Sustained Wind Speed Research Articles

Dependence of Tropical Cyclone Intensification Rate on Sea Surface Temperature, Storm Intensity, and Size in the Western North Pacific

Abstract This study extends the statistical analysis on the dependence of tropical cyclone (TC) intensification rate (IR) on sea surface temperature (SST), storm initial intensity (maximum sustained surface wind speed Vmax), and storm size, in terms of the radius of maximum wind (RMW), the radius of 34-kt (AR34; 1 kt = 0.51 m s−1) wind, and the outer-core wind skirt parameter DR34 (= AR34 − RMW), for North Atlantic TCs to western North Pacific (WNP) TCs during 1982–2015. Results show that the relationship between the TC maximum potential intensification rate (MPIR) and SST also exists in the WNP. TC IR depends strongly on TC intensity and structure, consistent with the findings for North Atlantic TCs. TC IR is positively (negatively) correlated with storm intensity when Vmax is below (above) 70 kt and negatively correlated with the RMW. Rapid intensification (RI) occurs only in a relatively narrow range of parameter space in storm intensity and both inner- and outer-core sizes, with the highest IR appearing for Vmax = 70 kt, RMW ≦ 40 km, AR34 = 150 km, and DR34 = 100 km. The highest frequency of occurrence of intensifying TCs occurs for Vmax ~ 40–60 kt, RMW ~ 20–60 km, AR34 = 200 km, and DR34 = 120 km. Overall, these values are very similar to those for TCs in the North Atlantic. These results suggest the need for the realistic initialization of TC structure in numerical models and the inclusion of size parameters in statistical TC intensity prediction schemes.

A Climatology of Southwest Indian Ocean Tropical Systems: Their Number, Tracks, Impacts, Sizes, Empirical Maximum Potential Intensity, and Intensity Changes

AbstractA 17-yr “climatology” of tropical-system activity, track, size, and 24-h intensity change in the southwest Indian Ocean (SWIO) is developed and analyzed in comparison with other intensively studied basins such as the North Atlantic Ocean. A first formulation of the empirical maximum potential intensity of SWIO tropical systems is also proposed, along with the climatology of sea surface temperatures from September to June. Systems with a 34-kt (1 kt = 0.514 m s−1) wind radius that does not exceed 46 km are considered to be very small or midget systems, on the basis of the 5th percentile of storm size distribution. Using the 95th percentile of overwater intensity changes, rapid intensification (RI) is statistically defined by a minimum increase of 15.4 m s−1 day−1 in the maximum 10-min mean surface wind speed (VMAX). This value is similar to the 30-kt threshold commonly used in the North Atlantic basin for 1-min sustained wind speeds. Rapid decay (RD) can be statistically defined by a minimum weakening of 13.9 m s−1 day−1, although the spread in the 5th percentile of intensity changes among the different intensity classes indicates that it is not as appropriate to use a unique RD threshold for all systems. It is shown that 43% of all tropical systems and all very intense tropical cyclones (VMAX ≥ 59.6 m s−1) underwent RI at least once during their lifetimes. It is highlighted that systems have a greater propensity to intensify rapidly for an initial intensity between 65 and 75 kt. Statistics indicate that operational intensity forecast errors are significantly greater at short range for RI cases while track errors are reduced.

Point Downscaling of Surface Wind Speed for Forecast Applications

AbstractA statistical downscaling algorithm is introduced to forecast surface wind speed at a location. The downscaling algorithm consists of resolved and unresolved components to yield a time series of synthetic wind speeds at high time resolution. The resolved component is a bias-corrected numerical weather prediction model forecast of the 10-m wind speed at the location. The unresolved component is a simulated time series of the high-frequency component of the wind speed that is trained to match the variance and power spectral density of wind observations at the location. Because of the stochastic nature of the unresolved wind speed, the downscaling algorithm may be repeated to yield an ensemble of synthetic wind speeds. The ensemble may be used to generate probabilistic predictions of the sustained wind speed or wind gusts. Verification of the synthetic winds produced by the downscaling algorithm indicates that it can accurately predict various features of the observed wind, such as the probability distribution function of wind speeds, the power spectral density, daily maximum wind gust, and daily maximum sustained wind speed. Thus, the downscaling algorithm may be broadly applicable to any application that requires a computationally efficient, accurate way of generating probabilistic forecasts of wind speed at various time averages or forecast horizons.

