GOES-16 Cirrus Channel and Dust

March 23rd, 2017 |

GOES-16 Visible (0.64 µm) images, 2132-2232 UTC on 23 March [click to play animated gif]

GOES-16 Visible (0.64 µm) images, 2132-2232 UTC on 23 March [click to play animated gif]

GOES-16 data posted on this page are preliminary, non-operational data that are undergoing testing.

The visible animation from late afternoon over west Texas, above, shows a characteristic signature of a shroud of dust around El Paso, TX behind a dryline associated with a developing cyclone in the lee of the Rocky Mountains. This pall of dust was visible in many of the 16 channels on the Advanced Baseline Imager (ABI) that sits on GOES-16. The toggle below cycles through the Red visible (0.64 µm), the Blue visible (0.47 µm), the Cirrus channel (1.38 µm), the Snow/ice channel (1.61 µm) and the Upper-Level and Lower-Level water vapor channels (6.19 µm and 7.34 µm, respectively) (Click here for a faster image toggle). In  addition, a 2-panel comparison of GOES-16 Visible and Cirrus band imagery is available here.

GOES-16 Visible (0.64 µm and 0.47 µm), Cirrus (1.38 µm), Snow/Ice (1.61 µm), Upper level Water Vapor (6.19 µm) and Lower Level Water Vapor (7.34 µm) images, 2132 UTC on 23 March [click to enlarge]

GOES-16 Visible (0.64 µm and 0.47 µm), Cirrus (1.38 µm), Snow/Ice (1.61 µm), Upper level Water Vapor (6.19 µm) and Lower Level Water Vapor (7.34 µm) images, 2132 UTC on 23 March [click to enlarge]

Several aspects of the toggle above bear comment. Note that the blue channel (0.47 µm) has in general a ‘hazier’ appearance than the 0.64 µm red channel. Atmospheric scattering is more important at shorter wavelengths, and that is picked up by the satellite. The 1.38 µm ‘Cirrus’ Channel generally does not see the surface because of water vapor absorption at that wavelength. However, the atmosphere behind the dry line is sufficiently parched (total Precipitable Water in the El Paso sounding on 0000 UTC 24 March is less than 6 mm; sounding from this site) that complete attenuation by water vapor is not occurring; dust is highly reflective at 1.38 µm and a signal becomes apparent in the dry air from west Texas southwestward into central Mexico.

Thin dust is very difficult to detect in the 1.61 µm snow/ice channel because solar energy at that wavelength reflected from the surface moves unimpeded through thin dust; thus you can generally see the surface in dusty regions in the 1.61 µm channel. On this date the 1.61 µm channel nimbly discriminated between water clouds (over central Mexico) and ice clouds (over much of the rest of the domain, as shown in this toggle between 0.64 µm and 1.61 µm : only the clouds composed of water are reflective (white) in both channels.

The atmosphere was sufficiently dry on this date that the lower-level (7.34 µm) water vapor channel detected surface features (horizontal convective rolls) associated with the blowing dust. (click here for the 6.19 µm image; surface features are not so apparent). Weighting functions computed at those wavelengths show a significant contribution from the surface at 7.4 µm (the red line), and also at 7.0 µm, (the green line), so the mid-level water vapor imagery from GOES-16 likely also shows surface influences); the 6.5 µm weighting function (the blue line) does not extend to the surface (These GOES-13 Sounder Weighting Functions that are similar to those from the GOES-16 ABI are from this site) so it’s unlikely that the 6.19 µm imagery shows surface features.

The GOES-R Website has fact sheets on the 0.47 µm, 0.64 µm, 1.38 µm, 1.61 µm, 6.19 µm and 7.34 µm channels.

Added: The RAMSDIS GOES-16 Loop of the Day from 23 March showed the Dust RGB product.

GOES-16 Visible and Cirrus Channels

March 21st, 2017 |

GOES-16 Visible (0.64 µm) images, 1202-1732 UTC on 21 March [click to play animated gif]

GOES-16 Visible (0.64 µm) images, 1202-1732 UTC on 21 March [click to play animated gif]

GOES-16 data posted on this page are preliminary, non-operational data that are undergoing testing.

GOES-16 Visible imagery captured the erosion of near-surface clouds over Ohio on 21 March 2017. A benefit of the routine 5-minute imagery is that it allows better estimates of exactly when the low clouds will clear out. There is ample suggestion in the animation above of the presence of cirrus clouds. The GOES-16 ABI has a channel at 1.38 µm that is specifically designed to detect cirrus clouds because that is a region in the electromagnetic spectrum where strong water vapor absorption occurs. The animation of ‘cirrus channel’ imagery, below, confirms the presence of widespread cirrus clouds.

GOES-16 Cirrus Channel (1.38 µm) images, 1202-1732 UTC on 21 March [click to play animated gif]

GOES-16 Cirrus Channel (1.38 µm) images, 1202-1732 UTC on 21 March [click to play animated gif]

The MODIS instrument also has a similar near-infrared Cirrus spectral band — and a comparison of Terra MODIS Visible (0.65 µm) and Cirrus (1.375 µm) images at 1601 UTC is shown below.

