Extreme Surface Wind Research Articles

Dynamics of Extreme Surface Winds Inside North Atlantic Midlatitude Cyclones

AbstractNorth Atlantic midlatitude cyclones are among the most severe weather systems, causing enormous economic damages and threatening human lives. The cyclone is typically characterized by cyclonic convergent surface winds, strong updrafts, and precipitation. However, extreme surface winds are often observed within the cyclone where downdrafts develop. The present study investigates the dynamical and thermodynamical characteristics of the horizontal winds impinging on the cold frontal surface and the associated downdrafts. It is shown that the cyclonic winds into the cold frontal surface are mainly responsible for the downdrafts that transport the high‐altitude horizontal momentum to the surface and cause intense surface winds. About half of the North Atlantic midlatitude cyclones are accompanied by the downdrafts especially in the southern and western parts of the cyclone center.

Effect of turbulent integral scale on non-Gaussian characteristics of surface wind pressure on square cylinder

This study investigated the statistical properties of the pressure fluctuations on a square cylinder across three distinct turbulence fields characterized by varying turbulent integral scales. The effect of turbulent integral scale on the non-Gaussian characteristics and extreme surface wind pressure acting on square cylinders beneath the separating flow were studied in detail. The findings indicated that the pressure distribution on the windward surface generally conformed to a Gaussian distribution, whereas notable non-Gaussian characteristics were observed in the pressure distribution on the side and leeward surfaces. The fluctuating pressure, skewness, kurtosis, peak factor, and extreme pressure increase with an increasing ratio of turbulent integral scale to structural depth (Lux/D), whereas the mean pressure remains unaffected by variations in Lux/D. As Lux increased, the energy of the internal vortices in the shear layer also increased. As a result, the non-Gaussian features of the pressure caused by vortex breakdown become more pronounced. Compared with Lux/D = 1.96, the underestimated value of the extreme pressure on the square cylinder had a maximum difference of up to 15.4% at Lux/D = 0.53. Therefore, the corresponding turbulent integral scale should be accurately simulated when measuring wind loading on a structure through wind tunnel tests.

Dynamical Projections of the Mean and Extreme Wave Climate in the Bohai Sea, Yellow Sea and East China Sea

Few studies have focused on the projected future changes in wave climate in the Chinese marginal seas. For the first time, we investigate the projected changes of the mean and extreme wave climate over the Bohai Sea, Yellow Sea, and East China Sea (BYE) during two future periods (2021–2050 and 2071–2100) under the RCP2.6 and RCP8.5 scenarios from the WAM wave model simulations with a resolution of 0.1°. This is currently the highest-resolution wave projection dataset available for the study domain. The wind forcings for WAM are from high-resolution (0.22°) regional climate model (RCM) CCLM-MPIESM simulations. The multivariate bias-adjustment method based on the N-dimensional probability density function transform is used to correct the raw simulated significant wave height (SWH), mean wave period (MWP), and mean wave direction (MWD). The annual and seasonal mean SWH are generally projected to decrease (-0.15 to -0.01 m) for 2021–2050 and 2071–2100 under the RCP2.6 and RCP8.5 scenarios, with statistical significance at a 0.1 level for most BYE in spring and for most of the Bohai Sea and Yellow Sea in annual and winter/autumn mean. There is a significant decrease in the spring MWP for two future periods under both the RCP2.6 and RCP8.5 scenarios. In contrast, the annual and summer/winter 99th percentile SWH are generally projected to increase for large parts of the study domain. Results imply that the projected changes in the mean and 99th percentile extreme waves are very likely related to projected changes in local mean and extreme surface wind speeds, respectively.

