The North Atlantic hurricane season on 2017 has proved to be both devastating and record-breaking so far.
By Jochen Kerkmann, Mark Higgins, Jose Prieto, Anne O'Carroll and Remko Scharroo (EUMETSAT), Ian Mills (Ian Mills Training), HansPeter Roesli (Switzerland) and Sancha Lancaster (Pactum), William Straka III (CIMSS).
Summer 2017 saw a number of strong hurricanes, up to Category 5 in strength, in the Atlantic, starting with Harvey in mid-August to Maria in mid September.
Hurricanes Irma, Jose and Katia can be seen on the GOES-16 Tropical Airmass RGB image, 6 September 08:00 UTC (Figure 2, source: CIRA).
In August parts of Texas were left devastated following severe floods caused by Hurricane Harvey. Harvey began as a tropical wave off the western coast of Africa on 13 August, but didn't become a fully formed hurricane in the Gulf of Mexico until 11 days later. It reached peak intensity as a Category 4 hurricane, with winds of 215 km/h (130 mph) and an atmospheric pressure of 938 hPa, as it made landfall in Rockport, Texas on 25 August.
Harvey is the wettest tropical cyclone on record in the contiguous United States. The resulting floods inundated hundreds of thousands of homes, displaced more than 30,000 people and killed around 70 people.
As this first major hurricane of the season hit Texas, another tropical wave was forming off the west African coast. Irma was classified as a Tropical Storm at 15:00 UTC on 30 August and with warm sea surface temperatures and low wind shear, strengthening was anticipated.
According to the National Hurricane Center early on 31 August, shortly after the development of a central dense overcast (CDO) and an eye feature, Irma rapidly intensified, with winds increasing from 110 km/h (70 mph) to 185 km/h (115 mph ) in just 12 hours.
On September 2, a ship to the west of the centre of Irma, recorded maximum winds of 70 km/h (45 mph). A strengthening subtropical ridge over the central North Atlantic pushed Irma from a western to southwestern direction on 2 and 3 September.
The first aircraft reconnaissance mission departed from Barbados on the afternoon of 3 September and discovered the eye was 47 km (29 mi) in diameter and the surface winds were 185 km/h (115 mph).
By 21:00 UTC on 4 September Hurricane Irma had strengthened into a Category 4 hurricane, with winds of 215 km/h (130 mph).
Favourable conditions helped deepen Irma and by 11:45 UTC on 5 September it was a Category 5 hurricane, with winds of 280 km/h,(175 mph), making Irma the easternmost Atlantic hurricane of this strength on record, surpassing Hurricane David in 1979.
Within a few hours Hurricane Irma had maximum sustained winds of 285 km/h (180 mph). By 00:15 UTC on 6 September, they'd reached 185 mph (295 km/h) and with a minimum pressure of 916 hPa — making Irma the strongest Atlantic hurricane since Hurricane Wilma (2005) in terms of sustained wind speed, and the most intense Atlantic hurricane since Hurricane Dean (2007). In addition, Irma was the strongest hurricane in the Atlantic basin outside of the Caribbean Sea and Gulf of Mexico in recorded history.
The eyewall of the hurricane moved over Barbuda near its record peak intensity overnight 5/6 September. Irma damaged or destroyed 95% of the structures on Barbuda, including its hospital, schools and both of its hotels. At least 13 people have died as a result of Irma. Figure 3 is the Suomi-NPP VIIRS Day Night Band (DNB) imagery of Irma as the eye passed over Barbuda on 6 September at 05:35 UTC.
The ASCAT instrument on Metop measures surface wind speeds and directions over the ocean. This is crucial for monitoring the formation and development of the storms and is used to pinpoint the storm centre.
Early on 8 September Irma made landfall on the Bahamian island Little Inagua and weakened into a Category 4 hurricane. But the hurricane regained Category 5 status 18 hours later as it hit Cuba, leaving a reported 10 dead.
At 13:10 UTC the next day, Irma made landfall in Cudjoe Key, Florida with maximum sustained winds of 215 km/h 130 mph (130 mph) and a central pressure of 929 hPa, bringing severe floods and large storm surges.