Terra MODIS Visible (0.65 µm) and Cirrus (1.375 µm) images [click to enlarge]

Terra MODIS Visible (0.65 µm) and Cirrus (1.375 µm) images [click to enlarge]

The GOES-16 ABI Cirrus Channel

March 1st, 2017 |

GOES-16 Band 4 (1.37 µm) Imagery from 1236-1406 UTC on 1 March 2017 [click to animate]

Note: GOES-16 data shown on this page are preliminary, non-operational data and are undergoing on-orbit testing.

The Advanced Baseline Imager (ABI) on the GOES-R Series of satellites (including GOES-16) includes a band that detects radiation at 1.37 µm (Fact Sheet Link). This 2-km resolution band is unique to GOES-16 among geostationary satellites. The animation above shows a subset of Full-Disk imagery at 15-minute intervals (GOES-16 produces a full disk every 15 minutes, in contrast to GOES-13/GOES-15’s 3-hour Full Disk cadence). The Cirrus channel highlights only the highest clouds associated with the wave cyclone over the central part of the United States. Clouds are not initially obvious early in the animation over the northern Plains: this band detects reflected solar radiation and therefore gives little information at night.

The Band 4 Cirrus Channel to the Band 2 visible (0.64 µm) toggle, below, enables an observer to distinguish between low/middle cloud levels and high clouds quite easily. Water vapor in the atmosphere above the low clouds in Illinois and Missouri (and elsewhere) is absorbing any reflected radiation at 1.37 µm there. If precipitation is being produced by a seeder/feeder mechanism, the presence of high clouds as detected in the Cirrus channel could help refine analyses of falling precipitation.

GOES-16 Band 2 (0.64 µm) and Band 4 (1.37 µm) Imagery from 1447 UTC on 1 March 2017 [click to enlarge]

A similar band (with 1-km resolution) is present on Terra and Aqua as part of MODIS and there are numerous CIMSS Satellite Blog Posts that incorporate snapshots from this MODIS cirrus-detection channel:  Detecting thin cirrus and contrails over Arkansas and Tennessee; Thin Cirrus over the Midwest; Cirrus associated with Haloes; The Cirrus Canopy of Hurricane Matthew; Transverse Banding, for example.

Although this band on ABI is called the Cirrus Channel, it has other uses.  It can be used to detect any highly reflective aerosol, such as volcanic ash or blowing dust, as long as the features are not obscured by water vapor.  It can also view the surface if the atmosphere is sufficiently dry:  Research suggests that a total precipitable water of about 12 mm is sufficient to attenuate the radiation.

Orographic cirrus over Colorado and Wyoming

December 18th, 2013 |
GOES-13 10.7 µm IR channel images (click to play animation)

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

AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data (above; click image to play animation) showed the development of a large shield of orographic cirrus clouds immediately downwind (to the east of) the high terrain of the Rocky Mountains in eastern Colorado and southeastern Wyoming on 18 December 2013. The development and persistence of such cloud features is important to monitor, due to their potential impact on daytime temperatures: if incoming solar radiation is significantly reduced by a canopy of dense cirrus, surface temperatures may not be as warm as forecast. Early in the cirrus shield development during the overnight hours, GOES-13 IR brightness temperatures were as cold as -66º C (darker red color enhancement) along the western edge of the cirrus shield, suggestive of a high and potentially dense ice cloud feature — but during the early morning hours the cirrus was seen to begin to rapidly dissipate after about 15 UTC.

While not directly related to the orographic cirrus cloud shield per se, the strong westerly winds interacting with the complex terrain of the Rocky Mountains produced some areas of turbulence across Colorado. At 14:05 UTC a pilot reported Moderate turbulence throughout a very deep layer (5,500 feet to 20,000 feet), and at 15:22 UTC a pilot reported occasional Severe turbulence between the altitudes of 13,000 and 16,000 feet.

Night-time comparisons of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel images at 08:12 UTC or 1:12 AM local time (above) and 09:51 UTC or 2:51 AM local time (below) revealed that the cirrus along the western (upwind) edge was quite cold on the IR images (with IR brightness temperatures as cold as -71º C at 08:12 UTC, and -76º C at 09:51 UTC), and also appeared much more optically thick along the western portion on the DNB images. However, on the DNB images the cirrus shield — although still exhibiting fairly cold IR temperatures — appeared to be much more optically thin along the eastern portion. Due to ample illumination by a nearly-full moon, the DNB provided vivid “visible images at night” to compliment the IR images; in the upper right corner of the DNB images, snow on the ground could also be seen across northern Nebraska into far southern South Dakota.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

A 1-km resolution POES AVHRR Cloud Top Height product at 11:39 UTC or 4:39 AM local time (below) indicated that the tops of the cirrus shield were at 12 km (darker green color enhancement).

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

The 12 UTC Denver rawinsonde data (below) showed that the tropopause was around 13 km, with an air temperature of -70º C. Winds at that altitude were from the west at 100 knots (with strong westerly winds at all altitudes throughout the tropopause).

Denver, Colorado rawinsonde data

Denver, Colorado rawinsonde data

Additional details and imagery of this orographic cirrus event can be found on the CIRA RAMMB GOES-R Proving Ground Blog.