Tropical Cyclone Development Research Articles

Active Role of Sea Surface Temperature Changes Over the Kuroshio in the Development of Distant Tropical Cyclones in Boreal Fall

AbstractThis study examined the active role of sea surface temperature (SST) changes over the Kuroshio in the development of distant tropical cyclones (TCs) approaching Japan in October, using a cloud‐resolving model. We designed a real SST experiment and SST sensitivity runs by adding and subtracting SST anomalies equivalent to a spread of year‐to‐year October‐mean Kuroshio's SST changes. Under a synoptic environment that low‐level northeasterlies prevailed over the Kuroshio, systematic differences in TC intensity between the real and modified SST runs were simulated despite the developing TCs being far away from the modified SST areas. In the warm (cool) SST runs, active (inactive) moisture import from the Kuroshio increased (decreased) the equivalent potential temperature within the planetary boundary layer (PBL) in the inner core, thereby enhancing (weakening) the latent eyewall heating and the associated secondary circulation. The differences in the secondary circulation led to those in the inward advection of absolute angular momentum in the PBL, affecting a vortex spin‐up in the inner core. We also found that the surface heat and moisture supply from the Kuroshio facilitated the warming and moistening processes of midlatitude air parcels traveling over the underlying current, causing further reinforcement of the moisture import into the distant TC. These thermodynamic processes were more and less apparent in the warm and cool SST runs, respectively. Since the synoptic environment in this study often arises in boreal fall, the Kuroshio's SST warming in recent decades may influence the intensities of TCs approaching Japan in fall.

Development of Tropical Cyclone over the Bay of Bengal during South West Monsoon

South West Monsoon (SW Monsoon) and Tropical Cyclone (TC) are two important weather systems for Bangladesh. During SW Monsoon i.e. during rainy season Bangladesh gets 70% to 85% of her annual rainfall. TC accompanied with strong gale winds, tornadoes, torrential rains and storm surges is considered as a deadly natural disaster. TC’s are mostly formed during pre-monsoon and post-monsoon season, but not uncommon during SW monsoon. This study consults the best track data (cyclone e-atlas) of India Meteorological Department (IMD) containing tracks of cyclones and depression over North Indian Ocean (NIO) for the years 1891 to 2020 i.e. of 130 years. In these 130 years, among total 1219 storms, 608 had formed during SW monsoon. If only Cyclonic Storms (CS) and Severe Cyclonic Storms (SCS) are considered then, 150 storms formed during SW Monsoon. This paper studied two cyclogenesis factors; vertical wind shear and upper level anticyclone for eight cases of tropical storms formed during SW Monsoon. Besides cyclogenesis factors, influence of Madden Julian Oscillation (MJO) is also studied. Threat analysis associated with tropical storms during SW Monsoon has been done which might help in planning of National Disaster Management Program. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 10(1), 2021, P 57-65

Impacts of the Saharan air layer on the physical properties of the Atlantic tropical cyclone cloud systems: 2003–2019

Abstract. It is generally known that the tropical cyclone (TC) cloud systems (TCCSs) in the North Atlantic region frequently occur during boreal summer, while the Saharan dust outbreaks occur concurrently. The Sahara air layer (SAL), an elevated layer containing Saharan dry air and mineral dust, has crucial impacts on the generation and evolution of TCs. However, the effects of SAL on the physical (macro and micro) characteristics of the Atlantic TCCSs have not been well constrained, and the interaction mechanisms between them still need further investigation. In this study, our primary interest is to distinguish the various effects of SAL on different intensities of TCs and further find out the probable causes of the varied feedback mechanisms. Therefore, we attempt to identify whether and how the effects of the SAL play a positive or negative role on the TCCSs and to draw a qualitative conclusion on how SAL affects the various intensities of the TCs. This paper focuses on the 70 TC samples from July to September in the years of 2003–2019 to investigate the physical effects of SAL on three intensities of TCs, i.e., the tropical depression (TD), tropical storm (TS), and hurricane (HU). The results show that SAL has a positive impact on the macro properties of HU but significantly suppresses the TD. It appears that the SAL attributes little to the variation of the ice cloud effective radius (CERi) for TS, whereas CERi changes significantly and differentially for TD and HU. When affected by SAL, the probability density function (PDF) curve of CERi generally shifts to the smaller value for TD, whereas the PDF curve becomes flatter for HU. Our analysis indicates that the various responses of TCCSs to SAL are determined by the combined effects of dry air masses, the dust aerosols as ice nuclei, and the thermodynamic and moisture conditions. Based on the observation data analysis, a concept scheme description has been concluded to deepen our recognition of the effects of SAL on the TCCSs.