Extreme Surface Winds During Landfalling Atmospheric Rivers: The Modulating Role of Near-Surface Stability

AbstractAtmospheric rivers (ARs) are long and narrow regions of strong horizontal water vapor transport. Upon landfall, ARs are typically associated with heavy precipitation and strong surface winds. A quantitative understanding of the atmospheric conditions that favor extreme surface winds during ARs has implications for anticipating and managing various impacts associated with these potentially hazardous events. Here, a global AR database (1999–2014) with relevant information from MERRA-2 reanalysis, QuikSCAT and AIRS satellite observations are used to better understand and quantify the role of near-surface static stability in modulating surface winds during landfalling ARs. The temperature difference between the surface and 1 km MSL (ΔT; used here as a proxy for near-surface static stability), and integrated water vapor transport (IVT) are analyzed to quantify their relationships to surface winds using bivariate linear regression. In four regions where AR landfalls are common, the MERRA-2-based results indicate that IVT accounts for 22-38% of the variance in surface wind speed. Combining ΔT with IVT increases the explained variance to 36-52%. Substitution of QuikSCAT surface winds and AIRS ΔT in place of the MERRA-2 data largely preserves this relationship (e.g., 44% compared to 52% explained variance for USA West Coast). Use of an alternate static stability measure–the bulk Richardson number–yields a similar explained variance (47%). Lastly, AR cases within the top and bottom 25% of near-surface static stability indicate that extreme surface winds (gale or higher) are more likely to occur in unstable conditions (5.3%/14.7% during weak/strong IVT) than in stable conditions (0.58%/6.15%).

The Impact of Atmosphere\u2013Ocean\u2013Wave Coupling on the Near-Surface Wind Speed in Forecasts of Extratropical Cyclones

Accurate modelling of air–sea surface exchanges is crucial for reliable extreme surface wind-speed forecasts. While atmosphere-only weather forecast models represent ocean and wave effects through sea-state independent parametrizations, coupled multi-model systems capture sea-state dynamics by integrating feedbacks between the atmosphere, ocean and wave model components. Here, we investigate the sensitivity of extreme surface wind speeds to air–sea exchanges at the kilometre scale using coupled and uncoupled configurations of the Met Office’s UK Regional Coupled Environmental Prediction system. The case period includes the passage of extra-tropical cyclones Helen, Ali, and Bronagh, which brought maximum gusts of 36 m s^{-1} over the UK. Compared with the atmosphere-only results, coupling to the ocean decreases the domain-average sea-surface temperature by up to 0.5 K. Inclusion of coupling to waves reduce the 98th percentile 10-m wind speed by up to 2 m s^{-1} as young, growing wind waves reduce the wind speed by increasing the sea-surface aerodynamic roughness. Impacts on gusts are more modest, with local reductions of up to 1 m s^{-1}, due to enhanced boundary-layer turbulence which partially offsets air–sea momentum transfer. Using a new drag parametrization based on the Coupled Ocean–Atmosphere Response Experiment 4.0 parametrization, with a cap on the neutral drag coefficient and reduction for wind speeds exceeding 27 m s^{-1}, the atmosphere-only model achieves equivalent impacts on 10-m wind speeds and gusts as from coupling to waves. Overall, the new drag parametrization achieves the same 20% improvement in forecast 10-m wind-speed skill as coupling to waves, with the advantage of saving the computational cost of the ocean and wave models.

Estimates of changes in surface wind and temperature extremes in southwestern Norway using dynamical downscaling method under future climate

Extreme surface winds and temperatures were estimated by the dynamical downscaling method combined with the generalized extreme value theory for the construction of Hardanger Suspension Bridge and the maintenance of Sotra Bridge in southwestern Norway. The Weather Research and Forecasting Model was used to downscale the Norwegian Earth System Model data from 2.5° × 1.8° to 1 km × 1 km horizontal grids. Simulations were performed for the control period, the 1990s, and the projection period, the 2050s, under the RCP8.5 radiative forcing scenario. Monthly maximum winds were compared with observations at three nearby observation stations for the warm and the cold seasons as well as the annual period. The simulated extreme wind distributions are in good agreement with the observed distributions at the coastal area, but have systematic positive deviations on the mountain. An extrapolation method was used to project extreme winds in the early and the late this century. Comparison of the simulated extreme winds between the 1990s and the 2050s shows that future extreme winds are unlikely to change with statistical significance during the cold season, but tend to decrease at mountainous and coastal areas with statistical significance during the warm season. They are possibly the reflections of the shift in the regional storm activities associated with the changes of the North Atlantic Oscillations and the effects of the local mountain topography. For surface maximum and minimum temperatures, the model can well reproduce the spreads of the pdf distributions. Both distributions shift towards higher temperatures in the 2050s.

