The Value of Geostationary Satellite Imagery in IOOS, Now and Future

Transcription

1 The Value of Geostationary Satellite Imagery in IOOS, Now and Future A. S. Lomax Itri Corporation D. W. Colburn Lockheed Martin Civil Space M. K. Galbraith Itri Corporation Abstract- The United States is embarking on the implementation of an Integrated Ocean Observing System (IOOS), as part of a broader global environmental observation effort, in order to increase the understanding of the ocean environment to derive societal benefits. These benefits will be achieved by making timely, accurate observations of the environment leading to quality predictions and forecasts of the environment and the consequences on the public. To this end, IOOS will consist of an observing subsystem, a modeling and analysis subsystem, and a data management and communications subsystem that ties it all together. The observing subsystem will provide sustained in situ and remotely-sensed measurements of the coastal and global ocean. Initially, IOOS will derive observations from existing operational and pre-operational programs and subsequently incorporate observations from novel programs as they attain operational status. The Geostationary Operational Environmental Satellite (GOES) program, and its follow-on, GOES-R, will play a key role in IOOS. The IOOS Development Plan calls for the GOES program to support the coastal and global components of the observing subsystem. Coordination of technology and applications across the various international geostationary satellite programs (GOES, MTSAT, METEOSAT, INSAT) will achieve a truly global observation system. In the near-term, GOES will be able to provide global and coastal zone sea surface temperature, sea ice, and heat flux observations that will impact some of the IOOS societal goals. As GOES-R comes online in the next decade, the planned sensor suite will permit the measurement of a variety of new core parameters including ocean color, optical properties, surface currents, and possibly bathymetry. Observing those new parameters in a frequency, spatial extent, and spatial resolution never before achieved will open up a new horizon of potential impacts on the societal benefits of IOOS. I. INTRODUCTION The global and coastal oceans play a significant impact on the economy and daily livelihood of our society. The broad global ocean impacts climate and weather and provides a thoroughfare for commerce and a buffer of security. The coastal ocean, namely the exclusive economic zone, likewise contributes to weather, marine transportation, and security but more significantly provides a wealth of natural resources that must be managed and understood. The United States is embarking on the implementation of an Integrated Ocean Observing System (IOOS), as part of a broader global environmental observation effort, in order to increase the understanding of the ocean environment to derive societal benefits. These societal benefits have been defined as (1) improve predictions of climate change and weather and their effects on coastal communities and the nation; (2) improve the safety and efficiency of maritime operations; (3) more effectively mitigate the effects of natural hazards; (4) improve national and homeland security; (5) reduce public health risks; (6) more effectively protect and restore healthy coastal ecosystems; and (7) enable the sustained use of living ocean and coastal resources. These benefits will be achieved by making timely, accurate observations of the environment leading to quality predictions and forecasts of the environment and the consequences on the public. To this end, IOOS will consist of an observing subsystem, a modeling and analysis subsystem, and a data management and communications subsystem that ties it all together. The observing subsystem will provide sustained in situ and remotely-sensed measurements of the coastal and global ocean. To achieve its objectives, the observing subsystem, as a whole, must measure core variables that will lead to societal benefits. These core variables are: temperature, salinity, bathymetry, sea level, surface waves, surface currents, ice distribution, ocean color, optical properties, heat flux, bottom character, and a variety of biological and chemical parameters. Initially, IOOS will derive observations from existing operational and pre-operational programs and subsequently incorporate observations from novel programs as they attain operational status. The Geostationary Operational Environmental Satellite (GOES) program, and its follow-on, GOES-R, will play a key role in IOOS. The GOES satelites provide a staring, synoptic-scale sensor with a quick resample cycle. The IOOS Development Plan [1] calls for the GOES program to support the coastal and global components of the observing subsystem. Coordination of technology and applications across the various international geostationary satellite programs (GOES, MTSAT, METEOSAT, INSAT) will achieve a truly global observation system. The current generation of GOES satellites carry two earthoriented environmental instruments: an infrared and visible imager and an atmospheric sounder. The future generation GOES-R satellite is also slated to carry a Hyperspectral Environmental Suite for Coastal Waters (HES-CW). Of these

