Parallax shifts in VIIRS views of Fiona

September 19th, 2022 |

VIIRS Day Night Band visible (0.7 µm) imagery from Suomi NPP (0549 UTC) and NOAA-20 (0638 UTC) on 19 September 2022 (Click to enlarge)

Suomi NPP and NOAA-20 both overflew Hurricane Fiona (NPP flew overhead to the east, NOAA-20 flew overhead to the west) in the early morning of 19 September 2022, as shown above in imagery created at AOML (The Atlantic Oceanagraphic and Meteorological Laboratory) and displayed at the Direct Broadcast site there. The images appear to show an eastward motion of the eye — but GOES-16 animations, below, show a persistent west-northwest motion (landfall occurred in the Dominican Republic around 0730 UTC).

GOES-16 Band 13 Infrared Imagery (10.3 µm), 0301 – 0946 UTC on 19 September 2022 (Click to enlarge)

The apparent eastward motion of the eye also shows up in the infrared imagey, which rules out artifacts related to shadowing.

VIIRS M15 (10.8 µm) infrared imagery from Suomi-NPP (0549 UTC) and NOAA-20 (0636 UTC) on 19 September 2022 (Click to enlarge)
Suomi NPP Day Night Band imagery and GOES-16 Band 13 Infrared imagery, ca. 0556 UTC on 19 September 2022 (Click to enlarge)

This might be an example of a Parallax shift in VIIRS imagery causing a shift in a feature. NOAA-20’s nadir was over Jamaica, considerably to the east of the Mona Passage where Fiona’s eye was developing. A parallax error may be responsible, because satellite navigation will place the tall clouds farther from the sub-satellite point than observed.

The full-resolution Day Night band imagery from Suomi NPP, and from NOAA-20 (both available from the CIMSS ftp site here and here) show strong convection starting ca. 0530 UTC and continuing through ~0630 UTC near the eye.

Ring Structures in GOES-17 infrared imagery over the Hawai’ian peaks

May 27th, 2022 |
GOES-17 Clean Window Infrared (10.3 µm) imagery, 0301 – 1601 UTC 27 May 2022 (click to enlarge)

An animation of GOES-17 Band 13 10.3 µm infrared imagery, above, shows the development of concentric circles of cold, warm, cold and warm surrounding the high peaks of the island of Hawai’i (Click here to see a topographic view). The warmth apparent around the peaks at the start of the animation above (dark in the enhancement chosen) suggests that the peaks are peeking through a stratus deck; the toggle below between sunset and sunrise imagery (the sunrise imagery has been brightened considerably) shows that clouds dissipated overnight.

GOES-17 Band 2 (0.64 µm) visible imagery at 0401 UTC and 1601 UTC on 27 May 2022 (Click to enlarge)

The animation below shows brightness temperatures sampled through the rings at 1121 UTC surrounding Mauna Kea. Mountain-top temperatures are sub-freezing, warming to around 5oC/41oF just off the peak, before cooling again to 2oC/36oF and then warming to 5+oC/41+oF farther down the slope. This interesting behavior results from a combination of cooling in the presence of an inversion, and cooling interrupted by the presence of cloudiness. The 1200 UTC sounding from Hilo, on the eastern part of the north coast of Hawai’i, is shown here (courtesy K. Kodama, WFO HNL).

GOES-17 Infrared Band 13 imagery (10.3 µm) at 1121 UTC on 27 May 2022, with brightness temperatures sampled at select points (Click to enlarge)

The toggle below compares GOES-17 Bands 13, 10 and 8 (10.3 µm, 7.3 µm and 6.19 µm, respectively) at 1121 UTC. The ringing is obvious in Band 13; it is not obvious in Bands 10 and 8: in those two bands any surface cooling differences are masked by water vapor emissions higher in the atmosphere, even at the high altitudes along the slopes of Mauna Kea and Mauna Loa.

