Suomi NPP VIIRS 0.7 µm Day/Night Band, 1.6 µm near-IR, 3.9 µm shortwave IR, and 11.45 µm longwave IR images

There was an explosive eruption of the Villarrica volcano in central Chile on the morning of 03 March 2015; the Buenos Aires VAAC issued their first volcanic ash advisory based upon initial detection on 06:38 UTC GOES-13 imagery, although media report and blog sources indicated that the eruption started closer to 06:00 UTC (3 am local time). A comparison of 06:07 UTC Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB), 1.6 µm near-IR, 3.9 µm shortwave IR, and 11.45 µm longwave IR images (above; courtesy of William Straka, SSEC) revealed a bright glow on the DNB and near IR images, with a pronounced “hot spot” evident on the shortwave IR (yellow to orange pixels; the hottest shortwave IR brightness temperature was over 600 K!) and even the longwave IR (darker black pixels) images. The DNB image was particularly striking, with nearby clouds and surface features being illuminated by the eruption.

MODIS and GOES-13 multispectral false-color Red/Green/Blue (RGB) images from the NOAA/CIMSS Volcanic Cloud Monitoring site (below; click image to play animation) showed that there was detection of a thermal anomaly or “hot spot” (indicated by a red box) as early as 04:20 UTC (MODIS) and 05:45 UTC (GOES-13); the volcanic cloud filament — which was estimated to be at an altitude of 30,000 feet — could be seen drifting to the southeast following the eruption.

MODIS and GOES-13 false-color RGB images (click to play animation)

On GOES-13 10.7 µm IR channel images (below; click image to play animation), the volcanic cloud initially exhibited an IR brightness temperature as cold as -42ºC  (green color enhancement), but the cloud filament quickly became very diffuse and difficult to identify on the IR images by 09:38 UTC.

GOES-13 10.7 µm IR images (click to play animation)

The 12 UTC rawinsonde profiles from Puerto Montt, Chile (station identifier SCTE) on 02 March and 03 March are shown below. On the 02 March profile, the -42º C temperature was at an altitude around 9400 meters or 30,800 feet; on the 03 March profile, -42º C was around 9100 meters or 29,900 feet.

Puerto Montt, Chile 12 UTC rawinsonde profiles on 02 March and 03 March

On GOES-13 3.9 µm shortwave IR images (below; click image to play animation) a “hot spot” (black to yellow to red color enhancement) was seen for several hours after the initial eruption. The highest shortwave IR brightness temperature observed by GOES-13 was 340.8 K — much lower than than the >600 K observed with the higher spatial resolution Suomi NPP VIIRS instrument.

GOES-13 3.9 µm shortwave IR channel images (click to play animation)

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GOES-13 0.63 µm visible channel images (click to play animation)

GOES-13 (GOES-East) 0.63 µm visible channel images (above; click image to play animation) showed the development and motion of a long single-band lake effect cloud feature over Lake Michigan on 26 February 2015. Snowfall from this band helped to boost total event accumulations (including other lake effect snow bands on the previous day) as high as 8 inches in the Chicago area, bringing this to the 3rd snowiest February on record there.

A comparison of the 18:39 UTC Suomi NPP VIIRS 0.64 µm visible channel image with the corresponding false-color Red/Green/Blue (RGB) is shown below. On the RGB image, snow, ice, and ice crystal clouds appear as varying shades of pink to red — and it can be seen that portions of the lake effect cloud band looked to be glaciated. Supercooled water droplet clouds appear as varying shades of white on this type of snow/ice-vs-cloud discrimination RGB image.

Suomi NPP VIIRS 0.64 µm visible channel and False-color RGB image (click to enlarge)

The 18:39 UTC Suomi NPP VIIRS 11.45 µm IR channel image (below) showed that cloud-top IR brightness temperatures were in the -20 to -30º C range (cyan to dark blue color enhancement) along the entire length of the lake effect cloud band, which also suggested that glaciation was likely.

Suomi NPP VIIRS 11.45 µm IR channel image (click to enlarge)

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GOES-13 6.5 µm water vapor channel images (click to play animation)

A cold and dry arctic air mass (morning minimum temperatures) was in place over the Great Lakes region on 23 February 2015. This arctic air mass was sufficiently cold and dry throughout the atmospheric column to allow the outlines of portions of the surface geography of the Great Lakes to be seen on GOES-13 (GOES-East) 6.5 µm water vapor channel images (above; click image to play animation).

