Imperial College London

ProfessorRalfToumi

Faculty of Natural SciencesDepartment of Physics

Professor of Atmospheric Physics
 
 
 
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Contact

 

+44 (0)20 7594 7668r.toumi Website

 
 
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Location

 

713Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

140 results found

wang S, Toumi R, 2019, Impact of dry midlevel air on the tropical cyclone outer circulation, Journal of the Atmospheric Sciences, ISSN: 0022-4928

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 due to 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.

Journal article

Parks R, Mclaren M, Toumi R, Rivett Uet al., 2019, Experiences and lessons in managing water from Cape Town

Water shortages will become more common in cities around the world during the 21st century due to climate change.• Cape Town, South Africa experienced an especially severe drought in 2017-2018 after several years of low rainfall. This drought prompted an estimate of Day Zero, when freshwater reservoir levels supplying the city would fall below 13.5% of capacity and the majority of the municipal water network would be shut down.• In response to this crisis, the City of Cape Town municipal government significantly extended an existing set of rules and regulations, and introduced additional measures, to limit water demand. These actions included restricting available water; new tariffs to penalise excess water usage; water management devices installed in domestic properties; and novel communication strategies.• The water crisis has had widespread economic and social impacts, with damage to the tourist and agriculture industries; and tensions between sections of society and government. • Any city under water stress, like Cape Town, needs a long-term strategy for water supply and demand. Such a strategy should include diversity of water sources, equity of service provisions, thoughtful but forceful messaging, early warning systems and co-operation between local, regional and national levels of government.

Report

Camp J, Roberts MJ, Comer RE, Wu P, MacLachlan C, Bett PE, Golding N, Toumi R, Chan JCLet al., 2019, The western Pacific subtropical high and tropical cyclone landfall: Seasonal forecasts using the Met Office GloSea5 system, Quarterly Journal of the Royal Meteorological Society, Vol: 145, Pages: 105-116, ISSN: 0035-9009

We investigate the relationship between the western Pacific subtropical high (WPSH) and tropical cyclone (TC) landfall in the ERA‐Interim reanalysis and two configurations of the UK Met Office Global Seasonal forecasting system version 5 (GloSea5): Global Atmosphere 3.0 (GA3) and Global Coupled configuration 2 (GC2). Both model configurations have the same horizontal and vertical resolution in the ocean and the atmosphere, but differ in terms of model physics. The WPSH strongly modulates TC activity over the subtropical western North Pacific (WNP) and TC landfall over East Asia (Japan, Korea and East China). Here we show that both model configurations GA3 and GC2 show significant skill for predictions of the WPSH and TC variability over the subtropical WNP, as well as TC frequency along the coast of East Asia, during the boreal summer (June–August). An extension of the analysis to include the full WNP typhoon season (June–November) is also examined; however, only a weak significant relationship between the WPSH index and the observed TC frequency over East Asia is found during this period, and no significant relationship is present in either GloSea5 GA3 or GC2. Results highlight the potential for operational seasonal forecasts of TC landfall risk for Japan, Korea and East China over the June–August period using predictions of the WPSH indices from GloSea5.

Journal article

Parks RM, Bennett J, Foreman K, Toumi R, Ezzati Met al., 2018, National and regional seasonal dynamics of all-cause and cause-specific mortality in the USA from 1980 to 2016, eLife, Vol: 7, ISSN: 2050-084X

In temperate climates, winter deaths exceed summer ones. However, there is limited information on the timing and the relative magnitudes of maximum and minimum mortality, by local climate, age group, sex and medical cause of death. We used geo-coded mortality data and wavelets to analyse the seasonality of mortality by age group and sex from 1980 to 2016 in the USA and its subnational climatic regions. Death rates in men and women ≥ 45 years peaked in December to February and were lowest in June to August, driven by cardiorespiratory diseases and injuries. In these ages, percent difference in death rates between peak and minimum months did not vary across climate regions, nor changed from 1980 to 2016. Under five years, seasonality of all-cause mortality largely disappeared after the 1990s. In adolescents and young adults, especially in males, death rates peaked in June/July and were lowest in December/January, driven by injury deaths.

