Potential Tropical Cyclone Sixteen (author: Madeline Scheinost)

The National Weather Service has issued storm surge warnings and tropical storm warnings across the Gulf Coast in anticipation of the development of Tropical Storm Nestor. The system is currently an invest region in the center of the Gulf of Mexico. The system is expected to reach tropical storm force in the morning hours of 18 October, and will likely make landfall near Panama City, FL in the early morning hours on 19 October 2019. The system is forecasted to move quickly across the Southeast before making its way to the Atlantic, shown in Figure 1. It is not expected to reach hurricane status, but instead become a sub-tropical storm. This means the system will have characteristics of both a tropical storm and a regular storm system. The main threats are flooding and strong winds. Parts of the Gulf Coast could see up to six inches of rain and two to five feet of storm surge.

Figure 1. National Hurricane Center released forecasted storm track from 12Z 18 October 2019. The image depicts the location of the center of the system over the Gulf of Mexico, and the anticipated motion of the storm along the cone.


Figure 2. Both images taken from NESDIS GOES-16 satellite image viewer. Image on the left is a visible satellite image of the system at 1321Z 18 October 2019. Image on the right is an IR Cloudtop image taken at 1326Z 18 October 2019.


Using the satellite images above (Figure 2), we can get an idea of the relative size and strength of the system as it makes its way towards the coast. There is no defined eye, indicating that the system is not well organized. Systems with a defined eye typically have hurricane force winds and are stronger in nature. However, we can note that there is strong convection in the center of the system. The black and white color in the IR image indicates high level cloud tops, which is a characteristic of strong convection. This indicates the system is strengthening, which we would expect to see as it is over a body of warm water.





CYGNSS: A Small but Mighty Satellite Fleet for Tropical Cyclone Research (author: Gigi Pavur)

Figure 1: One of eight microsatellites that comprise the Cyclone Global Navigation Satellite System (CYGNSS), which can be used for tropical cyclone research. (Source: NASA JPL)


While accurate forecasting of tropical cyclone intensity is inherently challenging, it is of high interest to society. Intensity predictions can be used to categorize a hurricane on the Saffir-Simpson Hurricane Wind Scale and prompt decision makers to issue storm warnings in a timely manner to protect lives and infrastructure. But with model biases, subjectivity, and limitations in analyses like the Dvorak Technique, Advanced Dvorak Technique, and ASCAT, how on Earth do meteorologists determine the intensity of a hurricane? Perhaps from space? (pun intended)

Microsatellites in low Earth orbit, which are only about the size of a microwave oven, offer an innovative and promising perspective for improving tropical cyclone intensity predictions. Researchers at NASA and the University of Michigan designed the Cyclone Global Navigation Satellite System, or CYGNSS. This eight microsatellite constellation, launched in December 2016, orbits the tropics at approximately 510km above the equator. Fig. 3 shows how the microsatellites deploy. Unlike traditional instruments such as the GOES Infrared Radiation and Visible products, CYGNSS detects reflected GPS signals which have L-band frequency. This long wavelength (~7.5 inches) is unobstructed by precipitation and atmospheric particles, meaning that CYGNSS can determine surface level sea roughness (which correlates to surface wind speeds) even within the most intense convection region of a hurricane: the eyewall. Surprisingly, CYGNSS is a passive satellite fleet. The GPS signal actually originates form GPS satellites in higher orbit, which are operated by the United States Air Force. Additionally, with an orbit period of only 95 minutes, each microsatellite passes within 12 minutes of the previous one. According to the mission’s Primary Investigator, Dr. Christopher Ruf, the data collected from CYGNSS is analogous to “a fleet of Hurricane Hunter airplanes distributed everywhere in the tropics.”


