The MODVOLC algorithm, its development, and implementation, is described in detail in the following three papers: Wright et al., (2002), Wright et al., (2004), and Wright, (2016).

Using the web interface one can access all of the “hot-spots” that the algorithm has detected since February 2000. On that page, the user can navigate using the left hand panel with the mouse, zooming in/out as they choose. This automatically adjusts the spatial extent of the data retrieved. On the left is shown a summary of the hot-spot data for the geographic area in the window (and the parameters selected from the options given below). The colored circles (and numbers that overlay them) indicate the total number of hot-spots in a “cluster” (to give a relative impression of the scale of the thermal anomaly in that geographic region). On the right are the hot-spot data themselves. Each polygon represents the geographic area of a MODIS hot-spot pixel. The dimensions of the polygons are scaled to the approximate size of the MODIS instantaneous field of view (“pixel size”) which varies from 1 km at spacecraft nadir to approximately 2 km by 5 k at the edges of the image swath. A single hot-spot polygon is green, but a group of red boxes signifies that many hot-spots lie atop each other at that geographic location. Floating over the volcano icon gives links to the relevant page of the Smithsonian Institution’s Global Volcanism Program, where further information about the volcano can be retrieved. The relevant link to the Jet Propulsion Laboratory’s ASTER Volcano Archive is also given, where free high resolution (15-90 m) ASTER data of the volcano in question can be downloaded.

The data displayed in the right hand window (and any plot generated, or data table created for download) are automatically updated every time an option (i.e. date range, geographic extent, night/day flag) is applied, so what you see in the window is what you get in the plot (and the file). For any plot you create, the data in that plot, and the raw MODVOLC file used to create it, can be downloaded as space delimited ASCII text files.

The “MODVOLC Data” are the MODVOLC entries themselves, and give, for each thermal anomaly detected: Field #1. Time of observation in Unix time; Field #2. Whether the anomaly was detected by the Terra or Aqua MODIS sensor; Fields #3 to #7. Time of observation in universal time; Fields #8 and #9. Latitude and longitude of the center of the hot-spot pixel (decimal degrees, registered to WGS-84); Fields #10 to #14. At-satellite spectral radiance in the five noted MODIS wave channels in units of W/m2/sr/micron; Fields #15 to #18. The sun-sensor viewing geometry at the time of detection (in degrees); Fields #19 and #20. The line and sample location in the original MODIS L1B granule; Field #21. The Normalized Thermal Index computed for the anomaly; Field #22. The sun-glint vector (in degrees, see Giglio et al., 2003); Field #23. An estimate of the excess radiant flux (i.e. the volcanogenic flux, above the background flux) in W for that hot-spot pixel; Field #24. The decadal-averaged temperature of Earth’s surface at that location computed from the monthly MODIS Land Surface Temperature Product (in K). In these files saturation of a particular wave-channel is denoted by a flag value of -10.000. A nighttime observation can be identified when the solar zenith is greater than 90 degrees (i.e. the Sun is below the horizon).

Plots of the data shown in the upper right hand window can be generated. These are 1. The number of hot-spot pixels for the geographic area displayed at each individual UNIX time, 2. An estimate of the radiant power from those hot-spots (in W) (see Wright et al., 2015) and 3. The summed 3.959 micron spectral radiance from those hot-spots. For computing the radiant power, band 21 data are used when band 22 is saturated. In the rare occurrences when band 21 is also saturated the maximum measurable spectral radiance for band 21 (60 and 85 W/m2/sr/micron for Terra MODIS and Aqua MODIS, respectively) is used instead.

Questions? Email Eric Pilger, Ethan Kastner, or Robert Wright

© University of Hawaii at Manoa, 2016