Publications
180 results found
Lai T-K, Toumi R, 2023, Has there been a recent shallowing of tropical cyclones?, Geophysical Research Letters, Vol: 50, Pages: 1-9, ISSN: 0094-8276
Many aspects of tropical cyclone (TC) properties at the surface have been changing but any systematic vertical changes are unknown. Here, we document a recent trend of high thick clouds of TCs. The global inner-core high thick cloud fraction measured by satellite has decreased from 2002 to 2021 by about 10% per decade. The TC inner-core surface rain rate is also found to have decreased during the same period by a similar percentage. This suppression of high thick clouds and rain has been largest during the intensification phase of the strongest TCs. Hence, these two independent and consistent observations suggest that the TC inner-core convection has weakened and that TCs have become shallower recently at least. For this period, the lifetime maximum intensity of major TCs has not changed and this suggests an increased efficiency of the spin-up of TCs.
Gangopadhyay A, Seshadri AK, Toumi R, 2023, Beneficial role of diurnal smoothing for grid integration of wind power, ENVIRONMENTAL RESEARCH LETTERS, Vol: 18, ISSN: 1748-9326
Li Y, Tang Y, Toumi R, et al., 2022, Revisiting the Definition of Rapid Intensification of Tropical Cyclones by Clustering the Initial Intensity and Inner-Core Size, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 127, ISSN: 2169-897X
Sparks N, Toumi R, 2022, The Dependence of Tropical Cyclone Pressure Tendency on Size, GEOPHYSICAL RESEARCH LETTERS, Vol: 49, ISSN: 0094-8276
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Sparks N, Toumi R, 2022, A physical model of tropical cyclone central pressure filling at landfall, Journal of the Atmospheric Sciences, Vol: 79, Pages: 2585-2599, ISSN: 0022-4928
We derive a simple physically based analytic model which describes the pressure filling of a tropical cyclone (TC) over land. Starting from the axisymmetric mass continuity equation in cylindrical coordinates we derive that the half-life decay of the pressure deficit between the environmentand TC centre is proportional to the initial radius of maximum surface wind speed. The initial pressure deficit and column-mean radial inflow speed into the core are the other key variables. The assumptions made in deriving the model are validated against idealised numerical simulations of TC decay over land. Decay half-lives predicted from a range of initial TC states are tested against the idealized simulations and are in good agreement. Dry idealised TC decay simulations show that without latent convective heating, the boundary layer decouples from the vortex above leading to a fast decay of surface winds while a mid-level vortex persists.
Wang S, Toumi R, 2022, Author Correction: On the intensity decay of tropical cyclones before landfall., Sci Rep, Vol: 12
Xu H, Tian Z, Sun L, et al., 2022, Compound flood impact of water level and rainfall during tropical cyclone periods in a coastal city: the case of Shanghai, NATURAL HAZARDS AND EARTH SYSTEM SCIENCES, Vol: 22, Pages: 2347-2358, ISSN: 1561-8633
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Tsui EYL, Toumi R, 2022, Pacific subsurface temperature as a long‐range indicator of El Niño, regional precipitation, and fire, Quarterly Journal of the Royal Meteorological Society, Vol: 148, Pages: 2102-2117, ISSN: 0035-9009
The SubNiño4 index based on the subsurface potential temperature around the thermocline beneath the west Pacific warm pool, the Niño 4 region, is examined as a long-range indicator of the surface El Niño–Southern Oscillation (ENSO) and ENSO-driven atmospheric response. The SubNiño4 index captures the evolution of subsurface ocean heat content between the El Niño and La Niña phases of the ENSO cycle, allowing it to serve as a long-range indicator of surface ENSO and hence also many ENSO-driven atmospheric anomalies. The SubNiño4 index has more temporally stable correlations with Niño 3.4 than the widely used western equatorial Pacific warm-water volume indicator. For a lead time of the order of 12 months, Niño 3.4 correlations afforded by the lead observed SubNiño4 index become similar to and can exceed those produced by typical dynamical ENSO predictions. The value and viability of the SubNiño4 index as a simple statistical long-range indicator of ENSO-driven atmospheric response is shown for regional precipitation anomalies throughout the Tropics and fires in Continental and Maritime Southeast Asia.