Weak Tropical Cyclones Dominate the Poleward Migration of the Annual Mean Location of Lifetime Maximum Intensity of Northwest Pacific Tropical Cyclones since 1980

The poleward migration of the annual mean location of tropical cyclone (TC) lifetime maximum intensity (LMI) has been identified in the major TC basins of the globe over the past 30 years, which is particularly robust over the western North Pacific (WNP). This study has revealed that this poleward migration consists mainly of weak TCs (with maximum sustained surface wind speed less than 33 m s−1) over the WNP. Results show that the location of LMI of weak TCs has migrated about 1° latitude poleward per decade since 1980, while such a trend is considerably smaller for intense TCs. This is found to be linked to a significant decreasing trend of TC genesis in the southern WNP and a significant increasing trend in the northwestern WNP over the past 30 years. It is shown that the greater sea surface temperature (SST) warming at higher latitudes associated with global warming and its associated changes in the large-scale circulation favor more TCs to form in the northern WNP and fewer but stronger TCs to form in the southern WNP.

Whitecap Coverage Dependence on Wind and Wave Statistics as Observed during SO GasEx and HiWinGS

AbstractConcurrent wavefield and turbulent flux measurements acquired during the Southern Ocean (SO) Gas Exchange (GasEx) and the High Wind Speed Gas Exchange Study (HiWinGS) projects permit evaluation of the dependence of the whitecap coverage W on wind speed, wave age, wave steepness, mean square slope, and wind-wave and breaking Reynolds numbers. The W was determined from over 600 high-frequency visible imagery recordings of 20 min each. Wave statistics were computed from in situ and remotely sensed data as well as from a WAVEWATCH III hindcast. The first shipborne estimates of W under sustained 10-m neutral wind speeds U10N of 25 m s−1 were obtained during HiWinGS. These measurements suggest that W levels off at high wind speed, not exceeding 10% when averaged over 20 min. Combining wind speed and wave height in the form of the wind-wave Reynolds number resulted in closely agreeing models for both datasets, individually and combined. These are also in good agreement with two previous studies. When expressing W in terms of wavefield statistics only or wave age, larger scatter is observed and/or there is little agreement between SO GasEx, HiWinGS, and previously published data. The wind speed–only parameterizations deduced from the SO GasEx and HiWinGS datasets agree closely and capture more of the observed W variability than Reynolds number parameterizations. However, these wind speed–only models do not agree as well with previous studies than the wind-wave Reynolds numbers.

Analysis of a Mediterranean tropical-like cyclone and its sensitivity to the sea surface temperatures

This study investigates an intense tropical-like cyclone which formed south of Sicily on 7 November 2014 and affected the central Mediterranean. The cyclone made landfall at Malta and eastern Sicily. The sustained surface wind speed reached 23.7m/s (named tropical cyclone strength) at Lampedusa island, while a minimum mean sea-level pressure of 984hPa was recorded at Malta. Baroclinic instability appeared to be important for its formation since it took place in a region with deep convection and was associated with a pre-existing low-level depression, a baroclinic zone and a dynamic tropopause anomaly. Phase space diagrams determined objectively the symmetry and the warm core structure of the system. The medicane was simulated by the non-hydrostatic WRF-ARW numerical weather prediction model. The role of the sea surface temperatures (SSTs) on the development of the medicane was investigated through numerical experiments in which climatological SSTs and uniform warm and cold SST anomalies were imposed. The motivation for such experiments has been provided by the occurrence of warm SST anomalies along its track. A strong medicane, with shorter lifetime, would have developed over the central Mediterranean Sea even if the SSTs were near the normal conditions of November.

