High-wind-speed evaluation in the Southern Ocean

Space-based scatterometer instruments provide crucial surface wind measurements with high resolution over global oceans. Midlatitude regions in the Southern Ocean are unique places to evaluate scatterometer winds at high-wind bands because these regions host the strongest wind fields at the ocean surface. The objective of this study is to evaluate high wind speeds observed by Quick Scatterometer (QuikSCAT) wind measurements and produced by simulation models and compare them with weather station data in the Southern Ocean. The occurrence and intensity of high-wind events in scatterometer measurements are compared with that of reanalysis winds, and the spatial and seasonal variability of high-wind characteristics is examined. The results show that the speeds of scatterometer winds are similar to model simulations in the monthly mean field but consistently stronger than both European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction/National Center for Atmospheric Research winds in high-wind bands. When scatterometer winds are compared with the weather station observations at Macquarie Island, the present study finds no systematic bias at high-wind bands across all months. However, both weather station and QuikSCAT winds are higher than the model simulations in high-wind bands most of the time. This suggests that model simulations may underestimate surface wind strength in high-wind bands. Such underestimation would lead to up to an 80% reduction in energy flux between the atmosphere and ocean. Even though high winds occur only sporadically and the reanalysis underestimation in high wind speed is not in itself of great magnitude, they have a significant impact on global climate. INDEX TERMS: 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; KEYWORDS: QuikSCAT winds, Southern Ocean, wind evaluatio

Advances in Hurricane Research

This book provides a wealth of new information, ideas and analysis on some of the key unknowns in hurricane research. Topics covered include the numerical prediction systems for tropical cyclone development, the use of remote sensing methods for tropical cyclone development, a parametric surface wind model for tropical cyclones, a micrometeorological analysis of the wind as a hurricane passes over Houston, USA, the meteorological passage of numerous tropical cyclones as they pass over the South China Sea, simulation modelling of evacuations by motorised vehicles in Alabama, the influence of high stream-flow events on nutrient flows in the post hurricane period, a reviews of the medical needs, both physical and psychological of children in a post hurricane scenario and finally the impact of two hurricanes on Ireland. Hurricanes discussed in the various chapters include Katrina, Ike, Isidore, Humberto, Debbie and Charley and many others in the North Atlantic as well as numerous tropical cyclones in the South China Sea

Tropical cyclones in the South-West Indian Ocean : intensity changes, oceanic interaction and impacts

Includes abstract.Includes bibliographical references (p. 229-253).This study investigates the climatology, intensification and ocean atmosphere interaction in relation to the passage of tropical cyclones (TCs) in the South-West Indian Ocean (SWIO). A Climatology of TCs in the SWIO including landfall in the area of Mozambique and Madagascar was developed for the 1952-2007 and 1980-2007 periods

Insights on the OAFlux ocean surface vector wind analysis merged from scatterometers and passive microwave radiometers (1987 onward)

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 5244–5269, doi:10.1002/2013JC009648.A high-resolution global daily analysis of ocean surface vector winds (1987 onward) was developed by the Objectively Analyzed air-sea Fluxes (OAFlux) project. This study addressed the issues related to the development of the time series through objective synthesis of 12 satellite sensors (two scatterometers and 10 passive microwave radiometers) using a least-variance linear statistical estimation. The issues include the rationale that supports the multisensor synthesis, the methodology and strategy that were developed, the challenges that were encountered, and the comparison of the synthesized daily mean fields with reference to scatterometers and atmospheric reanalyses. The synthesis was established on the bases that the low and moderate winds (<15 m s−1) constitute 98% of global daily wind fields, and they are the range of winds that are retrieved with best quality and consistency by both scatterometers and radiometers. Yet, challenges are presented in situations of synoptic weather systems due mainly to three factors: (i) the lack of radiometer retrievals in rain conditions, (ii) the inability to fill in the data voids caused by eliminating rain-flagged QuikSCAT wind vector cells, and (iii) the persistent differences between QuikSCAT and ASCAT high winds. The study showed that the daily mean surface winds can be confidently constructed from merging scatterometers with radiometers over the global oceans, except for the regions influenced by synoptic weather storms. The uncertainties in present scatterometer and radiometer observations under high winds and rain conditions lead to uncertainties in the synthesized synoptic structures.The project is sponsored by the NASA Ocean Vector Wind Science Team (OVWST) activities under grant NNA10AO86G.2015-02-1

Confidence and sensitivity study of the OAFlux multisensor synthesis of the global ocean surface vector wind from 1987 onward

