Symposium on Microseisms Held at Arden House, Harriman, N.Y. 4-6 September 1952, Sponsored by the Office of Naval Research, and the Geophysical Research Directorate of the U.S. Air Force. (1953) / Chapter Skim
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Amplitude Distribution of Storm Microseisms
Amplitude Distribution of Storm Microseisms
Pages 20-55
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From page 20... ...
It is possible, by the use of special microseismic charts, to detect, to track, and to determine changes in the intensity of a storm when it is within range of three or more microseismic stations. The Problem of Recording Storm Microseisms -- The Naval Aerological Service initiated the Microseismic Research Project in 1943 with one major objective, which was to determine if severe tropical storms could be detected and tracked by recording changes in the amplitude and period of microseisms.
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From page 21... ...
I " i ,.' ' " i .' ' A'i, "_'''i ' i '" ' 'il ' ' ' 19/14 30 2 BEARING SW MNUM* i f i ^ 20/1430 Z BEARING SW Figure 1.
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From page 22... ...
The tracks of six hurricanes are drawn in Figure 3, and along each track are listed the corresponding amplitude of recorded microseisms in mm on top of line and the intensity of the storm in knots on the bottom. The storm intensities and corresponding microseismic amplitudes are very consistent throughout the map, especially at points where the tracks cross BEARINGS CALCULATED FROM STORM MICROSEISMS 1800 2 19 AUGUST 1950 61*
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From page 23... ...
Hurricane tracks around Miami each other and for the beginning of first increase in microseisms. From such data it is possible to draw lines of equal microseismic amplitude for a storm of any intensity around a station where sufficient storm microseisms have been recorded.
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From page 24... ...
Use of Amplitude Charts -- The Fleet Weather Centrals at Guam and Miami direct aircraft to fly reconnaissance into and around tropical storms, when they are far from land. Some reconnaissance planes are now equipped with radar for tracking storms at night, but because of various interferences these radar reports are not always reliable.
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From page 25... ...
An amplitude of 30 mm was recorded at 1200 GCT, 7 September, lower right of Figure 6, at which time the storm was reported by aircraft to have 100 knots of wind and its position as reported was located between the 35 and 40 mm lines on the 90 knot amplitude chart. That position and the 100 knot wind agreed with the amplitude chart because the 100 knot storm was located just outside the 30 mm line of a 90 knot chart.
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From page 26... ...
The microseisms increased rapidly at all three stations when the storm was about 740 miles from each station. The maximum increase was greater and a day later at Okinawa because the storm was approaching that station and crossing increasingly larger lines of equal microseismic amplitude.
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From page 27... ...
Forecasting movement of storm by microseismic amplitude at Guam, Okinawa and Manila crossing decreasing lines of equal amplitude, and the microseisms should have been decreasing if the intensity of the storm had remained constant. The fact that the observed microseisms were too large at each station to fit the proper amplitude chart gave every evidence that the storms were increasing in intensity.
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From page 28... ...
This feature of storm detection, that of giving a good estimate of storm intensity, is an especially valuable aid in the forecasting of hurricanes, especially at night and at other times when there are no direct observation by planes and when no ship reports are available. The Micro-Ratio Charts -- The microseismic amplitude charts are primarily used for detection of storms, and after the storms are located, to 18001 1 SEPTEMBER I»80 MBER 1980 DATA)
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From page 29... ...
Amplitude charts, therefore, are very important in storm forecasting. The micro-ratio technique mentioned briefly before, is still another step forward in the tracking of severe storms.
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From page 31... ...
NAVY MICROSEISMIC RESEARCH PROJECT ^Boi/0400 MS)
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From page 32... ...
Gilmore on the use of empirical amplitude relationships of storm microseisms as a method of tracking tropical storms is highly interesting and shows promise. However, a large amount of observational data is needed before its usefulness can be verified.
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From page 33... ...
Although Mr. Gilmore favors the idea that storm microseisms which are recorded in force on land are generated beneath the eye of the storm, he nevertheless reports that the amplitude relationships at the several stations will hold regardless of how the microseisms are generated.
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From page 34... ...
Past and future verification of the amplitude ratio technique of every circumstance may possibly verify the contention that hurricane microseisms are generated under the ocean beneath the storm area. But I have strong evidence that this condition does not generally apply to extra-tropical storms in the western half of the Northern Hemisphere.
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From page 35... ...
Since Berkeley records microseisms Figure 3. Weather chart of North America for 15 January 1950 at 18h 30m GCT showing a storm moving off the coast of labrador which is believed responsible for the generation of the microseisms shown in figure 2, courtesy of U
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From page 36... ...
Weather charts 19-21 December 19 50 showing a storm moving off the coast of northeast North America and microseisms recorded at Chicago during this time. In this and other illustrations, the flags show wind velocity in 10 knot intervals and the numerals represent days, hours and minutes.
