It has been challenging to detect trends of tropical cyclone (TC) properties due to temporal heterogeneities and short duration of the direct observations. TCs impact the ocean surface temperature by creating cold wakes as a “fingerprint”. Here we infer changes of the lifetime maximum intensity (LMI), size and integrated kinetic energy from the cold wakes for the period 1982–2019. We find a globally enhanced local cold wake amplitude 3 days after the LMI of − 0.12 ± 0.04 °C per decade whereas the cold wake size does not show any significant change. Multivariate regression models based on the observed ocean cooling, the TC translation speed and the ocean mixed layer depth are applied to infer LMI and TC size. The inferred annual mean global LMI has increased by 1.0 ± 0.7 m s −1 per decade. This inferred trend is between that found for two directly observed data sets. However, the TC size and the TC destructive potential measured by the integrated kinetic energy, have not altered significantly. This analysis provides new independent and indirect evidence of recent TC LMI increases, but a stable size and integrated kinetic energy.

Poleward migrations of tropical cyclones have been observed globally, but their impact on coastal areas remains unclear. We investigated the change in global tropical cyclone activity in coastal regions over the period 1982–2018. We found that the distance of tropical cyclone maximum intensity to land has decreased by about 30 kilometers per decade, and that the annual frequency of global tropical cyclones increases with proximity to land by about two additional cyclones per decade. Trend analysis reveals a robust migration of tropical cyclone activity toward coasts, concurrent with poleward migration of cyclone locations as well as a statistically significant westward shift. This zonal shift of tropical cyclone tracks may be mainly driven by global zonal changes in environmental steering flow.

Climate change leads to sea level rise worldwide, as well as increases in the intensity and frequency of tropical cyclones (TCs). Storm surge induced by TC's, together with spring tides, threatens to cause failure of flood defenses, resulting in massive flooding in low-lying coastal areas. However, limited research has been done on the combined effects of the increasing intensity of TCs and sea level rise on the characteristics of coastal flooding due to the failure of sea dikes. This paper investigates the spatial variation of coastal flooding due to the failure of sea dikes subject to past and future TC climatology and sea level rise, via a case study of a low-lying deltaic city- Shanghai, China. Using a hydrodynamic model and a spectral wave model, storm tide and wave parameters were calculated as input for an empirical model of overtopping discharge rate. The results show that the change of storm climatology together with relative sea level rise (RSLR) largely exacerbates the coastal hazard for Shanghai in the future, in which RSLR is likely to have a larger effect than the TC climatology change on future coastal flooding in Shanghai. In addition, the coastal flood hazard will increase to a large extent in terms of the flood water volume for each corresponding given return period. The approach developed in this paper can also be utilized to investigate future flood risk for other low-lying coastal regions.

Mesoscale ocean temperature anomalies modify a tropical cyclone (TC). Through a modeling study we show that, while the maximum wind speed is rapidly restored after the TC passes a warm- or cold- (eddy size) sea surface temperature (SST) anomaly, the storm size changes are more significant and persistent. The radius of gale force winds and integrated kinetic energy (IKE) can change by more than 10% per degree and this endures several days after crossing an SST anomaly. These properties have a long memory of the impact from the ocean fluxes and depend on the integrated history of SST exposure. They are found to be directly proportional to the storm total precipitation. Accurate continuous forecast of the SST along the track may therefore be of central importance to improving predictions of size and IKE, while instantaneous local SST near the TC core is more important for the forecast of maximum wind speed.

The impact of dry midlevel air on the outer circulation of tropical cyclones is investigated in idealized simulations with and without a moist envelope protecting the inner core. It is found that a dry midlevel layer away from the cyclone center can broaden the outer primary circulation and thus the overall destructive potential at both developing and mature stages. The midlevel outer drying enhances the horizontal gradient of latent heating in the rainbands and drives the expansion of the outer circulation. The moist convection at large radii is suppressed rapidly after the midlevel air is dried in the outer rainbands. An enhanced horizontal gradient of latent heating initiates a radial–vertical overturning circulation anomaly in the rainbands. This anomalous overturning circulation accelerates the radial inflow of the main secondary circulation, increases the angular momentum import, and thus increases the cyclone size. The dry air, mixed into the boundary layer from the midtroposphere, is “recharged” by high enthalpy fluxes because of the increased thermodynamical disequilibrium above the sea surface. This recharge process protects the eyewall convection from the environmental dry-air ventilation. The proposed mechanism may explain the continuous expansion in the tropical cyclone outer circulation after maturity, as found in observations.

There have been no high-frequency aircraft observations of tropical cyclone (TC) eyewall boundary layer turbulence since two flights into Atlantic hurricanes in the 1980s. We present an analysis of the first TC boundary layer flight observations in the South China Sea by the Hong Kong Observatory comprising four eyewall penetrations. We derive the vertical flux of momentum and vertical momentum diffusivity from observed turbulence parameters. We observe negative (upward) vertical fluxes of tangential momentum near the eyewall consistent with a jet below the flight level near the radius of maximum wind. Our observations of vertical momentum diffusivity support a superlinear relationship between diffusivity and wind speed at the high wind speeds in the inner-core of TCs (power-law exponent of 1.73 ± 0.20) while the few existing boundary layer hurricane observations in the North Atlantic suggest a more linear relationship.

