Aerology and Amphibious Warfare
The Assault Landings on Leyte Island
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).
The Assault Landings on Leyte Island
The significance of weather in these operations, as noted in a review of action reports:
BY COMMANDER, THIRD AMPHIBIOUS FORCE:
1. The lack of weather restrictions was directly responsible for a large part of the success of the operation.
2. It was recognized that the existence of unreported typhoon weather can outweigh security considerations dictating radio silence.
3. Air operations were not seriously affected by the weather.
4. Weather conditions of wind and temperature distribution favored the efficient use of smoke in the transport area.
5. Ground operations were slowed by unfavorable soil trafficability conditions resulting from precipitation.
6. There was no attempt to use weather for concealment as our intentions had been disclosed when minesweeping operations started.
7. Good weather over the Philippine Islands favored enemy air opposition by facilitating the staging in of aircraft from other locations.
BY COMMANDER, SEVENTH AMPHIBIOUS FORCE:
1. The use of carrier support insures local air superiority as bad weather in the area has the same adverse effect on enemy operations.
2. Weather conditions were excellent for the landing of troops and for protection by friendly aircraft.
3. Close air support was hampered by afternoon shower activity.
4. Enemy aircraft made excellent use of cloud cover for concealment during harassing raids.
BY COMMANDER, FIRE SUPPORT GROUPS:
1. The proximity of a tropical storm on A-3 day handicapped preliminary landing operations and supporting air operations.
BY A CVE [Escort aircraft carrier] COMMANDING OFFICER:
1. Weather conditions curtailed air operations only once, on A-3 day.
BY VARIOUS OBSERVERS ASHORE:
1. High winds on A-3 day removed camouflage from many Japanese installations and positions.
A-Day for the assault landings on Leyte Island was 20 October, 1944, two months and two years after the first landings were conducted in the Guadalcanal Area of the Solomon Islands.
During this intervening period, the remainder of the Solomon Islands has been occupied or neutralized by the South Pacific Forces. The Gilberts, the Marshalls, the Marianas, and the Southern Palau Islands had been taken over by the Central Pacific Forces,1 and thirteen landing operations in the New Guinea area had been completed by Southwest Pacific Forces.2
The decision to conduct landing operations against the Japanese forces on Leyte Island during October, 1944, was dictated by considerations of higher strategy. As a result, there could be no attempt to select the most advantageous landing time from the standpoint of long-term weather probability.
The weather planning phase of this operation was, therefore, concerned with two over-all weather considerations related directly to the accomplishment of landings at a specified future time:
1. The weather to be expected during the period of the operation.
2. Strategic and tactical considerations introduced by the possible effects of weather on the various phases of the operation.
Average Weather Determined
Weather planning for this operation was accomplished in a joint conference between major Force Commanders assigned to the landing operation - COMMANDER, THIRD AMPHIBIOUS FORCE and COMMANDER, SEVENTH AMPHIBIOUS FORCE. The first step in the weather planning phase consisted of a critical examination of all available climatological information for the CENTRAL PHILIPPINES area, in an effort to determine the average weather which might be expected in the operating area. As set forth by the COMMANDER, SEVENTH AMPHIBIOUS FORCE, expected weather conditions were:
"Partly cloudy to cloudy conditions will prevail. Being in the doldrums belt, squalls and thunderstorms will bve encountered, especially in the afternoon and evening. During October, rain averages about 9 inches with about 20 days of rain. The prevailing wind direction is from the northeast about 4 to 7 knots in the protected gulf area and 8 to 16 knots at the entrance to the gulf. Winds up to 70 knots are encountered in typhoons while local squalls produce 30 knot winds of short duration. Land and sea breezes are prevalent. Sea and swells average 0 to 3 feet in the TACLOBAN area increasing to 2 to 5 feet along exposed coasts except during severe weather when they are considerably higher. Clouds average 75 percent with cumulus type prevailing. Cloud bases are at about 2,000 feet. Strong southerly winds, called 'COLLA,' caused by a low northeast of LUZON are frequently encountered."
