A Postlandfall Hurricane Classification System for the United States

Determining the proper relationship between the minimum sea level pressures and maximum sustained winds in tropical cyclones has been a long-standing problem. The major obstacle has been the lack of sufficient ground truth, i.e., actual measurements of maximum wind speeds in tropical cyclones with a wide range of central pressures. In this study 28 years of maximum wind measurements made at coastal and island stations in the western North Pacific were collected and analyzed. Because of problems in measuring and interpreting sustained surface wind speeds, only recorded peak gust values were used. These peak gust values were reduced to a standard anemometer level of 10 m using a power law relationship and then converted to 1 min sustained wind speeds using gust factors representative of an overwater environment. The sample was restricted to cases where it was reasonably certain that the station experienced the cyclone's winds during its passage. The resulting equation,where pc is the minimum sea level pressure (mb) and Vm the maximum sustained (1 min) wind speed (kt), indicates maximum wind speeds that are significantly lower than many previous studies.

Tropical cyclone-generated storm surges create natural disasters that are among the most deadly and costly global catastrophes. Individual disasters have inflicted hundreds of thousands of fatalities and billions of dollars in damage. In 1970, a tropical cyclone in the Bay of Bengal generated a 9.1-meter surge which killed approximately 300,000 people in Bangladesh (Frank and Husain 1971; Dube et al. 1997; De et al. 2005). More recently, and well into the age of satellite meteorology, a storm surge in 1991 killed approximately 140,000 people in Bangladesh (Dube et al. 1997). Although the magnitude of storm surge heights and loss of life are highest along the shores of Bangladesh and India, such disasters are not limited to countries with developing economies. The 1900 Galveston Hurricane generated a 6.1-meter surge (Garriott 1900), which killed between 6,000 and 8,000 people in Galveston, Texas (Rappaport and Fernandez-Partagas 1995), producing the most deadly natural disaster in United States history (National Oceanic and Atmospheric Administration 1999). More recently, Hurricane Katrina (2005) generated an 8.47-meter surge (Knabb et al. 2006), which claimed more than 1,800 lives along the coasts of Louisiana and Mississippi, and inflicted $81 billion dollars in damage (McTaggart-Cowan et al. 2008). While tropical cyclone-generated storm surge is a deadly and costly hazard, it is also scientifically complex, because meteorological, oceanographic and geographic factors influence the height, extent and duration of storm surge flooding. Such factors include maximum sustained hurricane wind speed at landfall and offshore, hurricane size, hurricane forward speed, the angle of hurricane approach to the coastline, bathymetry of coastal waters, coastline shape and the presence of barriers or obstructions to surge waters on land. Although relationships between some of these factors and resultant surge heights may seem intuitive (e.g. the assumption that stronger hurricanes always generate higher surges), storm surge observations reveal that a combination of physical factors influence surge characteristics. As a result, weaker hurricanes sometimes generate higher surges than stronger hurricanes. For example, Hurricane Ike (2008) generated a 5.33-meter surge along the Upper Texas Coast, although maximum sustained winds at landfall were only 175 km/ hour (Berg 2009), whereas Hurricane Charley in Western Florida had maximum sustained winds of 240 km/ hour at landfall, but only generated a 2.13-meter surge, partly because the storm rapidly intensified just before making landfall (Pasch et al. 2004). The complex nature of storm surge makes this phenomenon difficult to forecast and difficult for coastal populations to understand. In cases where storm surge heights and extents are

Previous work, in which Advanced Microwave Sounding Unit (AMSU) data from the Atlantic Ocean and east Pacific Ocean basins during 1999–2001 were used to provide objective estimates of 1-min maximum sustained surface winds, minimum sea level pressure, and the radii of 34-, 50-, and 64-kt (1 kt ≡ 0.5144 m s−1) winds in the northeast, southeast, southwest, and northwest quadrants of tropical cyclones, is updated to reflect larger datasets, improved statistical analysis techniques, and improved estimation through dependent variable transforms. A multiple regression approach, which utilizes best-subset predictor selection and cross validation, is employed to develop the estimation models, where the dependent data (i.e., maximum sustained winds, minimum pressure, wind radii) are from the extended best track and the independent data consist of AMSU-derived parameters that give information about retrieved pressure, winds, temperature, moisture, and satellite resolution. The developmental regression models result in mean absolute errors (MAE) of 10.8 kt and 7.8 hPa for estimating maximum winds and minimum pressure, respectively. The MAE for the 34-, 50-, and 64-kt azimuthally averaged wind radii are 16.9, 13.3, and 6.8 n mi (1 n mi ≡ 1852 m), respectively.

