CHIEF OF NAVAL OPERATIONS
This is one of a series of pamphlets dealing with the weather aspects of Naval and Amphibious Warfare. The data on which these studies are based are taken from official documents and reports submitted to the Navy Department. The material has been collated and presented in a semi-technical form with particular attention given to the operational aspects of weather.
During the preparation of this study, it was found that weather data submitted by the various commands were occasionally at variance. An effort has been made to reconcile these differences in order to provide an accurate account of the sequence of weather conditions as forecast for the Force Commander, to describe the actual weather conditions observed during the operation, and to give a practical explanation of these weather conditions.
It is hoped that these studies will afford a clear view of the use of weather information during the planning, strategical, and tactical phases of the operation, and that they will form a basis for a better understanding of the application of weather information in future operations.
The primary objective of these analyses is to assist those officers who are charged with the responsibility for the planning and execution of similar operations.
Vice Admiral, U.S.N.,
Deputy Chief of Naval Operations, (Air).
1. WOODLARK AND KIRIWINA ISLANDS--30 JUNE 1943.
2. LAE, NEW GUINEA--4 SEPTEMBER 1943.
3. FINSCHHAFEN, NEW GUINEA--22 SEPTEMBER 1943.
4. ARAWE, NEW BRITAIN--15 DECEMBER 1943.
5. CAPE GLOUCESTER, NEW BRITAIN--26 DECEMBER 1943.
6. SAIDOR, NEW GUINEA--2 JANUARY 1944.
Operations of the Seventh Amphibious Force
A Summary of the Significance of Weather in These Operations
- Rule-of-Thumb Weather Adages Common to the Area Were Often Incorrect and Misleading.
- Modern Methods of Weather Analysis Were Able to Indicate Favorable Periods for Landing Operations, Even During a Season When the Weather Was Generally Unfavorable.
- Weather Reports From Observers Aboard P.T. Boats Operating in an Advanced Area Were of Great Value.
- Expected Weather Conditions Were Employed to Great Advantage During the Strategic and Tactical Phases of These Operations.
Weather in the New Guinea--Solomon Islands area can be attributed to the operation of two factors. The first, which is the more important since it is in control for the greater fraction of time, is the wind circulation around the South Pacific high pressure area. As a first approximation, modification in the weather in this region is due to modification in the pressure system. Since the system shows only a seasonal variation, weather experienced on these islands remains reasonably constant for a considerable period of time. The second factor, however, results in the occasional interruption of this seasonal trend. Small-scale pressure systems in the form of fronts or wave cyclones occasionally move across the region and during their passage produce atypical weather at the islands.
A weather forecast for this region requires a consideration of both factors. For the most part, climatic averages give an accurate representation of the actual weather to be encountered, but the possibility of a small-scale pressure disturbance entering the region and altering the weather must always be considered before a final forecast is issued.
Tropical South Pacific Weather
Type I--Trade Weather
When consideration is given the different varieties of weather experienced on the islands, it becomes apparent that these can be divided into four major types. In order to provide a background for the discussion of weather and the operations, these principal types will be discussed here. Trade winds flowing around the semipermanent high pressure area produce a weather pattern so definite that it must be labelled "Trade Weather." Trade air originates in the polar regions but it is much modified in its lower layers by a long trajectory over water. Clouds are of the typical "trade wind cumulus type" covering large fractions of the sky. Widespread precipitation is rare in this air mass; scattered showers of short duration are more usual. Flying conditions are uniformly good.
The strength of the trades is quite variable depending upon the intensity of the high pressure area around which they flow. During periods of abnormally intense development, they may blow for a few days with a velocity of 15--20 knots. Then the sea becomes quite rough and swells are generated which often exceed the safe operating limit for small craft.
On the other hand, the trades may become quite weak, 5-8 knots, when the high pressure area is less intense. In the low latitudes around the Solomon Islands--New Guinea area this condition may persist sufficiently long to produce light seas ideal for small craft operations.
The trade winds are characterized in these regions by their east-northeast to south-southeast direction, the east-southeast and southeast winds showing predominance.
The trade circulation is strongest during the Southern Hemisphere winter. As a result of this circulation, the equatorial front (zone of convergence between the trades of the two hemispheres) with its accompanying bad weather remains well to the north from May through October. Throughout most of the island area, weather is best during these months. Along the coast of New Guinea, however, marked uplift by the coastal range produces high cloud cover and frequent rains.
