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Observing the earth radiation budget from satellites: Past, present, and a look to the future
Adv. Space Re~;. Vol. 5, No. 6, p p . 8 9 - 9 8 , 1985 P r i n t e d in G r e a t B r i t a i n .
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0273-1177/85 $0.00 + .50 C o p y r i g h t © COSPAR
OBSERVING THE EARTH R A D IA TIO N BUDGET FROM SATELLITES: PAST, PRESENT, AND A LOOK TO THE FUTURE Frederick B. House* Department of Physics and Atmospheric Science, Drexel University, Philadelphia, PA 19104, U.S.A.
ABSTRACT Satellite measurements of the radiative exchange between the planet Earth and space have been the objective of many experiments since the beginning of the space age in the late 1950's. The on-going mission of the Earth Radiation Budget (rsRB) experiments has been and will be to consider flight hardware, data handiin~ and scientific analysis methods in a single design strategy. Research and development on observational data has produced an analysis model of errors associated with ERB measurement systems on polar satellites. Results show that the variability of reflected solar radiation from changing meteorology dominates measurement uncertainties. As an application, model calculations demonstrate that measurement requirements for the verification of climate models may be satisfied with observations from one polar satellite,provided we have information on diurnal variations of the radiation budget from the ERBH mission.
INTRODUCTION It is quite clear that a key component or the observational portion of the World Climate Program will depend on satellitemeasurements. These space observations demand new ideas and advances in measurement techniques, better accuracy and stability of instrumentation, and coordinated data management plans. In the study of the earth radiation budget and its relationship to climate, it is essential to know the totalradiative inputs and losses at the upper and lower boundaries of the atmosphere. There is a need to monitor the planetary radiation balance at the top=of-the-atmosphere from satellites. Observations of the radiation balance or net radiation include the measurements of direct solar radiation, and the upwellin~ components of shortwave reflected exitance and longwave emitted exitance from the earth. An example of one measurement application is the need to observe the upwelUng components of the balance with spatial scales of about 250 k m for purposes of el/mate model verification. Instruments should be in orbit for at least I0 years and perhaps 20 years to accompl/sh this monitoring goal, and uncertainties of monthly observations should be within 5-I 0 W/m*'2. See /I/. The focus of this paper is to review the progress and perspectives for future improvements in measuring the radiation balance of the Earth-atmosphere system. The organization of the paper is divided into two sections. First, time is turned back and excerpts from the history of ERB measurements are traced from the beginning of the space effort. Second, an error analysis of current instrument systems is developed to assess the uncertainty of EZB observations for different space and time scales. "Currently on a leave of absence at the Atmospheric Sciences Division, NASA Langley
Research Center, Hampton, VA 23665 USA. JASR 5:6-(; 89
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HISTORY OF ERB M E A S U R E M E N T SYSTEMS Accurate satellite measurements of the radiation balance have been the objective of many experiments since the beginning of the space age. However, observations have been sporadic for most of the period up to the last decade. Even the most recent observations do not meet the monitoring goals mentioned above. Several factors ir~luence the design of ERB instrument systems on satellites. These include orbital and attitude constraints of the spacecraft, the viewing geometry of the radiometers, the spectral band-pass of the measurements, and methods of on-board calibration in orbit. Orbital characteristics affect the time/space sampling of observations on earth. In addition, measurement systems must adapt to the spacecraft attitude and spin characteristicswith radiometers isolated from the spacecraft. Both non-scanning (fixed) and scanning radiometers have been flown as experiments on satellites. The fixed instruments are referred to as medium field-of-view (MFOV) and wide field-of-view ( W P O V ) radiometers, and .the scanning instruments as the narrow field-of-view (NIFOV) radiometers. These instruments must observe two broad regions of the electromagnetic spectrum. Color differences of detectors or the use of filtersisolated the shortwave solar spectrum from 0.3 ~um to 4.0 ,urn, and the longwave infrared region from 4.0 ,urn to 50+~m. On-board calibration of both spectral regions is essential for the monitoring of measurement stability. Shortwave calibration methods included direct observations of solar irradiance, possibly reflected or transmitted solar radiation, and the darkness of space. Longwave calibration techniques included observations of a w a r m black-body source and the cold space reference.
(a)
(b)
Fig. I. SRB satellitesthat were part of the Vanguard Project during the 1950's. Early SatelliteSystems. The evolution of SRB satellite systems closely followed the development of space technology. As space hardware improved and rocket payload capacity increased, sateUites became more sophisticated in their construction and monitoring applications. The Vanguard Project was one of the first series of satellites to perform Earth measurements. The Vanguard 2 satellite shown in Fig. l.a is the first spacecraft built to monitor the radiation balance components. This spacecraft was developed by a team at The University of Wisconsin, headed by Professors Verner Suomi and Robert Parent. The spherical body of the satellite was about 60 cm in diameter and housed batteries, electronics and telemetry systems. Spherical thermistor bolometers on four extended booms performed radiation measurements of upweUing exitance fields from the earth. In the photograph, the black radiometer measured both the shortwave and Iongwave ~mponents while the white radiometer measured only the longwave component (white
Observing the Earth Radiation Budget
91
reflects shortwave radiation). The shortwave .exitance field.,is computed from the difference or the two measurements. The two attempts made to launch this satelliteinto orbit were unsuccessful due to rocket failures. The N e w York Times headlines for February 18, 1959, in Figure l.b announced the successful launch or a Vanguard satellite that measured reflected solar radiation. The instrument had telescope optics with radiometers which employed the spin and motion or the satelliteto scan successive swaths or the Earth scene. The stabilityor the spin vector degraded shortly after launch causing the satellite to tumble. However, this spacecraft made the first measurements of solar irradiance and cloud reflection. Note that the newstand price or the N e w York Times is 5 cents.
~_ ~s~,',~ ,," ~"t~'~:' ," .~
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(a) Fig. 2.
(b)
(a) Explorer 7 satellite hemispherical bolometers mounted on
rectangular mirrors at the equator of the spacecraft. (b) Comparison of emitted exitance measurements (cal/(cm*'2-min)) with synoptic weather patterns. See Weinstein and Suomi 12/. The first successful launch or an ERB instrument system was on Explorer 7 satellitein 1959. The instrumentation in Figure 2.a was built by the University or Wisconsin team and consists of • modification of the measurement principle for the Vanguard 2 experiment in Figure l.m The black and white radiometers were hemispheres attached to rectangular mirrors at the equator of the spacecraft. Figure l.b shows results from Explorer 7 measurements where emitted exitance observations are compared with synoptic weather patterns. These observations confirm that large exitances are associated with cloud-free high pressure areas, and smaller exitances correspond to cloudy low pressure systems. ~..~,..~.~:~ ....