Journal article

Li YI, Toumi R, 2018, Improved tropical cyclone intensity forecasts by assimilating coastal surface currents in an idealized study, Geophysical Research Letters, Vol: 45, Pages: 10019-10026, ISSN: 0094-8276

High‐frequency (HF) radars can provide high‐resolution and frequent ocean surface currents observations during tropical cyclone (TC) landfall. We describe the first assimilation of such potential observations using idealized twin experiments with and without these observations. The data assimilation system consists of the Ensemble Adjustment Kalman Filter and a coupled ocean‐atmosphere model. In this system, synthetic HF radar‐observed coastal currents are assimilated, and the 24‐, 48‐ and 72‐hr forecast performances are examined for TCs with various intensities, sizes, and translation speeds. Assimilating coastal surface currents improves the intensity forecast. The errors of the maximum wind speed reduce by 2.7 (33%) and 1.9 m/s (60%) in the 72‐hr forecast and 2.8 (40%) and 1.4 m/s (62%) in the 48‐hr forecast, for Category 4 and 2 cyclones, respectively. These improvements are similar to the current operational TC forecast errors, so that assimilating HF radar observations could be a substantial benefit.

Journal article

Wang S, Toumi R, 2018, Reduced sensitivity of tropical cyclone intensity and size to sea surface temperature in a radiative-convective equilibrium environment, Advances in Atmospheric Sciences, Vol: 35, Pages: 981-993, ISSN: 1861-9533

It has been challenging to project the tropical cyclone (TC) intensity, structure and destructive potential changes in a warming climate. Here, we compare the sensitivities of TC intensity, size and destructive potential to sea surface warming with and without a pre-storm atmospheric adjustment to an idealized state of Radiative-Convective Equilibrium (RCE). Without RCE, we find large responses of TC intensity, size and destructive potential to sea surface temperature (SST) changes, which is in line with some previous studies. However, in an environment under RCE, the TC size is almost insensitive to SST changes, and the sensitivity of intensity is also much reduced to 3% °C−1–4% °C−1. Without the pre-storm RCE adjustment, the mean destructive potential measured by the integrated power dissipation increases by about 25% °C−1 during the mature stage. However, in an environment under RCE, the sensitivity of destructive potential to sea surface warming does not change significantly. Further analyses show that the reduced response of TC intensity and size to sea surface warming under RCE can be explained by the reduced thermodynamic disequilibrium between the air boundary layer and the sea surface due to the RCE adjustment. When conducting regional-scale sea surface warming experiments for TC case studies, without any RCE adjustment the TC response is likely to be unrealistically exaggerated. The TC intensity–temperature sensitivity under RCE is very similar to those found in coupled climate model simulations. This suggests global mean intensity projections under climate change can be understood in terms of a thermodynamic response to temperature with only a minor contribution from any changes in large-scale dynamics.

Journal article

Bruneau N, Toumi R, Wang S, 2018, Publisher correction: Impact of wave whitecapping on land falling tropical cyclones, Scientific Reports, Vol: 8, ISSN: 2045-2322

Journal article

Wang S, Toumi R, 2018, A historical analysis of the mature stage of tropical cyclones, International Journal of Climatology, Vol: 38, Pages: 2490-2505, ISSN: 0899-8418

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.

