The Vertical Profile of Wind and Temperature in Cyclones and Anticyclones over the Eastern Two ...

30 Pages·2002·1.75 MB·English

MARCH2002 BLUESTEIN AND BANACOS 477
The Vertical Profile of Wind and Temperature in Cyclones and Anticyclones over the
Eastern Two-Thirds of the United States: A Climatology
HOWARD B. BLUESTEIN AND PETER C. BANACOS*
SchoolofMeteorology,UniversityofOklahoma,Norman,Oklahoma
(Manuscriptreceived7December2000,infinalform4September2001)
ABSTRACT
Aclimatologicalanalysis,baseduponoperationalsurfaceandupper-airdatafrom1957to1994,ofthewind
and temperature profiles composited with respect to each quadrant of surface cyclones and anticyclones, is
presented for the eastern two-thirds of the United States. The cyclones and anticyclones are located via an
objectiveprocedure.Hodographsandsoundingsarealsocompositedwithrespecttoseason,geographicregion,
timeofday,and,forcyclonesonly,intensity.Verticalprofilesofthestatic-stabilityparameterarecomposited
with respect to season and quadrant forboth cyclones and anticyclones.Thestructuresof meancyclones and
anticyclonesareshownanddiscussed.
Adiurnalvariationinhodographs(verticalshear)isfound,whichshowsupinbothcyclonesandanticyclones
as a rotation in the counterclockwise direction between 0000 and 1200 UTC above the boundary layer. The
effect is greater in cyclones than in anticyclones. This variation is hypothesized to be in part due to a tidal
oscillationandinpartduetoradiative–thermaleffects.
Inthe mean,awell-pronouncedequatorward-directedlow-leveljetisresolvedinthenorthwestquadrantof
surfacecyclones.Low-leveljetsdonotshowupinthemeaninotherquadrantsofcyclonesorinanticyclones.
Thecurvatureofhodographsnearthetropopauseisclockwiseincyclonesandcounterclockwiseinanticyclones.
1. Introduction dependsonthem.Forexample,thenatureofconvective-
scale and mesoscale precipitation systems embedded
Climatological analyses of extratropical synoptic-
within synoptic-scale features depends upon the wind
scale systemshave, uptorecently,documentedthefre-
and temperature structure of their parent system.Inthe
quency, tracks, and preferred regions of cyclogenesis,
last 20 years we have learned, from numerical simu-
cyclolysis, anticyclogenesis, and anticyclolysis in both
lation experiments (e.g., Weisman and Klemp 1982,
theNorthern(e.g.,Petterssen1956;Taylor1986;Zishka
1984), theory (e.g., Rotunno and Klemp 1982, 1985),
and Smith 1980; WhittakerandHorn1984)andSouth-
and observations (e.g., Maddox 1976; Bluestein and
ern (e.g., Taljaard 1967; Sinclair 1994) Hemispheres.
Jain 1985), how the nature of convective storms is re-
The interannual variability of cyclone frequency
lated to the convective available potential energy and
(Changnon et al. 1995) and the climatology of meteo-
to the vertical-shear profile. We have also learned how
rological‘‘bombs’’(SandersandGyakum1980)areex-
the mesoscale organization of stratiform precipitation
amples of other more specialized investigations. How-
systems,particularlybandedprecipitationinthevicinity
ever, the authors are unaware of climatological assess-
of frontal zones embedded in extratropical cyclones,is
mentsofhowthethermodynamicandvertical-shearpro-
related to the temperature, moisture, and vertical-shear
filesofbaroclinic,extratropicaldisturbancesvarywithin
profiles (e.g., Wolfsberg et al. 1986; Schultz and Schu-
their synoptic-scale, parent disturbances.
macher 1999).