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Fig. 3. Front page or the N e w York Times announcing the launch or Tiros I Satellite. (b) Comparison or cloud patterns and synoptic weather systems.
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The N e w York Times headlines for April 2, 1960, in Figure 3.a announed the successful launch of Tiros I satellite into a prograde orbit. This satellitecarried the first imaging system or TV camera on board which observed the patterns of reflected solar radiation. The spacecraft was space-spin stabilized which caused daytime observations in the northern hemisphere to preceed those in the southern hemisphere as the orbit precessed westward relative to the earth-sun reference. Figure 3.b shows an early nephanalysis of cloud patterns and their relationship to synoptic weather patterns. These observations illustrate the large-scale organization of weather systems. Tiros 2 satellite was launched in 1960, and the observations included shortwave and longwave scanning radiometers in addition to the video systems. For the first time, quality observations by scanning radiometers on satelliteswere available for analysis with synoptic weather patterns. Figure 4.a and 4.b show shortwave and longwave radiation patterns for the region from the eastern Atlantic Ocean to northern Africa. The maps illustrate observed patterns in relative counts. Note the detailed structure of the reflected shortwave scene compared to the smoothness of the longwave observation,,. These observations were the forerunners of current imaging systems on satellitestoday. Optical degradation of these radiometers emphasized the need for on-board calibrations. ('. : "."~; ...... " ::. i.': ::..i;" ~ii:
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Fig. 4. Broad-band scanning radiometer measurements from Tiros 2 satellite on November 29, 1960. Maps show patterns of (a) reflected shortwave and (b) emitted longwave observations in units of relative counts. Observations from the Serie~ of Nimbu~ Spacecraft, The Nimbus spacecraft was a three-axis stabilized satellite that maintained its attitude relative to the Earth. Several experiments related to KRI~ measurements were aboard some of the seven successful launches of the spacecraft from the mid 1960's through the late 1970's. One of the instruments that significantly impacted our knowledge of the radiation balance was the 5-channel M R I R scanning infrared radiometer on Nimbus 3.
~aj (b) Fig. 5. (a) The medium resolution infrared radiometer (MRIR) flown on Nimbus 3 satellite. (b) World map of the annual radiation balance.
93
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Earth scanning was accomplished, between the yoke structure of the M R I R instrument shown in Figure 5.a. A rotating ellipticalmirror projects the image of the earth scene on to five radiometers located to the right. The top of the yoke contains a w a r m black-body source for calibration and comparison with measurements of cold space during each sca~ Observations by the M R I R instrument were used in analyses of the Earth radiation budget. Figure 5.b presents an example of the detailed world map of the snnual radiation balance from Raschke et al,/3/. ERB instruments were flown on both Nimbus 6 and Nimbus 7 satellitesin 1975 and 1978. This was the first successful experiment to monitor all components of the radiation balance. In fact,three separate experiments were combined into one system. Figure 6 is a photograph of the complete sensor assembly where the instrument is attached to the bottom of the spacecraft in the orientation shown. The first experiment consists of ten solar sensors which measure the total solar irradiance and the radiation from selected spectral intervals for each orbit of the satellite. The second experiment monitors the Earth exitance with four fixed W F O V radiometers including the visible, near infrared and longwave regions of the spectrum. The third experiment ~ ,~L,~, ............. I monitors the angular distribution of the emerging ..... radiance fields from the Earth scene. The scanning Fig. 6. The ERB instrument flown NFOV radiometers are mounted on a bi-axial gimbal I on Nimbus 6 & 7 satellites. assembly to enhance the spatial coverage of scanning data. Both the shortwave and longwave regions of the spectrum are monitored by four telescopes in the scanhead. Additional details of the ERB experiment and the data archive are available in references /4,5,6/. The Future Earth Radiation BudRet Exoeriment (ERBE). W h e n considering the total of available ERB measurements from satellites,there remains one o u t s t a n d ~ sNent~ic problem for investigation. This problem concerns the diurnal variation of radiation balance components for all spatial scales. The principal scientific objective of the ERBE mission is to study the diurnal variation of the planetary radiation
balance/7/. Figure 7.a illustrates the t~ee-satellite configuration planned for RRBE. Additional details of the measurement system are summarized in the discussion summary of Figure 7.b. MEASUREMENTSYSTEM
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(a)
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Fig. 7. (a) The orbital configuration of the ERBE mission. (b) ERBli instruments are scheduled for launch on N O A A - F satellitein 1984, N O A A - G satellitein 198~. and the ERBS satellitefrom the Space Shuttle in 1984.
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F.B. House
Some important observations have been overlooked in this historical survey of ERB measurement systems. In the 1960's, several additional Wisconsin experiments were flown on the Tiros, Essa and Itos series of satellites. The M R I R scanning radiometer on the early Nimbus satellites lead to the development of the H R I R and the T H I R instruments on later satellitesin the series. In addition, the N O A A operational spacecraft employed a series of scanning radiometers including the SR, the V H R R and the AVHIUI instruments. One must also include the wealth of observations from satellites in geosynchronous orbit, especially those from the S M S I G O K S and M E T E O S A T spacecraft. All of these observations provide a wealth of satellite measurements pertainin~ to observations of the earth radiation budget.
