TAB 8 Marine Meteorology - Chapter 38 - Weather Routing.pdf - N. Bowditch - was72

CHAPTER 38

WEATHER ROUTING

PRINCIPLES OF WEATHER ROUTING

3800. Introduction

route, or if the forecasts permit, diverting to a shorter track

to take advantage of favorable weather and sea conditions.

The greatest potential advantage for this ship weather rout-

ing exists when: (1) the passage is relatively long, about

1,500 miles or more; (2) the waters are navigationally unre-

stricted, so that there is a choice of routes; and (3) weather

is a factor in determining the route to be followed.

Use of this advisory service in no way relieves the

commanding officer or master of responsibility for prudent

seamanship and safe navigation. There is no intent by the

routing agency to inhibit the exercise of professional judg-

ment and prerogatives of commanding officers and

masters.

Ship weather routing develops an optimum track for

ocean voyages based on forecasts of weather, sea condi-

tions, and a ship’s individual characteristics for a particular

transit. Within specified limits of weather and sea condi-

tions, the term optimum is used to mean maximum safety

and crew comfort, minimum fuel consumption, minimum

time underway, or any desired combination of these factors.

The purpose of this chapter is to acquaint the mariner with

the basic philosophy and procedures of ship weather rout-

ing as an aid to understanding the routing agency’s

recommendations.

The mariner’s first resources for route planning in rela-

tion to weather are the Pilot Chart Atlases and the Sailing

Directions (Planning Guides). These publications give cli-

matic data, such as wave height frequencies and ice limits,

for the major ocean basins of the world. They recommend

specific routes based on probabilities, but not on specific

conditions.

The ship routing agency, acting as an advisory service,

attempts to avoid or reduce the effects of specific adverse

weather and sea conditions by issuing initial route recom-

mendations prior to sailing, recommendations for track

changes while underway (diversions), and weather adviso-

ries to alert the commanding officer or master about

approaching unfavorable weather and sea conditions which

cannot be effectively avoided by a diversion. Adverse

weather and sea conditions are defined as those conditions

which will cause damage, significant speed reduction, or

time loss.

The initial route recommendation is based on a survey

of weather and sea forecasts between the point of departure

and the destination. It takes into account the hull type, speed

capability, cargo, and loading conditions. The ship’s

progress is continually monitored, and, if adverse weather

and sea conditions are forecast along the ship’s current

track, a recommendation for a diversion or weather adviso-

ry is transmitted to the ship. By this process of initial route

selection and continued monitoring of the ship’s progress

for possible changes in the forecast weather and sea condi-

tions along a route, it is possible to maximize the ship’s

speed and safety.

In providing optimum sailing conditions, the advisory

service also attempts to reduce transit time by avoiding the

adverse conditions which may be encountered on a shorter

3801. Historical Perspective

The advent of extended range forecasting and the develop-

ment of selective climatology, along with powerful computer

modeling techniques, have made ship routing systems possible.

The ability to effectively advise ships to take advantage of favor-

able weather was hampered previously by forecast limitations

and the lack of an effective communications system.

Development work in the area of data accumulation and

climatology has a long history. Benjamin Franklin, as deputy

postmaster general of the British Colonies in North America,

produced a chart of the Gulf Stream from information supplied

by masters of New England whaling ships. This first mapping of

the Gulf Stream helped improve the mail packet service be-

tween the British Colonies and England. In some passages the

sailing time was reduced by as much as 14 days over routes pre-

viously sailed. In the mid-19th century, Matthew Fontaine

Maury compiled large amounts of atmospheric and oceano-

graphic data from ships’ log books. For the first time, a

climatology of ocean weather and currents of the world was

available to the mariner. This information was used by Maury to

develop seasonally recommended routes for sailing ships and

early steam powered vessels in the latter half of the 19th century.

In many cases, Maury’s charts were proved correct by the sav-

ings in transit time. Average transit time on the New York to

California via Cape Horn route was reduced from 183 days to

139 days with the use of his recommended seasonal routes.

In the 1950’s the concept of ship weather routing was

put into operation by several private meteorological groups

and by the U.S. Navy. By applying the available surface and

upper air forecasts to transoceanic shipping, it was possible

to effectively avoid much heavy weather while generally

539

540

WEATHER ROUTING

sailing shorter routes than previously.

