Moderate Resolution Imaging Spectroradiometer (MODIS)
Product Details
TRANSITIONED PRODUCT DETAILS
Sea Surface Temperature Composite
The MODIS sea surface temperature (SST) composite product is produced by SPoRT from the sea surface temperature data provided by the University of South Florida. The primary advantage of the SST composite over other products is the temporal availability and the resolution. The SST composite is available four times each day. Additionally, the SST composite has a resolution up to 1 km. The product is based on the assumption that SST values have small day-to-day changes. Therefore, for a given region, the SST values from a preceding day are representative of the current day's values. The assumption's validity may vary both spatially and seasonally as oceanic and atmospheric forcing create higher frequency changes. The SPoRT approach uses the three most recent, cloud free SST values for each pixel within the domain. These are subdivided further into the daytime and nighttime passes. The next step is to combine the three most recent pixels. This requires some effort to remove cloud contamination beyond the use of the cloud mask. This is done by averaging the two warmest of the three pixels, which removes any cloud-contaminated pixels that were not excluded by the cloud mask. Finally, noise had to be dampened along with any remaining cloud contamination while maintaining the spatial gradients. An average of the 5x5 array surrounding each pixel was computed and is then assigned to the pixel for the final SST composite value. A more thorough discussion is in Haines et al. (2007) in IEEE Transactions on Geoscience and Remote Sensing, Volume 45, Number 9.
TRANSITIONED PRODUCT DETAILS
Sea Surface Temperature Latency
Each of the MODIS composite products use the same latency value. The latency is simply the average age of the two pixels used in each composite. The latency is a good measure to determine how much confidence can be placed in a given MODIS composite. Composite products with a low latency, such as a few days, will have a much greater degree of confidence than those with larger latencies. The larger latencies indicate that the input data for each pixel is older due to extensive cloud cover blocking MODIS observations.
TRANSITIONED PRODUCT DETAILS
Sea Surface Temperature Single Swath
The sea surface temperature is provided by the University of South Florida and distributed by the SPoRT Center to partner WFOs. The algorithms are from the Earth Observing Systems and use MODIS calibrated mid- and far-infrared radiances of 3.660-3.840 µm, 3.929-3.989 µm, 4.020-4.080 µm, 10.780-11.280 µm, and 11.770-12.270 µm, which correspond to MODIS bands 20, 22, 23, 31, and 32. The algorithm exploits the differences in atmospheric transmissivity in the different infrared bands. This provides a very accurate estimation of the atmospheric effects, which enable ancillary input to the algorithm, along with a land mask to mark non-water pixels and an ice-extent mask that limits polar sea coverage. A sequence of spatial and temporal homogeneity tests is applied to validate the quality of the cloud-free observations. This provides up to 1 km resolution of the sea surface temperature in cloud free regions. Additional information is available at the NASA MODIS Product Page.
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Spectral Difference
The Spectral Difference product, also known as the Thermal Difference product, is commonly referred to by its most common use; the Fog product. This product takes advantage of the lower thermal emissivity of water clouds (3.9 µm) versus land surfaces (11 µm). This difference is characterized by the 11 – 3.9 µm difference image calculated during the pre-dawn hours over a given region. Currently, a single subjectively determined threshold value (2.5 K) defines the cutoff region in the image. Locations with values greater than the threshold are labeled as fog. Conversely, regions with values lower than the threshold are clear. In reality, this threshold is not constant and can change spatially, temporally (time of night), and seasonally. There are two items to keep in mind when using this product. First, this product is only valid at night for fog detection. Secondly, regions of multi-layered stratus clouds could be indicated as fog, when fog may not actually be present.
TRANSITIONED PRODUCT DETAILS
Natural Color Composite
Also known as the True Color Composite, this MODIS product is one of two color composites produced by SPoRT. The Natural Color Composite is designed to enhance visible features and is therefore valid only during the day. In particular, ocean, land surface, cloud, and other atmospheric features (such as smoke and dust) are emphasized. This is known as a composite as the product is derived from combining three channels from MODIS, with each channel being assigned a color. The channels correspond to red, green, and blue where the intensity is proportional to the radiance values of the MODIS channels 1 (.620 - .670 µm), 4 (.545 - .565 µm), and 3 (.459 - .479 µm). This will approximate the actual (natural) colors in the field of view and represent what a person would see if they were looking down at the Earth. Additionally, several corrections are necessary in order to use the three channels. These atmospheric and geometric corrections account for atmospheric radiative interactions and cross track variation of the satellite field of view, respectively.
