Radiative Feedback From Dry Environmental Air Accelerates Tropical Cyclogenesis

A number of recent studies have highlighted how radiative feedback from clouds accelerates tropical cyclone (TC) development by amplifying spatial gradients in radiative heating. This study extends this work by examining how spatial gradients in free tropospheric moisture influence TC development through their impact on environmental radiative heating. We conduct a series of idealized model experiments in which only the longwave radiative heating due to water vapor is modified in the environmental region outside of the TC area. These experiments demonstrate that a vortex in a drier environmental free troposphere experiences faster development. Moreover, weaker vortices actually require a dry environmental free troposphere to develop. The accelerated genesis mainly results from a stronger spatial gradient in moisture‐induced radiative heating, which enhances energy convergence through a stronger transverse circulation. These results highlight a potentially important and overlooked role of dry air in facilitating TC genesis. Plain Language Summary Recent studies have shown that clouds play an important role in making hurricanes stronger by affecting how heat is distributed. This study builds on that work by looking at how moisture in the environmental air away from the storm center also influences hurricane development through its impact on heat distribution. Computer simulations in which only the heating due to the absorption of infrared heat by water vapor outside the hurricane area was varied showed that hurricanes with drier environmental air develop faster. Further analysis revealed that this faster development is mainly because of a stronger difference in how heat is distributed due to moisture. Our results suggest that dry air might play a crucial and overlooked role in helping hurricanes form. Key Points Dry environment accelerates tropical cyclone genesis through moisture‐induced gradients in radiative heating A dry environmental free troposphere enhances the radiative heating gradient between TC area and its surroundings The moisture‐induced radiative feedback enhances the energy convergence and is critical to TC genesis in the early stages of development