Type II--Cold Front
From time to time, the flow of the trades is interrupted by a cold front. Daring the Southern Hemisphere winter, a series of cold fronts passes across Australia. Although all of these fronts do not maintain their identity after leaving the continent, an occasional strong cold front will move into the tropical latitudes. Some of these fronts are quite pronounced and are accompanied by widespread thunderstorm activity, heavy showers, and generally poor weather. For the few days that they persist before the trades are re-established, flying conditions are below average or bad over a considerable area.
Most of these dissipate gradually in the tropics but, on occasion, a portion of the stagnant front will move back to the south as a warm front, giving widespread areas of bad weather. This condition may persist for a considerable period of time as different portions of the front are involved. Until the front finally dies, the frontal area is marked by a widespread cloud cover regardless of the direction of motion.
Type III--Equatorial Front Over The Area
During the Southern Hemisphere summer, the high pressure over Australia weakens and is gradually replaced by a low pressure area. The trades then become intermittent or vanish and the effect of circulation from the Northern Hemisphere is felt in this region. The equatorial front moves south eventually reaching the Solomon Islands--New Guinea area.
The frontal zone may vary in width between 25 and 150 miles and is generally characterized by cloudy or overcast skies, heavy precipitation, and lowered visibility. Weather is usually unsatisfactory for military operations employing aircraft or small boats.
Type IV--Equatorial Front South of the Area
When the equatorial front moves beyond the islands, the region is covered with an abnormally warm and moist equatorial air mass. Over ocean areas, good weather conditions are usually experienced but the inherent instability is frequently manifest over land when heating at the ground or orographic lifting produces widespread cloud cover, showers, and thunderstorms.
Although air and landing operations are little affected by this weather type, the consistently high relative humidity and consequent slow rate of evaporation hamper ground activity. With frequent rain and showers, traffic is bogged down. Consolidation and construction is severely handicapped by trafficability conditions. Despite this handicap, weather is more favorable for military operations with the front south of the islands than over them.
Sea and Swell
The state of the sea is unquestionably a vital element In the execution of a landing operation. A disturbed ocean surface is the result of the combination of sea and swell. The former refers to the disturbances resulting from the action of local winds, while swell is the result of a periodic motion of the water which, generated by wind action at one locality, is felt at some distance from the point of origin. At any beach, the complexity due to the combination of sea and swell is further increased by the effect of ocean-floor contour and beach topography.
Even in areas where sea and swell observations are numerous, the problem of forecasting surf height is difficult. In a region like the South Pacific where data are few or entirely absent, the difficulty is tremendously magnified.
Strategic and Tactical Use of Weather
The solution of such problems as the selection of landing beaches, the maintenance of air coverage, and the planning of the operational time-table is determined by a study of average weather conditions. Compilation of critical data for the area of projected operations is the first phase of strategic weather planning.
Tactical considerations which are concerned with the occurrence or absence of particular weather conditions on the scheduled D-Day cannot be related to average weather. The process of averaging, by its very nature, removes or smooths out all extremes. To obtain information concerning weather on a particular day, the current weather situation as portrayed on a synoptic
map must be projected forward 24-36 hours. The degree of reliance to be placed upon forecasts of this type is determined largely by the number of weather reports available, the skill of the forecaster, and the period of time covered by the forecast.
Auxiliary Weather Aids
For the occupation of Woodlark and Kiriwina, the weather reporting network was sufficient to permit accurate analysis and forecasting. During the planning stage of the assault against Lae, it was realized that regular reports from the area outside our own network were necessary. Accordingly, four aerographer's mates were transferred to an advanced P.T. base where, as members of combat crews, they could transmit weather reports from enemy territory upon completion of each patrol. During the period of this duty assignment, each aerographer's mate made over 50 combat patrols. The weather information from this source proved invaluable in the operations at Lae, Finschhafen, Arawe, Cape Gloucester, and Saidor.
THE OCCUPATION OF WOODLARK AND KIRIWINA
30 June 1943
Without giving consideration to the military urgency requiring the occupation of these islands, it is possible to determine the specific weather requirements for the operation, and the probability of fulfilling these requirements during the month of June. Finally, the actual weather encountered can be evaluated with respect to its effect on the outcome.