Hurricanes Erin, Opal, Luis, Marilyn, and Roxanne were the most destructive hurricanes of 1995. At landfall, Luis and Marilyn contained maximum sustained winds (marine exposure) estimated at near 60 and 46 m s21, respectively. The strongest landfalling storm of the 1995 season, Luis, decreased in intensity from a category 4 to 3 on the Saffir‐Simpson scale shortly before the eyewall crossed the Islands of Antigua, Barbuda, St. KittsNevis, St. Barthelemy, St. Martin, and Anguilla. Hurricane Marilyn strengthened as it approached the U.S. Virgin Islands, with St. Thomas bearing the brunt of the north and south eyewall winds of 46 m s21 (marine exposure) and St. Croix being affected by the relatively weak western eyewall peak winds of 35‐40 m s21 (marine exposure). For Luis and Marilyn only surface winds with marine exposures were analyzed because of unknown small-scale interactions associated with complex island terrain with 500‐1000-m elevations. Wind engineering studies suggest that wind acceleration over blunt ridges can increase or ‘‘speed up’’ winds by 20%‐80%. Topographic effects were evident in damage debris analyses and suggest that an operational method of assessing terrain-induced wind gusts (such as a scaled down mesoscale model) is needed. After landfall as a marginal hurricane over central Florida, Hurricane Erin regained strength over the Gulf of Mexico with a well-defined radar reflectivity structure. Erin struck the Florida panhandle near Navarre Beach with maximum sustained surface winds of 35‐ 40 m s21 affecting the Destin‐Ft. Walton area. Hurricane Opal made landfall in nearly the identical area as Erin, with maximum sustained surface winds of 40‐45 m s21, having weakened from an intensity of nearly 60 m s21 only 10 h earlier. Opal was characterized by an asymmetric structure that was likely related to cold front interaction and an associated midlevel southwesterly jet. Roxanne struck Cozumel, Mexico, with sustained surface winds (marine exposure) of 46 s21, crossed the Yucatan, and meandered in the southwest Gulf of Mexico for several days. While in the Bay of Campeche, Roxanne’s large area of hurricane-force winds disabled a vessel, which lead to the drowning deaths of five oil industry workers. High-resolution wind records are critical to preserving an accurate extreme wind climatology required for assessment of realistic building code risks. Unfortunately, power interruptions to Automated Surface Observing Stations on the U.S. Virgin Islands (St. Croix, St. Thomas) and Destin, Florida, prevented complete wind records of the eyewall passages of Marilyn and Opal, respectively.

The predictability limits of tropical cyclone (TC) intensity over the western North Pacific (WNP) are investigated using TC best track data. The results show that the predictability limit of the TC minimum central pressure (MCP) is ~102 h, comparable to that of the TC maximum sustained wind (MSW). The spatial distribution of the predictability limit of the TC MCP over the WNP is similar to that of the TC MSW, and both gradually decrease from the eastern WNP (EWNP) to the South China Sea (SCS). The predictability limits of the TC MCP and MSW are relatively high over the southeastern WNP where the modified accumulated cyclone energy (MACE) is relatively large, whereas they are relatively low over the SCS where the MACE is relatively small. The spatial patterns of the TC lifetime and the lifetime maximum intensity (LMI) are similar to that of the TC MACE. Strong and long-lived TCs, which have relatively long predictability, mainly form in the southwestern WNP. In contrast, weak and short-lived TCs, which have relatively short predictability, mainly form in the SCS. In addition to the dependence of the predictability limit on genesis location, the predictability limits of TC intensity also evolve in the TC life cycle. The predictability limit of the TC MCP (MSW) gradually decreases from 102 (108) h at genesis time (00 h) to 54 (84) h 4 days after TC genesis.

