Tropical Cyclone Genesis Research Articles

Influence of Different La Niña Decay Types on Tropical Cyclone Genesis Over the Western North Pacific

Influence of Different La Niña Decay Types on Tropical Cyclone Genesis Over the Western North Pacific

Possible Roles of the Sea Surface Temperature Warming of the Pacific Meridional Mode and the Indian Ocean Warming on Tropical Cyclone Genesis over the North Pacific for the Super El Niño in 2015

Possible Roles of the Sea Surface Temperature Warming of the Pacific Meridional Mode and the Indian Ocean Warming on Tropical Cyclone Genesis over the North Pacific for the Super El Niño in 2015

A statistics and physics-based tropical cyclone full track model for catastrophe risk modeling

Catastrophe (CAT) risk modeling of perils such as typhoon and earthquake has become a prevailing practice in the insurance and reinsurance industry. The event generation model is the key component of the CAT modeling. In this paper, a physics-based tropical cyclone (TC) full track model is introduced to model typhoons events in the western North Pacific basin. At the same time, a comprehensive test of the model is presented from the perspective of CAT risk modeling for insurance and reinsurance applications. The full track model includes the genesis, track, intensity, and landing models. Driven by the global environmental circulations, the model employs the advection and beta drift theory in atmospheric dynamics to model the track of typhoons. The proposed model is novel in the way of modeling the genesis of TCs with three-dimension kernel distributions in space and time. This enables the simulation of seasonal characteristics of TCs. By generating 10,000-year TC events, we comprehensively test the model from the standpoint of CAT insurance and reinsurance applications. The typhoon hazard model and the generated events can serve as the inputs for assessing the typhoon risk and insured loss caused by winds, rains, floods, and storm surges.

Contributions of the Large-Scale Environment to the Typhoon Genesis of Faxai (2019)

This study investigated the atmospheric and oceanic contributions to the genesis of Typhoon Faxai in 2019. Our statistical analysis using the tropical cyclone genesis score (TGS) attributed the tropical disturbance that developed into Faxai (Pre-Faxai) to easterly waves (EWs). The EW score evaluated by a grid version of the TGS (Grid-EW) averaged around the occurrence of Pre-Faxai was approximately twice as large as the climatological mean, and it was the second largest value in the past 38 years. The Pre-Faxai area with high Grid-EW scores could be traced back to the eastern North Pacific (ENP) around August 25, 2019. The lower-troposphere environment characterized by high Grid-EW scores was favorable for vortex formation because it provided a containment area for moisture entrained by the developing circulation or lofted by the deep convection therein. The Pre-Faxai area with high Grid-EW scores moved westward due to the background easterly flow over the ENP and then entered the western North Pacific (WNP). The Typhoon Intensity Forecast Scheme (TIFS) showed that the important environments for its genesis were ocean conditions and the vertical wind shear. The oceanic conditions contributed to the development of Pre-Faxai as it traveled over the WNP. The enhancement of vertical wind shear and subsequent suppression of the development of Pre-Faxai were caused by the lower-troposphere easterly winds associated with high EW scores. They were also caused by upper-troposphere westerly winds associated with an upper cold low northwest of Pre-Faxai. When the vertical shear decreased with weakening of the upper cold low, Pre-Faxai reached tropical storm intensity on September 4. Therefore, TGS and TIFS detected Pre-Faxai 10 days before the typhoon arose, which indicates that monitoring environmental factors such as EW and vertical wind shear are important for disaster prevention.

Effects of Tropical Sea Surface Temperature Variability on Northern Hemisphere Tropical Cyclone Genesis

Effects of Tropical Sea Surface Temperature Variability on Northern Hemisphere Tropical Cyclone Genesis

Exploring Western North Pacific Tropical Cyclone Activity in the High‐Resolution Community Atmosphere Model