Detailed consideration further disclosed that adverse weather conditions during the landings would most probably be in connection with either an unfavorable development and movement of the Equatorial Front or in connection with a typhoon in the area.
The Equatorial Front
During October, the Equatorial Front has begun its movement to the south under the influence of the increasing circulation from the northern (winter) hemisphere, and on the average lies through the middle of Mindanao, oriented in an east-west direction. As can be seen on the chart on page 4, the Gulf of Leyte is well within the zone of influence of this frontal system. Because of this location of the Equatorial Front, Leyte Island experiences two distinct weather types during the month of October - one type associated with the northward oscillation of the Equatorial Front, and the other type associated with the establishment of the northeast trade winds in the area.
Paradoxically, the onset of what is known as the "rainy" season is in connection with the establishment of the northeast trades rather than an accompaniment of the Equatorial Front. In October, however, the trades are not yet fully developed, and as a result, generally good weather can be expected during this period, unless the area is under the influence of a storm area. Partly cloudy skies, scattered showers (increasing over land in the afternoon), and a good visibility present no undue handicap to air and amphibious operation.
On the other hand, a well developed Equatorial Front oscillating northward over the area could give rise to very unfavorable operating conditions - cloudy to overcast skies with frequent shower activity and torrential rains. poor visibility, and bad soil trafficability conditions resulting from the heavy precipitation.
Even more important from the standpoint of operational significance, however, is the location of Leyte Island in respect to the average typhoon track for the month of October (page 5). The critical conditions arising from the presence of a typhoon in the operating are were expressed by COMMANDER, SEVENTH AMPHIBIOUS FORCE as follows:
AVERAGE WEATHER CONDITIONS - OCTOBER.
"If a typhoon threatened the Leyte Gulf area, severe weather conditions would result with high winds, heavy rains, high seas and tides."
Strategic and Tactical Weather Considerations
In an effort to foresee possible difficulties arising from an unfavorable weather development, many contingencies were accounted for in the formulation of operation orders.
Relating to Preliminary Operations:
1. While good weather was expected for the approach of the various forces, it was realized that the formation of a typhoon within the general area could disrupt many movement plans. In preparation for such a development, the following doctrine was outlined by the COMMANDER, SEVENTH AMPHIBIOUS FORCE:
"In the event of a typhoon affecting the Task Forces en route, the plan for delaying or retiring will be dictated by the most advantageous route for avoiding the storm, coupled with a consideration of the position and course of following echelons."
2. An important phase of preliminary operations was the capture and occupation of certain small islands which control the entrance to Leyte Gulf. Since these landings were to be accomplished by relatively small forces in advance of the main operation, sea and swell on the beaches was an important consideration. The problem of selecting the most advantageous landing places became a function of expected wind direction. Landing beaches were, therefore, designated in so far as possible on the southwest side of these islands, where they were protected from the prevailing northeast winds.
Relating to the Final Assault:
1. The importance of local air superiority in support of a landing operation having been adequately demonstrated by previous experience, positive measures were taken to insure such superiority during the landings on Leyte. Operations conducted by the Seventh Amphibious Force in the New Guinea area had underlined the fundamental weakness of depending entirely upon land-based aircraft for support. In several instances, a strong front between friendly land bases and the objective area had turned back our land-based aircraft. A large number of aircraft carriers were committed in support of the Leyte landings, effectively solving the problem of local air superiority. In the words of the COMMANDER, SEVENTH AMPHIBIOUS FORCE:
"Since carrier aircraft were supporting the operation, the problem of having a front between the objective area and our nearest friendly base did not have to be considered. Weather which would seriously affect flying conditions for our carrier aircraft would also be a disadvantage to the Japs."
2. Perhaps the most important single weather consideration during the assault phase of a landing operation is the condition of the sea and surf on the landing beaches. In this operation, our forces were favored by the
location of Leyte Gulf in respect to the prevailing northeast winds. This advantage was noted by the COMMANDER, SEVENTH AMPHIBIOUS FORCE, as follows:
"From the information obtained on average weather conditions, it was believed that landings on the main beaches could be made without difficulty. (Landing beaches) . . . were well within the Gulf and protected from extreme surf conditions with the exception of surf caused by a typhoon."