Sedimentological characteristics of the 2015 Tropical Cyclone Pam overwash sediments from Vanuatu, South Pacific

The interpretation of sediments deposited by prehistoric tropical cyclones (TC's) is limited by a lack of modern analogues, particularly in the South Pacific. On 13 March 2015, TC Pam made landfall on Vanuatu, reaching Category 5 intensity with 10-minute sustained wind speeds as high as 270km/h. Three months after landfall, we measured flow height (terrain elevation plus storm flow depth) and inland extent of TC Pam's maximum coastal inundation (composed of astronomical tides, storm surge, and superimposed storm waves), and described the sedimentological characteristics of the TC Pam overwash sediments from trenches and transects at two sites (Manuro and Port Resolution Bay).At Manuro (a mixed-carbonate embayment), the maximum flow height was 5.29m mean sea level (MSL), with an inland extent of 106m. The TC Pam sediments transition from a coarse to medium grained (mean: 1.07 Φ) carbonate sand (≤10cm thick) to pumice (≤18cm thick) that extends 400m inland into Lake Otas. The TC Pam overwash sediments are characterized by a coarsening upward sequence (1.45 to 0.23 Φ) followed by a finer grained eolian cap. At Port Resolution Bay (a volcaniclastic beach, PRB), the maximum flow height was 3.30m MSL (1.51m flow depth), with an inland extent of 117m. The TC Pam overwash sediments transition from a medium grained (mean: 1.76 Φ) volcanic sand (≤44cm thick) to pumice (≤5cm thick) that extends 320m from the shoreline. A subtle fining upward sequence was present in trench PRB2, whereas PRB1, PRB3, and PRB4 contained TC Pam sediments that were laminated and showed little to no vertical gradation in grain size. At PRB, we applied an inverse sediment transport model to reconstruct maximum flow depths using laboratory derived settling velocities and the distance from the berm. The reconstructed flow depths at PRB2 (1.43m), PRB3 (1.36m), and PRB4 (1.34m) compare favorably with the observed estimate (1.51m), illustrating the applicability of the inverse sediment transport model to reconstruct flow depths of prehistoric landfalling TC's.

More intensive summer tropical cyclone near 30°N of East Asia

The present study revealed that a climate regime shift occurred during the 1988–1991 period involving changes in tropical cyclone (TC) intensity (central pressure, maximum sustained wind speed) during the summer near 30°N in East Asia. Climatologically, TC intensity at 110°–125°E near 30°N (over Mainland China) is the weakest at that latitude while the strongest is found at 125°–130°E (over Korea). The TC intensity during the 1991–2015 (91–15) period had strengthened significantly compared to that of the 1965–1988 (65–88) period. The strengthening was due to a significantly lower frequency of TCs that passed through Mainland China during the 91–15 period. This lower frequency of was due to anomalous northeasterlies blown from the anomalous anticyclonic circulation located over continental East Asia and that had strengthened along the coast. Instead, TCs mainly followed a path from eastern regions in the subtropical western North Pacific to Korea and Japan via the East China Sea due to anomalous cyclonic circulations that had strengthened in the western North Pacific. In addition, low vertical wind shear had formed along the mid-latitude region in East Asia and along the main TC track in the 91–15 period, and most regions in the western North Pacific experienced a higher sea surface temperature state during the 91–15 period than in the previous period, indicating that a favorable environment had formed to maintain strong intensities of TCs at the mid–latitudes. The characteristics of TCs at the lower latitudes caused a strong TC intensity at the time of landfall in Korea and a gradual shifting trend of landing location from the western to southern coast in recent years.

Swarm intelligence and neural nets in forecasting the maximum sustained wind speed along the track of tropical cyclones over Bay of Bengal

