WINDS OF THE UPPER TROPOSPHERE AND LOWER STRATOSPHERE OVER THE UNITED STATES

Transcription

1 JANUARY 1957 MONTHLY EATHER REVIE 11 INDS OF THE UPPER TROPOSPHERE AND LOER STRATOSPHERE OVER THE UNITED STATES MARVIN. BURLEY, EARL M. RITCHIE, AND CHARLES R. GRAY National eather Records Center, U. S. eather Bureau, Asheville, N. C. [Manuscript received Noveer ; revlsed January 8, ABSTRACT Daily wind data fro United States stations for six pressure surfaces (2, 15, 1, 8, 5,.) for the period April 1951 through January 1956 are used to study upper winds along the 8th and 12th eridians. At 5degree intervals along the two eridians, idseason average wind speeds and prevailing directions have een coputed and are copared. 1. INTRODUCTION The cliatology of the upper air, particularly that area aove 4, feet has een neglected ecause of the lack of sufficient data. During recent years, with rapid iproveents in instruentation and tcchniqurs, an increasing supply of wind data at higher elevations has hccoe availale [I]. Although t,he aount of data has not reached its axiu, sufficient data are now availale to estalish certain cliatological relationships. inds over the United States for four idseasonal onths are copared fro latitude 25' X. through 45' N. along longitude 8'. and fro latitude 5' N. through 5' N. along longitude 12'. This study iscon:i?ned to the upper troposphere and the lower st'ratosphere, specifically etween the 2. level (aout 4, feet) and the. level (aout 8, feet'). Data used are 15 GMT rawin oservations for a period eginning April 1951 and ending with January Pilot alloon oservations were not used ecause they are few in nuer and often considered questionale at these altitudes. For ost of the period, sufficient data were availale to otain cliatological averages for single stations. However, this was not possile for the first years of t'he period for the idseason onths of January, April, and Octoe here necessary, data fro several neary stations were coined to arrive at a representative value. The persistence of winds at higher altitudes justifies this procedure. The 5year wind speed averages were plotted on aps. Isolines and wind roses were drawn and dat'a for the 5 degree latitudinal intersections of the eridian were interpolated. The arithetical wind speed averages were then plotted on graphs, and vertical profiles were drawn (figs. 18). No sujective interpretations were ade in drawing the curves. A very few sujective int,erpretations were ade for prevailing wind directions. In no case was the change &ore than to the adjacent direction. In the graphs, prevailing directions are to 16 copass points (i. e., N, NNE, etc.). The first nuer in parenthesesis the percentage of occurrence of the prevailing wind direction. The second nuer is the percentage of occurrence of the prevailing direct'ion plus the two adjacent directions. (Exaple: E(5, 75). Prevailing direction east 5 percent of the oservations; directions ESE, E, ENE 75 percent of the oservations). Sall prevailing direction percentages indicate that the directions are fairly well dispersed around the copass. The phrase "transition zone" is used in the discussion of the graphs to denote the region of iniu wind speed. Decreasing westerly winds in this region usually change rapidly with height t,o increasing easterlies. This rapid change in the winds is ost coon during the spring, suer, and fall seasons for southern latitudes and the suer season in northern latitudes. A good exaple of the seasonal arch of the transition zone is found in coparing two curves which are nearly identical. These are shown y the curves for July 4' N., 8'. (fig. 5) and Octoer 25O N., 8'. (fig. 7). Since this study was ased on oserved values alone, the possiilit,y of ias toward lower wind speeds in the final results was considered. To assess this effect, synoptic sit'uations for various periods and for stations with aundant data during this period were studied rather thoroughly. The following conclusions earing on this paper were reached: (I) the layer aove the 2. level is aove the axiu wind level a large percentage of the tie, and therefore decreasing values would e expected with height until the transition zone is reached; (2) high winds would affect the aount of data priarily ecause the rawinsonde alloon was lown out of range efore the 2. level was reached, therefore the levels aove the 2. level theselves could not e classifkd "highwind" levels for this reason; () the aundance of data for the &year period would tend to correct the ias toward a reasonale cliatological average ; (4) the persistence of winds fro day to day at these elevations ade it possile to detect trends of oth lower and higher speeds when values were issing. For these reasons it is elieved that the ean values derived can e considered sustantially accurate. The values derived fro this study copare closely with those of Kochanski and asko [2,, 41. These siilarities occur despite different techniques used, and should, therefore, increase confidence in the cliatological value of oth ethods. Siilar agreeent ay e found etween these values and geostrophic values fro

