It has been believed that tropical cyclones are likely to strengthen when strong thunderstorms (called convection) almost completely surround the storm’s center. In order to better understand what prevents this convection from surrounding the storm’s center in some storms (but not others), this study analyzes data collected on NOAA aircraft flights in two storms that occurred in the Atlantic Ocean in 2014.
1. When rain falls from in convection, it can bring air down with it, causing strong downward currents of air (called downdrafts). Since the temperature goes down with height, these downdrafts bring cool, dry air down to the ocean surface. This cool, dry air lowers the amount of energy available for convection to form over half the storm, making it more difficult for convection to surround the storm’s center.
2. To replace air rising in the convection, air slowly descends in the rain-free parts of the storm. When air descends, it warms and dries, creating a “lid” that stops new thunderstorms from growing there.
3. Wind can also bring in dry air from outside of the storm directly into it. This dry air can also limit thunderstorms that try to form in the rain-free parts of the storm.
You can read the paper at http://journals.ametsoc.org/doi/abs/10.1175/MWR-D-17-0073.1.