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.

Undular bores over the Gulf of Maine

April 27th, 2017 |

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

As pointed out by NWS Caribou:



numerous packets of wave clouds associated with undular bores were seen on GOES-16 Visible (0.64 µm) imagery over the Gulf of Maine on the morning of 27 April 2017. A longer animation with surface wind plots (below; also available as an MP4 animation) revealed the presence of 3 distinct bore structures: the largest and most well-defined which was moving eastward; a second (and much smaller) off the coast of Cape Cod which was moving southeastward; and a third which as moving northwestward  (and eventually intersected the northern end of the primary eastward-moving bore).

GOES-16 Visible (0.64 µm) images, with surface winds (knots) plotted in cyan [click to play animation]

GOES-16 Visible (0.64 µm) images, with surface winds (knots) plotted in cyan [click to play animation]

A comparison of GOES-16 and GOES-13 (GOES-East) Visible images (below; also available as an MP4 animation) showed that undular bore wave cloud structures were more clearly clearly seen with the higher spatial spatial resolution of GOES-16 (0.5 km at satellite sub-point, vs 1.0 km for GOES-13). The comparison also showed that the visible imagery from GOES-13 (launched in May 2006, and operational as GOES-East since April 2010) was not as bright as that from GOES-16; this is due to the fact that the performance of GOES visible detectors tends to degrade over time.

GOES-16 Visible (0.64 µm, left) and GOES-13 Visible (0.63 µm, right) images [click to play animation]

GOES-16 Visible (0.64 µm, left) and GOES-13 Visible (0.63 µm, right) images [click to play animation]

So what caused these undular bores to form and propagate across the Gulf of Maine? Such gravity waves are ducted within strong temperature inversions — and rawinsonde data from Chatham, Massachusetts and Yarmouth, Nova Scotia indicated that such inversions were in place above the surface that morning. The northwestward-moving bore could have been initiated by surface outflow from thunderstorms associated with a mid-latitude cyclone (which was producing storm force and gale force winds: surface analyses) — GOES-16 Infrared Window (10.3 µm) images (below; also available as an 88 Mbyte animated GIF) showed these thunderstorms which developed within the warm sector of the coastal low pressure system. However, the forcing mechanism(s) that generated the eastward and southeastward moving bores remains somewhat of a mystery.

GOES-16 Infrared Window (10.3 µm) images [click to play MP4 animation]

GOES-16 Infrared Window (10.3 µm) images [click to play MP4 animation]

GOES-16 daytime and nighttime images of the West Mims Fire in Georgia

April 25th, 2017 |

GOES-16 Blue Visible (0.47 µm, top), Red Visible (0.64 µm, center) and Shortwave Infrared (3.9 µm, bottom) images, with hourly surface plots in yellow [click to play animation]

GOES-16 “Blue” Visible (0.47 µm, top), “Red” Visible (0.64 µm, center) and Shortwave Infrared (3.9 µm, bottom) images, with hourly surface plots in yellow [click to play animation]

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

A daytime comparison of GOES-16 ABI “Blue” Visible (0.47 µm), “Red” Visible (0.64 µm) and Shortwave Infrared (3.9 µm) images (above; also available as an MP4 animation) displayed the smoke plume and “hot spots” (black to yellow to red pixels) associated with the West Mims Fire that was burning in far southeastern Georgia on 25 April 2017 (this fire complex had been burning since 06 April, during which time the drought conditions had been worsening across that region). Downwind of the fire, in far northeastern Florida, smoke reduced the surface visibility to 2 miles at Jacksonville and 5 miles at Fernandina Beach.

During the subsequent nighttime hours — as the fires were beginning to decrease in both intensity and areal coverage — a comparison of “Snow/Ice” Near-Infrared (1.61 µm), “Cloud-Top Phase” Near-Infrared (2.24 µm) and Shortwave Infrared (3.9 µm) images (below; also available as an MP4 animation) showed that a bright glow from the most intense fires was evident in both of the Near-Infrared spectral bands.

GOES-16

GOES-16 “Snow/Ice” Near-Infrared (1.61 µm, top), “Cloud-Top Phase” Near-Infrared (2.24 µm, center) and Shortwave Infrared (3.9 µm, bottom) images, with hourly surface plots in yellow [click to play animation]

Although the spatial resolution of the 1.61 µm Band 5 is 1 km (at satellite sub-point) versus 2 km for the 2.24 µm Band 6, the bright nighttime fire signature was more defined on the 2.24 µm imagery; this is explained by examining a plot of the Spectral Response Function (SRF) for each band (below; courtesy of Mat Gunshor, CIMSS). For a very hot fire target — represented by the red 1200 K line — the 2.24 µm Band 6 SRF is located near the peak of the 1200 K curve, so more of the fire-emitted radiance can be sensed by Band 6 (in spite of its lower spatial resolution).

Spectral Response Function plots for GOES-16 ABI Band 5 (1.61 µm), Band 6 (2.24 µm) and Band 7 (3.9 µm) [click to enlarge]

Spectral Response Function plots for GOES-16 ABI Band 5 (1.61 µm), Band 6 (2.24 µm) and Band 7 (3.9 µm) [click to enlarge]

GOES-16: Earth Day 2017

April 22nd, 2017 |
Full Disk images of all 16 bands of the GOES-16 ABI {click to enlarge]

Full Disk images of all 16 bands of the GOES-16 ABI {click to enlarge]

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

To commemorate Earth Day 2017, Full Disk images using all 16 spectral bands on the GOES-16 ABI instrument at 17:25:22 UTC on 22 April are displayed above. One feature that was prominent in most of the shorter-wavelength bands — which are able to sense a good deal of radiation/reflectance from the Earth’s surface — was the large area of sun glint near the center of the images (over the Pacific Ocean, just west of Panama/Costa Rica). The westward migration of this sun glint signature could be followed on an animation of Visible (0.64 µm) images (below). GOES-16 remained in a Mode 4 scan strategy during much of the day (until 1930 UTC), providing Full Disk images every 5 minutes.

GOES-16 Visible (0.64 µm) images [click to play animation]

GOES-16 Visible (0.64 µm) images [click to play animation]

Taking a closer look at a portion of the Amazon River in Brazil, a comparison of Blue Visible (0.47 µm), Red Visible (0.64 µm) and Near-Infrared Vegetation (0.86 µm) band imagery (below) highlighted the ability of the 0.86 µm images to discriminate between land and water (water appears very dark). This makes 0.86 µm imagery useful for identifying and monitoring areas of inland flooding.

GOES-16 Visible (0.47 µm and 0.64 µm) and Near Infrared (0.86 µm) images [click to enlarge]

GOES-16 Visible (0.47 µm and 0.64 µm) and Near Infrared (0.86 µm) images [click to enlarge]