Blowing dust along the southwest coast of Greenland

October 14th, 2018 |

As noted by Santiago Gassó, a long and very narrow plume of airborne dust was evident just off the southwest coast of Greenland on 14 October 2018. Terra MODIS and Suomi NPP VIIRS True Color Red-Green-Blue (RGB) images as viewed using RealEarth are shown below. An exposed (free of snow cover) glacial outlet between Qeqertarsuatsiaat and Paamiut was the point source of the dust plume — the change in water colors (shades of cyan) highlighted the offshore flow of meltwater from this glacier into the Labrador Sea, which then began to curve northward within the West Greenland Current. The strong pressure gradient between high pressure over southern Greenland and a low pressure southeast of the island (surface analyses) along with a passing trough axis caused brisk northerly winds, which lofted the aerosols into the boundary layer.

Terra MODIS and Suomi NPP VIIRS True Color RGB images [click to enlarge]

Terra MODIS and Suomi NPP VIIRS True Color RGB images [click to enlarge]

The plume of aerosols was also apparent on GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (below). The appearance of the plume on 1.61 µm imagery was due to the bright color of the “glacial flour” particles, which were efficient reflectors of incoming solar radiation — this brighter signature showed up well against the dark appearance of the water (which strongly absorbs radiation at the 1.61 µm wavelength).

GOES-16

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 Near-Infrared

GOES-16 Near-Infrared “Snow/Ice” (1.61 µm) images [click to play animation | MP4]

The plume of airborne dust was also seen on GOES-17 Visible and Near-Infrared images (below), although the viewing angle was less favorable than from GOES-16.

* GOES-17 images shown here are preliminary and non-operational *

GOES-17 "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 Near-Infrared "Snow/Ice" (1.61 µm) images [click to play animation | MP4]

GOES-17 Near-Infrared “Snow/Ice” (1.61 µm) images [click to play animation | MP4]

Unfortunately, there were no surface observations in the vicinity of the plume source to indicate how strong the surface winds were blowing; the closest active reporting sites along the southwest coast of Greenland were Godthaab/Nuuk to the distant north and Narsarsuaq to the distant south (large-scale Near-Infrared image). However, Metop-B ASCAT winds (source) just offshore of the plume origin area were in the 30-40 knots range around 1440 UTC (below).

Metop-B ASCAT surface scatterometer winds [click to enlarge]

Metop-B ASCAT surface scatterometer winds [click to enlarge]

Hurricane Michael reaches Category 3 intensity

October 9th, 2018 |

GOES-16

GOES-16 “Clean” Infrared Window (10.3 µm) image, with Metop-A ASCAT surface scatterometer winds [click to enlarge]

Metop-A ASCAT scatterometer data (above) showed surface wind speeds as high as 64 knots near the storm center while Michael was at Category 2 intensity just northwest of Cuba at 0307 UTC on 09 October 2018.

A toggle between Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images at 0752 UTC (below; courtesy of William Straka, CIMSS) revealed transverse banding north of the storm center on the Infrared image, and mesospheric airglow waves propagating westward away from Michael on the Day/Night Band image.

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

5-minute GOES-16 (GOES-East) “Clean” Infrared Window (10.3 µm) images from 0517-1332 UTC (below) showed a series of relatively brief convective bursts around the storm center, but in general Michael exhibited a somewhat disorganized appearance during that time period.

GOES-16

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

After sunrise, 1-minute Mesoscale Domain Sector GOES-16 “Red” Visible (0.64 µm) images (below) revealed the gradual formation of a more well-defined eye during the day, with episodic clusters of convective “hot towers” developing in the southeastern and eastern portions of the eyewall — which then rotated around to the north and northwest of the eye. By 18 UTC Michael had intensified to a Category 3 storm.

GOES-16

GOES-16 “Red” Visible (0.64 µm) images [click to play MP4 animation]

1-minute GOES-16 “Clean” Infrared Window images (below) indicated that infrared brightness temperatures associated with these hot towers were often as cold as -80º to -89ºC (violet to darker purple enhancement).

GOES-16

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

Michael had been encountering unfavorable deep-layer wind shear and had also been moving over a pocket of water with low Ocean Heat Content northwest of Cuba (below). However, once the hurricane began to move over waters having higher OHC in addition to warm Sea Surface Temperature, it gradually began to intensify from a Category 2 to a Category 3.

Ocean Heat Content and Sea Surface Temperature, with a plot of the track of Michael [click to enlarge]

Ocean Heat Content and Sea Surface Temperature, with the track of Michael [click to enlarge]

Land breeze convergence cloud band in Lake Michigan

September 23rd, 2018 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with hourly plots of surface and buoy reports [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed a narrow cloud band that had developed in Lake Michigan in response to land breeze induced convergence on the morning of 23 September 2018. With inland temperatures cooling overnight into the 30s and 40s F (the coldest in both Wisconsin and Michigan was 29ªF) and lake water temperatures of 64ºF (at the North Michigan buoy 45002) to 69ºF (at the South Michigan buoy 45007), a well-defined nocturnal land breeze was established along the western and eastern shorelines of the lake.

