NOAA-20 VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images (above) showed cold overshooting tops (darker black infrared enhancement) over the Leeward Islands as well as subtle mesospheric airglow waves propagating southward away from the center of Tropical Storm Dorian at 0606 UTC on 28 August 2019.

In a toggle between GOES-16 (GOES-East) “Clean” Infrared Window (10.35 µm) and DMSP-18 SSMIS Microwave (85 GHz) images from the CIMSS Tropical Cyclones site (below), the Microwave image revealed a convective band that was wrapping around the northern portion of the center of Dorian at 0930 UTC.

1-minute Mesoscale Domain Sector GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (below) also showed a convective burst wrapping around the eastern and northern edges of the center of Dorian after 15 UTC. The coldest cloud-top infrared brightness temperature associated with that early convective burst was -83ºC.

Dorian was upgraded to a Category 1 Hurricane at 18 UTC. Prior to that time, the tropical cyclone had been moving through an environment of low deep-layer wind shear (below), one factor that is favorable for intensification. Dorian was also passing over water possessing warm sea surface temperatures and modest ocean heat content.

VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP as viewed using RealEarth are shown below, from around the time when Dorian was upgraded from a Tropical Storm to a Hurricane.

A comparison of GOES-16 Infrared (at 2330 UTC) and GMI Microwave (at 2341 UTC) images (below) revealed Dorian’s small eye.

===== 29 August Update =====

On 29 August, 1-minute GOES-16 Visible and Infrared images (above) showed that periodic convective bursts persisted around the center of Category 1 Hurricane Dorian.

During one of those convective bursts from 1800-1900 UTC, an increase in GOES-16 GLM Flash Extent Density was evident (below).

GOES-16 Visible and Infrared images at 1852 UTC with and without an overlay of GLM Flash Extent Density are shown below. At that particular time, the overshooting top infrared brightness temperature reached a minimum value of -82.5C.

===== 30 August Update =====

The eye of Dorian became more well-defined in 1-minute GOES-16 Visible and Infrared images (above) during the morning hours on 30 August.

A DMSP-17 Microwave (85 GHz) Microwave image at 1141 UTC (below) did not yet show a completely closed eyewall structure at that earlier time.

Dorian was upgraded to a Category 3 hurricane at 18 UTC — the storm was moving into a narrow corridor of weaker deep-layer wind shear around that time. During the 3 hours leading up to 18 UTC, animations of 1-minute GOES-16 Visible and Infrared imagery — with and without an overlay of GLM Flash Extent Density — are shown below.

===== 31 August Update =====

Overlapping 1-minute GOES-16 Mesoscale Domain Sectors provided imagery at 30-second intervals — Visible and Infrared animations of the Category 4 hurricane from 1430-1900 UTC are shown above and below, respectively. A longer Visible animation from 1100-2259 UTC is available here (courtesy of Pete Pokrandt, AOS).

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The animation above shows the 1721 UTC GOES-16 Visible (0.64 µm) image along with NUCAPS profile locations from a NOAA-20 overpass. Convection is approaching from the west, from central Wisconsin. NUCAPS soundings can give a good estimate for how far south that convective line might develop, and a north-south series of profiles is shown in the imagery above.  Note in particular how soundings show increasing mid-level stability;  a strong inversion between becomes apparent between the NUCAPS Sounding just south of Door County on the western short of Lake Michigan and over eastern Lake Michigan on the Michigan shoreline.  This thermodynamic snapshot would argue that convection should not develop much farther south than central Lake Michigan!  the 1926 UTC Visible image, below, toggled with radar, confirms this forecast.

 

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NUCAPS from one satellite will periodically, north of about 40 N, supply profiles on two consecutive passes.  That happened on 27 August over Lake Michigan as might be expected given that the 1718 UTC pass had its westernmost swath over Lake Michigan.  The animation below shows the swath from 1901 UTC.  The strengthening inversion as you move south over Lake Michigan is apparent at 1901 UTC as well.

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A sequence of 4 consecutive Suomi NPP VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.4 µm) images (above) showed a small swirl of clouds associated with a weak area of low pressure near the North Pole — north of Greenland (surface analyses) — on 22 August 2019.

There were Suomi NPP NUCAPS soundings available in the vicinity of the surface low (above) — profiles from the 4 squared green dot locations (green dots indicate successful sounding retrievals from both the CrIS and ATMS instruments) which were closest to both the surface low and the North Pole (below) revealed characteristically-low arctic tropopause heights of around 7-8 km, and surface temperatures dropping to below freezing at the 2 most northerly points of 88.28º and 88.57º N latitude. Note: the Suomi NPP (SNPP) CrIS anomaly that began on 24 March 2019 was resolved via a switch to the redundant Side-2 electronics on 24 June — so CrIS data once again became available for incorporation into SNPP NUCAPS soundings beginning on 01 August. Training material for NUCAPS in AWIPS is available here.

According to GCOM-W1 AMSR2 data (source), this weak surface low was over a portion of the Arctic Ocean where sea ice concentration was still high (below).

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The Split Window Difference field (10.3 µm – 12.3 µm), shown above in the south Pacific around Samoa and American Samoa (Leone is on the island of Tutuila just west of 170º W Longitude; Fitiuta is on the island of Ta’u just east of 170º W Longitude), can be used to estimate the horizontal distribution of water vapor. The Split Window Difference can give a good estimate of moisture distribution in the atmosphere over the ocean where conventional moisture measurements are limited. The image above shows greater values (3.5 – 4 K, in yellow and orange) over the northern part of the image and smaller values (2-3 K, in yellow and blue) over the southern part of the image, divided by a band of cloudiness that passes through 20º S, 170º W.

NOAA-20 overflew this region at 0056 UTC, and NUCAPS profiles were available, as shown below.

The animation below steps through soundings at different locations. Total precipitable water as determined from the sounding is indicated. In the region where the Split Window Difference field was around 4 K, precipitable water values were in the 1.5-1.7″ range; in regions where the Split Window Difference was closer to 2 K, precipitable water values were closer to 0.5-0.75″.

Microwave-only data, shown below from the MIMIC website, shows a sharp gradient at 20º S, 170º W.

At ~1200 UTC, when NUCAPS again passed over this region, profiles could again be used to discern gradients in total precipitable water.  At that time, however, the Split Window Difference field was not computed because warming of the Advanced Baseline Imager (ABI) associated with the sub-optimal performance of the Loop Heat Pipe meant that Band 15 data were not available.  (Baseline Level 2 Products, such as total precipitable water, are also unavailable from GOES-17 because of the Loop Heat Pipe issue) The Split Window Difference field could be computed from Himawari-8 data however.

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