The relationship between minimum central surface pressure and the maximum sustained surface wind in tropical cyclones has been studied for many years, motivated by the fact that minimum pressure is generally easier to measure, but maximum wind is a much more relevant metric when considering tropical cyclone risk and potential impacts. It is well understood that tropical cyclone wind is closely related to the radial gradient of pressure through gradient or cyclostrophic balance assumptions, and not to a single point value of the minimum pressure near the storm center. But it is often the case that the maximum wind must be inferred from this single value. To accomplish this, a number of statistical relationships have been documented, such as those used in the Dvorak technique for estimating tropical cyclone intensity from satellite imagery. Here, the relationship between tropical cyclone maximum wind and minimum pressure is explored during eyewall replacement cycles (ERCs) that have been observed in North Atlantic hurricanes. It is shown that the wind–pressure relationship (WPR) can vary substantially during an ERC and generally moves away from the statistically fitted WPR used by the Dvorak technique in that basin. The changes in WPR during an ERC can be quite different depending on the intensity of the hurricane at the start of the ERC.

Using the Atlantic Hurricane Database (HURDAT2) and the General Bathymetry Chart of the Oceans (GEBCO‐2014), the probability of tropical cyclones of various intensities (hurricane, tropical storm, or a tropical depression) making landfall through the Lesser Antilles (i.e., the Windward and Leeward Islands—from 12.5°N to 17.5°N and 59.0°W to 63.0°W) from 1991 to 2016 was evaluated. HURDAT2 data from 1979 to 2016 also were used to assess storm location, maximum sustained winds, and minimum central pressure as they passed through the Lesser Antilles.In this region, tropical depressions generally did not intensify with respect to maximum sustained wind speeds and only one intensified with respect to a drop in its central pressure. By contrast, hurricanes were more likely to show intensification with respect to maximum sustained wind speed and central pressure. Tropical storms were equally likely to intensify or dissipate. Also, the data indicate that with the passage of a hurricane, maximum sustained winds were less likely to increase despite a drop in the central pressure. For tropical storms and particularly with tropical depressions, the effect of the islands is more pronounced.

Atlantic hurricane seasons have a long history of causing significant financial impacts, with Harvey, Irma, Maria, Florence, and Michael combining to incur more than 345 billion USD in direct economic damage during 2017–2018. While Michael’s damage was primarily wind and storm surge-driven, Florence’s and Harvey’s damage was predominantly rainfall and inland flood-driven. Several revised scales have been proposed to replace the Saffir–Simpson Hurricane Wind Scale (SSHWS), which currently only categorizes the hurricane wind threat, while not explicitly handling the totality of storm impacts including storm surge and rainfall. However, most of these newly-proposed scales are not easily calculated in real-time, nor can they be reliably calculated historically. In particular, they depend on storm wind radii, which remain very uncertain. Herein, we analyze the relationship between normalized historical damage caused by continental United States (CONUS) landfalling hurricanes from 1900–2018 with both maximum sustained wind speed ( V max ) and minimum sea level pressure (MSLP). We show that MSLP is a more skillful predictor of normalized damage than V max , with a significantly higher rank correlation between normalized damage and MSLP ( r rank = 0.77) than between normalized damage and V max ( r rank = 0.66) for all CONUS landfalling hurricanes. MSLP has served as a much better predictor of hurricane damage in recent years than V max , with large hurricanes such as Ike (2008) and Sandy (2012) causing much more damage than anticipated from their SSHWS ranking. MSLP is also a more accurately-measured quantity than is V max , making it an ideal quantity for evaluating a hurricane’s potential damage.

A study on radial characteristics of North Indian Ocean tropical cyclones and associated energy indices through numerical modeling

Public health risks from urban floods are a global concern. A typhoon is a devastating natural hazard that is often accompanied by heavy rainfall and high storm surges and causes serious floods in coastal cities. Affected by the same meteorological systems, typhoons, rainfall, and storm surges are three variables with significant correlations. In the study, the joint risk of rainfall and storm surges during typhoons was investigated based on principal component analysis, copula-based probability analysis, urban flood inundation model, and flood risk model methods. First, a typhoon was characterized by principal component analysis, integrating the maximum sustained wind (MSW), center pressure, and distance between the typhoon center and the study area. Following this, the Gumbel copula was selected as the best-fit copula function for the joint probability distribution of typhoons, rainfall, and storm surges. Finally, the impact of typhoons on the joint risk of rainfall and storm surges was investigated. The results indicate the following: (1) Typhoons can be well quantified by the principal component analysis method. (2) Ignoring the dependence between these flood drivers can inappropriately underestimate the flood risk in coastal regions. (3) The co-occurrence probability of rainfall and storm surges increases by at least 200% during typhoons. Therefore, coastal urban flood management should pay more attention to the joint impact of rainfall and storm surges on flood risk when a typhoon has occurred. (4) The expected annual damage is 0.82 million dollars when there is no typhoon, and it rises to 3.27 million dollars when typhoons have occurred. This indicates that typhoons greatly increase the flood risk in coastal zones. The obtained results may provide a scientific basis for urban flood risk assessment and management in the study area.