Projected Changes to Mean and Extreme Surface Wind Speeds for North America Based on Regional Climate Model Simulations

This study evaluates projected changes to surface wind characteristics for the 2071–2100 period over North America (NA), using four Global Environmental Multiscale regional climate model simulations, driven by two global climate models (GCMs) for two Representative Concentration Pathway scenarios. For the current climate, the model simulates well the climatology of mean sea level pressure (MSLP) and associated wind direction over NA. Future simulations suggest increases in mean wind speed for northern and eastern parts of Canada, associated with decreases in future MSLP, which results in more intense low-pressure systems situated in those regions such as the Aleutian and Icelandic Lows. Projected changes to annual maximum 3-hourly wind speed show more spatial variability compared to seasonal and annual mean wind speed, indicating that extreme wind speeds are influenced by regional level features associated with instantaneous surface temperature and air pressure gradients. The simulations also suggest some increases in the future 50-year return levels of 3-hourly wind speed and hourly wind gusts, mainly due to increases in the inter-annual variability of annual maximum values. The variability of projected changes to both extreme wind speed and gusts indicate the need for a larger set of projections, including those from other regional models driven by many GCMs to better quantify uncertainties in future wind extremes and their characteristics.

Can We Improve Parametric Cyclonic Wind Fields Using Recent Satellite Remote Sensing Data?

Parametric cyclonic wind fields are widely used worldwide for insurance risk underwriting, coastal planning, and storm surge forecasts. They support high-stakes financial, development and emergency decisions. Yet, there is still no consensus on a potentially “best” parametric approach, nor guidance to choose among the great variety of published models. The aim of this paper is to demonstrate that recent progress in estimating extreme surface wind speeds from satellite remote sensing now makes it possible to assess the performance of existing parametric models, and select a relevant one with greater objectivity. In particular, we show that the Cyclone Global Navigation Satellite System (CYGNSS) mission of NASA, along with the Advanced Scatterometer (ASCAT), are able to capture a substantial part of the tropical cyclone structure, and to aid in characterizing the strengths and weaknesses of a number of parametric models. Our results suggest that none of the traditional empirical approaches are the best option in all cases. Rather, the choice of a parametric model depends on several criteria, such as cyclone intensity and the availability of wind radii information. The benefit of using satellite remote sensing data to select a relevant parametric model for a specific case study is tested here by simulating hurricane Maria (2017). The significant wave heights computed by a wave-current hydrodynamic coupled model are found to be in good agreement with the predictions given by the remote sensing data. The results and approach presented in this study should shed new light on how to handle parametric cyclonic wind models, and help the scientific community conduct better wind, wave, and surge analyses for tropical cyclones.

Sub synoptic scale features of the South Australia Storm of September 2016 – Part II: analysis of mechanisms driving the gusts

AbstractAn extreme extratropical cyclone (ETC) struck South Australia on 28 September 2016, causing state‐wide blackouts and damage. In the second part of this two‐part study, we examine the extreme surface wind producing mechanisms within the ETC. ETCs have been extensively studied in the Northern Hemisphere (particularly in western Europe), highlighting the gust‐producing mesoscale features within. Before now, no Southern Hemisphere ETC has been examined in this way. There were a number of extreme gust‐producing features within the ETC, comparable to those observed in storms over western Europe. One such feature was a convective line, which caused many of the most extreme gusts and knocked out the state power grid. However, dry slot convection also contributed to the extremes, and this feature rarely causes extreme gusts in ETCs over the UK. Thus, further analysis is warranted to examine whether this is a common extreme‐gust‐producing ETC feature over Southern Australia. The strongest winds recorded throughout the event occurred on 29 September, and these were associated with the cold conveyor belt which spiralled around the low‐pressure centre.