Journal article

Bruneau N, Toumi R, Wang S, 2018, Impact of wave whitecapping on land falling tropical cyclones, Scientific Reports, Vol: 8, ISSN: 2045-2322

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

Journal article

Phillipson L, Toumi R, 2018, The Crossover Time as an Evaluation of OceanModels Against Persistence, Geophysical Research Letters, Vol: 45, Pages: 250-257, ISSN: 0094-8276

A new ocean evaluation metric, the crossover time, is defined as the time it takes for a numerical model to equal the performance of persistence. As an example, the average crossover time calculated using the Lagrangian separation distance (the distance between simulated trajectories and observed drifters) for the global MERCATOR ocean model analysis is found to be about six days. Conversely, the model forecast has an average crossover time longer than six days, suggesting limited skill in Lagrangian predictability by the current generation of global ocean models. The crossover time of the velocity error is less than three days, which is similar to the average decorrelation time of the observed drifters. The crossover time is a useful measure to quantify future ocean model improvements.

Journal article

Arcucci R, Carracciuolo L, Toumi R, 2018, Toward a preconditioned scalable 3DVAR for assimilating Sea Surface Temperature collected into the Caspian Sea, Journal of Numerical Analysis, Industrial and Applied Mathematics, Vol: 12, Pages: 9-28, ISSN: 1790-8140

© 2018 European Society of Computational Methods in Sciences and Engineering. Data Assimilation (DA) is an uncertainty quantification technique used to incorporate observed data into a prediction model in order to improve numerical forecasted results. As a crucial point into DA models is the ill conditioning of the covariance matrices involved, it is mandatory to introduce, in a DA software, preconditioning methods. Here we present first results obtained introducing two different preconditioning methods in a DA software we are developing (we named S3DVAR) which implements a Scalable Three Dimensional Variational Data Assimilation model for assimilating sea surface temperature (SST) values collected into the Caspian Sea by using the Regional Ocean Modeling System (ROMS) with observations provided by the Group of High resolution sea surface temperature (GHRSST). We present the algorithmic strategies we employ and the numerical issues on data collected in two of the months which present the most significant variability in water temperature: August and March.

Journal article

D'Amore L, Arcucci R, Li Y, Montella R, Moore A, Phillipson L, Toumi Ret al., 2018, Performance Assessment of the Incremental Strong Constraints 4DVAR Algorithm in ROMS, 12th International Conference on Parallel Processing and Applied Mathematics (PPAM), Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 48-57, ISSN: 0302-9743

Conference paper

bruneau N, zika J, Toumi R, 2017, Can the ocean’s heat engine control horizontal circulation? Insights from the Caspian Sea., Geophysical Research Letters, Vol: 44, Pages: 9893-9900, ISSN: 0094-8276

We investigate the role of the ocean's heat engine in setting horizontal circulation using a numerical model of the Caspian Sea. The Caspian Sea can be seen as a virtual laboratory - a compromise between realistic global models which are hampered by long equilibration times and idealized basin geometry models which are not constrained by observations. We find that increases in vertical mixing drive stronger thermally direct overturning and consequent conversion of available potential to kinetic energy. Numerical solutions with water mass structures closest to observations overturn 0.02 − 0.04 x 106m3/s(Sv) representing the first estimate of Caspian Sea overturning. Our results also suggest that the overturning is thermally-forced increasing in intensity with increasing vertical diffusivity. Finally, stronger thermally direct overturning is associated with a stronger horizontal circulation in the Caspian Sea. This suggests the ocean's heat engine can strongly impact broader horizontal circulations in the ocean.

Journal article

Li Y, Toumi R, 2017, A balanced Kalman filter ocean data assimilation system with application to the South Australian Sea, Ocean Modelling, Vol: 116, Pages: 159-172, ISSN: 1463-5003

