GOES-R Clear Sky Mask
Frequently Asked Questions about the GOES-R Clear Sky Mask Product
1) What is this product?
The GOES-R Cleark Sky Mask product produces output as a binary "cloud" or "no cloud" pixel. The resolution of this product is 2 km. The GOES-R Clear Sky Mask product is produced both day and night. It has an accuracy of 87%.
This product is useful when trying to assess the presence of clouds, especially high cirrus clouds or very low stratus clouds. Such clouds are often not easily discernable in satellite imagery.
2) How often do I receive this data?
The cadence of the Clear Sky Mask product is dependent upon which image from the satellite a person is looking at. For Full Disk and CONUS imagery, an image is produced every 15 minutes. For a mesoscale scan, it is produced every 5 minutes.
3) How do I display this product in AWIPS-II?
To display this product in AWIPS-II, go to the "GOES-R" tab of the CAVE menu, then select "Derived Products." From there, select the region of interest (GOES-E, GOES-W, or GOES Test and Full Disk, CONUS, and Mesoscale). Then, select the "Clear Sky Mask" product.
Alternatively, use the AWIPS Product Browser. Select "Sat", then either "GOES-16" or "GOES-17". From there, choose "Full Disk", "CONUS", or "Meso", then select "ACM."
4) How do I interpret the color maps associated with this product?
The colormap associated with this product is binary. Areas of cloud are colored white, while clear areas are colored black.
5) What other imagery/products might I use in conjunction with this product?
This product can be overlayed with satellite imagery. Of specific use may be the 0.64 um channel and the 10.33 um channel. These channels are the primary channels used to identify clouds in the atmosphere, yet they do not always make it clear to the naked eye if a cloud is present or not. That is, very high cirrus clouds or very low stratus clouds can be difficult to discern in imagery alone. This product can help take the guesswork out of cloud cover forecasting, which can directly impact temperature forecasts.
6) How is this product created?The GOES-R Clear Sky Mask product is calculated for all pixels. It utilizes GOES-R bands 0.64 um, 1.38 um, 1.61 um, 3.9 um, 7.0 um, 7.4 um, 8.5 um, 11.2 um, and 12.3 um. The algorithm relies on spectral, spatial and temporal tests. The performance of the cloud mask is therefore sensitive to any imagery artifacts or instrument noise. Calibrated observations are critical because the cloud mask compares the observed values to those from a forward radiative transfer model.
The ancillary data used to calculate the cloud top height includes1:
- Sun earth distance
- Surface elevation - Both the surface height and maximum surface elevation in a 3x3 box are used in the algorithm.
- Land mask - Using the land mask, each pixel is flagged internally as land or water.
- Desert mask - Using the desert mask, each pixel is flagged internally as desert or clear
- Snow mask - Using the snow mask, each pixel is flagged internally as snow or clear. In addition, if a pixel has a 11 μm brightness temperature of greater than 277K, the snow mask is turned off
- Surface emissivity of channel 7 from SEEBOR - Surface emissivity for each pixel and neighboring warm center (NWC) for each pixel are required
- NWP level associated with the surface
- NWP level associated with the tropopause
- Local Zenith Angle bin
- NWP Line and element indices
- Surface temperature from NWP
- Surface temperature uniformity from NWP
- Total precipitable water from NWP
- Total column ozone from NWP
- Clear-sky infrared RTM calculations:
- Clear-sky top-of-atmosphere (TOA) radiances for channel 7
- Clear-sky top-of-atmosphere (TOA) brightness temperatures computed for channels 14 and 15
- Clear-sky transmission profiles for channel 7
- Equivalent blackbody radiance of a cloud emitting at the temperature of the Tropopause for channel 14
- Clear-sky TOA channel-14 brightness temperature from the image 15 minutes prior
- Clear-sky reflectance:
- The clear sky reflectance is first corrected for atmospheric scattering by adding in the Rayleigh single scattering reflectance and transmission
- In the terminator region, the clear sky reflectance is renormalized
- Both the clear sky reflectance for each pixel as well as the standard deviation of the clear sky channel 2 reflectance over a 3x3 pixel array are used
The derived data used to calculate the cloud top height includes1:
- Valid pixel mask - A pixel is determined to be valid if it is not a space pixel, has a local zenith angle of less than 40 degrees, and has a valid measured and clear sky 11 μm brightness temperature.
