Transitory Solar Reflectance in GOES-R Series Imagery

March 5th, 2018 |

GOES-16 Visible (0.64 µm) animation, 1637-1732 UTC on 5 March 2018 (Click to enlarge)

Animations of GOES-16 Visible, near-Infrared and shortwave Infrared over North America shortly before the Vernal Equinox, and shortly after the Autumnal Equinox, (that is, when the Sun is overhead in the Southern Hemisphere) show bright spots that propagate quickly from west to east (these features were first noted by Frank Alsheimer of the National Weather Service). The animation above shows the visible imagery (0.64 µm) over the Continental United States on 5 March 2018 (Click here for a slower animation speed). Brightening over regions between 30 and 40 N between 1637 UTC and 1732 UTC is apparent. The animation below of the shortwave infrared (3.9 µm) shows slight warming (Click here for a slower animation), as might be expected with reflected solar energy. The brightening is also apparent in the Band 4 “Cirrus”  (1.37 µm) — in fact, a closer look at southern Colorado reveals the bright signature of sunlight reflecting off solar panels at the Alamosa Solar Generating Facility (Google maps).

GOES-16 Shortwave Infrared (3.9 µm) animation, 1637-1732 UTC on 5 March 2018 (Click to enlarge)

The increased reflectance can cause the ABI Clear Sky Mask to mis-characterize clear regions as cloudy (See the animation below; click here for a slower animation). Thus, Cloud properties (Cloud-top Height, Temperature, Pressure, etc.) can be identified in clear regions.

GOES-16 Clear Sky Mask (White: Clouds ; Black : No Clouds) from 1637 UTC – 1732 UTC on 5 March 2018 (Click to enlarge)

The bright spots in the visible, and warms spots in the shortwave infrared, occur when the Earth’s surface, the GOES Satellite and the Sun are aligned on one line. If you were within the bright spot with a powerful telescope trained on the Sun, you would see the GOES Satellite transecting the solar disk. The location of these bright spots changes with season: they appear in the Northern Hemisphere shortly before the (Northern Hemisphere) vernal equinox and shortly after the (Northern Hemisphere) autumnal equinox. Similarly, they appear in the Southern Hemisphere shortly before the (Southern Hemisphere) vernal equinox and shortly after the (Southern Hemisphere) autumnal equinox. On the Equinox, the bright spots are centered on the Equator.

This animation (courtesy Daniel Lindsey, NOAA/CIRA and Steve Miller, CIRA) shows where the reflection disk moves during the days around the Northern Hemisphere Autumnal Equinox; a similar animation for the Northern Hemisphere vernal equinox would show a disk starting at the North Pole and moving southward with time.

The animation below (from this link that is used for calibration exercises), shows the difference in reflectance (Bands 1-6) or Brightness Temperature (Bands 7-16) between 1657 and 1652 UTC on 3 and 5 March 2018. Two things are apparent: The centroid of the largest difference in solar reflectance has moved southward in those two days, as expected; the effect of this solar backscatter is most obvious in the visible, near-infrared and shortwave infrared channels (that is, bands 1-7 on the ABI).  The effect is most pronounced in clear skies.

Time Difference in each of the 16 ABI Channels (1657 – 1652 UTC) on 3 and on 5 March 2018 (Click to enlarge)

This reflectance feature is also detectable in legacy GOES Imagery. However, the great improvements in detection and calibration in the GOES-R Series ABI (and AHI on Himawari-8 and Himawari-9) and the better temporal resolution with the GOES-R Series allows for better visualization of the effect.

The feature also shows up in “True Color” Imagery, shown below (from this site). Geocolor imagery (shown here), from CIRA, also shows the brightening.

CIMSS Natural True Color Animation ending 1757 UTC on 5 March 2018 (Click to enlarge)

Thanks to Daniel Lindsey and Tim Schmit, NOAA/ASPB, Steve Miller, CIRA and Mat Gunshor, CIMSS, for contributions to this blog post.

