GOES-R Aerosol Optical Depth
Frequently Asked Questions about the GOES-R Aerosol Optical Depth Product
1) What is this product?
The GOES-R Aerosol Optical Depth product calculates the optical depth of detected aerosols. The resolution of this product is 2 km. The GOES-R Aerosol Optical Depth (AOD) product is created only during the daytime hours. Aerosol optical depth is a unit-less quantity, with a range from -1 to 5. The product's accuracy depends upon the surface beneath it (land or water) and also the concentration of aerosol. Over land, for optical depths less than 0.4, it is accurate to within 0.06. For optical depths from 0.4 to 0.8, it is accurate to within 0.04. For optical depths greater than 0.8, it is accurate to within 0.12. Over water, for optical depths less than 0.4, it is accurate to within 0.02. For optical depths greater than 0.4, it is accurate to within 0.10. The GOES-R AOD product is considered quantitative out to 60 degrees local zenith angle and qualitative beyond that.1
This product is useful when trying to identify the thickness of aerosol present in the atmosphere. The higher the optical depth, the thicker the aerosol. This can inform parameters such as visibility, air quality, and also imply the strength of an active fire (amount of smoke produced) or amount of dust in a dust storm.
2) How often do I receive this data?
The cadence of the AOD product is the same regardless of scan mode. That is, the product is produced every 15 minutes for Full Disk, and every 5 minutes for CONUS.
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 or CONUS). Then, select the Aerosol Optical Depth product.
Alternately, use the AWIPS Product Browser. Select "Sat", then either "GOES-16" or "GOES-17". From there, choose "Full Disk" or "CONUS", then select "AOD."
4) How do I interpret the color maps associated with this product?
Color tables have not yet been created for this product.
5) What other imagery/products might I use in conjunction with this product?
The product can be used in conjunction with Aerosol Detection product. Aerosol Detection product informs the user about the type of aerosol present. This product can also be used in conjunction with individual visible band imagery or false color RGB imagery. This product can be used in conjunction with visible 0.64um satellite imagery to identify the location of aerosols. The 0.47um channel can also assist with identifying the location of aerosols. Although these channels are used in the algorithm itself, it can be beneficial to see these individual components in case the algorithm is incorrectly identifying aerosol that is not present or missing aerosol that is present. This product can give a forecaster confidence in the amount of aerosol being lofted into the atmosphere by having a quantitative measurement. Otherwise, the only way to assess the aerosol amount is by implying it by the opacity of the aerosol in the visible satellite channels. Having this information can be beneficial when assessing the strength of a dust storm (by the amount of dust lofted) or when assessing the "strength" of an active wildfire (by the amount of smoke produced). Parameters such as visibility (high amounts of aerosols can lead to low visibility) and air quality (high amounts of aerosols as seen in a hazy environment coupled with hot and humid temperatures) can also be inferred from this product.
6) How is this product created?
The algorithm is designed to retrieve the aerosol optical depth (AOD)/suspended matter (SM) and aerosol size parameter (ASP) from ABI clear-sky spectral reflectances. The output is at 2-km spatial resolution and 5- and 15-minute temporal resolution during daytime. Due to the relatively weak aerosol signal and large uncertainties associated with surface reflectance, the current ABI algorithm does not attempt to do retrieval over bright surfaces, which include sun glint areas over water, desert, and bare soil surface over land. The primary retrieval product is AOD at 550 nm, however AOD in five ABI channels (0.47, 0.64, 0.86, 1.61, and 2.25 µm) are also calculated based on the selected aerosol model. Aerosol size parameter is represented by two Ångström Exponents corresponding to two pairs of wavelengths (0.47/0.86 and 0.86/2.25 µm). In addition, a
single aerosol type over land, and a fine mode and a coarse mode aerosol type along with the corresponding fine-mode weight over ocean are retrieved. The aerosol products are intended for air quality and weather applications.