Predicting tropical cyclone structure and evolution remains challenging. Particularly, the surface wave interactions with the continental shelf and their impact on tropical cyclones have received very little attention. Through a series of state-of-the-art high-resolution, fully-coupled ocean-wave and atmosphere-ocean-wave experiments, we show here, for the first time, that in presence of continental shelf waves can cause substantial cooling of the sea surface. Through whitecapping there is a transfer of momentum from the surface which drives deeper vertical mixing. It is the waves and not just the wind which become the major driver of stratified coastal ocean ahead-of-cyclone cooling. In the fully-coupled atmosphere-ocean-wave model a negative feedback is found. The maximum wind speed is weaker and the damaging footprint area of hurricane-force winds is reduced by up to 50% due to the strong wave induced ocean cooling ahead. Including wave-ocean coupling is important to improve land falling tropical cyclone intensity predictions for the highly populated and vulnerable coasts.

The characteristics of tropical cyclone intensity and size during the mature stage are presented. Rooted in the classic description by Herbert Riehl, the mature stage is identified as the period from the time of lifetime maximum intensity to the time of lifetime maximum size. This study is the first to analyse the global climatology of the mature stage of tropical cyclones in detail. Three basic features at the mature stage are observed: the reduction of intensity, the outward expansion of the eyewall, and the increase of tangential wind in the outer primary circulation. Globally, about a quarter of tropical cyclones undergo the mature stage. High intensity at the end of the immature stage favours the likelihood of the occurrence of the mature stage. The intensity reduction during the mature stage is considerable with nearly three-quarters of cyclones decreasing by more than 10%, which makes the conventional 'steady-state' presumption questionable. The increase in the radius of damaging-force wind is typically about 50 km, while the decrease in maximum wind speed is typically 20% at the mature stage. However, the average integrated kinetic energy and hence destructive potential increases substantially by about 70%. This is consistent with our finding that most of the highly damaging landfalling hurricanes undergo a mature stage. Intensity downgrades during the mature stage may be misinterpreted as they are mostly not accompanied by an overall danger reduction.

9 It has been challenging to project the tropical cyclone intensity, structure and destructive 10 potential changes in a warming climate. Here we compare the sensitivities of tropical cyclone 11 intensity, size and destructive potential to sea-surface warming with and without a pre-storm 12 atmospheric adjustment to an idealised state of Radiative-Convective Equilibrium (RCE). 13 Without RCE, we find large responses of tropical cyclone intensity, size and destructive 14 potential to sea surface temperature changes, which is in line with some previous studies. 15 However, in an environment under RCE, the tropical cyclone size is almost insensitive to sea 16 surface temperature changes, and the sensitivity of intensity is also much reduced to 3-4% o C -17 1 . Without the pre-storm RCE adjustment, the mean destructive potential during the mature 18 stage increases by about 25% o C -1 . However, in an environment under RCE, the sensitivity of 19 destructive potential to sea-surface warming is only about 3-4% o C -1 . Further analyses show 20 that the reduced response of tropical cyclone intensity and size to sea-surface warming under 21 RCE can be explained by the reduced thermodynamic disequilibrium between the air 22 boundary layer and the sea surface due to the RCE adjustment. When conducting large-scale 23

It is challenging to identify metrics that best capture hurricane destructive potential and costs. Although it has been found that the sea surface temperature and vertical wind shear can both make considerable changes to the hurricane destructive potential metrics, it is still unknown which plays a more important role. Here we present a new method to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the correlation of damage with integrated power dissipation and integrated kinetic energy of all hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are much better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also ultimately the cost of hurricanes. © 2016 IOP Publishing Ltd.

In this paper, a dense sea fog event that occurred over the Yellow Sea (YS) on 9 March 2005 is investigated using the Weather Research and Forecasting Model version 3.1.1 (WRF v3.1.1). It is shown that the WRF can reasonably reproduce the main features of this fog case with a newly implemented planetary boundary layer (PBL) scheme developed by Mellor–Yamada–Nakanishi–Niino (MYNN). The low-level jet (LLJ) associated with this fog episode played an important role in triggering the turbulence. During the fog formation, sea fog extended vertically with the aid of turbulence. The mechanical production term resulting from wind shear contributed to the generation of the turbulence. WRF simulation results showed that the fog layer was thicker in the northeastern part of the YS than that in the southwestern part due to the intensity of the inversion layer and the LLJ. The topography test in which the mountain region in Fujian Province was removed showed that the roles of topography were to prevent the moisture from extending to land, to intensify the inversion layer, and to enhance the intensity of LLJ, as well as to elevate its altitude.

In this paper, almost all available observational data and the latest 6.0 version of Regional Atmospheric Modeling Sys-tem (RAMS) model were employed to investigate a heavy sea fog event occurring over the Yellow Sea from 2 to 5 May 2009. The evolutionary process of this event was documented by using Multifunctional Transport Satellites-1 (MTSAT-1) visible satellite im-agery. The synoptic situation, sounding profiles at two selected stations were analyzed. The difference between the air temperature and sea surface temperature during the sea fog event over the entire sea region was also analyzed. In order to better understand this event, an RAMS modeling with a 15 km×15 km resolution was performed. The model successfully reproduced the main characteris-tics of this sea fog event. The simulated height of fog top and the area of lower atmospheric visibility derived from the RAMS mod-eling results showed good agreement with the sea fog area identified from the satellite imagery. Examinations of both observational data and RAMS modeling results suggested that advection cooling seemed to play an important role in the formation of this sea fog event.