Current status of satellite remote sensing and related data systems

S. I. Rasool

(1)as

of the ocean. They are essential i f we are to make optimal use of the potential of space measurements of ocean color to estimate ocean biomass and production. The related parameters to be measured from space are: • • • • • •

Ocean color (chlorophyll, sediment) Tropospheric gases over oceans (from space and from aircraft) Ocean topography Wind stress Sea surface temperature Volume of land and sea ice C.

GEOLOGICAL PROCESSES

The major i n i t i a t i v e within the domain of the geological processes, past and present, w i l l be in the "elucidation of the geological record of the evolution of our global ecosystems, with emphasis on the past 2 million years and particularly the last 10,000 years". However, s a t e l l i t e measurements can make contributions to the following areas: • • • • • •

Features and mineralization on the earth's surface. Ancient and actual hydrographic net. Mineralization regarding the evolution of ]andsystem and geocycle since the Archaean Era. An inventory of the world's current soil resources, including their physical and chemical properties and the relative balance or imbalance between soil formation and erosion. Quantitative assessment of the amount of material which runs off the continents and deposits in world estuaries, deltas and ocean basins. Measurements of the amounts and distribution of material injected into the atmosphere by gigantic volcanic eruptions, and also monitoring emission of S02 and possibly other gases.

The related parameters which need to be acquired globally are: • • • •

Soil type and time related changes Volcanic eruptions and distribution of aerosols around the Sediments in coastal zone waters Multispectral evaluation of dynamic and permanent features

globe

In the next section we w i l l discuss the current space capability for providing these measurements with consistency and c r e d i b i l i t y over the globe.

III. C U R R E N T STATUS OF SATELLITE R E M O T E SENSING AND R E L A T E D D A T A SYSTEMS A.

CURRENTSATELLITE OBSERVING SYSTEMS

Remotely sensed earth observations are available from the operational s a t e l l i t e system and from research s a t e l l i t e s . The operational system consists of geosychronous and polar orbiting weather s a t e l l i t e s , and polar orbiting land observing s a t e l l i t e s , as shown in Figure i . The weather satellites were designed mainly to provide information for weather analysis and forecasting• However, they also provide a number of environmental measurements of importance to a Global Change Program (Table 1). The land observing satellites provide information for agricultural, geological, and land use applications (see, for

G M S (Japan) 140 ° E

I N S A T (INDIA) 74 ° E

oo

L

:igure 1.

Operational Earth Ubservation S a t e l l i t e s

0730 L

VIETEOSAT (ESA) 0 ° LONGITUDE

75°W

GOES-E

~o

E C~

c~

B 0

(1)20

S.I. Rasool

Table i:

ENVIRONMENTAL PARAMETERS FROM UNITED STATES OPERATIOI~AL WEATHER SATELLITES

Satellite

Parameter

Instrument

NOAA

Vertical temperature profile

TOVS

IR/microwave sounder

1978

"

IR/H20 absorption

1978

Total ozone

"

IR/O3 absorption

1980

AVHRR

Vis & IR imagery

1960

Sea surface temperature

"

S p l i t IR window

1979

Snow line (NH)

"

Interpretation of vis. imagery

1966

Sea ice line

"

Vis. imagery & SMMR

1973

Vegetation index

"

NIR/Vis. contrast

1981

Albedo, longwave flux

"

Vis. window: albedo IR window: longwave flux

1974 1979

Stratospheric ozone

SBUV

UV backscatter

1984

Storm location

VISSR

Vis. & IR imagery

1966- f i r s t geosynchronus satellite

" Vis. cloud motions IR brightness temps.

1966 1974 1984

IR sounder

1980

Vis. reflectance

1982

Cloud cover image Winds Precip. index (tropics) GOES

Since

Precipitable water

Cloud cover image

GOES, Meteosat GMS, Insat

Method

Temperature p r o f i l e (currently experimentaloperational in 1990) Insolation - for U.S. and Mexico - through 1987

" " " VAS

VISSR

Remote Sensing for Global Change Study

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example, Table 2). Research satellites are flown to conduct short term experiments or to demonstrate instrument capability. Examples of some recent research satellites (see Table 3) include the Nimbus s e r i e s - which served as a testbed for the development of earth environmental sensors; Seasat, which demonstrated the feasibility of global ocean observations; the Earth Radiation Budget Experiment (ERBE) which is actually a three satellite experiment involving two NOAA polar orbiting weather satellites and a NASA s a t e l l i t e ; the Solar Maximum Mission, which obtained measurements of the solar flux; the Solar Mesosphere Explorer (SME); the Heat Capacity Mapping Mission (HCMM); and the Shuttle experiment ATMOS, which demonstrated the feasibility of extremely high resolution precision spectroscopy from space. Improvements in the operational weather satellite system w i l l take place in the 1989 - 1990 time frame. See Tables 4 and 5 for examples of modifications of the NOAA and GOES satellites which w i l l occur• The major improvement in the NOAA polar orbiting satellites w i l l be the installation of the f i r s t Advanced Microwave Sounding Unit (AMSU). The AMSU w i l l provide all weather temperature and moisture soundings. In addition, the AMSU w i l l provide information on precipitation rates, snow cover and sea ice. The AVHRR w i l l be modified to include a 1.6um near IR channel. The Global Change Program w i l l require this particular channel to permit cloud/snow-ice discrimination during the day. The modified GOESw i l l be a three axis stabilized platform with separate imaging and temperature sounding capability. An IR s p l i t window w i l l permit sea surface temperature determinations• B.

CURRENTDATA SYSTEMS 1.

Introduction

Investigators participating in the Global Change Program (GCP) must have access to long-term, continuous record, global-coverage data sets. They must also have access to processing and analysis capabilities, tools, and systems to aid them in their extraction of information from these data sets. Many data sets and data systems are already available or are planned for acquisition/ development. The purpose of this section is to provide a brief description of data sets and data systems already extant or that will be available in the near future, and to assess their potential contributions to the GCP. Section IV.E w i l l identify future needs for a GCP data system and provide recommendations on how to close gaps in the future needs which are l e f t by current systems. 2.

Current Data and Systems Capabilities

A number of powerful data systems containing important and valuable data sets already exist. These systems are capable of data acquisition, management, storage, and distribution. I t is believed that the foundation of a GCP data system can be found i f these data systems with their data and capabilities are brought together. 3.

Available or Planned Data Sets

Many of the required data sets are available now or are planned through the World Climate Research Program (WCRP), such as: • • • •

Cloud Climatology (ISCCP) Radiation Budget (ERB Nimbus 7; ERBE) Global Sea Surface Temperature (NOAANESDIS) Atmosphere - Ocean Fluxes (from ships and satellites - new program)

um

Repetition rate

18 days (L-4 & 5, 16 days )

185 km

9:30 a.m.

Equatorial crossing time (local time)

Swath width

80 m 240 m (thIR)

10.5 - 12.4 pm (Landsat 3)

0.~ - i.I

0.5 - 0.6 pm 0.6 - 0.7 pm 0.7 - 0.8 ~m

Instantaneous Field-of-View

Spectral bands

(MSS)

I

18 days

185 km

9:30 a .m.

