Extratropical Cyclogenesis over the western Pacific

March 30th, 2015
Himawari-8 AHI 0.64 µm visible channel images (click to play animation)

Himawari-8 AHI 0.64 µm visible channel images (click to play animation)

The AHI Instrument on Himawari-8 has 16 different channels sensing the atmosphere. The instrument is still in Post-Launch Testing, a period when instrument performance is monitored and adjusted. Extratropical cyclogenesis that occurred east of Japan on 30 March was captured by the different channels.

The 0.64 µm visible imagery, above, is the highest-resolution channel on AHI, with nominal 0.5-km resolution at the subsatellite point. The imagery above — at 1.5 km resolution and every 10 minutes — shows the development of an extratropical cyclone east of the main island of Japan (visible at the left edge of the imagery). Thin cirrus is spreading north of the storm and convection is developing both in the cool air north of the surface circulation center and along the cold front that is just to the west of the cirrus shield associated with the warm conveyor belt. Northerly surface winds north of the system and southern surface winds south of the system speak to the strengthening of the frontal boundary along which the storm is developing.

Himawari-8 AHI 0.85 µm infrared channel images (click to play animation)

Himawari-8 AHI 0.85 µm infrared channel images (click to play animation)

The 0.85 µm imagery, above, is in the near-infrared part of the electromagnetic spectrum, at wavelengths just a bit longer than red visible light (which is at 0.7 µm). It does an excellent job highlighting the land/water contrast (because bodies of water strongly absorb 0.85 µm solar radiation and land and clouds reflects it). This channel also is sensitive to vegetation. The larger-scale view shows jetstream cirrus south and southwest of the developing storm and an occluded system decaying to the east of Kamchatka.

The 0.46 µm imagery, below, is in the visible part of the electromagnetic spectrum, and is quite sensitive to aerosols (Click here for a fact sheet on ABI’s 0.46 µm “Blue Band”; fact sheets for all ABI Bands will be here in the future). The smog and pollution that surrounds Tokyo is more apparent in this imagery. Smog is also indicated near Osaka and Nagoya. A toggle between 0.64 µm, 0.46 µm and 0.85 µm imagery, here, from 30 March 2015 at 0000 UTC allows a comparison of the imagery.

Himawari-8 AHI 0.46 µm visible channel images (click to play animation)

Himawari-8 AHI 0.46 µm visible channel images (click to play animation)

The 1.60 µm imagery on AHI is useful because it can distinguish between clouds with water droplets (that scatter and reflect solar 1.60 µm radiation very effectively) and clouds with ice crystals (that absorb 1.60 µm radiation). In a standard enhancement, clouds with ice crystals appear grey, clouds with water droplets appear white. In the animation below, the glaciated cirrus canopy of the warm conveyor belt is readily apparent. Note also how the convection developing along the warm front has glaciated by the end of the animation.

Himawari-8 AHI 1.60 µm infrared channel images (click to play animation)

Himawari-8 AHI 1.60 µm infrared channel images (click to play animation)

The 3.9 µm on Himawari-8 provide detailed information about the sea surface temperature if clouds are not present, as was the case over the Kuroshio Current just east of Japan on 30 October. The animation below shows little change over 2 hours, as expected, except along the north wall of the current. Brightness Temperatures drop 10 C across the temperature gradient at the north end of the current.

Himawari-8 AHI 3.90 µm infrared channel images (click to play animation)

Himawari-8 AHI 3.90 µm infrared channel images (click to play animation)

Himawari-8 Water Vapor Imagery

March 25th, 2015
Himawari-8 6.2 µm infrared water vapor channel images (click to play animation)

Himawari-8 6.2 µm infrared water vapor channel images (click to play animation)

Full-resolution animations of Himawari-8 6.2 µm water vapor imagery suggest what a gamechanger Himawari-8 data is, and what a gamechanger GOES-R data will be. Full-disk imagery at every 10 minutes, and at 2-km resolution, means atmospheric motion vectors computed from water vapor imagery (or other channels, such as the 10.35 µm) will cover a larger area and be far more accurate than with present GOES (or MTSAT).

In addition, because there are multiple water vapor channels on Himawari-8 (as will be on GOES-R), water vapor at different levels in the atmosphere can be tracked easily. In the animation below, low-level cloud streets between Japan and the large storm over the northern Pacific are clearly evident, whereas they are shielded from view in the animation above by high-level cirrus — although their presence is evident once they emerge from under the cirrus. Weighting Functions for two similar channels on the GOES Sounder (from here) show that the shorter-wavelength 6.2µm channel will have a larger response to upper level moisture; the longer-wavelength 7.3 µm channel will have a larger response to lower level moisture. Interactions with land features that are evident in the 7.3 µm imagery below do not show up in the 6.2 µm imagery above because most of the signal at 6.2µm is emitted from the upper troposphere. Combining the three water vapor channels (there is also a 6.9 µm channel, not shown here) gives an excellent three-dimensional representation of water vapor in the atmosphere at high temporal resolution.

Himawari-8 7.3 µm infrared water vapor channel images (click to play animation)

Himawari-8 7.3 µm infrared water vapor channel images (click to play animation)

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