanese Space Agency are also proposinga series of satellites for oceanographic observation. Althoughthe USA has recently abandoned plans for a large oceano-graphic satellite observing system, they are considering flyinga satellite called TOPEX (topographic experiment) carrying aradar altimeter in an orbit optimised for that particular instru-ment. Because of the possibility that a number of oceanobserving satellites will be flown by various agencies in the late1980s, the oceanographic community through the WorldClimate Research Programme are making plans for particularlyintensive study of the oceans during this period. Satellite-and surface-based observations will need to be closelyco-ordinated, and means for dealing with the vast quantitiesof data involved will have to be set up. If these plans material-ise, a substantial step forward in climate research will havebeen made.

There has only been time to cover a small part of the fieldof climate-related observations from space. I have not forinstance mentioned passive microwave measurements forobserving precipitation, ice cover or soil moisture. Nor hasthere been mention of infra-red observations of atmosphericcomposition, or of sea-surface temperature, where there areimportant recent developments, or the possibilities beingpursued for instrumentation suitable for measuring importantmeteorological parameters, such as wind and surface pressurefrom space.

I hope, however, I have been able to give you a feeling forthe fascination and the challenge of the subject of earthobservation from space. Combining as it does the mostdemanding of instrumentation, requirements for data manage-ment and organisation on a very large scale, very difficultproblems in data interpretation and application, together withthe glamour of an association with space and the relevance ofclimate research, it is virtually unsurpassed as a field forscientific endeavour. Finally, it is an enterprise about whichall of mankind is concerned, and in which the whole worldcan be involved. The climate problem is not one which willbe solved quickly or easily; I hope I have convinced you thattackling it is very worthwhile.

6 References

1 HAYS, J.D., IMBRIE, J., and SHACKLETON, M.J.: 'Variations inthe earth's orbit; pacemaker of the ice ages', Science, 1976, 194,pp. 1121-1132

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2 LAMB, H.H.: The changing climate: selected papers' (Methuen,London,1966)

3 MITCHELL, J.M., Jr.: 'The changing climate' in 'Energy and cli-mate, studies in geophysics' (National Academy of Sciences,Washington DC, 1977)

4 MANABE, S., and WETHERALD, R.T.: 'On the distribution ofclimate change resulting from an increase of CO2 content of theatmosphere',/. Atmos. Sci., 1980, 37, pp. 99-118

5 WARK, D.Q., and HILLEARY, D.T.: 'Atmospheric temperature:successful test of remote probing', Science, 1969, 165, pp. 1256-1258

6 HANEL, R.A., and CONRATH, B.J.: 'Interferometer experiment onNimbus 3; preliminary results', ibid., 1969, 165, pp. 1258-1260

7 HOUGHTON, J.T., and TAYLOR, F.W.: 'Remote sounding fromartificial satellites and space probes of the atmospheres of the earthand the planets', Rep. Prog. Phys., 1973, 36, pp. 827-919

8 HOUGHTON, J.T.: The stratosphere and mesosphere', Q. J.R.Meterol. Soc, 1978, 104, pp. 1-29

9 RODGERS, CD.: 'Retrieval of atmospheric temperature andcomposition from remote measurements of thermal radiation',Rev. Geophys. & Space Phys., 1976, 14, pp. 609-624

10 BARNETT, J.J.: 'Satellite measurements of middle atmospheretemperature structure', Philos. Trans. R. Soc. London A, 1980,296, pp. 41-57

11 HOUGHTON, J.T.: 'The future role of observations from meteoro-logical satellites', Q. J.R. Meteorol. Soc, 1979, 105, pp. 1-23

12 HOUGHTON, J.T.: 'Calibration of infra-red instruments for theremote sounding of atmospheric temperature', AppL Opt., 1975,16, pp. 319-321

13 SCHWALB, A.: 'The Tiros-N NOAA A-G satellite series'. NOAAtechnical memorandum NESS 95, Washington DC, 1978

14 SMITH, W.L., WOOLFE, H.M., HAYDEN, CM., WARK, D.Q.,andMcMILLAN, L.M.: The Tiros-N operational vertical sounder',Bull. Am. Meteorol. Soc, 1979,60, pp. 1177-1197

15 BENGTSSON, L., and KALLBERG, P.: 'Numerical simulation -assessment of FGGE data with regard to their assimilation in a glo-bal data set'. Paper presented at COSPAR, Budapest, 1980

16 STOMMEL, H.: 'Asymmetry of interoceanic fresh-water and heatfluxes', Proc. Natl. Acad. Sci. USA, 1980, 77, pp. 2377-2381

17 OORT, A.H., and VONDER HAAR, T.H.: 'On the observed annualcycles in the ocean-atmosphere heat balance over the northernhemisphere', J. Phys. Oceanogr., 1976, 6, pp. 781-800

18 BORN, G.H., DUNNE, J.A., and LAME, D.B.: 'Seasat missionoverview', Science, 1979, 204, pp. 1405-1406

19 LINWOOD JONES, W., BLACK, P.G., BOGGS, D.M.,BRACKALENTE, E.M., BROWN, R.A., DOME, G., ERNST, J.A.,HALBERSTAM, I.M., OVERLAND, J.E., PETEHERYCH, S.,PIERSON, W.J., WENTZ, F.J., WOICESHYN, P.M., andWURTELE, M.G.: 'Seasat scatterometer: results of the Gulf ofAlaska workshop', ibid., 1979, 204, pp. 1413-1415

20 TOWNSEND, W.F.: 'An initial assessment of the performanceachieved by the Seasat-1 radar altimeter', IEEE J. Oceanic Eng.,1980, OE-5, pp. 80-92

21 'Study on satellite radar altimeter in climatological and oceano-graphic research' (European Space Agency, Paris, 1980)

22 CHENEY, R.E., and MARSH, J.G.: 'Seasat altimeter observationsof dynamic ocean currents in the Gulf Stream region', /. Geophys.Res., 1981, 86, pp. 473-483

23 LAI, D.Y., and RICHARDSON, P.L.: 'Distribution and movementof Gulf Stream rings', /. Phys. Oceanogr., 1977, 7, pp. 670-683

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