Nighttime VIIRS Day/Night Band (0.7 µm) images from Suomi NPP at 0743 UTC and NOAA-20 at 0832 UTC (below) showed that the cloud band had not yet formed at those times.

VIIRS Day/Night Band (0.7 µm) images from Suomi NPP at 0743 UTC and NOAA-20 at 0832 UTC [click to enlarge]

VIIRS Day/Night Band (0.7 µm) images from Suomi NPP at 0743 UTC and NOAA-20 at 0832 UTC [click to enlarge]

The Terra and Aqua MODIS Sea Surface Temperature product (below) confirmed that mid-lake water temperatures were generally in the middle to upper 60s F (green to light yellow enhancement) across the entire length of Lake Michigan.

Terra/Aqua MODIS Sea Surface Temperature product [click to enlarge]

Terra/Aqua MODIS Sea Surface Temperature product [click to enlarge]

An examination of the MODIS SST product with overlays of RTMA surface winds (below) showed that there was no clear signature in the model wind field of enhanced convergence either before or after the mid-lake cloud band had formed.

Terra/Aqua MODIS Sea Surface Temperature product, with RTMA surface winds [click to enlarge]

Terra/Aqua MODIS Sea Surface Temperature product, with RTMA surface winds [click to enlarge]

However, an overpass of the Metop-A satellite at 1559 UTC provided ASCAT surface scatterometer winds that did a better job than the RTMA at highlighting the mid-lake convergence that was helping to sustain the cloud band (below). This example underscores the value that satellite-derived winds can have over even high resolution models.

Terra MODIS Sea Surface Temperature product, with RTMA surface winds and Metop ASCAT winds [click to enlarge]

Terra MODIS Sea Surface Temperature product, with RTMA surface winds and Metop ASCAT winds [click to enlarge]

Remnants of Post-Tropical Cyclone Florence north of Bermuda

September 20th, 2018 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with plots of Derived Motions Winds [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images and their Derived Motion Winds (above) revealed the partially exposed low-level circulation associated with the indirect remnants of Post-Tropical Cyclone Florence north of Bermuda on 20 September 2018 (surface analyses). The strongest Visible winds — calculated by tracking cloud features having a height assignment at or below the 700 hPa pressure level — located west and northwest of the circulation center were generally in the 35-40 knot range during the later part of the day, with one target being tacked at 56 knots (though this seemed to be an anomalous outlier).

However, ASCAT scatterometer data from an overpass of the Metop-A satellite at 1335 UTC (below) only sensed surface winds speeds (deduced from ocean surface roughness) as high as 25 knots around the center of the circulation.

Metop-A ASCAT surface scatterometer winds [click to enlarge]

Metop-A ASCAT surface scatterometer winds [click to enlarge]

Using a GOES-16 satellite-winds-derived 850 hPa Relative Vorticity product from the CIMSS Tropical Cyclones site (below), motion of the lower-tropospheric vorticity associated with Florence could be followed from landfall on 14 September to the current position north of Bermuda 6 days later. While the bulk of the vorticity became elongated (as Post-Tropical Cyclone Florence transformed into more of a weak baroclinic frontal wave over the Northeast US on 18 September: surface analyses), a small portion of the remnant 850 hPa vorticity became separated and then moved southeastward across the Atlantic.

GOES-16 Relative Vorticity product [click to play animation | MP4]

GOES-16 Relative Vorticity product [click to play animation | MP4]

Sea Surface Temperature and Ocean Heat Content [click to enlarge]

Sea Surface Temperature and Ocean Heat Content [click to enlarge]

A tropical Invest (98L) was initiated by the National Hurricane Center to gather additional data and more closely monitor this feature. Although the circulation had been moving over the Gulf Stream where warm Sea Surface Temperature and modest Ocean Heat Content existed (above), deep-layer wind shear was increasing over the area due to the approach of a branch of the polar jet stream (below).

GOES-16 Mid-level Water Vapor (6.9 µm) images, with deep-layer wind shear analyzed at 22 UTC [click to enlarge]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with deep-layer wind shear analyzed at 22 UTC [click to enlarge]

Although deep convection was displaced to the southeast of the low-level circulation center, the GOES-16 Total Precipitable Water derived product (below) showed that ample moisture remained in place farther to the northwest over the Invest 98L.

GOES-16 Low-level Water Vapor (7.3 µm) images + Total Precipitable Water derived product [click to play MP4 animation]

GOES-16 Low-level Water Vapor (7.3 µm) images + Total Precipitable Water derived product [click to play MP4 animation]

===== 21/22 September Update =====

GOES-16

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images (above) showed the cyclonic spin of Invest 98L as it moved south of Bermuda on 21 September.

On 22 September, the circulation continued to drift a bit farther south of Bermuda (below), a few hundred miles north of an area of Saharan Air Layer dust (discussed here) — note the hazy signature of the dust on Visible imagery, along with elevated Aerosol Optical Depth values of 0.6 to 0.7 having a good coverage of medium to high confidence Dust Detection.

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, along with Aerosol Optical Depth and Dust Detection products [click to play MP4 animation]