Demonstration of the Temporal Evolution of Tropical Cyclone “Phailin” Using Gray-Zone Simulations and Decadal Variability of Cyclones over the Bay of Bengal in a Warming Climate

The intensity and frequency variability of cyclones in the North Indian Ocean (NIO) have been amplified over the last few decades. The number of very severe cyclonic storms (VSCSs) over the North Indian Ocean has increased over recent decades. “Phailin”, an extreme severe cyclonic storm (ESCS), occurred during 8–13 October 2013 over the Bay of Bengal and made landfall near the Gopalpur coast of Odisha at 12 UTC on 12 October. It caused severe damage here, as well as in the coastal Odisha, Andhra Pradesh, and adjoining regions due to strong wind gusts (~115 knot/h), heavy precipitation, and devastating storm surges. The fidelity of the WRF model in simulating the track and intensity of tropical cyclones depends on different cloud microphysical parameterization schemes. Thus, four sensitivity simulations were conducted for Phailin using double-moment and single-moment microphysical (MP) parameterization schemes. The experiments were conducted to quantify and characterize the performance of such MP schemes for Phailin. The simulations were performed by the advanced weather research and forecasting (WRF-ARW) model. The model has two interactive domains covering the entire Bay of Bengal and adjoining coastal Odisha on 25 km and 8.333 km resolutions. Milbrandt–Yau (MY) double-moment and WRF single-moment microphysical schemes, with 6, 5, and 3 classes of hydrometeors, i.e., WSM6, WSM5, and WSM3, were used for the simulation. Experiments for Phailin were conducted for 126 h, starting from 00 UTC 08 October to 06 UTC 13 October 2013. It was found that the track, intensity, and structure of Phailin are highly sensitive to the different microphysical parameterization schemes. Further, the precipitation and cloud distribution were studied during the ESCS stage of Phailin. The microphysics schemes (MY, WSM3, WSM5, WSM6), along with Grell–Devenyi ensemble convection scheme predicted landfall of Phailin over the Odisha coast with significant track errors. Supply of moisture remains a more crucial component than SST and wind shear for rapid intensification of the Phailin 12 h before landfall over the Bay of Bengal. Finally, the comparison of cyclone formation between two decades 2001–2010 and 2011–2020 over the Bay of Bengal inferred that the increased numbers of VSCS are attributed to the supply of abundant moisture at low levels in the recent decade 2011–2020.

Tropical Cyclone Footprints in Long-Term Mean State and Multiscale Climate Variability in the Western North Pacific as Seen in the JRA-55 Reanalysis

AbstractThe monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of tropical cyclones (TCs) in the western North Pacific (WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity. Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.

Impact of Assimilation of Satellite Retrieved Ocean Surface Winds on the Tropical Cyclone Simulations Over the North Indian Ocean

AbstractThe study intends to examine the impact of assimilation of ocean surface winds on the simulation of the tropical cyclones Ockhi, Mekunu, and Luban formed over the north Indian Oceans. We have conducted two assimilation experiments, cyclic control (CyclicCntl) and Hybrid, using the Weather Research and Forecast model. The CyclicCntl experiment and Hybrid experiment are the analysis generated without and with adding ocean surface winds, respectively. The ocean surface winds from Ocean Scatterometer, Advanced Scatterometer, and Windsat are assimilated using the ensemble three‐dimensional variational assimilation technique. The analysis generated by Hybrid and CyclicCntl is compared with the ocean surface winds observations from buoys, and it is found that the assimilation of ocean surface winds reduces the root mean square error between observation and analysis for zonal and meridional winds by 2–3 m s−1 (42%) compared to CyclicCntl simulations. The comparison with the International Best Track Archive for Climate Stewardship locations of cyclones derived from the analyses indicates that track error is maximum in genesis and landfall stage for both the Hybrid and CyclicCntl experiments, and the average track error is less than 80 km for the three cyclones. In a Hybrid experiment, during the intensifications from deep depression to the very severe cyclonic storm, the wind speed enhancement is dominant over the southeast and northwest sectors of the eyewall regions and subsequently in the rainband region. Thus, the assimilation of ocean surface winds from the three sensors improves the prediction of intensity and the evolution of tropical cyclones in general.