The critical nature of surf conditions was also recognized by the COMMANDER, THIRD AMPHIBIOUS FORCE:
"The possibility that a heavy surf might persist after the passage of cyclonic activity was covered by the assignment of especially trained beach observers. These two observers, one assigned to cover each landing area, were embarked in CVE's [Escort aircraft carriers] of the Air Support Group and were to be airborne and ready to make their observations early on the day of the landings. Both officers were aerologists, graduates of the Scripps Institute course in Surf and Swell Forecasting, and further trained in airborne observation of surf and beach conditions."
3. Of importance nearly equal to sea and surf are considerations related to terrain and soil trafficability in the landing areas. In this respect, at least one of our forces was very fortunate. In the words of the COMMANDER, THIRD AMPHIBIOUS FORCE:
"The possibility that ground operations might be hindered by the wet and boggy terrain was to a large extent automatically provided for by the fact that the assault was to be made in amphibious vehicles."
Additional details of the strategic and tactical role which weather was expected to play in this operation, along with a summary of the various weather factors related to operations is given by the COMMANDER, THIRD AMPHIBIOUS FORCE:
"Discounting the possibility of storms, however, it was pointed out that normal conditions would be favorable for naval and amphibious operations. Ground operations were expected to be hampered by wet terrain occasioned by the onset of the rainy season. Air operations were expected to be limited to low levels by the prevalence of lower clouds. Incendiary bombing and bombardment were expected to be less effective than normal due to the heavy rainfall and high humidity. Chemical and smoke operations over land by day were expected to be hampered by the high convective turbulence, and over water as well as over land by the lightness and variability of the prevailing winds. It was expected, however, that the use of area smoke screening over the transport areas would be effective."
Relating to a Possible Delay:
The likelihood that a delay in this operation might be made necessary because of weather conditions in the landing area was recognized by the Force Commanders.
COMMANDER, SEVENTH AMPHIBIOUS FORCE:
"If a typhoon is present . . . (in the operating area), H-Hour and possibly A-Day would be delayed."
COMMANDER, THIRD AMPHIBIOUS FORCE:
"Recognition of the possibility of unfavorable weather was taken in detailed planning by this Force. The realization that a delay in the target date might be necessary was appreciated by the Force Commander, and he was prepared to issue the necessary orders for a retirement or diversion of the Force."
Relating to the Vital Importance of Certain Weather Information:
The extreme importance of weather conditions to the success of this operation is emphasized by the following order given by the COMMANDER, THIRD AMPHIBIOUS FORCE to all Ship and Task Unit Commanders after the planning phase of the operation had been completed.
"Commanders of Ships and Tactical Units operating independently may break radio silence to report signs of typhoon activity whenever the strategic situation permits, keeping in mind that the existence of unreported typhoon weather may out-weight [sic] the security considerations dictating radio silence."
The account of the influence of weather upon preliminary operations is mainly concerned with a tropical disturbance of near-typhoon intensity which affected the operating area from A-6 to A-2 days.
At least one of the forces en route to Leyte Gulf was affected by this storm during the approach phase, and from records submitted by a CVE, the path of this storm has been reconstructed for the period 14 to 17 October. This chart appears on page 9, along with the track of the ship which furnished the weather reports.
For the most part, excellent weather conditions were encountered during the approach by the major Task Units. Routine operations, such as air searches and fueling operations, were accomplished under good weather conditions.