TRANSITIONED PRODUCT DETAILS
False Color Composite
As opposed to the Natural Color Composite, which is designed to enhance visible features, the False Color product combines one visible channel with two shortwave infrared channels to highlight features with infrared signatures. This particular False Color Composite has been developed to distinguish between snow and clouds, both of which appear white on Natural Color imagery. While snow may look like clouds in visible imagery (i.e. what our eyes see), snow reflects radiation differently than clouds in other portions of the spectrum. Spectrally, snow is different from clouds at wavelengths greater than 1.4 µm. MODIS channels at 1.63 and 2.13 µm can therefore be used to distinguish between snow and cloud cover. To make this distinction more obvious, a visible channel combined with the two channels just mentioned is used to produce the “false color” image. A few other procedures are necessary to create this product. The MODIS imagery is “stretched” to enhance the contrast between the features to assure a good color differentiation between various features of interest. The MODIS data are combined such that features with large reflectance in the visible, 1.63, and 2.13 µm channels take on color characteristics corresponding to red, green, and blue, respectively. Green locations have clear skies and no snow cover. White locations are water clouds, while pink locations are clouds with ice. Finally, snow cover is seen as dark red. While standard visible imagery animations can distinguish between snow cover and clouds (since clouds move), the False Color product enhances the difference with these colors. The Great Falls, Montana Weather Forecast Office has been receiving this product since 2004. This has been used to map snow cover on the ground to improve flood forecasts from springtime snow melt. A case study from February 2008 can be found in Loss et al. (2009) and additional information can be found in the MODIS False Color Snow product section located in the SPoRT Training page.
TRANSITIONED PRODUCT DETAILS
Land Surface Temperature
This product is generated using a physical split window retrieval algorithm (Suggs et al. 2004). Produced at 1 km resolution and with retrievals available both day and night, the Land Surface Temperature product provides a spatial resolution superior to that of the existing ASOS surface observation network. The trade-off is the low temporal resolution. The values obtained through this product have been found to compare favorably with the results produced by the Earth Observing System science team as shown by (Suggs et al. 2004).
TRANSITIONED PRODUCT DETAILS
Lifted Index
This is another University of Wisconsin product distributed by the SPoRT Center. The lifted index is a measure of atmospheric stability, which indicates how likely the atmosphere will produce thunderstorms. Positive values indicate fair weather, while negative values indicate the potential of thunderstorms. Large negative values begin to indicate the potential for severe weather. This particular product is derived, both day and night, with a box made of 5 × 5 1-km pixels. If nine of the 10 pixels in this box are cloud free, the lifted index is calculated. When this condition is met, the lifted index is computed using the infrared temperature and moisture profile data. These MODIS profiles are produced at 20 vertical levels for temperature and 15 levels for moistureWith a higher resolution compared to GOES, the MODIS instrument can take measurements in partly cloudy regions that GOES would be unable to. Additionally, the early afternoon overpass provides a measure of the atmospheric stability just before convection climatologically starts and provides updated information from the morning radiosonde data. Additional discussions on this product can be found in the MODIS Product Description PDF.
TRANSITIONED PRODUCT DETAILS
Total Precipitable Water
The SPoRT Center provides the University of Wisconsin/Earth Observing System (EOS) real-time product (MOD 05) to SPoRT's partner Weather Forecast Offices. During the daytime, a near-infrared algorithm is applied over clear land areas of the globe and above clouds over both land and the ocean. Over clear ocean areas, water vapor estimates are provided over the extended glint area. An infrared altorithm for deriving atmospheric profiles is also applied both day and night for the Level 2 data. The Level 2 data are generated at the 1 km spatial resolution of the MODIS instrument using the near-infrared algorithm during the day. During both day and night, they are generated at 5x5 1 km pixel resolution using the infrared algorithm when at least nine fields of view are cloud free. This information is provided at NASA's MODIS Product Page.
TRANSITIONED PRODUCT DETAILS
Visible Image
The MODIS Visible imagery is produced at a single band of 10.65 µm. Obviously, this is available only during the daytime hours. This typically provides two overpasses per day; one by the Aqua and Terra satellites at a resolutions down to 250m. At these high resolutions, the Visible channel can detect cloud features that cannot be resolved by GOES, thus providing forecasters better situational awareness.
TRANSITIONED PRODUCT DETAILS
Cloud Mask
The MODIS Cloud Mask product is a Level 2 product generated at 1-km and 250-m (at nadir) spatial resolutions. This is produced by the University of Wisconsin and NASA’s Earth Observing System. The SPoRT Center distributes this product to our partner WFOs. The algorithm employs a series of visible and infrared threshold and consistency tests to specify confidence that an unobstructed view of the Earth’s surface is observed. An indication of shadows affecting the scene is also provided. The 250-m cloud-mask flags are based on the visible channel data only. Radiometrically accurate radiances are required, so holes in the Cloud Mask will appear wherever the input radiances are incomplete or of poor quality. More technical information can be found with the MODIS Cloud Mask User's Guide and at the Goddard Space Flight Center MODIS Page.