A climatological assessment of the theromodynamic
In addition, local maxima (with respect to height)in
and vertical-shear profiles is important for several rea-
wind speed at low levels have been documented in the
sons.First,thenatureofanumberofmesoscalesystems
polewardairstreamaheadofsurfacecyclones(Bluestein
1993, p. 390) and in the equatorward airstreambehind
surface cyclones (Bluestein 1993, p. 391). The former
*Currentaffiliation:NOAA/NWS/NCEP/StormPredictionCenter,
often play a role in transporting warm, moist air pole-
Norman,Oklahoma.
wardandthelatteroftenplayaroleintransportingcold,
dry air equatorward. The authors are unaware of any
Correspondingauthoraddress: Dr.HowardB.Bluestein,School
long-term climatologies of these low-level jets.
of Meteorology, University of Oklahoma, 100 E. Boyd, Rm. 1310,
In recent years compact disctechnologyhasallowed
Norman,OK73019.
E-mail:[email protected] us to access easily large meteorological datasets(Mass
(cid:113)2002AmericanMeteorologicalSociety
478 MONTHLY WEATHER REVIEW VOLUME130
FIG. 1. The region of the Western-Northern Hemisphere in which surface cyclones and anti-
cyclones were objectively located (region A). Cyclones and anticyclones only within region B
wereconsideredincomposites.Soundingsforcompositestobedescribedlatercamefromregion
BandtheareajustoutsideregionB(i.e.,within500kmofthecycloneandanticyclonecenters
inregionB).
et al. 1987; Mass 1993); in addition, large datasets ar- section 6, mean cross sections of wind and potential
chivedonmass-storagedevicesareeasilyavailableover temperaturethrough cyclonesandanticyclonesarepre-
the Internet. It is therefore appropriate now to compile sentedanddiscussed.Section7containsasummaryand
a climatology of the vertical profile of wind and tem- discussion of the results and suggestions for further
perature in synoptic-scale, extratropical cyclones and studies.
anticyclones.Thepurposeofthispaperistopresentand
discusstheimplicationsofsuchclimatologicalanalyses,
2. Data and methodology of data processing
withanemphasisontheseasonalanddiurnalvariations,
and to a lesser extent on regional variations. Thefocus
a. Strategy for locating surface cyclones and
of this study is on systems in the central and eastern
anticyclones
sectionsoftheUnitedStates.Webelievethatourstudy
isuniqueinthattheresultsarevalidinareferenceframe We first compiled an objective climatology of the
moving with cyclones and anticyclonesthemselves,in- locations of the centers of surface cyclones and anti-
stead of in a reference frame fixed with respect to the cyclones over a broad area covering all of the United
ground ashasbeendonebefore[e.g.,WallaceandHar- Statesandnearbysurroundingregions(Fig.1).Thecli-
tranft (1969) examined the diurnal wind variation in a matologyisbasedonsealevelpressuredatafromtwice-
ground-based reference frame]. daily (0000 and 1200 UTC) hemispheric,griddedanal-
In section 2, the data sources and methodology of yses from version III of the National Meteorological
processing the data are detailed. Climatologies of sur- Center [NMC, now known as the National Centers for
face cyclones and anticyclones are presented and dis- EnvironmentalPrediction(NCEP)]–NationalCenterfor
cussed in section 3. Climatological analyses of hodo- Atmospheric Research (NCAR) hemispheric gridpoint
graphs are presented in section 4 and corresponding data archive from 1946 to 1994. These data, stored on
analysesofthermalstructureareshowninsection5.In the NMC octagonal (polar stereographic) grid (with
MARCH2002 BLUESTEIN AND BANACOS 479
381-km grid spacing at 60(cid:56)N)wereobtainedfromaset data,whicharenotaffectedbymodel-relatedprocesses
of two compact discs from the Department of Atmo- suchastheparameterizationoflatentheatrelease.How-
sphericSciencesattheUniversityofWashington(Mass ever, we did accept the use of the sea level pressure
et al. 1987; Mass 1993). The analyses in this dataset datafromthenavy,whichwereinfactaffectedbymodel
come from those done by the United States Navy for processes. The upper-air dataset, to be discussed later,
1946–93andfromNMC‘‘finalanalyses’’for1994(i.e., were in fact collected from real observations, notfrom
although the dataset is from the NMC–NCAR archive, model-affected fields.)