40 m

O. 5-0.75 pm

I -

0.52 0.60 0.69 0.90 1.75 2.35 12.5

pm pm ~m pm pm ~m pm

16 days

185 km

9:30 a .m.

30 m 12U m (thlR)

0.45 0.52 0.63 0.76 1.b5 2.08 10.4

(TM)

- U.9

~m (panchromatic)

26 days (any point on the globe is accessible at least every bth day)

2 x 60 km ( t o t a l accessible swath width: ±400 km)

IU: 30 a .m.

20 m ( m u l t i s p e c t r a l ) 10 m (panchromatic)

0.5

0.5 - 0.59 lJm 0.61 - 0.69 pm 0.79 - 0.90 ~m

--gFOT (HRV)

SOMELAND OBSERVING SATELLITE SYSTEMS

(RBV)

Table 2:

o_

o

Remote Sensing for Global Change Study

Table 3:

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RECENTENVIRONMENTAL RESEARCH SATELLITES

MISSIONS

PARAMETERS

Nimbus 7

Solar flux, earth radiation budget, sea surface temperature & chlorophyl, sea ice, stratospheric temperature, 03, HNO3, NO2, H20, CH4, N20

Seasat

Sea surface winds, waves, sea ice, sea surface temperature,.current fluctuations

ERBE 3 satellite experiment ERBS, NOAA9, NOAA10

Earth radiation budget, solar flux

ERBS/SAGE

Stratospheric aerosols and 03

SMM

Solar flux

SME

Stratosphere/mesosphere chemistry

HCMM

Land heat capacities

ATMOS Shuttle experiment

Stratospheric and upper tropospheric chemistry

(2)CHANGE BEGINS ON NOAA-H

0)CHANNELS 3a AND 3b ARE TIME SHARED

1698.0MHz 1702.5MHz 1707.0MHz

136.77MHz 137.77MHz

DIRECT SOUNDING BROADCAST

HIGH RESOLUTION PICTURE TRANSMISSION (HRPT)

137.50MHz 137.62MHz

CH 4: 10.3-11.3um CH 5: 11.5-12.4um

CHI: 0.58-0.68um CH 2:0.7 .1.1 um CH 3: 3.55-3.93um

0.5km 1.0km

AUTOMATIC PICTURE TRANSMISSION (APT)

COMMUNICATIONS

SPECTRAL SPECIFICATIONS

VISIBLE IR

5

NOAA.E THROUGH -J

NO CHANGE

S-BAND AT APPROX. 1609.5MHz

NO CHANGE NO CHANGE

CH 1: SAME CH 2: 0.82-0.87um CH 3a: 1.57.1.78um{I) CH 3b: 3.55-3.93um(1) CH 4: SAME CH 5: SAME

SAME SAME

6

NOAA.K THROUGH -M

TWO SATELLITE SYSTEM

EQUATOR CROSSING TIMES AM 07:30 LOCAL PM 14:30 LOCAL

INCLUDED INCLUDED

DATA ,COLLECTION SYSTEM SEARCH AND RESCUE

INCLUDED

SPACE ENVIRONMENT MONITOR

SBUV (PM ONLY)

15.0kin (NADIR) 15.0km (NADIR)

NIA NIA

OZONE

40.0km (NADIR)

110.0km (NADIR)

AM NO CHANGE PM 13:30 LOCAL(2)

INCREASED CAPACITY NO CHANGE

SBUV (PM ONLY)

NO CHANGE

SAME

20

4 17.5km (NADIR)

20

NOAA-K THROUGH -M

23

NOAA-E THROUGH -J

SPATIAL RESOLUTION IR (HIRS) MICROWAVE (TEMPERATURE) MICROWAVE (WATER VAPOR) MICROWAVE (ICE)

NUMBER OF CHANNELS IR (HIRSI2) MICROWAVE (MSU, AMSU)

SOUNDING

FUNCTION

Comparison of Sensors and Systems for NOAA-Series, Polar-Orbiting Satellites

NUMBER OF CHANNELS |AVHRR) SPATIAL RESOLUTION

IMAGING

FUNCTION

Table 4.

v

o

FUNCTION

TIME SHARED NONEI11

3000 x 31~0km (MINUTES}

COMMUNICATIONS WEFAX SEARCH AND RESCUE

IIISTARTS 0~1 GOES G AND H

5 (5 CHANNELSI

5 (3 CHANNELS)

DEDICATED CHANNEL OPERATIONAL

SAME

10% OF THE IFOV

2O

NOT SPECIFIED

REGISTRATION CHANNEL TO CHANNEL

SAME 4 4 4 SAME

SAME 10.2-11.2 11.5-12.5 SAME SAME

5 OPERATIONAL

GOES'Next

TIMELINESS (MINUTESI EARTH DISK

1 8 8 8 8

SPATIAL RESOLUTION (kin) VISIBLE THERMAL IR THERMAL IR THERMAL WINDOW IR WATER VAPOR

0.55-0.75 9.7-12.8 12.3-13.0 3.8,-4.0 6.5-7.0

2 OPERATIONAL 3 PROTOTYPE OPERATIONAL

GOES-EIH

SOLAR X-RAY tMAGER

SOLAR ENVIRONMENTAL MONITOR

OTHER SYSTEMS DATA COLLECTION SYSTEM

IMAGING/SOUNDING EARTH LOCATION OF SENSOR DATA: ABSOLUTE PICTURE TO PICTURE

NONE

INCLUDED

INCLUDED

10km 5km

EXPERIMENT TIME SHARED WITH IMAGER

NOT SPECIFIED

REGISTRATION CHANNEL TO CHANNEL OPERATION

5 HOURS 2.5 HOURS

SAME AS GOES-E/H OPTIONAL

SAME AS GOES-E/H

2km 2kin (1 km GOAL)

10% OF AN IFOV (2% DESIRED} SEPARATE OPERATIONAL INSTRUMENT

4 HOURS 40 MINUTES

8km

14 km

SPATIAL RESOLUTION TIMELINESS 50 OEGREES OF LATITUDE 3(XX)x3000km

14 (MINIMUM)

GOES-Next

12

GOES-EIh

NUMBER OF CHANNELS

SOUNDING

FUNCTION

Comparison of Sensors and Systems for Present and Future GOES Spacecraft

SPECTRAL CHARACTERISTICS (urn} VISIBLE THERMAL IR THERMAL IR THERMAL WINDOW IR WATER VAPOR

NUMBER OF CHANNELS

IMAGING

TABLE 5:

E

C~

O

S. I. Rasool

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• • • • •

Ocean Circulation (from TOPEX . . .) Tropical Wind (derived from GARP & FGGE effort) (ECMWF) Global Precipitation and Snow Cover (SMMR Nimbus 7) Continental Run-off Land Surface Al bedo Net Radiation (from TIROS Operational Surface Temperature Vegetation Density (Index) Soil Moisture

Vertical

Sounder -TOVS)

The following are a few examples of data sets that are not planned within the current programs, and as such must be given high p r i o r i t y in planning the ICSU program on Global Change: • • •

Ozone tropospheric and stratospheric Other Trace Gases (CH4, NxO, CCIF's, HC's) (from s a t e l l i t e s , balloons and optical measurements from surface) Aerosols tropospheric and stratospheric 4.

