JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D24, PAGES 31,109-31,125,DECEMBER 27, 1999

Diurnal and semidiurnal variations in global surface
wind and divergence fields
Aiguo Dai and Clara Deser
National Center for Atmospheric Research, Boulder, Colorado

Abstract.

Diurnal and semidiurnal variations in surfacewinds and wind divergence

overthe globe(50øS-70øN) are documentedusing3-hourlywind observations
from-o10,000 land stations and available marine reports during 1976-1997. A strong
diurnal cyclein surfacewindsis found over land areas(strongestover high terrain
and in summer)with an amplitudeof 0.6-1.1 m/s for wind speedand 0.5-0.7 m/s
for the zonal and meridional wind components. Surface wind speed peaks in theearly afternoon over most of the globe. It is suggestedthat increased downward
turbulent mixing of momentum during the day could be one of the main causes
for the early afternoon maximum of surface wind speed. The diurnal anomalies of
surfacewind divergencetend to be out of phase in adjacent regions. In particular,

land areas (except for extreme inland locations)exhibit maximum divergencearounddawn (0600-0800 localsolartime(LST)), while nearbyoceanicregionshave
their maximumdivergencein the evening(1700-1900 LST). Thus, there is evidence
for a large-scalediurnal circulation in which surfaceair rises and convergesoverthe continents and sinks and diverges over nearby oceans in the afternoon andearly evening, and the opposite occurs in the early morning. Over the tropicalAtlantic and PacificOceans(10øS-10øN), a zonally coherentpattern of maximum
convergence(divergence)north (south) of the equatoraround 0900-1200LST isgenerally similar to the latitudinal profile of the mean daily divergence,indicativeof an enhancementof the local Hadley cell around 1030 LST relative to the daily
mean. Another zonally coherent north-south dipole occursover the North Pacific,

with maximumsurfacewind divergence(convergence)around0600-0800 LST in
the subtropics(midlatitudes). This phasepattern correlateswith cloudcoverover
the two regions. Over the United States, surfacedivergencepeaks around 0600 LST
in the west and the east and around2000 LSTin the center.

al., 1992],the openoceansin the tropicalPacific[Gut-

1. Introduction

Solarheatingin the atmosphere,combinedwith otherregionalforcings,generatesinternal gravity wavesin theatmosphereat periods of integral fractions of a solarday, especiallyat the diurnal and semidiurnalperiods.
These waves cause regular oscillationsin atmosphericpressure,temperature, and wind fields which are of-ten referredto as atmospherictides [Wallaceand Har-

zler and Hartten, 1995; Deser, 1994; Deser and Smith,1998] and tropical Atlantic [Pedder,1978; Nitta andEsbensen,1974],the centralUnited States[Heringand
Borden,1962; Reiter and Tang, 1984],southernIndia[Sivaramakrishnan
et al., 1993],northeastern
Australia[May, 1995],and easternAfrica [Savijarvi,1997]. For
example,Heino [1978]findsthat diurnal variationsof

surface wind speed accountsfor more than 50% of the
tranft, 1969; Chapmanand Lindzen, 1970; Haurwitz and total variance in summer over Finland. Over the tropiCowley, 1973; Hsu and Hoskins, 1989; Whiteman and cal Pacific,DeserandSmith[1998]find that the semid-

Bian, 1996;Dai and Wang,1999]. Althoughthe tidal
variations in winds are much larger in the upper atmo-

iurnal cycle dominates the daily variation of the surface

zonal wind component(u) with an averageamplitudespherethan at the surface[WallaceandHartranft, 1969; of 0.15 m/s and a phase(time of maximum,or Tmax)
Williamsand Avery,1996],largediurnaland semidiur- of 0310 and 1510localsolartime (LST), whilethe diurnal variations in surfaceor lower tropospheric winds arenal cycle dominatesfor the meridional wind component

foundovertropicalislands[Aspliden,1977; Williams et (v), whichpeaksaround0600-0900 (1800-2000) LST
north (south)of the equatorand hasan averageamplitudeof 0.19 m/s.
Copyright 1999 by the American GeophysicalUnion.
Paper number 1999JD900927.

0148-0227/ 99/ 1999JD900927509.00

Many of the features of daily wind variations documented by the above regional studies have been foundto be related to local forcingsuchas solar heating asso-

31,109

31,110DAI AND

DESER:DIURNAL

VARIATIONS

ciated with topography, latent heating in moist convection, and differential solar heating over adjacent waterand land surfaces. To separate the migrating, global
tidal waves from the nonmigrating, regional tidal signals, one needs to analyze the daily variations on a
global scale.
The daily variations in surface wind divergencehave
been documented only for limited regions in the trop-

IN SURFACE

WINDS

AND

DIVERGENCE

resultsin terms of causal mechanismsfor the strong diurnal cyclesin surfacewind speedand wind divergence.

