Tropical cyclone forecasting

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Tropical cyclone forecasting is the science of forecasting where a tropical cyclone's center, and its effects, are expected to be at some point in the future. There are several elements to tropical cyclone forecasting: track forecasting, intensity forecasting, rainfall forecasting, storm surge, tornado, and seasonal forecasting. While skill is increasing in regard to track forecasting, intensity forecasting skill remains nearly unchanged over the past several years. Seasonal forecasting began in the 1980s in the Atlantic basin and has spread into other basins in the years since.

History

Short term

The methods through which tropical cyclones are forecast have changed with the passage of time. The first known forecasts in the Western Hemisphere were made by Lt. Col. William Reed of the Corps of Royal Engineers at Barbados in 1847. Reed mostly utilized barometric pressure measurements as the basis of his forecasts. Benito Vines introduced a forecast and warning system based on cloud cover changes in Havana during the 1870s. Before the early 1900s, though, most forecasts were done by direct observations at weather stations, which were then relayed to forecast centers via telegraph. It wasn't until the advent of radio in the early twentieth century that observations from ships at sea were available to forecasters. The 1930s saw the usage of radiosondes in tropical cyclone forecasting. The next decade saw the advent of aircraft-based reconnaissance by the military, starting with the first dedicated flight into a hurricane in 1943, and the establishment of the Hurricane Hunters in 1944. In the 1950s, coastal weather radars began to be used in the United States, and research reconnaissance flights by the precursor of the Hurricane Research Division began in 1954. [1]

The launch of the first weather satellite, TIROS-I, in 1960, introduced new forecasting techniques that remain important to tropical cyclone forecasting to the present. In the 1970s, buoys were introduced to improve the resolution of surface measurements, which until that point, were not available at all oversea surfaces. [1]

Long term

In the late 1970s, William Gray noticed a trend of low hurricane activity in the North Atlantic basin during El Niño years. He was the first researcher to make a connection between such events and positive results led him to pursue further research. He found numerous factors across the globe influence tropical cyclone activity, such as connecting wet periods over the African Sahel to an increase in major hurricane landfalls along the United States East Coast. However, his findings also showed inconsistencies when only looking at a single factor as a primary influence. [2]

Utilizing his findings, Gray developed an objective, statistical forecast for seasonal hurricane activity; he predicted only the number of tropical storms, hurricanes, and major hurricanes, foregoing specifics on tracks and potential landfalls due to the aforementioned inconsistencies. [2] Gray issued his first seasonal forecast ahead of the 1984 season, which used the statistical relationships between tropical cyclone activity, the El Niño–Southern Oscillation (ENSO), Quasi-biennial oscillation (QBO), and Caribbean basin sea-level pressures. [3] [4] The endeavor proved modestly successful. [2] He subsequently issued forecasts ahead of the start of the Atlantic hurricane season in May and before the peak of the season in August. [5] Students and colleagues joined his forecast team in the following years, including Christopher Landsea, Paul W. Mielke Jr., and Kenneth J. Berry. [6]

Track

The large-scale synoptic flow determines 70 to 90 percent of a tropical cyclone's motion. The deep-layer mean flow is considered to be the best tool in determining track direction and speed. If storms are significantly sheared, use of a lower-level wind is a better predictor. Knowledge of the beta effect can be used to steer a tropical cyclone, since it leads to a more northwest heading for tropical cyclones in the Northern Hemisphere. It is also best to smooth out short term wobbles of the storm center in order to determine a more accurate trajectory. [7]

Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system. Combining forecast models with increased understanding of the forces that act on tropical cyclones, and a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. [8] An accurate track forecast is important, because if the track forecast is incorrect, forecasts for intensity, rainfall, storm surge, and tornado threat will also be incorrect.

1-2-3 rule

The 1-2-3 rule (mariner's 1-2-3 rule or danger area) is a guideline commonly taught to mariners for severe storm (specifically hurricane and tropical storm) tracking and prediction. It refers to the rounded long-term NHC/TPC forecast errors of 100-200-300 nautical miles at 24-48-72 hours, respectively. These numbers were close to the 10-year average for the 1982–1991 time frame. [9] However, these errors have decreased to near 50-100-150 as NHC forecasters become more accurate. The "danger area" to be avoided is constructed by expanding the forecast path by a radius equal to the respective hundreds of miles plus the forecast wind radii (size of the storm at those hours). [10]