Cloud glaciation caused by perturbations

February 8th, 2018 |

GOES-16 ABI Band 2 (0.64 µm) Visible Imagery, 1222 – 1712 UTC on 8 February 2018 (Click to animate)

The animation above shows visible imagery from GOES-16 (0.64 µm) over Pennsylvania on 8 February 2018.  Northwest flow over the ridges of the Appalachians is causing stable waves clouds that are parallel to the topography.  However, the animation shows point sources over Somerset and Cambria counties — in southwestern Pennsylvania — that are changing the character of the clouds and disrupting the linear cloud features.  The animation of the GOES-16 ABI 1.61 µm “Snow/Ice” channel, below, shows that the point sources are causing glaciation in the clouds.  Glaciated clouds contain ice, and ice strongly absorbs energy at 1.61 µm, so glaciated clouds appear dark.  The point sources, likely smokestacks, are perturbing the flow and likely introducing freezing nuclei into the supercooled clouds.  As a result, supercooled cloud liquid water droplets freeze.  The toggle between visible 0.64 µm and near-infrared 1.61 µm at 1312 UTC and at 1402 UTC suggests that different smokestacks are operating at different times of the day.  Note that later in the animations, mid-level clouds move in that obscure the view of the lowest clouds.

In addition to glaciated clouds, snow on the ground appears dark as well.  Snow on the ground in the Susquehanna River Valley, is very bright in the 0.64 µm imagery, and darker in the 1.61 µm.  The darkest regions over south central Pennsylvania and northern Maryland are likely regions where snowfall was followed by freezing rain:  the layer of ice on top of the snow will absorb 1.61 µm energy more readily than the snow itself.  This chart from the National Weather Service Eastern Region shows ice accumulations less than 0.10″ in that region.

GOES-16 ABI 1.61 µm Near-Infrared Imagery, 1222 – 1712 UTC on 8 February 2018 (Click to animate)

There is a GOES-16 Baseline Product that determines cloud-top phase.  The toggle below, showing imagery 1412 UTC on 8 February, suggests a change from supercooled (bright green) to mixed phase (dark green) to ice (red) in the region.  The 2-km native resolution of the Cloud Phase product (ATBD can be read here) vs. 1-km for 1.61 µm (and 0.5-km for 0.64 µm ) might account for some of the differences between what the 1.61 µm channel suggests over southwestern Pennsylvania and what the Cloud Phase product diagnoses.  (In addition, the GOES-16 Baseline Cloud Phase product has not yet reached Provisional Maturity Status).

GOES-16 Baseline Cloud Phase Product and GOES-16 Snow/Ice 1.61 µm Near-Infrared Imagery, 1412 UTC on 8 February 2018. (Click to enlarge)

So, glaciation of clouds can be induced as shown above by turbulence/freezing nuclei introduced by large smokestacks. The 1-minute animations below shows a region of supercooled clouds from 1515 UTC to 1715 UTC. Note the periodic appearance of hole-punch clouds. In this case, aircraft to/from Chicago O’Hare are likely penetrating the thin supercooled cloud layer, and the passage of the planes is causing glaciation. The clouds within the hole punch cloud are glaciated, and therefore dark in the 1.61 µm imagery: energy at that wavelength is absorbed, not reflected as happens in the visible wavelengths.

Ice in Lake Michigan is visible in the 0.64 µm, but not apparent in the 1.61 µm. Lake Ice and water both absorb 1.61 µm energy. Lake ice reflects 0.64 µm energy.

GOES-16 ABI Visible (0.64 µm) Imagery, 1515-1715 UTC on 8 February 2018 (Click to animate)

GOES-16 ABI Near-Infrared “Snow/Ice” (1.61 µm) Imagery, 1515-1715 UTC on 8 February 2018 (Click to animate)

Rope Cloud over the northwest Gulf of Mexico

January 22nd, 2018 |

GOES-16 “Red Visible” 0.64 µm imagery from 1402-2142 UTC on 22 January 2018. (Click to animate)

Visible GOES-16 Satellite Imagery over the northeastern Gulf of Mexico on 22 January 2018 showed the development of a Rope Cloud. Such features have been discussed before on the CIMSS Blog — here, here, here and here! Rope Clouds are handy features in satellite analysis over the ocean because they indicate distinctly where the surface cold front exists. Note that the WPC surface analysis, shown here for 1500 UTC, has the front in the same location as the rope cloud, with convection noted out in advance of the surface cold front. The hourly animation below, showing surface observations and the GOES-16 Red Visible (0.64 µm) Imagery, confirms the windshifts that were observed when the Rope Cloud/Cold Front passed any station.

