Why a Cirrus Channel is useful

June 28th, 2017 |

GOES-16 Visible Image (0.64 µm) at 1557 UTC on 28 June 2017 (Click to enlarge)

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

Consider the visible image above. Can you tell where the eastern and southern edge of the cirrus shield is over the eastern United States? When the Sun is not on the horizon, thin cirrus can be very hard to detect in visible imagery because cirrus clouds are not efficient back-scatterers of solar radiation (all clouds forward-scatter very effectively, but cirrus are optically thin). In a still image, then, with a high sun angle, cirrus can be hard to discern.  There are brightness temperature difference fields that can also be used to infer the presence of cirrus.  For example, the Split Window Difference field (10.3 µm – 12.3 µm) and the Cloud Phase Difference (8.5 µm – 11.2 µm), toggled below, will highlight regions of cloudiness.  But do they also capture very thin cirrus?

Split Window Difference field (10.3 µm – 12.3 µm) and the Cloud Phase Difference (8.5 µm – 11.2 µm) fields at 1557 UTC on 28 June 2017 (Click to enlarge)

 

The Cirrus Channel on GOES-16 (1.38 µm), below, better captures the areal extent of the cirrus.  This is because it is very sensitive to reflective features such as cirrus clouds, and because it is in a region of the electromagnetic spectrum where water vapor absorption occurs — so surface features that might complicate the interpretation are masked (Note, for example, that cumuliform clouds over Northwestern Pennsylvania are not apparent in the Cirrus Band imagery). Click here to toggle between all 4 images.

GOES-16 “Cirrus Channel” (Band 4, 1.38 µm) fields at 1557 UTC on 28 June 2017 (Click to enlarge)

One of the GOES-16 Baseline Products is a Cloud Mask — this is important because many other Baseline Products use the output from the Cloud Mask in decision trees. The toggle below shows the Cirrus Band (1.38 µm), the Red Visible (0.64 µm) and the Cloud Mask for 1557 UTC on 28 June 2017.

 

GOES-16 “Cirrus Band” (Band 4, 1.38 µm), “Red Visible” (0.64 µm) and Cloud Mask Baseline Product (White=Cloud, Black= Clear) (Click to enlarge)

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.