The primary driver of the Earth’s climate system is the distribution of net downward radiation at the top of the atmosphere (TOA) owing principally to the Sun–Earth geometry. Clouds are an essential component of the re-distribution of this solar energy in the climate system through their effect on shortwave (SW) and longwave (LW) radiative fluxes, as well as their contribution to energy re-distribution through phase changes of water. Thus, they are an important component of the radiation budget and earth energy balance.
The interactions among clouds and other components of the Earth climate system are quite complex, and they are an area of active investigation (1-3). A large amount of the incoming solar energy is directly reflected at the TOA by clouds, and this effect dominates in the SW radiation regime (wavelengths less than 3 mm). In the LW regime (wavelengths longer than 3 mm), thermal processes dominate. Thermal radiative energy is released by all elements of the earth’s surface with the spectral peak in the LW regime dependent on the apparent temperature of the emitting components. Since clouds generally reduce LW penetration (4), their presence limits the amount of LW radiation released from the surface of the earth (5-7) that can escape into space. The cloud then re-emits at a colder apparent temperature that depends on cloud height, reducing the net upwelling LW radiative flux. Of the two mechanisms, SW reflectance increases the total upwelling radiation and acts as a cooling effect on the earth energy balance, while LW attenuation reduces the upwelling radiation and acts as a warming effect.
The overall contribution of clouds to the radiation budget is complicated and depends on the specifics of the Earth-ocean-atmosphere system. With a globally averaged cloud-radiative effect of 20 W m−2, clouds act to strongly cool the planet (8) . However, as the climate warms, clouds and their radiative effects are expected to increase, providing a feedback signal that is most likely negative (i.e., a cooling effect in a warming planet), but highly uncertain (8-10). Improving our understanding of the role of clouds in climate is crucial to understanding the effects of global warming.
In recent decades, a great deal has been learned about clouds and their radiation contribution to the earth energy balance (11, 12). However, the specific impact of clouds due to organised mesoscale convective systems such as tropical cyclones on the earth energy balance is relatively unknown. Most tropical cyclone studies examine intensity, track and precipitation, but little is known about their radiation contribution to the earth energy balance. There are three reasons for the lack of research in this area. First, compared with tropical cyclone characteristics such as intensity and precipitation, their radiation does not cause direct major damage and is not of immediate concern. Second, as tropical cyclones are surrounded by other cloud systems with which they have complicated interactions, it is difficult to decide the boundary of the tropical cyclone and non-tropical cyclone cloudy regions. Third, many tropical cyclones need to be analyzed over a long period of time in order to draw conclusions about their radiation impact on the earth energy balance.
The question of how tropical cyclones may change in a warmer climate has been discussed broadly. A theoretical study showed that a significant increase in tropical cyclone intensity will likely occur in a warming climate (13). Webster, et al., examined the number of tropical cyclones and cyclone days as well as tropical cyclone intensity over 35 years in an environment of increasing sea surface temperature. They found that there has been a large increase in the number and proportion of hurricanes reaching categories 4 and 5, while the overall number of cyclones and cyclone days has decreased (14). However, their study was limited by inconsistencies in the global tropical cyclone best track archives that underpin their study. Global model studies have documented the potential effects of climate change on tropical cyclones (15-19). While there is wide variation in the results of these studies, the consensus is for a tendency toward decreasing frequency, increasing proportion of intense, and increasing precipitation of tropical cyclones as the climate warms (19-21). However, there is currently little information on how the structure, size, convective activity, or seasonal activity of tropical cyclones will change. One study did find a positive feedback mechanism between tropical cyclones and climate warming through the strengthening of the Kuroshio current over the western Pacific Ocean (22). Although there has been a significant number of research studies on the variation of tropical cyclone activity due to climate change, there are few studies about the impact of a warming climate on tropical cyclone cloud net radiation or on the feedback that tropical cyclone cloud net radiation provides to the climate system.
With the advancement of remote sensing technology, satellite images have been used extensively to assess tropical cyclone intensity (23-27), tropical cyclone genesis (28), tropical cyclone location and tracking (23-25, 29), wind structure (30-32), and precipitation (33-35). In addition, there have been some attempts to isolate the tropical cyclone in satellite images using segmentation algorithms mostly for tracking purposes (30, 36).
We recently presented a semi-automated remote sensing framework that successfully segments tropical cyclone clouds in satellite images (37). We then used the segmented images to compute the global contribution of tropical cyclone clouds to the upwelling radiation during 2016 and found that tropical cyclones were responsible for a net increase in upwelling radiation compared to the clear-sky condition that is significant compared with the surplus energy in the earth energy balance (40.5 TW upwelling (cooling) in 2016 due to tropical cyclones, compared with the ~300 TW energy surplus (warming) in the earth energy balance (38)). Although the overall impact of tropical cyclones on the earth energy balance in this single year of data is net-cooling, there is a distribution of results for individual storms that range from slightly warming to significantly cooling (37). To date, there is no understanding of what drives this distribution. In addition, while we demonstrated that tropical cyclones may produce an increase in upwelling radiation that is large enough to have an impact on the global earth energy balance, the original study did not consider a long enough data set to robustly quantify the overall result, consider whether there are regional differences, or consider how changes in tropical cyclone activity might affect the earth energy balance. In this paper, we expand the data set to include all tropical cyclones globally from 2001 to 2020 to verify the conclusions in (37) over a longer-term period. In addition, we explore the interannual, regional, and diurnal variation of the signature in order to understand how tropical cyclone cloud activity impacts the earth energy balance. Finally, we develop an understanding of the types, physical characteristics, seasonality, and locations of tropical cyclones that produce net positive (cooling) or negative (warming) totals to the earth energy balance and discuss the implications of these findings for a warming climate.
 Throughout this paper we use the convention that upwelling radiation is positive. An increase in upwelling radiation represents a cooling effect.