the surface dataset was mostly produced by the U.S. Forthepurposeofqualitycontrol,eachsetofgridded
Navy).TheU.S.Navy’soperationalanalysissystemhas datawasinspected;grosslycorruptedanalysesweredis-
evolved in asimilarfashiontothatofotheroperational carded.Inaddition,eachmonthwecomparedarandom
forecasting centers (e.g., Dey 1989) while employing subset of the surface pressure analyses with analyses
largelythesameobservationaldatasources(Goerssand from the Daily Weather Maps series published by the
Phoebus 1992). The navy changed from a successive United States Department of Commerce. On the basis
corrections technique to a global multivariateoptimum ofourcomparisons,itwasconcludedthattheevolution
interpolationanalysisbeginningin1988(Barker1992), of operational objective analysis methodology did not
and new preanalysis objective quality control proce- impact the synoptic-scale sea level pressure analyses
dures were also implemented that year (Baker 1992). significantly in a qualitative sense.
Since these analyses were used as part of a dataassim- In order to locate cyclones and anticyclones more
ilation procedure for running the Navy Operational precisely (but not necessarily more accurately), we re-
Global Atmospheric Prediction System (NOGAPS) analyzed the data from the relatively coarse NMC grid
model, which became operational in 1982 (Rosmond onto a polar stereographic grid having the same corner
1992), it is expected that the redistribution of sealevel points,andexactlyhalfthespacingofa256-pointsubset
pressure is influenced to some degree bymodel-related of the NMC grid (Fig. 1, region A). We used a three-
processes, especially in later years of the analysis pe- passBarnes(1964)schemelargelyfollowingtheguide-
riod. During the early years of our dataset (1946–75) a lines of Koch et al. (1983); the scheme was configured
‘‘fields of information blending’’ (FIB) technique de- so that the rms difference between the initial and col-
velopedbyM.Holl attheNavalResearchLaboratories located analysis points was on the order of 0.2 mb,
(NRL)/FleetNumericalMeteorologyandOceanography which is very small compared to the amplitude of the
Center (FNMOC) was used to generate the sea level pressure variations in cyclones and anticyclones.
pressure analyses (P. Pauley 2001, personal communi- Errors in determining the locations of cyclones and
cation). This methodology involved the use of an iter- anticyclones from a discrete set of pointsarefromspa-
ative weak-constraint variational analysis of a recent tial-truncation errors (Changnon et al. 1995) and lack
numerical forecast and a previous analysis. Some ver- of sufficient resolution. Lack of spatial resolution did
sionoftheFIBanalysiswasusedforsealevelpressure not allow us to determine accurately the value of the
untilthemultivariateoptimuminterpolationschemewas pressure extremum at the center of the most intense
implemented in 1988 and used through the end of our pressure systems. For most extratropicalsystems,how-
dataset. So, all the sea level pressure analyses wereaf- ever, the difference between the subjectively estimated
fected by model processes to some extent. pressureextremum(determinedfromtheDailyWeather
Missinganalyseshadbeenreplacedbyanalysesfrom Map series) and the objectively determined extremum
theairforce,NMCoperationaland‘‘final’’hemispheric was not greater than 3–4 mb. Dickinson et al. (1997)
analyses,andongoingresearchprojects.Subjectivesur- found a slightly greater difference (5 mb) between the
faceanalyseshadbeendigitizedmanuallyandincluded NMC gridded sea level central pressure and the NMC
aspartoftherecord,especiallyfordatathroughtheend subjectivesurfaceanalysiscentralpressureinanintense
of the 1950s. From 1946 to 1954, and for shorter in- cyclone over the Gulf of Mexico.
termittent periods between 1960 and 1962, analyses Theobjectiveprocedureforlocatingpressuresystems
were available just once daily at 1200 UTC. from the reanalyzed data involved first identifyinggrid
Since the dataset was not homogeneous over the en- points having a sea level pressure less than 1000 mb
tire 1946–94 period, it is possible that there might be forcyclonesandgreaterthan1026mbforanticyclones.
