Gravity Wave over New England , April 12 , 1961

1, SYNOPTIC SITUATION An unusual rise in pressure was lrotcd 011 t l r c AI.1.T. Depart8nlent of Meteorology Irricrob:trograph during tlw early morning of April 12, 1961. T h fact. t'hat no st80r1rr;\or squally weather was observed at, the t'irrre aroused the author's curiosity to irlvestigat8e the pl~errotr~cr~on in greater detail. Stud!of t,llc 3-hr. t'erldencies reported by the net'work of hourly reporting stat,iorrs in a ~ ~ d around New England revealed an area ol' unusual1,v largc prcssuro rises moving across thc region i n a, generally nort tlcast,erly direction. The surface synoptic pat)t'ern rorlsist'ed ol a l'airly d w p Low rnovirlg slowly east#ward I'ronl Novn Scotia ant1 a narrow ridgc tthrough t,he Mid-At,lantic Statcs. A rat,llclr vigorous Jlow was rrlovirlg eastward in thc low^ Mississippi Valle?-, preceded by an a(*tivC ovcrrunrling squall line. The main features are sllowll in figure 1. 7'lre pattern is such t,frat the pressure rise over S e w England could not have been caused by t8he ra,pid t11ovemcnt8 of a small, intense Jlow or sharp trough as reported by Tryggvason [l] and Emersoll [2J. Sinw t8he direction of movement of the pressure rise was nearly perpendicular to the winds in the lower and rrriddlt. troposphere, it' could not have been an advcvtivc! Eeat'ure. The presence ol the squall line in t8hc South suggcsts that, the pressure wave rrlay have hcen associat'ed wit8tr t8hc outbreak of convective act'ivity over eastern 'I'cvas lat,c in the afternoon of' t h e preceding day. T h e 0100 and 0700 EST positions oE t'hc pressure rise are slrown b ~ dt~slred lines in figure 1, as well as an cxt,rapolat#ed position for 1900 EST the day bef'orc. The extmrapolat8iorl was lrrade with allowance for the orienta.tion of t'he winds in t,hc 850to 700-nlb. layer relative t'o the direct'ion of propaga,tion of t,he pressure rise, since the macroscale n~ovenlent of the air in which the disturbance was embedded would contribute tBo it's apparent motion. Wit'l-1 t'hc, aid of tllc FIGURE 1.-General synoptic situation at 0100 EST, April 14, 1961. Observed and extrapolated positions of the pressure rise (short dashes) and squall liue (long and short dashes) are also shown :tt 6-hr. intervals. 432 ~ION'I'ITLY WEATHER J3EVIli:W' OCTOBER 1962 This t'ype of situation has been suggcsted by Yot'trrcary [3] as a nlechanistlr for triggering gravity waves on an inversion or frontal surface, a ~ l d rrla>~ be similar t,o ccrt,ain aspects of t,lte sit,uation artalyzcd by Brunk [4]. I n t,llc lat'ter cases, however, t'he dist,urbance resulted in wind fluct'uations at tlre surface, whereas only the prcssurr seemed to be affected in the New England example discussed here. I t is intmesting t'o 11ot.e that Tepper [SI, on the other hand, postulat,es t,llat' t8he pressure jump is first, caused by acceleration ol a cold front,, ant1 the pressure jwnp t'hen t,riggers a squall line in a suit'able air mass. 2. ANALYSIS AND DISCUSSION Tn order to obtain as fine resolution in space and titllc as possible, wit8110ut, obtaining lnicroharograllrs for cac~ll stat8ion, tlle 1-lrr. changes in prcssure at stat.iorls in t'lle area of int,erest report,ing on Service A teletypewriter were computed from all availablr observations during thc. period 0100 to 1200 EST, April 12, 1961. Isallobars wcrc drawn with some snloot hing, allowing for reasonable errors in t,he readings ol t,lle barotnzt,er. A few reports were assumed erroneous, as when two st,ations within the satlie city area had indicated l-hr. changes differing by a millibar or more. The I-hr. pressurc. changes are shown in figures 2A, 2U, and 2C: a t times near t'hc beginning, peak intensity, and end of t,he pressure rise, as detected in thc Northcast. During its earlier stages, t'hc pressure risc was preceded by a11 area of falls of similar nlagrlit8udc, but by t8he tinle i t reached Maine, thc falls were negligible. 