n this paper, an Ensemble Kalman Filter (EnKF) based regional ocean data assimilation system has been developed and applied to the South Australian Sea. This system consists of the data assimilation algorithm provided by the NCAR Data Assimilation Research Testbed (DART) and the Regional Ocean Modelling System (ROMS). We describe the first implementation of the physical balance operator (temperature-salinity, hydrostatic and geostrophic balance) to DART, to reduce the spurious waves which may be introduced during the data assimilation process. The effect of the balance operator is validated in both an idealised shallow water model and the ROMS model real case study. In the shallow water model, the geostrophic balance operator eliminates spurious ageostrophic waves and produces a better sea surface height (SSH) and velocity analysis and forecast. Its impact increases as the sea surface height and wind stress increase. In the real case, satellite-observed sea surface temperature (SST) and SSH are assimilated in the South Australian Sea with 50 ensembles using the Ensemble Adjustment Kalman Filter (EAKF). Assimilating SSH and SST enhances the estimation of SSH and SST in the entire domain, respectively. Assimilation with the balance operator produces a more realistic simulation of surface currents and subsurface temperature profile. The best improvement is obtained when only SSH is assimilated with the balance operator. A case study with a storm suggests that the benefit of the balance operator is of particular importance under high wind stress conditions. Implementing the balance operator could be a general benefit to ocean data assimilation systems.

Journal article

Arcucci R, Celestino S, Toumi R, Laccetti Get al., 2017, Toward the S3DVAR data assimilation software for the Caspian Sea, International Conference on Numerical Analysis and Applied Mathematics (ICNAAM), Publisher: AIP Publishing, ISSN: 1551-7616

Data Assimilation (DA) is an uncertainty quantification technique used to incorporate observed data into a prediction model in order to improve numerical forecasted results. The forecasting model used for producing oceanographic prediction into the Caspian Sea is the Regional Ocean Modeling System (ROMS). Here we propose the computational issues we are facing in a DA software we are developing (we named S3DVAR) which implements a Scalable Three Dimensional Variational Data Assimilation model for assimilating sea surface temperature (SST) values collected into the Caspian Sea with observations provided by the Group of High resolution sea surface temperature (GHRSST). We present the algorithmic strategies we employ and the numerical issues on data collected in two of the months which present the most significant variability in water temperature: August and March.

Conference paper

Sparks NJ, Hardwick SR, Schmid M, Toumi Ret al., 2017, IMAGE: a multivariate multi-site stochastic weather generator for European weather and climate, Stochastic Environmental Research and Risk Assessment, Vol: 32, Pages: 771-784, ISSN: 1436-3240

Capturing the spatial and temporal correlation of multiple variables in a weather generator is challenging. A new massively multi-site, multivariate daily stochastic weather generator called IMAGE is presented here. It models temperature and precipitation variables as latent Gaussian variables with temporal behaviour governed by an auto-regressive model whose residuals and parameters are correlated through resampling of principle component time series of empirical orthogonal function modes. A case study using European climate data demonstrates the model’s ability to reproduce extreme events of temperature and precipitation. The ability to capture the spatial and temporal extent of extremes using a modified Climate Extremes Index is demonstrated. Importantly, the model generates events covering not observed temporal and spatial scales giving new insights for risk management purposes.

Journal article

Corsaro CM, Toumi R, 2017, A self-weakening mechanism for tropical cyclones, Quaterly Journal of the Royal Meteological Society, Vol: 143, Pages: 2585-2599, ISSN: 1477-870X

A mechanism leading to the self-weakening of tropical cyclones is proposed using the Weather and Research and Forecasting model. A comparison between an experiment with variable Coriolis parameter f and one on an f-plane shows that after the initial intensification the former is characterized by a smaller intensity. As opposed to the tropical cyclone on the f-plane, the one with variable f weakens significantly after reaching maturity. Analyses of the 3-D circulation show that the main reason for the weakening is dry intrusion in the mid-upper troposphere from the west. Once the dry intrusion reaches the inner vortex, strong downdrafts reduce the high equivalent potential temperature in the boundary layer inflow. The subsequent updrafts in the eyewall, characterized by lower equivalent potential temperature, are considerably reduced and, consequently, the secondary circulation weakens.Back-trajectories are used to determine the origin of the dry intrusion. It is found that the air parcels expelled from the storm deep convection into the outflow layer recirculate anticyclonically back into the vortex, causing a self-weakening of the tropical cyclone. A time span for the recirculation of at least 48 h allows the air parcels to sink substantially before reaching the vortex circulation. Some implications of the intrinsic nature of this process are briefly discussed.