Wang S, Toumi R, 2022, An analytic model of the tropical cyclone outer size, npj Climate and Atmospheric Science, Vol: 5, ISSN: 2397-3722
There are simple conceptual models of tropical cyclone intensification and potential intensity. However, such a framework has been lacking to describe the evolution of the outer circulation. An analytic growth model of the tropical cyclone outer size is derived from the angular momentum equation. The growth model fits a full-physics idealized tropical cyclone simulation. The lifecycle composite of the best-track outer size growth shows a strong super-linear nature, which supports an exponential growth as predicted by the growth model. The climatology of outer size growth measured by the radius of gale-force wind in the North Atlantic and Eastern Pacific during the period 2004–2017, can be understood in terms of four growth factors of the model: the initial size, the growth duration, the mean growth latitude, and the mean top-of-boundary-layer effective local inflow angle. All four variables are significantly different between the two basins. The observed lifetime maximum size follows a lognormal distribution, which is in line with the law of the proportionate effect of this exponential growth model. The growth model fits the observed outer size well in global basins. The time constant of the exponential size growth is approximately equal to the product of the Coriolis parameter and the mean effective inflow angle above the boundary layer. Further sensitivity experiments with the growth model suggest that the interannual variability of the global lifetime maximum size is largely driven by the variation of growth duration.
Lok CCF, Chan JCL, Toumi R, 2022, Importance of air-sea coupling in simulating tropical cyclone intensity at landfall, Advances in Atmospheric Sciences, Vol: 39, Pages: 1777-1786, ISSN: 0256-1530
An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones (TCs) along the South China coast is upgraded by including ocean and wave models. A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea. Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size. In particular, interaction with the ocean does not necessarily imply a weakening of the TC, with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC. These results suggest that to simulate the evolution of TC structure more accurately, it is essential to use an air-sea coupled model instead of an atmosphere-only model.
Gangopadhyay A, Seshadri AK, Sparks NJ, et al., 2022, The role of wind-solar hybrid plants in mitigating renewable energy-droughts, Renewable Energy, ISSN: 0960-1481
Gangopadhyay A, Sparks NJ, Toumi R, et al., 2022, Risk assessment of wind droughts over India, Current Science, Vol: 122, Pages: 1145-1153, ISSN: 0011-3891
Wind power growth makes it essential to simulateweather variability and its impacts on the electricitygrid. Low-probability, high-impact weather events suchas a wind drought are important but difficult to identify based on limited historical datasets. A stochasticweather generator, Imperial College Weather Generator (IMAGE), is employed to identify extreme eventsthrough long-period simulations. IMAGE capturesmean, spatial correlation and seasonality in wind speedand estimates return periods of extreme wind eventsover India. Simulations show that when Rajasthan experiences wind drought, southern India continues tohave wind, and vice versa. Regional grid-scale winddroughts could be avoided if grids are strongly interconnected across the country.
Kotova L, Costa MM, Jacob D, et al., 2022, Climateurope festival: an innovative way of linking science and society, Climate Services, Vol: 26, ISSN: 2405-8807
The Climateurope Festivals were designed to create synergies between different European, national and international initiatives in the fields of Earth-system modelling & Climate Services and enhance the transfer of information between suppliers and users. It gave an opportunity to display best in class outcomes and engage in world class networking in a less rigid environment than a scientific conference.A number of formats were adopted in the Festival, from traditional impulse talks to innovative interactive sessions, and the thought-provoking discussions allowed the participants to share their experiences and knowledge around the advantages and challenges that Climate Services face within different sectors.Three Climateurope Festivals were originally planned to be held across Europe. Two Festivals were successfully organised, the first in Valencia in 2017, and the second in Belgrade in 2018. Due to the COVID-19 pandemic and associated lockdowns and travel restrictions, the third and final Festival was held online as a series of virtual web-based Festivals in 2020/2021.The Festivals were highly valued by participants. There was a strong desire by the Climateurope network to continue a science-stakeholder dialogue and make the Climateurope Festivals a regular event.