different biases during some periods, owing to the dif- The threshold value in cyclones was chosen subjec-
ferent analysis procedures, which have not been docu- tively; the threshold value for anticyclones waschosen
mented very well; however, we did not think it worth- to be 13 mb greater than standard atmospheresealevel
while to standardize and reanalyze the entire dataset pressuretobeconsistentwiththe13-mbpressure-deficit
ourselves, which would have been an overwhelming (withrespecttostandardatmospheresealevelpressure)
task. It was also decided not to use the recently com- thresholdincyclones.Theuseofthesethresholdvalues
pleted NCEP–NCAR reanalysisprojectdata(Kalnayet for cyclones and anticyclones may exclude weak cy-
al. 1996) mainly because it was not available at the clones and anticyclones, especially during thesummer,
outsetofthisstudy.(Inaddition,itwasfeltthatitwould but include many weak cyclones and anticyclones, es-
bebestinitiallytouseanalysesmainlyofobservational pecially during the winter. Thus the dataset may be bi-
480 MONTHLY WEATHER REVIEW VOLUME130
FIG. 2. Relative frequency of synoptic-scale, extratropical (a) cyclones and (b) anticyclones over the eastern two-thirds of the United
Statesfrom1946to1994bymonthandtime(0000and1200UTC).‘‘Relativefrequency’’isthenumberofsystemsmeetingtheobjective
criteriadescribedinthemainbodyofthepaperdividedbythenumberofmaptimes.
ased toward stronger systems during the summer and fromthepreviouslyidentifiedone,inordertobecertain
weakersystemsduringthewinter.Toseehowsensitive it was truly a separate system.
climatologiesaretothresholdvalues,cycloneswerealso Sincetheobjectofourstudyisextratropicalsystems,
located for threshold values of 994 and 988 mb and tropical warm-core systems near shore were excluded
anticyclones were also located for threshold values of from the sample. Tropical warm-core systems were
1032 and 1038 mb, that is, for deviations of 6 and 12 identified from 6-h Atlantic tropical cyclone data com-
mb from the other thresholds. Systems within one grid piledbytheNationalHurricaneCenter;113caseswere
point of the edge of the boundaries in Fig. 1 were not removedfromthesample.Sincethelocationsoftropical
considered. Since elongated surface pressure troughs systems over the ocean are usually known more accu-
and ridges do not have well-defined centers, we ex- ratelythanthoseoftheirextratropicalcounterparts,ow-
cluded them by imposing the followingisobar-circular- ing to satellite, operational weather radar, and in situ
ity criteria: For both cyclones and anticyclones, reconnaissance aircraft observations, it was possible to
determine the differences between the actual tropical
|(cid:93)2p/(cid:93)x2| and |(cid:93)2p/(cid:93)y2| (cid:46) 1/3.3|(cid:185)2p|.
cyclone locations and the estimates based on the re-
The circularity criteria were chosen subjectively.Elon- analyzed gridded data. We were therefore able to esti-
gated Arctic anticyclones eastoftheRockiesandweak mate the spatial errors involved in determining system
cyclonesalongfrontalboundariesarenotgenerallycap- location by our procedure. In conjunction with other
tured. However, a sample composed of more circular qualitycontrolproceduresbasedontherawinsondedata
systems is better suited for the quadrant-averaging ap- (cf. section 2b), these spatial errors do not affect our
proachusedtoexaminetheirstructure.Ifasystemwere resultsappreciably.Themeandifferenceinlocationwas
identified in an analysis, any additional one had to be 86km,whichislessthantheaveragedifferencebetween
at least 1500 km (approximately one-half wavelength surfaceobservingsitesoverlandand25%oftheaverage
of most synoptic-scale, extratropical systems) away spacing between rawinsonde sites. Since extratropical
MARCH2002 BLUESTEIN AND BANACOS 481
FIG.2.(Continued)
systems tend to be broader than tropical systems, the exhibit a much higher frequency of occurrence of cy-
locations of them over land are probably known with clones and anticyclones. It is recognized that central
even better accuracy. pressureisnotnecessarilyanindicationofintensity;for
Altogether,31082NMCgridswereanalyzedoverthe example, during the winter a cyclonemayberelatively
49-yr period from 1946 to 1994. In the eastern two- intense (i.e., the magnitude of the horizontal pressure
thirdsoftheUnitedStatesandadjacentportionsofCan- gradient is intense), even when its central pressure is
ada, the Gulf of Mexico, and the Atlantic Ocean (Fig. not relatively low.