Intensity

Forecasters say they are less skillful at predicting the intensity of tropical cyclones than cyclone track. [11] Available computing power limits forecasters' ability to accurately model a large number of complex factors, such as exact topology and atmospheric conditions, though with increased experience and understanding, even models with the same resolution can be tuned to more accurately reflect real-world behavior. [12] Another weakness is lack of frequent wind speed measurements in the eye of the storm. The Cyclone Global Navigation Satellite System, launched by NASA in 2016, is expected to provide much more data compared to sporadic measurements by weather buoys and hurricane-penetrating aircraft. [13]

An accurate track forecast is essential to creating accurate intensity forecasts, particularly in an area with large islands such as the western north Pacific and the Caribbean Sea, as proximity to land is an inhibiting factor to developing tropical cyclones. A strong hurricane/typhoon/cyclone can weaken if an outer eye wall forms (typically around 80–160 kilometers (50–100 miles) from the center of the storm), choking off the convection within the inner eye wall. Such weakening is called an eyewall replacement cycle, and is usually temporary. [14]

Maximum potential intensity

Dr. Kerry Emanuel created a mathematical model around 1988, called the maximum potential intensity or MPI, to compute the upper limit of tropical cyclone intensity based on sea surface temperature and atmospheric profiles from the latest global model runs. Maps created from this equation show values of the maximum achievable intensity due to the thermodynamics of the atmosphere at the time of the last model run (either 0000 or 1200 UTC). However, MPI does not take vertical wind shear into account. [15] MPI is computed using the following formula:

${\displaystyle V=A+B\cdot e^{C(T-T_{0})}}$

Where ${\displaystyle V}$ is the maximum potential velocity in meters per second; ${\displaystyle T}$ is the sea surface temperature underneath the center of the tropical cyclone, ${\displaystyle T_{0}}$ is a reference temperature (30˚C) and ${\displaystyle A}$, ${\displaystyle B}$ and ${\displaystyle C}$ are curve-fit constants. When ${\displaystyle A=28.2}$, ${\displaystyle B=55.8}$, and ${\displaystyle C=0.1813}$, the graph generated by this function corresponds to the 99th percentile of empirical tropical cyclone intensity data. [16]

Rainfall

Tropical cyclone rainfall forecasting is important, since between 1970–2004, inland flooding from tropical cyclones caused a majority of the fatalities from tropical cyclones in the United States. [17] [18] While flooding is common to tropical cyclones near a landmass, there are a few factors which lead to excessive rainfall from tropical cyclones. Slow motion, as was seen during Hurricane Danny and Hurricane Wilma, can lead to high amounts. The presence of topography near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan acts to magnify amounts due to upslope flow into the mountains. Strong upper level forcing from a trough moving through the Westerlies, as was the case during Hurricane Floyd, can lead to high amounts even from systems moving at an average forward motion. A combination of two of these factors could be especially crippling, as was seen during Hurricane Mitch in Central America. [19] Therefore, an accurate track forecast is essential in order to produce an accurate tropical cyclone rainfall forecast. [20] However, as a result of global warming, the heat that has built up on the ocean's surface has allowed storms and hurricanes to capture more water vapour and, given the increased temperatures in the atmosphere also, retain the moisture for a longer capacity. [21] This results in incredible amounts of rainfall upon striking land which can often be the most damaging aspect of a hurricane.

Operational methods

Historically, tropical cyclone tracking charts were used to include the past track and prepare future forecasts at Regional Specialized Meteorological Centers and Tropical Cyclone Warning Centers. The need for a more modernized method for forecasting tropical cyclones had become apparent to operational weather forecasters by the mid-1980s. At that time the United States Department of Defense was using paper maps, acetate, grease pencils, and disparate computer programs to forecast tropical cyclones. [22] The Automated Tropical Cyclone Forecasting System (ATCF) software was developed by the Naval Research Laboratory for the Joint Typhoon Warning Center (JTWC) beginning in 1986, [23] and used since 1988. During 1990 the system was adapted by the National Hurricane Center (NHC) for use at the NHC, National Centers for Environmental Prediction and the Central Pacific Hurricane Center. [23] [24] This provided the NHC with a multitasking software environment which allowed them to improve efficiency and cut the time required to make a forecast by 25% or 1 hour. [24] ATCF was originally developed for use within DOS, before later being adapted to Unix and Linux. [23]

Storm surge

The main storm surge forecast model in the Atlantic basin is SLOSH, which stands for Sea, Lake, Overland, Surge from Hurricanes. [25] It uses the size of a storm, its intensity, its forward motion, and the topography of the coastal plain to estimate the depth of a storm surge at any individual grid point across the United States. An accurate forecast track is required in order to produce accurate storm surge forecasts. However, if the landfall point is uncertain, a maximum envelope of water (MEOW) map can be generated based on the direction of approach. If the forecast track itself is also uncertain, a maximum of maximums (MoM) map can be generated which will show the worst possible scenario for a hurricane of a specific strength. [26]

The location of most tropical cyclone-related tornadoes is their northeast quadrant in the Northern Hemisphere and southeast quadrant in the Southern Hemisphere. [27] Like most of the other forecasts for tropical cyclone effects, an accurate track forecast is required in order to produce an accurate tornado threat forecast.

