Landfalling tropical cyclones accelerate due to land–sea thermal and roughness contrasts

The National Hurricane Center (NHC) is one of three national centers operated by the National Weather Service (NWS). It has national and international responsibilities for the North Atlantic and eastern North Pacific tropical and subtropical belts (including the Gulf of Mexico and the Caribbean Sea) for tropical analyses, marine and aviation forecasts, and the tropical cyclone forecast and warning programs for the region. Its roots date back to the 1870s, and it is now in the forefront of the NWS modernization program. Numerous changes and improvements have taken place in observational and forecast guidance tools and in the warning and response process over the years. In spite of all these improvement, the loss of property and the potential for loss of life due to tropical cyclones continues to increase rapidly. Forecasts are improving, but not nearly as fast as populations are increasing in hurricane prone areas such as the United States East and Gulf Coast barrier islands. The result is that longer and longer lead times are required for communities to prepare for hurricanes. The sea land over lake surge from hurricanes (SLOSH) model is used to illustrate areas of innudation for the Galveston/Houston, Texas; New Orleans, Louisiana, southwest Florida; and the Atlantic City, New Jersey areas under selected hurricane scenarios. These results indicate the requirement for lengthy evacuation times. The forecast and warning process is then illustrated, starting with tropical analyses, numerical guidance, the meteorological/hydrological coordination of the forecast, and finally the warning coordination and response process. Examples are used to illustrate the sensitivity of the warning and response process to preplanning based upon SLOSH model results, the coordination between NWS and local and state officials, and the critical role played by the media for motivating people to take the desired action in an orderly fashion. These examples illustrate how this process worked to near perfection during Hurricane Hugo, but was disrupted in the Galveston/Houston area by conflicting information reaching local officials and the public during Hurricane Gilbert. Finally, a brief look into the future is attempted, with emphasis upon new observing systems, next generation numerical models and expected improvements in tropical cyclone track and intensity forecasts and the warning process at landfall and inland. The next generation weather radar (NEXRAD) systems in the modernized and restructured NWS are expected to play a major role in improving short-term warnings of flash floods, high winds, and possible tornadoes as hurricanes move inland and start to decay.

Tropical Cyclone Oswald (2013) is considered to be one of the highest-impact storms to make landfall in northern Australia even though it only reached a maximum category 1 intensity on the Australian category scale. After making landfall on the west coast of Cape York Peninsula, Oswald turned southward, and persisted for more than 7 days moving parallel to the coastline as far south as 30°S. As one of the wettest tropical cyclones (TCs) in Australian history, the favorable configurations of a lower-latitude active monsoon trough and two consecutive midlatitude trough–jet systems generally contributed to the maintenance of the Oswald circulation over land and prolonged rainfall. As a result, Oswald produced widespread heavy rainfall along the east coast with three maximum centers near Weipa, Townsville, and Rockhampton, respectively. Using high-resolution WRF simulations, the mechanisms associated with TC Oswald’s rainfall are analyzed. The results show that the rainfall involved different rainfall mechanisms at each stage. The land–sea surface friction contrast, the vertical wind shear, and monsoon trough were mostly responsible for the intensity and location for the first heavy rainfall center on the Cape York Peninsula. The second torrential rainfall near Townsville was primarily a result of the local topography and land–sea frictional convergence in a conditionally unstable monsoonal environment with frictional convergence due to TC motion modulating some offshore rainfall. The third rainfall area was largely dominated by persistent high vertical wind shear forcing, favorable large-scale quasigeostrophic dynamic lifting from two midlatitude trough–jet systems, and mesoscale frontogenesis lifting.

This study examines the potential impacts of large-scale atmospheric circulations that are forced by sea surface temperatures (SST) on global tropical cyclone (TC) formation. Using the Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere and land surface model, version 4 (AM4), under different SST distributions, it is found that the east–west clustering of global TC formation is mainly governed by large-scale circulations in response to given SSTs, instead of direct ocean surface fluxes associated with zonal SST anomalies. Our zonally homogeneous SST simulations in the presence of realistic surface coverage show that TC clusters still emerge as a result of the breakdown of zonal circulations related to land–sea distribution, which produce specific “hotspots” for global TC formation. Sensitivity experiments with different climate warming scenarios and model physics confirm the persistence of these TC clusters in the absence of all zonal SST variations. These robust results offer new insights into the effects of large-scale circulation and terrain forcing on TC clusters beyond the traditional view of direct SST impacts, which are based on the direct alignment of the warmest SST regions and TC clusters. In addition, our experiments also capture internal variability of the global TC frequency, with an average fluctuation of 6–8 TCs at several dominant frequencies of ∼3, 6, and 9 years, even in the absence of all SST interannual variability and ocean coupling. This finding reveals an intrinsic “noise” level of the global TC frequency that one has to take into account when examining the past and future trends in TC activity and their related significance or detectability. Significance Statement In this study, the clustering of global tropical cyclone (TC) formation is investigated, using global simulations under different idealized sea surface temperature (SST) distributions. Our results show that it is the response of the large-scale tropical circulations to SST anomalies that is mostly responsible for the clustering of global TC formation rather than surface flux differences. It is also found that the tropical atmosphere contains inherent fluctuations in the global TC frequency of 6–8 TCs every 3–9 years, even in the absence of all SST interannual and zonal variability. These results offer new insight into the role of tropical dynamics in governing TC climatology and suggest possible mechanisms underlying the clustering of global TC formation under different climate conditions.