1, region B), 3808 (nontropical)cyclonesand3040an-
ticyclones were identified. Since 57% of the grid times
b. Strategy for compositing hodographs and
were at 1200 UTC, these statistics could be biased to-
thermodynamic soundings
ward systems more prevalent at that time of day. Any
significantdifferencescouldbearesultofdiabaticpro- Rawinsonde data from 1946 to 1994 that had been
cesses or topography. The overall relativefrequencyof archived by the National Weather Service and the mil-
occurrence of cyclones was 0.1225, or roughly one cy- itary were obtained, for North America, from the Na-
clone every 8 maps; it is greater than the relative fre- tional Climatic Data Center (NCDC). Owing to a dif-
quency of occurrence of anticyclones, which was ferent standard timefor balloonlaunchesbefore1June
0.0978,orroughlyoneanticycloneevery10maps.(Our 1957, we decided to consider data only from 1 June
counting procedure does not identify independent cy- 1957to31December1994forthisportionofthestudy.
clones; i.e., the same cyclone appearing on a sequence It was still possible to obtain a large statistical sample
of maps does not count as one cyclone. The relative from this subset of all the data available.
frequency of occurrence of cyclones is the number of Data from all rawinsonde launches within 500 km
cyclones on each map meeting the central pressurecri- (less than a quarter wavelengthofmostsystems)ofthe
terion divided by the numberof individualmapsused.) center of surface pressure systems in the eastern two-
Although we did not compute a climatology that in- thirds of the United States and adjacent regions were
cluded weaker systems, it is obvious that one would compiled and the quadrant(northeast,southeast,south-
482 MONTHLY WEATHER REVIEW VOLUME130
FIG.3.Relativefrequencyofsynoptic-scale,extratropicalcyclonesandanticyclonesfrom1946to1994,byyear.
west, and northwest) relative to the center of each sys- kmofthecenterofsurfacesystemsandwhenthesurface
temwasnoted.Wedidnotconsiderdatafromthewest- wind was in excess of 5 m s(cid:50)1. We made sure that the
ern one-third of the United States becausethecomplex observed wind direction was within 90(cid:56) of the wind di-
terrain there makes interpretation of the data more dif- rection estimated from the geostrophic wind modifiedby
ficultandevenimpossibleinareasofveryhighterrain. a 30(cid:56) cross-isobar (toward lower pressure) component to
The dataset is not homogeneous over thedurationof accountforfrictionandbaroclinicity(Hoxit1974).Inrare
the sample because instruments and operating proce- instances when the surface wind direction did not satisfy
dures have undergone changes (Friedman 1972; Pratt this constraint, the wind observation wasrelocatedtothe
1985), there have been some flawed automated pro- quadrant in which it was satisfied.
cessing procedures (Schwartz 1990), the network has We then imposed a statistical quality control on the
undergone changes (Bosart 1990; Elliott and Gaffen data by checking manually all data that were three or
1991),andtheequipmentisnotuniform(Schwartzand more standard deviations from the seasonal and quad-
Doswell1991).However,thevariousproblemswithop- rant means. If these data were associated with an ex-
erational rawinsonde data have been well documented treme event, they were retained; otherwise, if they
in the literature. lacked spatial and/or temporal coherence (i.e., if they
Toaddresstheseproblemspriortocompilingthedata, were grossly unrepresentative as a result of instrument
several objective and subjective quality control mea- or other problems), they were discarded. We anticipate
sureswereimplementedtoeliminatesuperadiabaticlay- thatbycarefulqualitycontrolproceduresandexamining
ers and supersaturated levels. In addition, height data a very large data sample, these problems will not sig-
computedabovethehighestlevelforwhichtemperature nificantly affect our results.
data were available were discarded in order to avoid
hydrostatic inconsistencies (Gandin 1988; Collins and
3. Climatology of surface cyclones and
Gandin 1990).