SATMOS (France)

The Service d'Archivage e t a Traitement Meteorologique des Observations Satellitaires (SATMOS: a service for the archive and processing of meteorological observations from satellites) is a relatively simple d a t a assembly, processing, and distribution center in Lannion, France• SATMOSacquires NOAA AVHRR HRPT data from NOAA NESDIS in Washington, DC and makes i t available to its local meteorlogical s c i e n t i f i c community. I t operates on a " r o l l i n g archive" principle with i year's worth of the most recent data, but does retain some retrospective data sets which are of exceptional interest to their user community. The data are cataloged only by time of acquisition. Access to the data is through the French national telecommunications systems called TRANSPAC. SATMOSoffers quick look browsing capabilities, has a local image processing system, and offers navigation and calibration processing services. 5.

National Space Science Data Center

The National Space Science Data Center (NSSDC) of the NASA assembles, reprocesses, stores, and distributes virtually a l l data acquired by NASA space and earth science s a t e l l i t e s . These data sets, or information about the data, are accessible through networks such as the Space Physics Analysis Network with speeds currently ranging up to 9.6 k baud. In addition, the NSSDC offers data manipulation, display, graphics, and reproduction services to the world's space and earth sciences community• The NSSDC also conducts i t s own development of systems for data management and data storage which are injected into NSSDC operational capabilities when they are mature• 6.

Earthnet (ESA)

The Earthnet data system performs a similar but more extensive mission for the European s c i e n t i f i c community than the NSSDC does for the U•S. scient i f i c community. I t is responsible for archiving, managing, distributing, and encouraging the use of s a t e l l i t e remote sensing data. However, i t differs

Remote Sensingfor Global Change Study

(])27

from the NSSDC in that i t is also responsible for the acquisition and preprocessing of data, and i t operates in a decentralized manner with various national data centers responsible for specific data types. (e.g., Italy for Landsat, U.K. for Seasat). The f a c i l i t i e s are very much up-to-date technologi c a l l y . Earthnet already has Meteosat data, some Nimbus and HCMMdata, and will soon be capable of acquiring and providing NOAAAVHRR LAC data through i t s soon-to-be operational station at Maspalomas, Canary Is. (Spain). An extensive high speed network is in place connecting the various national data centers which can be used to access information about the data holdings, or to transmit the data. 7.

GRID

The Global Resources Information Database (GRID) was established to assemble, manage, and make available datasets which are important to the U.N. resource assessment, monitoring, and management activities. GRID currently does not have many data sets but has considerable image processing and image data manipulation power such as a geographic information system, digitization capability, and map projection transformation support. 8.

Pilot Climate Data System

The Pilot Climate Data System (PCDS) is an advanced data management, manipulation, and display system developed to support NASA's climatological scientific community. The PCDS is operational within NASA's NSSDC and has an on-line catalog, inventory, and browse, ~utomatic data access and r e t r i e val; and powerful manipulation and device-independent graphics display capab i l i t i e s . About 20 datasets, which are kept in a common data format structure, are currently supported including International Satellite Cloud Climatology Program cloud data, Earth Radiation Budget data, Nimbus data, precipitation and global solar flux data, and some long-term climatological data records. 9.

Pilot Land Data System

The Pilot Land Data System (PLDS) is an advanced data/information management, computing, processing/analysis, and communications system developed to support NASA's land processes scientific community. The data management component of PLDS w i l l contain a central on-line directory of data bases and archives, a distributed set of on-line data catalogs, full information query capability on the data sets, and state-of-the-art spatial data management functionality. The PLDS w i l l provide system access to a variety of NASA computer systems from interactive minicomputers for image analysis to supercomputers for process modeling. The PLDS will develop and offer the use of a full range of software tools for radiometric/geometric correction, classification, etc. All of these capabilities will be available through a high speed network (0.6 k band) covering the U.S. Many satellite and aircraft data sets are already being assembled in the PLDS, including Landsat TM and MSS, NOAA AVHRR, TMS, PBMR, Nimbus SMMRand GOES VISSR, to support 2 key projects in the International Satellite Land Surface Climatology Program (ISLSCP): the Retrospective Analyses Program and the First ISLSCP Field Experiment (FIFE). 10.

NASAOcean Data System

The NASA Ocean Data System (NODS) is a distributed data management system, designed to provide easy and timely access to NASA's oceanographic satellite data and allied in situ data sets. The primary functions are to archive, catalog, and disseminate these data using state-of-the-art technologies for computer database management, high volume data storage and retrieval, and computer networking. NODS catalog, bibliography, data selection requests, and browse f i l e displays are all available interactively at a user's terminal. J~l

?zI-C

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S.I. Rasool

Communications links to most U.S. oceanographic institutions are currently provided via the SPAN network. NODScatalog and archive software are being designed for portability to several sites, to foster development of a distributed data system with common standards and user interface. Currently archived satellite data include Seasat SCAT, SMMR, ALT; GEOS-3 ALT; AVHRR/CZCS time series; and in future will include data from the DMSP, NROSS, and TOPEX missions, among others. 11.

Canadian Center for Remote Sensing

The Canadian Center for Remote Sensing (CCRS) has developed a geocoded satellite imaging data base which contains an extensive set of Landsat MSS and TM, and now SPOT data, covering all of Canada. The database has been produced at 1:150,000 scale by resampling each of the different data types. While probably not a complete record of all of Canada, a 10 year record of several select sites in Canada, such as the plains, does exist. 12.

NOAA/NESDIS

The National Environmental Satellite, Uata, and Information Service (NESDIS) is a component of the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA). NESDIS is responsible for the development and operation of the nation's civil operational environmental satellites (the Geostationary Operational Environmental Satellites, GOES; the Polar-Orbiting Operational Environmental Satellites, POES, also known as the TIROS series; and Landsat). In addition, NESDIS manages the nation's long-term global data bases for meteorology, oceanography, solid-earth physics, and solar-terrestrial sciences. A wide variety of digital and nondigital data come to NOAA's national climatic, oceanographic, and geophysical data centers after the original collection purposes have been served. NESDIS'digital data base consists of over 63,000 gigabytes, excluding Landsat data, and is expected to more than quadruple in the next ten years. Digital data come from various sources, including polar-orbiting and geostationary satellites, a i r c r a f t , ships, buoys, radar, and several types of surface instruments. The non-satellite digital data base is located in Asheville, North Carolina; digital data from satellite instruments are kept in Suitland, Maryland. The non-digital data base contains nearly 300 million printed pages, photoprints, and charts, and over 80 million microfiche, reels of film, negatives, and transparencies. Non-digital data include products derived from digital sources as well as maps, publications, and analog photographs/ negatives. Non-digital data are located at and maintained by the data center responsible for the discipline from which they derive. 13.