Journal article

Arcucci R, D'Amore L, Toumi R, 2017, Preconditioning of the background error covariance matrix in data assimilation for the Caspian Sea, 1st International Conference on Applied Mathematics and Computer Science (ICAMCS), Publisher: AIP Publishing, ISSN: 1551-7616

Data Assimilation (DA) is an uncertainty quantification technique used for improving numerical forecasted results by incorporating observed data into prediction models. As a crucial point into DA models is the ill conditioning of the covariance matrices involved, it is mandatory to introduce, in a DA software, preconditioning methods. Here we present first studies concerning the introduction of two different preconditioning methods in a DA software we are developing (we named S3DVAR) which implements a Scalable Three Dimensional Variational Data Assimilation model for assimilating sea surface temperature (SST) values collected into the Caspian Sea by using the Regional Ocean Modeling System (ROMS) with observations provided by the Group of High resolution sea surface temperature (GHRSST). We also present the algorithmic strategies we employ.

Conference paper

Phillipson L, Toumi R, 2017, Impact of data assimilation on ocean current forecasts in the Angola Basin, OCEAN MODELLING, Vol: 114, Pages: 45-58, ISSN: 1463-5003

The ocean current predictability in the data limited Angola Basin was investigated using the Regional Ocean Modelling System (ROMS) with four-dimensional variational data assimilation. Six experiments were undertaken comprising a baseline case of the assimilation of salinity/temperature profiles and satellite sea surface temperature, with the subsequent addition of altimetry, OSCAR (satellite-derived sea surface currents), drifters, altimetry and drifters combined, and OSCAR and drifters combined. The addition of drifters significantly improves Lagrangian predictability in comparison to the baseline case as well as the addition of either altimetry or OSCAR. OSCAR assimilation only improves Lagrangian predictability as much as altimetry assimilation. On average the assimilation of either altimetry or OSCAR with drifter velocities does not significantly improve Lagrangian predictability compared to the drifter assimilation alone, even degrading predictability in some cases. When the forecast current speed is large, it is more likely that the combination improves trajectory forecasts. Conversely, when the currents are weaker, it is more likely that the combination degrades the trajectory forecast.

Journal article

Cahill B, Toumi R, Stenchikov G, Osipov S, Brindley Het al., 2017, Evaluation of thermal and dynamic impacts of summer dust aerosols on the Red Sea, Journal of Geophysical Research: Oceans, Vol: 122, Pages: 1325-1346, ISSN: 2169-9275

The seasonal response of upper ocean processes in the Red Sea to summer-time dust aerosol perturbations is investigated using an uncoupled regional ocean model. We find that the upper limit response is highly sensitive to dust-induced reductions in radiative fluxes. Sea surface cooling of −1°C and −2°C is predicted in the northern and southern regions, respectively. This cooling is associated with a net radiation reduction of −40 W m−2 and −90 W m−2 over the northern and southern regions, respectively. Larger cooling occurs below the mixed layer at 75 m in autumn, −1.2°C (north) and −1.9°C (south). SSTs adjust more rapidly (ca. 30 days) than the subsurface temperatures (seasonal time scales), due to stronger stratification and increased mixed layer stability inhibiting the extent of vertical mixing. The basin average annual heat flux reverses sign and becomes positive, +4.2 W m−2 (as compared to observed estimates −17.3 W m−2) indicating a small gain of heat from the atmosphere. When we consider missing feedbacks from atmospheric processes in our uncoupled experiment, we postulate that the magnitude of cooling and the time scales for adjustment will be much less, and that the annual heat flux will not reverse sign but nevertheless be reduced as a result of dust perturbations. While our study highlights the importance of considering coupled ocean-atmosphere processes on the net surface energy flux in dust perturbation studies, the results of our uncoupled dust experiment still provide an upper limit estimate of the response of the upper ocean to dust-induced radiative forcing perturbations.

