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.

Daily record maximum precipitable water at Miami, Florida

May 17th, 2016 |

H/T to Brian McNoldy for sending this out on Twitter:

Hourly images of the MIMIC Total Precipitable Water (TPW) product (below) revealed the northward transport of deep tropical moisture from the Caribbean during the 16 May17 May 2016 period, with TPW values near Miami (KMFL) around 55 mm or 2.17 inches at 12 UTC on 17 May.

MIMIC Total Precpitable Water product [click to play animation]

MIMIC Total Precpitable Water product [click to play animation]

A comparison of the 00 UTC and 12 UTC Miami soundings (below) showed the increase of moisture within the middle to upper troposphere that helped contribute to the daily record maximum TPW value of 2.18 inches at 12 UTC.

00 UTC and 12 UTC Miami rawinsonde data [click to enlarge]

00 UTC and 12 UTC Miami rawinsonde data [click to enlarge]

The northward surge of tropical moisture also helped to fuel the development of a large mesoscale convective system over the eastern Gulf of Mexico, as seen in 4-km resolution GOES-13 Infrared Window (10.7 µm)  imagery (below). Cloud-top IR brightness temperatures were as cold as -80º C (violet color enhancement) at times with this storm.

GOES-13 Infrared Window (10.7 µm) images [click to play animation]

GOES-13 Infrared Window (10.7 µm) images [click to play animation]

A comparison of 1-km resolution Terra MODIS Visible (0.65 µm) and Infrared Window (11.0 µm) images at 1632 UTC is shown below.

Terra MODIS Visible (0.65 µm) and Infrared Window (11.0 µm) images [click to enlarge]

Terra MODIS Visible (0.65 µm) and Infrared Window (11.0 µm) images [click to enlarge]

Christmas Full Moon Day/Night Band images

December 25th, 2015 |

Suomi NPP VIIRS Day/Night Band image swaths [click to enlarge]

Suomi NPP VIIRS Day/Night Band image swaths [click to enlarge]

A rare Full Moon on Christmas — the last occurrence was in 1977, and the next will be in 2034 — was reached at 1111 UTC, and provided some compelling “visible images at night” from the Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB). The animation above shows the coverage of 4 consecutive DNB image swaths across much of North America, as viewed using RealEarth.

Taking a closer look at various regions and features, we will begin with the Northeast US at 0610 UTC (below). One item of interest was the narrow fingers of valley fog that were forming in parts of Pennsylvania and New York, where strong radiational cooling under cloud-free skies was allowing the surface air temperatures to cool into the 30s and 40s F.

Suomi NPP VIIRS 0.7 µm Day/Night Band image centered over the Northeast US [click to enlarge]

Suomi NPP VIIRS 0.7 µm Day/Night Band image centered over the Northeast US [click to enlarge]

Over the Southeast US at 0749 UTC (below), ample illumination by the Full Moon provided a very detailed nighttime view of the tops of numerous thunderstorms that had developed along and ahead of a cold frontal boundary (surface analysis). Note the appearance of several bright white areas at the tops of some thunderstorms, a signature of cloud illumination by intense lightning activity. In addition, temperatures ahead of the cold front were unusually warm for 25 December — for example, new records for the warmest low temperature for the date were set at both Mobile, Alabama and Pensacola, Florida (with 75º F and 71º F, respectively).

Suomi NPP VIIRS Day/Night Band image centered over the Southeast US [click to enlarge]

Suomi NPP VIIRS Day/Night Band image centered over the Southeast US [click to enlarge]

Farther to the north, a DNB image centered over South Dakota (below) showed a great deal of variability in snow cover across that area; the effect of deeper snow cover on surface air temperatures could also be seen in the surface observations at that time, with the colder readings generally coinciding with sites having snow on the ground.

Suomi NPP VIIRS Day/Night Band image centered over South Dakota [click to enlarge]

Suomi NPP VIIRS Day/Night Band image centered over South Dakota [click to enlarge]

Even farther to the north, it could be seen that ice covered a significant portion of Hudson Bay, Canada (below). In southern parts of Hudson Bay where open water still existed, numerous “lake effect” cloud bands could be seen due the northwesterly flow of very cold arctic air across the water.

Suomi NPP VIIRS Day/Night Band image centered over Hudson Bay, Canada [click to enlarge]

Suomi NPP VIIRS Day/Night Band image centered over Hudson Bay, Canada [click to enlarge]

A DNB image centered over the Western US at 0930 UTC (below) revealed features such as snow pack over the higher terrain of the Sierra Nevada in California (where temperatures at the time ranged from 55º F in the desert at Needles KEED to 1º F in the mountains at South Lake Tahoe KTVL), and a variety of open-cell and closed-cell convection over the adjacent offshore waters of the Pacific Ocean.

Suomi NPP VIIRS Day/Night Band image centered over the western US [click to enlarge]

Suomi NPP VIIRS Day/Night Band image centered over the western US [click to enlarge]

Finally, a DNB image centered over Alaska and the Yukon Territory of Canada at 1111 UTC (below) showed widespread snow cover over the cloud-free interior regions, with the thick cloud shield from a Gulf of Alaska storm moving over southwestern and southcentral Alaska. Very cold surface air temperatures at or below -30º F could be seen at a number of sites in the interior areas, with -42º F being reported at the time in Arctic Village, Alaska PARC (their minimum temperature later dropped to -45º F).

Suomi NPP VIIRS Day/Night Band image centered over Alaska and the Yukon Territory [click to enlarge]

Suomi NPP VIIRS Day/Night Band image centered over Alaska and the Yukon Territory [click to enlarge]