IBM Center (France)

The IBM Center in Paris, France has developed a powerful relational database for image data. The system is hosted on an IBM 3081 computer using the SQL language. I t currently contains AVHRRand Meteosat image data which are used by the IBM scientific community. 14.

Commercially Available Data Products

Many of the data mentioned above are available commercially. While a c o , fete l i s t of available products is beyond the purview of this report, two important datasets (Landsat and SPOT) are listed in Tables 6 and 7.

x x x

x x

Black and White

Accession

Aids

Micro Catalogs Micro Image Fiche

•

Micro Image Film Worldwide Reference System Maps

Computer Compatible Tapes (CCTs)- 9 Track, 2600 and 6250 BPI High Density Tapes (HDTs)-- MSS, 14 Track, 20,000 BPI; TM, 28 Track, 33,000 BPI Partially Corrected Et Fully Corrected Data

Digital

Positive Positive Positive

9" x 9" 20" x 20" 40" x 40"

Paper

•

Pos & Neg Pos & Neg Positive

70 mm 9" x 9" 9" x 9"

Products

Film

Data

Format

Photographic

Landsat

!mage Size

•

TABLE

X

X

X

X

Color

v

E

c/}

=_=

O 5"

==

,-t

O

~o

(1)30

S.I. Rasool

Table 7. 1.

SPOT Data Products

General. SPOT Scene:

A SPOT scene covers an area of approximately bU x 6D kilometers. Stereo coverage of a specific a r e a requires two scenes. Information and s p e c i f i cations on all acquired and processed SPOT scenes are available f r o m the SPOT catalog. Processing Leve]s: Level 1A - Equalization of detector responses. Relevant provided for interband calibration and geometric correction.

coefficients

are

Level 1B - Same as Level 1A, but resampled to correct for systematic geometric distortions. Level 2 - Same as Level IB, but geometrically corrected to a map projection u t i l i z i n g ground control points (derived item). Level S - Same as Level 2, but registered and resampled to a reference scene.

2.

Digital

Items

Computer Compatible Tapes Packing density: either 1600 or 625U bpi Levels - IA, 1B, 2, S Detectors - panchromatic, multispectral 3.

Photographic Items Black & White

Transparencies: Levels 1A, 1B, 2, S Multispectral and panchromatic Scales 1:250,0UD and I:40D,DUO Prints:

Levels 1B, 2, S Multispectral and panchromatic Scales l:lO0,OO0 and I:25U,OUO

Color

Levels 1B, 2, S 3 bands multispectra] Scales I:2bU,OOD and

1:400,000 Levels 1B, 2, S 3 bands multispectral Scales I:IOU,OUO and 1:2bO,OOU

Remote Sensing for Global Change Study

C.

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CURRENTSATELLITE DATAACQUISITION AND ANALYSIS PROGRAMS

I nt roduct i on Space-based remote sensing systems are well suited to provide global, consistent, repetitive and long-term data sets on the state of the atmosphere, oceans and land. The current observing system has been operational for close to a decade, thereby providing an i n i t i a l data set for the study of global change. However, the present mix of space flights, sensors calibration procedures, ground validation efforts and data distribution schemes are not f u l l y adequate for the purpose of assessing and predicting global change over periods of years to centuries. The current satellite observing systems and the complement of instruments on board (as discussed in section A of this chapter) were put in place for purposes other than studying long-term global changes on the earth. They were designed for improving weather forecasting, geological mapping, agricultural monitoring, short term research programs, and demonstration of instrument capability. Operational systems were intended for qualitative applications, or for quantitative applications in which the signal was strong, rather than for monitoring long-term, subtle changes in the earth system. Some satellites have demonstrated the feasibility of new techniques, have obtained good quantitative observations and functioned longer than expected. A few motivated individuals have been able to produce a time series of some of the crucial parameters over a period of years. Validation requires extensive and carefully planned programs to make "ground truth" observations and the intercomparison of these observations with simultaneous space-based measurements, due allowance being made for the differences in the scale of the measurements. Some validation efforts have been conducted to a fully satisfactory level, while other satellite data products have received only limited validation, or no validation at a l l . In the sections which follow, we briefly review the status of a wide variety of satellite data products. The sections below summarize a range of interrelated activities being conducted under the auspices of two international programs. 1.

World Climate Research Program (WCRP) Activities

The data projects of the WCRPare intended to obtain validated data sets of climatic parameters.

global

International Satellite Cloud Climatolo~ Project (ISCCP) The f i r s t major data project of the World Climate Research Program began its operational phase in duly 1983. Its basic objective is to collect and analyze satellite observed radiance data to infer the global distribution of cloud radiative properties, to improve the modeling of cloud effects on climate. ISCCPhas two components: operations and research. The operational component takes advantage of the global coverage afforded by the current and planned international array of geostationary and polar orbiting operational meteorological satellites during the 1980's to produce a five-year global radiance and cloud data set. The most important characteristic of these data will be their globally uniform coverage of various indices of cloud cover. The research component of ISCCP will coordinate studies to validate the cloud climatology, to improve cloud retrieval and analysis algorithms, to improve the parameterization of clouds and radiation in climate models, and to investigate the role of clouds in the atmosphere's radiation budget and hydrologic cycle. Validation involves comparative measurements at a number of test areas selected as representative of major, or d i f f i c u l t , cloud types and meteorological conditions. Complementary efforts within the framework of the WCRPw i l l promote the use of resulting ISCCP data sets in climate research.