Examining the Role of Cloud Radiative Interactions in Tropical Cyclone Development Using Satellite Measurements and WRF Simulations

AbstractThis study examines the role of cloud‐radiative interactions in the development of tropical cyclones using satellite measurements and model simulations. Previous modeling studies have found that the enhanced cloud radiative heating from longwave radiation in the convective region plays a key role in promoting the development of tropical convective systems. Here, we use satellite measurements and Weather Research and Forecasting Model (WRF) simulations to further investigate how critical cloud radiative interactions are to the development of tropical cyclones (TCs). Clouds and the Earth's Radiant Energy System measurements show that intensifying TCs have greater radiative heating from clouds within the TC area than weakening ones. Based on this result, idealized WRF simulations are performed to examine the importance of the enhanced radiative heating to TC intensification. Sensitivity experiments demonstrate that removing cloud‐radiative interactions often inhibits tropical cyclogenesis, suggesting that cloud‐radiative interactions play a critical role.

Uncertainty of Tropical Cyclone Wind Radii on Sea Surface Temperature Cooling

AbstractThe sea surface temperature (SST) beneath a tropical cyclone (TC) is of great importance to its dynamics; therefore, understanding and accurately estimating the magnitude of SST cooling is of vital importance. Existing studies have explored important influences on SST of TC translation speed, maximum surface winds, ocean thermal condition, and ocean stratification. But the influence of the TC wind radii (or collectively called the TC size) on SST has been largely overlooked. In this study, we assess the influence of wind radii uncertainty on SST cooling by a total of 15,983 numerical simulations for the western North Pacific during the 2014–2018 seasons. Results show a 6%–20% SST cooling error induced using wind radii from the Joint Typhoon Warning Center official forecast and a 35%–40% SST cooling error using wind radii from the operational runs of the Hurricane Weather Research and Forecasting (HWRF) model. Our results indicate that SST cooling is most sensitive to the radius of 64 kt winds (R64) due to its effects on the integrated kinetic energy of the TC and subsequent mixing of the ocean surface layer. It is also found that the correlation between SST cooling induced by the TC and its size is 0.49, which is the highest among all the parameters tested. This suggests that it is extremely important to get TC size correct in order to predict the SST cooling response, which then impacts TC evolution in numerical weather prediction models.

Tropical cyclone life cycle in a three‐dimensional numerical simulation

AbstractAn idealized, three‐dimensional, numerical simulation of tropical cyclone evolution in a quiescent environment on an f‐plane is used to explore aspects of the cyclone's life cycle in the context of the rotating‐convection paradigm. In the 20‐day simulation, the vortex undergoes a life cycle including a gestation period culminating in genesis, a rapid intensification phase, a mature phase, a transient decay and re‐intensification phase, a second mature phase and a rapid decay phase. During much of the life cycle, the flow evolution is highly asymmetric, although important aspects of it can be understood within an azimuthally averaged framework, central to which are a boundary‐layer control mechanism and a new ventilation diagnostic. The boundary‐layer control mechanism provides an explanation for the gradual expansion of the inner core of the vortex. The ventilation diagnostic characterizes the ability of deep convection within a given radius to evacuate the mass of air ascending out of the boundary layer within that radius. The transient decay and re‐intensification phase is not associated with an eyewall replacement cycle, but rather with a hitherto undescribed process in which the eyewall becomes fragmented as a rainband complex forms beyond it. This process is interpreted as an interplay between the boundary layer and ventilation. The final rapid decay of the vortex results from the ever increasing difficulty of deep convection to ventilate the air exiting the boundary layer. Any unventilated air flows radially outwards in the lower troposphere and leads to spin‐down because of the approximate conservation of mean absolute angular momentum. If found in real cyclones, such transience or final decay might be erroneously attributed to ambient vertical wind shear. The results support the hypothesis that, even in a quiescent environment, isolated tropical cyclone vortices are intrinsically transient and never reach a globally steady state.

On the Hyperbolicity of the Bulk Air–Sea Heat Flux Functions: Insights into the Efficiency of Air–Sea Moisture Disequilibrium for Tropical Cyclone Intensification

AbstractSea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulas, these fluxes are a function of surface wind speed U10 and air–sea temperature and moisture disequilibrium (ΔT and Δq, respectively). Although many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies have found that TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under conditions of relatively low wind. Herein, a new perspective on the bulk aerodynamic formulas is introduced to evaluate the relative contribution of wind-driven (U10) and thermodynamically driven (ΔT and Δq) ocean heat uptake. Previously unnoticed salient properties of these formulas, reported here, are as follows: 1) these functions are hyperbolic and 2) increasing Δq is an efficient mechanism for enhancing the fluxes. This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady-state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δq > 5 g kg−1 at moderate values of U10 led to intense inner-core moisture fluxes of greater than 600 W m−2 during RI. Peak values in Δq preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δq is a very effective way to increase surface heat fluxes—this can easily be achieved as a TC moves over deeper warm oceanic regimes.