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6 16 MONTHLY EATHER REVIE JANUARY 1957 upper air aps [5, 61. Percentage frequencies of the prevailing direction and adjacent directions facilitate assessent of the relative variaility or constancy of wind direction. 2. MIDSEASONAL IND PROFILES JANUARY Longitude 8' est (jig. I).The wind speeds at all latitudes decrease with height to the 5. level and t'hen ecoe steady or increase 1 or 2 eters per second to the. level. The faster the wind speed at 2. the ore rapid is the decrease fro level to level. Between the 2. and 8. levels the speed increases fro latitudes 25' to 5' N. A sall decrease in speed occurs, at these levels, at latitude 4' N. and a decided decrease is oserved at latitude 45' N. The prevailing wind direction for all latitudes etween the 2. and 5. levels is west. At the. level fro 25' to 4' N. the prevailing direction is westsouthwest. Short periods of easterly winds are oserved at the higher levels. This slight tendency for easterly winds is ore prevalent in the lower latitudes. Longitude 12' est (fig.,%').the wind speeds at all latitudes decrease with height to the 5. level and then increase fro 1 to 2 eters per second to. Speeds are approxiately the sae, along the eridian, fro 2. through 8. There is a tendency for increasing speed in the higher latitudes aove 8. The unfority of wind speed, with increasing latitude along the 12th eridian, contrast sharply with the orextree changes along the 8th eridian. The average speeds, level y level for coparale lat,it'udes, are lower than those along the 8th eridian. The prevailing wind direction is west etween the 2. and 15. levels for allatitudes. Between the 1. and. levels fro 4' to 5' N. the prevailing direction is westnorthwest. Easterly winds occur ost frequently at, the higher levels and are of short duration. APRIL Longitude 8' est V;g. ).The wind speed curves for January and April are siilar in appearance ut several iportant changes have taken place. At the 2. level the average speeds for April are approxiately the sae fro 25' to 4' N. The speeds in the lower levels at 25' N. are greater than in January. Speeds in the lower level fro ' to 45' N. are lower than they were in January. There is a 1 eters per second decrease in speed at 2. fro 4' to 45' N. At 25' N. the speed decreases fro 2. to 5. and then increases slightly. At ot,her latitudes it decreases to 5. and then decreases ore slowly or reains constant. est winds prevail at all latitudes and levels with the exception of the. level at ' N. and the 5. and. levels at 25' N. where the prevailing wind direction is eastsoutheast. Longitude 12' est (jig. 4).The wind speed fro 5' to 4' N. is greater in the lower levels and less in the upper levels than fro 45' to 5' N. The speed decreases with height to 8. and then decreases ore slowly or reains constant at all latitudes. Coparing the wind speed curves along the two eridians it is found that the greatest difference is in the lower levels. The higher speeds are found along the 8th eridian. For t'he higher levels, all curves are nearly identical. The prevailing win direction for all latitudes and levels is west. Short periods of east,erlies are oserved in the upper levels. JULY Longitude 8C' est (jig. 5).The wind speeds and directions for July differ sustantially fro those of the other idseasonal onths in that the t8ransition zone is easily found at all latitudes. The transition zone is lower (generally at the 1. level) fro 25' lo 5' N., and oves upward (8. to 5.) with increasing latitude. Below the transition zone, wind speeds are light in the lower latitudes, increasing northward, so tjhat at 45' N. the 2. speeds are higher than those of April. Aove the transition zone there is a rapid increase in speeds in the southern latitudes. This increase ecoesaller with increasing latitude and appears to e associated with the variaility of the altitude of the transition zone. East'erly winds prevail at all levels fro 25' to ' N. ith increasing latitude, elow the transit'ion zone, directions gradually change until at 45' N. the prevailing direction is west. Aove the transition zone, easterly winds predoinate at all latitudes. Generally, winds that are westerly at the 2. level reain westerly, with increasing height, t'o t'he transition zone and then turn east'erly. The sense of the change to an easterly direction ay e either clockwise or counterclockwise depending upon the final westerly direction (i. e., northwesterly winds turn through north and nort'heast (2g. 5); southwesterly winds turn through south and southwest (fig. 6)). Longitude 12' U7est (fig. 6).The trend of changing speeds during July at 12'. copares very closely with that of the 8'. eridian. There is a definite transition zone at all latitudes ranging fro 1. at 5' N. to 5. at 5' N. Speeds elow the transition zone are generally light at 5' Y., increasing north ward. There is a ore rapid decrease fro level to level at the higher latitudes. Speeds in the transition zone are the sae for all latitudes. Aove the transition zone, speeds increase oderately in southern latitudes, and only slightly in northern latitudes. Southwest winds prevail elow tjhe transition zone; easterly winds prevail aove the transition zone. OCTOBER Longitude 8' est (fig. 7).The t'ransition zone which was present for all latitudes for July, has now disappeared athe northerly latitudes, and has oved

7 JANUARY 195i EATHER MONTHLY REVIE 17 upward to t'he 8. level at 25' and ' N. Speeds aove this zone increase only slightly (2 to eters per second). It is int'eresting to note the steady increase in the 2. speed fro 25' to 45' N., which produces a rapid decrease fro 2. to 8. at, the northerly latitudes. The curves at, 5', 4', and 45' N., are siilar except that the speeds at. t,he higher latitudes are greater, level for level, and tend t'o ecoe ore unifor with increasing height. Prevailing directions are westerly at all levels fro 5' to 45' X. Fro 2.5' to ' X. t'hey are westsout'hwest elow t,he transit'ion zone and east aove it. Periods of easterlies are oserved at all lat,itudes, hut' t'hey are of short duration and ecoe less frequent with increasing latitude. Longitude 12' "est (fig. 8).Thc Oct'oer trend at 12'. differs fro hat at 8'. only slightly. Speeds in the lower levels increase at a unifor rate with incrrasing latitude ut tend t'o reain nearly constant aove the 5. level. At the 12th eridian speeds in the lower levels average aout 2 percent' less t'han those for equal latitudes at the 8th eridian, and in the upper levels aout 1 percent less. Prevailing direct'ions are fro the west in the lower levels at all latitudes. 'I'hey are westnort'hwest in the higher levels wit'h the exception of slight,, perhaps insignificant, predoinance of easterly winds at 5' S. Short periods of easterlies are oscrved ut are ore frequent', of longer duration, and at lower eleva,tions in the southerly latitudes.. CONCLUSION The ost striking dissiilarity et'wecn the speed curves for January and curves for the idseasonal onths of April, July, and Oct'oer for 8'. is the consistently higher speeds that occur fro 2. t'lroug11,5. Speeds in January along t'his longitude averegc 2 percent' higher than in April, 7 percent higher than in July, and percent higher than in Octoe At 8'. the ean January speed averages 25 percent higher than for the sae points at longitude 12'. This ratio is greater (aout 4 percent) at 5' and 4' X. than for other latitudes. The reason sees to e that the untlerlying jet strea activity is ore persistent across 8' M at these two latitudes. Latitudinally, the largest decrease in speed with height at oth eridians is at 5' and 4' N. for all seasons. This, of course, t'akes into consideration the increase in speeds fro the 1. to. levels during July. Along the 8th eridian the highest8 2. speeds are found at 5' N. during January, and the highest. speeds are found at 25' N. during July. At 12'., 2. speeds are approxiately the sae in January at all Iatitudes, and the :. speeds reach a axiu at. 5' N. in July. Coparison of the relative speeds along the two eridians shows t,he greatest difference for the four seasons is along latitude 5O N. The speeds along the 8'. eridian are uch higher than those along the 12'. eridian. 'The curves for the two longitudes shown on t'he accopanying graphs are approxiately the sae at 45' N. with the exception of t'he Octoer curve. It can readily e seen that, disregarding the relative speeds, the curves closely resele each other in nearly all cases. The prevailing windirections are westerly at all latitudes, elow the transition zone, with the exception of easterly winds during July at 25' and ' N. Aove the t'ransition zone in suer, the wind seldo varies fr9 a direct'ion etween east'rlortheast and east,southeast. Variaility of wind dirwtion is greatest, in the region of the transition zone. hen no transition zone is present dispersion is great'est' in the highest levels. ACKNOLEDGMENTS Sincere appreciation is expressed to M Leslie Sith, Director of the National eather Records Center, for his aid and encourageent in pursuing this study, and to M Xoran L. Chnfield, Chief of the Cliatic, Analysis Section for his editing a,nd technical advice in the preparation of t"rke pape