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S.I. Rasool

The basic global cloud climatology data consists of regional and three hour monthly means of cloud parameters. The ISCCP data set w i l l also contain sampled global radiance data and correlative global data for a number of atmospheric and surface parameters. Radiation Budget Climatology Earth Radiation Budget (ERB) observations are needed for climate studies to document spatial and temporal v a r i a b i l i t y ; to understand climate forcing and feedback mechanisms; to validate climate models; and to monitor and understand climate v a r i a b i l i t y and change. Precision requirements for measurements of the ERB at the top of the atmosphere have been presented in WCP-70 (1984). Required precisions range from 5-10 Wm"2 for regional and zonal monthly means down to 1Wm"2 for global decadal means. The Earth Radiation Budget Experiment (ERBE) -- a current three s a t e l l i t e research mission involving two NOAA weather satellites and a research s a t e l l i t e -- appears to meet these requirements, but i t w i l l terminate in 1989. A continuing long-term data set of ERB estimates is available from the AVHRR instrument on the NOAA s a t e l l i t e s , but these instruments were designed for cloud imagery and i t is unlikely that they can be used for longterm monitoring of global change. The WCRP is in the process of establishing precision requirements for the radiation budget at the top of the atmosphere and at the earth's surface and assessing the accuracy of current measurements. These measurements are needed for diagnostic studies, model validation and i n i t i a l i z a t i o n , process studies and monitoring of climate trends. The surface radiation budget is especially relevant for air-sea, land-air and cloud radiation studies. Global Sea Surface Temperature (SST) Data The WCRP plans to obtain global SST fields by combining ship and satell i t e observations. The WCRP requirements for SST data are for global coverage in b° x 5° regions at 30 day intervals accurate to 1.5°C; and for tropical coverage (2u°N - 20°S) in 2° x 2° regions at 15 day intervals accurate to 0.5% for sea surface temperature less than 28°C and to 0.3% for sea surface temperature greater than 28%. The AVHRR observations meet most of the WCRP requirements. NOAAmaintains a continuous program of validating i t s s a t e l l i t e SST observations against d r i f t i n g buoy observations. The bias and random difference of the NOAA SSTs, when compared to d r i f t i n g buoy observations, are on the order of 0.5 - l.O°C. Comparisons of different s a t e l l i t e instruments (AVHRR, TOVS, SMMR) show larger differences. NASA and NOAA are sponsoring a series of workshops to identify and improve these measurements. In addition, an international workshop sponsored by COSPAR has assessed the current capabilities for measuring SST (WCP - 110). Global Precipitation The WCRP requirements for precipitation data are summarized in WCP-111 (1985). Basically they call for accuracies of I-3 cm/mo for regional, monthly means. An attempt w i l l be made to obtain s a t e l l i t e precipitation estimates from two sources: the infrared radiances observed by the geostationary satell i t e s , which are correlated with tropical ocean precipitation, and the microwave observations of the Special Sensor Microwave Imager (SSMI), to be flown on the US DMSP s a t e l l i t e , which can provide estimates of instantaneous precipitation rates from analysis of microwave absorption/emission (over the ocean) or microwave scattering (ocean and land) by rain drops and ice particles. Both methods are essentially based on empirica! calibration, which requires special "ground" and "ocean" truth measurements. Unfortunately, because of i t s spotty nature, ground truthing of s a t e l l i t e observations is d i f f i c u l t . As part of i t s global precipitation data project, the WCRP is planning a special validation

Remote Sensing for Global Change Study

(1)33

program. COSPARis sponsorin~ an international workshop to be held in Washin9ton, DC., in November, 1986, to help formulate the 9round truth program for this project. Z.

International 5atellite Land Surface Climatology Project Activities

The major objective of the International Satellite Land Surface Climatology Project (ISLSCP) is to develop, validate and implement methods for exploiting currently available s a t e l l i t e observations to determine land surface parameters of importance to climate. Such information is required to monitor globally the changes of the land surface brought about by climatic fluctuations or by man himself; to further develop the mathematical models for predicting climate on various time scales; and to permit the inclusion of land surface variables in diagnostic and empirical studies of climatic variations. The land surface parameters of major interest include skin temperature, albedo, radiation budget, and vegetation c o v e r . Aside from their importance to the climate, these land surface properties also influence biogeochemical cycles. ISLSCP is conducting major field programs to validate s a t e l l i t e techniques for determination of land surface climatological variables. Une of these is currently taking place in central France as part of the HAPEX project of the WCRP. Another (FIFE) is planned for the central U.S. in 1987. In these programs, areas of the size 1UKm X IUKm are covered with small networks of instruments to measure fluxes of radiation, sensible and latent heat, vegetation skin temperature, soil moisture and other meteorological and surface parameters to provide ground truth for evaluating s a t e l l i t e algorithms (see Fig. 2). In addition to the ground observations, radiative, latent and sensible heat fluxes are measured in situ by a i r c r a f t ; skin temperature and soil moisture are measured remotely. Special efforts are made to collect all available s a t e l l i t e data for the area, including both land observing and weather s a t e l l i t e s . Examples of some of the results currently bein9 obtained on land surface parameters from space are given below. Land Surface Temperature Chahine (198b) has recently used the observations of the TIROS Operational Vertical Sounder (TOVS), which provides atmospheric temperature prof i l e s for use as i n i t i a l conditions in numerical weather prediction models, to derive global distributions of surface skin temperature. Fig. 3 shows the global distribution of the change in surface skin temperature from July 1979 to July 1982. Values in excess of 6°C can be seen over some of the continental areas. Such observations, once they are validated, can provide the data required to determine, for example, whether or not the CO2 greenhouse effect is actually producing i t s predicted effect on global climate. Al bedo Surface albedo plays a c r i t i c a l role in climatic feedback processes since, on the one hand, i t is affected by human a c t i v i t y and climatic fluctuations and, on the other hand, through i t s control of the amount of solar energy absorbed at the surface, i t affects the climate. A recent review of the work on determination of surface albedos from s a t e l l i t e observations has been given by Pinker (1985). Because of their large fields of view, ERB instruments are not particularly suitable for surface albedo determinations for which cloud free observations are needed. Investigators have used the narrow spectral interval reflectance observations of the Landsats or weather satellites to derive surface albedo. In such derivations, we must account for the effect of the atmosphere, the bi-directional reflectance function of the surface, and the conversion from a narrow spectral interval to the entire solar spectrum. An example of the results of recent work is Fig. 4, after

(1)34

S.I. Rasool

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(1)37

Change in Global Vegetation Index (Green Leaf Density) from August 1982 to February 1983 (after Tucker, et a l . , 1986).

(1)38

Figure 6.

S.I. Rasool

Long-term Change in Global Normalized Difference Vegetation Index (NDVl) -- 1981 to 1984. (Tucker, et a l , , 1987)

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(1)40

S.I. Rasool

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(1)41

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(1)43

Deschampset al.(1985),which shows albedo mapsof Africa, derived from Meteosat visible observations. Vegetation Monitoring of l a n d surface vegetation is of direct relevance to land surface hydrology and also to the surface energy balance. Evaporation from soil and from vegetation d i f f e r b o t h under water-unlimited and - limited conditions. Potential evaporation from deep-rooted plants may continue for a significantly longer period than from soil. Non-potential soil evaporation is determined to a significant degree by the soil hydraulic properties, while plant physiologic characteristics (leaf stomata) determine canopy evaporation. Fractional vegetation cover also determines surface temperature, because dry soil surfaces are considerably warmer than canopies. A measure of the amount of vegetation is obtained from s a t e l l i t e observations of reflected sunlight in the visible and near infrared spectral regions. Green vegetation has a sharp increase in r e f l e c t i v i t y from the visible to the near infrared, and this difference can be used to construct a veyetation index. The global distribution of vegetation index is currently produced on a weekly basis from the NOAA AVHRR observations. An example of a global vegetation index map is shown in Fig. 5, after Tucker (1986). Short-term variations in this index can be used to detect areas of drought; long-term variations can be used to detect more subtle changes such as trends in deforestation, desertification, and global change (Fig. 6). It has been d i f f i c u l t to determine exactly what the quantitative i n t e r pretation of the s a t e l l i t e vegetation index is on the ground, i . e . , leaf area index, vegetation vigor, etc. However, Fig. 7, after Sellers (1985~, shows that the vegetation index may be a good measure of photosynthetically active radiation (PAR). The carbon cycle is affected by vegetation and this relationship is evident in Fig. 8, after Tucker (1986), ~lhich shows a time series of locally averaged atmospheric carbon dioxide and vegetation indices. D.