While both COMMANDER, THIRD AMPHIBIOUS FORCE, and COMMANDER, SEVENTH AMPHIBIOUS FORCE, had suspected the presence of an incipient typhoon in the area west of Yap as early as A-6, the first positive information of such an unfavorable weather development was received from COMMANDER, FIRE SUPPORT GROUPS at noon on 17 October (paraphrased):
"Preliminary A-3 day landings on the islands at the entrance to Leyte Gulf made on schedule. Tropical disturbance approaching typhoon intensity is
TROPICAL STORM OF 14-17 OCTOBER 1944.
expected to keep up for two days, becoming more intense tomorrow. Rough sea, heavy rain, zero ceiling and visibility. Believe center is 75 miles north of Leyte, estimated direction of movement 290 degrees, speed 10 knots. Small ships and CVE's experiencing rough weather. Air operations not possible. CVE's turning back."
This disturbing information was supplemented four hours later by a dispatch from a CVE COMMANDER operating in the Leyte area (paraphrased):
"Due to the presence of a typhoon, air operations have been impossible this date. Estimate center of typhoon at 124 degrees east and 12 degrees north. Speed 13 knots to west-northwest. One destroyer escort lost mast overboard in high winds. Weather should permit aircraft operations by noon tomorrow."
Daring the evening of 17 October, the COMMANDER, FIRE SUPPORT GROUPS, reported that weather had delayed all operations that day, but that all the preliminary landings had been effected, and that the weather was now improving slightly.
This last report was most reassuring, and by dawn of 18 October it was felt that the operation could commence on schedule unless mine sweeping in the assault area had been delayed to such an extent that a delay in A-day might be necessary.
The Final Decision
A short time later, the Fleet Weather Central, Pearl Harbor, confirmed the opinion of the COMMANDER, THIRD AMPHIBIOUS FORCE, that the storm was not of typhoon proportion. This report, coupled with conditions noted on the weather map for 0000 OCT, 18 October, permitted a decision to conduct landing operations as scheduled at 1000, 20 October. No rough seas, heavy swells or high surf were expected since the passage of the storm on 17 October through Leyte Gulf had been rapid and no wave-generating areas were within effective distance of the beaches.
The remainder of the approach phase is outlined by COMMANDER, THIRD AMPHIBIOUS FORCE, as follows:
Remainder of Approach
"With the passage of this small storm on 17 October, the Equatorial Front weakened considerably in the Visayan area, and the approach of the transports continued through a region of calm seas, partly cloudy skies and widely scattered rain squalls. Most of the enemy air had been neutralized by our previous strikes and the mine sweeping in Leyte Gulf had already disclosed our intentions so the fact that the expected cover afforded by the doldrum belt did not materialize was not a source of concern to this force. The wind remained light from the southeast quadrant and the combination of a wind from astern and a hot sun during the day made the ships very hot and uncomfortable. Despite the heat and discomfort, however, the transports and landing craft arrived off Leyte Gulf with personnel in good physical condition and eager for the assault."
In summarizing the influence of weather on preliminary operations, the following specific effects are noted:
1. Air strikes by land-based aircraft were not unduly handicapped by weather conditions. Very few missions were turned back because of weather associated with the Equatorial Front.
2. There was no attempt to use weather for concealment during the approach.
3. With the exception of A-3 day, the operation of carrier aircraft was unhampered by weather conditions.
4. High winds with resulting unfavorable sea and swell conditions handicapped preliminary landings on the entrance islands and minesweeping on A-3 day.
5. It was later learned that these same high winds had presented our attacking forces with an unexpected advantage. Many Japanese installations which had been cleverly concealed were disclosed when camouflage was blown away on A-3 day.
6. Under the influence of the strong wind circulation associated with the storm, the Equatorial Front weakened and moved to the south, thus contributing to the extended period of good weather observed on A-day and the following period.
FINAL LANDING OPERATIONS
The weather forecasts for the day of the Assault Landing was issued by COMMANDER, SEVENTH AMPHIBIOUS FORCE:
Forecast for the Operation
"Scattered intermediate and low clouds becoming broken over land in the afternoon with shower and thunderstorm activity late in the afternoon. Visibility good except in shower areas. Surface winds northwesterly 4 to 8 knots in the morning increasing to 8 to 12 knots in the afternoon. Surf 1 to 2 feet from the east."