2. Data and Analysis Method
The 3-hourly station data of surfacewind speedanddirection, from which the zonal (u) and meridional(v) wind componentsare derived,wereextractedfrom
System(GTS) synoptic
ical Atlantic [Nitta and Esbensen,1974;Pedder,1978; the GlobalTelecommunication
weatherreportsarchivedattheNational Center for AtDewart, 1978] and tropical Pacific [Deser and Smith,Research(NCAR) (DS464.0,http://www.scd
1998], where diurnal variationsin surfacedivergence mosphericThis surfacedata
appear to be out of phase between 5øN and 5øS. Dai ßucar.edu/dss/datasets/dsq64.0.html).
et al. [1999]analyzedthe diurnal variationsof surface set, which covers the time period from July 1976 todivergenceassociatedwith the diurnal and semidiur- April 1997 and has a volume of about 2.5 GB per
nal tides in surface pressurefields over the contiguous year, contains3-hourly (0000, 0300, 0600 CoordinatedUnited States.
UniversalTime (UTC), etc.) surfacewind measureThe diurnal cycle in low-level wind convergencemay ments (typically made at about 10 meters abovethe
be relatedto that in precipitation.Dai et al. [1999]sug- ground) from about 15,000 land and island stations.
gestthat the diurnal cyclein low-levelconvergenceover We also processedthe individual marine reports fromthe United States largely controlsthe diurnal timing in the COADS data set [Woodruffet al., 1993)from Jansummerprecipitation(late afternoonmaxima overthe uary 1976to December1995 (data after 1995werenot
Rockies and the East and nocturnal maxima in the cenreleasedat the time of our analysis).The COADS retral United States). Limited shipobservations
suggests ports include hourly data from a small number of buoys
in the equatorial Pacific, and 3-hourly data from ships
and buoys. We used the COADS reports at the GTS
the northern(30øN-58øN) AtlanticandPacific[Kraus, 3-hourly reporting times.
Surface wind speedsare typically reported in units
1963]. Satelliteand surfaceobservations
alsoshowearlymorning or predawnmaxima in tropical oceanicrainfall of knots (1 knot = 0.514 m/s). Althoughthis preci[Janowiaket al., 1994;Changet al., 1995]anddeepcon- sion is rather low compared to the diurnal amplitudes

that maritime precipitation is significantly more frequent at night, with a maximum around 0300 LST overvection[Gray and Jacobson,
1977;McGarry and Reed, of surfacewinds,the averagingovera largenumber(up1978;Albright,1987;Fu et al., 1990;Hendonand Wood- to 1800)of reportsshouldsubstantiallyreducethe ranberry,1993]. However,a late afternoonmaximumof dom errors in the measurements. While the winds areconvectionwas found over the tropical eastern Atlantic measuredat a relatively stable height abovethe groundat the land and island stations, ships and buoysmeaReedand Jaffe, 1981].
Several complementarymechanismshave been pro- sure winds at various levels. The Tropical Atmosphereposedto explain the diurnal cycle in deep convective Ocean(TAO) and other mooredbuoysmeasurewinds,

rainfall over the tropical oceans. These include day- at -•2 m above the sea surface. Anemometers on fishnight variationsin troposphericradiative coolingrates ing and other small boats can be 2-20 m above the[GrayandJacobsen,
1977;FoltzandGray,1979],cloud- sea surface, while big commercial ships may measureradiation interactions[Randall et al., 1991; Xu and winds at more than 20 m above the sea surface. The
Randall,1995],and daytimeheatingof the seasurface biasesresultingfrom the variablemeasurementheights
Hendonand Woodberry,
1993; Chenand Houze,1997]. in marine surface wind data are discussedby Kent etthat
However,the relative importanceof theseprocesses
has al. [1993]. A preliminary evaluationsuggested
wind is as good a measureof the largenot yet been resolved.Resultsof diurnal variationsof ship-measured
surfacewinds and divergenceshould provide insights scale wind flow as the winds recorded at island stainto the mechanismsfor the diurnal cycle in the oceanic tions [Morrisseyet al., 1988]. Despitetheir differences
rainfall and convection.
in the measuringheight, winds measuredby shipsandIn this study, we document the mean diurnal and buoysmatch fairly well [Quayle, 1984]. No attempts
semidiurnal variations in surfacewind speed, zonal and were made to adjust these biases in this study nor in
meridional wind components, and surface wind diver- COADS.
In contrastto land stations, which are fixed, the ship
genceoverthe globe(50øS-70øN)by analyzing3-hourly
data from approximately 10,000 land and island sta- and many of the buoys are moving, and thus their retions and individual marine reports from the Compre- portsare madeat variablelongitude/latitudelocations.
hensiveOcean-Atmosphere
Data Set (COADS) during We averagedindividual marine reports within each 2.5ø
1976-1997. To the best of our knowledge,this work longitude x 2ø latitude box to obtain a mean value forrepresentsthe first attempt to examinethe mean daily each grid box at each reporting time. Land and fixedvariations of surface winds and wind divergence on a ocean stations were analyzed individually for the amglobal basis. We also discussthe implicationsof our plitudeand phaseof diurnal ($1) and semidiurnal(5'2)

