Moderate Resolution Imaging Spectroradiometer (MODIS)
Product Details
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Visible Image
The MODIS Visible imagery is produced at a single band of 0.645 µm, available only during the daytime hours. This typically allows for two satellite overpasses per day: one by Aqua and one by Terra. At a resolution of 250m, the Visible channel can detect cloud features that cannot be resolved by GOES, thus providing forecasters better situational awareness.
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Longwave Infrared
MODIS Longwave Infrared imagery uses a wavelength of 11.03 µm (band 31) and is capable of 1-km resolution. This is nearly identical to the Infrared Imagery produced by the GOES satellites, but at a higher resolution, 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. Longwave Infrared 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 3.959 µm wavelength (band 22) 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.
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Water Vapor
As with the Visible Imagery, the Water Vapor Imagery uses a single band but at 6.7 µm (band 27) 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.
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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, band 22) versus land surfaces (11 µm, band 31). 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.
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Air Mass
The MODIS "Air Mass" RGB composite uses a total of four infrared channels to characterize moisture, cloud cover, and synoptic-scale features associated with air masses. Red shades are generated by differencing bands 27 and 28 (6.7 µm and 7.2 µm), greens difference bands 30 and 31 (9.7 µm and 11.0 µm), and blue is produced by an inversion of the band 27 (6.7 µm) brightness temperature. Temperature differences in each channel pairing or single channel brightness temperature are constrained and enhanced to develop color combinations relevant to air mass characterization. EUMETSAT developed the air mass composite to improve detection of rapid cyclogenesis, jet streaks, and potential vorticity anomalies. In the final color combination, warm air masses are indicated in shades of green, while colder air masses are indicated in deep shades of blue and purple. Jet streaks and regions of subsidence are identified as red shades. EUMETSAT provides additional training references at the following web link: http://oiswww.eumetsat.org/IPPS/html/bin/guides/msg_rgb_airmass.pptg page.
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True Color Composite
Also known as the True Color Composite, this MODIS product is one of two color composites produced by SPoRT. The True 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 (true) 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.
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Snow/Cloud Composite
As opposed to the True Color Composite, which is designed to enhance visible features, the Snow/Cloud product combines one visible channel with two shortwave infrared channels to highlight features with infrared signatures. This particular Snow/Cloud Composite has been developed to distinguish between snow and clouds, both of which appear white on True 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 µm (band 6) and 2.13 µm (band 7) 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 "snow/cloud" 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 Snow/Cloud 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 Snow/Cloud product section located in the SPoRT Training page.
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Dust
The MODIS "Dust" RGB uses five infrared channels and their differences to identify dust plumes that are being lofted into the lower atmosphere. Red shades relate to the difference in bands 32 and 31 (11.7 and 10.7 µm), green shades relate to differences in bands 31 and 29 (10.7 µm and 8.4 µm), and the blue coloring relates to brightness temperatures in band 31 (10.7 µm). The RGB composite is designed to identify dust plumes in a bright pink color during daytime imagery, and a darker pink to purple shade in nighttime imagery, due to the contrast in surface and low level temperatures that occur in the diurnal cycle. Cold, thick and high level clouds will also appear as dark reds, while thin cirrus and contrails will appear in shades of green to black depending upon the background surface. The dust RGB is primarily useful in identifying blown dust associated with dust storms and comparable events in desert or dry areas where these events are more frequent.
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Nighttime Microphysics
The MODIS "Nighttime Microphysics" or "Fog" RGB uses a total of three infrared channels to identify low clouds and fog, low stratus, and thin cirrus. Red shades are related to the difference in bands 32 and 31 (11.7 and 10.7 µm), green shades relate to the difference in bands 31 and 29 (11.7 and 8.4 µm), and the blue shades relate to the brightness temperature in band 31. The RGB composite recipe was developed by EUMETSAT to provide a cloud analysis, detection of fog, contrails, and snow. The composite is designed to characterize fog with a light green color. The actual shade of the fog color will depend upon the depth of the fog layer and the temperature contrast between the cloud top and the underlying surface. As the surface temperature warms, the color of fog in the composite will transition from a light green to a light blue, due to the inclusion of the infrared channel assigned to the blue color. Deep convective clouds appear in deep reds. High level ice clouds, such as cirrus, appear in dark shades of blue.
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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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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.
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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). This product is inactive.