GOES-17 Bands 13, 10 and 8 (Clean Window, Low-Level Water Vapor and Upper-Level Water Vapor, i.e., 10.3 µm, 7.3 µm and 6.19 µm, respectively) at 1121 UTC on 27 May 2022 (click to enlarge)

Weighting Functions for the three water vapor bands (taken from this website) underscore why ringing around the peaks is far less likely to be observed in the water vapor channels on this day. The sounding shown below (or here) shows a moist layer above 500 mb, between 6 and 8 km above ground; Mauna Loa and Mauna Kea peak at about 4.2 km. The surface-based infrared signal at wavelengths between 6.2 and 7.3 µm will be obscured by water vapor emissions from higher up in the atmosphere. If that high-altitude moist layer were missing, the cold-warm-cold-warm signal would have a better chance of appearing in the water vapor imagery.

Because skies are clearing, one might expect these ring features to appear in the Land Surface Temperature field. However, a toggle that field at 1201 UTC, along with the Clear Sky Mask, shows that is not the case. Land Surface Temperature is computed in regions where skies are “Clear” or “Probably Clear”, but not where skies are “Cloudy” or “Probably Cloudy” (the four possible states in the Clear Sky Mask; note however that AWIPS displays on “Cloudy” skies as white, and all other states are transparent). Thus, the ‘warm’ parts of the rings are displayed; the cold parts are not.

GOES-16 Land Surface Temperature display with/over, Band 13 infrared (10.3 µm) imagery, Topography, and Clear Sky Mask at 1201 UTC on 27 May 2022 (click to enlarge)

Thanks very much to Kevin Kodama, WFO HNL, and Jordan Gerth, NWS/OBS, for bringing this interesting case to our attention!

Quiz Time: What county in the USA has all boundaries visible from satellite?

December 2nd, 2021 |
MODIS-derived BRDF from 1 December 2021 (Click to enlarge)

MODIS-derived (from Terra and Aqua satellites) Bidirectional Reflectance Distribution Function (BRDF), above, (as noted in this blog post), shows the (meager) snow distribution as of early December. How many counties (or parishes) in the United States (out of more than 3000!) are clearly delineated in Satellite Imagery such as what is shown above? Counties that are primarily islands (or peninsulas) — Dare County in North Carolina, for example — show up well (False Color image shown here, in an image taken from VIIRS Today), but the inland borders do not.

For a county to be recognizable from Space, its landcover must differ significantly from adjacent counties. In the zooming-in animation below (from RealEarth, click the image to zoom in), users will note that Menominee County in northeast Wisconsin becomes apparent. Menominee County is almost entirely forest (unlike its neighbors) and as such has a much different signal in the (for example) 0.87 µm channel on VIIRS (or 0.86 µm on GOES-16). When it is zoomed in, the outlines of the County are obvious.

MODIS-derived BRDF from 1 December 2021 at various zoom levels (Click to animate)

The county also shows up well in the VIIRS True Color/False Color toggle below, from 30 November. The southern edge of the snow at that time was just southeast of Menominee County, and the land-use change across the county border is apparent. Snow in the county (cyan in the False Color enhancement) is difficult to view from the imagery — because of the pine forests!

VIIRS True-Color and False-Color imagery over northeastern WI, 1838 UTC on 30 November 2021 (Click to enlarge)

Menominee County has been on this blog before! In 2007, a tornado tracked through Menominee County and left a visible scar in satellite imagery (link). Eight years later (link), the scar was still apparent! November 6 2021 was a clear day over the upper Midwest. Suomi-NPP True Color imagery, below (link to original large image), still shows vestiges of the scar!

Suomi NPP True-Color imagery, 6 November 2021. The outline of Menominee County is apparent, as is the southwest-to-northeast tornado scar

Hole punch clouds over the Upper Midwest

November 7th, 2021 |

On the morning of Sunday, November 7th, numerous elongated hole punch clouds were visible over the Upper Midwest, including parts of Wisconsin, Illinois, Iowa, and Minnesota. Also called fall streak clouds, these are a relatively rare phenomenon that form because of the unusual properties of cloud droplets.