Climatic variability during the last ∼90 ka of the southern and northern Levantine Basin as evident from marine records and speleothems

This study investigates the main climatological features of extreme precipitation (TCER) induced by tropical cyclones (TCs) affecting Guangxi (GX), South China using multiple datasets and a 99th percentile threshold during 1981–2020, with an emphasis on the rainfall diversities of different high-impact TC groups and their associated mechanisms. Results show that there are large regional differences and a seasonal imbalance in the climatological features of TCER in GX. In summer (fall), TCs with TCER events primarily move northward or eastward (northwestward or westward), namely, S-NWTCs and S-ETCs (F-WTCs and F-NWTCs). The rainfall centers exhibit asymmetrical features with S-NWTCs and F-NWTCs located in the northeast quadrant, but S-ETCs and F-WTCs in the southwest and northeast quadrants, respectively. Comparisons of atmospheric circulations and environmental factors indicate that the intense rainfall of F-WTCs is mainly attributed to the trough–TC interaction, which is accompanied by stronger upper-level westerly jet and cold air intrusion, thus increasing baroclinic energy and uplifting for the strongest rainfall among these four groups. This interaction is absent for other groups due to a greater South Asian high and western North Pacific subtropical high. Instead, the increased rainfall in S-NWTCs and F-NWTCs can mainly be attributed to the stronger low-level southwesterly jet, which, in combination with low-level warm advection and convergence induced by land–sea friction, promotes the release of latent heat through moisture condensation. S-ETCs differ from S-NWTCs and F-NWTCs in that moisture convergence is weaker due to the much-weakened TC circulation.

Tropical cyclone (TC) is considered as the most dangerous and devastating hydrometeorological natural hazards in the coastal regions. On average, one severe cyclone strikes Bangladesh coast every three years. On the other hand, under significant sediment discharge from the Ganges-Brahmaputra-Meghna (GBM) River system (1.0 ~ 2.4 billion tons/year), Bengal delta dramatically changes its shorelines and bathymetry and has been gaining about 400 km2 land at the eastern part of the Meghna Estuary for the last twenty-seven (1991–2018) years. This study aims to investigate the impact of morphological changes on storm surge induced inundation characteristics on the newly reclaimed coastal lands along with different hypothetical land elevations and Sea level Rise (SLR) scenarios. Five different cyclone tracks are used to generate different cyclonic scenarios with the same strength as TC-1991. This study involves the application of the Delft-3D numerical model and ArcGIS to simulate, calculate, and visualize inundation. The results show that inundation heights strongly depend on the cyclone tracks even if the strength (wind speed and pressure drop) remains the same for all tracks. Also, with the accretion of lands, the inundation depth and extent will decrease at the mainland but increase at the accreted lands and the offshore islands, while with higher land elevation of the accreted lands, it will decrease. With SLRs, the offshore islands and accreted lands are more susceptible than the mainland. The impact of the all over land along with a 1.0 m Sea Level Rise (SLR) on the inundation depth and extent pattern will strongly depend on the elevation of the accreted lands.

Recent composite analysis of landfalling tropical cyclones (TCs) suggests a rain rate peak in the early morning, which contradicts the typically observed peak in convective precipitation over land seen in the late afternoon to early evening. We conducted a set of idealized simulations of TCs and analyzed observational data from TC Bebinca (2018), which stalled near the shoreline of southern China. We show a distinct land–sea contrast in the diurnal variation of TC precipitation and an 8–12 hr offset between the peak precipitation time over land compared with that over the sea in a TC that stalls at the shoreline. The highest land surface temperature and maximum low‐level buoyancy during the afternoon led to peak precipitation over land at this time. However, the peak precipitation over the sea in the early morning was generated by the increase in relative humidity caused by nighttime radiative cooling and enhanced instability.

