Diurnal Cloud and Circulation Changes in Simulated Tropical Cyclones.

Observations of the diurnal cycle in tropical cyclones (TCs) systematically indicate a ∼12‐hr offset between peak rainfall rate and the maximum height of anvil clouds in the TC cloud canopy. This phasing conflicts with archetypal models of organized deep convection, which suggest a tight coupling between rainfall, vertical cloud growth, and anvil clouds. We show that this phasing owes to the bimodal diurnal evolution of the transverse circulation, which peaks nocturnally from low–midlevels, and during daytime in the upper troposphere. The bottom‐heavy nocturnal circulation state is driven by latent heating from nocturnally invigorated deep convection, while the top‐heavy daytime state is the thermally direct circulation response to strong shortwave‐cloud warming in the optically thick TC cloud canopy. This daytime upper‐level circulation response manifests in a lifting of the maximum height of the TC outflow and, in turn, a lifting and invigoration of the upper‐level anvil clouds of the TC cloud canopy. Plain Language Summary: We typically expect the rainfall, deeply growing thunderstorm clouds, and outward‐spreading anvil clouds in tropical cyclones (e.g., hurricanes) to closely coincide in time—that is, when rainfall peaks, we expect deep clouds to be vigorously growing, and anvil clouds in the upper‐level cloud canopy to be spreading outward and at their greatest height. Previous observational studies, however, indicate that in the 24‐hr diurnal cycle, anvil clouds behave distinctly from the deep thunderstorm clouds that produce the rain: while rainfall and deep clouds peak overnight, the height and spreading of the anvil clouds peaks during the day. This study indicates that this distinct behavior of anvil clouds owes to the interaction between solar radiation and ice clouds. Namely, the abundant ice crystals in anvil clouds absorb much of the incoming solar radiation, leading to strong warming there. This daytime warming, in turn, enhances the upward motion at the height of these anvil clouds, causing the storm's upper‐level outflow to lift. The lifting of outflow promotes the anvil clouds to lift and spread. This lifting of anvil clouds in the outflow maximizes in the early afternoon, even though rainfall is suppressed during daylight hours. Key Points: Solar heating of optically thick canopy clouds lifts outflow in the afternoonThis afternoon outflow lifting explains the afternoon peak in cloud top heightThe diurnal variation of insolation does not impact the timing of spontaneous cyclogenesis in RCE [ABSTRACT FROM AUTHOR]