Small Eddy and coastal jet off the coast of Northern California

May 4th, 2017 |

GOES-16 Visible (0.64 µm) from 1245 through 2200 UTC on 4 May 2017 (Click to play mp4 animation)

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

One of the two GOES-16 Mesoscale Sectors was moved from its default position over the eastern United States and placed over the west coast of the United States on 4 May 2017. This allowed 1-minute imagery of a small-scale coastal eddy between Cape Mendocino and Pt. St. George near Crescent City, above, and an associated coastal jet. (Click here to play 300-meg Animated Gif; alternatively, this animation shows the eddy from 1600-1900 UTC as displayed in AWIPS (courtesy Dan Miller, WFO DLH))

A zoomed-in Visible animation of the coastal eddy is shown below; NWS Eureka described it as “one of the best examples of these coastal eddies seen in quite a while”.

GOES-16 Visible (0.64 µm) images, with hourly surface reports plotted in yellow (Click to animate)

GOES-16 Visible (0.64 µm) images, with hourly surface reports plotted in yellow (Click to animate)

GOES-16 Visible 0.64 µm imagery is able to capture not only the eddy, but also the northerly low-level jet that develops off the coast of Cape Mendocino, swiftly moving clouds southward around that feature. A small eddy also develops south of Cape Mendocino. Note also the abundance of cirrus clouds flowing northward along the coast.

The dimensions of this eddy are approximately 70 km in the along-shore direction and 55 km perpendicular to the shore, yet GOES-16 is able to capture and resolve many small-scale cloud bands. The small cloud band streaming south around Cape Mendocino, for example, is only about 6 km wide and is well-resolved; if GOES-16 becomes GOES-East at 75 W Longitude, this is the type of resolution that can be expected in Salt Lake City.

It should be noted that none of the models (including the hourly RTMA, below) resolved this eddy feature.

Suomi NPP VIIRS Visible (0.64 µm) image, with RTMA surface winds {Click to enlarge)

Suomi NPP VIIRS Visible (0.64 µm) image, with RTMA surface winds {Click to enlarge)

Thanks to Dan Miller, Science and Operations Officer (SOO) in Duluth for calling this awesome feature to our attention!

Using GOES-16 to view clouds over snow

May 1st, 2017 |

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

A late-season snow storm dropped a band of heavy snow over Colorado, western Kansas and western Nebraska on 29-30 April 2017. In the visible image above from sunrise on 1 May 2017, it is difficult to guess where the cloud features sit on top of the snow (Click here for a visible image with a map), even with the knowledge that they are casting shadows in this early morning imagery. GOES-16 includes a 1.61 µm channel, however; radiation at that wavelength is absorbed strongly by ice — either in the form of cirrus clouds, or snow, so that reflectance is small over ice features. The toggle below between the 1.61 µm “Snow/Ice” Channel and the 0.64 µm “Red Visible” channel shows ice and snow as dark. Clouds that are made up of water droplets are highly reflective in the 1.61 µm and in the 0.64 µm channels; such water clouds (there are only a few of them!) show up as very bright against the dark background of snow in the 1.61 µm channel.

Note in the toggle above that shadows are much darker in the 1.61 µm channel. Why?

Atmospheric scattering is stronger at shorter wavelengths in the atmosphere; there is more scattering of 0.64 µm radiation than of 1.61 µm radiation. In the shadow regions, more 0.64 µm radiation than 1.61 µm is being scattered back towards the satellite for detection. Shadows in the 0.47 µm “Blue Visible” band should be even less distinct. Non-annotated versions of imagery are available here for 0.64 µm and here for 1.61 µm.

During the subsequent late morning and early afternoon hours, the edges of the long swath of snow cover were seen to melt quickly — due to heating from the high May sun angle — on GOES-16 Visible (0.64 µm) and Snow/Ice (1.61 µm) images, below. The Snow/Ice images helped to highlight bright cumulus clouds (composed of supercooled water droplets) drifting southeastward across the snow cover.

GOES-16 Visible (0.64 µm, left) and Snow/Ice (1.61 µm, right) images [click to animate]

GOES-16 Visible (0.64 µm, left) and Snow/Ice (1.61 µm, right) images [click to animate]

Animations of GOES-16 Visible vs Snow/Ice images from the previous day (when the southwestern portion of the swath of fresh snow cover first became evidentt as clouds from the parent storm departed) are available here: Animated GIF | MP4.

Adjustments to GOES-16 Radiances in Visible and near-Infrared Channels

April 29th, 2017 |

GOES-16 Visible (0.64) Imagery at 1145 UTC and 1200 UTC on 29 April 2017, before and after, respectively, a calibration change (Click to enlarge)

Reflectance values detected near Solar Noon over thick clouds can exceed 1 (i.e., the albedo will exceed 100%) because of contributions (caused by scattering) to a pixel from neighboring pixels. A fix that allows for these larger-than-one reflectance values in the GOES-16 Processing is scheduled to take place in July. Until July the values exceeding one will be treated as missing in AWIPS. The change discussed in this blog post, below, implemented on 29 April 2017, should reduce the number of those missing pixels.

GOES-16 has on-orbit calibration of visible and near-infrared channels. This is in contrast to legacy GOES (GOES-13, GOES-14 and GOES-15); the lack of on-orbit calibration means that visible imagery from GOES-13 and GOES-15 show different reflectance values, as shown in this image from 2000 UTC on 01 May 2017: GOES-13 shows smaller reflectance (the right half of the image) than GOES-15 (the left half of the image). Such degradation should not occur with GOES-16.

On-orbit calibration is achieved by viewing the Sun for a set period and determining the photons detected. That value is then related to the amount of solar energy reflected back from the Earth from bright surfaces. It’s important that the exact amount of time for the solar view is known; otherwise the reflectance cannot be calibrated correctly.

NESDIS Engineers and the NOAA CWG (Calibration Working Group) determined in April that visible and near-infrared reflectances were too large (by up to 10%) because the amount of time viewing the Sun was different than the time-value used in the calibration. Shortly before 1200 UTC on 29 April a change in the calibration module was made to mitigate this mis-match. Reflectances dropped about 10% because of this calibration change.

The animation above shows GOES-16 0.64 imagery before and after the calibration change and it’s difficult to note a reduction in the brightness; however, a scatterplot of the reflectance over the convection that is over the tropical Atlantic east of the Amazon (click here to see the region, it is shaded in purple; this image shows the 1145 UTC image with the region toggled on and off) does show higher values at 1145 vs. 1200 UTC, at a time during the day when the rising Sun should cause increases in reflectance. (A similar plot of 1215 UTC vs. 1200 UTC shows reflectance values with no bias).

Density Diagram of GOES-16 Reflectance Values at 1145 UTC (x-axis) vs. GOES-16 Reflectance Values at 1200 UTC (y-axis) (Click to enlarge)

Imagery in this blog post was created using the Satellite Information Familiarization Tool (SIFT).

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

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)