The precipitation pattern of a landfalling tropical cyclone (TC) with and without a weak environmental vertical wind shear (VWS) is investigated using WRF/NCAR model simulations under idealized conditions. In the simulations without VWS, results show that for the outer band (r ∼ 100–300 km), the cold and dry air originating over smooth land is advected offshore, reduces the stability and develops a band of rainfall on the eastern side of the TC, while the rough land surface tends to trigger more rainfall to the west. For the inner core (r< 100 km), there is only small rainfall asymmetry when the land surface is smooth and dry, but with a rough land surface, the rainfall asymmetry becomes evident and generally stronger rainfall is found over the land areas to the west. Further experiments are performed to compare the effects from weak environmental VWS and land–sea contrast. It is found that the storm‐scale (within 400 km from TC centre) VWS changes continuously in direction and magnitude due to asymmetric diabatic heating and accompanying upper‐level winds. The rainfall pattern in the inner‐core region follows closely the storm‐scale VWS with a downshear‐left relationship regardless of the surface properties, while in the outer‐band region, rainfall distribution is first strongly affected by the surface roughness before landfall and by the environmental VWS afterwards. Therefore, an evolving rainfall–VWS (both environmental and due to storm‐scale dynamics) relationship during TC landfall results.

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate tropical cyclone (TC) wind distribution near landfall. On an f plane at 15°N, the effects of the different surface roughness between the land and sea on the wind asymmetry is examined under a strong constraint of a dry atmosphere and time-invariant axisymmetric mass fields. The winds are found to adjust toward a steady state for prelandfall (50, 100, and 150 km offshore), landfall, and postlandfall (50, 100, and 150 km inland) TC positions. The TC core is asymmetric even when it lies completely offshore or inland. The surface (10 m) wind asymmetry at the core for pre- (post) landfall position is apparently related to the acceleration (deceleration) of the flow that has just moved over the sea (land) as a response to the sudden change of surface friction. For prelandfall TC positions, the resulted strong surface inflow to the left and front left (relative to the direction pointing from sea to land) also induces a tangential (or total) wind maxima at a smaller radius, about 90° downstream of the maximum inflow, consistent with the absolute angular momentum advection (or work done by pressure). The surface maximum wind is of similar magnitude as the gradient wind. There is also a small region of weak outflow just inside the wind maxima. For postlandfall TC positions, inflow is weakened to the right and rear right associated with the onshore flow. Both onshore and offshore flows affect the surface wind asymmetry of the core in the landfall case. Above the surface and near the top of the planetary boundary layer (PBL), the wind is also asymmetric and a strongly supergradient tangential wind is primarily maintained by vertical advection of the radial wind. Much of the steady-state vertical structure of the asymmetric wind is similar to that forced by the motion-induced frictional asymmetry, as found in previous studies. The associated asymmetry of surface and PBL convergences has radial dependence. For example, the landfall case has stronger PBL convergence to the left for the 0–50-km core region, due to the radial inflow, but to the right for the 100–500-km outer region, due to the tangential wind convergence along the coastline. The strong constraint is then removed by considering an experiment that includes moisture, cumulus heating, and the free adjustments of mass fields. The TC is weakening and the sea level pressure has a slightly wavenumber-1 feature with larger gradient wind to the right than to the left, consistent with the drift toward the land. The asymmetric features of the wind are found to be very similar to those in the conceptual experiments.

Severe Tropical Cyclone Veronica impacted the Australian northwest coast in March 2019. Synthetic aperture radar observations showed that the eyewall wind maximum shifted from the left-forward quadrant, characteristic of the motion-induced asymmetry for Southern Hemisphere storms, to the right-forward quadrant as Veronica approached land. Analysis of a large body of similar observations showed that the majority of cyclones within 200 km of land have an eyewall surface wind asymmetry with the strongest winds in the offshore-flow semicircle, similar to that in Veronica, rather than an asymmetry consistent with motion. In contrast, cyclones further from land mostly have the expected motion-induced asymmetry. Simulations with a dynamical tropical cyclone boundary layer demonstrate that the rightward shift of the wind maximum in Veronica was consistent with asymmetric surface friction due to the proximity to land. Analysis of the simulated vertical structure revealed that the asymmetry comprised two counterrotating spirals, similar to those described in a companion paper. The effect of this change in surface wind asymmetry on storm impact is argued to be likely modest, because it occurs largely in the offshore flow, usually away from people and infrastructure. Significance Statement Landfall is the time at which tropical cyclones are generally most dangerous, so understanding the changes that occur during landfall helps to mitigate impact. Significant changes were observed in the near-surface winds of Tropical Cyclone Veronica as it approached the coast, and the wind maximum in this Southern Hemisphere cyclone shifted from its normal position in the left-forward quadrant to the right front. We use a climatological analysis to show that similar changes are common, and a dynamical model to show that they are caused by asymmetric friction due to the land–sea boundary.

