Summary: The main model that forecasters use to predict what a hurricane will do is the Hurricane Weather Research and Forecast (HWRF) model. Because we cannot measure what is currently happening in the hurricane exactly, we run the model many times at once with different measurements to get an idea of the different forecasts that are possible, and this set of runs is called an ensemble and the individual runs are called members. In this study, forecasts from an HWRF ensemble for Hurricane Edouard (2014) are studied to learn the differences between hurricane that intensify rapidly, what we call rapid intensification (RI), and those that do not (NI).
Important Conclusions:
- Hurricanes are made up of thunderstorms, what we call convection. The strongest thunderstorms in the hurricane are called convective bursts (CBs). In hurricanes undergoing RI, CBs move around the hurricane center, but they stay in the same place for the NI members.
- If the wind around the top but outside the hurricane moves the same way the hurricane wind moves (for example, counter-clockwise north of the Equator), this allows the CBs to move around the hurricane center and undergo RI. If that wind blows in the opposite direction, the hurricane will not undergo RI.
- The center of the hurricane is not in exactly the same place from the surface to the top, what we call tilt. Because of this tilt, strong wind in the middle and top of the hurricane can flow over the center at the surface. This wind can push strongly spinning wind over the surface center and increase the spinning wind and reduce the tilt in hurricanes undergoing RI. In contrast, in NI members, this does not happen. Therefore, this increase in the spinning wind is a key ingredient for RI.
- Shear is the difference in wind between low and high levels of the atmosphere around the tropical cyclone. The direction this shear points toward what we call the downshear part of the hurricane. The RI member have more moisture in the part of the storm to the right of the downshear part than the NI members, so this is important for the formation of CBs in the RI members.
Read the article at http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-17-0171.1
