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)

Scattering and Shadows

June 19th, 2017 |

GOES-16 Imagery over New England from 1022 through 1117 UTC on 19 June 2017. Blue Band (0.47 µm), upper left; Red Band (0.64 µm), upper right; ‘Veggie’ Band (0.86 µm), lower left; ‘Snow/Ice’ Band (1.61 µm), lower right. Click to enlarge.

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

The animation of GOES-16 imagery, above, showing, clockwise from upper left, the GOES-16 0.47 µm, 0.64 µm, 1.61 µm and 0.86 µm channels, shows lows clouds over southeast New England, with a few mid-level clouds aloft. The moving mid-level clouds are casting shadows on the lower clouds beneath. Note that the shadows appear darkest at the longest wavelength. This is shown in the AWIPS cursor readout below as well — for the point selected, the 1.61 µm reflectance is 5.4%, and it increases to 18.9% at 0.47 µm. Why does it change by wavelength?

AWIPS read-out of reflectance in a shadow east of Cape Cod. Note the reflectance increases as wavelength decreases. (Click to enlarge)

Rayleigh scattering in the atmosphere is a function of wavelength: scattering is more effective at shorter wavelengths. Thus, the atmosphere is scattering the greatest amount of 0.47 µm radiation (compared to the longer wavelengths shown here). Shadows are darker (there is less detected reflectance) at longer wavelengths because less longer-wavelength radiation is scattered towards the satellite sensor.

The tweet from the National Weather Service in Melbourne, below, shows another shadow scene with 0.86 µm imagery.


The Split Window Difference as a measurement of Atmospheric Moisture

April 7th, 2017 |

GOES-16 Split Window Difference (10.33 µm – 12.30 µm) with 850-mb Dewpoint Temperatures from the Rapid Refresh overlain (Click to enlarge)

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

GOES-16 includes both a clean infrared window (10.33 µm) and a so-called ‘dirty’ infrared window channel (12.30 µm). The clean infrared window is in a part of the electromagnetic spectrum where there is very little absorption of energy by water vapor; in the dirty infrared window, modest amounts of water vapor absorption occur. The brightness temperature difference, nicknamed the Split Window Difference (SWD for short), can highlight differences in moisture in clear skies.

The toggle above shows the SWD (10.33 µm – 12.30 µm) at 1430 UTC on 7 April 2017. A pronounced gradient stretches southeast to northwest from Louisiana to northeast Kansas and extreme southeastern Nebraska.  Values over Missouri, for example, are around 0.9-1.0 K vs. 1.7-2.2 K over Oklahoma.  The gradient in the brightness temperature difference aligns very neatly with the 850-mb dewpoint temperature from the Rapid Refresh. You can use this product to monitor moisture return from the Gulf of Mexico.

AWIPS Note: The Default enhancement in AWIPS for the Split Window Difference, shown above, does not include large enough negative values. The Split Window Difference value can exceed -5 K in regions of dust. See this link for a different enhancement for this case with a wider range of temperature differences. A similar image uses the mean 1000-700 mb dewpoint temperature rather than values from the single 850-mb level.

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An animation of this imagery (not shown) shows general increases in the SWD values with time.  A consistent signal of moisture will be present only if the temperature decreases with height in the moist layer (that is — if there is no inversion).  An increase in the SWD does not necessarily show an increase in moisture — it can, rather, signify an increase in near-surface temperature (for more information, consult this article by Lindsey et al.). The gradient in the field can remain, however, as in this example.

The Split Window Difference field does an exemplary job of detecting contrails over the southern Plains. The toggle below shows that the SWD signal of cirrus is more distinct than in the 1.378 µm Cirrus Channel! (Thanks to Matt Bunkers of WFO Rapid City for noting this!)

Cirrus Channel (1.378 µm ) and Split Window Difference (10.33 µm – 12.30 µm) at 1607 UTC on 7 April 2017 (Click to enlarge)

(Note that the SWD was something that was available from GOES-8 through GOES-11. Link)

The GOES-16 ABI Veggie channel at 0.86 µm

March 1st, 2017 |

GOES-16 Red Visible (0.64 µm) and Veggie (0.86 µm) bands over Florida, 21:11 UTC on 01 March 2017 (Click to enlarge)

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

The ABI Band at 0.86 µm (Fact Sheet) allows superior land/sea discrimination. This occurs because land is more reflective to radiation at 0.86 µm than to radiation at 0.64 µm. The toggle above shows Florida in the standard visible (0.64 µm) and at 0.86 µm. Coastal boundaries and islands (such as the Keys and the Bahamas) are far more distinct in the near-infrared so-called ‘veggie’ channel at 0.86 µm. Inland lakes are also better defined with the 0.86 µm channel. Because the land is so bright, land/cloud contrast is reduced in the 0.86 µm imagery, so clouds over land appear more distinct in the 0.64 µm imagery.

The toggle below shows a similar scene over the Tidewater region of southeast Virginia and points to the south.  Again, inland lakes and rivers and the coastal boundary is more apparent in the 0.86 µm imagery than in the 0.64 µm imagery.

GOES-16 Red Visible (0.64 µm) and Veggie (0.86 µm) bands over the mid-Atlantic States, 20:01 UTC on 01 March 2017 (Click to enlarge)

Use the 0.86 µm band when land/water distinction is important!

Because ABI does not have a spectral band in the ‘green’ part of the electromagnetic spectrum (Band 1 at 0.47 µm is in the blue, Band 2 at 0.64 µm is in the red), information from the 0.86 µm band is used in construction of simulated ‘true color’ imagery (as discussed here).

In addition, the 0.86 µm channel provides useful burn scar information in ‘False Color’ imagery (that combines 2.2 µm, 0.86 µm and 0.64 µm imagery) because burn scars appear dark in 0.86 µm imagery.