TRANSITIONED PRODUCT DETAILS
Cloud Phase Indicator
Utilizing visible, near-infrared, and shortwave infrared retrievals the Cloud Phase Indicator attempts to define the state of water within the cloud’s environment. This product, produced by the University of Wisconsin, lists six individual states as well as indicating where there are clear skies via the MODIS Cloud Mask product. The product graphically shows the location of pure liquid water clouds, pure ice clouds, and mixed phase clouds for daytime passes only. Additionally, locations of non-opaque liquid and ice clouds are shown. Finally, regions where the algorithm cannot determine a state are flagged as uncertain.
TRANSITIONED PRODUCT DETAILS
Cloud Top Pressure
The Cloud Top Pressure product is produced by the University of Wisconsin and distributed to partner National Weather Service Offices by the SPoRT Center. The cloud top pressure is determined from radiances measured in spectral bands located within the broad 15 µm CO2 absorption region. The radiances are sensitive to different atmospheric layers since the atmosphere becomes more opaque as the wavelength increases from 13.3 to 15 µm. This technique has been widely used as discussed in Menzel et al. (2008). MODIS provides measurements at 1 km resolution in four wavelengths in the 15 µm CO2 band. The cloud top pressure is produced for cloudy regions of a 5 × 5 pixel array where the radiances for the cloudy pixels, which are determined by the 1 km cloud mask, are averaged. Additional details can be found at the MODIS Science Page and the aforementioned Menzel et al. (2008) on the American Meteorological Society web site.
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Longwave Infrared
This is nearly identical to the Infrared Imagery produced by the GOES satellites, but at a higher resolution (down to 1 km), which means cooler (and therefore higher) cloud tops may be identified. This assists with the recognition of convective versus stratiform cloud types, particularly at night. This serves as a demonstration product for the future capabilities of GOES-R.
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Shortwave Infrared
The MODIS shortwave infrared imagery uses the 3.9 µm wavelength and is capable of 4 and 1 km resolutions. This imagery is particularly useful in detecting hot spots on the Earth’s surface, such as from wild fires. Hot spots will appear as single white pixels.
TRANSITIONED PRODUCT DETAILS
Water Vapor
As with the Visible Imagery, the Water Vapor Imagery uses a single band but at 6.7 µm that is sensitive to the amount of water vapor in the atmosphere. The Water Vapor Imagery has a resolution of 1 km, and provides observations of features that cannot be resolved by GOES.
RESEARCH DETAILS
Enhanced SST v6.3a Composite
SPoRT, in collaboration with the Jet Propulsion Laboratory (JPL), is developing an experimental, enhanced sea surface temperature (SST) composite incorporating the Moderate Resolution Imaging Spectroradiometer (MODIS), the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) and the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA). The primary advantage of this SST composite over other products is its temporal availability and resolution. The current approach generates composites over a given region at four times each day corresponding to Terra and Aqua equator crossing times (i.e., Terra day, Aqua day, Terra night, Aqua night). Day-time (night-time) AMSR-E SST data from Aqua plus the most recent daily OSTIA SSTs are used with both Terra and Aqua MODIS day-time (night-time) SST data sets. For a given day and region, the SSTs from the previous seven days form a collection used in the compositing based on the assumption that SST values have small day-to-day changes. At each 1 km pixel data from the collection (MODIS, AMSR-E and OSTIA) cloud-free SST values are used to determine a weighted average based on their quality and latency. In this way recent SST data is given more weight than older data.
MODIS SST data cannot be produced in the presence of clouds and therefore is limited in coverage to cloud-free regions. However, persistent cloud cover increases the latency of the temporally composited SST product data and consequently reduces the accuracy of the SSTs present in the composite. By incorporating AMSR-E data, latency decreases, as this product is able to provide accurate SSTs in areas of persistent cloud cover. Note neither product can be used during times of precipitation. With the addition of AMSR-E data, new limitations arise since it is not available within 125 km of the coastline. This leads to potential "false gradients" in the SSTs when combined with MODIS data. To lessen these limitations, OSTIA data are introduced in areas where MODIS and AMSR-E data are not available. This decreases the latency (particularly along the coast) therefore increasing the accuracy of the SST composite. In order to retain a more detailed spatial representation of the SSTs, AMSR-E and OSTIA are weighted 20% that of the higher-resolution MODIS data.
SPoRT and JPL are working together to settle on a consensus algorithm to be used as a real-time composite. Before the end of the year, the plan is to transition the final algorithm to the Physical Oceanography Distributed Active Archive Center (PODAAC) at JPL for formal operations and dissemination to the public.