AbstractHigh‐resolution climate models (∼28 km grid spacing) can permit realistic simulations of tropical cyclones (TCs), thus enabling their investigation in relation to the climate system. On the global scale, previous works have demonstrated that the Community Atmosphere Model (CAM) version 5 presents a reasonable TC climatology under prescribed present‐day (1980–2005) forcing. However, for the Western North Pacific (WNP) region, known biases in simulated TC genesis frequency and location under‐represent the basin's dominant share in observations. This study addresses these model biases in WNP by evaluating WNP TCs in a decadal simulation, and exploring potential improvements through nudging experiments. Among the major environmental controls of TC genesis, the lack of mid‐level moisture is identified as the leading cause of the deficit in simulated WNP TC genesis over the Pacific Warm Pool. Subsequent seasonal experiments explore the effect of constraining the large‐scale environment on TC development by nudging WNP temperature field toward reanalysis at various strengths. Temperature nudging elicits a significant response in TC genesis and intensity development, as well as in moisture and convection over the Warm Pool. These responses are sensitive to the choice of nudging timescale. Overall, the nudging experiments demonstrate that improvements in the large‐scale environment can lead to improvements in simulated TCs, suggesting future model developments in relation to model physics. In this way, the potential improvements in model fidelity will contribute to the understanding of how the mean state of current or future climates may give rise to extremes such as TCs.

Tropical Cyclone Precipitation Response to Surface Warming in Aquaplanet Simulations With Uniform Thermal Forcing

AbstractWhile many modeling studies have attempted to estimate how tropical cyclone (TC) precipitation is impacted by climate change, the multitude of analysis techniques and methodologies have resulted in varying conclusions. Simplified models may be able to help overcome this problem. Radiative‐convective equilibrium (RCE) model simulations have been used in various configurations to study fundamental aspects of Earth's climate. While many RCE modeling studies have focused on TC genesis, intensification, and size, limited work has been done using RCE to study TC precipitation. In this study, the response of TC precipitation to sea surface temperature (SST) change is analyzed in global Community Atmosphere Model (CAM) aquaplanet simulations run with Radiative‐Convective Equilibrium Model Intercomparison Project protocols, with the addition of planetary rotation. We expect that the insight gained about how TC precipitation responds to SST warming will help predict how TCs in the real world respond to climate change. In the CAM RCE simulations, the warmer SST simulations have less TCs on average, but the TCs tend to be larger in outer size and more intense. As simulation SST increases, more extreme precipitation rates occur within TCs, and more of the TC precipitation comes from these extreme rates. For extreme (99th percentile) TC precipitation, SST, and TC intensity increases dominate the 8.6% per K increase, while TC outer size changes have little impact. For accumulated TC precipitation, SST, and TC intensity contributions are still the majority, but TC outer size changes also contribute to the 6.6% per K increase.

Decadal shift of the interannual relationship between western North Pacific tropical cyclone genesis frequency and South China Sea monsoon trough around 1980s

AbstractThe present study investigates the decadal shift of the interannual relationship between the South China Sea (SCS) monsoon trough (MT) and the genesis frequency of tropical cyclone (TC) over the western North Pacific during June–October 1949–2015. Significant change was found around the 1980s. The weak TC (categories 1–3) number has a decreasing relationship with the SCS MT. Their correlation coefficients are statistically remarkable during 1949–1978 but reduce to around 0.1. In contrast, the connection between strong TC (categories 4–5) number and SCS MT is strengthened after 1980s with correlation reaching up to 0.7. Such a shift can be explained by the changes in large‐scale environmental factors associated with MT movement. After 1980s, when the SCS MT extends southeastward, there exits enhanced cyclonic relative vorticity in lower level, increased moisture in middle level, reduced vertical wind shear and intensified divergence in upper level. These conditions are favourable for weak TC intensifying and developing into strong TC. In addition, more strong TC occurred in the southwest quadrant of WNP after 1980s; thus, the SCS MT would affect the TC for a longer time. Therefore, the genesis frequency of weak TCs has diminished, while more strong TCs are formed along with the SCS MT extending southeastward, leading to their strengthened interannual relationship.

A comparison of tropical cyclone formation over the western North Pacific in August between 1996 and 2014

The tropical cyclogenesis over the western North Pacific (WNP) displays distinct difference in August of 1996 and 2014. Total 9 tropical cyclones (TCs) formed in August of 1996 over the WNP (5 and 4 TCs in the northeastern and southwestern part, respectively), whereas no TC formed in August of 2014. The present study investigates the reasons for the above difference. In August 1996, an obvious wave train with a southwest–northeast orientation is detected at the lower-level over the WNP on the monthly time scale. Anomalous cyclone and enhanced convection are induced over the northeastern WNP by the wave train, providing a favorable condition for TC formation. Analysis shows that the intraseasonal positive vorticity anomalies contributed to the TC genesis over the southwestern WNP though monthly mean state was unfavorable. In August 2014, both monthly mean and daily environment conditions were hostile to the TC formation.