Figure 2: Comparison of tropical cyclone measurements from Advanced Dvorak Technique, CYGNSS, and a combination of products. When used in combination with other products, CYGNSS improves the overall model predictions. (Source: AGU Geophysical Research Letters)


In a paper published by AGU’s Geophysical Research Letters, when CYGNSS data from 2017 was combined with other tropical cyclone intensity prediction methods for hurricanes Irma and Harvey, the overall predictions improved. Fig. 2 shows wind speed and vector comparisons by the Advanced Dvorak Technique, CYGNSS, and a combination of methods, as well as surface latent heat flux. Since CYGNSS has only been in operation since 2017, researchers are still exploring and improving the applications of this small but mighty microsatellite fleet’s capabilities to improve tropical cyclone intensity forecasting.


Figure 3: Click the video to view how the eight microsatellites of CYGNSS deploy in low Earth orbit (Source: NASA)







“Weather: A Concise Introduction” by Gregory Hakim and Jérôme Patoux

Lumbering Lorenzo (author: Amy Weng)

While the United States has had some tropical storm events happen closer to home, there’s now a formidable cyclone building in the mid-Atlantic, and its name is Lorenzo. On September 26, 2019, Lorenzo is classified as a category 3 hurricane, and expected to strengthen to category 4 in the near future. Fortunately for readers, Lorenzo is not expected to reach the United States as a hurricane, or any other landmass. Even if Lorenzo doesn’t make landfall, it is a great opportunity to observe hurricane formation unimpeded by landmasses. The eye is particularly striking in Figure 1, standing out from the cloud outflow surrounding it due to the angle of the Sun at the time of viewing.

Fig. 1. GOES-16 full disc visible imagery with Lorenzo west of the African coast, taken 26 September 2019 at 17:20 GMT (or 1:20 PM EDT).


Lorenzo is projected to become a category 4 hurricane, but it’s also likely to be demoted back to a category 3 soon after. Taking a look at the Airmass RGB data from GOES-16 and Meteosat may help answer why – green regions represents moist, tropical airmasses and rusty orange regions represent dry airmasses, and Lorenzo’s got a dry orange region similar in size to itself just to its west (Fig. 2). I’ve taken imagery from two satellites in this case because Airmass RGB tends to wrap the horizon with a blue/purple hue due to how it detects water vapor. If Lorenzo continues west, it will have to interact with the dry air region, which could weaken its development by halting moisture intake. If Lorenzo moves north, it will have to contend with cooler water temperatures that could also weaken its development, as colder seawater will evaporate less readily.

Fig. 2. GOES-16 (left) and Meteosat 0 degree (right) full disc Airmass RGB composite imagery of Lorenzo taken 26 September 2019 at 21:00 GMT (or 5 PM EDT).


The good news is that Lorenzo in its current form likely won’t directly impact continental landmasses. The bad news (for Lorenzo) is its possible trajectories all lead to a weakening system in the near future. Incidentally, the dry air region to Lorenzo’s west could be a fragment of the Saharan Air Layer, a hot and dry airmass that advects out of the Sahara during the warmer months. The SAL isn’t always loaded with dust, but it often is, and some dust particles on Dust RGB imagery can be seen to the west of Africa at the time of this post (Fig. 3). We could have Africa to thank for stopping Lorenzo from getting out of control!

Fig. 3. Meteosat 0 degree Dust RGB over Africa, taken 26 September 2019 at 18:00 GMT. Dust is represented in pink hues.


GOES-16 full disc visible imagery can be found at https://whirlwind.aos.wisc.edu/~wxp/goes16/vis/goes16_fulldisk.html

GOES-16 full disc Airmass RGB imagery can be found at https://whirlwind.aos.wisc.edu/~wxp/goes16/multi_air_mass_rgb/goes16_fulldisk.html

Meteosat full disc Airmass RGB imagery can be found at https://eumetview.eumetsat.int/static-images/MSG/RGB/AIRMASS/FULLDISC/index.htm

Meteosat full disc Dust RGB imagery can be found at https://eumetview.eumetsat.int/static-images/MSG/RGB/DUST/FULLRESOLUTION/