The contemplated strategy demanded the following weather-dependent conditions:
1. Landing beaches selected in such a way as to obtain maximum expectancy of favorable sea and swell.
2. Protection for the repair ship (which also served as flagship) at its anchorage in Milne Bay.
3. Air coverage for the landing parties. The solution to these problems could be found in a consideration of average weather conditions in June.
WEATHER AT WOODLARK AND KIRIWINA REFLECTS THE TRADE WIND REGIME OF THE SURROUNDING OCEAN AREA. CLOUD COVER AVERAGES FOUR-TENTHS TO SIX-TENTHS. PRECIPITATION IN THE FORM OF HEAVY SHOWERS TOTALS 10--14 INCHES DURING THE MONTH. DIURNAL RANGE OF TEMPERATURE IS FROM 73°F TO 88°F.
AVERAGE WIND DIRECTION AND FORCE (KNOTS)
The month of June marks the "beginning of winter in the Southern Hemisphere with the result that the southeast trades become well established in most of the tropical area. In the southern part of the Coral Sea, the average wind velocity is about 15 knots, decreasing to 8 knots near the Solomons. Scattered to broken cumulus clouds at 2,000 feet, scattered showers, good visibility, and uniformly good flying conditions prevail about 90 percent of the time during this month. (See Chart page 5).
Between 15 June and 15 August when the trades are strongest, the uplift action of the mountains on Eastern New Guinea results in the formation of an extensive cloud cover. Otherwise, flying conditions throughout the area are good.
Fulfillment of Strategic Requirements
The landing beaches could be selected in accordance with average weather conditions. A beach located on the lee side of the island with respect to the average southeast winds would most probably meet sea and surf requirements.
The cloud cover produced by the lifting of the southeasterly current over the mountainous ridge on the southern coast of Eastern New Guinea could be expected to extend over Milne Bay. It was forecast that this cloud cover would provide the repair ship effective concealment from enemy observation.
Once the operation was scheduled, only two sets of weather conditions would be permitted to interfere with its execution. Arrangements were made to postpone the landings if air activity were entirely impossible. Also, if sea and surf conditions at the beaches made amphibious operations dangerous, D-Day could be delayed. Since the weather development which would create either of these undesirable conditions was the movement of a front off the Australian continent, it was expected that the aerological unit attached to the Seventh Amphibious Force could furnish adequate forecasts to permit the necessary postponement. The local squalls, which are frequent in the area during all seasons, were known to be of short duration and, therefore, relatively inconsequential.
Weather and The Operation
All phases of the operation were completed under expected weather conditions. Despite the fact that aircraft scouting operations were only 60--70 percent effective because of cloud cover and that the advance party on Kiriwina was hampered by a local squall, the landings were accomplished on schedule. Supply and reinforcement were aided by favorable weather during the entire month following the operation.
WEATHER MAP FOR 0300, 30 JUNE 1943
AVERAGE WIND DIRECTION AND FORCE (KNOTS)
THE ASSAULT LANDINGS AGAINST LAE, NEW GUINEA--4 SEPTEMBER 1943
FINSCHHAFEN, NEW GUINEA--22 SEPTEMBER 1943.
Both the assault against Lae on 4 September and the landing on Finschhafen on 22 September were operations of necessity dictated by considerations which could not include expected weather as an element of strategy. As a result, weather requirements for this operation were limited to considerations related directly to the execution of the assault.
BECAUSE OF TOPOGRAPHY IN NEW GUINEA, WEATHER AT LAE AND FINSCHHAFEN IS SOMEWHAT COMPLICATED BY VARIOUS LOCAL EFFECTS. DURING THE NIGHT AND EARLY MORNING, SURFACE WINDS ARE FROM THE NORTHWEST. IN THE AFTERNOON, WINDS ARE FROM THE SOUTHWEST. THE SHARP RISE FROM THE NARROW COASTAL PLAIN TO THE MOUNTAINS BEHIND LAE AND FINSCHHAFEN PRODUCES A LIFTING OF THE SOUTHEAST TRADES. RESULTANT CLOUDINESS AND PRECIPITATION ARE HIGH DURING THE MONTH. FLYING CONDITIONS ARE AVERAGE TO GOOD. THE AVERAGE TEMPERATURE IS 80°F,WITH LITTLE VARIATION FROM MONTH TO MONTH OR FROM YEAR TO YEAR.