Optimum Track Ship Routing (OTSR), the ship rout-

ing service of the U.S. Navy, utilizes short range and

extended range forecasting techniques in route selection

and surveillance procedures. The short range dynamic fore-

casts of 3 to 5 days are derived from meteorological

equations. These forecasts are computed twice daily from a

data base of northern hemisphere surface and upper air ob-

servations, and include surface pressure, upper air constant

pressure heights, and the spectral wave values. A signifi-

cant increase in data input, particularly from satellite

information over ocean areas, can extend the time period

for which these forecasts are useful.

For extended range forecasting, generally 3 to 14 days,

a computer searches a library of historical northern hemi-

sphere surface pressure and 500 millibar analyses for an

analogous weather pattern. This is an attempt at selective

climatology by matching the current weather pattern with

past weather patterns and providing a logical sequence-of-

events forecast for the 10 to 14 day period following the dy-

namic forecast. It is performed for both the Atlantic and

Pacific Oceans using climatological data for the entire peri-

od of data stored in the computer. For longer ocean transits,

monthly values of wind, seas, fog, and ocean currents are

used to further extend the time range.

Aviation was first in applying the principle of mini-

mum time tracks (MTT) to a changing wind field. But the

problem of finding an MTT for a specific flight is much

simpler than for a transoceanic ship passage because an air-

craft’s transit time is much shorter than a ship’s. Thus,

marine minimum time tracks require significantly longer

range forecasts to develop an optimum route.

Automation has enabled ship routing agencies to de-

velop realistic minimum time tracks. Computation of

minimum time tracks makes use of:

Ship weather routing services are being offered by

many nations. These include Japan, United Kingdom, Rus-

sia, Netherlands, Germany, and the United States. Also,

several private firms provide routing services to shipping

industry clients.

There are two general types of commercial ship routing

services. The first uses techniques similar to the Navy’s

OTSR system to forecast conditions and compute routing

recommendations. The second assembles and processes

weather and sea condition data and transmits this to ships at

sea for on-board processing and generation of route recom-

mendations. The former system allows for greater

computer power to be applied to the routing task because

powerful computers are available ashore. The latter system

allows greater flexibility to the ship’s master in changing

parameters, selecting routes, and displaying data.

3802. Ship And Cargo Considerations

Ship and cargo characteristics have a significant influ-

ence on the application of ship weather routing. Ship size,

speed capability, and type of cargo are important consider-

ations in the route selection process prior to sailing and the

surveillance procedure while underway. A ship’s character-

istics identify its vulnerability to adverse conditions and its

ability to avoid them.

Generally, ships with higher speed capability and less

cargo encumbrances will have shorter routes and be better

able to maintain near normal SOA’s than ships with lower

speed capability or cargoes. Some routes are unique be-

cause of the type of ship or cargo. Avoiding one element of

weather to reduce pounding or rolling may be of prime im-

portance. For example, a 20 knot ship with a heavy deck

cargo may be severely hampered in its ability to maintain a

20 knot SOA in any seas exceeding moderate head or beam

seas because of the possibility of damage resulting from the

deck load’s characteristics. A similar ship with a stable car-

go under the deck is not as vulnerable and may be able to

maintain the 20 knot SOA in conditions which would dras-

tically slow the deck-loaded vessel. In towing operations, a

tug is more vulnerable to adverse weather and sea condi-

tions, not only in consideration of the tow, but also because

of its already limited speed capability. Its slow speed adds

to the difficulty of avoiding adverse weather and sea

conditions.

Ship performance curves (speed curves) are used to es-

timate the ship’s SOA while transiting the forecast sea

states. The curves indicate the effect of head, beam, and fol-

lowing seas of various significant wave heights on the

ship’s speed. Figure 3802 is a performance curve prepared

for an 18 knot vessel.

With the speed curves it is possible to determine just

how costly a diversion will be in terms of the required dis-

tance and time. A diversion may not be necessary where the

duration of the adverse conditions is limited. In this case, it

may be better to ride out the weather and seas knowing that

1. A navigation system to compute route distance,

time enroute, estimated times of arrival (ETA’s),

and to provide 6 hourly DR synoptic positions for

the range of the dynamic forecasts for the ship’s

current track.

2. A surveillance system to survey wind, seas, fog,

and ocean currents obtained from the dynamic and

climatological fields.