ERROR ANALYSIS OF KIIB M E A S U R E M E N T
SYSTEMS
In the design of instrument systems on satellites, it is important to consider flight hardware in relationship to problems of data handling and analysis. A c o m m o n example of a system mismatch is the situation where the volume of data is too large and the costs too expensive for complete scientific utilization of the measurements. It is unwise to separate the measurement hardware from the data handlin~ procedures and the data products for scientificinvestigations. The physics of radiative transfer limits the accuracy of satelliteobservations. A perfect instrument system on a satellite yields imperfect observationsl Consider longwave observations as an example. Instrument calibrations before launch and in orbit ere based on the measurement o!'a black-body source of radiation. The spectral radiance emerging from the source is known from Planck's radiation law. W h e n the radiometer views the earth scene, the spectrum of the emer~in~ radiation is complicated because of clouds, the surface, absorbtion bands in the infrared spectrum, etc. The only calibration information is for a Planckian spectral distribution of radiation, not that observed from the earth. The primary calibration can be transfered directly to the measurement if the radiometer has a uniform spectral response. To date, seanmng radiometers are far from being spectrally uniform. However, the M F O V and W F O V radiometers for the ERBE mission meet this design goal. Shortwave measurements have similar calibration transfer problems. The error in satellite measurements is described in terms of absolute accuracies and random uncertainties. Absolute accuracies pertain to offset errors of the measurement. Random uncertainties refer to statisticalerrors that reduce in magnitude with repeated observations. For example, transient weather systems passing an area of the earth are "random" in nature. Averaging such observations in time reduces the uncertainty to the true mean value. The radiation balance equation for an area of the Earth is M . = Eo cos(8o) - M s w - M~-w
(1)
M N is the net radiation at the top-of-the-atmosphere, Eo is the solar irradiance incident at angle (~o,M s w and M L W are the shortwave and Iongwave exitances, respectively. Typical annual global values of the net radiation are 0 = 342.5 - 105.5 - 237.0 W/re"2. These values assume a solar constant of 1370 W / r e " 2 and a planetary albedo of 30.8%. If the absolute accuracy of the three terms tothe right of (I) are +_0.5%, +_2.0% and +=2.0%, the rms accuracy of the radiation balance is about I ~ = 0 +_5.4 W/re"2. Thus, one is confident that the Earth is in radiative balance to an absolute accuracy of about +_5.4W/re"2.
Observing the Earth Radiation Budget
95
Aqalysis Model. The interpretation goal of satell/te measurements is to estimate the 24-hr radiation balance for a period in days that is longer than the correlation time of meteorological events in a region. In this context, the total uncertainty of one observation 0='T is a combination of the root variance of scene meteorology O's. the interpretation uncertainties for angular models O'M and filterfunctions of the instrument OF, and noise from the radiometric observation 0~. These are combined in an rms sense by 2
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2
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!/2
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(2)
Preparation of data utilizes either time averaging of observations for a given region or spatial averaging over many regions. The uncertainty of time averaging depends on the samp.n@ rate R (obs./day) of an area and the period T (days) for the averaging period of the product. The uncertainty or spatial averaoino_, depends on the effective area Ao (km**2) OFeach observation and the area OFthe data product AP (kin**2), where AP > A o . The uncertainty of the data product is reduced by the square root OF ( I/RT) in the case OF time averaging and by the square root OF (Ao/A~) For spatial averaging of regions. Both terms apply for time and space averaging. The equation for estimating the uncertainty (T'p of data products is I/2
0"~ = + O~ (I/RT)
I/2
(Ao/AF)
(3)
Equations (2) and (3) assume independent observations and a normal distribution of statistics. The results of calculations in the next section employ observations by the EEB instrument on Nimbus 7 satellite (ERB=7). Observational statistics from ImR=7 measurements are used to define the shortwave and Iongwave meteorological uncertainties 0"s. It is noted that this variable dominates all other terms alTecting the uncertainty of observations in (2). Model Calculations. Model calculations focus on the current WW-7 observations and the forthcoming ElIBl; measurements. F/gure 8 portrays the satellite view of the Earth scene for the fixed Idlq)V and W F O V radiometers, and for the sonnninE H F O V instruments. Table I presents data for the average scene area of useful observations for dLrferent instrument systems. The diameter of each area is given in degrees great circle arc (GCA). The enhanced fixed-field areas pertain to observations after enhancement with a numerical filter. This procedure improves the spatial resolution of the observations by a factor of four.
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House
Table 2 presents data concerning independent sampling rates for polar orbiting Satellites. These values are computed from the spatial sampling of a Scanning radiometer. The rates for longwave (LW) exitances are twice those for shortwave (SW) exitances because L W data are acquired on both ascending and descendlna nodes of the orbit. Smaller magnitudes are assigned to the Wlq)V and M F O V instrument systems. TABLS 2 ~g
INDEPENDENT SAMPLING RATES FOR POLAR ORBITING SATELLITES
60
R(0Os. per day) Instrument system
~*-oT 1%} 40 ~ N F O V
Reflectedexitance
Emittedexitance
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\ t-MFOV(ERBE) , , j ~ L ~ . ~ . . L. ~ - ~ wFOv (ERB'7)
1 2 3 4 5 6 Obse~ationareaAo (KM2)
7 Log (Ao.)
Fig. 9. Total uncertainty of an observation in eqn. (2) for different instrument systems.
Figure 9 shows the percent uncertainty for both S W and L W observations as a function of their effecthre area A o in Table I. Although reasonable estimates were made for other terms in (2), the variability of an observat/on dominates the magn/tude of of the total uncertainty. Data points for (ERB-7) are based on Nimbus 7 observations, and those points for (KIBE) are estimates relative to these observations. EIBE values are larger than those for B~S-7 because its scene area is six times smaller than the EIB-7 area (see Table I). Clearly,the percent uncertainty for S W observations is about two and one halt times greater than those for the L W measurements. This characteristicwas apparent in the structure of the maps in Figures 4.a and 4.b. Meteorolog/cal variab/J/ty is the dominant source of uncertainty affecting the results. For example, assume that onJy one observation is made for a period of one week. The percent uncertainty that this one observation is representative of the average observation during the week is about +_50% and +10% for S W and L W exitances, respectively. These values apply to the NIFOV(ImB=7) observations. Other instrument systems would have dilTerent uncertainties according to the functions in Figure 9. RESULTS A N D DISCUSSION Data for model calculations in the last section are used with equation (3) to compute uncertainties of data products for different time periods and sizes of geographical areas. The minimum spatial resolution of observations is noted in Table I. Figure 10.a (next page) shows the uncertainty of shortwave-albedo data products for averaging periods from a day to a week, month, season and a year. All functions pertain to the minimum resolution of each instrument. Thus, high resolution scanning radiometers have much larger uncertainties than their fixed WI~OV counterparts. In other words, the spatial averaging properties of WI~)V radiometers reduce measurement uncertainties, but ground resolution is degraded. The results indicate that uncertainties decrease rapidly during the first two weeks of observations and then much more slowly. For example, the uncertainties of S W products for the NFOV(I.5) ERIB-7 data are +40% for daily values, +15% for weekly averages, +7% for monthly averages and +4% for seasonal dat=L The averaging period of the data products should be two weeks or longer, Ze., for periods longer than the turning point of the functions in Figure 10.=L The corresponding L W results vary from +6% to +0.6% for the same periods. Simultaneous observations from two and three satellitesystems would reduce the uncertainties by factors of 0.71 and 0.58.
