Great Lakes surface geographical outlines evident on water vapor imagery

February 23rd, 2015
GOES-13 6.5 µm water vapor channel images (click to play animation)

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)

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

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)

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)

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

Himawari-8 Water Vapor Imagery, and AHI Webapps

January 25th, 2015
Himawari-8 Water Vapor Imagery at 0230 UTC on 25 January 2015 (click to enlarge)

Himawari-8 Water Vapor Imagery at 0230 UTC on 25 January 2015 (click to enlarge)

Himawari-8, launched by the Japanese Meteorological Agency in October 2014, is in its check-out phase with the satellite located near 0º North 140º East. The animation above shows the three water vapor bands (Bands 8, 9 and 10 centered at 6.2 µm, 6.9 µm and 7.3 µm, respectively) from the AHI on Himawari-8.

The strength of three water vapor channels is that they provide information about moisture at three different levels in the atmosphere. Water vapor channel weighting functions (computed from this website) for ABI on GOES-R (an instrument that is very similar to the AHI on Himawari-8) show a peak response near 350-400 mb for the 6.2 µm channel but a peak response near 600-700 mb for the 7.3 µm channel (the 6.9 µm channel is in between). The longer-wavelength water vapor channel can provide information about features located farther down into the atmosphere. In the imagery above, the 7.3 µm imagery shows open cellular convection in the cold advection south of the occluded low pressure system over the northern Pacific, east of Japan. In contrast, the 6.2 µm imagery shows only the higher clouds and moisture.

The effect is far more pronounced at full resolution, below. The 6.2 µm data shows only high clouds and moisture; those high-altitude features are not well represented at 7.3 µm. In contrast, low clouds that cannot be seen in the 6.2 µm data are very apparent in the 7.3 µm imagery.

Full Resolution Himawari-8 Water Vapor Imagery at 0230 UTC on 25 January 2015 (click to enlarge)

Full Resolution Himawari-8 Water Vapor Imagery at 0230 UTC on 25 January 2015 (click to enlarge)

Similarly, over south central Australia, there is a strong cold signal in the 6.2 µm imagery east of Adelaide. The 6.9 µm and 7.3 µm imagery does not show such a strong signal, suggesting that only high clouds are present.

Multiple water vapor channels are present now on the GOES Sounder (see here), and those data are used in the CIMSS NearCasting product. GOES Sounder data has a limited domain, however, and relatively coarse resolution. Himawari-8 (and GOES-R) offers a great increase in spatial and temporal resolution over the three GOES Sounder water vapor channels.

These AHI Images are from data posted at JMA‘s AHI webpage: Link. A comparison of Himawari-8 and MTSAT-2 visible and IR images is available here.

Himawari-8 AHI Satellite Band Webapp page

Himawari-8 AHI Satellite Band Webapp page

A collection of “webapps” (above) was created which allows one to explore the different spectral bands of the Himawari-8 AHI from the 25 January 2015 First Images. An example from the Full Disk webapp is shown below.

http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2015/01/ahi_webapp_full_disk.png

Example from the AHI Full Disk image webapp

First images from Himawari-8

December 18th, 2014
Himawari-8 0.64 µm visible channel image (click to enlage)

Himawari-8 0.64 µm visible channel image (click to enlarge)

The Japan Meteorological Agency has released the first images from the AHI instrument on the Himawari-8 satellite, which was launched on 7 October this year.

This link shows full disk imagery from all 16 spectral bands. The AHI on Himawari-8 is very similar to the ABI that will fly on GOES-R.

A comparison of images using each of the 16 spectral bands is shown below, centered over the Sea of Japan. Cloud streets are seen over much of the open waters, due to the southeastward and eastward transport of very cold air from Siberia (surface analysis). Lee waves (or “mountain waves”) are evident on the water vapor bands (8, 9  and 10) downwind or southeast of the higher terrain areas on the main Japanese island of Honshu.

Comparison of the 16 AHI spectral bands, centered on the Sea of Japan (click to enlarge)

Comparison of the 16 AHI spectral bands, centered on the Sea of Japan (click to enlarge)

 Band 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
µm  0.47   0.52   0.64   0.86   1.6   2.3   3.9   6.2   6.9   7.3   8.6   9.6   10.4   11.2   12.4   13.3 

Similar comparisons of Himawari-8 images covering Hawaii, western Australia, and the far Southern Hemisphere are available on the First Light AHI Satellite Band Webapp.

As seen on the MTSAT-2 vs Himawari-8 comparison below, even at large satellite viewing angles over the far southern portion of the Southern Hemisphere (for example, between Australia/Tasmania and Antarctica) AHI imagery such as that from water vapor channels exhibits higher quality (due to factors such as improved spatial resolution, signal-to-noise ratio, data bit depth, etc).

MTSAT-2 vs Himawari-8 water vapor channel images

MTSAT-2 vs Himawari-8 water vapor channel images

Airborne glacial silt from the Copper River Valley in Alaska

October 28th, 2014
GOES-15 0.63 µm visible channel images (click to play animation)

GOES-15 0.63 µm visible channel images (click to play animation)

McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) showed the hazy signature of airborne glacial silt drifting southward out of the Copper River valley and over the adjacent waters of the Gulf of Alaska on 28 October 2014. The strong winds lofting the silt were very localized to the Copper River valley itself, with cold dense arctic air from further inland (air temperatures were 8 to 10º F at Gulkana, PAGV) accelerating through narrow mountain passes — note how winds at nearby Cordova (PACV) were generally calm during much of the period. As the western edge of the airborne silt reached Middleton Island (PAMD), the surface visibility dropped as low as 5 miles.

AWIPS II images of Suomi NPP VIIRS data provided a better view of the aerial coverage of the glacial silt: a comparison of VIIRS 0.64 µm visible channel and 1.61 µm near-IR “snow/ice channel” images (below) showed that the 1.61 µm image offered better contrast to help locate the edges of the feature. This 1.61 µm channel imagery will be available from the Advanced Baseline Imager (ABI) on GOES-R.

Suomi NPP VIIRS 0.64 µm visible channel and 1.61 µm near-IR

Suomi NPP VIIRS 0.64 µm visible channel and 1.61 µm near-IR “snow/ice channel” images

Two consecutive VIIRS 1.61 µm images (below) revealed the changes in aerosol coverage between 21:43 UTC and 23:22 UTC.

Suomi NPP VIIRS 1.61 µm near-IR

Suomi NPP VIIRS 1.61 µm near-IR “snow/ice channel” images

The more dense portion of the airborne glacial silt particle feature exhibited a slightly warmer (darker gray) appearance on VIIRS 3.74 µm shortwave IR images, due to efficient reflection of incoming solar radiation.

Suomi NPP VIIRS 3.74 µm shortwave IR images

Suomi NPP VIIRS 3.74 µm shortwave IR images

A VIIRS true-color Red/Green/Blue (RGB) image from the SSEC RealEarth site (below) offered a good view of the coverage of the glacial silt at 21:45 UTC.

Suomi NPP VIIRS true-color image

Suomi NPP VIIRS true-color image