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The Critical Role of Cloud–Infrared Radiation Feedback in Tropical Cyclone Development

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The Critical Role of Cloud–Infrared Radiation Feedback in Tropical Cyclone Development The critical role of cloud–infrared radiation feedback in tropical cyclone development James H. Ruppert Jra,b,1, Allison A. Wingc, Xiaodong Tangd, and Erika L. Durane aDepartment of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, PA 16802; bCenter for Advanced Data Assimilation and Predictability Techniques, The Pennsylvania State University, University Park, PA 16802; cDepartment of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306; dKey Laboratory of Mesoscale Severe Weather, Ministry of Education, and School of Atmospheric Sciences, Nanjing University, Nanjing 210093, China; and eEarth System Science Center, University of Alabama in Huntsville/NASA Short-term Prediction Research and Transition (SPoRT) Center, Huntsville, AL 35805 Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved September 21, 2020 (received for review June 29, 2020) The tall clouds that comprise tropical storms, hurricanes, and within an incipient storm locally increase the atmospheric trapping typhoons—or more generally, tropical cyclones (TCs)—are highly of infrared radiation, in turn locally warming the lower–middle effective at trapping the infrared radiation welling up from the sur- troposphere relative to the storm’s surroundings (35–39). This face. This cloud–infrared radiation feedback, referred to as the mechanism is a positive feedback to the incipient storm, as it “cloud greenhouse effect,” locally warms the lower–middle tropo- promotes its thermally direct transverse circulation (38, 39) sphere relative to a TC’s surroundings through all stages of its life (Fig. 1A). Herein, we examine the role of this feedback in the cycle. Here, we show that this effect is essential to promoting and context of TC development in nature. accelerating TC development in the context of two archetypal In contrast to idealized study frameworks, TCs in nature de- storms—Super Typhoon Haiyan (2013) and Hurricane Maria (2017). velop from transient precursor disturbances, such as African Namely, this feedback strengthens the thermally direct transverse easterly waves and monsoon depressions (40). Given such a circulation of the developing storm, in turn both promoting satura- disturbance, the variable large-scale environment conspires to tion within its core and accelerating the spin-up of its surface tan- promote its intensification into a strong TC only when sea surface gential circulation through angular momentum convergence. This temperature (SST) is high and vertical wind shear is suppressed feedback therefore shortens the storm’s gestation period prior to (40), with few exceptions. Such constraints explain why TC de- its rapid intensification into a strong hurricane or typhoon. Further velopment is fickle in nature. Assuming such conditions, the air EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES research into this subject holds the potential for key progress in TC within the precursor storm warms and moistens, as the conver- prediction, which remains a critical societal challenge. gence of angular momentum at low levels leads to the formation hurricane | radiation | tropical cyclone | clouds | feedback of a surface cyclone that can subsequently intensify through WISHE (14–17). Here, we test the hypothesis that cloud–infrared radiation feedback promotes and accelerates this evolution in the andfalling tropical cyclones (TCs) are among the most cata- context of two archetypal storm events: Super Typhoon Haiyan strophic natural disasters to affect humankind, which alone L (2013) (or Yolanda) and Hurricane Maria (2017). We conduct explain half of all weather- and climate-related deaths and eco- numerical modeling experiments using a small ensemble of large- nomic losses in the United States (1). Upward trends in both population density and wealth in global coastal areas will con- scale, convection-resolving simulations and sensitivity tests for both storms using the Weather Research and Forecasting (WRF) tinue exacerbating this issue (2), as will the rising sea level (3) Materials and Methods and the likely increase of TC intensity with climate change (4–6). atmosphere model ( ). Given our objective, Major progress has been achieved in the prediction of TC mo- tion in recent decades, owing to the increasing quantity and Significance quality of satellite measurements and advancements in data as- similation (7–9). There has been comparatively little progress in The deep clouds that make up tropical disturbances, the pre- the prediction of TC intensity and intensification, however, cursors to more intense tropical cyclones (TCs) (including hur- which