Cloud Phase Determined by GOES-16 Brightness Temperature Differences
Channel Difference Field (8.4 µm – 11.2 µm), 2027 UTC on 16 March 2017 (Click to enlarge)
The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing.
One of the GOES-16 Band Difference Products available in AWIPS is shown above (click here for the same image with a default enhancement), the 8.4 µm – 11.2 µm ‘Cloud Phase’ Channel Difference. There has been considerable work showing a good correlation between the 8.4 µm – 11 µm brightness temperature difference and cloud-top phase. Click here for example. The key take-aways:
- “Radiative transfer simulations indicate that the brightness temperature difference between the 8.5- and 11-micron bands (hereafter denoted as BTD[8.5-11]) tends to be positive in sign for ice clouds that have an infrared optical thickness greater than approximately 0.5. Water clouds of relatively high optical thickness tend to exhibit highly negative BTD[8.5-11] values of less than -2K.”
- “Clear-sky BTD[8.5-11] values tend to be negative because the surface emittance at 8.5 microns tends to be much lower than at 11 microns, especially over non-vegetated surfaces.”
- “Small particles tend to increase the BTD[8.5-11] values relative to large particles because of increased scattering.”
This Algorithm Theoretical Basis Document (ATBD) contains further information and references on the topic.
In the color enhancement above (generated using just 3 colors — blue, red and yellow, and easily changeable for those whose eyesight is color challenged), negative brightness temperature differences — ice clouds — are denoted by red to yellow values. Positive values are blue. Water-based clouds are white or light blue, ice clouds are red. Large Brightness Temperature Differences occur over the Desert southwest and strong negative values (blue in the enhancement) are present because of emissivity differences from the soil at the two wavelengths. The toggle below cycles through the Band Difference field, the 8.4 µm and 11.2 µm imagery, the upper level water vapor (6.19 µm), the cirrus channel (1.378 µm) and the Veggie channel (0.86 µm). Click here for a toggle between the Cloud Phase Product and Water Vapor and Cirrus channels only (to highlight ice clouds), or here for a toggle between the Cloud Phase and Veggie Band (a band in which clouds composed of water droplets are visible).
One of the GOES-16 Baseline Products will be Cloud Phase. A beta version of this product should be flowing before the end of May. The ATBD for that product shows that both 8.4 µm and 11.2 µm data are used in its creation, and the channel difference shown here shows why.
Cloud Phase Brightness Temperature Difference (8.4 µm – 11.2 µm), Window Channel (11.2 µm), Infrared Cloud Phase (8.4 µm), Upper Level Water Vapor (6.19 µm), Cirrus Channel (1.38 µm) and Veggie Band (0.86 µm), all at 2027 UTC on 16 March 2017 (Click to enlarge)
Fact Sheets are available for the 8.4 µm and 11.2 µm Channels on GOES-16 from the GOES-R website.
AWIPS Notes: AWIPS sometimes mislabels the 8.4 µm channel as 8.5 µm. The Channel Difference field shows missing data where the brightness temperature difference field is exactly zero. Such points are apparent in the image above over Minnesota and North Dakota as black speckles.