WEATHER MAP FOR 0900 4 SEPTEMBER 1943
Since both objectives were heavily defended by enemy forces, the use of weather in these operations, although limited, was important.
1. The most advantageous use of our air superiority was dependent upon the existence of satisfactory flying weather--at the base, en route to the target, and in the target area. To insure adequate air support, the Fifth Air Force was delegated authority to postpone D-Day in the event of an unfavorable weather forecast.
2. In spite of the fact that these were landings of necessity, surf conditions had to permit the efficient handling of landing craft if the operations were to succeed. For this reason, authority to postpone D-Day was given the Commander, Seventh Amphibious Force, in the event of a forecast of unfavorable landing conditions.
Although the Southern Hemisphere's spring season starts in September, the atmospheric circulation produced by the strong anticyclone which covers Australia during winter continues with little diminution. The southeast trades prevail throughout the Coral Sea. With the equatorial front remaining in the Northern Hemisphere, bad weather of appreciable duration in the New Guinea area is produced only by a cold front moving off the Australian continent. Climatic averages for New Guinea, reflecting the infrequency of such cold frontal passage, indicate that favorable weather is encountered at least 90 percent of the time during the month of September.
Forecasting for The Operation
Since the movement of cold fronts from the Australian continent to the New Guinea area is subject to a fairly refined analysis and forecast technique, weather conditions in the New Guinea area could be forecast for a 36--48 hour period in the future. Thus, if necessary, the order to postpone D-Day could be given in sufficient time to be effective.
During the planning stage, it was thought that landings on beaches exposed to the prevailing southeast wind would be extremely hazardous due to the swell created by the steady trades. Closer examination, however, disclosed the fact that landings were feasible at all times except during a period when the trades were abnormally strong. As this condition was found to exist less than 10 percent of the time during the month and then under circumstances which were recognizable in advance, it was forecast that the landings would not be handicapped by unfavorable conditions on the beaches. Weather reports from the Advanced Aerological Unit operating with the P.T. boats were expected to furnish ample warning of any unfavorable development.
Weather and The Operation
The preliminary and final phases of both operations were carried out as planned. Average conditions of scattered cumulus clouds and widely scattered showers did not handicap air operations. Surface operations were particularly favored by the calm seas and low swell. During the operations, there was no deliberate use of weather either by our own forces or by the enemy.
WEATHER MAP FOR 0900 22 SEPTEMBER 1943
ASSAULT LANDING AGAINST ARAWE, NEW BRITAIN
15 December 1943
The assault against Arawe, New Britain, on 15 December 1943 was another operation of necessity in which expected weather could not be an element of strategy. Weather requirements were, therefore, limited to conditions existing on D-Day.
Limiting weather conditions for the assault against Arawe were nearly identical with those at Lae and Finschhafen:
1. The use of air superiority was dependent upon the existence of favorable flying weather over the entire operational area. D-Day could be postponed if, in the opinion of the Fifth Air Force, weather conditions preventing their support of the assault did not simultaneously prevent enemy air activity.
2. Since, in the initial phase of this landing, rubber boats were to be used in going over a coral reef, a calm sea at the beaches was an absolute necessity. If the Commander of the Seventh Amphibious Force anticipated unfavorable surf conditions on D-Day, the landings were to be postponed.
3. Movement of small craft from Finschhafen across Vitiaz and Dampier Straits was also a problem of considerable magnitude. Since this water area is exposed to the full sweep of both the southeast trades and the northwest monsoon, any abnormal development of either would produce dangerous conditions for this phase of the operation. Postponement would be necessary with the occurrence of unfavorable sea conditions on D minus one day.
Average December Weather
Examination of the accompanying chart of average December weather, page 14, shows that the equatorial front during this month ordinarily lies north of Arawe--across New Ireland. It is also apparent that the landing beaches are completely exposed to the southeast trades. The effect of this undesirable location is, however, diminished to some extent by the general weakening of the trade winds at the beginning of the Southern Hemisphere summer. Specifically, the average strength of the southeast trades in the Coral Sea decreases from 15 knots in September to 7 knots in December.