3. An environmental constraint system imposed as

part of the route selection and surveillance process.

Constraints are the upper limits of wind and seas

desired for the transit. They are determined by the

ship’s loading, speed capability, and vulnerability.

The constraint system is an important part of the

route selection process and acts as a warning sys-

tem when the weather and sea forecast along the

present track exceeds predetermined limits.

4. Ship speed characteristics used to approximate

ship’s speed of advance (SOA) while transiting the

forecast sea states.

WEATHER ROUTING

541

a diversion, even if able to maintain the normal SOA, will

not overcome the increased distance and time required.

At other times, the diversion track is less costly be-

cause it avoids an area of adverse weather and sea

conditions, while being able to maintain normal SOA even

though the distance to destination is increased. Based on in-

put data for environmental conditions and ship’s behavior,

route selection and surveillance techniques seek to achieve

the optimum balance between time, distance, and accept-

able environmental and seakeeping conditions. Although

speed performance curves are an aid to the ship routing

agency, the response by mariners to deteriorating weather

and sea conditions is not uniform. Some reduce speed vol-

untarily or change heading sooner than others when

unfavorable conditions are encountered. Certain waves

with characteristics such that the ship’s bow and stern are in

successive crests and troughs present special problems for

the mariner. Being nearly equal to the ship’s length, such

wavelengths may induce very dangerous stresses. The de-

gree of hogging and sagging and the associated danger may

be more apparent to the mariner than to the ship routing

agency. Therefore, adjustment in course and speed for a

more favorable ride may be initiated by the commanding

officer or master when this situation is encountered.

important for route selection and surveillance, optimum

routing is normally considered attained if the effects of

wind and seas can be optimized.

Wind: The effect of wind speed on ship performance

is difficult to determine. In light winds (less than 20-knots),

ships lose speed in headwinds and gain speed slightly in fol-

lowing winds. For higher wind speeds, ship speed is

reduced in both head and following winds. This is due to the

increased wave action, which even in following seas results

in increased drag from steering corrections, and indicates

the importance of sea conditions in determining ship per-

formance. In dealing with wind, it is also necessary to know

the ship’s sail area. High winds will have a greater adverse

effect on a large, fully loaded container ship or car carrier

than a fully loaded tanker of similar length. This effect is

most noticeable when docking, but the effect of beam

winds over several days at sea can also be considerable.

Wave Height: Wave height is the major factor affect-

ing ship performance. Wave action is responsible for ship

motions which reduce propeller thrust and cause increased

drag from steering corrections. The relationship of ship

speed to wave direction and height is similar to that of wind.

Head seas reduce ship speed, while following seas increase

ship speed slightly to a certain point, beyond which they re-

tard it. In heavy seas, exact performance may be difficult to

predict because of the adjustments to course and speed for

shiphandling and comfort. Although the effect of sea and

swell is much greater than wind, it is difficult to separate the

two in ship routing.

In an effort to provide a more detailed description of

the actual and forecast sea state, the U.S. Navy Fleet Nu-

merical Meteorology and Oceanography Center,

3803. Environmental Factors

Environmental factors of importance to ship weather

routing are those elements of the atmosphere and ocean that

may produce a change in the status of a ship transit. In ship

routing, consideration is given to wind, seas, fog, ice, and

ocean currents. While all of the environmental factors are

Figure 3802. Performance curves for head, beam, and following seas.

542

WEATHER ROUTING

sectors) and 15 frequency bands for wave periods from 6

to 26 seconds with the total wave energy propagated

throughout the grid system as a function of direction and

frequency. It is based on the analyzed and forecast plane-

tary boundary layer model wind fields, and is produced

for both the Northern and Southern Hemispheres out to 72

hours. For OTSR purposes, primary and secondary waves

are derived from the spectral wave program, where the

primary wave train has the principal energy (direction and

frequency), and the secondary has to be 20 percent of the

primary.

Fog: Fog, while not directly affecting ship perfor-

mance, should be avoided as much as feasible, in order to

maintain normal speed in safe conditions. Extensive areas

of fog during summertime can be avoided by selecting a

lower latitude route than one based solely upon wind and

seas. Although the route may be longer, transit time may

be less due to not having to reduce speed in reduced visi-

bility. In addition, crew fatigue due to increased

watchkeeping vigilance can be reduced.