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From page 37... ...
Case II. The source of heavy microseisms shown in Figure 4 is undoubtedly a violent storm accompanied by 75-knot winds on its northwest limb, moving oceanward off Newfoundland.
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From page 38... ...
Weather charts 7-9 December 1949 showing a storm moving inland from the coast of Alaska, and a time-period-amplitude graph of microseisms recorded at certain North American stations. This storm is believed to have been responsible for the microseisms shown in figure 5, but the microseisms appeared after the storm had moved inland.
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From page 39... ...
Weather charts 20-23 January 1951 showing a violent storm off the west coast of North America and time-amplitude graph of 7 sec. microseisms recorded at Sitka and Chicago during the same interval.
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From page 40... ...
Yet two late August 1952 hurricanes on the coast of the Carolinas or southward sent microseisms as far as 1700 miles inland to Bozeman, Mont. Referring to Figure 5 of Gilmore's paper the amplitudes at Whiting Field are about the same as at Jacksonville which is much closer to the storm center; and about twice the amplitude at Miami which is about the same distance as Whiting.
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From page 41... ...
The observations given all negate ocean waves or swell as being significant in the generation of the type of microseisms under discussion. Introduction -- A study of the intensity and variations of microseism amplitudes affords much information applicable to the practical use, and the problem of origin of microseisms.
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From page 42... ...
Time shown is GCT. western North Atlantic Ocean two and occasionally three hurricanes were present simultaneously on 25 days and were in a position to cause ambiguity at most of the hurricane tracking stations.
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From page 43... ...
Part II -- Amplitude Studies by Means of Resonant Seismometers -- It is considered an empirical fact at least for east-coast stations, that the microseisms period-spectrum varies Figure 4. Cold front positions (A)
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From page 44... ...
In order to attempt rapid study of the onset times of microseism storms, and the envelope of amplitudes without the laborious measurements heretofore used, these instruments record on drums making one rotation in 25 hours. Although a time scale is given on the illustrations to follow, since no time marks were made on the early records, automatic time marks are being imposed on current records.
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From page 45... ...
showing the 0030 and 0630 positions of the cold front associated with the microseism storm. (The dot below "P" on the chart marks the position of the Palisades station; the dotted line off the coast represents the 1,000 fathom depth contour.)
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From page 46... ...
Figure 6 indicates a 5/13 -01 04 Figure 8A. Wave-gauge records of May 11-13, 1946 made near Woods Hole, Massachusetts.
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From page 47... ...
Gilmore refers to the commencement of microseism storms with the primary generating hurricanes at a distance so great as to preclude the simultaneous arrival of swell in local waters. It is believed c/i i.in *
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From page 48... ...
show ocean bottom pressure records of high magnitude near Woods Hole, Massachusetts, and the simultaneous long-period vertical microseism records made at Weston Observatory some 50 miles to the north. The ocean waves were generated by protracted onshore southerly winds of 4 to 6.
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From page 49... ...
Case 1. Analyzed wave records of the Beach Erosion Board made from a bottom pressure gauge off Long Branch, New Jersey, give sea conditions as "calm" from February 12 to 15, 1952, with surface heights of 0.4 to 1.0 ft.
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From page 50... ...
Much more severe atmospheric disturbances commenced at Palisades at 0200 probably in the main mass of cold air following an unmarked secondary cold front (which is actually shown on earlier weather maps)
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From page 51... ...
Case 2. Calm seas existed in the same area from March 4 at 2000 to March 6 at 1200 according to wave records of the Beach Erosion Board.
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From page 52... ...
This suggests the effect of gustiness or turbulence as being of special significance in microseism origin." "It is suggested that pulsations or oscillations in the air striking the water resulting from instability or turbulence in the cold air are coupled to the sea surface by some mechanism." Roschke -- "It is shown that high-velocity flows of cold air are much more efficient mechanisms for producing extended intervals of maximum-amplitude micro-oscillations in the air than corresponding warm air flows." Lee (1934) observed that although microseism storms can always be associated with some atmospheric disturbance, the intensity of the microseisms varied despite similar conditions of pressure gradient and winds within the atmospheric disturbance.
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From page 53... ...
Microseism period is shown to increase continuously until the generating cyclone reached deepest ocean water. The reverse effect is expected when fronts or storms approach coastal stations from shoaling seas.
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From page 54... ...
The only unique method of origin seems to lie in excitation within the area of an atmospheric disturbance and by direct coupling of the energy of some impulsive air disturbances to the sea surface. At lower than hurricane wind velocities, cold air is a much more efficient microseism source than warm air.
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From page 55... ...
For some time, I have been concerned about the terms by which tropical storms are described. One hears discussion and comparison of "90-knot storms" or "120-knot storms." Actually, the magnitude of the wind at the center of the storm is only one factor in the description of a storm.
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