Tropical cyclones are well-known extreme weather and the cause of considerable damages, injuries and loss of life. The assessment of the maximum sustained wind speed along the track of the tropical cyclones is very important for estimating the strength of the cyclones. The swarm intelligence in the form of ant colony optimization (ACO) technique is introduced in this study to compute the pheromone deposition along the track of tropical cyclones followed by neural nets to forecast the maximum sustained wind speed of the cyclones occurring over the Bay of Bengal of North Indian Ocean. The ACO is a nonlinear problem-based meta-heuristic optimization method for finding approximate solutions to discrete optimization problems and simulates the decision-making processes of ant colony similar to other adaptive learning techniques. The method has shown its application potential in various fields including the prediction of monsoon rainfall. In this study, the amount of pheromone deposition during the successive stages of the cyclones has been estimated. A range of minimum central pressure (MCP), central pressure drop (PD), maximum sustained wind speed (MSWS) and intensity (T-No) associated with the cyclones of Bay of Bengal are utilized to form the input matrix of the neural nets. The neural nets are trained to forecast the maximum sustained wind speed along the track of the tropical cyclones over Bay of Bengal. The result reveals that the errors in forecasting the MSWS along the track of tropical cyclones with 6, 12, 18 and 24 h lead time are 2.6, 2.9, 3.1 and 4.8, respectively. The result is compared with the existing dynamical, statistical and adaptive models to evaluate the skill of the present model. The result is well validated with observation.

Characterization and Comparison of Aerodynamic Roughness Lengths Using Ground-Based Photography and Sonic Anemometry

In this study, two methods used to estimate surface terrain conditions (z0) surrounding Automated Surface Observing System (ASOS) stations in tropical cyclone-prone regions were analyzed. The first method utilized ground-based photography from the National Oceanic and Atmospheric Administration Hurricane Research Division's Tropical Cyclone Wind Exposure Documentation Project (WEDP). The second applied a modified effective z0 method using ASOS wind data. Comparisons reveal that WEDP z0 estimates are typically larger in magnitude and do not account for changes in upstream z0. Variability in the z0 estimates showed a distinctly skewed nature in the probability distributions, which may have some physical meaning. It was also shown that standardized maximum 1-min sustained wind speeds using both the WEDP and MM2010 median z0 estimates for short fetch lengths can have differences as large as 15%. The MM2010 z0 estimates were also compared with single weighted mean z0 estimates to be incorporated in the next version of the U.S. wind loading standard and suggest that a single weighted mean z0 value does not effectively capture the variability of the terrain with respect to wind direction.

Storm impact on sea surface temperature and chlorophyll a in the Gulf of Mexico and Sargasso Sea based on daily cloud‐free satellite data reconstructions

AbstractUpper ocean responses to tropical storms/hurricanes have been extensively studied using satellite observations. However, resolving concurrent sea surface temperature (SST) and chlorophyll a (chl a) responses along storm tracks remains a major challenge due to extensive cloud coverage in satellite images. Here we produce daily cloud‐free SST and chl a reconstructions based on the Data INterpolating Empirical Orthogonal Function method over a 10 year period (2003–2012) for the Gulf of Mexico and Sargasso Sea regions. Daily reconstructions allow us to characterize and contrast previously obscured subweekly SST and chl a responses to storms in the two main storm‐impacted regions of the Atlantic Ocean. Statistical analyses of daily SST and chl a responses revealed regional differences in the response time as well as the response sensitivity to maximum sustained wind speed and translation speed. This study demonstrates that SST and chl a responses clearly depend on regional ocean conditions and are not as universal as might have been previously suggested.

Characteristics of lightning activity in tropical cyclones developed over North Indian Ocean basin during 2010–2015

The characteristics of lightning activity in tropical cyclones (TCs) over North Indian Ocean (NIO) are presented using sample of 21 TCs developed during 2010–2015 using TRMM and World Wide Lightning Location Network (WWLLN) datasets along with information from annual reports of Regional Specialized Meteorological Center (RSMC), New Delhi. The microphysical features such as Polarization Corrected Brightness Temperature (PCT), attenuation corrected reflectivity factor, Ice Water Path (IWP) play a pivotal role in development of convective systems within TCs. The TCs exhibited systematic variation in lightning flashes per day within 300km of estimated center during their life cycle irrespective of their severity and flash rate within 300km of storm center. The lightning flashes ranged from 1 to 3500 flashes per day during pre-cyclone stage, 100 to 8000 flashes per day during cyclone stage and 0 to 4300 flashes per day during post-cyclone stage. The TCs produced maximum flash density in eyewall region (20–40km) and outer rainband region (200–260km). The WWLLN recorded 165,512 flashes within 300km of TC centers during their life cycle. The maximum flashes occurred during cyclone stage followed by pre-cyclone and post-cyclone. The time variation of flash rate for all cyclones was episodic and primarily peaked during late night and early morning hours. The diurnal variation of lightning flashes during TCs due to variation in detection efficiency of WWLLN also controls temporal distribution of lightning activity. During rapid intensification, TCs produced profuse lightning flashes per hour. Episodic lightning flashes per hour occurred during weakening stage and prior to landfall. It is evident that outbreaks of lightning flashes prior to maximum intensity change as manifested in maximum sustained wind speed and fall in estimated central pressure (ECP) indicate potential predictive value of lightning activity for TC intensity change.