Effects of ocean eddies on the tropical storm Roanu intensity in the Bay of Bengal.

A tropical storm (TS) Roanu occurred in northern Sri Lanka in 2016, which transported northwards along the west coast of the Bay of Bengal (BoB). During the development of the TS, ocean eddies on its track had an important effect on the intensity of Roanu. The dynamic mechanism was investigated with multisource reanalysis and Argo float data in this study. The results show that ocean eddies were the main reason why Roanu first enhanced, weakened, and then enhanced again. Warm eddy W1 supports the initial development of the TS, cold eddy C1 weakens Roanu, and warm eddy W2 continues to support Roanu. On May 19, 2016, the maximum average latent heat flux over W1 was 260.85 w/m2, while that of C1 was only 200.71 w/m2. After the passage of Roanu, the tropical cyclone heat potential (TCHP) of eddies significantly decreased. The TCHP of W1, W2, C1 and C2 decreased by 20.95 kJ/cm2, 11.07 kJ/cm2, 29.82 kJ/cm2, 9.31 kJ/cm2, respectively. The mixed layer of warm eddies deepened much more than that of cold eddies, supporting Roanu development. In addition, changes in potential vorticity (PV) values caused by the disturbance of eddies may also reflect changes in the TS intensity. This study offers new insights on the influence of ocean eddies in regulating the development of tropical cyclone (TC) in the BoB.

Combined Effects of Midlevel Dry Air and Vertical Wind Shear on Tropical Cyclone Development. Part I: Downdraft Ventilation

AbstractThis study examines how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via downdraft ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS. A strong, positive, linear relationship exists between the low-level vertical mass flux in the inner core and TC intensity. The linear increase in vertical mass flux with intensity is not due to an increased strength of upward motions but, instead, is due to an increased areal extent of strong upward motions (w > 0.5 m s−1). This relationship suggests physical processes that could influence the vertical mass flux, such as downdraft ventilation, influence the intensity of a TC. The azimuthal asymmetry and strength of downdraft ventilation is associated with the vertical tilt of the vortex: downdraft ventilation is located cyclonically downstream from the vertical tilt direction and its strength is associated with the magnitude of the vertical tilt. Importantly, equivalent potential temperature of parcels associated with downdraft ventilation trajectories quickly recovers via surface fluxes in the subcloud layer, but the areal extent of strong upward motions is reduced. Altogether, the modulating effects of downdraft ventilation on TC development are the downward transport of low–equivalent potential temperature, negative-buoyancy air left of shear and into the upshear semicircle, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development.

Combined Effects of Midlevel Dry Air and Vertical Wind Shear on Tropical Cyclone Development. Part II: Radial Ventilation

AbstractThis study demonstrates how midlevel dry air and vertical wind shear (VWS) can modulate tropical cyclone (TC) development via radial ventilation. A suite of experiments was conducted with different combinations of initial midlevel moisture and VWS environments. Two radial ventilation structures are documented. The first structure is positioned in a similar region as rainband activity and downdraft ventilation (documented in Part I) between heights of 0 and 3 km. Parcels associated with this first structure transport low–equivalent potential temperature air inward and downward left of shear and upshear to suppress convection. The second structure is associated with the vertical tilt of the vortex and storm-relative flow between heights of 5 and 9 km. Parcels associated with this second structure transport low–relative humidity air inward upshear and right of shear to suppress convection. Altogether, the modulating effects of radial ventilation on TC development are the inward transport of low–equivalent potential temperature air, as well as low-level radial outflow upshear, which aid in reducing the areal extent of strong upward motions, thereby reducing the vertical mass flux in the inner core, and stunting TC development.