DAI AND DESER: DIURNAL VARIATIONS IN SURFACE WINDS AND DIVERGENCE
Station,

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Figure 1. Stations(big dots)and 2.5ø longitudex 2.0ø latitudeoceanicgrid boxes(smalldots)with 4 or moreyearsof surfacewind data duringthe 1976-1997periodat the 0900 UTC (top)and 1200UTC (bottom) observationtimes. N is the numberof total stationsand oceanicboxes.

harmonicswhich then were gridded onto a 2.5ø longitude x 2ø latitude grid using the natural neighbor in-

terpolationmethod[Watson,1992]. The griddeddataof amplitude and phase were used in the spherical harmonic analysis.
The number of reports of surface winds at 0300,
0900, 1500 and 2100 UTC is generally smaller than at
0000, 0600, 1200, and 1800 UTC for both the GTS and

1800UTC (cf. Figure 1) by usingreports(after daily
meanis removed)10ø to the east (40 min later) and to
the west(40 min earlier). This resultsin 12 reportsper
day (0000, 0040, 0520, 0600, 0640 UTC, etc.) at theseoceanic boxes. Sampling tests with given diurnal andsemidiurnal harmonicsshowedthat the 3-hourly evensampling(i.e., 0000, 0300, 0600 UTC, etc.) and the
supplemented
12 reports/daysamplingare sufficientto

COADS data sets (cf. Figure 1). Over much of the exactly reproduce the original harmonics. Tests alsoSouthernOcean(southof ..•30øS)and partsof the equa- showed that the supplementation of data points does
torial Pacific, surface wind data are available only at not changethe diurnal harmonicssignificantly.
0000, 0600, 1200, and 1800 UTC. While these 6-hourly
The individual observationsat each synoptic hour
data are sufficientfor sampling the diurnal harmonic, were averagedby season(DJF, December-February;
they are not adequatefor capturing the semidiurnal cy- MAM, March-May; JJA, June-August; and SON,cle. Fortunately, the semidiurnal cycle of surfacewinds September-November)for eachyear. For the seasondoes not change rapidly in the longitudinal direction ally averagedvaluesto be usedin our harmonic analyoverthe oceans[Deserand Smith,1998].Thuswe were sis we required at least 7 of the 8 synoptic hours havable to supplementthe 6-hourly data at those oceanic ing data for the land stations and 11 of the 12 supgrid boxesthat have data only at 0000, 0600, 1200, and plemented synoptic hours having data for the oceanic

31,112DAI AND DESER: DIURNAL

VARIATIONSIN SURFACE WINDS AND DIVERGENCE

grid boxes. About 10,000 GTS stationsand 5000-7000 Yo is the daily mean value, $• and $2 are the diurnal(2.5ø x 2.0ø) oceanicgrid boxesmeet this requirement and semidiurnalharmonics,respectively,An is the amwith four or more years of data during the 1976-1997 plitude(notethat the peak-to-peakamplitudeis 2An),
period. GTS stations and oceanic grid boxes with less an is the phase,andt• ismeanlocalsolartime (LST) exthan four years of data were excluded in our analysis. pressedin degrees
or radians(i.e., t• - 2•rh/24, wheret•
The stations and oceanicgrid boxeswith sufficientdata is LST in hours).The residualin equation(1) contains
are shownin Figure I for the 0900 (similar for 0300, the higher-orderharmonicsof the daily cyclewhichare
1500,and 2100) UTC and 1200(similarfor 0000,0600, not resolved by the 3-hourly sampling. Since previ1800) UTC observation
times. It can be seenthat the ous analysesof hourly data showedthat these highstation and marine observationsprovide good coverage frequency harmonics are much smaller than $1 and $2
overmuchof the globeexceptthe Arctic Ocean,Green- overthe tropicalPacificOcean[DeserandSmith,1998],
land, the Antarctic,and the southernPacific(southof the error inducedby the aliasingof the high-frequency50øS)wherethe dataaresparse.We shallfocuson the harmonics
is likelyto be smallin equations(1) and (2).
region from 50øSto 70øN.

Surfacedivergence(D = V ß V, whereV is horizontal wind vector)wascomputed(on sigmasurface)using finite differencingfrom the observed2.5ø longitude
x 2.0ø latitude gridded winds. Tests showedthat thefinite-differencemethod yielded divergencefieldssimilar
to those derived using spectral methods.
The mean 3-hourly surfacewind and divergencedata(Y) at eachlocationmay be representedbyY(t') - Yoq-S•(t') q-S2(t') q-residual(1)

