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Abstract The properties of diurnal variability in tropical cyclones (TCs) and the mechanisms behind them remain an intriguing aspect of TC research. This study provides a comprehensive analysis of diurnal variability in two simulations of TCs to explore these mechanisms. One simulation is a well-known Hurricane Nature Run (HNR1), which is a realistic simulation of a TC produced using the Weather Research and Forecasting (WRF) Model. The other simulation is a realistic simulation produced using WRF of Hurricane Florence (2018) using hourly ERA5 data as input. Empirical orthogonal functions and Fourier filtering are used to analyze diurnal variability in the TCs. In both simulations a diurnal squall forms at sunrise in the inner core and propagates radially outward and intensifies until midday. At midday the upper-level outflow strengthens, surface inflow weakens, and the cirrus canopy reaches its maximum height and radial extent. At sunset and overnight, the surface inflow is stronger, and convection inside the RMW peaks. Therefore, two diurnal cycles of convection exist in the TCs with different phases of maxima: eyewall convection at sunset and at night, and rainband convection in the early morning. This study finds that the diurnal pulse in the cirrus canopy is not advectively driven, nor can it be attributed to weaker inertial stability at night; rather, the results indicate direct solar heating as a mechanism for cirrus canopy lifting and enhanced daytime outflow. These results show a strong diurnal modulation of tropical cyclone structure, and are consistent with other recent observational and modeling studies of the TC diurnal cycle.
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This content will become publicly available on July 1, 2026
Reconsideration of the Mass and Condensate Sources for the Tropical Cyclone Outflow
Abstract The widely accepted view of the secondary circulation of a mature tropical cyclone (TC) consists of boundary layer inflow that turns upward through the eyewall and then turns outward to form the outflow layer and the cirrus shield. This view can be traced to schematics shown in several foundational studies of TCs and persists in both the peer-reviewed and popular literature in numerous diagrams and cartoons. Updrafts in rainbands are nearly always depicted as not supplying the primary outflow. However, examination of the mass and moisture budgets of the cirrus outflow shield—i.e., the outflow layer from about 100- to 300-km radius—in mesoscale model simulations of hurricanes reveals a different picture. A significant fraction of the dry airmass flux (varying widely but around 50%) and even larger fraction of the condensate in the outflow comes from rainbands. The mass flux from the eyewall is limited by its small size, and condensate is falling out rapidly. Instead, the condensate shield and outflow mass flux are significantly supplied by deep convection in the surrounding rainbands. These findings are consistent with the recently developed appreciation of the diurnally forced rainband complexes that have been shown to expand the cirrus shield. The simulations show that moist air and condensate can be lifted into the outflow in either narrow convective towers or in mesoscale ascending updrafts, and these features can be found in airborne Doppler radar observations. These findings update our understanding of the physical significance of changes in size and thickness of the cirrus shield. Significance StatementTropical cyclones are recognized from satellite images of their high clouds that spiral outward from the storm center. The size and evolution of this outflow are used by experts and algorithms to estimate the intensity and future behavior of these storms. Conventional wisdom holds that the overwhelming source of these high-altitude clouds is the upward transport of moisture in thunderstorms around the calm center. Computer simulations of tropical cyclones and radar observations taken by aircraft show that in fact most of these clouds come from thunderstorms in the surrounding rainbands. These findings highlight the importance of the rainband convection in controlling the size and thickness of the outflow clouds, which in turn inform our estimates of storm intensity.
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- Award ID(s):
- 2334173
- PAR ID:
- 10657249
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Bulletin of the American Meteorological Society
- Volume:
- 106
- Issue:
- 7
- ISSN:
- 0003-0007
- Page Range / eLocation ID:
- E1342 to E1359
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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