underscores the need for further research into the physical ricanes and typhoons), effectively trap infrared radiation processes that govern TC development (7). emitted by Earth’s surface and lower atmosphere. Our results The primary energy source for TCs is evaporation from the demonstrate that the local atmospheric warming caused by ocean surface (10). A long history of research indicates that TCs this “cloud greenhouse effect” is a key trigger for promoting intensify through the WISHE feedback (wind-induced surface and accelerating the evolution of such precursor storms into heat exchange), whereby the rate of evaporation increases with intense TCs. The forecasting of TC formation remains extremely surface wind speed (11–13). A sufficiently strong initial surface challenging, while the representation of cloud processes and cyclone is required for this process to ensue, however, and the their feedback with radiation is a large source of uncertainty in pathway to this stage of TC genesis remains a subject of debate the numerical models that forecasts rely upon. Our results (14–17). This pathway is the focus of this study and is where we suggest that focusing future research on constraining these argue that cloud–radiation interaction plays a crucial role. processes in models holds promise for key progress in the Two important discoveries suggest that TC development in prediction of these devastating storms. nature may be acutely sensitive to cloud–radiation interaction. First, recent research highlights a pronounced diurnal variation Author contributions: J.H.R. designed research; J.H.R., A.A.W., X.T., and E.L.D. performed in TCs—in their precipitation, clouds, and winds (18–25). This research; J.H.R. analyzed data; and J.H.R. wrote the paper. diurnal cycle is caused by the interaction between solar radiation The authors declare no competing interest. and clouds in the TC (26–28), and thus implies that cloud– This article is a PNAS Direct Submission. radiation interaction can substantially influence TC behavior. The Published under the PNAS license. second pertinent discovery is that the localized greenhouse effect 1To whom correspondence may be addressed. Email: [email protected]. of deep convective clouds has an important influence on TC de- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ velopment in idealized model frameworks (29–34). Namely, due doi:10.1073/pnas.2013584117/-/DCSupplemental. to their extremely high emissivity, the deep convective clouds www.pnas.org/cgi/doi/10.1073/pnas.2013584117 PNAS Latest Articles | 1of9 Downloaded by guest on September 23, 2021 A Incipient Storm B Intensifying Hurricane Fig. 1. The cloud greenhouse effect accelerates tropical cyclone development. Schematic depiction of how the trapping of infrared radiation by deep convective clouds leads to locally increased warming (red shading), and how this warming promotes the thermally direct transverse circulation (thin arrows) of the tropical cyclone (TC). (A) An incipient storm, characterized by a weak, broad primary circulation (thick, circular arrows). (B) An intensifying hurricane characterized by a well-defined eye and a strong primary circulation. − we focus herein on the initial formation and intensification of Haiyan reached a maximum intensity of 87 m·s 1 by 1200 UTC these events, neglecting their later evolution. An overview of the 07 November, just before making landfall in the Philippines near real and simulated storm events is provided next, followed by Guiuan, Eastern Samar, around 0000 UTC 08 November testing of the study hypothesis. (Fig. 3B and SI Appendix, Fig. S1B). For both Maria and Haiyan, control simulations are conducted Overview of the Storm Events. Hurricane Maria and Super Ty- with the intent to reproduce their genesis and intensification phoon Haiyan were both catastrophic, record-breaking storms realistically. The simulations are initialized 48 h prior to official due to their tracks and ideal environmental conditions for in- storm identification in the case of Maria, and ∼36 h prior in the tensification prior to landfall—in the Lesser Antilles and Puerto case of Haiyan (start times of the observed time series are in- Rico in the case of Maria, and in the Philippines in the case of dicated by vertical ticks in Fig. 2 A and B). The intensity of the Haiyan. However, both Haiyan and Maria developed along the simulated storms is quantified by invoking two separate methods typical pathways of storms in the Western North Pacific and to calculate maximum wind speed. Fig. 2 A and B depicts the Atlantic basins. They are therefore considered archetypal TC maximum azimuthally averaged 10 m wind speed, which effec- development cases, representative of a much larger sample of tively captures the intensity of the complete, closed circulation of events. Below,
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