Impact of Antarctic Ozone Depletion and Recovery on Southern Hemisphere Precipitation, Evaporation, and Extreme Changes

The possible impact of Antarctic ozone depletion and recovery on Southern Hemisphere (SH) mean and extreme precipitation and evaporation is examined using multimodel output from the Climate Model Intercomparison Project 3 (CMIP3). By grouping models into four sets, those with and without ozone depletion in twentieth-century climate simulations and those with and without ozone recovery in twenty-first-century climate simulations, and comparing their multimodel-mean trends, it is shown that Antarctic ozone forcings significantly modulate extratropical precipitation changes in austral summer. The impact on evaporation trends is however minimal, especially in twentieth-century climate simulations. In general, ozone depletion has increased (decreased) precipitation in high latitudes (midlatitudes), in agreement with the poleward displacement of the westerly jet and associated storm tracks by Antarctic ozone depletion. Although weaker, the opposite is also true for ozone recovery. These precipitation changes are primarily associated with changes in light precipitation (1–10 mm day−1). Contributions by very light precipitation (0.1–1 mm day−1) and moderate-to-heavy precipitation (>10 mm day−1) are minor. Likewise, no systematic changes are found in extreme precipitation events, although extreme surface wind events are highly sensitive to ozone forcings. This result indicates that, while extratropical mean precipitation trends are significantly modulated by ozone-induced large-scale circulation changes, extreme precipitation changes are likely more sensitive to thermodynamic processes near the surface than to dynamical processes in the free atmosphere.

Extreme wind conditions observed by satellite synthetic aperture radar in the North West Pacific

In this paper some recent results on SAR observation of extreme surface wind conditions are summarized. Particular emphasis is put on the investigation of typhoons occurring in the North West Pacific. The study is based on the use of ENVISAT ASAR wide swath images (400 km×400 km), which allow synoptic measurements of the complete mesoscale system at high resolution (150 m). Surface wind speed for typhoon cases is determined from SAR measurements using the geophysical model function CMOD5. Further structures observed in the image like streaks indicating wind direction and the ring of maximum wind speed are additionally taken into account to reconstruct the typhoon wind field. The influence of heavy rain on the radar cross section is estimated from an existing radiative transfer model and compared to the SAR measurements. A new technique for the estimation of typhoon intensity from SAR data is presented, which makes use of a parametric type model. The main goal of the paper is the improvement of the estimation of maximum typhoon intensity using SAR data.

Development of a model-based high-resolution extreme surface wind climatology for Switzerland

An innovative methodology aimed at establishing a numerical model-based high-resolution climatology of extreme winds over Switzerland is described, that makes use of the Canadian Regional Climate Model where a new windgust parameterization has been implemented. Self-nesting procedures allow windstorms to be studied at resolution as high as 2-km. The analysis of ten major windstorms concludes that the average spatial pattern and magnitude of the simulated windspeeds are well captured, and the areas that experienced extreme winds correspond well with observations and to the location where forest damage was reported following the last two of these storms. This climatology would eventually serve to form risk assessment maps based on the exceedance of windspeed thresholds. There is, however, a need for further investigations to encompass the full range of potential extreme wind cases. The ultimate goal of this methodology is to assess the change in the behaviour of extreme winds for a climate forced by enhanced greenhouse gas concentrations, and the impact of future windstorms over the Alpine region at high resolution.

