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Discussion papers
https://doi.org/10.5194/acp-2019-569
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-569
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Aug 2019

Submitted as: research article | 23 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

A numerical modelling study of the physical mechanisms causing radiation to accelerate tropical cyclogenesisand cause diurnal cycles

Melville E. Nicholls1, Warren P. Smith1, Roger A. Pielke Sr.1, Stephen M. Saleeby2, and Norman B. Wood3 Melville E. Nicholls et al.
  • 1Cooperative Institute for Research in Environmental Sciences, Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO 80309, USA
  • 2Department of Atmospheric Science, Colorado State University, Fort Collins, CO80523, USA
  • 3Space Science and Engineering Centre, University of Wisconsin, Madison, WI 53706, USA

Abstract. Numerical modeling studies indicate that radiative forcing can significantly accelerate tropical cyclogenesis. The primary mechanism appears to be nocturnal differential radiative forcing between a developing tropical disturbance and its relatively clear-sky surroundings. This generates weak ascent in the system core, which promotes enhanced convective activity. The goal of this study is to examine this hypothesis in more detail and in doing so shed light on the particular physical mechanisms that are responsible for the accelerated development of the system. In order to clarify the effects of radiation the radiative forcing occurring in a full physics simulation is imposed as a forcing term on the thermodynamic equation in a simulation without microphysics, surface fluxes or radiation included. This gives insight into the radiatively induced circulations and the resultant changes to the temperature and moisture profiles in the system core that can influence convective development. Simulations with separate environment and core radiative forcings support the hypothesis that differential radiative forcing due to nocturnal longwave cooling in the environment is the main factor responsible for accelerating the rate of tropical cyclogenesis. Simple idealized cloud experiments indicate that both cooling and moistening caused by the induced ascent significantly influence convective development, with the cooling having the largest impact. Diurnal cycles of Convective Available Potential Energy (CAPE), outgoing longwave radiation, deep convection and upper level outflows are examined and their relation to the radiative forcing is discussed.

Melville E. Nicholls et al.
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Short summary
Numerical modeling simulations indicate that radiation significantly accelerates tropical cyclogenesis. This study provides evidence that the primary physical mechanism is nocturnal longwave cooling of the environment. This generates weak upward motion in the core of the system that over the course of a night promotes convective activity and is responsible for a diurnal cycle. Understanding the role of radiation is likely to lead to improved forecasting of these major weather events.
Numerical modeling simulations indicate that radiation significantly accelerates tropical...
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