'This tnay hc at,tribut,ed partmially but, not entireljto superposition of t,he norrrlal diurnal rise in pressurc. A small atrrount 01' rise may also have been due t'o slow east'ward ret'reat of t hr Low near Nova Scot,ia. Xeverthcless, the isallobaric: gradient clearly had a maximum arolmd 0600 EST, as sccn fro111 t#he entire series of pressure change maps (not, a11 shown here). When the rise had reached t,he Gull' of St,. Lawrence, its magnitude had decreased to only a littlr over 2 mb./lrr. and it appeared to be flattenirlg out in its spatmid imensions. The hourly positions of t,he axis of nlaxinlum pressure rise are shown in figure 2D. The speed of propagtltion was found to vary between about 50 rlnd 85 kt'., with the fastest rates of tritvel around 0200 t t n d 0700 EST, tt slight retardwt,ion around 0400 and 0500, and :x more nlttrketl slowing Lifter 0900. Part of the variabilit'y in speed 11l:t.v have been spuriously introduced by the :tn:tlysis, hut most of t'he variation is probably real. The velocity of the pressure rise aretr suggests t h t t t it was a gravity-wave type of phenomenon. A prelirrlinary check of the morning radiosonde observations indicated it fairly strong inversion ut most of the stations where the pressure rise WHS pronounced, and isotherlnal or nlarkedly stable layers at several other stations in the vicinity. The lower t opospheric soundirgs for S e w York, Albany, Nantucket', Portland, and Sable Islitrid are shown in figure 3. The air at Kew York and Albany had evidently subsided throughoutj the layer frorn 600 nib. to 800 mb., producing :L stable lapse rate tlrrouglr this depth. Farther east) :it Smtucket , the stttble layer W ~ L S more concentrated into tl nltrrked inversion, nnd the eEects of subsidence were :lgrLin present. At Portlund and Sable Island the, air was gener:dly moist :It , d l levels because of the proximity ol' the Low, but, inversions were noted at these stations d s o , suggesting t h a t :I renlntmt of lifted tropical air may have been present following t'he earlier occlusion of the T1ow. Very little, if any, subsidence had occurred in this m 3 r . A nctrrly moist' w1iab:ltic lqme rate extended for ; h o s t 2,000 l't. above the inversion at' Port'land. The structure of t,he inversion is better displayed by 111e:~ns of cross-sectiond nndysis derived from the soundings. The thcrtnul struvture of' the atmosphere along the direction of propagstion of' tlre pressure rise is shown in figure 4 A t t n t l :L section nornlt~l t'o this through central S e w Eng1:rntl is shown in figure 4R. The principal inversion slopes down tow2lrd the south : r n d west, and is nlost 111:ukecl over central New England. I t is interesting to note that the pressure rise wws most intense in this s tme area where the inversion was strongest. A second, weaker inversion n1ay be traced tlt lligher levels from wrltr;rl New Englrrntl to the south and west. 'I'heoretic~nl speeds were computed frorn several of the sonntlings, using t l r c wave velocit'y f'or~nul:~. >IS fount1 i n ttlhlt 1 of Tepper's ttrticle [ 5 ] . The computed speed w : ~ obttlined by designtrting RS 8, tlle mean potential terrlpertlturc of the ntrlrospherc from the base ol' the inversion or stable layer to the surface, trnd as e2 the trletln potential temperature from t'he top of the inversion or st>lble layer to tlle 500-tnb. level. The choice ol' this upper level was not too critical, as the lapse rates were generdly quite steep from 500 rrlb. to t'he base of the tropopause. 1 he height of the middle of the inversion or st:Lble layer wtrs designated RS h. For exn~nple, at Xnntuclwt (fig. 3C), the inversion was judged to extend from 790 nib. t o 745 mb., centered about 2,:s ktrr. above the ground. The mean potent'ial tctnperxture of the layer l'rotn the ground to the base of the inversion W R S 280' K., :md from the top ol t'lle inversion to 500 r r l b . it T ~ L S 302" K. With these values, the forrrrula in 'I'epper's article gives :I cornputed value of 78 kt. for the speed of' propxgation o f :I gravity wave. Since this fortnult~ is for w11 itical ntrrrospllere consisting of two utlifortll layers of potential tetnperat.ures B1 and e2, sepatxted by an inversion a t height h above the ground, exact8 :yreetnent with observed speeds would not be rxpected. There was fairly good agreement, however, as seen in table 1. Iheomtic.:tl speeds as corrected by the advective cornponetlt of wind trre shown in parentheses. The advective r ,