A few examplesof the fitted diurnal and semidiurnal oscillationsof wind speedare shownin Figure 2. Itshould be emphasizedthat while the diurnal and semidiurnal harmonics can represent the diurnal variations
reasonablywell over majority of the locations,they fit
the data poorly at somestationsand oceanboxes,espe-

cially whenthe numberof observations
is smalland/or
the diurnal variationsare weak. We shall use the percentageof the meandaily varianceexplainedby the two
harmonicsas a measureof the goodnessof fit.
The harmoniccoefficientsan and bnare then interpolated onto a 2.5ø longitudex 2ø latitude grid usingthe

naturalneighborinterpolationmethod[Watson,1992].
$n(t') = An sin (nt' + an)= an cos(nt') + bnsin (nt') At each latitude, the gridded coefficientsare then ex(2)panded using trigonometricseriesof the longitudesothat the relative importance of various zonal modes

wheren=l, 2 (for diurnaland semidiurnaloscillations), with differentwavenumberscanbe examined(seeHauWestern U.S. (108.23W, 36.75N)

Ctrl. North Atlantic (33.75W, 47.00N)

---'+

30

Tropical Africa (35.00E, 1.02N)

0

12

18

Local Time (hr)

24

0

+ '--

6

12

18

24

30

Midla[. S. America (67.45W, 45.7•

30

0

0

12

18

24

30

Local Time (hr)Figure 2. Observedmean DJF surfacewind speed(pluses)togetherwith the fitted diurnal(short-dashedcurve)andsemidiurnal(long-dashedcurve)harmonics
at fourlocations.The solid
curve is the sum of the two harmonics.

DAI AND DESER: DIURNAL VARIATIONS IN SURFACE WINDS AND DIVERGENCE

7o

0000 UTC, WindSpeed(m/s)

mode predominates. The dominant wave mode will become more clear in the zonal harmonic analysis of thedata. Superposedon top of the migrating global mode,
there are various regional features. For example, diurnal wind variations are larger over land than ocean andstrongest over high terrain such as the Rocky Mountains, the Andes, and the Tibetan Plateau.

40•

31,113

lO

-g0

3.

Results

-50

- 180

- lg0

-60

0

60

120

180

3.1. Daily Variations

0600 UTC, Wind Speed (m/s)

Figure 4 shows the spatial distributions of the amplitudes of the diurnal and semidiurnal harmonics inannual mean wind speed, zonal, and meridional windcomponentsaveragedover the 1976-1997 period. It canbe seen that the amplitudes of $1 are generally larger

-0.4,
40

•

of Surface Winds•oover land (especiallyover high terrain suchas the Tibetan Plateau and the westernUnited States)than over

-g0

-180

-120

-60

0

60

120

•80

1200 UTC, Wind Speed (m/s)

7040

ocean. It should be pointed out that large values overcontinental boundaries were interpolated onto adjacentoceansby the contouringpackage(samein Figure 5).
The amplitude of $1 for wind speed ranges from 0.6
to over 1.0 m/s over land areas. For the zonal andmeridional wind components,the amplitudes of $x are..

about 0.3-0.4 m/s over the oceansand 0.5-0.7 m/sover middle•

toand low-latitude

land areas. These are con-siderably smaller than those of wind speed,suggestingthat diurnal variations in wind directions are relatively-20

small. Analysesof wind directiondata (not shown)indicate that the diurnal amplitude of wind direction is
-t80

70

-tZ0

-60

0

60

- ••

.

--

-

40

•

10 ø over the oceans and 10ø-40 ø over land areas(largerin summerand overhigh terrains).

1800 .UTC, Wind Speed (m/s)

- -•

about

tZ0

The semidiurnal

harmonics

in surface winds have anamplitudeof about 0.2-0.3 m/s, whichis substantiallysmallerthan that of the diurnal harmonics(Figure 4).
This is in contrast to surfacepressurewhosesemidiurnal
oscillation is stronger than or comparableto the diurnall0

oscillationovermostof the globe[Dai and Wang,1999].
There is also no significant land-sea differencein theamplitude of $2 (consistentwith the surfacepressure-50

field), suggestingthat surfaceforcingof the semidiurnal

'
-lB0

-120

-60

0

Longitude

60

120

lB0

oscillationis small.

Seasonalvariations in the amplitudesof surfacewinds

Figure 3. Annual mean surfacewind speedanomalies are significantonly for $• and only over the land areas(m/s) relative to the daily mean at the 6-hourlyob- outside the tropics. Figure 5 shows that the ampliservation times derived by averaging and gridclingthe tudes of $1 of surface wind speed over North America
station a•.d marine data of the 197•-1997 period. Neg- and Eurasia vary from about 0.3-0.5 m/s in DJF to
ative valuesare hatched.Contourintervalsare 0.2 m/s. 0.7-1.1m/s in JJA. Similarseasonalvariationsare also
Plots for seasonal means are similar.
seenfor the zonaland meridionalwind components(notshown).
Figure 6 showsthe local solar time Tmax when therwitz and Cowley[1973]and Dai and Wang[1999]for diurnal harmonic peaks for annual mean wind speed.
The diurnal harmonic of wind speedpeaksaround early
detailsof the method).
The raw data of annualmeansurfacewind speed(ex- afternoon(1400 LST) overmostland areasand slightly
pressedas departuresfrom the daily mean) are shown earlier (1200-1400 LST) over the oceans. Tmax overin Figure 3 for the 6-hourly observationtimes. It can the Pacific off Mexico is in late afternoon (1600-1800
be seenthat the westwardpropagatingwave number I LST). This feature seemsto be robust sincethere are31,114DAIAND

DESER:DIURNAL

VARIATIONSS1 of Wind Speed (0.1m/s),

70

• IY

40

" • • ..