anticyclones
Because there is an uncertainty about the precise lo-
cation of the center of surface systems, we imposed an Over the 49-yr period from 1946 to 1994 surface
additionalcheckontherawinsonde-sitequadrantsrelative cyclones (anticyclones) in the eastern two-thirdsofthe
to the center of the systems when they were within 250 United States were more common at 0000 (1200 UTC)
MARCH2002 BLUESTEIN AND BANACOS 483
FIG.4.Stddev,asafunctionofheight(kmAGL),inthenortheast(darksolid
line),southeast(darkdashedline),southwest(lightsolidline),andnorthwest(light
dashedline)quadrants(within500kmofthecenters)of(a)thezonal(u)component
ofthewindincyclones,(b)themeridional((cid:121))componentofthewindincyclones,
(c)thezonalcomponentofthewindinanticyclones,and(d)themeridionalcom-
ponentofthewindinanticyclones.
than at 1200 (0000 UTC) (Fig. 2) when anticyclones Duringthefallandearlywinter(October–January),the
whosecentralpressureexceededanyofthethreethresh- deeper(994-mbthreshold)cyclonesweremorecommon
olds were counted, but only when cycloneswhosecen- at 1200 UTC than 0000 UTC. The deepest cyclones
tral pressure was below 1000 mb were counted; when (988-mbthreshold),ontheotherhand,weremorecom-
the threshold pressure for cyclones was decreased to mon at1200UTCthan0000UTCformostoftheyear.
994and988mb,cycloneswerefoundtobemorecom- The opposite diurnal behavior of cyclone and anticy-
mon at 0000 UTC than 1200 UTC mainly during the clone frequency of occurrence when the cyclones and
spring (March–June) and early fall (September) only. anticyclones were relatively weak can be explained as
484 MONTHLY WEATHER REVIEW VOLUME130
TABLE1.Numberofsoundingsusedineachcomposite.
Compositetype NEquadrant SEquadrant SWquadrant NWquadrant
Allcyclones 3182 4183 3890 3112
0000UTC 1687 2228 1981 1635
1200UTC 1495 1955 1909 1477
Allanticyclones 2853 3259 3271 2815
0000UTC 1075 1226 1275 1179
1200UTC 1778 2033 1996 1636
Springcyclones 1439 1822 1663 1367
0000UTC 790 996 882 722
1200UTC 649 826 781 645
Summercyclones 268 427 327 276
0000UTC 162 253 182 168
1200UTC 106 174 145 108
Fallcyclones 570 843 772 562
0000UTC 286 452 384 292
1200UTC 284 391 388 270
Wintercyclones 905 1093 1131 908
0000UTC 451 529 536 454
1200UTC 454 564 595 454
Springanticyclones 528 554 625 516
0000UTC 176 176 228 201
1200UTC 352 378 397 315
Summeranticyclones 145 225 225 160
0000UTC 35 56 54 48
1200UTC 110 169 171 112
Fallanticyclones 1101 1285 1242 1039
0000UTC 416 465 480 414
1200UTC 685 820 762 625
Winteranticyclones 1083 1203 1183 1103
0000UTC 449 532 515 518
1200UTC 634 671 668 585
Northeastregioncyclones 1174 1754 1845 957
0000UTC 532 833 847 445
1200UTC 642 921 998 512
Southeastregioncyclones 596 524 676 877
0000UTC 296 263 335 442
1200UTC 300 261 341 435
Plainsregioncyclones 1410 1905 1370 1281
0000UTC 861 1133 801 750
1200UTC 549 772 569 531
Northeastregionanticyclones 1120 1548 1588 938
0000UTC 420 581 613 381
1200UTC 700 967 975 557
Southeastregionanticyclones 1313 1048 1155 1409
0000UTC 490 382 449 601
1200UTC 823 666 706 808
Plainsregionanticyclones 426 664 530 474
0000UTC 169 264 214 199
1200UTC 257 400 316 275
Cyclones,p (cid:44)980mb 36 69 58 47
Cyclones,(cid:122)mi(cid:46)n 2f 320 363 356 362
Cyclones,(cid:122)g(cid:46)3f 39 26 46 49
g
Albany,NY,cyclones
0000UTC 28 52 61 25
1200UTC 21 48 39 33
GreenBay,WI,cyclones
0000UTC 59 52 95 31
1200UTC 92 54 119 38
OklahomaCity,OK,cyclones
0000UTC 34 72 18 10
1200UTC 18 76 16 8
Greensboro,NC,cyclones
0000UTC 14 19 23 20
1200UTC 13 17 28 14
MARCH2002 BLUESTEIN AND BANACOS 485
TABLE1.(Continued)
Compositetype NEquadrant SEquadrant SWquadrant NWquadrant
