Marine weather forecasting

Marine weather forecasting is the process by which mariners and meteorological organizations attempt to forecast future weather conditions over the Earth's oceans. Mariners have had rules of thumb regarding navigation around tropical cyclones for many years, dividing a storm into halves and sailing through the normally weaker and more navigable half of their circulation. Marine weather forecasts by various weather organizations can be traced back to the sinking of the Royal Charter in 1859 and the RMS Titanic in 1912.

The wind is the driving force of weather at sea, as wind generates local wind waves, long ocean swells, and its flow around the subtropical ridge helps maintain warm water currents such as the Gulf Stream. The importance of weather over the ocean during World War II led to delayed or secret weather reports, in order to maintain a competitive advantage. Weather ships were established by various nations during World War II for forecasting purposes, and were maintained through 1985 to help with transoceanic plane navigation.

Voluntary observations from ships, weather buoys, weather satellites, and numerical weather prediction have been used to diagnose and help forecast weather over the Earth's ocean areas. Since the 1960s, numerical weather prediction's role over the Earth's seas has taken a greater role in the forecast process. Weather elements such as sea state, surface winds, tide levels, and sea surface temperature are tackled by organizations tasked with forecasting weather over open oceans and seas. Currently, the Japan Meteorological Agency, the United States National Weather Service, and the United Kingdom Met Office create marine weather forecasts for the Northern Hemisphere.

History

There are various origins for government-issued marine weather forecasts, generally following maritime disasters.

United Kingdom In October 1859, the steam clipper Royal Charter was wrecked in a strong storm off Anglesey. 450 people lost their lives. Due to this loss, Vice-Admiral Robert FitzRoy introduced a warning service for shipping in February 1861, using telegraph communications. This remained the United Kingdom Met Office's primary responsibility for some time afterwards. In 1911, the Met Office had begun issuing marine weather forecasts which included gale and storm warnings via radio transmission for areas around the British Isles. This service was discontinued during and following World War I, between 1914 and June 1921, and again during World War II between 1939 and 1945.

United States

The first attempt as a marine weather program within the United States was initiated in New Orleans, Louisiana by the United States Army Signal Corps. A January 23, 1873 memo directed the New Orleans Signal Observer to transcribe meteorological data from the ship logs of those arriving in port. Marine forecasting responsibility transferred from the United States Navy to the Weather Bureau in 1904, which enabled the receipt of timely observations from ships at sea. The sinking of RMS Titanic in 1912 played a pivotal role in marine weather forecasting globally. In response to that tragedy, an international commission was formed to determine requirements for safer ocean voyages. In 1914, the commission's work resulted in the International Convention for the Safety of Life at Sea. In 1957, in order to help address marine issues, the United States Weather Bureau started to publish the Mariners Weather Log bi-monthly publication to report past weather conditions primarily over Northern Hemisphere oceans, information regarding the globe's tropical cyclone seasons, to publish monthly climatologies for use of those at sea, and to encourage voluntary ship observations from vessels at sea.

Within the United States National Weather Service (NWS), forecast weather maps began to be published by offices in New York City, San Francisco, and Honolulu for public use. North Atlantic forecasts were shifted from a closed United States Navy endeavor to a National Weather Service product suite via radiofacsimile in 1971, while northeast Pacific forecasts became publicly available by the same method in 1972. Between 1986 and 1989, the portion of the National Meteorological Center (NMC) known as the Ocean Products Center (OPC) was responsible for marine weather forecasting within the NWS. Between August 1989 and 1995, the unit named the Marine Forecast Branch also was involved in providing objective analysis and forecast products for marine and oceanographic variables. The Marine Prediction Center, later renamed the Ocean Prediction Center, assumed the U.S. obligation to issue warnings and forecasts for portions of the North Atlantic and North Pacific oceans once it was created in 1995.

Importance of the wind

Development of warm ocean currents The trade winds blow westward in the tropics, and the westerlies blow eastward at mid-latitudes.Glossary of Meteorology (2009). Westerlies. American Meteorological Society. Retrieved on 2009-04-15. This wind pattern applies a stress to the subtropical ocean surface with negative curl across the north Atlantic Ocean.Matthias Tomczak and J. Stuart Godfrey (2001). Regional Oceanography: an Introduction. Matthias Tomczak, pp. 42. . Retrieved on 2009-05-06. The resulting Sverdrup transport is equatorward.Earthguide (2007). Lesson 6: Unraveling the Gulf Stream Puzzle – On a Warm Current Running North. University of California at San Diego. Retrieved on 2009-05-06. Because of conservation of potential vorticity caused by the poleward-moving winds on the subtropical ridge's western periphery and the increased relative vorticity of northward moving water, transport is balanced by a narrow, accelerating poleward current, which flows along the western boundary of the ocean basin, outweighing the effects of friction with the western boundary current known as the Labrador current. The conservation of potential vorticity also causes bends along the Gulf Stream, which occasionally break off due to a shift in the Gulf Stream's position, forming separate warm and cold eddies.Maurice L. Schwartz (2005). Encyclopedia of Coastal Science. Springer, p. 1037. . Retrieved on 2009-05-07. This overall process, known as western intensification, causes currents on the western boundary of an ocean basin, such as the Gulf Stream, to be stronger than those on the eastern boundary.National Environmental Satellite, Data, and Information Service (2009). Investigating the Gulf Stream . North Carolina State University. Retrieved on 2009-05-06.

