Paper on how Hurricane Michael (2018) intensified rapidly in the Gulf of America published in Monthly Weather Review
Weather forecasters use computer models to make predictions. These forecasts of the future depend on knowing what is happening now, what we call initial conditions. However, we can’t measure the weather at every location on earth all the time, so we don’t know exactly what these initial conditions are. Therefore, we run forecast models many times using different initial conditions that are similar to what we think is happening now, and we call this large set of forecasts an ensemble. In this paper, a version of a model that forecasts the weather over the entire globe, NOAA’s finite-volume cubed sphere, or FV3, model is used to study the strengthening of Hurricane Michael in the Gulf of America. The model is run at high resolution over the Atlantic and Gulf of America, meaning that the distance between points it uses to calculate forecasts is about 3 km (just under 2 miles). The model is run 40 times, and difference between the forecasts that correctly show the storm strengthening versus those that did not, is studied. This tells us how Michael may have been able to intensify despite the strong upper-level wind shear (the difference in wind velocity between the top and the bottom of the hurricane), which usually weakens hurricanes. The computer forecasts of Michael’s wind and rainfall were compared to observations from NOAA P3 Hurricane Hunter aircraft.
■ Important Conclusions:
1. Because of wind shear, rainfall tends to occur on different sides of the storm. In the forecasts that correctly predicted that Michael would intensify rapidly, it isn’t until the rainfall completely surrounds the hurricane’s center, not just on one side of the storm, does rapid intensification occur. The top three panels of the figure (above) show, on the left, what a weather radar sees when the rain surrounds the center (in the center of the black circles). In the middle panel, we see what it looks like when the rain does not surround the center. The right panel shows what the radar on the NOAA Hurricane Hunter aircraft saw at that time, with rain surrounding the center.
2. Similarly, the wind blowing through the hurricane at different levels is different; this pushes the center of the hurricane in different directions, so the center is not in the same place. In the forecasts that correctly predicted that Michael would intensify rapidly, it isn’t until the centers are in the same place at different heights that rapid intensification of Michael began. The bottom three panels show the wind in Hurricane Michael at different levels (red about 1 mile above the surface, and blue about 3.5 miles above the surface). In the left panel, you can see the centers are in about the same place at the two levels. In the middle panel, the centers are in different places at the two levels. The wind data from the Doppler radar on the NOAA Hurricane Hunter aircraft show that the two centers were in nearly the same place.
3. Observations from NOAA Hurricane Hunter aircraft showed that these two things (the center in the same place with height and rainfall surrounding the center) occurred in reality in Hurricane Michael as it began its rapid intensification. These two things did not occur in the forecasts that did not predict as much intensification.