Hourly Surface Observations and GOES-16 “Red Visible” 0.64 µm imagery from 1400-2200 UTC on 22 January 2018. (Click to enlarge)

Explosive cyclogenesis off the East Coast of the United States

January 4th, 2018 |

GOES-16 Clean Window (10.3 µm) Imagery, 0102-1337 UTC on 4 January 2018 (Click to animate)

A strong extratropical cyclone that deposited snow in the deep south developed explosively during the early morning hours of 4 January 2018. The GOES-16 Clean Window (10.3 µm) animation, above, from 0102 – 1337 UTC on 4 January, brackets the explosive development: from 993 hPa at 0000 UTC to 968 mb at 0900 UTC, a strengthening that easily meets the “Bomb” criteria set forth by Sanders and Gyakum (1980). The Clean Window animation shows the strong surface circulation with well-defined conveyor belts. Convection develops at the leading edge of the dry slot that is approaching southern New England at the end of the animation. The Low-Level Water Vapor (7.3 µm) animation for the same time, below, suggests very strong descent behind the storm, where brightness temperatures warmer than -10º C (orange in the enhancement used) are widespread.

GOES-16 Low-Level Water Vapor (7.3 µm) Infrared Imagery, 0102-1332 UTC on 4 January 2018 (Click to animate)

This storm can also be viewed using Red-Green-Blue composites (in addition to the single-channel animations shown above). The Airmass RGB, below, combines the Split Water Vapor Difference (6.2 µm – 7.3 µm) as Red, Split Ozone (9.6 µm – 10.3 µm) as Green, and Upper level Water Vapor (6.2 µm) as Blue. (Other storms analyzed with the Airmass RGB can be seen here, here, and here). The strong red signal in the Airmass RGB south of the storm suggests very strong sinking motion.

GOES-16 AirMass RGB Product, 0102-1332 UTC (Click to animate)

ASCAT Scatterometer winds over the system at 0205 UTC showed an elongated surface circulation with multiple observations of winds exceeding 50 knots (in red), and a large region (in yellow) of winds exceeding 35 knots.

GOES-16 ABI Clean Window (10.3 µm) and ASCAT Scatterometer winds, 0205 UTC on 4 January 2018 (Click to enlarge)

GOES-16 ABI Red Visible (0.64 µm) and ASCAT Scatterometer winds, 1520 UTC on 4 January 2018 (Click to enlarge)

The 1520 UTC ASCAT pass, above, sampled half the storm, and hurricane-force winds were indicated.

The snow that was deposited in the Deep South by this storm (also discussed here) persisted through a cold night and was visible in the GOES-16 Visible (0.64 µm) imagery, below. Highly reflective snow can be difficult in a still image to distinguish from clouds — but the Snow/Ice Channel on GOES-16 (1.61 µm) detects energy at a wavelength that is strongly absorbed by ice. Thus, snow (and ice) on the ground (or in clouds), has a different representation. (Here are toggles between the two images, with and without a map). The snow cover over coastal Georgia, South and North Carolina appears dark in the Snow/Ice channel because the snow is absorbing, not reflecting, the 1.61 µm radiation.  It is noteworthy that the 1.61 µm image is especially dark over far southeastern Georgia northeastward along the immediate coastline of South Carolina.  These are regions where freezing rain and sleet fell, versus predominantly snow to the north and west (as also noted here; The National Weather Service in Tallahassee tweeted out an ice/snow accumulation map that also agrees with the 1.61 µm image).  Ice in the cirrus clouds northeast of North Carolina is also apparent in the Snow/Ice 1.61 µm imagery.

GOES-16 Band 2 Visible (0.64 µm) Imagery, 1412 UTC on 4 January 2018 (Click to enlarge)

GOES-16 ABI Band 5 Snow/Ice (1.61 µm) Imagery, 1412 UTC on 4 January 2018 (Click to enlarge)

Suomi NPP overflew the storm shortly after midnight on 4 January; Day Night band visible imagery (courtesy Kathleen Strabala, CIMSS), below, shows a well-developed cyclone covering much of the northeast Atlantic Ocean. Snow cover is apparent over the deep south of the United States.

Suomi NPP Day Night Band Visible (0.7 µm) Imagery, 0614 UTC on 4 January 2018 (Click to enlarge)

(Added, 5 January 2018: This website shows a during-the-day CIMSS True Color Image animation of the storm on 4 January 2018. Animation courtesy Dave Stettner, CIMSS).