97
Observing the Earth Radiation Budget (D~'~e(er ~GCA)
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6
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(a) (b] Fig. I0. (a) Uncertainty of S W data products for different averaging periods. (b) Uncertainty of observations after spatial averaging. Figure 10.b presents results of uncertainties for both S W and L W measurements =Lfter spatial averaging. Averaging areas range from that for the BRB=7 observations to I0 degree zones, hemispheres and the globe. Separate lines show averaging periods of a day, a month and one year. Data are plotted on a log-log scale, and the lines have a slope of -I/2 which follows from the logarithm of equation (3). These results are useful in evaluating particular instrument systems. Consider the requirement mentioned in the Introduction of monthly products with 5-10 W/re"2 errors and 62,500 kin"2 areas. The c o m m o n log of the area is about 4.8 in Figure 10J) giving uncertainties of +_5% and +0.6% for S W and L W observations, respectively. These values correspond to +_5.3 W l m " 2 and +1.4 W/re"2 for average values of the radiation balance in equation (I). The total rms error for both accuracies and uncertainties is +7.7 W l m " 2 (including the +_5.4 W/re"2 absolute accuracy in (I)). One can conclude that for an average 250 k m by 250 k m region of the earth, a scanning radiometer on one polar orbiting satellite meets the +_5 to I0 W/re"2 monthly error requirement for climate model verifications. This conclusion presumes a satisfactory conclusion of the ~tlBS mission regarding diurnal variations of the radiation balance. Figure 11 illustrates a comparison of data product uncertainties for each instrument system. The differences among the systems for L W exitance measurements are within the +I % level of inaccuracy. In the case of the S W products, the scsnnino radiometers have +8% to +9% uncertainties. This level of error decreases to +1% for resulting areas of about one million square kin. Finally, the fixed M F O V and W F O V instruments indicate uncertainties or 1% or less for all their averaaine areas. LW Exitance
SW Exitance-Albedo tO
COp (%) 1.0
0.1
NFOV (O.6~ OV (1.5 °) °MFOV {5") OV (10") WFOV(7.5") 115"1
~
4 5 6 7 8 9 Log (Ap} Area of eta ~xoduct AD (KM 2)
t0 r
o
~-NFOV10.6 )
cop(%l to~- ~ ' ~ . E r
~-'MFOV(SO~
\ \ ' , , q - ' ~ o v (z5°)
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Area of data product Ap (KM 2)
(a) (b) Fig. I I. Uncertainty of monthly data products for different instrument systems and areas of spatial averaging. SUMMARY AND CONCLUSION
Earth radiation budget (ImR) experiments have a long history spnnnin~ from the first development of measurement systems to the future EglBI¢ mission this year. The progress in this area of satellite observation has been the work and accomplishment of
98
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many individuals from different countries of the world, supported by their institutions. Other papers in Symposium 4 of C~)SPAR will present research results pertaining to recent measurements of the radiation balance. The forthcoming three-sateUite ERBE mission will provide needed observations to study the dirunal variation of the planetary radiation balance. Error analysis of ERB measurement systems includes all aspects of the project from the design and fabrication of the flight hardware to the data handling and scientific evaluation procedures for research. An analysis model is developed to determine the inaccuracy of observations (eqn. 2), and the uncertainty of data products after time andlor spatial averaging (eqn. 3). EI~=7 observations from Nimbus satelliteprovide the basic statisticaldata for variability of meteorology. Results indicate that the minimum time period for averaging scanner measurements is about two weeks for observations from a polar orbiting satellite. The time period shortens for experiments with multiple satellites like the I~I~E mission. Shortwave observations are the limiting factor controlling the level of error in radiation balance measurements -- uncertainties for Iongwave measurements are 40% of those for shortwave exitances. For an average 250 k m by 250 k m region of the earth, the total error in the radiation balance is +7.7 W/re"2 for observations from a polar orbiting satellite. Although with knowledge from the ERBE mission, it may be concluded that a seamninR radiometer on one polar orbiting satellite meets the 5-10 W / m " 2 monthly error requirement for climate model verifications.
ACIKNOWIJB]~KMF~IT Heartfelt thanks are expressed to William R. Bandeen of NASA Goddard Space Flight Center, Greenbelt, MD, USA. who kindly shared his excellent photographs of ERB measurement systems which provided the basis for this historicalreview. This research was supported by the ERBE Project of N A S A La~ley Research Center, Hampton, VA, USA. under Contract No. NSA I-16428 with Drexel University.
I. Satellite systems to measure the Earth's radiation budget parameters and climate change signals,Renort of an International Meetin2 of Exoerts. Igls,Austria (I 983). 2. M. Weinstein and V. Suomi, Analysis of satelliteinfrared radiation measurements on a synoptic scale,Mo~. Wea. Rev.. 89, pp.419-428 (1961).
3. E Raschke, T.H. Yonder Haar, W.R. Bandeen and M. Pasternak, The radiation balance of the Earth-atmosphere system from Nimbus 3 radiation measurements, I. Atmos. Sc,i.. 30, #3, (1973). 4. H. Jacobowitz, H.V. Soule, HJ~. Kyle, F J3. House and The Nimbus-7 ~ Experiment Team, The earth radiation budget (I~R) experiment - an overview, I.GeoDhv. Res.. 89, #D4, pp. 5021-5038, (1984). 5. H.L. Kyle, F J}. House, P£. Ardanuy, H. Jaeobowitz,R.H. Maschhoff and J.R.Hickey, N e w in-flightcalibration adjustment of the Nimbus 6 and 7 earth radiation budgbet wide field of view radiometers, I. GeoDhv. Res.. 89, #D4, pp. 5057-5076, (1984). 6. FJ~. House, Analysis of the dynamic behavior of the I~11 instrument on Nimbus 7 satellite,submitted to the I.Geonhv. Res.. (1984). 7. B. Barkstrom and J. Hall, Earth radiation budget experiment (ERBIi): An overview, ].of Energy, 6, pp.141-146, (1982).
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reserved.