Wang S, Toumi R, 2022, More tropical cyclones are striking coasts with major intensities at landfall, Scientific Reports, Vol: 12, ISSN: 2045-2322
In this study we show that the number of annual global tropical cyclone (TC) landfalls with major landfall intensity (LI≥50 m s-1) has nearly doubled from 1982 to 2020. The lifetime maximum intensity (LMI) of global major landfalling TCs has been increasing by 0.8 m s-1 per decade (p<0.05), but this significance of intensity change disappears at landfall (0.3 m s-1 per decade, p=0.69). The lack of a significant LI trend is caused by the much larger variance of LI than that of LMI in all basins and explains why a significant count change of TCs with major intensity at landfall has only now emerged. Basin-wide TC trends of intensity and spatial distribution have been reported, but this long-term major TC landfall count change may be the most socio-economic significant.
Wang S, Toumi R, 2022, On the intensity decay of tropical cyclones before landfall, Scientific Reports, Vol: 12, ISSN: 2045-2322
It remains unclear how tropical cyclones (TCs) decay from their ocean lifetime maximum intensity (LMI) to landfall intensity (LI), yet this stage is of fundamental importance governing the socio-economic impact of TCs. Here we show that TCs decay on average by 25% from LMI to LI. A logistic decay model of energy production by ocean enthalpy input and surface dissipation by frictional drag, can physically connect the LMI to LI. The logistic model fits the observed intensity decay as well as an empirically exponential decay does, but with a clear physical foundation. The distance between locations of LMI and TC landfall is found to dominate the variability of the decay from the LMI to LI, whereas environmental conditions are generally less important. A major TC at landfall typically has a very large LMI close to land. The LMI depends on the heating by ocean warming, but the LMI location is also important to future landfall TC intensity changes which are of socio-economic importance.
Wang S, Toumi R, 2021, Recent tropical cyclone changes inferred from ocean surface temperature cold wakes, Scientific Reports, Vol: 11, ISSN: 2045-2322
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.
Phillipson LM, Toumi R, 2021, A physical interpretation of recent tropical cyclone post‐landfall decay, Geophysical Research Letters, Vol: 48, Pages: 1-9, ISSN: 0094-8276
The decay of landfalling tropical cyclones is important to the damage caused. We examine a simple physically based decay model of maximum surface winds driven by frictional turbulent drag and a modification accounting for partial to complete land roughness. The model fits an algebraic decay with a parameter determined by the ratio of the surface drag coefficient to the effective vortex depth. This parameter has been decreasing from 1980 to 2018. There is also a global mean increase of wind speed 24 h after landfall of +1.13 m/s per decade. We cannot exclude the possibility that this trend is driven by the initial wind speed increase, but it is most likely due to a slowing of the decay. This weaker decay amounts to an additional 7 h of gale force winds for a typical Category 1 at landfall.
Lok CCF, Chan JCL, Toumi R, 2021, Tropical cyclones near landfall can induce their own intensification through feedbacks on radiative forcing, COMMUNICATIONS EARTH & ENVIRONMENT, Vol: 2
Tsui EYL, Toumi R, 2021, Hurricanes as an enabler of Amazon fires, Scientific Reports, Vol: 11, Pages: 1-8, ISSN: 2045-2322
A teleconnection between North Atlantic tropical storms and Amazon fires is investigated as a possible case of compound remote extreme events. The seasonal cycles of the storms and fires are in phase with a maximum around September and have significant inter-annual correlation. Years of high Amazon fire activity are associated with atmospheric conditions over the Atlantic which favour tropical cyclones. We propose that anomalous precipitation and latent heating in the Caribbean, partly caused by tropical storms, leads to a thermal circulation response which creates anomalous subsidence and enhances surface solar heating over the Amazon. The Caribbean storms and precipitation anomalies could thus promote favourable atmospheric conditions for Amazon fire.
