VIIRS, a scanning radiometer, collects visible and infrared imagery and radiometric measurements of the land, atmosphere, cryosphere, and oceans. VIIRS data is used to measure cloud and aerosol properties, ocean color, sea and land surface temperature, ice motion and temperature, fires, and Earth's albedo. Climatologists use VIIRS data to improve our understanding of global climate change.
VIIRS extends and improves upon a series of measurements initiated by the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS).
Accurate satellite measurements of ocean pigment concentrations and sea surface temperature (SST) were first demonstrated with the Nimbus-7 Coastal Zone Color Scanner (CZCS) and the NOAA-7 Advanced Very High Resolution Radiometer (AVHRR) launched in 1978 and 1981, respectively. Subsequent missions like the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) have provided high quality time series of these and other properties by expanding the spectral information and calibration accuracy, and by improving the processing algorithms.
Like MODIS, VIIRS is a multi-disciplinary sensor providing data for the ocean, land, aerosol, and cloud research and operational users. VIIRS spectral coverage will allow for data products similar to those from SeaWiFS as well as SST, a standard MODIS product. SST is an Essential Climate Variable (ECV) and, through validation with instruments traceable to NIST standards, is a Climate Data Record. Also, as with SeaWiFS and MODIS, the VIIRS scan and orbit geometries will provide global coverage every two days.
The VIIRS design incorporates a SeaWiFS-like rotating telescope assembly which protects the optical components from on-orbit contamination. This will result in greater on-orbit stability than other designs. VIIRS also has a solar diffuser assembly with a stability monitor similar to MODIS for tracking on-orbit performance in visible wavelengths, and a MODIS-like black body calibration target for the infrared bands.
Two-day coverage is a general requirement for ocean ecology and carbon research because microscopic marine plant (phytoplankton) concentrations are highly variable, particularly in coastal zones. The VIIRS 750 m resolution across the entire scan will provide twice the coverage of MODIS and SeaWiFS, which is a substantial improvement for coastal and estuarine studies in particular. VIIRS also has shortwave infrared bands that can be used for turbid water aerosol corrections.
Measurement of pigment concentrations, water clarity, suspended particulates and other properties in coastal regions are critical for coastal zone management, fisheries management, and naval operations. Specific applications that VIIRS can continue include operational forecasts of harmful algal blooms in the Gulf of Mexico, detection of areas at high risk for coral bleaching due to temperature, assessing fish recruitment and climate impacts, and assessing primary productivity and ecosystem health in ocean basins and coasts.
Similarly, accurate estimates of SST are essential for many applications such as hurricane prediction and weather forecasting. The VIIRS SST data will be used to continue the decadal global SST time series currently derived from the AVHRR, MODIS and other sensors that is critical for climate change research. VIIRS products will augment other satellite products to provide important real-time ocean products for the U.S. Navy.
VIIRS ocean products will be used to understand, monitor, and forecast coastal processes as part of the Navy's 3-D ocean nowcast - forecast modeling system. Specifically, SST products are used in real time for characterizing ocean fronts and water masses. Additionally, SST from VIIRS will be assimilated into real time ocean circulation models for global and regional ocean prediction of currents and fronts. Similarly, real time ocean color products such as surface chlorophyll and optical properties are used for defining water clarity conditions on global and regional scales. Real-time VIIRS bio-optical products will provide daily model initialization fields with are assimilated into ecological models and used for ocean optical forecast and predicting 3-D coastal bio-optical properties. Ocean optical and circulation forecast systems are used daily to support navy operations.
It was always intended that the NPP VIIRS instrument would provide a bridge between EOS MODIS and the operational JPSS (formerly NPOESS) VIIRS. In that context, the land science of VIIRS will build and expand on the heritage of land science from the NOAA AVHRR and EOS MODIS.
The strength of these systems lies in their time-series of daily multi-spectral observations, which are used to characterize and monitor the land surface at regional to global scales. MODIS provided a new standard in calibrated, science-quality, coarse-resolution satellite observations which will continue with VIIRS. Although the primary goal of the VIIRS instrument design was to the meet the needs of the operational weather community, much of the MODIS for capability for land science has been retained.