0273-1177/85 $0.00 + .50 C o p y r i g h t © COSPAR
OBSERVING THE EARTH R A D IA TIO N BUDGET FROM SATELLITES: PAST, PRESENT, AND A LOOK TO THE FUTURE Frederick B. House* Department of Physics and Atmospheric Science, Drexel University, Philadelphia, PA 19104, U.S.A.
ABSTRACT Satellite measurements of the radiative exchange between the planet Earth and space have been the objective of many experiments since the beginning of the space age in the late 1950's. The on-going mission of the Earth Radiation Budget (rsRB) experiments has been and will be to consider flight hardware, data handiin~ and scientific analysis methods in a single design strategy. Research and development on observational data has produced an analysis model of errors associated with ERB measurement systems on polar satellites. Results show that the variability of reflected solar radiation from changing meteorology dominates measurement uncertainties. As an application, model calculations demonstrate that measurement requirements for the verification of climate models may be satisfied with observations from one polar satellite,provided we have information on diurnal variations of the radiation budget from the ERBH mission.
INTRODUCTION It is quite clear that a key component or the observational portion of the World Climate Program will depend on satellitemeasurements. These space observations demand new ideas and advances in measurement techniques, better accuracy and stability of instrumentation, and coordinated data management plans. In the study of the earth radiation budget and its relationship to climate, it is essential to know the totalradiative inputs and losses at the upper and lower boundaries of the atmosphere. There is a need to monitor the planetary radiation balance at the top=of-the-atmosphere from satellites. Observations of the radiation balance or net radiation include the measurements of direct solar radiation, and the upwellin~ components of shortwave reflected exitance and longwave emitted exitance from the earth. An example of one measurement application is the need to observe the upwelUng components of the balance with spatial scales of about 250 k m for purposes of el/mate model verification. Instruments should be in orbit for at least I0 years and perhaps 20 years to accompl/sh this monitoring goal, and uncertainties of monthly observations should be within 5-I 0 W/m*'2. See /I/. The focus of this paper is to review the progress and perspectives for future improvements in measuring the radiation balance of the Earth-atmosphere system. The organization of the paper is divided into two sections. First, time is turned back and excerpts from the history of ERB measurements are traced from the beginning of the space effort. Second, an error analysis of current instrument systems is developed to assess the uncertainty of EZB observations for different space and time scales. "Currently on a leave of absence at the Atmospheric Sciences Division, NASA Langley
Research Center, Hampton, VA 23665 USA. JASR 5:6-(; 89
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F.B.
ltouse
HISTORY OF ERB M E A S U R E M E N T SYSTEMS Accurate satellite measurements of the radiation balance have been the objective of many experiments since the beginning of the space age. However, observations have been sporadic for most of the period up to the last decade. Even the most recent observations do not meet the monitoring goals mentioned above. Several factors ir~luence the design of ERB instrument systems on satellites. These include orbital and attitude constraints of the spacecraft, the viewing geometry of the radiometers, the spectral band-pass of the measurements, and methods of on-board calibration in orbit. Orbital characteristics affect the time/space sampling of observations on earth. In addition, measurement systems must adapt to the spacecraft attitude and spin characteristicswith radiometers isolated from the spacecraft. Both non-scanning (fixed) and scanning radiometers have been flown as experiments on satellites. The fixed instruments are referred to as medium field-of-view (MFOV) and wide field-of-view ( W P O V ) radiometers, and .the scanning instruments as the narrow field-of-view (NIFOV) radiometers. These instruments must observe two broad regions of the electromagnetic spectrum. Color differences of detectors or the use of filtersisolated the shortwave solar spectrum from 0.3 ~um to 4.0 ,urn, and the longwave infrared region from 4.0 ,urn to 50+~m. On-board calibration of both spectral regions is essential for the monitoring of measurement stability. Shortwave calibration methods included direct observations of solar irradiance, possibly reflected or transmitted solar radiation, and the darkness of space. Longwave calibration techniques included observations of a w a r m black-body source and the cold space reference.
(a)
(b)
Fig. I. SRB satellitesthat were part of the Vanguard Project during the 1950's. Early SatelliteSystems. The evolution of SRB satellite systems closely followed the development of space technology. As space hardware improved and rocket payload capacity increased, sateUites became more sophisticated in their construction and monitoring applications. The Vanguard Project was one of the first series of satellites to perform Earth measurements. The Vanguard 2 satellite shown in Fig. l.a is the first spacecraft built to monitor the radiation balance components. This spacecraft was developed by a team at The University of Wisconsin, headed by Professors Verner Suomi and Robert Parent. The spherical body of the satellite was about 60 cm in diameter and housed batteries, electronics and telemetry systems. Spherical thermistor bolometers on four extended booms performed radiation measurements of upweUing exitance fields from the earth. In the photograph, the black radiometer measured both the shortwave and Iongwave ~mponents while the white radiometer measured only the longwave component (white
Observing the Earth Radiation Budget
91
reflects shortwave radiation). The shortwave .exitance field.,is computed from the difference or the two measurements. The two attempts made to launch this satelliteinto orbit were unsuccessful due to rocket failures. The N e w York Times headlines for February 18, 1959, in Figure l.b announced the successful launch or a Vanguard satellite that measured reflected solar radiation. The instrument had telescope optics with radiometers which employed the spin and motion or the satelliteto scan successive swaths or the Earth scene. The stabilityor the spin vector degraded shortly after launch causing the satellite to tumble. However, this spacecraft made the first measurements of solar irradiance and cloud reflection. Note that the newstand price or the N e w York Times is 5 cents.
~_ ~s~,',~ ,," ~"t~'~:' ," .~
~
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, I
(a) Fig. 2.
(b)
(a) Explorer 7 satellite hemispherical bolometers mounted on
rectangular mirrors at the equator of the spacecraft. (b) Comparison of emitted exitance measurements (cal/(cm*'2-min)) with synoptic weather patterns. See Weinstein and Suomi 12/. The first successful launch or an ERB instrument system was on Explorer 7 satellitein 1959. The instrumentation in Figure 2.a was built by the University or Wisconsin team and consists of • modification of the measurement principle for the Vanguard 2 experiment in Figure l.m The black and white radiometers were hemispheres attached to rectangular mirrors at the equator of the spacecraft. Figure l.b shows results from Explorer 7 measurements where emitted exitance observations are compared with synoptic weather patterns. These observations confirm that large exitances are associated with cloud-free high pressure areas, and smaller exitances correspond to cloudy low pressure systems. ~..~,..~.~:~ ....
~,.~~,~~,.:~!~:~:~....