Effect of the Initial Vortex Vertical Structure on Early Development of an Axisymmetric Tropical Cyclone

AbstractWith the nonhydrostatic axisymmetric cloud‐resolving model CM1, two sets of ensemble runs are conducted to study the influence of initial vortex depth on the early development of an idealized tropical cyclone (TC). It is found that with the shallow initial vortex, the intensification rate (IR) is smaller and the TC takes a longer time to reach a state of slantwise moist neutrality. These behaviors are primarily ascribed to the less active convection in the shallow case. In contrast to the deep initial vortex, the shallow initial vortex is characterized by a more stable stratification in the lower troposphere and a smaller gradient of angular momentum (M) in the mid‐to‐upper troposphere. The former tends to hinder the occurrence of deep convection in the TC while the latter makes it difficult to intensify the mid‐to‐upper level vortex and to establish a deep vortex. Thus, deep convection bursts intermittently in the early developing stage of the TC originating from the shallow vortex. During the periods of inactive convection, the TC weakens by the decline of M source associated with reduced horizontal advection, which is overwhelmed by sinks of M from moderately decreased vertical advection and nearly‐constant dissipation term. Therefore, the IR of the TC is smaller in the early developing stage and the development of TC from the shallow vortex usually needs a longer time to build the steady deep secondary circulation favorable for more rapid intensification.

Impact of Microphysical Parameterizations on Simulated Hurricanes—Using Multi-Parameter Satellite Data to Determine the Particle Size Distributions that Produce Most Realistic Storms

Understanding and forecasting hurricanes remains a challenge for the operational and research communities. To accurately predict the Tropical Cyclone (TC) evolution requires properly reflecting the storm’s inner core dynamics by using: (i) high-resolution models; (ii) realistic physical parameterizations. The microphysical processes and their representation in cloud-permitting models are of crucial importance. In particular, the assumed Particle Size Distribution (PSD) functions affect nearly all formulated microphysical processes and are among the most fundamental assumptions in the bulk microphysics schemes. This paper analyzes the impact of the PSD assumptions on simulated hurricanes and their synthetic radiometric signatures. It determines the most realistic, among the available set of assumptions, based on comparison to multi-parameter satellite observations. Here we simulated 2005′s category-5 Hurricane Rita using the cloud-permitting community Weather Research and Forecasting model (WRF) with two different microphysical schemes and with seven different modifications of the parametrized hydrometeor properties within one of the two schemes. We then used instrument simulators to produce satellite-like observations. The study consisted in evaluating the structure of the different simulated storms by comparing, for each storm, the calculated microwave signatures with actual satellite observations made by (a) the passive microwave radiometer that was carried by the Tropical Rainfall Measuring Mission (TRMM) satellite—the TRMM microwave imager TMI, (b) TRMM’s precipitation radar (PR) and (c) the ocean-wind-vector scatterometer carried by the QuikSCAT satellite. The analysis reveals that the different choices of microphysical parameters do produce significantly different microwave signatures, allowing an objective determination of a “best” parameter combination whose resulting signatures are collectively most consistent with the wind and precipitation observations obtained from the satellites. In particular, we find that assuming PSDs with larger number of smaller hydrometeors produces storms that compare best to observations.

Understanding the Processes Causing the Early Intensification of Hurricane Dorian through an Ensemble of the Hurricane Analysis and Forecast System (HAFS)

The early stages of a tropical cyclone can be a challenge to forecast, as a storm consolidates and begins to grow based on the local and environmental conditions. A high-resolution ensemble of the Hurricane Analysis and Forecast System (HAFS) is used to study the early intensification of Hurricane Dorian, a catastrophic 2019 storm in which the early period proved challenging for forecasters. There was a clear connection in the ensemble between early storm track and intensity: stronger members moved more northeast initially, although this result did not have much impact on the long-term track. The ensemble results show several key factors determining the early evolution of Dorian. Large-scale divergence northeast of the tropical cyclone (TC) appeared to favor intensification, and this structure was present at model initialization. There was also greater moisture northeast of the TC for stronger members at initialization, favoring more intensification and downshear development of the circulation as these members evolved. This study highlights the complex interplay between synoptic and storm scale processes in the development and intensification of early-stage tropical cyclones.