Evaluation of downscaled wind speeds and parameterised gusts for recent and historical windstorms in Switzerland

Assessments of local-scale windstorm hazard require highly resolved spatial information on wind speeds and gusts. In this study, maximum (peak) sustained wind speeds on a 3-km horizontal grid over Switzerland are obtained by dynamical downscaling from the Twentieth Century Reanalysis (20CR) employing the Weather Research and Forecasting (WRF) model. Subsequently, simulated peak gusts are derived using four wind gust parameterizations (WGPs). Evaluations against observations at 63 locations in complex terrain include four high-impact windstorms (occurring in 1919, 1935, 1990, and 1999) and 14 recent windstorms (occurring between 1993 and 2011). Peak sustained wind speeds and directions are generally well simulated, although wind speeds are mostly overestimated. In general, performance and skill measures are best for locations on the Swiss Plateau and inferior for Alpine mountain and valley locations. An independent ERA-Interim WRF downscaling configuration produces overall comparable results, implying that the 20CR ensemble mean is a reliable data set in dynamical downscaling exercises. The four evaluated WGPs largely reproduce the observed gustiness, although the timing and magnitude of the peak gusts are not regularly captured. None of the WGPs stands out as single best for the complex topography of Switzerland. Differences among the WGPs are small compared to the biases inherited from the sustained-wind part in the WGP formulations. All WGPs transform overestimated peak sustained winds into underestimated peak gusts, which points to an underrepresentation of the turbulent part in the WGP formulations. The range of simulated peak gusts from downscaling all 20CR ensemble members does not reliably include the observed peak gust, indicating limited benefit in applying an ensemble approach. Despite the limitations, we infer that with spatial optimisations of the simulation (e.g. by bias correction or adaptation of the WGP schemes), downscaling of 20CR input is an efficient option for high-resolution assessments of windstorm hazard and risk in Switzerland.

Applying Machine Learning Methods for Predicting Tropical Cyclone Rapid Intensification Events

The aim of this study is to improve the intensity prediction of hurricanes by accounting for Rapid Intensification (RI) events. Modern machine learning methods offer much promise for predicting meteorological events. One application is providing timely and accurate predictions of Tropical Cyclone (TC) behavior, which is crucial for saving lives and reducing damage to property. Current TC track prediction models perform much better than intensity (wind speed) models. This is partially due to the existence of RI events. An RI event is defined as a sudden change in the maximum sustained wind speed of 30 knots or greater within 24 hours. Forecasting RI events is so important that it has been put on the National Hurricane Center top forecast priority list. The research published published on usingmachinelearning methods for RI prediction is currently very limited. In this study, we investigate the potential of popular machine learning methods to predict RI events. The evaluated models include support vector machines, logistic regression, naive-Bayes classifiers, classification and regression trees and a wide range of ensemble methods including boosting and stacking. We also investigate dimensionality reduction and feature selection and we address class imbalance using the Synthetic Minority Over-sampling Technique (SMOTE). The evaluation shows that some of the investigated models improve over the current operational Rapid Intensification Index model finally; we use RI predictions to make improved storm intensity predictions.