SELECTEDSATELLITE DATA ACQUISITION AND ANALYSES PROGRAMS I.

Solar Flux

The amount of solar energy reaching the earth-atmosphere system is a parameter which cannot be measured from space because as much as 40% is either absorbed by the atmosphere or reflected back into space by the clouds. Ever since the beginning of the s a t e l l i t e era, the objective has been to measure precisely and continuously the total amount of solar energy reaching to the top of the earth's atmosphere. A small (1%) but persistent change in the solar input to earth could have important effect on the global climate. A number of rocket launches in the 1960's attempted to establish a trend but with not much success. Problems related to sensor s t a b i l i t y and their c a l i bration delayed the i n i t i a t i o n of a long-term space based observing program until 1978, when Nimbus-7 was launched carrying an Earth Radiation Budget Experiment. Today, six years of continuous data on solar flux are available and show a decreasing trend of 0.02% per year (Fig. 9). A different instrument named Active Cavity Radiometer Irradiance Monitor was launched in 198U on board the Solar Maximum Mission and has also acquired d a t a continuously for most of the same period. I t is interesting that these independent measurements show a similar decreasing trend (Fig. 10). Is the solar output really diminishing or is i t the space environment which is de~rading both sensors equally? This important question can only be answered by: o

assuring the continuity of such measurements throuqh the next solar cycle, especially until 1991-1992, when the solar activity is supposed to go through i t s maximum phase;

(1)44

S. I, Rasool

performing occasional measurements of solar irradiance (on Shuttle f l i g h t s ) , at various frequencies, in order to determine in which region of the spectrum the change, i f any, is taking place. The accuracy and precision next decade is given in Table 8. 2.

requirement for

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Ozone and Stratospheric Temperature Trends from Satellites

Comparison of three d i s t i n c t s a t e l l i t e measurements (SBUV, LIMS and SAGE) suggests that we can determine ozone in an absolute sense to about 15% (one standard deviation) from 25-30 km and about 6% from 3U-55 km. Comparison of ozone s a t e l l i t e and balloonsonde data suggests that the balloonsondes provide systematically lower readings than SBUV above about 32 km. The cause of this descrepancy is not currently recognized. U t i l i z i n g SBUV as the basic data source for four year zonal average profiles, the random uncertainties appear to be about 4% (one standard deviation) in mid-and high-latitude winter and about 2% elsewhere. A careful analysis of the ozone data for the last decade allows us to make the following statements: • • •

•

•

•

Global trend estimates of total ozone determined from the Dobson spectrophotometer network indicate l i t t l e overall support for a s t a t i s t i c a | l y significant trend during the 14-year period 1970-1983. Recent evidence has been presented that indicates a considerable decrease in Antarctic total ozone during the spring period since about 1968. This is presently the subject of further analysis (Fig. 11).(Watson et al.) Trend e~timates from 13 ozone balloonsondes indicate statiscally s i g n i f i cant positive trends in the lower troposphere and negative trends in the lower stratosphere. The interpretation of these results, however, is clouded by uncertainties in instrument behavior and lack of a global station network. Ozone trend estimates from 13 Umkehr stations indicate s t a t i s i c a l l y significant negative trends from 197U to 1980 in the middle stratosphere that are in substantive agreement with results from one-dimensional numerical models. The observational results are sensitive to the inclusion of a term to acccount for stratospheric aerosol impact on the measurements and the spatial distribution of the sites, but do not appear sensitive to the inclusion of a 20.7 cm flux variation (an indicator for solar flux variation). Examination of the NOAA SBUV-2 s a t e l l i t e measurement program indicates that i f the system operates as designed, i t is capable of global ozone trend detection in the middle to upper stratosphere, as well as total ozone measurements to within about 1.5% over a period of one decade at the 95% confidence level. As with other long-term measurement programs, however, i t is necessary to examine continually the SBUV-2 instrument performance and s a t e l l i t e measurements and compare them with independent data.

Also, i t should be noted that the SBUV-2 data are inherently total ozone and ozone profiles between 25 and 55 km. I f ozone trends can be determined unambiguously from the earth's surface to the overlap region with the SBUV-2 profiles, a high quality measurement program would e x i s t . Two independent analyses of lower stratospheric temperatures during the period 1965-1979 are suggestive of a downward temperature trend. The large cooling in rocketsonde temperatures reported for the early 1970's appears now to be due to a change in the rocketsonde temperature measurement system. Taking this into account, s t a t i s t i c a l l y significant negative trends are observed in June rocketsonde data at 40-45 km from 19731983 that are in substantive agreement with results from one-dimensional

Figure 11.

Satellite Observations (Nimbus-7) of the So-Called "Ozone Hole" over the South Pole.

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(1)47

TABLE8. SOLAR ELECTROMAGNETICRADIATION SPACE MONITORING. REQUIREMENTS Speclral Interval

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(1)48

S.I. Rasool

numerical models. These preliminary results w i l l have to be examined further with a more complete data set. Examination of the NOAA TOVSstratospheric s a t e l l i t e temperature measurement program indicates that i t is essential that the instrument-to-instrument consistency be verified by a high quality, independent data system. Such a system does not exist. I t i s , therefore, important to develop a long-term s a t e l l i t e and 9round-based temperature measurement program sufficient to measure a mid-stratospheric temperature trend to a 95% confidence level of 1.5°K/decade. 3.

Middle Atmosphere Program (MAP)

The MAP is coming to an end within the next year or two, but has contributed in a major way to validating data from instruments in the Nimbus-7, ERBE, and ATMOS projects, as well as providing comprehensive non-satellite data sets for the testing and development of geophysical theories of interest to future global change programs. The results of MAP have been previously reported in a series of reports. Among the most notable achievements in the f i r s t half of the 1980's have been: o The establishment of accurate (± 20% level) near-global d a t a sets for 03 , HNO3, NO2, N20, H20 and CH4. • The theoretical understanding of spatial and temporal v a r i a b i l i t y of these species has been substantially improved with work, for example, on high latitude sources of NOx, total hydrogen budgets (H20 + CH4), and the role of temporary reservoir species such as CIONO2 and N205. o The success of the ATMOS project is providing a well-understood very high resolution infrared spectrum of the atmosphere. o Demonstration of the value of integrating complementary s a t e l l i t e , rocket, balloon, and surface measurements. • Improved understanding of the interaction between dynamics, radiation and chemistry, throughout the middle atmosphere, and also in terms of exchange processes at the upper and lower boundaries. The results of MAP have laid an extremely valuable base for the forthcoming UARS mission which seeks to extend the number of simultaneously-observed parameters and the accuracy of their measurement, and to assure e f f i c i e n t processes of interplay with modelers. 4.

