GOES-R Volcanic Ash
Frequently Asked Questions about the GOES-R Volcanic Ash: Detection & Height Product
1) What is this product?The GOES-R Volcanic Ash product detects areas of volcanic ash in the atmosphere and estimates a height of that ash in kilometers (which is then converted to feet locally in AWIPS) for all pixels that are not obscured by weather clouds. It also calculates the amount of volcanic ash (called mass loading) in tons/kilometer2 of ash lofted into the atmosphere. The VA product's horizontal resolution is 2km. The GOES-R VA product is created both day and night. It is accurate to within 3 km for ash height 2 tons/km2 for mass loading, with a range of 0 - 50 tons/km2. The GOES-R SST product data is quantitative out to 60 degrees local zenith angle, and can be considered qualitative beyond that.1
2) How often do I receive this data?
The cadence of the GOES-R Volcanic Ash product is every 15 minutes, regardless of scan mode. The image will always be a full disk image.
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 then select Full Disk. Then, select the Volcanic Ash product.
Alternately, use the AWIPS Product Browser. Select "Sat", then either "GOES-16" or "GOES-17". From there, choose "Full Disk", then select "VA."
4) How do I interpret the color maps associated with this product?
Color maps have not yet been established for these products. There is a baseline ability for AWIPS to display them, however the colormaps have not been vetted with any forecasters and therefore may not be yet be ideal.
5) What other imagery/products might I use in conjunction with this product?The GOES-R VA product can give a forecaster an idea of the location, height, and direction of volcanic as (either from an erupting volcano or from the ground lofted by the wind). Overlaying the visible satellite imagery (if during the day) and toggling back and forth between the product and the imagery can give an idea of how well the algorithm is capturing the location of the ash. The same can be done using the 10.33 IR channel (although often times the ash may be too light to be seen in this channel). Satellite derived winds can also be overlayed onto this product to get an idea of the speed and direction of the movement of the ash. A basic movement vector can also be calculated using two or three successive images of the product, which can then be compared to winds fields (as wind barbs, streamline, etc.) from various weather models. This can lend insight into which model should be trusted over the next 12 to 18 hours in terms of where the ash might move to.
6) How is this product created?
The GOES-R Volcanic Ash product is only calculated on pixels that are determined to have volcanic ash present. It utilizes GOES-R bands 7.4 um, 8.5 um, 11.2 um, 12.3 um, and 13.3 um channels. The product relies on the infrared observations to avoid discontinuities associated with the transition from day to night. The algorithm performance is sensitive to imagery artifacts or instrument noise. Channel 16 provides the needed sensitivity to cloud height for optically thin mid and high level ash clouds while channels 10, 11 and 14-15 provide the needed sensitivity to cloud microphysics (including composition). .1
The ancillary data used to calculate the volcanic ash algorithm includes1:
- Land cover / Surface type
A global land cover classification collection created by The University of Maryland Department of Geography (Hansen et al. 1998). Imagery from the AVHRR satellites acquired between 1981 and 1994 were used to distinguish fourteen land cover classes (http://glcf.umiacs.umd.edu/data/landcover/). This product is available at 1 km pixel resolution.
- Surface emissivity of ABI channels 14 and 15
A global database of monthly mean infrared land surface emissivity is required for ABI channels 14 and 15. The algorithm utilizes surface emissivity derived using the Moderate Resolution Imaging Spectroradiometer (MODIS). Emissivity is available globally at ten generic wavelengths (3.6, 4.3, 5.0, 5.8, 7.6, 8.3, 9.3, 10.8, 12.1, and 14.3 microns) with 0.05 degree spatial resolution. The ten wavelengths serve as anchor points in the linear interpolation to any wavelength between 3.6 and 14.3 microns. The monthly emissivities have been integrated over the ABI spectral response functions to match the ABI channels.
- Profiles of pressure and temperature
The calculation of cloud emissivity requires profiles of pressure and temperature from a global Numerical Weather Prediction (NWP) model. In addition, knowledge of the location of the surface and tropopause levels is required. While six-hour forecasts were used in the development of the volcanic ash algorithm, and, as such, are recommended, any forecast in the 0 to 24 hour range is acceptable.
- Black cloud radiance profiles for channels 10, 11, 14, 15 and 16
The ABI-VAA requires the radiance emitted upward by a black body surface and transmitted through a non-cloudy atmosphere, with gaseous absorption, to the top of the atmosphere as a function of the atmospheric level of the black surface. The black cloud radiance is computed as a function of NWP grid cells and viewing angle (it is not computed at the pixel resolution).
Top-of-atmosphere clear-sky radiance estimates of channels 10, 11, 14, 15 and 16
The ABI-VAA forward model requires knowledge of the radiance ABI would sense under clear-sky conditions at each pixel.
As described earlier, the ash height and mass loading retrieval requires a priori knowledge of which pixels contain volcanic ash. Thus, prior to calling the ash retrieval algorithm, an ash detection algorithm must be applied to determine which pixels likely contain volcanic ash (based upon ash confidence). Given this requirement, the algorithm processing precedence is as follows: ash detection routine --> ash retrieval routine. Both ash routines require multiple scan lines of ABI data due to the spatial analysis that is applied within each. Complete scan line segments should consist of at least the minimum number of scan lines required by the Gradient Filter. While overlap between adjacent scan line segments is beneficial, scan line overlap was not used in the development and validation of this algorithm.
1Pavalonis, Michael, and Justin Sieglaff. NOAA NESDIS Center for Satellite Applications and Research GOES-R Advanced Baseline Imager (ABI) Algorithm Theoretical Basis Document for Volcanic Ash (Detection and Height) v.2.0. 15 September 2010. http://www.goes-r.gov/products/ATBDs/baseline/Aviation_VolAsh_v2.0_no_color.pdf