Ke Q, Yin J, Bricker JD, et al., 2021, An integrated framework of coastal flood modelling under the failures of sea dikes: a case study in Shanghai (Jun, 10.1007/S11069-021-04853-Z, 2021), NATURAL HAZARDS, Vol: 109, Pages: 705-706, ISSN: 0921-030X
Ke Q, Yin J, Bricker JD, et al., 2021, An integrated framework of coastal flood modelling under the failures of sea dikes: a case study in Shanghai, NATURAL HAZARDS, Vol: 109, Pages: 671-703, ISSN: 0921-030X
Wang S, Toumi R, Ye Q, et al., 2021, Is the tropical cyclone surge in Shanghai more sensitive to landfall location or intensity change?, ATMOSPHERIC SCIENCE LETTERS, Vol: 22, ISSN: 1530-261X
Phillipson L, Li Y, Toumi R, 2021, Strongly coupled assimilation of a hypothetical ocean current observing network within a regional ocean-atmosphere coupled model: an OSSE case study of typhoon hato, Monthly Weather Review, Vol: 149, Pages: 1317-1336, ISSN: 0027-0644
The forecast of tropical cyclone (TC) intensity is a significant challenge. In this study, we showcase the impact of strongly coupled data assimilation with hypothetical ocean currents on analyses and forecasts of Typhoon Hato (2017). Several observation simulation system experiments (OSSE) were undertaken with a regional coupled ocean–atmosphere model. We assimilated combinations of (or individually) a hypothetical coastal current HF radar network, a dense array of drifter floats, and minimum sea level pressure. During the assimilation, instant updates of many important atmospheric variables (winds and pressure) are achieved from the assimilation of ocean current observations using the cross-domain error covariance, significantly improving the track and intensity analysis of Typhoon Hato. Relative to a control experiment (with no assimilation), the error of minimum pressure decreased by up to 13 hPa (4 hPa/57% on average). The maximum wind speed error decreased by up to 18 kt (5 kt/41% on average) (1 kt ≈ 0.5 m s−1). By contrast, weakly coupled implementations cannot match these reductions (10% on average). Although traditional atmospheric observations were not assimilated, such improvements indicate that there is considerable potential in assimilating ocean currents from coastal HF radar and surface drifters within a strongly coupled framework for intense landfalling TCs.
Smith M, Toumi R, 2021, Using video recognition to identify tropical cyclone positions, Geophysical Research Letters, Vol: 48, Pages: 1-9, ISSN: 0094-8276
Tropical cyclone (TC) center fixing is a challenge for improving forecasting and establishing TC climatologies. We propose a novel objective solution through the use of video recognition algorithms. The videos of tropical cyclones in the Western North Pacific are of sequential, hourly, geostationary satellite infrared (IR) images. A variety of convolutional neural network architectures are tested. The best performing network implements convolutional layers, a convolutional long short-term memory layer, and fully connected layers. Cloud features rotating around a center are effectively captured in this video-based technique. Networks trained with long-wave IR channels outperform a water vapor channel-based network. The average position across the two IR networks has a 19.3 km median error across all intensities. This equates to a 42% lower error over a baseline technique. This video-based method combined with the high geostationary satellite sampling rate can provide rapid and accurate automated updates of TC centers.