In addition to the commonly-used 4-km resolution 6.5 µm water vapor channel on the GOES Imager instrument, there are also three 10-km resolution water vapor channels on the GOES Sounder instrument (centered at 6.5 µm, 7.0 µm, and 7.4 µm). A 4-panel comparison of these water vapor channel images (below; click image to play animation) provides the visual indication that each water vapor channel is sensing radiation from different layers at different altitudes — for example, the surface geographical outlines of the Great Lakes are best seen with the Sounder 7.4 µm (bottom left panels) and the Imager 6.5 µm (bottom right panels) water vapor channels.

GOES-13 Sounder 6.5 µm, 7.0 µm, 7.4 µm, and Imager 6.5 µm water vapor channel images (click to play animation)

An inspection of GOES Sounder and Imager water vapor channel weighting function plots (below) helps to diagnose the altitude and depth of the layers being sensed by each of the individual water vapor channels at a variety of locations. For example, the air mass over Green Bay, Wisconsin was cold and very dry (with a Total Precipitable Water value of 0.87 mm or 0.03 inch), which shifted the altitude of the various water vapor channel weighting functions to very low altitudes; this allowed surface radiation from the contrasting land/water boundaries to “bleed up” through what little water vapor was present in the atmosphere, and be sensed by the GOES-13 water vapor detectors. In contrast, the air mass farther to the south over Lincoln, Illinois was a bit more more moist, especially in the middle/upper troposphere (with a Total Precipitable Water value of 4.20 mm or 0.17 inch) — this shifted the altitude of the water vapor channel weighting functions to much higher altitudes (to heights that were closer to those calculated using a temperature/moisture profile based on the US Standard Atmosphere).

GOES-13 Sounder and Imager water vapor channel weighting function plots for Green Bay WI, Lincoln IL, and the US Standard Atmosphere

In addition to the temperature and/or moisture profile of the atmospheric column, the other factor which controls the altitude and depth of the layer(s) being detected by a specific water vapor channel is the satellite viewing angle (or “zenith angle”); a larger satellite viewing angle will shift the altitude of the weighting function to higher levels in the atmosphere. Recall that the water vapor channel is essentially an Infrared (IR) channel — it generally senses the mean temperature of a layer of moisture or clouds located within the middle to upper troposphere. In this case, the sharp thermal contrast between the cold land surfaces surrounding the warmer Great Lakes was able to be seen, due to the lack of sufficient water vapor at higher levels of the atmosphere to attenuate or block the surface thermal signature.

The new generation of geostationary satellite Imager instruments (for example, the AHI on Himawari-8 and the ABI on GOES-R) feature 3 water vapor channels which are similar to those on the current GOES Sounder, but at much higher spatial and temporal resolutions.

On a separate — but equally interesting — topic: successive intrusions of arctic air over the region allowed a rapid growth of ice in the waters of Lake Michigan. A 15-meter resolution Landsat-8 0.59 µm panochromatic visible image viewed using the SSEC RealEarth web map server (below) showed a very detailed picture of ice floes along the western portion of the lake, as well as a patch of land-fast ice in the far southern end of the lake.

Landsat-8 0.59 µm panochromatic visible image (click to enlarge)

The motion of the band of ice floes along the western  edge of Lake Michigan was evident in 1-km resolution GOES-13 0.63 µm visible channel images (below; click image to play animation) — along the east coast of Wisconsin, southwesterly winds gusting to around 20 knots were acting to move the ice floes away from the western shoreline of Lake Michigan.

GOES-13 0.63 µm visible channel images (click to play animation)

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GOES-13 0.63 µm visible channel images (click to play animation)

GOES-13 (GOES-East) 0.63 µm visible channel images (above; click to play animation) revealed the presence of a mesocale vortex (“mesovortex”) propagating eastward across the ice-free waters of western Lake Ontario on on 17 February 2015. At the beginning of the animation, also note that there were numerous “hole punch clouds” seen in the stratus cloud deck that covered the western Lake Ontario region during the early morning hours; these holes were likely caused by aircraft inbound/outbound from the Toronto International Airport — particles in jet engine exhaust act as ice nuclei, causing supercooled water droplets to turn into larger, heavier ice particles which then fall out of the cloud to create holes (sometimes described as “fall streaks” due to their appearance).

A closer view using a sequence of MODIS and VIIRS true-color Red/Green/Blue (RGB) images from the SSEC RealEarth web map server site is shown below. There was a significant amount of ice in the northeastern section of Lake Ontario, as well as a ring of offshore ice around other parts of the lake.

MODIS and VIIRS true-color images

A comparison of the 16:31 UTC Terra MODIS 0.65 µm visible channel and the corresponding Sea Surface Temperature product (below) showed that SST values in the ice-free portions of the mesovortex path were generally in the 30 to 34º F  range.

Terra MODIS 0.65 µm visible channel image and Sea Surface Temperature product

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