- Cloud mask - For the Terminator Thermal Stability Test, the cloud mask from one hour prior is required
- Local Radiative Centers
- Given a derived channel 14 top of troposphere emissivity, εstropo(11μm), the local radiative center (LRC) is defined as the pixel location, in the direction of the gradient vector, upon which the gradient reverses or when an emissivity value (εstropo(11μm)) greater than or equal to 0.75 is found, whichever occurs first. The gradient filter routine is provided by the framework and is required as an input to the algorithm. The required inputs to the gradient filter are:
- The line and element size of the segment being processed
- A binary mask for the segment of pixels that have non-missing εstropo(11μm) for the segment
- The minimum and maximum valid emissivity values (0.0 and 1.0 respectively)
- The maximum εstropo(11μm) value to be considered (0.75)
- Neighboring Warmest Center - The algorithm employs a check for the line and element location of the warmest (largest 11 μm brightness temperature) pixel limited to a 10x10 region. The 10x10 region is one that surrounds each pixel, and classifies those as Neighboring Warmest Center (NWC). The assumption here is that the NWC points represent the optically thinnest pixel in the local area.
- Correlation of channel 9/10 brightness temperature to channel 14 brightness temperature - The algorithm computes the Pearson Correlation Coefficient between the channel 10 and channel 14 brightness temperatures for each pixel. If channel 10 is not available, then the channel 9 brightness temperature can be used.
- Derived channel 14 top of the Tropopause emissivity - The algorithm derives the channel 14 top of troposphere emissivity using the measured channel 14 radiance, clear sky channel 14 radiance, space mask, latitude/longitude cell index from the NWP, Tropopause index from the NWP, local zenith angle bin index, and channel 14 micron blackbody radiance. Both the channel 14 top of Troposphere emissivity for each pixel as well as the LRC channel 14 top of Troposphere emissivity for each pixel are required.
- Minimum channel 2 reflectance over a 3x3 pixel array
- Mean channel 2 reflectance over a 3x3 pixel array
- Standard deviation channel 2 reflectance over a 3x3 pixel array
- Maximum channel 14 brightness temperature over a 3x3 array
- Standard deviation of the channel 14 brightness temperature over a 3x3 pixel array
- The standard deviation of either the channel 9 or channel 10 brightness temperature. NOTE: Channel 9 is only used if channel 10 is not available.
- Cold surface pixel - If a pixel has a surface temperature of less than 265K, it is defined as a cold surface.
- Glint mask - A glint mask is initially defined based upon the glint zenith angle. Any pixels that have a glint zenith of less than 40 degrees are classified as “glint.” However, those pixels that have been marked as glint and have an 11μm brightness temperature of less than freezing (273K), or the 11μm brightness temperature is less than the clear sky 11μm brightness temperature minus 5, have the glint flag turned off. Turning the glint mask off is an attempt to restore cold pixels in the glint zone. Further checks to look at pixels that have a uniform 0.64μm reflectance are performed. A check is done by checking to see if a glint pixel has a standard deviation channel 2 reflectance over a 3x3 pixel array less than 0.10 the mean channel 2 reflectance over a 3x3 pixel array. If it does, the pixel is restored to non-glint.
- Day/Night mask - A day/night mask is defined based upon the solar zenith angle. Any pixels that have a solar zenith of less than 87 degrees are classified as “day” and those greater than 87 degrees are classified as night.
- Terminator mask - We classify those pixels that are between 87 degrees and 93 degrees as pixels in the terminator region.
1Heidinger, Andrew. NOAA NESDIS Center for Satellite Applications and Research Algorithm Theoretical Basis Document: ABI Cloud Mask v.2.0. 6 June 2011. http://www.goes-r.gov/products/ATBDs/baseline/Cloud_CldMask_v2.0_no_color.pdf