Seasonal forecast

By looking at annual variations in various climate parameters, forecasters can make predictions about the overall number and intensity of tropical cyclones that will occur in a given season. For example, when constructing its seasonal outlooks, the Climate Prediction Center in the United States considers the effects of the El Niño-Southern Oscillation, 25–40 year tropical cycle, wind shear over the oceans, and ocean surface temperature. [28]

Related Research Articles

The National Hurricane Center (NHC) is the division of the United States' NOAA/National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 30th parallel north in the northeast Pacific Ocean and the 31st parallel north in the northern Atlantic Ocean. The agency, which is co-located with the Miami branch of the National Weather Service, is situated on the campus of Florida International University in University Park, Florida.

A storm surge, storm flood, tidal surge or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems. Its severity is affected by the shallowness and orientation of the water body relative to storm path, as well as the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges. It is a measure of the rise of water beyond what would be expected by the normal movement related to tides.

The 1988 Atlantic hurricane season was a near average season that proved costly and deadly, with 15 tropical cyclones directly affecting land. The season officially began on June 1, 1988, and lasted until November 30, 1988, although activity began on May 30 when a tropical depression developed in the Caribbean Sea. The June through November dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The first cyclone to attain tropical storm status was Alberto on August 8, nearly a month later than usual. The final storm of the year, Tropical Storm Keith, became extratropical on November 24.

The Weather Prediction Center (WPC), located in College Park, Maryland, is one of nine service centers under the umbrella of the National Centers for Environmental Prediction (NCEP), a part of the National Weather Service (NWS), which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the U.S. government bf t. Until March 5, 2013 the Weather Prediction Center was known as the Hydrometeorological Prediction Center (HPC). The Weather Prediction Center serves as a center for quantitative precipitation forecasting, medium range forecasting, and the interpretation of numerical weather prediction computer models.

The 1957 Atlantic hurricane season featured the one of longest travelling tropical cyclones in the Atlantic basin, Hurricane Carrie. Nevertheless, the season was generally inactive with eight tropical storms – two of which went unnamed – and three hurricanes, two of which intensified further to attain major hurricane intensity. The season officially began on June 15 and ended on November 15, though the year's first tropical cyclone developed prior to the start of the season on June 8. The final storm dissipated on October 27, well before the official end of the season. The strongest hurricane of the year was Carrie, which reached the equivalent of a Category 4 hurricane on the Saffir–Simpson hurricane scale on two separate occasions in the open Atlantic; Carrie later caused the sinking of the German ship Pamir southwest of the Azores, resulting in 80 deaths.

The 2006 Atlantic hurricane season was the least active since 1997 as well as the first season since 2001 in which no hurricanes made landfall in the United States, and was the first since 1994 in which no tropical cyclones formed during October. Following the intense activity of 2005, forecasters predicted that the 2006 season would be only slightly less active. Instead activity was slowed by a rapidly forming moderate El Niño event, the presence of the Saharan Air Layer over the tropical Atlantic, and the steady presence of a robust secondary high-pressure area to the Azores high centered on Bermuda. There were no tropical cyclones after October 2.

Hurricane Linda was the second-strongest eastern Pacific hurricane on record. Forming from a tropical wave on September 9, 1997, Linda steadily intensified and reached hurricane status within 36 hours of developing. The storm rapidly intensified, reaching sustained winds of 185 mph (295 km/h) and an estimated central pressure of 902 millibars (26.6 inHg); both were records for the eastern Pacific until Hurricane Patricia surpassed them in 2015. The hurricane was briefly forecast to move toward southern California, but instead, it turned out to sea and lost its status as a tropical cyclone on September 17, before dissipating on September 21. Linda was the fifteenth tropical cyclone, thirteenth named storm, seventh hurricane, and fifth major hurricane of the 1997 Pacific hurricane season.