Impact of North America snow cover on tropical cyclogenesis over the western North Pacific

This study reveals a connection of summer–fall (JJASO) tropical cyclone (TC) genesis over the western North Pacific (WNP) to preceding boreal spring (MAM) North America snow cover (NASC). Sea surface temperature (SST) anomalies in the tropical central Pacific and subtropical eastern Pacific play a crucial role in relaying influence of the MAM NASC on the following JJASO WNP TC genesis frequency. The increased NASC leads to a decrease in upward sensible heat flux and the atmospheric cooling over the North America. The atmospheric cooling enhances the meridional thermal contrast and geopotential height gradient, which is favorable for the occurrence of lower-level westerly wind anomalies and positive precipitation anomalies over the tropical eastern Pacific. The lower-level northeasterly wind anomalies over the subtropical northeastern Pacific as a Gill-type atmospheric response to positive precipitation anomalies induce ocean surface cooling via the enhanced wind speed. A positive feedback between the northeasterly wind anomalies and negative SST anomalies leads to a westward extension of the easterly flows to the western Pacific. The easterly wind anomalies along with the negative specific humidity anomalies and negative lower-level vorticity anomalies, and enhanced vertical wind shear suppress the TC genesis over the WNP during JJASO.

Spontaneous Cyclogenesis without Radiative and Surface-Flux Feedbacks

AbstractTropical cyclones (TCs) are among the most intense and feared storms in the world. What physical processes lead to cyclogenesis remains the most mysterious aspect of TC physics. Here, we study spontaneous TC genesis in rotating radiative–convective equilibrium using cloud-resolving simulations over anfplane with constant sea surface temperature. Previous studies proposed that spontaneous TC genesis requires either radiative or surface-flux feedbacks. To test this hypothesis, we perform mechanism-denial experiments, in which we switch off both feedback processes in numerical simulations. We find that TCs can self-emerge even without radiative and surface-flux feedbacks. Although these feedbacks accelerate the genesis and impact the size of the TCs, TCs in the experiments without them can reach similar intensities as those in the control experiment. We show that TC genesis is associated with an increase in the available potential energy (APE) and that convective heating dominates the APE production. Our result suggests that spontaneous TC genesis may result from a cooperative interaction between convection and circulation and that radiative and surface-flux feedbacks accelerate the process. Furthermore, we find that increasing the planetary rotation favors spontaneous TC genesis.

Tropical Cyclone Frequency

AbstractThe frequency with which tropical cyclones (TCs) occur controls all other aspects of tropical cyclone risk since a storm that does not occur can do no harm. Yet this frequency is poorly understood. There is no accepted theory that explains the average number of TCs that occur each year on the Earth, nor how that number will change with global warming. Arguments based on global budgets of heat or moisture do not yet appear helpful, nor does a detailed understanding of the physical processes of TC genesis. Empirical indices that predict TC frequency as a function of large‐scale environmental variables can explain some of its relative variations in space and time, but not its absolute value. Global numerical models with horizontal grid spacings on the order of 25–50 km have allowed much improved simulations of TC activity, however. Many such models project a decrease in frequency with warming, but some project an increase. Idealized simulations, including those at higher resolutions, offer promise by allowing a systematic, deductive investigation of the roles of individual environmental factors. In addition to the larger‐scale environmental modulation of genesis likelihood, precursor disturbances, or “seeds”, may exert an independent influence on TC frequency.

A New Time-Dependent Theory of Tropical Cyclone Intensification

AbstractIn this study, the boundary layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer, is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal-wind balance, with eyewall convection coming into moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies, as found recently based on idealized numerical simulations. An ad hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic Ocean and eastern and western North Pacific Ocean. As compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as seen in observations.

