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MODIS Volcanic Ash

Product Information:

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The Cooperative Institute for Research in the Atmosphere (CIRA) in Fort Collins, Colorado and the Naval Research Laboratory (NRL) in Monterey, California are developing and distributing the Volcanic Ash Enhancement (Blue Light Absorption) product.

These Volcanic Enhancement products are sent to the National Weather Service (NWS) Regional Headquarters from which they are distributed to Weather Forecast Offices (WFOs) for display on their local AWIPS systems. Imagery updates are available approximately two times per day from the MODIS sensors on board Terra (~10:30 AM local time) and Aqua (~1:30 PM local time), depending on location. More frequent updates of this daytime-only product are available at higher latitudes.

The size of the Volcanic Ash Enhancement product images is determined by the span and resolution of the AWIPS domain itself. Since current AWIPS system displays accommodate 1-byte per pixel, a good rule of thumb is that the size of the imagery (in bytes) corresponds roughly to the total number of pixels in a given AWIPS domain. For example, an AWIPS domain having dimensions of 1000 x 1000 pixels will require approximately 1 Megabyte (~106 bytes).

Product Description:

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The purpose of the Volcanic Ash Enhancement imagery product is to provide a visually intuitive depiction that is useful to experts and non-experts alike. Instead of toggling between visible imagery, infrared imagery, and various spectral differences, the current product attempts to put several of these elements together and depict the consensus of where the tests have high confidence in the presence of certain kinds of ash in the scene. While there may be other properties to volcanic ash plumes (such as high SO2 content, detectable via other spectral bands of GOES-R) the current enhancement keys in on ash plumes with a high silicate-based composition. These plumes pose a particular hazard to turbine jet engines as they can both damage and accrue material on the blades, compromising the aerodynamics of the engine system and leading to flame-outs. While the enhancement does not provide a quantitative approximation of how much ash is present, its height in the atmosphere, or its specific size properties (parameters of high importance for model predictions of how the ash plume will vary/transport), it does provide a rapid-assessment flag for the most hazardous (to aviation) variety of plume and a quick validation against other detection techniques.

The Volcanic Ash Enhancement product demonstrates the kind of imagery that will be possible in the GOES-R era. GOES-R will feature the Advanced Baseline Imager (ABI) sensor whish will be able to produce versions of the imagery shown here at much higher time resolution.

Since we do not have all the required channels on present-day GOES satellites to do the Volcanic Ash Enhancement described here, we can only demonstrate now it via sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments, which fly on polar-orbiting NASA Terra and Aqua satellites. The polar orbits limit the temporal frequency of coverage to about two passes (mid-morning and mid-afternoon) per day, recalling that this is a daytime-only product. The following provides a brief summary of the logic that goes into enhancing volcanic ash plumes from GOES-R “ABI-like” measurements.

In the most general sense, enhancing volcanic ash plumes in satellite imagery requires the isolation of the ash signal (in terms of measurable properties) in order to separate it in color-space from other components of the scene. The science literature presents many ways to do this. As the chemical properties of ash are very diverse, it is often the case that one approach will work better than another for a given volcano (and sometimes ash properties of a given volcano will vary over time, depending on the nature of the eruption). The current approach combines several of the more reliable discriminators which take advantage of the spectral bands that will be available to GOES-R. The principal considerations for the enhancement are:

  1. Color: similar to our discussion of mineral dust enhancements, to some extent we can distinguish volcanic ash from meteorological clouds with our own eyes through color differences (some ash clouds have earth tones, owing to their strong absorption of blue light). Since most meteorological clouds appear gray/white, the color information provides a useful discriminator. This is why true color imagery can also be useful for volcanic ash detection. A satellite sensor that has the ability to distinguish color (e.g., via multiple narrow-band channels in the visible part of the spectrum) can combine this information in an analogous way to color vision for volcanic ash detection. This approach works well over dark surface backgrounds, like water. In cases where the background color leads to ambiguity in discerning the ash cloud/plume, we need to appeal to other discriminators, and these are described below.
  2. Temperature: When hot ash is lofted into the atmosphere, its temperature rapidly adjusts to whatever the air temperature of the environment is. The ash layer will have its own radiative influence on the environmental temperature (via absorption of upwelling thermal radiation from below, absorption/reflection of incoming solar radiation from above, and the heat of the volcanic gasses themselves) but these effects are of secondary importance here, so we will ignore them. Since temperatures in the lower atmosphere generally decrease with height (especially during the daytime hours), the ash layer cools as it rises, and soon produces a “thermal contrast” against the warmer surface. A satellite sensor that is sensitive to heat (infrared radiation) can therefore assist us in detecting the lofted ash based on the temperature contrast it produces against the warmer background surface. If we combine this temperature contrast information with the color distinction information described above, now we have a more robust way of distinguishing between meteorological clouds (which are gray/white and cool) and ash plumes (which are earth-tones and cool) than either test can provide on its own.
  3. Spectral Differences in Transparency: Similar again to mineral dust detection, the chemical and physical properties of some volcanic ash clouds (particularly those with silicate-based particles like microscopic obsidian glass) give rise to different optical properties (scattering and absorption behavior) depending on what part of the infrared spectrum we view them in. In the same way that these properties result in preferred blue-light absorption in the visible part of the spectrum, differences occur in the middle and thermal infrared parts of the spectrum which gives rise to volcanic ash plumes appearing “thinner” (more transparent) or “thicker” (more opaque). As a result, measurements of a mid-level volcanic ash plume in a part of the infrared spectrum that corresponds to the more absorbing behavior may appear cooler than measurements of that same ash plume in a part of the infrared spectrum where it appears more transparent. In the case of more absorbing behavior, we’re seeing more of the cool emissions of the ash layer’s environmental temperature, while in the case of more transparent behavior we’re seeing warmer contributions from the underlying surface that are able to transmit through the ash. Computing a difference between two such measurements, chosen carefully, provides an effective way to identify ash. The two channels used here are the 11 and 12 micrometer thermal infrared bands, which provide an ash detection signal that is opposite in sign to most meteorological clouds.

