There is mounting evidence that warming oceans caused by climate change could intensify the destructive power of tropical cyclones, which include hurricanes, typhoons, and tropical storms. Yet scientists don’t understand many basic features of these storms. In a new study, Ruppert and O’Neill tackle a long-standing mystery about tropical cyclones that could improve forecasts of hurricane behavior.
Tropical cyclones form over oceans, drawing energy from warm, moist air rising off the sea surface. As the unstable air rises, more warm, moist air takes its place and rises too. Under the right conditions, the system starts to spin and grow, forming bands of massive towers of cumulonimbus clouds that can dump many inches of rain, topped by flat, anvil-shaped clouds.
A curious feature of these cloud formations is that they transform from day to night, following the diurnal cycle, due to their interaction with incoming and outgoing radiation. Rainfall increases overnight, when cloud tops radiate energy to space and cool, dumping the most rain in the early morning. Yet satellite cloud observations tell us that the anvil clouds reach their greatest heights during the afternoon. This roughly 12-hour offset goes against archetypal models of tropical convection, which suggest that the most rainfall should occur at around the same time that the clouds reach their greatest heights.
To explore this phenomenon, the authors attempted to re-create the offset using computer simulations of tropical cyclones. They found that the peak in anvil cloud top height occurs during the day because the Sun’s short-wave radiation heats the thick canopy of clouds, causing both the clouds and the stream of outward flowing air to lift. The lifting begins at the inner core of the tropical cyclone, where the clouds are thickest and radiative warming is strongest, they found. At night, while the storm continues to be driven by thermal energy released by the ocean, the clouds become bottom heavy and produce more rain. Understanding this daily cycle could help improve rainfall forecasts upon landfall, the authors note, while it resolves a long-standing mystery of cloud behavior in tropical cyclones.(Geophysical Research Letters, https://doi.org/10.1029/2018GL081302, 2019)
—Emily Underwood, Freelance Writer