The Atlantic hurricane season is the period in a year when hurricanes usually form in the Atlantic Ocean. Tropical cyclones in the North Atlantic are called hurricanes, tropical storms, or tropical depressions. In addition, there have been several storms over the years that have not been fully tropical and are categorized as subtropical depressions and subtropical storms. Even though subtropical storms and subtropical depressions are not technically as strong as tropical cyclones, the damages can still be devastating.

A Pacific hurricane is a mature tropical cyclone that develops within the northeastern and central Pacific Ocean to the east of 180°W, north of the equator. For tropical cyclone warning purposes, the northern Pacific is divided into three regions: the eastern, central, and western, while the southern Pacific is divided into 2 sections, the Australian region and the southern Pacific basin between 160°E and 120°W. Identical phenomena in the western north Pacific are called typhoons. This separation between the two basins has a practical convenience, however, as tropical cyclones rarely form in the central north Pacific due to high vertical wind shear, and few cross the dateline.

A tropical cyclone forecast model is a computer program that uses meteorological data to forecast aspects of the future state of tropical cyclones. There are three types of models: statistical, dynamical, or combined statistical-dynamic. Dynamical models utilize powerful supercomputers with sophisticated mathematical modeling software and meteorological data to calculate future weather conditions. Statistical models forecast the evolution of a tropical cyclone in a simpler manner, by extrapolating from historical datasets, and thus can be run quickly on platforms such as personal computers. Statistical-dynamical models use aspects of both types of forecasting. Four primary types of forecasts exist for tropical cyclones: track, intensity, storm surge, and rainfall. Dynamical models were not developed until the 1970s and the 1980s, with earlier efforts focused on the storm surge problem.

James Louis Franklin is a former weather forecaster encompassing a 35-year career with National Oceanic and Atmospheric Administration (NOAA). He served as the first branch chief of the newly formed Hurricane Specialist Unit (HSU) before his retirement in 2017.

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Tropical cyclone track forecasting involves predicting where a tropical cyclone is going to track over the next five days, every 6 to 12 hours. The history of tropical cyclone track forecasting has evolved from a single-station approach to a comprehensive approach which uses a variety of meteorological tools and methods to make predictions. The weather of a particular location can show signs of the approaching tropical cyclone, such as increasing swell, increasing cloudiness, falling barometric pressure, increasing tides, squalls, and heavy rainfall.

The Hurricane Databases (HURDAT), managed by the National Hurricane Center, are two separate databases that contain details on tropical cyclones, that have occurred within the Atlantic Ocean and Eastern Pacific Ocean since either 1851 or 1949.

The following is a glossary of tropical cyclone terms.

The history of Atlantic tropical cyclone warnings details the progress of tropical cyclone warnings in the north Atlantic Ocean. The first service was set up in the 1870s from Cuba with the work of Father Benito Viñes. After his death, hurricane warning services were assumed by the US Army Signal Corps and United States Weather Bureau over the next few decades, first based in Jamaica and Cuba before shifting to Washington, D.C.. The central office in Washington, which would evolve into the National Meteorological Center and the Weather Prediction Center, assumed the responsibilities by the early 20th century. This responsibility passed to regional hurricane offices in 1935, and the concept of the Atlantic hurricane season was established in order to keep a vigilant lookout for tropical cyclones during certain times of the year. Hurricane advisories issued every six hours by the regional hurricane offices began at this time.

The Automated Tropical Cyclone Forecasting System (ATCF) is a piece of software originally developed to run on a personal computer for the Joint Typhoon Warning Center (JTWC) in 1988, and the National Hurricane Center (NHC) in 1990. ATCF remains the main piece of forecasting software used for the United States Government, including the JTWC, NHC, and Central Pacific Hurricane Center. Other tropical cyclone centers in Australia and Canada developed similar software in the 1990s. The data files with ATCF lie within three decks, known as the a-, b-, and f-decks. The a-decks include forecast information, the b-decks contain a history of center fixes at synoptic hours, and the f-decks include the various fixes made by various analysis center at various times. In the years since its introduction, it has been adapted to Unix and Linux platforms.

A tropical cyclone tracking chart is used by those within hurricane-threatened areas to track tropical cyclones worldwide. In the north Atlantic basin, they are known as hurricane tracking charts. New tropical cyclone information is available at least every six hours in the Northern Hemisphere and at least every twelve hours in the Southern Hemisphere. Charts include maps of the areas where tropical cyclones form and track within the various basins, include name lists for the year, basin-specific tropical cyclone definitions, rules of thumb for hurricane preparedness, emergency contact information, and numbers for figuring out where tropical cyclone shelters are open.

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