REFERENCES 1. B. Ratner, inds and Fallcut: A Clirnatoloyical Appraisal, U. S. eather Bureau, June A. Kochanski and P. E. asko, "Analysis and ind Plow athe 5 and 25. Levels," U. S. Air Force, Air eather Seruzce Technical Report 1596, ashington, D. C., May A. Kochanski and P. E. asko, "Daily ind Flow at the 5 and 25. Levels," Bulletin of the Aerican Meteorological Society, vol. 7, No. 1, Jan. 1956, pp A. Kochanski and P. E. asko, "Mean ind Flow at the 5 and 25. Levels," Bulletin of the Aerican A4.eteoroZogica.l Society, vol. 7, No. 2, Fe. 1956, pp U. S. Air Force, "Mean Monthly Maps of, 2 loo, 5, and 25. Surfaces over North Aerica," Air eather Service Technical Report No. 1517, ashington, D. C., Spril U. S. eather Bureau, Dazly Series Synoptic eather Maps, Northern Heisphere 1rn. and 5. Charts, (Seven ont8hs of 195: January through AIay, July, and Octoer) ashington, D. C., ,649572

8 18 MONTHLY EATHER REVIEIV JANUARY 1987 SUGGESTIONS FOR AUTHORS Articles are accepted for the Monthly eather Review with the understanding that they have not een pulished or accepted for pulication elsewhere. Two copies of the anuscript should e suitted. All copy, including footnotes, references, tales, and legends for figures should e doule spaced with argins of at least 1 inch on sides, top, and otto. Soe inked corrections are acceptale ut pages with ajor changes should e retyped. The style of capitalization, areviation, etc., used in the Review is governed y the rules set down in the Governent Printing Office Style Manual Tales should e typed each on a separate page, with a title provided. They should e nuered consecutively in araic nuerals. In equations conventional syols in accordance with the Aerican Standards Association Letter Syols for Meteorology should e used. If equations are written into the anuscript in longhand, duiouslooking syols should e identxed with a penciled note. References should e listed on a separate sheet and nuered in the order in which they occur in the text; or, if there are ore than 1, in alphaetical order according to autho The listing should include author, title, source (if a agazine the volue, nuer, onth, year, and coplete page nuers; if a ook the pulisher, place of pulication, date, and page nuers). If reference is ade to a selfcontained pulication, the author, title, pulisher, place of pulicat ion, and dat,e should e given. ithin the text references should e indicated y araic nuers in rackets to correspond to the nuered list. Footnotes should e nuered consecutively in araic nuerals and indicated in the text y superscripts. Each should e typed at the otto of the page on which the footnote reference occurs. Illustrations. A list of legends for the illustrations should e typed (doule spaced) on a separate sheet. Each illustration should e nuered in the argin or on the ack outside the iage area. To fit into the Review page, illustrations ust take a reduction not to exceed % x 9 (colun size) or 7% x 9 (page size). Map ases should show only political and continental oundaries and latitude and longitude lines, unless data are to e plotted, when station circles will also e needed. Usually the less unnecessary detail in the ackground the etter will e the result fro t he standpoint of clear reproduction. Line drawings and graphs should also e uncluttered with fine ackground grids unless the graph deands very close reading. It is not necessary to suit finished drawings, as drafting work can e done at the tie the paper is prepared for pulication. Photographs should e sharp and clear, with a glossy surface. Bear in ind that arks fro paper clips or writing across the ack will show up in the reproduction. Drawings and photographshould e protected with cardoard in ailing. eather Notes Many years ago t he Monthly eather Review pulished detailed eyewitness accounts of exceptional stors. These accounts oth enrich t,he eteorologist, s knowledge of stors and provide hi wit,h particular details t>hat, cannot e found elsewhere. Because such inforat>ion ears directly upon questions the eteorologist. ust attept to answer aout weather phenoena (for exaple, the identification of stors as tornadoes), and ecause the inforation has potential value in oth t he research and service progras of the eather Bureau, pulication of eyewitness account s of exceptional stors and other eteorological phenoena was resued in the April 1955 issue. They appear fro tie to tie under the heading eather Notes. Contriutions to these Notes are invited fro readers of the Review. There is no liitation placed on length of description ut it is expected that ost will e short accounts. Any weather peculiarities, whether stors or ot>her phenoena, are acceptale suject at te Mat erial should e addressed to Editor, Monthly eather Reviclw, U. S. eather Bureau, ashington 25, D. C.