The western North Pacific Ocean is the most active tropical cyclone (TC) basin. However, recent studies are not conclusive on whether the TC activity is increasing or decreasing, at least when calculations are based on maximum sustained winds. For this study, TC minimum central pressure data are analyzed in an effort to better understand historical typhoons. Best-track pressure reports are compared with aircraft reconnaissance observations; little bias is observed. An analysis of wind and pressure relationships suggests changes in data and practices at numerous agencies over the historical record. New estimates of maximum sustained winds are calculated using recent wind–pressure relationships and parameters from International Best Track Archive for Climate Stewardship (IBTrACS) data. The result suggests potential reclassification of numerous typhoons based on these pressure-based lifetime maximum intensities. Historical documentation supports these new intensities in many cases. In short, wind reports in older best-track data are likely of low quality. The annual activity based on pressure estimates is found to be consistent with aircraft reconnaissance and between agencies; however, reconnaissance ended in the western Pacific in 1987. Since then, interagency differences in maximum wind estimates noted here and by others also exist in the minimum central pressure reports. Reconciling these recent interagency differences is further exasperated by the lack of adequate ground truth. This study suggests efforts to intercalibrate the interagency intensity estimate methods. Conducting an independent and homogeneous reanalysis of past typhoon activity is likely necessary to resolve the remaining discrepancies in typhoon intensity records.

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.

On 19 October 2000, Hurricane Michael merged with an approaching baroclinic trough over the western North Atlantic Ocean south of Nova Scotia. As the hurricane moved over cooler sea surface temperatures (SSTs; less than 25°C), it intensified to category-2 intensity on the Saffir–Simpson hurricane scale [maximum sustained wind speeds of 44 m s−1 (85 kt)] while tapping energy from the baroclinic environment. The large “hybrid” storm made landfall on the south coast of Newfoundland with maximum sustained winds of 39 m s−1 (75 kt) causing moderate damage to coastal communities east of landfall. Hurricane Michael presented significant challenges to weather forecasters. The fundamental issue was determining which of two cyclones (a newly formed baroclinic low south of Nova Scotia or the hurricane) would become the dominant circulation center during the early stages of the extratropical transition (ET) process. Second, it was difficult to predict the intensity of the storm at landfall owing to competing factors: 1) decreasing SSTs conducive to weakening and 2) the approaching negatively tilted upper-level trough, favoring intensification. Numerical hindcast simulations using the limited-area Mesoscale Compressible Community model with synthetic vortex insertion (cyclone bogus) prior to the ET of Hurricane Michael led to a more realistic evolution of wind and pressure compared to running the model without vortex insertion. Specifically, the mesoscale model correctly simulates the hurricane as the dominant circulation center early in the transition process, versus the baroclinic low to its north, which was the favored development in the runs not employing vortex insertion. A suite of experiments is conducted to establish the sensitivity of the ET to various initial conditions, lateral driving fields, domain sizes, and model parameters. The resulting storm tracks and intensities fall within the range of the operational guidance, lending support to the possibility of improving numerical forecasts using synthetic vortex insertion prior to ET in such a model.