•",o ,IN SURFACEAND

DIVERGENCESZ of Wind Speed (O.lm/s),

ANN

' -•

WINDS

I -

0 •. I Y-

o"

ANN

.
5

-20

-180

-120

-60

0

60

120

S1 of Zonal Wind (0.1m/s),

-•-'

I

40 --

'

lO

•

-•80

180 -180

ANN

60

-60

'-:-"

0

60

•20

180 -180

S1 of Meridional Wind (O.lm/s), ANN

40

180

ANN

t •.•.• 1 .-

40 --

20

lO

-60

120

SZ of Zonal Wind (0.1m/s),

3
.

'
-120

-120

2.0,•
-•20

-60

0

60

•20

180

S2 of Meridional Wind (O.lm/s),

ANN

40

J
-•80

-•20

-60

0

60

•20

Longitude•80-•80

-•20

-60

0

60

•20

•80

LongiLude

Figure 4. Amplitude (in units of 0.1m/s) of the diurnal (S•, left column)and semidiurnal(S2,right column)harmonicsof meanannualsurfacewind speed(top panels),zonal (middlepanels),
and meridional(bottom panels)wind components.The amplitudeshavebeensmoothedin space
to reducethe small-scalenoise.Contourintervalsare 0.1 m/s for S• and 0.05 m/s for S2. Values
over0.7 m/s are hatched.

sufficientship reportsfrom this area (cf. Figure 1). daily varianceover land areas and only about half ofCompared with surfacewind speed, the phase of the the daily varianceover the oceans.This partly reflects
diurnalharmonicsof the wind components(not shown) the fact that the sampling errors are larger over ocean
exhibitsmore regionalvariations,with Tmax generally than over land.
peaking around 1200 LST. The seasonaldifferencesof
Tmax are generally small.

3.2. Daily Variations
Divergenceof Surface Wind

A convenientmeasureof the relativestrengthsof the
To provide a context for diurnal variations in surface
diurnal and semidiurnalharmonicsis the percentageof
the meandaily varianceaccountedfor (Figure7). Over wind divergence,we showthe daily mean surfacewind
mostland areasthe diurnal (semidiurnal)harmonicex- and wind divergencefieldsfor the annualmean,winterplains50-70% (10-20%) of the mean daily variance; and summer(Figure 8). It can be seenthat the Interwhile over the oceans,the percent variance explained is tropicalConvergence
Zone (ITCZ) is well definedover
only slightlylargerfor the diurnal harmonic(30-40%) the Pacificand Atlantic by the bandsof maximum conthan for the semidiurnalharmonic (15-25%). To- vergence(1.5-2.0 x 10-6 s-1 for annualand DJF andgether, the two harmonicsexplain 60-90% of the mean 2-3 x 10-6 s-1 for JJA). OvertheIndianOcean,the

DAI AND DESER: DIURNAL
S1

VARIATIONSofIN SURFACE WINDS AND DIVERGENCE

Wind Speed(0.im/s),

31,115

DJF

40

5

-20

_ot
-180

-60

-120

0

"'

60

S1 of Wind Speed (0.1m/s),

x

4

120

1130

120

180

JJA

"

4O

-20

-180

-120

-60

0

60

Longitmde

Figure5. Same
asFigure
4 (S1only)butformeanDJF(top)andJJA(bottom)
surfacewind
speed.

tropical convergencezone spreadsover a much wider genceoccursin the westernand easternparts of thetendsto occurin the midregion. The subtropicaldivergencecentersover the continent,while convergenceoceans are close to the western boundaries of the condle. This pattern existsin all the seasonsalthough thetinents and to the east of the centers of the subtropi- convergencein the middleis muchreducedduringDJF
cal pressurehighs[e.g.,Shea,1986].Strongdivergence and extendsfarther to the westin JJA (the pattern in(1.5-2.5 x 10-6 s-1) alsooccursduringDJF overthe springand autumn is very similarto that for the aneastern coasts of Asia and North America where winds
nualmean).The land (oceanic)areastendto havemore(divergence)in summer(alsoseeFigure10)
comingoff the continentsfrom the northwestaccelerate convergenceover the oceans. Over North America, strong diver70

TITlaXofs1of Wind Speed,ANN

while the oppositeis true in winter. This is no surprise
becausethe boundary layer air over land is warmer than
the air over oceanduring summer,which leadsto sur-facewindconvergenceand risingmotionoverland (viceversain winter). We shall showthat this mechanism
may alsoexplainthe phasedifferencein the diurnal cycle of surfacedivergencebetweenland and ocean.
Figure 9 showsthe amplitudesof the diurnal and40

semidiurnal harmonics of the annual mean, D JF and.