During the first part of December 1943. the equatorial front had moved to a position far south of its average location for this month. Although it almost covered the southeast tip of New Guinea, the front was so diffuse that it was accompanied by very little bad weather. Moreover, this fortuitous development placed the south coast of New Britain to the leeward of the now prevailing northwest wind with the result that sea conditions at Arawe were most favorable throughout the month.
The extreme weakness of the equatorial front gave promise that the usually favorable flying weather would continue to prevail. Furthermore,
AVERAGE WIND DIRECTION AND FORCE (KNOTS)
cold front activity in the Southern Hemisphere was at a minimum, and the Northern Hemisphere winter had not advanced sufficiently to produce cold front intrusions into the area.
Weather and The Operation
Preliminary air operations were carried out as planned with little interference from the weather. Weather reports from the P.T. base were favorable and the final phase began. Landing craft en route to the beaches experienced fairly heavy seas for craft of their size, but the schedule was not broken. In the assault area, the absence of surf aided the landings and subsequent reinforcement operations.
SINCE THE EQUATORIAL FRONT LIES JUST NORTH OF NEW BRITAIN DURING DECEMBER ARAWE IS UNDER THE INTERMITTENT INFLUENCE OF THIS FRONT THROUGHOUT THE MONTH. THE CIRCULATION IS DECIDEDLY WEAK, VARYING FROM SOUTHEAST TO NORTHWEST DEPENDENT ON THE EXACT POSITION OF THE FRONT. HIGH CLOUDINESS, FREQUENT RAIN SHOWERS AND THUNDERSTORMS ARE CHARACTERISTICS OF THIS FRONT DURING PERIODS WHEN IT IS ACTIVE. OFTEN THE FRONT IS INACTIVE, AND ITS LIMITS ARE INDEFINABLE.
WEATHER MAP FOR 0900 15 DECEMBER 1943
ASSAULT LANDING AT CAPE GLOUCESTER, NEW BRITAIN
26 December 1943
The early movement of the equatorial front to the south, as pointed out in the "Assault on Arawe," had advanced the date of change from the southeast to the northwest season from January to December. The average conditions for January as shown on page 18 are, therefore, representative of conditions which prevailed in the area during the latter part of December, 1943.
This early shift to the season of northwest winds increased the difficulty of selecting landing beaches which would enjoy maximum protection from wind, sea, and swell, all coming from a northwesterly direction. According to time-honored local accounts, the great strength of the northwest monsoon would make any landing during this season unfeasible. Logical analysis based upon statistics indicated that abnormally high northwest winds were rare and that the northwest swell generated by the monsoon would not be excessive. This deduction was supported by wind and sea observations transmitted by the advanced unit.
Weather conditions which could postpone D-Day were the same as at Arawe. The Fifth Air Force could postpone the assault if flying conditions favored the enemy overwhelmingly. The Commander, Seventh Amphibious Force, could call for postponement if surf conditions made landings impossible. Also, if sea conditions in Dampier Strait were such as to interfere with the transfer of assault troops from LCI's to LCM's, D-Day would be delayed.
Weather and The Operation
Full strategic and tactical air support was implemented by favorable weather conditions. Surface operations were carried out as scheduled in good weather and calm seas. A definite northwest swell was present on the landing beaches, but the resulting breakers did not exceed four feet. One LCT was broached temporarily when the anchor cable parted, but otherwise the landing craft experienced little difficulty. Local shower and thunderstorm activity during the initial landings did not handicap the movement of troops and supplies.
AVERAGE WIND DIRECTION AND FORCE (KNOTS)
DURING JANUARY, CAPE GLOUCESTER IS UNDER THE INFLUENCE OF THE NORTHWEST MONSOON. HIGH CLOUD COVER, FREQUENT THUNDERSHOWERS, AND GENERALLY "WET" WEATHER RESULT FROM OROGRAPHIC LIFTING OF THIS EQUATORIAL AIR MASS. FLYING CONDITIONS ARE GENERALLY AVERAGE TO GOOD BUT GROUND OPERATION IS OFTEN HAMPERED BY HEAVY PRECIPITATION AGGRAVATED BY EXTREMELY SLOW EVAPORATION.