2 instruments, the Imager and HES-CW are applicable to IOOS as they provide the capability to observe and measure oceanic parameters. In the near-term, GOES will be able to provide global and coastal zone sea surface temperature, sea ice, and heat flux observations that will impact some of the IOOS societal goals. As GOES-R comes online in the next decade, the planned sensor suite will permit the measurement of a variety of new core parameters including ocean color, optical properties, surface currents, and possibly bathymetry. Observing those new parameters in a frequency, spatial extent, and spatial resolution never before achieved will open up a new horizon of potential impacts on the societal benefits of IOOS. II. NATURE OF CURRENT GEOSTATIONARY EARTH OBSERVATIONS IN THE WESTERN HEMISPHERE A. Sensors and Characteristics The current generation of GOES satellites carries two sensors that are applicable to earth science: an imager and a sounder. The imager is a 5-channel instrument that covers the visible and infrared (IR) portions of the electromagnetic spectrum with characteristics listed in Table 1. The imager is capable of scanning a full earth disc (a hemisphere) in 26 minutes in normal mode and a smaller area in rapid scan mode in 5 minutes. A new GOES imager image is available every 30 minutes. The GOES satellites also carry a sounding instrument which measures brightness temperatures in 19 discreet channels in 4 spectral bands (see Table II). The sounder has a nominal spatial resolution of 10 km in the horizontal and 5 km in the vertical. While the sounder is capable of scanning a 3000 km by 3000 km box in 42 minutes, the nominal scan cycle is once per hour. B. Value of Current GOES Observations to IOOS The value of current geostationary satellite observations to IOOS is best described in terms of the number of IOOS core variables and societal goals supported through either parameters measured or products derived from those parameters. The current generation GOES satellite provides limited capability as applied to IOOS core variables. The GOES imager measures two parameters that are of concern to IOOS: temperature with its IR channels and albedo with its visible channel. Temperature is easily analyzed, in cloud-free imagery, into sea surface temperature products as shown in Figure 1. Sea surface temperature analyses can be used to TABLE I GOES IMAGER INSTRUMENT CHARACTERISTICS Channel: Spectrum Visible Shortwave IR Moisture IR 1 IR 2 Wavelength ( m) IGFOV a (km) a Instantaneous Geographic Field of View at nadir TABLE II GOES SOUNDER INSTRUMENT CHARACTERISTICS Spectrum Channel Wavelength ( m) Longwave IR Medium wave IR Shortwave IR Visible determine the extent of ocean surface currents but not their speed. Albedo is used in determining sea ice coverage but this capability is largely limited to the Great Lakes as geostationary imagery is marginally useful in higher latitudes. Though it is not an explicit IOOS core variable, GOES imagery can be used to derive near-surface and also upperlevel winds from cloud motion. These measurements are important factors in determining steering currents and the shear environment which impact both hurricane track and intensity. In addition to the parameters measured by the imager, the GOES sounder measures radiative parameters that contribute to an understanding of global and regional heat flux. While GOES may not provide a significant contribution to the observation of IOOS core variables, its imagery and data do provide significant input to the IOOS societal goals. The following societal goals are addressed, if not satisfied, by the current iteration of geostationary satellites: - Weather & Climate. The current GOES satellite system is the primary source of remotely-sensed imagery for the National Weather Service and private-sector meteorological firms. Forecasters utilize visible and infrared imagery along with an array of derived products from the imager and sounder to analyze and forecast routine and severe weather. The numerical weather prediction community relies heavily on GOES to provide initial conditions for model in data sparse regions. Maintaining a long-term archive of GOES data will also increase out understanding of climate. - Safety of Marine Operations. While not a source of data for nearshore and port operations, GOES data fills a critical

3 Figure 1. Sea surface temperature analysis from GOES imagery. void of data in the open ocean for weather forecasting to ensure safe and efficient ship routing. - Natural Hazards. GOES imagery is a critical source of information to aid in the prediction of tropical cyclone formation, intensity, and track. GOES provides a persistent view of storms as they pass through their life cycle and give forecasters important information through derived products to assess the impacts on steering currents and intensity. - Healthy Ecosystems. While GOES imagery is typically at a resolution too coarse for ecosystem assessment, the derived sea surface temperatures from GOES are an important piece of information in ecosystem management. It is well documented that coral reef bleaching is impacted by increased ocean temperatures. GOES imagery provides possibly the best source for long-term, continuous, and wide-spread monitoring of sea surface temperatures for coral reef researchers and managers. III. NATURE OF FUTURE GEOSTATIONARY EARTH OBSERVATIONS IN THE WESTERN HEMISPHERE A. Sensors and Characteristics While the GOES-R satellite system is still in the preliminary design phase, the requirements for this nextgeneration satellite are well documented [2],[3]. GOES-R will carry two sensors applicable to IOOS with significantly improved capability. The first sensor is the Advanced Baseline Imager (ABI) which is a scanning 16-channel multispectral (infrared and visible spectra) imager with characteristics identified in Table III. The ABI will have 3 scan scenes in two operational modes. There will be a Full Disc scan that covers the full hemisphere visible to the satellite. There will also be a CONUS scan, covering an area slightly larger than the continental U.S. and a mesoscale scan, which can cover a smal scale anywhere in the