Image Above : NASA Terra satellite image of San Diego wildfires. Red pixels indicate fire activity; black horizontal line indicates the U.S.-Mexico border. (Oct. 23, 2007 at 11:25 a.m. PDT) Image credit: NASA/MODIS Rapid Response
The science VIIRS will address centers on four major areas: energy and water balance, vegetation dynamics, land cover land use change and the cryosphere. These research areas map to the NASA Science Focus Areas. Energy and water balance studies include quantifying surface albedo, photosynthetically active radiation (PAR), land surface temperature, evapotranspiration and the associated radiative forcing and surface atmosphere exchanges. Such data are used to parameterize regional to global scale climate and hydrological models.
The study of vegetation dynamics, including phenology will involve the use of products such as the Vegetation Index, Leaf Water Content and Leaf Area Index. These products are used in global dynamic vegetation, carbon and agricultural production models. The study of land cover change includes the use of Land Cover and Fire products. The former are also used as a basic stratification data layer, for example to initiate regional to global climate and integrated assessment models, and the latter are used to quantify vegetation disturbance and as inputs to model emissions from biomass burning. Cryospheric science utilizes the Snow and Ice products (land and sea-ice) to determine both their extent and inter-annual variability and as inputs to hydrological models.
Evaluation of the planned operational land products to be generated from the VIIRS (Environmental Data Records) by the science team indicates that new and enhanced products will be needed to meet the needs of the science community. Land science use of the VIIRS Products will depend on a well-calibrated and characterized instrument, operational product quality assessment, and a well-supported program of product validation, building on the experience and protocols developed for MODIS. Validation requires the collection of independent data from other sensors and field measurements.
To continue the science products from MODIS, which utilize both TERRA and AQUA data (e.g. Albedo, LAI, LST), will require the integration of VIIRS data with equivalent data from other coarse sensors such as METOP and Sentinel 3. Land science from VIIRS will also involve the use of multi-sensor and multi-resolution data. For example, the combined use of VIIRS and LDCM will enable the detection and quantification of land cover change. Measurement of land surface trends in the context of climate change will involve linking the VIIRS measurements to those from AVHRR and MODIS in the form of long-term data records.
VIIRS data will be used to expand upon the MODIS applications to fire and air quality monitoring, agriculture monitoring and production modeling, carbon modeling and flood and sea ice mapping.
In general, clouds cover about 70 percent of the planet on any given day. They influence the amount of sunlight reaching the surface, and they regulate the amount of energy--both solar and thermal--that reaches space.
Since 1980, polar-orbiting weather satellites have included both imagers and sounders. These types of sensors record data continuously, using different wavelengths to infer information about clouds on a global scale. They can determine cloud top height and thermodynamic phase (ice or water particles), and make estimates of microphysical and optical properties that indicate the amount of water and ice in the cloud layer.
The cloud products derived from VIIRS and CrIS will serve a range of communities. For example, information on cloud cover is needed by the solar energy community to optimize energy production. Cloud products are being used increasingly in nowcasting models (up to 9 hours in the future) and in numerical weather prediction models (perhaps several days in the future). Such products are used to determine the probability of precipitation or severe weather.
Since such models requires that cloud products be available soon after the data are collected, data reduction must be timely and efficient. On a longer time scale, these cloud data are used to build climate data records that are critical for establishing a decadal record of cloud properties. Technical issues associated with deriving a long-term data set of cloud properties require continual work on sensor calibration and refinement of algorithms.
Historically, cloud information was provided by the Advanced Very High Resolution Radiometer (AVHRR) and the High resolution InfraRed Sounder (HIRS). On the NASA Earth Observing System Aqua and Terra platforms, such measurements are provided by the MODerate resolution Imaging Spectroradiometer (MODIS) and the Atmospheric Infrared Sounder (AIRS). Over time, the technology behind these imager and sounder sensors has improved, with the latest imager being the Visible Infrared Imager Radiometer Suite (VIIRS) and the Cross-track Infrared Sounder (CrIS).
VIIRS will provide information about clouds, aerosol, and surface properties at a spatial resolution of about 750 m for most spectral measurements. VIIRS records data at a set of discrete wavelengths from the ultraviolet (0.45 _m) to the infrared (12 _m). CrIS is a hyperspectral (> 1000 spectral wavelengths) sensor that will provide complementary information about clouds, especially in complex regions such as the poles, over bright surfaces such as snow/ice, and in areas that have strong temperature inversions.
The cloud properties from VIIRS, and in some regions from VIIRS combined with CrIS, will maintain continuity with the decadal cloud record from 1980 to the present that is provided by historical and current sensors such as AVHRR, HIRS, MODIS, and AIRS. VIIRS cloud products will be available at higher spatial resolution, and will be well-calibrated, which is essential for its use in continuing the climate record.