In many ways the current algorithm is similar to the Blowing Dust (Blue Light Absorption) product. An important distinction here is in the handling of high-altitude signals. In the Blowing Dust product, it is assumed that upper-tropospheric signals are the result of semi-transparent cirrus clouds which are in fact false-alarms. The filtering is accomplished by examining the 1.38 µm shortwave water-vapor band reflectance. In the current volcanic ash product, we must allow for the possibility that explosive eruptions may loft ash into the upper troposphere or even the lower stratosphere. As such, the 1.38 µm logic is modified to allow for high reflectance to first be tested against coloration prior to filtering it out. If the reflectance exceeds a set of thresholds then it is retained as an enhanced pixel, whereas the Blowing Dust product would suppress this pixel.

While any one of the above discriminators alone may not be sufficient to fully distinguish volcanic ash, we can often provide a reasonable isolation from other components of the scene through combining the various tests. When all tests are satisfied simultaneously (i.e., an intersection) we can say with greater confidence that the pixel being examined contains ash. It is important to point out that in the current algorithm these tests are not strictly “yes/no” but are cast as confidence factors that range between values of 0 (low confidence) to 1 (high confidence). The extent to which the tests are satisfied (strength of the ash signal) is communicated in the enhanced imagery in terms of brightness of the enhanced ash features…with brighter regions being higher confidence.

Product Examples and Interpretation

Fig. 2. Another example of the Eyjafjallajökull volcanic eruption captured by Terra-MODIS during an earlier phase of the same eruption series shown in Fig. 1 (this image collected on 13 April 2010 at 1135 UTC), with true color shown on the left and the Volcanic Ash Enhancement shown on the right. The enhanced imagery does a much better job in identifying the plume source and extent as the plume becomes more diffuse to the east. Meteorological clouds appear as blue (cirrus) and gray/white (low clouds).

The Volcanic Ash Enhancement product is designed to simplify the detection of ash present in a potentially complex scene. We do this by gathering together all the ‘ash discriminators’ as outlined in Section 2 of this training module, and present it in a visual form that helps it stand out from other constituents of the scene. The end result is imagery that is ‘false color’ (i.e., it no longer looks ‘real’ in comparison to true color imagery) but provides a less ambiguous depiction of where ash resides.

Fig. 3. A two-overpass (Terra, followed by Aqua ~2 hrs later) sequence captures the initial stages of an explosive eruption of the Okmok volcano (located along the Aleutian chain of the Pacific “Rim of Fire”). True color imagery (left panels) is able to discern portions of the plume as regions of brown coloration. The Volcanic Ash Enhancement provides additional sensitivity to other portions of the plume, particularly evident in the eastern portions of the 2330Z image where optically thinner ash over bright white meteorological clouds is easy to miss.

What to look for: Regions of yellow against dark blue backgrounds. Sometimes ash plumes will display sharp boundaries, especially near the volcanic source or during the early phases of an eruption, but more often present a diffuse appearance as the ash cloud spreads downstream, and may even appear in visible imagery (or to pilots in the cockpit) as benign cirrus clouds.

What to watch out for: Cloud shadows, coastlines (both ocean and lake), and cold terrain (especially mountain ranges and elevated plateaus during the winter months) occasionally appear ‘enhanced’ as false areas of volcanic ash. If used regularly, analysts will quickly become adept at identifying and discarding these residual artifacts of the algorithm.

Other Considerations: With the advent of GOES-R ABI, detection of volcanic ash plumes over areas of coverage will be improved significantly due to much higher temporal refresh (including 15 minute full-disk).

Note: The current version of the Volcanic Ash Enhancement (Blue Light Absorption) product requires information from sunlight reflection, and thus is only valid for daytime observations. A 24-hr/day product that uses strictly infrared bands has been developed by EUMETSAT (developed for MSG-SEVIRI) and will be applied to MODIS observations and added to the GOES-R Proving Ground product suite soon. Since the appearance and capabilities of that product are different and complementary, a separate tutorial will accompany this EUMETSAT product.

Advantages and Limitations

The main advantage of Volcanic Ash Enhancement imagery over conventional single-channel imagery is the ability to make a rapid assessment of ash in the scene. Its advantage over other ash enhancements is the ability to do so while suppressing (in color/brightness) the non-ash components of the scene. Another advantage of imagery-based enhancements (vs. pure quantitative products such as an ‘ash mask’) is that imagery retains the meteorological context of the scene. Ash plumes track with the environmental flow patterns, and this pattern can often be inferred by noting the plume’s observed orientation with respect to the meteorological cloud field.

Ambiguity between volcanic ash and mineral dust will occur. Typically we do not observe the two features in the same scenes, but it is possible (e.g., Gobi dust storm crossing the Pacific Rim). The need for sunlight reflectance limits the product to daytime-only application, and it is further limited in the GOES-R Satellite Proving Ground by the availability of MODIS which offer only a few passes per day. A transient limitation is the depiction of the product in AWIPS as an 8-bit color (256 colors) image. Since the Volcanic Ash product is in fact a 24-bit red/green/blue composite, there will be some degradation in quality when attempting to represent a broad color palette (256^3 = almost 17 million) in a reduced number of color tones. This challenge will go away in AWIPS II, which is advertised to have the capability to display 24-bit imagery.