Photo of a hole punch cloud and the associated fall streaks, taken on the east side of Madison, WI, at 11:20 AM CST on Sunday, November 7th. Photo by the author.

While most people know the freezing temperature of water is 0 °C (32 °F), that’s only true when dealing with a flat surface.  A curved droplet has more energy in it due to surface tension squeezing the droplet together, and so the air temperature has to be colder in order to make the droplet cold enough to freeze.  As a result, clouds of liquid water below freezing are relatively common, especially in the spring and fall when temperatures at cloud level are just below freezing.  These are called supercooled clouds.

Another commonly-known fact about water is if the relative humidity of the air is less than 100%, liquid water will evaporate.  Again, that’s not necessarily true for cloud droplets. What is especially interesting is that the relative humidity required to support growth is bigger for a cloud droplet than it is for an ice crystal.  Given an environment with both cloud droplets and ice crystals, the droplets will evaporate and the ice crystals will grow.  This is known as the Bergeron-Findeisen process and is a key part of forming precipitation from cold clouds. 

Both cloud droplets and ice crystals require a nucleus to form. Dust, pollen, and other aerosols are common nuclei.  While water can condense on many different aerosols, ice crystals are much more selective. Due to the rigid crystal shape of ice, it can only form on aerosols that have a similar structure. This is, in part, why supercooled clouds are relatively common: there’s just not enough ice nuclei around for ice crystals to form.  

That brings us to Sunday morning: a rather large altostratus deck was present across the upper midwest. Even though the surface temperature was approaching 16 °C (60 °F), the clouds were high enough above the surface that their temperature was below freezing.  The morning sounding from Davenport, IA, showed that the freezing level was around 3300 m (11,000 ft) above sea level, but airport observations around the region showed that cloud bases were around 5100 m (17,000 ft). Without a sufficient amount of ice nuclei present, they stayed in the liquid phase and were thus supercooled clouds.

1200 UTC (6 AM CST) sounding from Davenport, IA, showing the freezing level was approximately 3300 m (11,000 ft) above sea level. Image from the University of Wyoming sounding archive.

However, numerous aircraft were flying through those clouds as they ascended from or descended into airports across the region.  The moisture-rich exhaust from the planes was deposited into the low-pressure wake behind the airplane, where it cooled very quickly and formed ice.  Normally, this would form the classic contrails seen behind many aircraft in the sky.  However, in this case the contrail served as a nucleation site within the supercooled cloud.  The droplets near the ice rapidly evaporated and the ice crystals generated by the airplanes grew even larger. In some cases, the crystals grew so large that they could no longer be supported aloft, and they started falling to the ground as snow.  They didn’t reach the ground because the air was warm and dry beneath the cloud, and so the ice crystals either melted and evaporated, or they sublimated (going directly from solid to vapor).  

The Terra polar-orbiting satellite happened to be passing overhead at the right time to capture this phenomenon while it was happening around 10:30 AM CST.  Almost-clear holes are seen in northeastern Iowa and southeastern Minnesota, while in northern Illinois they appear as elongated ice clouds surrounded by a clear region embedded within a larger cloud.  

MODIS True-color image from the 10:30 AM CST overpass showing hole punch clouds, circled in white.

The loop from Band 2 (0.64 micron) from GOES-16 also shows these clouds propagating through the region. This view, over Dane County (Madison) Wisconsin, shows one hour of visible-wavelength satellite imagery. The embedded ice clouds are clearly visible as structures that propagate from the west to the east. While the airplanes that created these structures have long since departed to other locations, their impact remained for some time.

Animation of GOES-16 Band 2 reflectance over south central Wisconsin. Dane County, home of Madison, is outlined.

Other blog posts showing examples of hole punch clouds can be found here.