Journal article

Arcucci R, D'Amore L, Pistoia J, Toumi R, Murli Aet al., 2017, On the variational data assimilation problem solving and sensitivity analysis, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 335, Pages: 311-326, ISSN: 0021-9991

We consider the Variational Data Assimilation (VarDA) problem in an operational framework, namely, as it results when it is employed for the analysis of temperature and salinity variations of data collected in closed and semi closed seas. We present a computing approach to solve the main computational kernel at the heart of the VarDA problem, which outperforms the technique nowadays employed by the oceanographic operative software. The new approach is obtained by means of Tikhonov regularization. We provide the sensitivity analysis of this approach and we also study its performance in terms of the accuracy gain on the computed solution. We provide validations on two realistic oceanographic data sets.

Journal article

Nissan H, Toumi R, 2016, Regional modelling of rainfall erosivity: Sensitivity of soil erosion to aerosol emissions, Quarterly Journal of the Royal Meteorological Society, Vol: 143, Pages: 265-277, ISSN: 1477-870X

A rainfall erosivity scheme is implemented into the widely-used regional atmosphere-landmodel, WRF. Rainfall erosivity is parameterised from hourly precipitation and surface runoff in a high resolution (4 km) convection-permitting model. The scheme is used to examine thepotential effects of changes in atmospheric aerosol concentrations on soil erosion in a case study of northern India and the surrounding countries for the 2010 monsoon season, using a model which isolates the indirect effect on cloud microphysics only. This study offers a preliminary investigation into this emerging topic, but longer simulations would be needed to establish a robust signal.Summer precipitation is reduced in most areas and the monsoon circulation weakens for increases in cloud condensation nuclei concentrations. This can be attributed to localised cloud microphysical changes in the northeast of India, which induce a dynamic response opposing the monsoon circulation. The two regions of greatest decrease in erosion with increasing aerosolare in the Western Ghats and the Ganges Delta, both significant cropland areas. However, the effect is not uniform, with isolated local increases in soil erosion. These results suggest that, while efforts to reduce anthropogenic aerosol emissions may improve water availability for crops through enhanced rainfall, these benefits are likely to be tempered by an increase in soil erosion, though robust local changes were difficult to predict.

Journal article

Trivedi A, Toumi R, 2016, Mechanisms of bottom boundary fluxes in a numerical model of the Shetland shelf, Ocean Dynamics, Vol: 67, Pages: 1-21, ISSN: 1616-7341

Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connectthis region to deeper waters. Two proposed ways in which across-slope BBLfluxes can occur, in regions that have a slope current aligned to the bathymetry, are:the frictional veering of bottom currents termed the ‘Ekman drain’; and through localwind-forced downwelling (wind-driven surface Ekman flow with an associated bottomflow). We investigate the variability, magnitude and spatial scale of BBL fluxeson the Shetland shelf, which has a prominent slope current, using a high-resolution(∼ 2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL atthe shelf break near the 200 m isobath and are found to have a seasonal variabilitywith high/low volume transport in winter/summer respectively. By using a multivariateregression approach, we find that the locally wind-driven Ekman transport playsno explicit role in explaining daily bottom fluxes. We can better explain the variabilityof the across-slope BBL flux as a linear function of the speed and across-slopecomponent of the interior flow, corresponding to an Ekman plus mean-flow flux. Weestimate that the mean-flow is a greater contributor than the Ekman flux to the BBLflux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow,which has a much shorter decorrelation length compared to the Ekman flux. We concludethat both the speed and direction of the interior current determines the dailyBBL flux. The wind does not explicitly contribute through local downwelling, butmay influence the interior current and therefore implicitly the BBL fluxes on longertimescales.

Journal article

Wang S, Toumi R, 2016, On the relationship between hurricane cost and the integrated wind profile, Environmental Research Letters, Vol: 11, ISSN: 1748-9326

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.

Journal article

Wang S, Toumi R, 2016, On the relationship between hurricane cost and the integrated wind profile, Environmental Research Letters, Vol: 11, ISSN: 1748-9326

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.