Variations in the power dissipation index in the East Asia region

This study examines the variability of the power dissipation index (PDI) for different regions of the East Asia region during the period 1960–2013. The annual PDI (APDI) in the region is calculated as the sum of the PDI, defined as the cube of the maximum sustained wind speed at landfall of each tropical cyclone (TC) making landfall at that region. Upward and downward trends in APDI are found in the northern and southern parts of East Asia respectively, suggesting a possible northward shift in TC landfall locations. Interdecadal variations of the APDI can also be found in some regions. The APDI in various regions show a close relation with the PDI distribution over the western North Pacific (WNP) with three characteristic patterns. The ENSO and basin-wide mode represents the PDI patterns associated with ENSO events and the overall PDI over the WNP. The east–west dipole mode and the north–south dipole mode denote the east–west and north–south shifts of PDI respectively. Based on the steering flow (average winds within the 850–300 hPa layer) near the East Asian coast, a three-cell model for TC landfall in East Asia is proposed, which corresponds to three major modes of the atmospheric circulation in the WNP. Each of these modes shows an anomalous circulation located east of Taiwan, east of Japan and the South China Sea, respectively and each of which has a significant impact on the APDI in some regions along the coast of East Asia. A northward shift in the APDI along the East Asian coast is identified in the period 1997–2013 as a result of the change in steering flow pattern, northward shift in TC genesis location and weaker vertical wind shear over the ocean near the coastal areas.

Isotropic and anisotropic kriging approaches for interpolating surface-level wind speeds across large, geographically diverse regions

ABSTRACTWindstorms result in significant damage and economic loss and are a major recurring threat in many countries. Estimating surface-level wind speeds resulting from windstorms is a complicated problem, but geostatistical spatial interpolation methods present a potential solution. Maximum sustained and peak gust weather station data from two historic windstorms in Europe were analyzed to predict surface-level wind speed surfaces across a large and topographically varied landscape. Disjunctively sampled maximum sustained wind speeds were adjusted to represent equivalent continuously sampled 10-minute wind speeds and missing peak gust station data were estimated by applying a gust factor to the recorded maximum sustained wind speeds. Wind surfaces were estimated based on anisotropic and isotropic kriging interpolation methodologies. The study found that anisotropic kriging is well-suited for interpolating wind speeds in meso- and macro-scale areas because it accounts for wind direction and trends in wind speeds across a large, heterogeneous surface, and resulted in interpolation surface improvement in most models evaluated. Statistical testing of interpolation error for stations stratified by geographic classification revealed that stations in coastal and/or mountainous locations had significantly higher prediction errors when compared with stations in non-coastal/non-mountainous locations. These results may assist in mitigating losses to structures due to excessive wind events.

The Arbitrary Definition of the Current Atlantic Major Hurricane Landfall Drought

Abstract A major hurricane [96+ knots (kt; 1 kt = 0.51 m s–1) of maximum sustained wind] has not made landfall in the United States since Wilma (2005). Recent elegant stochastic–statistical modeling estimates the return period of a 9-yr streak for this metric as 177 yr, suggesting extraordinary rarity, especially in the context of the length of the record (1851–2014). Current awareness of the drought is increased given that the 2015 hurricane season is expected to be suppressed from El Niño and the recent anniversaries of several noteworthy landfalls. Yet, here we show that the significance or even existence of the current 9-yr drought is highly dependent on the metric used. Acknowledging that wind intensity estimates are binned every 5 kt and have approximate 10-kt uncertainty, we examine the same record using landfall thresholds of 95–105 kt. Using 105-kt landfall, 1993–2003 becomes a previously unreported yet more remarkable 11-yr drought and 1981–88 becomes an 8-yr drought. Further, landfall minimum sea level pressure is more reliably estimated than maximum sustained wind speed. For landfall intensities stronger than 960 hPa (a climatological threshold for 100 kt), the current drought disappears because of Irene (2011) and Sandy (2012). A coastline-independent yet nearby proximity metric is tested and reveals a nonexistent drought. Accordingly, this study suggests the following: 1) Caution is advised when identifying a hurricane drought and its historical significance. 2) Using hurricane landfall statistics to infer a climate signal is fraught with issues (threshold, coastline, and potentially nonscientific contributions), regardless of intensity metric. 3) From a societal context, human and financial losses matter most, and Irene [2011; $8 billion (U.S. dollars)] and Sandy (2012; $88 billion) occurred during the current drought.