Variability of Tropical Cyclone Landfalls in China

AbstractThe reported decreasing trend of the annual tropical cyclone (TC) landfalls in southern China and increasing trend in southeastern China in recent decades are confirmed to be an abrupt shift occurring at the end of the twentieth century, based on a statistical analysis. The opposite trends in the two adjacent regions are often considered to be a result of tropical cyclone landfalls in southern China being deflected northward. However, it is demonstrated in this study that they are phenomenally independent. In fact, the abrupt decrease of TC landfalls in southern China occurs as a result of an abrupt decrease of the westward events in the postpeak season (October–December), which in turn is a consequence of a significant decrease of the TC genesis frequency in the southeastern part of the western North Pacific (WNP) Ocean basin. On the other hand, the abrupt increase of TC landfalls in southeastern China occurs because of an abrupt increase of the northwest events in the peak season (July–September), as the consequence of a statistically westward shift of TC genesis. The relevant variations of TC genesis are shown to be mainly caused by decreased relative vorticity and increased vertical wind shear, which, however, are intrinsically related to the accelerated zonal atmospheric circulation driven by a La Niña–like sea surface warming pattern over the WNP that developed after the end of twentieth century.

Remote forcing effect of sea surface temperatures in the northern tropical Atlantic on tropical cyclone genesis over the Western North Pacific in July

AbstractIn July 2020, neither genesis nor landfall of typhoons occurred in the western North Pacific (WNP). This was the first time that the “no‐typhoon” phenomenon had occurred since 1949. This study analysed the variation in the tropical cyclone genesis frequency (TCGF) in the WNP in July over the past 40 years, including the corresponding atmospheric background circulation and the possible influence of tropical sea surface temperature (SST). The results revealed that the eastward and northward location of the western Pacific subtropical high (WPSH), the deepened and eastward expanded tropical monsoon trough (TMT) in the WNP, the active convection in the main body of TMT, the weakened southwest water vapour transport of the East Asian summer monsoon, as well as the strengthened and eastward expanded equatorial westerly wind were conducive to the tropical cyclone genesis in the WNP in July. Furthermore, we used regression analysis to explore the possible impact of tropical SSTs. We found that the relatively high SSTs in the northern tropical Atlantic (NTA) in June and July triggered a cyclonic circulation over the tropical East Pacific and NTA, resulting in strong upward motion. The airflow subsidence in the tropical central Pacific then caused the WPSH to extend westward and the easterly airflow on the south side of WPSH to strengthen, which weakened the WNP monsoon trough. Besides, the influence of warming NTA SSTs could be conveyed to the Indian Ocean and indirectly induce the enhancement of WPSH. Such a series of the large‐scale circulation variations were not favourable for tropical cyclone genesis in the WNP. As a result, the TCGF was relatively low in July. Thus, one of the causes of the July 2020 “no‐typhoon” phenomenon in the WNP could be the remote forcing effect of SST in NTA.

Seasonal Variation of Tropical Cyclone Genesis and the Related Large-Scale Environments: Comparison between the Bay of Bengal and Arabian Sea Sub-Basins

Using tropical cyclone data along with sea surface temperature data (SST) and atmospheric circulation reanalysis data during the period of 1980–2019, the seasonal variation of tropical cyclone genesis (TCG), and the related oceanic and atmospheric environments over the Arabian Sea (AS) and Bay of Bengal (BOB) are compared and analyzed in detail. The results show that TCG in both the BOB and AS present bimodal seasonal variations, with two peak periods in the pre-monsoon and post-monsoon season, respectively. The frequencies of TCG in the BOB and AS are comparatively similar in the pre-monsoon season but significantly different in the post-monsoon season. During the post-monsoon season of October–November, the TCG frequency in the BOB is approximately 2.3 times higher than that of the AS. The vertical wind shear and relative humidity in the low- and middle-level troposphere are the two major contributing factors for TCG, and the combination of these two factors determines the bimodal seasonal cycle of TCG in both the AS and BOB. In the pre-monsoon season, an increase in the positive contribution of vertical wind shear and a decrease in the negative contribution of relative humidity are collaboratively favorable for TCG in the AS and BOB. During the monsoon season, the relative humidity factor shows a significant and positive contribution to TCG, but its positive effect is offset by the strong negative effect of vertical wind shear and potential intensity, thus resulting in very low TCG in the AS and BOB. However, the specific relative contributions of each environmental factor to the TCG variations in the AS and BOB basins are quite different, especially in the post-monsoon season. In the post-monsoon season, the primary positive contributor to TCG in the AS basin is vertical wind shear, while the combined effect of vertical wind shear and relative humidity dominates in the BOB TCG. From the analysis of environmental factors, atmospheric circulations, and genesis potential index (GPI), the BOB is found to have more favorable TCG conditions than the AS, especially in the post-monsoon season.

