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Abstract The diurnal cycle (DC) in the cirrus canopy of tropical cyclones (TCs) is a well-documented phenomenon. While early studies linked the DC in the area of the cirrus canopy to a DC in the strength of eyewall convection, later studies considered it a direct response to the DC of radiation in the cirrus canopy. In this study, an idealized linear model is used to examine the extent to which linear dynamics can capture the DC in TCs, in particular the transition between balanced and radiating responses to diurnal heating. The model heat forcing is physically motivated by the diabatic heating output from a realistic simulation, which illustrates the presence of a DC in moist convective heating and radiative heating in the eyewall, and a DC in radiative heating in the cirrus canopy. This study finds that the DCs of heating in the eyewall yield a response that is restricted to inside the RMW by the high inertial stability in the inner core. The DC of radiative heating in the cirrus canopy yields a response throughout the entire cyclone. Lower-frequency responses, of diurnal and semidiurnal frequency, are balanced throughout much of the cyclone. High-frequency waves with periods under 8 h, created at sunrise and sunset, can radiate outward and downward. These results indicate that diurnal responses are balanced in the majority of a TC and originate in the cirrus canopy, instead of the eyewall. The DC in cirrus canopy vertical motion also appears to originate in the cirrus canopy.
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The Spatiotemporal Evolution of the Diurnal Cycle in Two WRF Simulations of Tropical Cyclones
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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- Award ID(s):
- 1654831
- PAR ID:
- 10367604
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 79
- Issue:
- 4
- ISSN:
- 0022-4928
- Format(s):
- Medium: X Size: p. 1021-1043
- Size(s):
- p. 1021-1043
- Sponsoring Org:
- National Science Foundation
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