North Wall Effect: During the Northern Hemisphere

fall and winter, the waters to the north of the Gulf Stream

in the North Atlantic are at their coldest, while the Gulf

Stream itself remains at a constant relatively warm tem-

perature. After passage of a strong cold front or behind a

developing coastal low pressure system, Arctic air is

sometimes drawn off the Mid-Atlantic coast of the United

States and out over the warm waters of the Gulf Stream by

northerly winds. This cold air is warmed as it passes over

the Gulf Stream, resulting in rapid and intense deepening

of the low pressure system and higher than normal surface

winds. Higher waves and confused seas result from these

winds. When these winds oppose the northeast set of the

current, the result is increased wave heights and a shorten-

ing of the wave period. If the opposing current is

sufficiently strong, the waves will break. These phenome-

na are collectively called the “North Wall Effect,”

referring to the region of most dramatic temperature

change between the cold water to the north and the warm

Gulf Stream water to the south. The most dangerous as-

pect of this phenomenon is that the strong winds and

extremely high, steep waves occur in a limited area and

may develop without warning. Thus, a ship that is labor-

ing in near-gale force northerly winds and rough seas,

proceeding on a northerly course, can suddenly encounter

storm force winds and dangerously high breaking seas.

Numerous ships have foundered off the North American

coast in the approximate position of the Gulf Stream’s

North Wall. A similar phenomenon occurs in the North

Pacific near the Kuroshio Current and off the Southeast

African coast near the Agulhas Current.

Ocean Currents: Ocean currents do not present a

significant routing problem, but they can be a determining

factor in route selection and diversion. This is especially

true when the points of departure and destination are at

relatively low latitudes. The important considerations to

be evaluated are the difference in distance between a

great-circle route and a route selected for optimum cur-

rent, with the expected increase in SOA from the

following current, and the decreased probability of a di-

version for weather and seas at the lower latitude. For

example, it has proven beneficial to remain equatorward

of approximately 22

N for westbound passages between

the Canal Zone and southwest Pacific ports. For east-

bound passages, if the maximum latitude on a great-circle

track from the southwest Pacific to the Canal Zone is be-

low 24

N, a route passing near the axis of the Equatorial

Countercurrent is practical because the increased distance

is offset by favorable current. Direction and speed of

ocean currents are more predictable than wind and seas,

but some variability can be expected. Major ocean cur-

rents can be disrupted for several days by very intense

weather systems such as hurricanes and by global phe-

nomena such as El Nino.

Ice: The problem of ice is twofold: floating ice (ice-

bergs) and deck ice. If possible, areas of icebergs or pack ice

should be avoided because of the difficulty of detection and

the potential for collision. Deck ice may be more difficult to

contend with from a ship routing point of view because it is

caused by freezing weather associated with a large weather

system. While mostly a nuisance factor on large ships, it

causes significant problems with the stability of small ships.

Latitude: Generally, the higher the latitude of a route,

even in the summer, the greater are the problems with the

environment. Certain operations should benefit from sea-

sonal planning as well as optimum routing. For example,

towing operations north of about 40

latitude should be

avoided in non-summer months if possible.

3804. Synoptic Weather Considerations

A ship routing agency should direct its forecasting

skills to avoiding or limiting the effect of weather and seas

associated with extratropical low pressure systems in the

mid and higher latitudes and the tropical systems in low lat-

itude. Seasonal or monsoon weather is also a factor in route

selection and diversion in certain areas.

Despite the amount of attention and publicity given to

tropical cyclones, mid-latitude low pressure systems gener-

ally present more difficult problems to a ship routing

agency. This is primarily due to the fact that major ship traf-

fic is sailing in the latitudes of the migrating low pressure

systems, and the amount of potential exposure to intense

weather systems, especially in winter, is much greater.

Low pressure systems weaker than gale intensity

(winds less than 34 knots) are not a severe problem for most

ships. However, a relatively weak system may generate pro-

longed periods of rough seas which may hamper normal

Monterey, California, produces the Global Spectral Ocean

Wave Model (GSOWM) for use by the U.S. Navy’s Opti-

mum Track Ship Routing (OTSR) service. This model

provides energy values from 12 different directions (30

Figure 3804a. Generalized 10% frequency isolines of gale force winds for October through January.

TAB 8 Marine Meteorology - Chapter 38 - Weather Routing.pdf

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