JJA divergencecalculatedfrom the observedwinds.
The diurnal amplitude of the divergencevariesfrom 2.0

-20

-50
-180

x 10-7 s-1 over the oceans to about 3.0-5.5 x 10-7

I
-120

-60

0

60

120

180

s-1 overland areas. During borealsummer,maximum

values occur over the western coast of North America,
northern
Africa, and the Middle East. During austral
Figure 6. Localsolartime Tmax (hours)whenthe disummer,
the diurnal amplitude is largest (4.0-5.5 x
urnalharmonic($1) of annualmeansurfacewindspeed
10-7 s-1) over Australia and southern Africa. Therepeaks.
Longitude

31,116DAI AND DESER: DIURNAL

VARIATIONSIN SURFACE WINDS AND DIVERGENCE% Var. Explainedby S1, ANl•V

-•

10

10

'

•

•

-•0

-•0

-1•0

1

'

0

ß

-•80

%Var. Explainedby S2, ANNV

-60

0

60

•0

•80 -•80

Longitude

ß •5
--1•0

•0 i
-60

0

60

•0

•80

Longitude

Figure 7. Percentageof the meandailyvarianceexplainedby the diurnal(left) andsemidiurnal(right) harmonics
of the annualmeansurfacewind speed.Plotsfor the zonaland meridional
wind componentsare similar to thoseof wind speed.

are, however, no high values of the diurnal amplitude
over the Bolivian highlandsin DJF, which is likely due

to the poordata samplingoverthe region(cf. Figure 1).
The semidiurnalamplitude of the divergenceis about

central United States (Plate 1). This alternatingfeature, which is seenin all the seasons,resemblesthe pattern in the mean divergencefield (Figure 8). Figure 11(top) showsthe meandailymarchof surfacedivergence

1.0-2.0

x 10-7 s-1 over most of the low and middle lat-averaged over the western, central, and eastern United

itudes and doesnot vary much from winter to summer.

Statesduringsummer.The diurnalamplitudeis largest

The valuesat the higher latitudes (especiallyfor $2)
are likely to have large uncertainties due to the limited(9-12 x 10-7 s-1) in the westernUnited Statesandsmallest(-•2 x 10-7 s-1) in the centralUnitedStates.

number

The diurnal cycledominatesover the semidiurnalcycleof observations

there.

Plate 1 showsthe Tmax for annual mean surface winddivergence.The estimatedphasemay havelarge uncertainties(+1.5 hours),especiallyoverthe oceans.Nevertheless,somelarge-scalepatterns are evident for many
regionsin Plate 1. It can be seenthat the diurnal cycleof divergencetends to be approximately out of phasefor many adjacent regions. For example, most land ar-in the westernand easternUnitedStates. Figure11(bottom) showsthe pattern of 1800 LST minus0600

LST differencesof surfacewindsand divergence.
At
1800LST relativeto 0600LST, strongwinds(up to

2 m s-1) fromthePacific(theAtlantic)andtheGulfof Mexicoconverge
overthe western(eastern)
UnitedStates,whilethe windanomalies
are relativelyweakeas (exceptfor the innermostparts of the continents) over the central United States.
tend to havemaximumdivergence(or minimumconverThe phasemap (Plate 1) of the diurnalharmonicof
gence)arounddawn(0600-0800 LST), whilethe oceans surface
winddivergenceshows
a zonallycoherent(aladjacentto the continentshave maximum divergencein thoughnoisy)patternwithinabout10ø of the equator
the evening(1700-1900 LST). This pattern is evident over the Pacificand Atlantic, with maximumconverover almost all the continental boundaries, especially gence(divergence)
north(south)of theequatoraroundaround Australia, North America, eastern Asia, the Indian subcontinent, and the Mediterranean Sea. Even
though soxneinnermost parts of Eurasia and North
America do not follow this land-seapattern, simpleav-

0900-1200LST. Accordingly,
wezonallyaveraged
thediurnal divergenceanomaliesacrossthe eastern andcentralPacific(180ø-85øW)andAtlantic(55øW- 15øE).
Figure12 showsthe zonallyaveraged
daily meanwind

eraging(weightedby area) of the JJA divergencedata divergenceprofile,togetherwith the zonallyaveraged
divergenceanomalyprofilearound 1030 LST for the Pa-

over the land and oceanic areas between 25øN and 60øNresults in two diurnal cycleswith near oppositephases cific(top)andAtlantic(bottom)domains
in eachsea-(Figure 10). Figure 10 alsoshowsthat the diurnal cycle son.Notethat thescaleforthedailymeandivergenceisdominatesover the semidiurnalcycle over both the land
and the oceanic areas at these northernlatitudes.

Theabove land-sea phase differenceis most pronouncedinsummerannualbut is also evident

in other seasons and in themean.