(a)
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.
(b)
Fig. 3. Front page or the N e w York Times announcing the launch or Tiros I Satellite. (b) Comparison or cloud patterns and synoptic weather systems.
F.B.
92
ilol~s~,
The N e w York Times headlines for April 2, 1960, in Figure 3.a announed the successful launch of Tiros I satellite into a prograde orbit. This satellitecarried the first imaging system or TV camera on board which observed the patterns of reflected solar radiation. The spacecraft was space-spin stabilized which caused daytime observations in the northern hemisphere to preceed those in the southern hemisphere as the orbit precessed westward relative to the earth-sun reference. Figure 3.b shows an early nephanalysis of cloud patterns and their relationship to synoptic weather patterns. These observations illustrate the large-scale organization of weather systems. Tiros 2 satellite was launched in 1960, and the observations included shortwave and longwave scanning radiometers in addition to the video systems. For the first time, quality observations by scanning radiometers on satelliteswere available for analysis with synoptic weather patterns. Figure 4.a and 4.b show shortwave and longwave radiation patterns for the region from the eastern Atlantic Ocean to northern Africa. The maps illustrate observed patterns in relative counts. Note the detailed structure of the reflected shortwave scene compared to the smoothness of the longwave observation,,. These observations were the forerunners of current imaging systems on satellitestoday. Optical degradation of these radiometers emphasized the need for on-board calibrations. ('. : "."~; ...... " ::. i.': ::..i;" ~ii:
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: :::i
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(a)
(b)
Fig. 4. Broad-band scanning radiometer measurements from Tiros 2 satellite on November 29, 1960. Maps show patterns of (a) reflected shortwave and (b) emitted longwave observations in units of relative counts. Observations from the Serie~ of Nimbu~ Spacecraft, The Nimbus spacecraft was a three-axis stabilized satellite that maintained its attitude relative to the Earth. Several experiments related to KRI~ measurements were aboard some of the seven successful launches of the spacecraft from the mid 1960's through the late 1970's. One of the instruments that significantly impacted our knowledge of the radiation balance was the 5-channel M R I R scanning infrared radiometer on Nimbus 3.
~aj (b) Fig. 5. (a) The medium resolution infrared radiometer (MRIR) flown on Nimbus 3 satellite. (b) World map of the annual radiation balance.
93
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Earth scanning was accomplished, between the yoke structure of the M R I R instrument shown in Figure 5.a. A rotating ellipticalmirror projects the image of the earth scene on to five radiometers located to the right. The top of the yoke contains a w a r m black-body source for calibration and comparison with measurements of cold space during each sca~ Observations by the M R I R instrument were used in analyses of the Earth radiation budget. Figure 5.b presents an example of the detailed world map of the snnual radiation balance from Raschke et al,/3/. ERB instruments were flown on both Nimbus 6 and Nimbus 7 satellitesin 1975 and 1978. This was the first successful experiment to monitor all components of the radiation balance. In fact,three separate experiments were combined into one system. Figure 6 is a photograph of the complete sensor assembly where the instrument is attached to the bottom of the spacecraft in the orientation shown. The first experiment consists of ten solar sensors which measure the total solar irradiance and the radiation from selected spectral intervals for each orbit of the satellite. The second experiment monitors the Earth exitance with four fixed W F O V radiometers including the visible, near infrared and longwave regions of the spectrum. The third experiment ~ ,~L,~, ............. I monitors the angular distribution of the emerging ..... radiance fields from the Earth scene. The scanning Fig. 6. The ERB instrument flown NFOV radiometers are mounted on a bi-axial gimbal I on Nimbus 6 & 7 satellites. assembly to enhance the spatial coverage of scanning data. Both the shortwave and longwave regions of the spectrum are monitored by four telescopes in the scanhead. Additional details of the ERB experiment and the data archive are available in references /4,5,6/. The Future Earth Radiation BudRet Exoeriment (ERBE). W h e n considering the total of available ERB measurements from satellites,there remains one o u t s t a n d ~ sNent~ic problem for investigation. This problem concerns the diurnal variation of radiation balance components for all spatial scales. The principal scientific objective of the ERBE mission is to study the diurnal variation of the planetary radiation
balance/7/. Figure 7.a illustrates the t~ee-satellite configuration planned for RRBE. Additional details of the measurement system are summarized in the discussion summary of Figure 7.b. MEASUREMENTSYSTEM
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(a)
~., ,.' :.
(b)
Fig. 7. (a) The orbital configuration of the ERBE mission. (b) ERBli instruments are scheduled for launch on N O A A - F satellitein 1984, N O A A - G satellitein 198~. and the ERBS satellitefrom the Space Shuttle in 1984.
94
F.B. House
Some important observations have been overlooked in this historical survey of ERB measurement systems. In the 1960's, several additional Wisconsin experiments were flown on the Tiros, Essa and Itos series of satellites. The M R I R scanning radiometer on the early Nimbus satellites lead to the development of the H R I R and the T H I R instruments on later satellitesin the series. In addition, the N O A A operational spacecraft employed a series of scanning radiometers including the SR, the V H R R and the AVHIUI instruments. One must also include the wealth of observations from satellites in geosynchronous orbit, especially those from the S M S I G O K S and M E T E O S A T spacecraft. All of these observations provide a wealth of satellite measurements pertainin~ to observations of the earth radiation budget.