Journal article

Bruneau N, Toumi R, 2016, A fully-coupled atmosphere-ocean-wave model of the Caspian Sea, Ocean Modelling, Vol: 107, Pages: 97-111, ISSN: 1463-5011

Located in the mid-latitudes, the Caspian Sea is the largest enclosed basin in the world. A fully-coupled atmosphere-ocean-wave model of the Caspian Sea at high resolution (8 km) for a period of three years is presented. After validating each component of the modelling platform, the wave state of the Caspian Sea is studied. Results show very different wave regimes between the three different basins, a strong seasonality and an almost swell-free state. It is shown here that waves modify the horizontal eddy viscosity and vertical heat diffusion. However, due to a reasonably weak annual wave state, these effects are restricted to the upper-ocean layer (< 30 m) except during the most severe events (100 m).Three main experiments are conducted: 1) the ROMS ocean model forced by atmospheric reanalysis (CFSR), 2) ROMS coupled with the atmospheric model WRF and 3) the impact of wave-induced processes. The seasonality of the Caspian Sea is accurately captured in each experiment which highlights a rapid warming of the sea surface temperature (SST) in spring while the mixed layer depths (MLD) become very rapidly shallow (shifting from over 100 m to 15 m in two months). Contrarily, a gentle cooling of the SST accompanied with a deepening of the MLD is modelled during autumn and winter. The results also show a significant improvement of the model skill in the representation of the dynamics when ROMS is coupled to WRF.Finally, as ocean surface waves imply feedback at the interface atmosphere-ocean through the transfer of momentum, mass and heat, we investigate their potential effects on the Caspian Sea dynamics. Results are mixed and show a reasonably weak impact of wave-induced processes. While waves have a negligible effect during the winter as wave-induced mixing is confined to the MLD, the summer global SST are less accurately modelled due to the enhancement of mixing in shallow MLDs. However the SST bias, temperature at a subsurface location are improved.

Journal article

Toumi R, wong B, 2016, Effect of extreme ocean precipitation on sea surface elevation and storm surges, Quarterly Journal of the Royal Meteorological Society, Vol: 142, Pages: 2541-2550, ISSN: 1477-870X

Ocean models that neglect mass and momentum contributions from precipitation can have asystematic bias in sea surface height (SSH). Here, a new rainfall scheme is introduced intothe Regional Ocean Modelling System (ROMS) to incorporate the effects of precipitationmass. When precipitation is added to the sea surface, it spreads out via surface gravity wavesthat increase in propagation speed with increasing water depth. Over several days, the SSHincrease due to the precipitation mass added created a geostrophic adjustment, generatinganti-cyclonic geostrophic currents around the SSH increase. The transfer of momentum fromprecipitation to the sea surface, or rain stress, can also be important. In the case study of areal tropical cyclone, Monica passing North Australia, the effect of incorporatingprecipitation mass is compared with other processes affecting the storm surge: surface wind,inverse barometer effect and rain stress. The maximum SSH response is 170.6 cm for thewind effect, 61.5 cm for the inverse barometer effect, 7.5 cm for the effect of rain stress and6.4 cm for the effect of rain mass. Each process has been shown to have different spatialinfluences. The effect of rain mass has a strong remote influence compared to the inversebarometer effect and the effect of rain stress. This is particularly seen in semi-enclosed bays.