Over the UnitedStates the diurnal

harmonic of sur-in 10-6 s-1, whilethescale
forthe1030LSTdivergenceanomalyis in 10-7 s-1. With the exceptionof the Pacificin JJAthelatitudinalstructureof the divergenceanomalyat 1030LST is generallysimilarto the profileof the meandaily divergence,indicativeof an enhance-ment(-•10%)of thelocalAtlanticandPacificHadleyface divergencepeaks around 0600 LST in the western cellsat 1030LST relativeto the daily mean. In JJA
and eastern UnitedStates and around 2000 LST in theoverthe Pacifictheprofileof anomalous
divergenceatDAI AND DESER: DIURNAL VARIATIONS IN SURFACE WINDS AND DIVERGENCE

60

31,117

SurfaceWinds(Max.length=S.6m/s)
and Divergence(E-6/s),-'•--•- •-"• - --• - ....," • .... ,.;•,'-,.•
•:'•
.....
r½.-.•.•:,•"•.'-"•'; '.'•.....

...............
-......
'-.,..•,
ß,-":•.........
:g-.:.-.2:..2-:?.'.:•g.:-:.;::[•.4•.•-•.'•/,.2•.,.
• .'...............
'.•'"L•......
: ½" ....
,.,...,:,•.,,.•:.•:.•.'.........
'"'.........
'"'!•........
,'>,'.-'*•'•2•'•• - •" ,•;.
ß•.............
'"'-'•'.....
..r%..::'

'i[•.

.'

..

•

- t ....

'•'•,'•:E."•i•:••'.•';"•"'•.t•.>:

/

! •

ß

'

•-•--

ANN

•-if

ß '"'}•i½•',--•.---•'-•$.

.

• "

I

• •• •

SurfaceWinds(Max-.length=8.8m/s)• •--andDivergence(E-6/s),........... ' :•........... -•,........
--::•-'• • ........•':x' - '• '•'•-•-'•'•":• ....' ............ ...-. ..--' ..-....' •i....
'-•• ' ..'.'"'rz•.;•-.....
'•'"' . •- -- •' '•6O

ß' •' •'--

45

o

-15

•1

•"••2-1.•••'•'•

•

'

'

-30

SurfaceWinds(Max.length-14.6 m/s) and Divergence(E-6/s),•2•4; •=•*, " ' ß - ........ • '",'•" ',6O'............

i-•- • .....

•- "•-_IT•" ...... '": '- '--•'"•

•

•

,

JJA
* •'• '• • "•

45

3O

•

•

.

,

-

-15

-3O

0

60

120

180

240

3OO

Longitude

Figure8. Observed(1976-1997)
meansurface
winds(arrows)
andwinddivergence(10-6 s-I,

contours;
dashedcontours
indicateconvergence)calculatedfromthe windsfor the annualmean(top), DJF (middle),and JJA (bottom).

36O31,118DAI AND DESER: DIURNAL VARIATIONS IN SURFACE WINDS AND DIVERGENCE

S1 of Divergence (1E-7/s),

'

•,, '

,, '

'

.. '

-20
-120

-60

0

60

120

180

-180

of

-120

-•0

0

•0

1•0

180

•

1.0

•0

1•0

180

120

180

of

40

-180

ANN

•..

10

•-20
-180

S2 of Divergence (1E-7/s),

..

'-•

10

ANN

•.•-

-1•0

•
•.5•

..

-•0

0

•0

ß0

40

120

180

-180

of

-120

-•0

0

se of

--

'-

1.5

.

-180

-120

-60

0

60

120

Longitude180•-180 -120

-60

0

60

Longitude

Figure 9. SameasFigure4 but for annummean,DJF andJJAsurface
divergence(10-? s-1)

calculated

from the observed winds.

1030 LST is displacedsouthwardby about 5ø of latitude genceoverthe equatorial
AtlanticITCZ (3.5øS-7.5øN)relative to the profileof daily meandivergence,and the peaksaround0100LST, whichis out of phasewith thestrongest diurnal cycles are over the transition zones, regions
to thenorth(7.5øN-19.5øN,Tmax_•1300
LST)
not in the ITCZ.

We do not understandthe reasons

andsouth(3.5øS-18.5øS,Tmax_•1400
LST). Compared

for this displacement. The diurnal divergenceresults with the PacificITCZ, wheresurfacedivergencepeaks
broadlysupportthe findingsof Deserand Smith[1998] around 2230 LST, Tmax over the Atlantic ITCZ is
for the tropical Pacific,althoughDeserand Smith found 2-3 hours later. Our Atlantic ITCZ results are conconsiderablylarger amplitudes over the equatorial Pa- sistentwith measurements
madeduringthe GARP Atcific. This may be partly due to the higher temporal lantic TropicalExperiment(GATE, about 5øN-10øN
sampling(hourlydata) and lesssmoothingin time and and 27øW-20øW):Pedder[1978]reportsan amplitude
spacein Deser and Smith [1998]than in the present of 0.2 x 10-6 s-1 anda Tmaxof 0310LST, whichis
study.
Fiõure 13 provides more detail on the diurnal varia-

slightlylaterthanthat shownin PlateI andFigure13.