Air–sea–land interaction is a branch of boundary‐layer geophysics including coastal meteorology, physical oceanography, and marine geology. Numerous topics related to the air–sea–land interactions during tropical cyclones (TCs) are discussed. Since most of these topics are related to the general public to save life and property during a TC, simplified formulas are presented for use including the rapid estimations of wind, wave, current, storm surge, wave setup, turbulence intensity (TI), gust factor, shoaling depth, and seabed scouring. In addition, when a storm track is located on the left of a major river such as the Mississippi river during Hurricane Isaac in 2012, it is found that the storm surges can propagate more than 228 river miles or 367 km inland and are approximately related to the square root of the distance upstream.

The tropical cyclone boundary layer (TCBL) connecting the underlying terrain and the upper atmosphere plays a crucial role in the overall dynamics of a tropical cyclone system. When tropical cyclones approach the coastline, the wind field inside the TCBL makes a sea–land transition to impact both onshore and offshore structures. So better understanding of the wind field inside the TCBL in the sea–land transition zone is of great importance. To this end, a semiempirical model that integrates the sea–land transition model from the Engineering Sciences Data Unit (ESDU), Huang’s refined TCBL wind field model, and the climate change scenarios from the Coupled Model Intercomparison Project Phase 6 (CMIP6) is used to investigate the influence of climate changes on the sea–land transition of the TCBL wind flow in Hong Kong. More specifically, such a semiempirical method is employed in a series of Monte-Carlo simulations to predict the wind profiles inside the TCBL across the coastline of Hong Kong under the impact of future climate changes. The wind profiles calculated based on the Monte-Carlo simulation results reveal that, under the influences of the most severe climate change scenario, slightly higher and significantly lower wind speeds are found at altitudes above and below 400 m, respectively, compared to the wind speeds recommended in the Hong Kong Wind Code of Practice. Such findings imply that the wind profile model currently adopted by the Hong Kong authorities in assessing the safety of low- to high-rise buildings may be unnecessarily over-conservative under the influence of climate change. On the other hand, the coded wind loads on super-tall buildings slightly underestimate the typhoon impacts under the severe climate change conditions anticipated for coastal southern China.

Atmospheric forcing of the eastern tropical Pacific: A review

Based on high-resolution surface observation and reanalysis data, this paper analyzes the extreme heat events (EHEs) over two densely populated urban agglomerations in southeast China, namely the Yangtze River Delta (YRD) and the Pearl River Delta (PRD), including the spatial–temporal distribution of heatwaves and warm nights and the synoptic backgrounds for regional heatwaves. The results show that the occurrence frequency of EHEs is modulated significantly by local underlying features (i.e., land–sea contrast, terrain), and the strong nocturnal urban heat island effects make warm nights much more likely to occur in cities than rural areas during heatwaves. About 80% of the YRD regional heatwaves occur from 15 July to 15 August, while a lower fraction (53%) of the PRD heatwaves is found during this mid-summer period, which partially explains the warm-season average intensity of the former being 2–3 times the latter. A persistent, profound subtropical high is the dominant synoptic system responsible for the mid-summer YRD heatwaves, which forces significant descending motion leading to long-duration sunny weather. The mid-summer PRD heatwaves involve both high-pressure systems and tropical cyclones (TCs). A TC is present to the east of the PRD region on most (about 72%) PRD heatwave days. The organized northerly winds in the planetary boundary layer in the outer circulation of the TC transport the inland warm air, which is heated by the foehn effect at the lee side of the Nanling Mountains and possibly also the surface sensible heat flux, towards the PRD region, leading to the occurrence of the extremely high temperatures.

Hourly rainfall from automatic weather stations and reanalysis data from MERRA-2 are used to investigate the diurnal variation of precipitation in Hong Kong, a site along the southeast China coast with strong interactions between the monsoonal circulation and the land–sea breeze. The precipitation in Hong Kong is characterized by a spatially uniform diurnal cycle with the peak at about 0800 local time (LT), with rather weak dependence on local terrain. Precipitation unrelated to tropical cyclones (TCs) dominates the diurnal variation of precipitation, especially in the summer. The diurnal cycle exhibits a notable seasonal dependence, with the strongest signal in the summer. The morning peak of precipitation over Hong Kong is coincident with deep rising motion, linking to near-surface convergence and overlying weak divergence. The convergence may be attributed to the prevalence of the southerly monsoonal flow over the South China Sea (SCS) and to the northerly land breeze induced by the land–sea thermal contrast in the morning. The overlying weak divergence could be ascribed to the nocturnal–early morning acceleration of southerly flow over southeast China. Linked to the inverse relationship between monsoon intensity and the land–sea thermal contrast, the diurnal cycle of precipitation is strengthened when the SCS monsoon is active and weakened when the land–sea thermal contrast is high. Both the cloud-top radiative cooling effect and the enhanced radiative cooling over inland cloud-free areas also play roles in the development of the morning rainfall peak over Hong Kong.