Albany,NY,anticyclones
0000UTC 41 43 30 27
1200UTC 98 53 69 34
GreenBay,WI,anticyclones
0000UTC 53 27 31 22
1200UTC 60 42 48 39
OklahomaCity,OK,anticyclones
0000UTC 2 19 13 12
1200UTC 13 22 17 21
Greensboro,NC,anticyclones
0000UTC 15 38 45 38
1200UTC 53 103 71 22
a hydrostatic consequence of radiative cooling during 988 mb. The midwinter relative minimum is probably a
thenight(highersurfacepressureintheearlymorning) resultoftheinhibitionofbaroclinicinstabilityduetohigh
and insolational heating during the day (lower surface staticstabilityatlowlevelsovertheNorthAmericancon-
pressurelateintheafternoon).Whencyclonesaredeep- tinentandlikelydoesnotoccurwithextratropicaloceanic
er, they may be overall more intense, so that the mag- cyclones.Anticyclonesweremostcommonduringthefall,
nitude of the surface pressure gradient is stronger, the relatively frequent during the winter, and least frequent
surfacewindsstronger,andtheeffectsofverticalmixing during the summer (Fig. 2b).
in the boundary layer more intense; the effect of the The annual relative frequencies of cyclones and an-
latter might be to reduce the effects of radiative tem- ticyclones(Fig.3)exhibitlargeyear-to-yearvariability.
perature changes. From Fig. 3 it may be inferred that there could be a
Cyclones were most common in late winter and early slight trend of decreasing cyclone frequency over the
spring, and least common in the summer (Fig.2a),when period of record, as has also been noted by Changnon
there is the least pole-to-equator baroclinicity over the et al. (1995). It is beyond the scope of this paper to
UnitedStates;therewasarelativemaximuminfrequency discuss or explain this finding, other than to note that
of occurrence in late fall and a relative minimum during wehaveindependentlysupportedtheirresults.Thespa-
midwinterwhenthepressurethresholdwas1000and994 tial distributions of cyclones and anticyclones over the
mb,butnotwhenthepressurethresholdwasdecreasedto 49-yr period (not shown) are generally consistent with
the climatologies of Whitaker and Horn (1984) and
ZishkaandSmith(1980),eventhoughtheirstudiesmay
haveincludedweakersystems.Thesimilaritiesbetween
our climatologies of surface cyclones and anticyclones
with those independently formulated by others is ad-
ditional confirmation that our objective techniques for
locatingsurfacecyclonesandanticyclonesareeffective.
4. Climatological analyses of hodographs
Table1liststhenumberofsoundingsusedtoprepare
each composite hodograph. For most composites, hun-
dreds of soundings were used; in many instances well
in excess of a thousand soundings were used. In the
worstcase,summertimeanticyclonesat0000UTC,only
several dozen soundings were used.
Wind composites were computed by averaging the
zonal and meridional components separately, as has
been done in another context by Carr and Millard
(1985). Owing to the ‘‘nonuniqueness’’ of vectorinter-
FIG. 5. Composite hodographs for region B of Fig. 1, for 1 Jun polation (Schaefer and Doswell 1979), the magnitude
1957–31Dec1994,forallcyclones(solidline)andallanticyclones
of the averaged winds is less than the average wind
(dashedline)regardlessofquadrant.Numbersplottedareheightsin
speed in each composite. The composite hodographs
kmAGL.Thesamplesize(N)ofeachcompositeisindicatedineach
panel;first(second)numberisforthesolid-line(dashed-line)cases. were calculated from data interpolated at 500-m inter-
486 MONTHLY WEATHER REVIEW VOLUME130
FIG.6.AsinFig.5butforeachquadrantofsurface(top)cyclonesand(bottom)anticyclonesforwinter(solidline)andsummer(dashed
line).WinterandsummerseasonsareDec–FebandJun–Aug,respectively.