he potential relationships between tropical cyclones and global climate change are scientifically and socially complex, with great implications for society. The exceptional nature of the 2005 North Atlantic hurricane season alone provides great incentives for better understanding the full range of interactions and causes and effects thereof. The 2005 season saw the largest number (27) of named storms (sustained winds over 17 m s–1) and the largest number (14) of hurricanes (sustained winds over 33 m s–1), and it was the only year with three category 5 storms (maximum sustained winds over 67 m s–1). Also recorded was the most intense storm on record (Wilma, minimum pressure 882 hPa), the most intense storm ever in the Gulf of Mexico (Rita, 897 hPa), and the most costly storm on record (Katrina; some estimates put the cost of Katrina at as high as $200 billion; see information online at www.rms.com/Publications/ KatrinaReport_LessonsandImplications.pdf). (Please see www.ncdc.noaa.gov/oa/climate/research/2005/ hurricanes05.html for additional details.) The climate is in the process of a rapid warming, caused in part by human activities, including emission of greenhouse gases (GHGs) and aerosols, and changes in land use (Houghton et al. 2001; Karl and Trenberth 2003). These climate changes may well be changing the properties of tropical cyclones, yet the potential relationships between climate change and tropical cyclones and the consequences for humans have been downplayed or dismissed by a number of recent articles, testimonies, and press releases (e.g., Michaels et al. 2005;1 Pielke et al. 2005; Mayfield 2005). For example, the recent article with the allencompassing title “Hurricanes and global warming” by Pielke et al. (2005) raises several important points, yet it is incomplete and misleading because it 1) omits any mention of several of the most important aspects of the potential relationships between hurricanes and global warming, including rainfall, sea level, and storm surge; 2) leaves the impression that there is no significant connection between recent climate change caused by human activities and hurricane characteristics and impacts; and 3) does not take full account of the significance of recently identified trends and variations in tropical storms in causing impacts as compared to increasing societal vulnerability. In a similar vein, public statements and testimonies by prominent hurricane forecasters have denied or minimized important connections between global warming and tropical cyclones, attributing interannual variations in tropical cyclones only to natural variability (e.g., Mayfield 2005). It is true that society’s vulnerability to hurricane damage is increasing rapidly, predominantly because of the rise in population and property values near

The Freeman/Hasling Hurricane Damage Potential [HDP] Scale gives a more accurate and complete description of the impact expected to offshore structures from a hurricane moving through the Gulf of Mexico [GOM] oil leases than current industry methods. The Freeman/Hasling HDP Scale is not only based on the maximum sustained winds in the hurricane but also the central pressure, storm speed, storm heading, radius of 34 and 64 knot winds, the area of 34 feet or greater significant wave heights and the duration of the winds greater than 64 knots. This scale can be used to determine the number of offshore properties at risk of being damaged or destroyed as the hurricane moves through the GOM oil leases. This will then give the user an idea of the impact of the storm surge and hurricane generated waves to their properties. Introduction Hurricane Ike 2008 demonstrated that Size Matters, where the size of the hurricane is defined as the radius of hurricane force winds, [64 knots, R64]. Ike was a very large storm which generated a very high storm surge and waves over the GOM oil and gas leases. The author used Weather Research Center's [WRC] Hurricane Wind and Wave Model in real time to forecast Hurricane Ike as it moved through the leases. During the 2008 hurricane season, WRC meteorologists noticed the significant difference in the hurricane wind field size between Hurricane Gustav and Hurricane Ike [See Figure 1]. Using ARCGIS, WRC was able to graph the results for the hurricane wind and wave model. Figure 1 shows the comparison of the maximum sustained wind fields for Hurricane Gustav and Hurricane Ike. The yellow wind barbs indicate winds greater than 64 knots [Category 1 on the Saffir/Simpson Scale], the light orange wind barbs indicate winds greater than 83 knots [Category 2] and the dark orange wind barbs indicate winds greater than 96 knots [Category 3]. The radius of hurricane force winds for Hurricane Gustav extended out to the right of the hurricane track 60 nautical miles compared with Hurricane Ike which had hurricane force winds extending out 110 nautical miles. WRC's Hurricane Wind and Wave Model also computes the significant waves, maximum waves and wave crests generated by the hurricane wind field so WRC's meteorologists were also able to plot the significant waves generated by Hurricane Ike using the ARCGIS software. A composite plot of the regions with significant waves greater than 30 feet, 34 feet, 40 feet, etc. can be made using the software. The GIS software also allows you to overlay other fields such as the Gulf of Mexico offshore platforms and pipelines. Hurricane Ike was classified as a Category 2 hurricane on the Saffir/Simpson Scale, which usually means the significant waves generated cause little damage offshore. However, the size of Ike allowed a very long fetch for the hurricane force winds, which in turn generated very high significant waves, maximum waves, wave crests and storm surges for this hurricane intensity. This led WRC meteorologists to review the past significant hurricanes that have influenced the GOM oil leases to determine if an improved damage potential scale could be generated. This research led to the development of the Freeman/Hasling Hurricane Damage Potential [HDP] Scale. The Freeman/Hasling HDP Scale is not only based on the maximum sustained winds in the storm but also the central pressure, storm speed, storm heading, radius of 34 and 64 knot winds, the area of significant wave heights and the duration of the winds greater than 64 knots. This scale can be used to determine the number of offshore properties at risk of being damaged or destroyed as the hurricane moves through the GOM oil leases which will give the user an idea of the impact of the hurricane generated waves to their properties.