Toumi R, 2021, 100 Years of meteorology at Imperial College, Weather, Vol: 76, Pages: 119-119, ISSN: 0043-1656
Sparks N, Toumi R, 2021, On the seasonal and sub-seasonal factors influencing East China tropical cyclone landfall, Atmospheric Science Letters, Vol: 22, Pages: 1-8, ISSN: 1530-261X
To date it has proved difficult to make seasonal forecasts of tropical cyclones, particularly for landfall and in East China specifically. This study examines sources of predictability for the number of landfalling typhoons in East China on seasonal (June–October) and sub‐seasonal time scales. East China landfall count is shown to be independent of basin‐scale properties of TC tracks, such the genesis location, duration, basin track direction and length, and basin total count. Large‐scale environmental climate indices which are potential basin scale drivers are also shown to be largely uncorrelated with landfall prior to and throughout the season. The most important factor is the steering in the final stages to landfall. The seasonal landfall is strongly anti‐correlated with the more local zonal mid‐tropospheric wind field over the East China sea (r = −.61, p < .001). It is proposed that geopotential height anomalies over Korea/Japan cause anomalous easterly winds in the East China Sea and enhance landfall rates by steering typhoons onto the coast. Early, peak, and late sub‐seasonal landfall counts are shown to be independent of each other yet share this predictor. This local feature may be dynamically predictable allowing a potential hybrid dynamical‐statistical seasonal forecast of landfall.
Toumi R, Wang S, 2021, Recent migration of tropical cyclones toward coasts, Science, Vol: 371, Pages: 514-517, ISSN: 0036-8075
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.
Smith M, Toumi R, 2021, A dipole of tropical cyclone outgoing long-wave radiation, Quarterly Journal of the Royal Meteorological Society, Vol: 147, Pages: 166-180, ISSN: 0035-9009
Large‐scale (500 ≤ r ≤ 2,200 km) outgoing long‐wave radiation (OLR) and water vapour (WV) fields are investigated in satellite observations over the Pacific, linked to ERA‐5 reanalysis data and the ECMWF ensemble forecasts of tropical cyclones (TC) globally. A large‐scale OLR dipole pattern of low and high fluxes are found in both the observations and model. As expected, a low OLR region is positioned within the TC circulation, but there is also a robust high OLR region poleward and west of the TC centre. A dry “black hole” on WV grey‐scale imagery occupies the same region of high OLR. Relative to the central low OLR TC signal, the typical dipole magnitude, distance and orientation of the high OLR regions are 230 W·m−2, 1,150 km, and 145° anticlockwise from east. From the reanalysis we find that the interaction between the vortex and the environmental flows produces upper‐level convergence, low‐level divergence and subsidence throughout the troposphere in the region of high OLR. Analysis of the ECMWF model shows that the position of the high OLR region rotates anticlockwise about the TC centre as the TC moves from westward to eastward. Through use of a sub‐ensemble, we test if capturing the high OLR anomaly has a significant relationship with TC track. We apply a perfect model approach and find that sub‐ensembles that are composed of models whose large‐scale OLR fields closely match the target TC also have a better track. This improvement is mostly attributed to the high OLR component of the dipole.
Parks RM, Bennett JE, Tamura-Wicks H, et al., 2020, Reply to: Concerns over calculating injury-related deaths associated with temperature, NATURE MEDICINE, Vol: 26, ISSN: 1078-8956
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Sparks N, Toumi R, 2020, Pacific subsurface ocean temperature as a long-rangepredictor of South China tropical cyclone landfall, Communications Earth & Environment, Vol: 1, ISSN: 2662-4435
Seasonal forecasts of the tropical cyclones which frequently make landfall along the densely populated South China coast are highly desirable. Here, we analyse observations of landfalling tropical cyclones in South China and of subsurface ocean temperatures in the Pacific warm pool region, and identify the possibility of forecasts of South China tropical cyclone landfall a year ahead. Specifically, we define a subsurface temperature index, subNiño4, and build a predictive model based on subNiño4 anomalies with a robust double cross-validated forecast skill against climatology of 23%, similar in skill to existing forecasts issued much later in the spring. We suggest that subNiño4 ocean temperatures precede the surface El Niño/Southern Oscillation state by about 12 months, and that the zonal shifts in atmospheric heating then change mid-level winds to steer tropical cyclones towards landfall in South China. We note that regional subsurface ocean temperature anomalies may permit atmospheric predictions in other locations at a longer range than is currently thought possible.
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