As Irma was rapidly intensifying on 31 August, yet another tropical wave was seen off the west African coast and another typical Cap Verde–type cyclone was forming. Jose was designated as Tropical Storm Jose at 15:00 UTC on 5 September, when the system was about 2,420 km east of the Lesser Antilles.
With the favourable conditions of warm water temperatures, low wind shear, and abundant moisture Jose intensified into a Category 3 hurricane on 7 September, with maximum sustained winds of 195 km/h (120 mph) and a minimum barometric pressure 966 hPa.
However, by 9 September it had moved away from the northern Leeward Islands of the Caribbean archipelago, and the warnings were downgraded to lesser alerts.
For days Hurricane Jose was in the loop over the Atlantic.
Similar to Hurricane Harvey, Hurricane Katia formed in the Gulf of Mexico and became a Category 1 hurricane, and the season's 11th hurricane, on 7 September.
Katia made landfall north of Tecolutla, Mexico late 8 September, bringing winds of 100 km/h (75 mph), killing at least two people.
By 9 September, it had deteriorated to a tropical storm, with winds dropping to around 72 km/h (40 mph).
Maria was classed as a Tropical Storm on 16 September and then rapidly intensified to a hurricane at 21:00 UTC on 17 September.
The two GOES-16 ABI animated gifs from 17 September, 16:30–17:30 UTC — near-infrared, band 06 (NIR2.3) and infrared band 13 (IR10.4) — show Maria before it strengthened. Close to the centre of the hurricane, a massive area of convection can be seen (almost like a blob), producing very low temperatures in band 13 (IR10.4) and high reflectivity in band 06 (NIR2.3).
What the new near infrared channels of ABI will contribute to the monitoring and classification of hurricanes is currently unknown, but the importance of the new NIR1.3 band is discussed below (in the section "forecasting and tracking hurricanes").
The hurricane continued to intensify, going from a Category 1 to a Category 4 hurricane east of Le Lamentin, Martinique. By 00:00 UTC on 19 September it had been upgraded to Category 5 intensity as the eye was just east of Dominica.
Maria made landfall in Dominica at 01:35 UTC on 19 September, with winds of 260 km/h (160 mph), causing widespread destruction and at least seven deaths.
Figure 4 is the Sentinel-3 OLCI RGB, 19 September 14:03 UTC, while the hurricane was over the islands of Dominica.
Maria went on to cause further destruction in Guadeloupe, Martinique, the Virgin Islands and Puerto Rico.
Forecasting and tracking hurricanes
In order to track storms such as these, weather forecasters use a variety of observations. Satellite information is very important for tracking and determining intensity trends of hurricanes and other tropical storms. When a hurricane is well offshore and out of effective radar range, forecasters use satellite imagery to continuously track the storm’s movement and development.
The imagery gives information about the top of the storm. Satellites can also give information about the winds speeds over the ocean surface. Forecasters are able to use satellite imagery over the west of Africa to spot the initial formation of the storms, before they even reach the classification of Tropical Depression.
Figure 5 shows Hurricane Irma on 5 September, before it hit the Lesser Antilles. Note the big difference as regards cloud position, due to the enormous parallax shift between the two satellites.
Note also that the high level clouds of Irma are over-enhanced in the standard airmass RGB, see the strong whitish colour. A special tuning of the Airmass RGB is needed to monitor high-level clouds and overshooting tops in tropical cyclones.
The 'tropical' Airmass RGB from GOES-16 alleviates this problem by using ranges more appropriate for cold, high clouds. In particular, for the green range (IR9.6–IR10.3) it uses a range from -25 to +25 K (instead of -40 to +5 K).
This makes this RGB very suitable for detecting overshooting tops (white) and other cloud top structures — in this case the spiral banding of Hurricane Irma.
Being able to see when a tropical cyclone is forming, and to continuously track where it is heading, means more timely warnings can be issued and mitigating actions taken.
Forecasters also pay attention to the sea surface temperature. As a cyclone travels over water, if it passes over warmer water it will pick up energy and intensify, and travelling over cooler water will weaken it.
The temperature of the sea surface is monitored from satellites. For a cyclone to form the sea surface temperature needs to be at least 26 °C.
The AVHRR Sea Surface Temperature (SST) anomaly versus climatology imagery from 3, 4 and 5 September (Figure 6) shows the high SST anomalies approaching and the cool wake where the energy has been taken up by Hurricane Irma.