ERROR ANALYSIS OF KIIB M E A S U R E M E N T
SYSTEMS
In the design of instrument systems on satellites, it is important to consider flight hardware in relationship to problems of data handling and analysis. A c o m m o n example of a system mismatch is the situation where the volume of data is too large and the costs too expensive for complete scientific utilization of the measurements. It is unwise to separate the measurement hardware from the data handlin~ procedures and the data products for scientificinvestigations. The physics of radiative transfer limits the accuracy of satelliteobservations. A perfect instrument system on a satellite yields imperfect observationsl Consider longwave observations as an example. Instrument calibrations before launch and in orbit ere based on the measurement o!'a black-body source of radiation. The spectral radiance emerging from the source is known from Planck's radiation law. W h e n the radiometer views the earth scene, the spectrum of the emer~in~ radiation is complicated because of clouds, the surface, absorbtion bands in the infrared spectrum, etc. The only calibration information is for a Planckian spectral distribution of radiation, not that observed from the earth. The primary calibration can be transfered directly to the measurement if the radiometer has a uniform spectral response. To date, seanmng radiometers are far from being spectrally uniform. However, the M F O V and W F O V radiometers for the ERBE mission meet this design goal. Shortwave measurements have similar calibration transfer problems. The error in satellite measurements is described in terms of absolute accuracies and random uncertainties. Absolute accuracies pertain to offset errors of the measurement. Random uncertainties refer to statisticalerrors that reduce in magnitude with repeated observations. For example, transient weather systems passing an area of the earth are "random" in nature. Averaging such observations in time reduces the uncertainty to the true mean value. The radiation balance equation for an area of the Earth is M . = Eo cos(8o) - M s w - M~-w
(1)
M N is the net radiation at the top-of-the-atmosphere, Eo is the solar irradiance incident at angle (~o,M s w and M L W are the shortwave and Iongwave exitances, respectively. Typical annual global values of the net radiation are 0 = 342.5 - 105.5 - 237.0 W/re"2. These values assume a solar constant of 1370 W / r e " 2 and a planetary albedo of 30.8%. If the absolute accuracy of the three terms tothe right of (I) are +_0.5%, +_2.0% and +=2.0%, the rms accuracy of the radiation balance is about I ~ = 0 +_5.4 W/re"2. Thus, one is confident that the Earth is in radiative balance to an absolute accuracy of about +_5.4W/re"2.
Observing the Earth Radiation Budget
95
Aqalysis Model. The interpretation goal of satell/te measurements is to estimate the 24-hr radiation balance for a period in days that is longer than the correlation time of meteorological events in a region. In this context, the total uncertainty of one observation 0='T is a combination of the root variance of scene meteorology O's. the interpretation uncertainties for angular models O'M and filterfunctions of the instrument OF, and noise from the radiometric observation 0~. These are combined in an rms sense by 2
0~-+_(
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2
0~.
2
+
O'P
2
+
!/2
0"o1
(2)
Preparation of data utilizes either time averaging of observations for a given region or spatial averaging over many regions. The uncertainty of time averaging depends on the samp.n@ rate R (obs./day) of an area and the period T (days) for the averaging period of the product. The uncertainty or spatial averaoino_, depends on the effective area Ao (km**2) OFeach observation and the area OFthe data product AP (kin**2), where AP > A o . The uncertainty of the data product is reduced by the square root OF ( I/RT) in the case OF time averaging and by the square root OF (Ao/A~) For spatial averaging of regions. Both terms apply for time and space averaging. The equation for estimating the uncertainty (T'p of data products is I/2
0"~ = + O~ (I/RT)
I/2
(Ao/AF)
(3)
Equations (2) and (3) assume independent observations and a normal distribution of statistics. The results of calculations in the next section employ observations by the EEB instrument on Nimbus 7 satellite (ERB=7). Observational statistics from ImR=7 measurements are used to define the shortwave and Iongwave meteorological uncertainties 0"s. It is noted that this variable dominates all other terms alTecting the uncertainty of observations in (2). Model Calculations. Model calculations focus on the current WW-7 observations and the forthcoming ElIBl; measurements. F/gure 8 portrays the satellite view of the Earth scene for the fixed Idlq)V and W F O V radiometers, and for the sonnninE H F O V instruments. Table I presents data for the average scene area of useful observations for dLrferent instrument systems. The diameter of each area is given in degrees great circle arc (GCA). The enhanced fixed-field areas pertain to observations after enhancement with a numerical filter. This procedure improves the spatial resolution of the observations by a factor of four.
TABLE] AVERAGE 84~N~ ~ FOR D u - r ~
OF ~ OB8~VATION$ IINST1M/MENT 8 ~
heading Instrument system
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96
F.B.
House
Table 2 presents data concerning independent sampling rates for polar orbiting Satellites. These values are computed from the spatial sampling of a Scanning radiometer. The rates for longwave (LW) exitances are twice those for shortwave (SW) exitances because L W data are acquired on both ascending and descendlna nodes of the orbit. Smaller magnitudes are assigned to the Wlq)V and M F O V instrument systems. TABLS 2 ~g
INDEPENDENT SAMPLING RATES FOR POLAR ORBITING SATELLITES
60
R(0Os. per day) Instrument system
~*-oT 1%} 40 ~ N F O V
Reflectedexitance
Emittedexitance
1.6
3.2
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~ N F O M (ER~!)
20 LW , 0
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2.0
• WFOV
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(ERB-T)
\ t-MFOV(ERBE) , , j ~ L ~ . ~ . . L. ~ - ~ wFOv (ERB'7)
1 2 3 4 5 6 Obse~ationareaAo (KM2)
7 Log (Ao.)
Fig. 9. Total uncertainty of an observation in eqn. (2) for different instrument systems.
Figure 9 shows the percent uncertainty for both S W and L W observations as a function of their effecthre area A o in Table I. Although reasonable estimates were made for other terms in (2), the variability of an observat/on dominates the magn/tude of of the total uncertainty. Data points for (ERB-7) are based on Nimbus 7 observations, and those points for (KIBE) are estimates relative to these observations. EIBE values are larger than those for B~S-7 because its scene area is six times smaller than the EIB-7 area (see Table I). Clearly,the percent uncertainty for S W observations is about two and one halt times greater than those for the L W measurements. This characteristicwas apparent in the structure of the maps in Figures 4.a and 4.b. Meteorolog/cal variab/J/ty is the dominant source of uncertainty affecting the results. For example, assume that onJy one observation is made for a period of one week. The percent uncertainty that this one observation is representative of the average observation during the week is about +_50% and +10% for S W and L W exitances, respectively. These values apply to the NIFOV(ImB=7) observations. Other instrument systems would have dilTerent uncertainties according to the functions in Figure 9. RESULTS A N D DISCUSSION Data for model calculations in the last section are used with equation (3) to compute uncertainties of data products for different time periods and sizes of geographical areas. The minimum spatial resolution of observations is noted in Table I. Figure 10.a (next page) shows the uncertainty of shortwave-albedo data products for averaging periods from a day to a week, month, season and a year. All functions pertain to the minimum resolution of each instrument. Thus, high resolution scanning radiometers have much larger uncertainties than their fixed WI~OV counterparts. In other words, the spatial averaging properties of WI~)V radiometers reduce measurement uncertainties, but ground resolution is degraded. The results indicate that uncertainties decrease rapidly during the first two weeks of observations and then much more slowly. For example, the uncertainties of S W products for the NFOV(I.5) ERIB-7 data are +40% for daily values, +15% for weekly averages, +7% for monthly averages and +4% for seasonal dat=L The averaging period of the data products should be two weeks or longer, Ze., for periods longer than the turning point of the functions in Figure 10.=L The corresponding L W results vary from +6% to +0.6% for the same periods. Simultaneous observations from two and three satellitesystems would reduce the uncertainties by factors of 0.71 and 0.58.
