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]

Severe thunderstorms in Iowa and Minnesota

September 20th, 2018 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with SPC storm reports plotted in red [click to play MP4 animation]

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed thunderstorms that developed in the vicinity of a surface low and associated cold front (surface analyses) that was moving northeastward across the Upper Midwest during the afternoon and evening hours of 20 September 2018. These storms produced damaging winds and several tornadoes across northern Iowa and southern Minnesota (SPC storm reports | NWS FSD summary | NWS MPX summary). An animation of Visible images with hourly plots of surface reports is available here.

GOES-16 “Clean” Infrared Window (10.3 µm) images (below) showed cloud-top infrared brightness temperatures as cold as -70 to -75ºC (black to light gray enhancement) with the more vigorous overshooting tops. These thunderstorms continued moving eastward as a squall line, causing additional damaging winds across northern Wisconsin.

GOES-16

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

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]

NUCAPS views Saharan Air over the Atlantic

September 20th, 2018 |

Saharan Air Layer Analysis over the Tropical Atlantic, 0600 UTC on 20 September 2018 (Click to enlarge)

There have been many episodes of Saharan Air over the tropical Atlantic within the past months, and another episode is in progress on 20 September. The Saharan Air Layer (SAL) analysis, above, from the CIMSS Tropical Weather website (Direct Link), shows dry air north and east of the Caribbean. The Clean Window ABI Band 13 (10.3 µm) Full-Disk ABI infrared imagery, below, from 0500 UTC, overlain with NUCAPS sounding points, shows where data were available from that morning overpass of Suomi NPP.

GOES-16 ABI 10.3 µm Infrared Imagery at 0500 UTC along with NUCAPS Sounding Points at approximately the same time (Click to enlarge)

The stepping animation below shows NUCAPS Soundings at a selection of points that starts north of the Saharan Air Layer and ends up within the SAL. The underlying figure is the Dust RGB from AWIPS, an RGB that combines the Split Window Difference (12.3 µm -10.3 µm; Red Component), Split Cloud Top Phase Brightness Temperature Difference (11.2 µm – 8.5 µm; Green Component) and 10.3 µm Infrared Imagery (Blue Component). Typically, regions with dust as might accompany a SAL have a pink tinge. The soundings are annotated to include Total Precipitable Water measurements, and mid-level Relative humidity. NUCAPS soundings identify the region where the SAL is present.

Dust RGB at 0433 UTC north and east of the Caribbean, and NUCAPS Soundings at selected points along a transect (Click to enlarge)

The SAL air continued its movement west during the day on 20 September.  The toggle below shows the Dust RGB, ABI Band 3 (0.86 µm) and the Baseline Aerosol Detection Product (in blue) at about the same time as the afternoon NUCAPS Sounding overpass (from Suomi NPP).  Suomi NPP overflew the eastern half of the SAL air (the overpass from NOAA-20 was more centered on the SAL air approaching the Caribbean, but NOAA-20 NUCAPS soundings are not yet in AWIPS;  they should be by the end of the year).

GOES-16 ABI Dust RGB, “Veggie Band” (Near-Infrared at 0.86 µm), and Baseline Aerosol Detection Product (Blue points), 1615 UTC on 20 September 2018 (Click to enlarge)

NUCAPS Soundings at 3 points (North of the SAL, within the SAL, and south of the SAL), below, show much different thermodynamics within the SAL.

NUCAPS Profiles at ~1600 UTC on 20 September 2018 at three locations as noted (Click to enlarge)

NOAA’s G-IV flew through this outbreak, deploying dropsondes to sample the event. The path of the aircraft (with the dropsonde locations) is here. Sonde #26, below, in the heart of the SAL, is shown below, with a nearby NOAA-20 NUCAPS sounding. (Flight path and Sonde imagery courtesy Chris Barnet, STC/NOAA) Refer to the caption for details.  Recall that the Dropsonde shows values at a point.  The NUCAPS profile is sampling a volume that is approximately a 50-km cylinder!  There is nevertheless excellent agreement.

Dropsonde #26 data (raw data in light grey; values averaged into the 100 NUCAPS vertical layers in black); GFS sounding in magenta. NUCAPS Microwave-only sounding in green; NUCAPS Microwave and infrared retrieval (as might be seen in AWIPS) in Red. Time offset from the Dropsonde is noted (Click to enlarge)

SAL outbreaks cause a significant deterioration in air quality over the Caribbean. The image below, courtesy Ernesto Rodriguez, SOO for the National Weather Service office in San Juan, Puerto Rico, compares Air Quality before and during a SAL outbreak in July, and during the current outbreak.

The view outside of the National Weather Service office in San Juan on 20 September and 13 July 2018 (during SAL outbreaks) and on 12 July 2018 (before a SAL outbreak). Imagery courtesy Ernesto Rodriguez, NWS SJU.