Figure 7 shows Hurricanes Irma, Jose and Harvey as seen from Sentinel-3 SLSTR infrared 12 µm brightness temperature (S9 channel). Cold temperatures below 190 K can be seen in all three storms. The eye is well defined in the image of Jose, with the warmer temperatures from the underlying ocean observed through the storm.
As in the case of Tropical Cyclone Dora (the first hurricane of the 2017 eastern Pacific hurricane season), we also looked at some new bands of ABI, namely the NIR1.3 band (band 04) and the NIR2.3 band (band 06) to get a feeling of what these bands could contribute to tropical cyclone analysis and forecasting.
The animation of the NIR1.3 band (Figure 8) shows Irma on 4 September, a day before the above Airmass RGB images.
The centre (eye) of Irma can be well seen, as well as the radiating cirrus clouds, the deep convective storms in the spiral bands, and the outer (spiral) cirrus outflow.
As this band does not see the lower level clouds, it gives a better contrast for these features than the standard solar bands, like the high resolution VIS0.6 band (band 02).
This is demonstrated in the animated gifs in Figure 9 and 10, which show the VIS0.6 and the NIR1.3 band on 5 September.
Animated gif, Band 02 (VIS0.6)
Animated gif, Band 04 (NIR1.3)
As regards the NIR2.3 micron band (Figure 11), it is interesting to note that this band (alone) is not very useful for cloud phase detection: unlike the NIR1.6 micron band (Figure 12), water and ice clouds have nearly the same reflectivity in this band. This can be best seen when you look at the water clouds in the eye of Hurricane Irma.
Animated gif, Band 06 (NIR2.3)
Animated gif, Band 05 (NIR1.6)
However, when this new band is used together with the NIR1.6 band, then cloud phase retrieval improves. This led to the invention/creation of the ‘cloud phase’ RGB (not shown here) which uses both bands, the NIR1.6 and the NIR2.3 bands to observe cloud properties, in particular clouds phase (see example in the case study Giant convective cells in centre of decaying Typhoon In-fa seen by Himawari-8).
Forecasters also use satellite instruments such as altimeters, instruments that can measure ocean surface wind speed and significant wave height.
The animated gif in Figure 13 shows a combination of infrared imagery of Hurricanes Katia, Irma and Jose, along with altimeter observations of wind speed and wave height by satellite altimetry.
The images are each shifted 8 hours in time, starting with 7 Sep 12:15Z. In the background is the GOES East image from NOAA for that moment in time.
Overlaid are the altimeter tracks of Jason-2 (JA2), Jason-3 (JA3), Sentinel-3A (S3A), CryoSat-2 (CS2) and SARAL/AltiKa (SRL) that crossed the same region between four hours before and four hours after the central time of the image.
The coloured strips indicate measured wind speeds (exceeding 80 km/h in some cases). The yellow 'comb' along the track indicates wave height. The width corresponding to 10 m is indicated in the legend.
On the pass of 7 September 20:15 UTC CryoSat-2 was crossing over the eye of Irma (at 20:22 UTC). At the edge of the eye, wind speeds of 50 m/s and wave heights of 12 m were measured (far beyond the requirement of the mission). As expected, inside the eye of the storm wind speeds and wave heights dropped to 30 m/s and 7 m/s respectively.
The other images show Jason-2, Jason-3 and Sentinel-3A passing near the centres of Katia, Irma, and Jose.
Note: NOAA's GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.
Hurricane Irma's progression on the CIMMS Blog
Hurricane Maria's progression on the CIMMS Blog
GPM Satellite Probes Dangerous Category Five Hurricane Irma (NASA)
Four days of Irma's eye, seen by GOES-16 ABI (NASA SPoRT/Twitter)
Stunning Photos from Space Show Florida Lights Before and After Irma (The Weather Channel)
National Hurricane Center
Hurricane Irma: 10 dead in Cuba as record flooding hits northern Florida – latest updates (The Guardian)
While Irma takes aim on Florida, Hurricanes Jose and Katia are also threatening land (USA Today)
Hurricane Irma kills 9 in Caribbean islands as Florida awaits storm (CNN)
Photos: Hurricane Harvey slams Texas (CNN)