Publications
188 results found
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.
Wang S, Rashid T, Thorp H, et al., 2020, A shortening of the life-cycle of major tropical cyclones, Geophysical Research Letters, Vol: 47, Pages: 28 Jul 2020-28 Jul 2020, ISSN: 0094-8276
In this study a comprehensive picture of the changing intensity life cycle of major (Category 3 and higher) tropical cyclones (TCs) is presented. Over the past decades, the lifetime maximum intensity has increased, but there has also been a significant decrease in duration of time spent at intensities greater than Category 1. These compensating effects have maintained a stable global mean‐accumulated cyclone energy of individual major TCs. The global mean duration of major TCs has shortened by about 1 day from 1982 to 2018. There has been both faster intensification (Categories 1 to 3) and weakening (Categories 3 to 1) by about 40%. The probabilities of rapid intensification and rapid weakening have both risen in the period 2000–2018 compared to 1982–1999. A statistically significant anticorrelation is found between the lifetime maximum intensity and the following duration of the final weakening. This suggests an element of self‐regulation of TC life cycles.
Bruneau N, Wang S, Toumi R, 2020, Long memory impact of ocean mesoscale temperature anomalies on tropical cyclone size, Geophysical Research Letters, Vol: 47, ISSN: 0094-8276
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.
Ito K, Wu C-C, Chan KTF, et al., 2020, Recent progress in the fundamental understanding of tropical cyclone motion, Journal of the Meteorological Society of Japan, Vol: 98, Pages: 5-17, ISSN: 0026-1165
While the fundamental understanding of tropical cyclone (TC) movement is fairly mature, notable advancements are still being made. This paper summarizes new concepts and updates to the existing fundamental theories on TC movement obtained from simplified barotropic models, full-physics models, and data analysis, particularly since 2014. The scope includes recent works on the interaction between a TC and its environment, and the predictability related to TC movement. Although conventional concepts of steering flow, β-gyre, and diabatic heating remain important, a more complete understanding of TC movement governing mechanisms can provide an important basis for further track forecast improvements.
Parks RM, Bennett JE, Tamura-Wicks H, et al., 2020, Anomalously warm temperatures are associated with increased injury deaths, Nature Medicine, Vol: 26, Pages: 65-70, ISSN: 1078-8956
Temperatures which deviate from long-term local norm affect human health, and are projected to become more frequent as the global climate changes.1 There is limited data on how such anomalies affect deaths from injuries. Here, we used data on mortality and temperature over 38 years (1980-2017) in the contiguous USA and formulated a Bayesian spatio-temporal model to quantify how anomalous temperatures, defined as deviations of monthly temperature from the local average monthly temperature over the entire analysis period, affect deaths from unintentional (transport, falls and drownings) and intentional (assault and suicide) injuries, by age group and sex. We found that a 1.5°C anomalously warm year, as envisioned under the Paris Climate Agreement,2 would be associated with an estimated 1,601 (95% credible interval 1,430-37 1,776) additional injury deaths. 84% of these additional deaths would occur in males, mostly in adolescent to middle ages. These deaths would comprise of increases in deaths 39 from drownings, transport, assault and suicide, offset partly by a decline in deaths from falls in older ages. The findings demonstrate the need for targeted interventions against injuries during periods of anomalously high temperatures, especially as these episodes are likely to increase with global climate change.
Phillipson L, Toumi R, 2019, Assimilation of satellite salinity for modelling the Congo River plume, Remote Sensing, Vol: 12, Pages: 1-20, ISSN: 2072-4292
Abstract:Satellite salinity data from the Soil Moisture and Ocean Salinity (SMOS) mission was recently enhanced, increasing the spatial extent near the coast that eluded earlier versions. In a pilot attempt we assimilate this data into a coastal ocean model (ROMS) using variational assimilation and for the first time, investigate the impact on the simulation of a major river plume (the Congo River). Four experiments were undertaken consisting of a control (without data assimilation) and5the assimilation of either sea surface height, SMOS and the combination of both. Several metrics specific to the plume were utilised, including the area of the plume, distance to the centre of mass, orientation and average salinity. The assimilation of SMOS and SMOS-SSH consistently produced the best results in the plume analysis. Argo float salinity profiles provided independent verification of the forecast. The SMOS or SMOS-SSH forecast produced the closest agreement for Argo profiles over the whole domain (outside and inside the plume) for three of four months analysed, improving over the control and a persistence baseline. The number of samples of Argo floats determined to be inside the plume were limited. Nevertheless, for the limited plume-detected floats the largest improvements were found for the SMOS or SMOS-SSH forecast for two of the four months.
Sparks N, Hon KK, Chan PW, et al., 2019, Aircraft observations of tropical cyclone boundary layer turbulence over the South China Sea, Journal of the Atmospheric Sciences, Vol: 76, Pages: 3773-3783, ISSN: 0022-4928
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.
wang S, Toumi R, 2019, Impact of dry midlevel air on the tropical cyclone outer circulation, Journal of the Atmospheric Sciences, Vol: 76, Pages: 1809-1826, 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.
Parks R, Mclaren M, Toumi R, et 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.
Camp J, Roberts MJ, Comer RE, et 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.
Hattermann FF, Wortmann M, Liersch S, et al., 2018, Simulation of flood hazard and risk in the Danube basin with the Future Danube Model, Climate Services, Vol: 12, Pages: 14-26, ISSN: 2405-8807
Major river and flash flood events have accumulated in Central and Eastern Europe over the last decade reminding the public as well as the insurance sector that climate related risks are likely to become even more damaging and prevalent as climate patterns change. However, information about current and future hydro-climatic extremes is often not available. The Future Danube Model (FDM) is an end-user driven multi-hazard and risk model suite for the Danube region that has been developed to provide climate services related to perils such as heavy precipitation, heat waves, floods, and droughts under recent and scenario conditions. As a result, it provides spatially consistent information on extreme events and natural resources throughout the entire Danube catchment. It can be used to quantify climate risks, to support the implementation of the EU framework directives, for climate informed urban and land use planning, water resources management, and for climate proofing of large scale infrastructural planning including cost benefit analysis. The model suite consists of five individual and exchangeable modules: a weather and climate module, a hydrological module, a risk module, an adaptation module, and a web-based visualization module. They are linked in such a way that output from one module can either be used standalone or fed into subsequent modules. The utility of the tool has been tested by experts and stakeholders. The results show that more and more intense hydrological extremes are likely to occur under climate scenario conditions, e.g. higher order floods may occur more frequently.
Parks RM, Bennett J, Foreman K, et 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.
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.
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.
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
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.
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.
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.
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.
D'Amore L, Arcucci R, Li Y, et 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
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.
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.
Arcucci R, Celestino S, Toumi R, et 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.
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