satelite s field of view. The two operational modes identified to date include: Mode Included Scans Scan Frequency Mode 4 Full Disk Every 5 minutes Mode 3 Full Disc Every 15 minutes CONUS Every 5 minutes Mesoscale Every 30 seconds The other sensor applicable to IOOS is the Hyperspectral Environmental Suite (HES) which is a combination hyperspectral infrared sounder and visible/solar reflective imaging radiometer. Because GOES-R is in the design phase, the spectral bands are still being finalized but the HES will have a capability for sounding the atmosphere and for imaging Coastal Waters (HES-CW). The sounder will be capable of collecting data over the continental U.S. from 4 spectral bands at a frequency of hourly. The HES-CW is the most intriguing capability GOES-R possesses for the IOOS community. This sensor will collect, as planned, data from at least 14 discrete channels in the visible/near-ir spectral range with a 300 m spatial resolution. Table IV lists these channels and their applicability along with additional channels that are considered goal requirements vice threshold requirements. B. Value of Future GOES-R Observations to IOOS Because of the planned improvements in the number of channels and spectral resolution of the GOES-R instruments, the next generation geostationary satellites will provide a significant value to IOOS in terms of both measuring core variables and achieving societal goals. Despite several years of further scientific research and algorithm development prior to GOES-R launch which could lead to additional capabilities, the system observational requirements give an idea of the value that GOES-R will provide to the IOOS observing subsystem [2],[3]. Additionally, an examination of the spectral wavelengths of the ABI and HES-CW give an indication of those core variables that could be measured from the proposed system. TABLE III GOES-R ABI CHARACTERISTICS Channel Wavelength ( m) Resolution (km)

4 Wavelength ( m) TABLE IV GOES-R HES-CW CHARACTERISTICS Application Colored dissolved organic matter Chlorophyll a Chlorophyll a and chlorophyll b Chlorophyll a and accessory pigments Accessory pigments Accessory pigments and suspended sediment Chlorophyll a, accessory pigments, suspended sediments Suspended sediments, coccolith concentration Chlorophyll a fluorescence Chlorophyll a fluorescence Atmospheric correction Oxygen band Atmospheric correction Column water (atmosphere) GOAL REQUIREMENTS Continuous coverage in 10 nm channels between for hyperspectral imaging to improve atmospheric correction and discriminate water column and bottom features 1.24 Atmospheric Correction 1.38 Daytime cirrus cloud 1.61 Daytime cloud water 2.26 Daytime cloud properties 11.2 Sea surface temperature 12.3 Sea surface temperature Of the 17 IOOS core variables, six are specifically listed as requirements in the GOES-R program documentation [2],[3]. From its Mission Requirements Document, GOES-R will be capable of measuring the following parameters: Temperature The ABI will be capable of measuring sea surface temperature at coastal, mesoscale, and hemispheric scales with a spatial resolution of at least 2 km and a thermal resolution of 1 K. Currents GOES-R will be capable of mapping mesoscale and hemispheric scale currents with a resolution of 2 km and an accuracy of 1 km. Additionally, current velocities will be measured to an accuracy of +/- 1 km/hr. Ice Distribution The combination of the muti-spectral capabilities of the ABI and HES-CW will permit increased capability in the area of sea and lake ice distribution. GOES- R will be able to determine ice concentrations with a resolution of 10 km for hemispheric applications and 3 km for coastal and inland waters to an accuracy of 10%. The proposed system will also be able to determine ice motion (speed and direction) with resolutions of 5 km and 15 km for coastal/inland and hemispheric applications, respectively. Ocean Color The HES-CW will be capable of measuring nearshore and offshore turbidity, chlorophyll, and reflectance which are all properties critical for understanding ocean color. In the coastal environment, these properties will be observed with a spatial resolution of 300 m and an accuracy of < 30% w and on the hemispheric scale with a resolution of 4 km with the same accuracy. Optical Properties In addition to the color properties, the HES-CW will be capable of measuring particulate absorption, backscatter, and chlorophyll fluorescence which describe the optical properties of the water column. These properties will be measured with an accuracy of < 30% and a spatial resolution of 300 m nearshore and 1 km offshore. Heat Flux While not called out as ocean observational requirements but rather as atmospheric observational requirements, the GOES-R instruments will be capable of measuring a host of heat flux-related parameters. Program documentation specifically calls for the measurement of absorbed shortwave radiation, downward longwave radiation, upward longwave radiation, downward solar insolation, and reflected solar insolation. In addition to the parameters specifically listed as observational requirements, there are other IOOS core variables that may be able to be derived from GOES-R observations. The Airlie House Report [4] specified that the core variables of bathymetry and bottom character could be measured through the use of multi- and hyperspectral remote sensing. The HES-CW, in its threshold requirements state, could be useful in these applications. If the HES-CW goal requirement of a hyperspectral sensor is realized, the GOES-R system could provide significant value to measuring bathymetry and bottom characteristics, within the constraints of its 300 m spatial resolution. Another core variable that