Stratospheric and Tropospheric Aerosols

For the purpose of observational methods and data set productions, i t is convenient to distinguish between tropospheric aerosols and aerosols in the stratosphere. Stratosphere Stratospheric aerosols have an atmospheric residence time of 1 to 2 years, resulting in a much more homogeneous spatial distribution than tropospheric aerosols. Simulations with General Circulation Models show that stratospheric aerosols are probably the most important aerosol type influencing the climate system. There are also some hints that they interact with ozone and other stratospheric trace gases and influence their concentration there. A nearly global data set characterizing the aerosol concentration versus height (optical depth at 2 km above the tropopause) has been building up since October 1978, from s a t e l l i t e s using solar occultation techniques (SAM, SAGE) -- see Fig. 12 and 13. Additionally, a few Lidar stations situated in midlatitudes of the northern hemisphere monitor stratospheric aerosols continuously (Fig. 14). The interac-

(1)49

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Remote Sensing ~ r Global Change Study

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tion of stratospheric aerosols with the chemical/photochemical cycle requires, however, knowledge of additional aerosol parameters (chemical composition, size d i s t r i b u t i o n ) which can in principle only be acquired by in s i t u measurements. As the spectral dependence of backscatter and optical depths gives the same i n f o r mation on the size d i s t r i b u t i o n , simultaneous in situ and s a t e l l i t e measurements can be used to derive concentration, size d i s t r i b u t i o n and chemical composition• I t is therefore recommended that s a t e l l i t e (SAGE) type measurements should continue in a multiwavelen~th model at least another decade to derive the v a r i a b i l i t y includin~ indications on trends in stratospheric aerosol concentration• • Comparisons of in situ and s a t e l l i t e measurements are made with the aim to get more information on the chemical nature of stratospheric aerosols ~nd to derive useful relations to remotely measurable parameters. • The present groundbased Lidar station network should continue to operate and be extended to at least one station in the southern hemisphere to support the SAGE mission. • Tne necessary c a r e should be taken to ensure a long term consistent data set. • Advanced space technologies (Lidar) are to be developed in conjunction with ground networks. • Theoretical studies on the interaction of aerosols with the chemical/ photochemical cycle and dynamics, including transport mechanisms, are encouraged. From the COSPAR point of view, the output concerning observing strategies is especial|y interesting• Troposphere Tropospheric aerosols, although having shorter lifetimes than stratospheric aerosols, may affect regional and global climate as a result of increased optical depths• NOAA is currently planning to obtain estimates of aerosol optical depths over the oceans from AVHRR v i s i b l e observations• Over land, t h i s approach needs to be verified by numerous ground truth data as the signal (radiance) measured by a s a t e l l i t e depends also on the surface r e f l e c t i v i t y which, on land, is quite variable. As dust outbreaks from desert regions contribute episodically to the tropospheric aerosol budget in a big way, t h e i r monitoring is of a special interest• At least, the number of dust outbreaks and forest f i r e s , the areal coverage of t h e i r sources as well as c l a s s i f i c a t i o n of t h e i r strength, can be done well enough from routine geostationary and polar orbiting s a t e l l i t e s • We recommend therefore that: • Satellites be used to track the number of aerosol outbreaks and t h e i r intensity as well as t h e i r horizontal d i s t r i b u t i o n . This should be done on an operational basis. One of the results would be an intercomparison of existing retrieval algorithms. • Due to the i n a b i l i t y of passive measurments from s a t e l l i t e s to e s t i mate composition, optical properties and vertical d i s t r i b u t i o n , ground based measurements of aerosol properties should be expanded• The results on aerosol optical properties should also be used to improve the retrieval techniques for s a t e l l i t e measurements. • Improved s a t e l l i t e techniques such as multispectral and active methods should be implanted. • Theoretical studies on aerosol transport as well as on the interaction of a~rosols with other atmospheric cycles are encouraged• • Experimental as well as theoretical studies on the impact of different kinds of aerosols on the biosphere are encouraged. From the COSPAR point of view, i t is especially interesting to measure these properties which are especially crucial in this respect• • The efforts in building up an aerosol climatology are i n t e n s i f i e d .

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S.I. Rasool

b.

OceanColor

The primary goal of the Coastal Zone Color Scanner (CZCS) Program was to demonstrate the f e a s i b i l i t y of measuring the visible radiance of the ocean from space and of inferring estimates of chlorophyll concentration and ocean primary productivity. Secondary goals were: • to provide flow visualization for ocean currents. • to acquire data on the optical properties of the oceans, in particular on the diffuse attenuation coefficient that expresses the attenuation of incoming solar radiation; and • to provide data on the density of atmospheric aerosols over the ocean that w i l l also affect this radiation. Considerable international effort has gone into developing analysis techniques for these data, and monitoring long-term degradation of the CZCS instrument. Now that satisfactory techniques are available, NASA is processing all data from launch in 1978 up to June 1986, to provide global estimates of monthly averaged near surface phytoplankton pigment concentrations with 18 km spatial resolution. CZCS operation was limited to less than 10% duty due to power and data storage capacity on Nimbus-7, and therefore coverage is very sparse. Major ocean areas lack sufficient data to estimate monthly or even annual means of primary productivity. However, these data should provide, for the f i r s t time, a time sequence showing the annual and interannual variab i l i t y of ocean pnytoplankton concentrations. Because of the c r i t i c a l role of phytoplankton in the biospheric interactions, these patterns should provide sensitive indicators of the effects of longer term climatic variations. Fig. lb shows the average plant abundance and distribution for a f i f t h of the earth's surface in May. Different scales are used for land and water. For land areas, the Vegetation Index is derived from the NOAA-6 Advanced Very High Resolution Radiometer (AVHRR) for May 1982 (dark green is the highest density of green vegetation). For oceans and lakes, the amount of phytoplankton in terms of chlorophyll pigment density (red is highest) is derived from the Coastal Zone Color Scanner (CZCS) on NASA's Nimbus-7 for May 1979. The Sahara Desert, tropical rainforests, and spring greening of temperate forests and fields are evident on land. The corn and wheat belt south of the Great Lakes is less green because crops are just beyinning to grow. In the oceans, productive upwelling areas along the c o a s t (especially off N.W. Africa) and the "spring bloom" in the North Sea and northern North Atlantic are very evident. CZCS data do not distinguish between high sediment and pigments, and values in coastal and lake waters carl be ambiguous. This composite image includes all daylight data collected by the two sensors during the periods in this region (10% to BO°N, and 1U°E to IO0°W). For the land, this is but a part of a multiyear global data set. For the ocean, this is the f i r s t view of biological a c t i v i t y on a basin scale. Black areas in the ocean indicate no observations, white indicates that clouds or sea ice were present during every observation. Striping a l s o indicates undersampling of oceanic v a r i a b i l i t y with the limited-duty CZCS. Due to further age-related spacecraft power limitations and sensor problems, the CZCS operations were suspended in June, 1986. I t is important for global change studies to f l y a follow-on sensor as soon as possible to continue the B-year observational data set begun by CZCS. 6.

Ice and Snow Data Sets

Changes in the volume and extent of the earth's snow and ice cover are principal indicators of global change and significantly affect the environment through changes in sea level, climate feedbacks, and direct impacts of extended ice cover. In particular, i t is not known whether the ice sheets of

Remote Sensing for Global Change Study

Figure 15.