According to the COMMANDER, THIRD AMPHIBIOUS FORCE:
"The assault proceeded on schedule following the preliminary bombardment by ships' gunfire and aircraft, a slight onshore tendency of the almost imperceptible wind conveniently drifting the smoke and dust of the bombardment off the beaches and into the interior. The airborne beach observer had made his required report earlier, but the report was unnecessary in this case
due to the almost complete absence of surf.
"The only smoke mission called for was the smoking-off of Catmon Hill which commanded the northern beach area. This was accomplished expeditiously by smoker aircraft and white phosphorous projectiles from the fire support ships. The smoke was very effective."
The weather during the assault phase was described by COMMANDER, THIRD AMPHIBIOUS FORCE, in detail as follows:
Weather during the Morning
"Weather at the entrance to Leyte Gulf at daybreak on A-day (20 October 1944) was cloudy with altostratus clouds and five-tenths swelling cumulus, bases at 2500 feet, visibility to seaward 12 miles, a barometer reading 1009.9 millibars and the wind force 2 from the southeast. By J-hour (the hour for the landing on the Dulag - San Jose beaches by this Force) the cloudiness had decreased to only partly cloudy with altocumulus and swelling cumulus, bases still at 2500 feet. There was a flat calm and correspondingly flat sea. The visibility had been reduced to 6 miles by haze."
Weather developments during the remainder of the day were noted by the COMMANDER, THIRD AMPHIBIOUS FORCE, as follows:
"Cloudiness increased during the day as convective activity progressed over the surrounding land areas and by mid-afternoon showers had begun to drift over the Gulf from the Island of Samar, to the eastward. Visibility in these showers was reduced to less than 3 miles."
With the exception of A plus 1 day, weather remained good throughout the initial assault phase. The unfavorable weather experienced on A plus 1 day was in connection with a severe thunderstorm of about an hour's duration. Although the wind reached 35 knots in gusts, it did not set up rough seas in the Gulf and unloading operations were halted only momentarily.
The Effects of Weather
The influence of weather on the assault phase of the operation has been noted as follows:
BY THE COMMANDER, THIRD AMPHIBIOUS FORCE:
"Air operations were not seriously affected by the weather, except for the usual afternoon rain squalls which could readily be avoided.
"The weather was used to good effect by this Force when, in a force 1 north-northwest wind, the transport area lay under the thick area smokescreen during the sunset and dawn air raids. Augmented by a screening line of LCI [Landing craft, infantry] smokers to windward of the area, and favored by continuing light winds and night-time inversions, the transports lay safely under a protective smoke blanket for a total of over
16 hours during the 4 days that the Force Flagship was in the southern transport area.
"Ground operations were somewhat slowed by the muddy terrain resulting from the heavy rain on 17 October, but in general, conditions were better than expected. The rain squalls from A-day to A plus 6 day were not heavy enough or of long enough duration to cause any continued discomfort by troops,
An Advantage for the Enemy
"Good flying conditions prevailing over the entire north and central Philippines made it easy for the enemy to launch frequent harassing raids from their beaches in Cebu, Negros, and other Visayan fields, and to replenish their losses by staging planes in from Luzon."
BY THE COMMANDER, SEVENTH AMPHIBIOUS FORCE:
"Weather conditions were excellent for the landing of troops and for protection by friendly aircraft.
Support by Aircraft
"Close support aircraft were hampered somewhat by lowering ceilings and showers over the land target areas in the late afternoon but not during the morning assault phase.
Enemy Use of Weather
"Enemy aircraft made excellent use of the heavy cumulus clouds for concealing their approach on bombing runs and also for cover against antiaircraft fire and from our planes."
The overall significance of weather during the Assault Landings on Leyte Island is well summarized by the COMMANDER, THIRD AMPHIBIOUS FORCE.
"On the whole, it is believed that the lack of weather was directly responsible for a large part of the success of the operation. The force went to Leyte, expecting the worst, were prepared to meet the worst, and found themselves with the best weather that could possibly be hoped for at this season.