Variability of Tropical Cyclone Frequency Over the Western North Pacific in 2018–2020

In this study, we examine the tropical cyclone (TC) activity over the western North Pacific (WNP) in 2018–2020 and its relationship with planetary scale convection and circulation anomalies, which play an important role for TC genesis. To determine the sea surface temperature (SST)-forced atmospheric variability, atmospheric general circulation model (AGCM) ensemble simulations are executed along with the observed SST. For AGCM experiments, we use two different convection schemes to examine uncertainty in convective parameterization and robustness of simulated atmospheric response. The observed TC activity and genesis potential demonstrated consistent features. In our AGCM ensemble simulations, the updated convection scheme improves the simulation ability of observed genesis potential as well as planetary scale convection and circulation features, e.g., in September–October–November (SON), a considerable increase in the genesis potential index over the WNP in SON 2018, WNP in SON 2019, and South China Sea (SCS) in SON 2020, which were not captured in the Emanuel scheme, have been simulated in the updated convection scheme.

Seasonal contrasts between tropical cyclone genesis in the South China Sea and westernmost North Pacific

AbstractClimatologically, tropical cyclone (TC) genesis in the South China Sea (SCS; 108°–120°E, 8°–22°N) and westernmost North Pacific (WmNP; 122°–140°E, 8°–22°N) exhibit different seasonal variability. Maximum TC genesis occurs in July over the WmNP, but in September over the SCS. Based upon genesis potential index (GPI) analysis for the period 1979–2018, the dominant cause of increased TC genesis in the SCS from July to September is a reduction of vertical wind shear of total winds (VWS), while decreased 600‐hPa relative humidity (RH600) acts to suppress TC genesis. Reduced VWS in the SCS is associated with weakening of southwesterly flows and cross‐equatorial southerly flows at 850 hPa and the Tibetan high at 200 hPa. In the WmNP, decreased TC genesis from July to September results from a minor increase in its northern section (north of 15°N) and a major decrease in its southern section (south of 15°N). Decreased TC genesis in the southern section is primarily affected by reduced RH600 and secondly by enhanced VWS. Reduced RH600 is associated with a decrease in total humidity and an increase in 600‐hPa temperature from July to September. Enhanced VWS is mainly caused by intensified entrances of 850‐hPa westerly/southwesterly flows from the SCS and cross‐equatorial flows from tropical oceans in company with weakening of the Pacific subtropical high. The effects of 850‐hPa absolute vorticity and potential intensity associated with the possible maximum wind speed on seasonal variability of TC genesis are insignificant and minor.

Hemisphere-asymmetric tropical cyclones response to anthropogenic aerosol forcing

How anthropogenic forcing could change tropical cyclones (TCs) is a keen societal concern owing to its significant socio-economic impacts. However, a global picture of the anthropogenic aerosol effect on TCs has not yet emerged. Here we show that anthropogenic aerosol emission can reduce northern hemisphere (NH) TCs but increase southern hemisphere (SH) TCs primarily through altering vertical wind shear and mid-tropospheric upward motion in the TC formation zones. These circulation changes are driven by anthropogenic aerosol-induced NH-cooler-than-SH and NH-increased versus SH-decreased meridional (equator to mid-latitudes) temperature gradients. The cooler NH produces a low-level southward cross-equatorial transport of moist static energy, weakening the NH ascent in the TC formation zones; meanwhile, the increased meridional temperature gradients strengthen vertical wind shear, reducing NH TC genesis. The opposite is true for the SH. The results may help to constrain the models’ uncertainty in the future TC projection. Reduction of anthropogenic aerosol emission may increase the NH TCs threat.

Climatological and Seasonal Variations of the Tropical Cyclone Genesis Potential Index Based on Oceanic Parameters in the Global Ocean

Climatological and Seasonal Variations of the Tropical Cyclone Genesis Potential Index Based on Oceanic Parameters in the Global Ocean