Swell dispersion and wave groups

Swells are often created by storms long distances away from the beach where they break, and the propagation of the longest swells is only limited by shorelines. For example, swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip.Directional recording of swell from distant storms, W. H. Munk, G. R. Miller, F. E. Snodgrass, and N. F. Barber, 1963: Phil. Trans. Roy. Soc. London A 255, 505 This distance allows the waves comprising the swells to be better sorted and free of chop as they travel toward the coast. Waves generated by storm winds have the same speed and will group together and travel with each other, while others moving at even a fraction of a metre per second slower will lag behind, ultimately arriving many hours later due to the distance covered. The time of propagation from the source t is proportional to the distance X divided by the wave period T. In deep water it is where g is the acceleration of gravity. As an example, for a storm located away, swells with a period T=15 s will arrive 10 days after the storm, followed by 14 s swells another 17 hours later.

This dispersive arrivals of swells, long periods first with a reduction in the peak wave period over time, can be used to tell the distance at which swells were generated. Whereas the sea state in the storm has a frequency spectrum with more or less always the same shape (i.e. a well defined peak with dominant frequencies within plus or minus 7% of the peak), the swell spectra are more and more narrow, sometimes as 2% or less, as waves disperse further and further away. The result is that wave groups (called sets by surfers) can have a large number of waves. From about seven waves per group in the storm, this rises to 20 and more in swells from very distant storms.

Sailing ship journeys Ocean journeys by sailing ship can take many months, and a common hazard is becoming becalmed because of lack of wind, or being blown off course by severe storms or winds that do not allow progress in the desired direction. A severe storm could lead to shipwreck, and the loss of all hands. Sailing ships can only carry a certain quantity of supplies in their hold, so they have to plan long voyages carefully to include appropriate provisions, including fresh water.

Tropical cyclone avoidance

Mariners have a way to safely navigate around tropical cyclones. They split tropical cyclones in two, based on their direction of motion, and maneuver to avoid the right segment of the cyclone in the Northern Hemisphere (the left in the Southern Hemisphere). Sailors term the right side the dangerous semicircle since the heaviest rain and strongest winds and seas were located in this half of the storm, as the cyclone's translation speed and its rotational wind are additive. The other half of the tropical cyclone is called the navigable semicircle since weather conditions are lessened (subtractive) in this portion of the storm. The rules of thumb for ship travel when a tropical cyclone is in their vicinity are to avoid them if at all possible and do not cross their forecast path (crossing the T). Those traveling through the dangerous semicircle are advised to keep to the true wind on the starboard bow and make as much headway as possible. Ships moving through the navigable semicircle are advised to keep the true wind on the starboard quarter while making as much headway as possible.The Pennsylvania State University. Lesson 21: Weather. Retrieved on 2007-05-26.

The 1-2-3 rule (mariners' 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 National Hurricane Center forecast errors of 100-200-300 nautical miles at 24-48-72 hours, respectively. However, these errors have decreased to near 50-100-150 as NHC forecasters become more accurate with tropical cyclone track forecasting. 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).Central Pacific Hurricane Center. Hurricane Awareness Week 2005. Retrieved on 2007-12-24.

Within numerical weather prediction

Ocean surface modeling

The transfer of energy between the wind blowing over the surface of an ocean and the ocean's upper layer is an important element in wave dynamics. The spectral wave transport equation is used to describe the change in wave spectrum over changing topography. It simulates wave generation, wave movement (propagation within a fluid), wave shoaling, refraction, energy transfer between waves, and wave dissipation. Since surface winds are the primary forcing mechanism in the spectral wave transport equation, ocean wave models use information produced by numerical weather prediction models as inputs to determine how much energy is transferred from the atmosphere into the layer at the surface of the ocean. Along with dissipation of energy through whitecaps and resonance between waves, surface winds from numerical weather models allow for more accurate predictions of the state of the sea surface.