WEATHER MAP FOR 0900 26 DECEMBER 1943
ASSAULT LANDING AT SAIDOR, NEW GUINEA
2 January 1944
On the average, January marks the arrival of the northwest monsoon in the New Guinea area. That this phenomenon had occurred a month early in the season, 1943--1944 has been pointed out previously. By January, the entire area was dominated by the northwest circulation.
Strategic Weather Consideration
With the coastline around Saidor oriented in such a way as to give considerable exposure to the northwest winds, the selection of landing beaches was of great strategic importance. The beach which presented the least amount of navigational difficulties was fully exposed to the northwest winds. Although, as was demonstrated in the assault at Cape Gloucester, this would not prevent a landing, it was decided to make every possible use of weather advantage by selecting instead three smaller beaches which were sheltered from the prevailing winds.
An additional consideration was introduced by local topography. Due to the sharp rise from the coastal plain to the mountains of the interior, high cloud cover with frequent showers was produced by northwest winds. Consistently good weather, as forecast and enjoyed in previous operations, could not be expected at Saidor. Nonetheless, it was thought that weather would not handicap the operation unduly.
The dependency of D-Day on the weather remained the same as in previous assaults. Provisions for the postponement of the operation were made in the event that weather would prohibit air support or would endanger the actual landing.
Weather and The Operation
With the relatively unimportant exception that H-Hour was delayed 30 minutes because of low visibility resulting from light rain at dawn, all operations were completed as scheduled.
In the execution of this operation, our forces enjoyed an unexpected weather advantage which had certainly not been included in the plans for D-Day. Throughout that day, unfavorable weather between Wewak and Rabaul interfered with enemy air activity and made the resistance to our landings rather less than expected.
AVERAGE WIND DIRECTION AND FORCE (KNOTS)
AT SAIDOR, JANUARY IS THE MIDDLE OF A DEFINITE RAINY SEASON. THE MOIST NORTHWEST AIR, LIFTED BY THE MOUNTAINS NEAR THE COAST OF NEW GUINEA, IS PRODUCTIVE OF HIGH CLOUD COVER AND INTENSE PRECIPITATION. FLYING IS FREQUENTLY RESTRICTED BY SHOWER ACTIVITY AND GROUND OPERATION IS HAMPERED BY SOIL TRAFFICABILITY CONDITIONS. AVERAGE WINDS ARE LIGHT NORTHWEST, AND LOCAL CIRCULATION IS DISTORTED BY A LAND BREEZE DURING THE NIGHT. SAIDOR RECEIVES OVER 140 INCHES OF RAIN DURING THE YEAR. TEMPERATURE IS CONSISTENTLY IN THE 80S.
WEATHER MAP FOR 0900 2 JANUARY 1944
From the standpoint of weather in these operations, the most interesting aspect was the practical operational use of modern methods of weather analysis as opposed to rule-of-thumb adages of the local "expert." Despite the oft-repeated, all-inclusive warnings that "landings cannot be accomplished during the southeast monsoon" and "high breakers prohibit small craft operation during the northwest season," the operations were planned and executed with unqualified success.
Weather data collected in the past, although neither complete nor entirely reliable, disputed these statements. First-hand observations did not give them support. By treating weather in the tactical phase as a specific forecasting problem which could be solved by sound practical forecasting principles, all six landings were made as planned and on schedule. Perhaps one of the most important lessons to be learned from this series of operations is that weather precepts based upon casual observation by not-too-well qualified observers may achieve "truth" by virtue of repetition alone.
The use of auxiliary weather aids did much to solve the forecasting problem. Reports from an advanced area provided the necessary assurance that unexpected weather situations would not be encountered by landing parties on D-Day. Auxiliary weather observers have been employed to advantage in several other operations. Their employment in the future may be equally valuable.
In summary, these operations of the Seventh Amphibious Force were facilitated by the excellent use of expected weather conditions, both in the strategic and tactical phases of each operation. In nearly every phase of the operation--selection of landing points, the timing of the actual assault, the problems related to supply and reinforcement--expected weather was an integral part of the over-all plan. In the words of the Commander, Seventh Amphibious Force, "General good weather, forecast and experienced, contributed considerably to the success of these operations."