The ancillary data used in the Aerosol Optical Depth product include:
- GOES-R Cloud Mask product: ABI cloud mask is needed to identify clear-sky pixels for aerosol retrievals.
- GOES-R Snow / Ice Mask product: ABI snow/ice masks are needed to identify snow/ice-free pixels for aerosol retrievals. When the ABI snow/ice mask is not available, the Interactive Multisensor Snow and Ice Mapping System (IMS) is used.
- GOES-R Total Precipitable Water product
- GOES-R Ozone product
- GFS model ocean surface wind speed & direction
- GFS model surface pressure
- GFS model surface height
- GFS model total precipitable water
- GFS model total ozone
- Global land/sea mask at 1km resolution
- Global surface elevation mask at 1km resolution
- Atmospheric lookup table (LUT): This contains atmospheric reflectance as function of aerosol model, aerosol optical depth, ABI channel and scattering angle. Also, atmospheric transmittance as function of aerosol model, aerosol optical depth, ABI channel and zenith angle. Finally, atmospheric spherical albedo as function of aerosol model, aerosol optical depth, and ABI channel.
- Sunglint LUT: This contains water sunglint direct-hemispheric reflectance as function of aerosol model, aerosol optical depth, ABI channel, solar zenith angle, local zenith angle, relative azimuth angle, and surface wind speed. Also, water spherical albedo as function of ABI channel and wind speed.
- Aerosol Normalized Extinction Coefficients: These are normalized aerosol extinction coefficient as function of aerosol model, aerosol optical depth (land only) and ABI channel.
- Aerosol Mass Extinction Coefficients: These are Mass extinction coefficients for each aerosol model.
The algorithm for aerosol retrieval over ocean is based on the VIIRS algorithm; however, the aerosol models used by the algorithm are from the MODIS Collection 5 algorithm. ABI channels 2, 3, 5 and 6, for which surface reflection can be estimated without information on ocean color, are used. Thus, for example, ABI channel 1 (0.47µm) is not included due to the large uncertainty in water leaving radiance.
It is assumed that the surface reflectance of water can be modeled with sufficient accuracy. Therefore, the essence of the multi-channel aerosol algorithm over water is the simultaneous retrieval of optical depth and aerosol model by matching calculated and observed TOA reflectances in selected ABI channels. The contribution of aerosol to the TOA reflectance is approximated by a linear combination of two aerosol modes corresponding to a fine and a coarse mode of the size distribution with a fine mode weight. Since an exact match of TOA reflectance in all selected channels is nearly impossible, the algorithm uses ABI channel 3 (0.86 µm) as the principal channel, where the aerosol optical depth at 550 nm and fine-mode weight (η ) retrieval is performed to match the observed TOA reflectance. The 0.86-µm channel is chosen because of its negligible underwater reflectance, while the contributions from the fine and coarse aerosol modes are still significant. The sum of squared differences of calculated and observed reflectances in other channels is calculated as the residual for the judgment of the degree of match, and the minimum residual is considered as the ‘best’ match. The process of checking for minimum residual is repeated and the solution that gives minimum residual is chosen for the retrieval of 550 nm and η . AOD at other wavelengths can be determined by using spectral dependence of aerosol optical properties unique to that aerosol model, which is included in the pre-calculated LUT.
Surface reflection, including water leaving, whitecaps and sun glint direct reflection and atmosphere-surface coupling are explicitly calculated in the algorithm. Dependence of ocean surface reflectance on surface wind is incorporated, which can be significant at mid-high latitude where relatively large wind speed dominates.1
1NOAA NESDIS Center for Satellite Applications and Research GOES-R Advanced Baseline Imager (ABI) Algorithm Theoretical Basis Document for Suspended Matter / Aerosol Optical Depth and Aerosol Size Parameter v.2.0. 25 September 2010. http://www.goes-r.gov/products/ATBDs/baseline/AAA_AODASP_v2.0_no_color.pdf