Changes in storm track and cyclone activity in three SRES ensemble experiments with the ECHAM5/MPI-OM1 GCM

Synoptic activity over the Northern Hemisphere is evaluated in ensembles of ECHAM5/MPI-OM1 simulations for recent climate conditions (20C) and for three climate scenarios (following SRES A1B, A2, B1). A close agreement is found between the simulations for present day climate and the respective results from reanalysis. Significant changes in the winter mid-tropospheric storm tracks are detected in all three scenario simulations. Ensemble mean climate signals are rather similar, with particularly large activity increases downstream of the Atlantic storm track over Western Europe. The magnitude of this signal is largely dependent on the imposed change in forcing. However, differences between individual ensemble members may be large. With respect to the surface cyclones, the scenario runs produce a reduction in cyclonic track density over the mid-latitudes, even in the areas with increasing mid-tropospheric activity. The largest decrease in track densities occurs at subtropical latitudes, e.g., over the Mediterranean Basin. An increase of cyclone intensities is detected for limited areas (e.g., near Great Britain and Aleutian Isles) for the A1B and A2 experiments. The changes in synoptic activity are associated with alterations of the Northern Hemisphere circulation and background conditions (blocking frequencies, jet stream). The North Atlantic Oscillation index also shows increased values with enhanced forcing. With respect to the effects of changing synoptic activity, the regional change in cyclone intensities is accompanied by alterations of the extreme surface winds, with increasing values over Great Britain, North and Baltic Seas, as well as the areas with vanishing sea ice, and decreases over much of the subtropics.

Early warning capabilities of the global model GME of DWD—a case study

The early warning capability of the global model GME of the Deutscher Wetterdienst (DWD) is investigated for the case of a storm in the western Mediterranean Sea in November 2002. The Balearic Islands and the Algerian coast were hardest hit by it with record amounts of precipitation producing floods and mud avalanches, and high winds. Signals for a big storm were predicted several days in advance by the operational GME model. Also, several forecasts with higher resolution are compared using initial data interpolated from the IFS analysis of ECMWF. As the extreme precipitation was produced by orographically enhanced rain, higher rain rates and total sums were predicted with higher resolution than the operational model. However, the resolution has only small or no impact on the position and minimum pressure of the cyclone, and on the extreme surface winds.

Evidence for Intensification of North Pacific Winter Cyclones since 1948

Abstract Using NCEP–NCAR reanalysis and in situ data, evidence of important changes in the winter (December–March) cyclone climatology of the North Pacific Ocean over the past 50 years is found. The frequency and intensity of extreme cyclones has increased markedly, with associated upward trends in extreme surface winds between 25° and 40°N and major changes in cyclone–related circulation patterns in the Gulf of Alaska. Related increases in extreme wave heights are inferred from wave measurements and wave–model hindcast results. The more vigorous cyclone activity has apparently resulted from increasing upper–tropospheric winds and vertical wind shear over the central North Pacific. Such changes, which create an environment more favorable for cyclone formation and intensification, may be related to the observed modulation of El Nino–related teleconnections at decadal and longer timescales. It is intriguing that this trend has been relatively steady rather than the sudden or stepwise shifts documented for o...

Extreme surface winds in mid-latitude storms: forecasting and changes in climatology

The meteorological conditions for high wind speeds and gusts are reviewed; it is pointed out that whereas wind engineers focus their attention on shear-generated turbulent flows produced in the presence of high synoptic pressure gradients, meteorological forecasters for mid-latitude climates are equally concerned with the production of gusts by the combined action of the boundary layer dynamics and thermal convection above it. Radar data, radiosonde observations and mean wind and gust measurements largely confirm the working hypotheses of forecasters. The theoretical analysis for how the upper level convective downdrafts interact with the boundary layer is developed in Section 3. The forecasting of extreme winds and other weather variables using ensemble techniques is now statistically significant for periods of up to 15 days ahead; recent results are reviewed in Section 4. Finally, in Section 5, using the results of different Global Climate Models, the effects of climate change on the frequency, intensity and tracks of storms and hurricanes are considered, in order to estimate how climate change might affect extreme surface winds.