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Biome Productivity Levels for the North Altlantic Ocean and Surrounding Landmass for May. Landmass Plant Abundance is Derived from NOAA AVHRR Data for May, 1982. Phytoplankton Concentrations in Oceans and Lakes is from the Nimbus CZCS in May lY79. Black areas aenote insufficient data for productivity assessment. (Esaias/GSFC, 1986, unpublished)

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Greenland and Antarctica are growing or shrinking, whether t h e i r ice flow is stable or subject to rapid changes, or what changes in ice mass might be induced by climatic change• Each year approximately 3000 kms of ice are exchanged between the oceans and the ice sheets, but the balance between snow accumulation and ice discharge is uncertain by 50% or more. This uncertainty in ice mass balance contributes an uncertainty in global sea level change of ±3 mm/year. Now, s a t e l l i t e altimetry offers a unique means of monitoring changes in ice volume, determining the overall mass balance and providing much of the data required for modeling ice behavior and predicting future changes. S a t e l l i t e observations of the seasonal snow and sea ice cover have provided a more than ten-year record of the seasonal and interannual variations in extent (Fig. 16&16a) but extended records are required to establish the natural v a r i a b i l i t y and detect long-term changes. Observations of the extent of the Arctic ~ I t i y e a r ice pack by passive microwave imagery are providing a means of monitoring the perennial sea ice cover and detecting changes that might be induced by polar warming or changes in river overflow, for example. Although maps of snow extent since 1966 and sea ice extent since 1973 are available as p a r t i a l l y described in Section I l l A, a c r i t i c a l analysis of the uniformity of the data products is needed. Passive microwave methods for measuring sea ice extent, for example, have high potential accuracy, provided the data are systematically processed. Current research on snow mapping by passive microwave may also lead to more consistent maps of snow extent than currently provided by visual image interpretation. Development and validation of snow and sea ice data sets have been objectives of the World Climate Research Program. However, potential changes in ice sheets volume, which may be significant on time scales of several decades, have not been d i r e c t l y addressed by WCRP. Consequently, determination of the present growth or shrinkage of the polar ice sheets and the detection of future changes in polar ice volume by laser altimetry should be a p r i o r i t y element of a global change program. Many of the required measurements for associated research with ice sheet growth or shrinkage can be acquired by radar, passive microwave, and v i s i b l e sensors. • Radar-altimetry measurements provide the best available estimates of ice-sheet topography for modeling ice flow, with an accuracy over smoother parts of the ice-sheet of ± 1 meter (Fig. 17). However, accurate measurements cannot be acquired over most near-coastal regions due to the effects of surface slopes and undulations. • Laser altimetry measurements by a future s a t e l l i t e is required for detection of changes in ice volume over the entire polar ice sheets by sequent i a l (approximately 5year intervals) mapping of elevation to ± 10 centimeters, and for detection of changes in c r i t i c a l ice rises and grounding lines. • Microwave radiation from the surface has been measured systematically since 1973 by NASA's Nimbus s a t e l l i t e s (Fig. 18). Near total coverage of icecovered regions has been obtained every few days. The measurements provide estimates (with a spatial resolution of tens of km) of: • Sea-ice characteristics: ice extent, ice concentration; and whether or not the ice has survived a summer m e l t season. • Snow cover on land: snow extent, and water equivalent of dry snow cover. • Ice-sheet characteristics: whether the ice surface is wet or dry; (possibly) estimates of snow-accumulation rates. • Imagery from Landsat and more recently from SPOT, with a spatial resolution of a few tens of meters, provides detailed information over selected areas, but coverage is sparse. Applications include mapping and, by comparing sequential images of the same area, estimate of ice movements• • Synthetic Aperture Radar measurements provide high-resolution, all weather images of the surface. The power requirements and high data rate of the SAR l i m i t coverage to selected areas within range of a ground receiving

(1)58

S.I. Rasool

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(1)60

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Remote Sensing for Global Change Study

Fi gure 18.

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Variations in the Northern Hemisphere Show Depth Derived from the Scanning Multichannel Microwave Radiometer for February 1979, 1982, 1983, and 1985. (Chang, GSFC, unpublished, 1985)

Remote Sensing ~ r Global Change Study

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station. Applications are similar to those for Landsat data (mapping and ice motion) with the added capability of all weather operation. • Visible and infrared measurements from weather satellites have lower resolution (1-4 km) but a larger swath width (3QOU km) and they provide global coverage everyday. Despite the coarse resolution, these data are being used to compile a map of Antarctica which, in many areas, w i l l represent a significant improvement over existing maps. In addition, thermal infrared measurements provide estimates of ice-surface temperatures. Laser altimeters have acquired ice elevation profiles from aircraft and have been proposed to fly as Shuttle experiments and on research platforms during the 1980's or early 1990's. We recommend that the highest p r i o r i t y be p|aced on includin 9 a laser altimeter on a polar-orbitin 9 s a t e l l i t e for the detection of changes in polar ice volume. All of the sensors (except laser altimeter) described above have been flown aboard satellites and their capabilities over ice are well-proven. Problems remain with validating some of the derived geophysical parameters, and we anticipate continued improvement in the accuracy of these parameters and in our a b i l i t y to assess errors. For some parameters, such as snow-accumulation rates derived from passive microwave data, there is a need for extensive research. Nevertheless, we can clearly state that s a t e l l i t e measurements provide our best estimates of sea-ice characteristics, snow extent on land, ice-sheet topography and extent, and the areal extent of summer melt zones on the ice sheets. Consequently,we recommend that a high p r i o r i t y be placed on continuin~ the measurements necessary to compile lonB time series of these data. This w i l l require continuous passive microwave coverage and repeated (but not necessarily continuous) surveys by nighresolution visible imagery, SARs~ and radar altimeters. In addition, there is a good probability of obtaining long-term surface temperature measurements from improved infrared radiometers such as ATSR. Missions that w i l l acquire the data needed to satisfy this recommendation are planned for launch during the next five to ten years. Associated with these missions are planned research and validation programs such as PIPER (Program for International Polar Oceans Research), and PISR~ (Polar Ice Sheet Research Group). Validation of Snow Measurements At present there is no coordinated program to improve s a t e l l i t e sensing of snow cover, but a number of investigators in several countries have been conducting research on developing microwave techniques. Specific validation tasks that should be conducted are: • o

Mapping of snow areal extent; Improvement of algorithms for snow water equivalent determination from Imaging Microwave Radiometer (IMR) data; and Validation and improvement of methods for determining snow shortwave albedo.

IV. GAPS IN THE C U R R E N T SPACE SYSTEM AND THE N E E D E D F U T U R E THRUSTS There are a number of gaps in the current s a t e l l i t e observing system that must be f i l l e d to meet the requirements of a Global Change Program. Some of these gaps can be f i l l e d by flying instruments on operational satell i t e s that have already been successfully tested on research s a t e l l i t e s . Others w i l l be f i l l e d by future s a t e l l i t e missions already approved for f l i g h t . S t i l l others w i l l require new technology to measure from space some of the more elusive c r i t i c a l parameters. In this section, we discuss all three means of bridging these gaps.