97
Observing the Earth Radiation Budget (D~'~e(er ~GCA)
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(a) (b] Fig. I0. (a) Uncertainty of S W data products for different averaging periods. (b) Uncertainty of observations after spatial averaging. Figure 10.b presents results of uncertainties for both S W and L W measurements =Lfter spatial averaging. Averaging areas range from that for the BRB=7 observations to I0 degree zones, hemispheres and the globe. Separate lines show averaging periods of a day, a month and one year. Data are plotted on a log-log scale, and the lines have a slope of -I/2 which follows from the logarithm of equation (3). These results are useful in evaluating particular instrument systems. Consider the requirement mentioned in the Introduction of monthly products with 5-10 W/re"2 errors and 62,500 kin"2 areas. The c o m m o n log of the area is about 4.8 in Figure 10J) giving uncertainties of +_5% and +0.6% for S W and L W observations, respectively. These values correspond to +_5.3 W l m " 2 and +1.4 W/re"2 for average values of the radiation balance in equation (I). The total rms error for both accuracies and uncertainties is +7.7 W l m " 2 (including the +_5.4 W/re"2 absolute accuracy in (I)). One can conclude that for an average 250 k m by 250 k m region of the earth, a scanning radiometer on one polar orbiting satellite meets the +_5 to I0 W/re"2 monthly error requirement for climate model verifications. This conclusion presumes a satisfactory conclusion of the ~tlBS mission regarding diurnal variations of the radiation balance. Figure 11 illustrates a comparison of data product uncertainties for each instrument system. The differences among the systems for L W exitance measurements are within the +I % level of inaccuracy. In the case of the S W products, the scsnnino radiometers have +8% to +9% uncertainties. This level of error decreases to +1% for resulting areas of about one million square kin. Finally, the fixed M F O V and W F O V instruments indicate uncertainties or 1% or less for all their averaaine areas. LW Exitance
SW Exitance-Albedo tO
COp (%) 1.0
0.1
NFOV (O.6~ OV (1.5 °) °MFOV {5") OV (10") WFOV(7.5") 115"1
~
4 5 6 7 8 9 Log (Ap} Area of eta ~xoduct AD (KM 2)
t0 r
o
~-NFOV10.6 )
cop(%l to~- ~ ' ~ . E r
~-'MFOV(SO~
\ \ ' , , q - ' ~ o v (z5°)
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Area of data product Ap (KM 2)
(a) (b) Fig. I I. Uncertainty of monthly data products for different instrument systems and areas of spatial averaging. SUMMARY AND CONCLUSION
Earth radiation budget (ImR) experiments have a long history spnnnin~ from the first development of measurement systems to the future EglBI¢ mission this year. The progress in this area of satellite observation has been the work and accomplishment of
98
F.B.
Itouse
many individuals from different countries of the world, supported by their institutions. Other papers in Symposium 4 of C~)SPAR will present research results pertaining to recent measurements of the radiation balance. The forthcoming three-sateUite ERBE mission will provide needed observations to study the dirunal variation of the planetary radiation balance. Error analysis of ERB measurement systems includes all aspects of the project from the design and fabrication of the flight hardware to the data handling and scientific evaluation procedures for research. An analysis model is developed to determine the inaccuracy of observations (eqn. 2), and the uncertainty of data products after time andlor spatial averaging (eqn. 3). EI~=7 observations from Nimbus satelliteprovide the basic statisticaldata for variability of meteorology. Results indicate that the minimum time period for averaging scanner measurements is about two weeks for observations from a polar orbiting satellite. The time period shortens for experiments with multiple satellites like the I~I~E mission. Shortwave observations are the limiting factor controlling the level of error in radiation balance measurements -- uncertainties for Iongwave measurements are 40% of those for shortwave exitances. For an average 250 k m by 250 k m region of the earth, the total error in the radiation balance is +7.7 W/re"2 for observations from a polar orbiting satellite. Although with knowledge from the ERBE mission, it may be concluded that a seamninR radiometer on one polar orbiting satellite meets the 5-10 W / m " 2 monthly error requirement for climate model verifications.
ACIKNOWIJB]~KMF~IT Heartfelt thanks are expressed to William R. Bandeen of NASA Goddard Space Flight Center, Greenbelt, MD, USA. who kindly shared his excellent photographs of ERB measurement systems which provided the basis for this historicalreview. This research was supported by the ERBE Project of N A S A La~ley Research Center, Hampton, VA, USA. under Contract No. NSA I-16428 with Drexel University.
I. Satellite systems to measure the Earth's radiation budget parameters and climate change signals,Renort of an International Meetin2 of Exoerts. Igls,Austria (I 983). 2. M. Weinstein and V. Suomi, Analysis of satelliteinfrared radiation measurements on a synoptic scale,Mo~. Wea. Rev.. 89, pp.419-428 (1961).
3. E Raschke, T.H. Yonder Haar, W.R. Bandeen and M. Pasternak, The radiation balance of the Earth-atmosphere system from Nimbus 3 radiation measurements, I. Atmos. Sc,i.. 30, #3, (1973). 4. H. Jacobowitz, H.V. Soule, HJ~. Kyle, F J3. House and The Nimbus-7 ~ Experiment Team, The earth radiation budget (I~R) experiment - an overview, I.GeoDhv. Res.. 89, #D4, pp. 5021-5038, (1984). 5. H.L. Kyle, F J}. House, P£. Ardanuy, H. Jaeobowitz,R.H. Maschhoff and J.R.Hickey, N e w in-flightcalibration adjustment of the Nimbus 6 and 7 earth radiation budgbet wide field of view radiometers, I. GeoDhv. Res.. 89, #D4, pp. 5057-5076, (1984). 6. FJ~. House, Analysis of the dynamic behavior of the I~11 instrument on Nimbus 7 satellite,submitted to the I.Geonhv. Res.. (1984). 7. B. Barkstrom and J. Hall, Earth radiation budget experiment (ERBIi): An overview, ].of Energy, 6, pp.141-146, (1982).