Hurricane-induced shoreline change and post-storm recovery: northeastern Yucatan Peninsula, Mexico

ABSTRACT Mulcahy, N.; Kennedy, D.M., and Blanchon, P., 2016. Hurricane-induced shoreline change and post-storm recovery: northeastern Yucatan Peninsula, Mexico. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1192 - 1196. Coconut Creek (Florida), ISSN 0749-0208. Storms are major drivers of shoreline change on barrier beach systems. In tropical environments many such systems are fronted by coral reefs; however, little is known about how these reefal structures influence beach response during hurricanes. This study provides a detailed assessment of the impacts of Hurricane Wilma on the coral-fringed barrier beaches of the northeastern Yucatan Peninsula, Mexico. Hurricane Wilma made landfall in late October 2005 as a Category 4 storm, bringing sustained wind speeds of 67 ms−1, and storm waves with significant wave heights (Hs) ≈ 13 m. To determine th...

Lightning activity and its relationship with typhoon intensity and vertical wind shear for Super Typhoon Haiyan (1330)

Super Typhoon Haiyan (1330), which occurred in 2013, is the most powerful typhoon during landfall in the meteorological record. In this study, the temporal and spatial distributions of lightning activity of Haiyan were analyzed by using the lightning data from the World Wide Lightning Location Network, typhoon intensity and position data from the China Meteorological Administration, and horizontal wind data from the ECMWF. Three distinct regions were identified in the spatial distribution of daily average lightning density, with the maxima in the inner core and the minima in the inner rainband. The lightning density in the intensifying stage of Haiyan was greater than that in its weakening stage. During the time when the typhoon intensity measured with maximum sustained wind speed was between 32.7 and 41.4 ms−1, the storm had the largest lightning density in the inner core, compared with other intensity stages. In contrast to earlier typhoon studies, the eyewall lightning burst out three times. The first two eyewall lightning outbreaks occurred during the period of rapid intensification and before the maximum intensity of the storm, suggesting that the eyewall lightning activity could be used to identify the change in tropical cyclone intensity. The flashes frequently occurred in the inner core, and in the outer rainbands with the black body temperature below 220 K. Combined with the ECMWF wind data, the influences of vertical wind shear (VWS) on the azimuthal distribution of flashes were also analyzed, showing that strong VWS produced downshear left asymmetry of lightning activity in the inner core and downshear right asymmetry in the rainbands.

Real-time prediction of movement, intensity and storm surge of very severe cyclonic storm Hudhud over Bay of Bengal using high-resolution dynamical model

Tropical cyclone (TC) ‘Hudhud’ (October 2014) over the Bay of Bengal was the first TC to strike the Port city, Vishakhapatnam, with intensity of 100 knots (180 km h−1) since 1891. The advanced research weather research and forecasting (ARW) model with 9-km grid spacing is coupled offline with dynamical storm surge model and is used for real-time prediction of Hudhud. In the current study, the successful story of prediction of Hudhud is substantiated. The ARW model was able to predict the genesis in advance of 3 days with reasonable accuracy in position and strength. The mean initial vortex position and intensity errors are 46 km and 6 knots, respectively. The model predictions showed that Hudhud would intensify to a very severe cyclonic storm (max. sustained wind speed ~97 knots from all the experiments) in advance of ~4 days which is consistent with the observation. The mean absolute intensity forecast error is significantly less (18 knots) up to 96-h forecast. The model could reproduce the upper-level warming of ~7.5 °C at ~50 km radial distance similar to that of satellite observation. The ARW model showed appreciable performance in track prediction (with errors ≤150 km up to 96-h forecast). On the landfall day, the rainfall patterns are realistic and consistent with IMD rainfall observation. The dynamical storm surge model, forced with the outputs of the ARW model, was able to predict realistic surge ranging between 1.4–2.2 m in 24- to 96-h forecast lengths. This study highlights the importance and need of high-resolution numerical models for operational TC and associated surge forecast over the region.