may be achievable with data from GOES-R is salinity. Salinity measurements are planned for measurement with microwave radiometers on the U.S. Aquarius and European Soil Moisture and Ocean Salinity (SMOS) satellites. However, another technique has been proposed for deriving salinity from outgoing longwave radiation [5]. If this technique proves feasible, GOES-R data could used in the measurement of salinity. This increased volume of core variable observations provided by GOES-R will have a significant impact on achieving IOOS societal goals: - Weather & Climate. GOES-R will continue to support the Weather and Climate goal in the same way as its predecessor satellite system. However, because of the increased spatial resolution, reduced scan cycles, and increased spectral coverage and resolution, GOES-R will provide drastically improved capability for forecasters and models. These improvements will enhance historical derived products and also allow for the creation of new products in the realm of aerosols and trace gases. - Safety of Marine Operations. GOES-R will continue to provide support to this goal but with some enhanced capabilities. First, GOES-R will be able to view the coastal environment at better fidelity than with GOES thus extending

5 the value of observations closer to shore. Also, GOES-R will be able to provide ice concentration and motion information at a sufficient spatial resolution which is extremely important to navigable waters in the Great Lakes. - Natural Hazards. GOES-R will continue to be a valuable tool in assessing tropical cyclone threats to the U.S. and Caribbean region. The increased spatial resolution, shorter scan times, and addition of a mesoscale scan mode will improve the accuracy of forecasts and assessment of damage leading to better decision-making on the part of local and state officials. - Public Health. The goal of public health in the context of IOOS is closely linked to harmful algal blooms (HABs). HABs infect coastal areas with potentially-toxic algae when water conditions are conducive for formation. Because of the spectral characteristics and increased spatial resolution near the coast of the HEW-CW, GOES-R will become a source of valuable information for the prediction, detection, and tracking of coastal HABs. Having a constantly-staring geostationary sensor capable of resolving nearshore HABs will provide a significant capability not in existence today to both the seafood industry and coastal managers who must make decisions about closing beaches and fishing areas in the presence of HABs. - Healthy Ecosystems. Once again, because of the increased spatial and spectral resolution of its sensor suite, a significant increase in value will be achieved by GOES-R in the realm of healthy ecosystems. GOES-R will continue the climatology of sea surface temperatures and extend it into coastal waters at a higher resolution. With the launch of GOES-R, climatologies of other key ecosystem indicators will begin including turbidity, chlorophyll, and other optical properties. Long-term, persistent monitoring of these parameters as will be afforded by GOES-R is the best way to assess and track the health of coastal ecosystems. These increases in value that will be provided by an operational GOES-R system, however, do present some challenges. GOES-R will provide a significant increase in the volume of data that must be processed and managed. While the GOES-R ground system will be designed to handle this data load, the design and development efforts for IOOS must be cognizant of the importance of GOES-R data and build a system that is extensible to the requirements imposed by GOES-R data. Also, new data assimilation techniques will need to be developed for getting higher-resolution and more frequently-available data into numerical models. Higher resolution and more frequent observations will naturally lead to improved forecasts and analyses provided sufficient computing power and models that are improved with finermesh grids and more frequent data assimilation. IV. CONCLUSION The current generation geostationary satellites will be an important observing element in an operational IOOS. Geostationary satellites provide unique capability in that they esentialy stare at their target and provide nearly continuous monitoring of the parameters of interest. The current generation of GOES satellites provides a significant value to achieving some of the IOOS societal goals. The nextgeneration GOES-R satellite will provide remote observational capabilities that meet or even exceed the capabilities provided by present-day polar-orbiting satellites in terms of spatial and spectral resolution. GOES-R will also provide increased capability over existing geostationary satellites in terms of scan modes and scan cycle times. All of these increased capabilities add up to significant value in the future inventory of IOOS observations and major strides in achieving the IOOS societal goals. REFERENCES [1] National Ocean Research Leadership Council, 2005: The first Integrated Ocean Observing System development plan, Ocean.US Publication No. 9, 86 pp. [2] GOES Operational Requirements Working Group, 2004: GOES-R program requirements document, NOAA/NESDIS, Silver Spring, MD, 225 pp. [3] GOES Operational Requirements Working Group, 2004: GOES-R mission requirements document 2B [DRAFT], NOAA/NESDIS, Silver Spring, MD, 361 pp. [4] Ocean.US, 2002: Building consensus: toward an integrated and sustained ocean observing system, Ocean.US, Arlington, VA, 175 pp. [5] Murty, V. S. N., B. Subrahmanyam, V. Tilvi, and J. J. O Brien, 2004: A new technique for the estimation of sea surface salinity in the tropical Indian Ocean from OLR, J. Geophys. Res., 109, C12006, doi: /2003jc