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 6842–6862, doi:10.1002/2014JC010194.This study presented an uncertainty assessment of the high-resolution global analysis of daily-mean ocean-surface vector winds (1987 onward) by the Objectively Analyzed air-sea Fluxes (OAFlux) project. The time series was synthesized from multiple satellite sensors using a variational approach to find a best fit to input data in a weighted least-squares cost function. The variational framework requires the a priori specification of the weights, or equivalently, the error covariances of input data, which are seldom known. Two key issues were investigated. The first issue examined the specification of the weights for the OAFlux synthesis. This was achieved by designing a set of weight-varying experiments and applying the criteria requiring that the chosen weights should make the best-fit of the cost function be optimal with regard to both input satellite observations and the independent wind time series measurements at 126 buoy locations. The weights thus determined represent an approximation to the error covariances, which inevitably contain a degree of uncertainty. Hence, the second issue addressed the sensitivity of the OAFlux synthesis to the uncertainty in the weight assignments. Weight perturbation experiments were conducted and ensemble statistics were used to estimate the sensitivity. The study showed that the leading sources of uncertainty for the weight selection are high winds (>15 ms−1) and heavy rain, which are the conditions that cause divergence in wind retrievals from different sensors. Future technical advancement made in wind retrieval algorithms would be key to further improvement of the multisensory synthesis in events of severe storms.The project is sponsored by the NASA Ocean Vector Wind Science Team (OVWST) activities under grant NNA10AO86G. The database of 126 buoys was established during the development of the OAFlux surface turbulent latent and sensible heat fluxes under the auspices of the NOAA grant NA09OAR4320129.2015-04-1

Mechanisms determining structural changes during the extratropical transition of Typhoon Sinlaku (2008): a modelling study

The transition from a tropical cyclone into an extratropical cyclone occurs worldwide several times per year. This so-called extratropical transition is often associated with high impact weather. At the same time the complexity of the structural changes makes it difficult for models to adequately forecast the development. In September 2008, Typhoon Sinlaku moved along the coast of Japan and re-intensified as a tropical cyclone shortly before it underwent extratropical transition. The structural changes of this typhoon are analyzed based on a unique measurement dataset and simulations from a numerical weather prediction model. Besides, several meteorological tools are used to analyze the dynamical processes that caused the structural changes of Sinlaku

Construction of a Tropical Cyclone Size Dataset using Retroactive Analysis Data with a Damage Application

This thesis introduces a new tropical cyclone (TC) size dataset. Using the radius of the outermost closed isobar (ROCI) as the size metric of focus, a comprehensive record of TC size at landfall was constructed for tropical storms and hurricanes that made landfall in the United States along the East Coast and in the Gulf of Mexico. The ROCI information was derived from mean sea level pressure maps generated using the NCEP/NCAR Reanalysis I dataset over a 69-year period (1948 – 2016). Construction of the dataset involved using two methodologies, one based on hourly interpolated HURDAT2 best tracks and the other based on landfall indicated by the reanalysis generated maps. Descriptive statistics were calculated for ROCI with respect to the dataset as a whole, intensity, and landfall location. Both methods were compared against each other, both with respect to ROCI as well as landfall locations. The results indicated that the two methods generated statistically identical ROCI, even though individual TCs could have differing ROCI values. The results also indicated that there was no significant trend in landfall ROCI over time. With respect to landfall locations, the results indicate that roughly two-thirds of all TCs in the dataset experienced a westward shift in TC center landfall location relative to the best track center location, with the displacement more prevalent in the Gulf of Mexico. A secondary analysis was conducted to ascertain the relationship between TC size and total economic damage, using damage data collected by Icat. The results of this analysis suggest a significant relationship between TC size and damage. This dataset serves as a prototype, with future work focusing on improving and extending the dataset

The low-level wind structure of tropical cyclones

The main effects of tropical cyclones include high winds, extensive storm surges, and widespread flooding. These main effects are all closely related to the low-level wind structure, the height of which is about 2 km from the underlying surface. A physically based analytic model (λ model) is presented to describe the low-level wind structure of tropical cyclones in terms of one intensity measure, one size measure, and the storm meridional position. The λ model provides an accurate fit of the near-surface azimuthal wind field simulated with an idealized full-physics numerical model. This numerical model is further applied to investigate the tropical cyclone low-level wind structure evolution during the mature stage with observations. Three basic features at the mature stage are observed: the outward expansion of eyewall, the reduction of intensity and the increase of tangential wind in the outer spiral bands. Model simulations show that the outer circulation expansion is accompanied by the outward migration of diabatic heating at mid-level in the eyewall. The outward movement of eyewall is caused by the opposing angular momentum transports. Consequently, the intensity decays due to the angular momentum conservation and cyclostrophic adjustment, which is captured in a new analytic pressure-wind relationship derived from the λ model. To investigate the relationship between the tropical cyclone damage and low-level wind structure, the λ model is used to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the correlation of damage with the integrated wind profile of all hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also ultimately the cost of hurricanes.Open Acces

Quantification of the Downstream Impact of Extratropical Transition for Typhoon Jangmi and Other Case Studies

The impact of extratropical transition on the midlatitude flow is quantified based on potential vorticity inversion. The detailed study of Typhoon Jangmi (2008) reveals the diabatically enhanced net transport of low-PV air to the tropopause as the key physical process determining the direct impact of ET. Relocation experiments and further case studies show the crucial role of the relative position of the TC and the midlatitude flow for the downstream impact of ET and the reduced predictability

Quantification of the downstream impact of extratropical transition for Typhoon Jangmi and other case studies

The impact of extratropical transition on the midlatitude flow is quantified based on potential vorticity inversion. The detailed study of Typhoon Jangmi (2008) reveals the diabatically enhanced net transport of low-PV air to the tropopause as the key physical process determining the direct impact of ET. Relocation experiments and further case studies show the crucial role of the relative position of the TC and the midlatitude flow for the downstream impact of ET and the reduced predictability