Journal article

Phibbs S, Toumi R, 2016, The dependence of precipitation and its footprint on atmospheric temperature in idealized extratropical cyclones, Journal of Geophysical Research: Atmospheres, Vol: 121, Pages: 8743-8754, ISSN: 2169-8996

Flood hazard is a function of the magnitude and spatial pattern of precipitation accumulation.The sensitivity of precipitation to atmospheric temperature is investigated for idealized extratropicalcyclones, enabling us to examine the footprint of extreme precipitation (surface area where accumulatedprecipitation exceeds high thresholds) and the accumulation in different-sized catchment areas. Themean precipitation increases with temperature, with the mean increase at 5.40%/∘C. The 99.9th percentileof accumulated precipitation increases at 12.7%/∘C for 1 h and 9.38%/∘C for 24 h, both greater thanClausius-Clapeyron scaling. The footprint of extreme precipitation grows considerably with temperature,with the relative increase generally greater for longer durations. The sensitivity of the footprint of extremeprecipitation is generally super Clausius-Clapeyron. The surface area of all precipitation shrinks withincreasing temperature. Greater relative changes in the number of catchment areas exceeding extremetotal precipitation are found when the domain is divided into larger rather than smaller catchment areas.This indicates that fluvial flooding may increase faster than pluvial flooding from extratropical cyclones in awarming world. When the catchment areas are ranked in order of total precipitation, the 99.9th percentile isfound to increase slightly above Clausius-Clapeyron expectations for all of the catchment sizes, from 9 km2to 22,500 km2. This is surprising for larger catchment areas given the change in mean precipitation. Wepropose that this is due to spatially concentrated changes in extreme precipitation in the occluded front

Journal article

Toumi R, Wong B, 2016, Model study of the asymmetry in tropical cyclone-induced positive and negative surges, Atmospheric Science Letters, Vol: 17, Pages: 334-338, ISSN: 1530-261X

Storm surges pose significant threats to coastal communities, yet negative surges are not aswell understood as positive surges. In this study, idealized experiments of a tropical cycloneforcing a 3D ocean model are conducted to investigate the asymmetry of positive andnegative surges. Negative surges are larger in magnitude and extend further across thecoastline than positive surges. While positive surges are driven by wind blowing onshore,negative surges are largely dominated by alongshore winds, with horizontal divergence as themain mechanism. This asymmetry also increases with decreasing depth and increasinglatitude.

Journal article

Poulain V, Bekki S, Marchand M, Chipperfield MP, Khodri M, Lefevre F, Dhomse S, Bodeker GE, Toumi R, De Maziere M, Pommereau J-P, Pazmino A, Goutail F, Plummer D, Rozanov E, Mancini E, Akiyoshi H, Lamarque J-F, Austin Jet al., 2016, Evaluation of the inter-annual variability of stratospheric chemical composition in chemistry-climate models using ground-based multi species time series, Journal of Atmospheric and Solar-Terrestrial Physics, Vol: 145, Pages: 61-84, ISSN: 1364-6826

The variability of stratospheric chemical composition occurs on a broad spectrum of timescales, ranging from day to decades. A large part of the variability appears to be driven by external forcings such as volcanic aerosols, solar activity, halogen loading, levels of greenhouse gases (GHG), and modes of climate variability (quasi-biennial oscillation (QBO), El Niño-Southern Oscillation (ENSO)). We estimate the contributions of different external forcings to the interannual variability of stratospheric chemical composition and evaluate how well 3-D chemistry-climate models (CCMs) can reproduce the observed response-forcing relationships. We carry out multivariate regression analyses on long time series of observed and simulated time series of several traces gases in order to estimate the contributions of individual forcings and unforced variability to their internannual variability. The observations are typically decadal time series of ground-based data from the international Network for the Detection of Atmospheric Composition Change (NDACC) and the CCM simulations are taken from the CCMVal-2 REF-B1 simulations database. The chemical species considered are column O3, HCl, NO2, and N2O. We check the consistency between observations and model simulations in terms of the forced and internal components of the total interannual variability (externally forced variability and internal variability) and identify the driving factors in the interannual variations of stratospheric chemical composition over NDACC measurement sites. Overall, there is a reasonably good agreement between regression results from models and observations regarding the externally forced interannual variability. A much larger fraction of the observed and modelled interannual variability is explained by external forcings in the tropics than in the extratropics, notably in polar regions. CCMs are able to reproduce the amplitudes of responses in chemical composition to specific external forcings. H

Journal article

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