Influence of dust aerosols on eastern Pacific tropical cyclone intensity

The thermodynamic state of the tropical atmosphere plays an important role in the development of tropical cyclone (TC) intensity. This study reports new results that demonstrate a negative association between eastern Pacific TC intensity in offshore regions and dust aerosol optical depth (AOD) for the years 1980–2019. At the same time, a negative response of cloud water in the lower troposphere to dust AOD is reported by analyzing MERRA-2 reanalysis data and GCM simulations from CMIP6. This response can be explained by the dust semi-direct effect, in which dust aerosols absorb solar radiation, promoting the evaporation of clouds. In principle, this aerosol-driven vaporization modification could affect the enthalpy of the air surrounding a TC. Using potential intensity theory, the authors demonstrate that change in TC intensity related to dust AOD conditions is a consequence of the anomalous enthalpy of the air surrounding a TC caused by the dust semi-direct effect.摘要热带地区大气的热力学状态对台风的发展起着重要作用.本项研究结果表明, 东太平洋台风强度与沙尘AOD之间存在一定的负相关.同时, MERRA-2再分析数据和CMIP6的GCM模拟的分析结果, 都表明了低层大气中云对沙尘AOD的负响应.沙尘颗粒物的准直接辐射效应可以解释上述响应:沙尘颗粒物吸收太阳辐射, 并用此热量促进云层蒸发.理论上, 这种气溶胶驱动的汽化作用会影响台风周围空气的焓.根据台风的潜在强度理论, 与沙尘相关的台风强度变化, 实则是沙尘的准直接辐射效应引起的台风周围空气焓值变化的结果.

The Role of Radiative Interactions in Tropical Cyclone Development under Realistic Boundary Conditions

AbstractThe impact of radiative interactions on tropical cyclone (TC) climatology is investigated using a global, TC-permitting general circulation model (GCM) with realistic boundary conditions. In this model, synoptic-scale radiative interactions are suppressed by overwriting the model-generated atmospheric radiative cooling rates with their monthly varying climatological values. When radiative interactions are suppressed, the global TC frequency is significantly reduced, indicating that radiative interactions are a critical component of TC development even in the presence of spatially varying boundary conditions. The reduced TC activity is primarily due to a decrease in the frequency of pre-TC synoptic disturbances (“seeds”), whereas the likelihood that the seeds undergo cyclogenesis is less affected. When radiative interactions are suppressed, TC genesis shifts toward coastal regions, whereas TC lysis locations stay almost unchanged; together the distance between genesis and lysis is shortened, reducing TC duration. In a warmer climate, the magnitude of TC reduction from suppressing radiative interactions is diminished due to the larger contribution from latent heat release with increased sea surface temperatures. These results highlight the importance of radiative interactions in modulating the frequency and duration of TCs.

Influence of Saharan Dust on the Large‐Scale Meteorological Environment for Development of Tropical Cyclone Over North Atlantic Ocean Basin

AbstractThe tropical cyclones (TCs) frequently occur in the North Atlantic Ocean Basin, which is adjacent to West Africa, the largest global source of atmospheric dust. However, few studies have conducted systematic observational research of how dust affects the genesis conditions of TCs at large scales which includes sea surface temperature (SST), vorticity, vertical wind shear, and specific humidity. This study focused on the period from June–September in the years 2000 to 2018 to investigate the horizontal and vertical distributions of aerosol optical depth (AOD) (dust) and meteorological parameters. Dust can be transported at 600 hPa upward and is mixed well within the troposphere over land, while it is mainly distributed in the lower troposphere over the ocean. The SST is significantly suppressed by dust due to direct radiation effects, but the atmospheric temperature is warmer at 20–40°W between 700 and 850 hPa. The vertical distributions of temperature and specific humidity are similar. Dust decreases specific humidity in the lower troposphere over the ocean, especially in high AOD regions, but enhances midlevel moisture. Dust heats the lower troposphere and favors the development of convection and positive vorticity at heights of approximately 800–1,000 hPa. The warming effect of dust on the lower atmosphere over land and the nearby oceans results in strengthened West African monsoons and vertical wind shear. Thus, dust suppresses the SST and low‐level specific humidities and favors wind shear and positive relative vorticity, which further influence environmental conditions in the TC genesis region.

The critical role of cloud–infrared radiation feedback in tropical cyclone development

The tall clouds that comprise tropical storms, hurricanes, and typhoons-or more generally, tropical cyclones (TCs)-are highly effective at trapping the infrared radiation welling up from the surface. This cloud-infrared radiation feedback, referred to as the "cloud greenhouse effect," locally warms the lower-middle troposphere relative to a TC's surroundings through all stages of its life cycle. Here, we show that this effect is essential to promoting and accelerating TC development in the context of two archetypal storms-Super Typhoon Haiyan (2013) and Hurricane Maria (2017). Namely, this feedback strengthens the thermally direct transverse circulation of the developing storm, in turn both promoting saturation within its core and accelerating the spin-up of its surface tangential circulation through angular momentum convergence. This feedback therefore shortens the storm's gestation period prior to its rapid intensification into a strong hurricane or typhoon. Further research into this subject holds the potential for key progress in TC prediction, which remains a critical societal challenge.