The alternatingpattern of divergenceseenin thetion of the localHadley cell in the Atlantic duringD JF. tropics also exists at higher latitudes over the central
The surfacedivergencedata confirm the phasepattern Pacific Ocean. Figure 14 showsthe mean annual diurshown in Plate 1. Figure 13 showsthat surfacediver- nal cycleof surfacewind divergencezonallyaveraged

DAI AND DESER: DIURNAL VARIATIONS IN SURFACE WINDS AND DIVERGENCE

Tmax of S1 of Divergence,

60

.....

30•

.

.

.,

'-•

ANN

.

•.•,,..

•

,'

-15 •

'.'

-3•0E

31,119

• 90E

150E

,.

150W

90W

30W

30E

,,

0

2

4

6

8

10

12

14

16

18

20

22

24

Plate 1. SameasFigure6 but for surface
winddivergencein colors.
acrossthe centralPacific(160øE- 130øW)asa function evidentalthoughstrongcoastaleffectsmakeit lesspronouncedthan overthe centralPacificOcean(Plate 1).

of latitude. It can be seen that the diurnal harmonicpeaksaround1800-2000LST from -•25øNto 42øN,
whichis out of phasewith the region10ø-25øN.Across 3.3. Zonal Wave Components
The diurnal and semidiurnal oscillations can be exthe equatorthe diurnalcycleis approximately
out ofphase
between0ø-10øN(Tmax=2100-2300
LST) and pandedintovariouszonalwavecomponents
at eachlat0o-12øS(Tmax=0900-1200LST). The phasetransi- itude.Figure15shows
theglobally(from50øSto 70øN)tion from 25øNto 12øSsuggeststhat the diurnalcycle averagedamplitudesfor the important wave compo-

of surface
divergence
overthe ITCZ (0ø-10øN)is cou- nents of the annual diurnal and semidiurnal harmonics.

pledwithboththezones
to thenorth(10ø-25øN)and The wavemodeswith positive(negative)wavenumbers
south(0ø-12øS).Figure14 alsoshowsthat the mean (s) traveltowardthe west(east),whilethe modewith
diurnalamplitude
increases
fromabout0.5x 10-7 s-• s=0 is a standingwave. In particular,the wavewith
aroundthe equatorto 1.0-1.5 x 10-7 s-1 in the sub- s=l for S1 (s = 2 for Sa) travelswestward
at the speed
tropicsin the Pacific. Overthe SouthPacific,Tmax of the mean Sun. When expressedin the local time,
is approximately
7 hoursearlierover12øS-28øS
than this waveis independentof the longitudeand is calledover 0ø-12øS. The amplitudeover 12øS-28øSis also the migratingtide, whereasthe otherwavesdepends
about twice that over the other latitudinal zones. Tests

on both the local time and the longitudeand are calledshowedthat 'the land stations within 12øS-28øS (cf.

nonmigratingtides[HaurwitzandCowley,
1973].
Figure15 showsthat both the diurnaland semidi-

Figure1) donotchange
thediurnalpatternin thiszone.

Overthe Atlantic Ocean,the alternatingpatternis also urnal oscillationsare dominatedby the migrating tides
whoseglobalmeanamplitudes
are about5 timesthose-u.a•, , . , , . , , , , , , , ' , ' ' ]of the nonmigratingtides. Contraryto surfacepressure,whoseglobalmeanamplitudeof the semidiurnal
oscillation
is greaterthan that of the diurnaloscilla-•-- 1.8tion[Haurwitz
andCowley,
1973;
DaiandWang,1999],• .25-60NLand(solidline)vsOcean(dashedline),JJAB.
0;'" '"X6.8 ,theglobal
meanamplitudes
ofthediurnaloscillationofsurfacewindsand divergenceare largerthan thoseof

ß

'

',

/
55

-- ' 0':% the semidiurnaloscillation,especiallyfor their migrat-

ingcomponents
andfor surface
windspeedanddivergence.
Thisisconsistent
withthepatterns
shownin thex,,--[-......
6

i

lg

lB

Hour (LST)g4

raw data (Figure3).
,- :.-,tø Globallyaveraged,theamplitudesofthetwoharmon,---..:......
icsof the zonalwind componentare only slightlylarger
6

Figure10. Meansurface
winddivergence(10-7 s-•)than thoseof the respectiveharmonicsof the meridionalwind component.The diurnalharmonicof sur-duringJJAaveragedat eachlocaltimeoverthe land facewindspeedis abouttwiceasstrongasthoseof the(solidcurve,leftordinate,
negativevaluesindica[econ-whilethe semidiurnal
harmonicofvergence)
andoceanic(dashedcurve,rightordinate)wind components,
areas from 25øN to 60øN.

surfacewind speedis considerably
weakerthan those31,120DAI AND DESER:DIURNAL

VARIATIONSIN SURFACE

WINDS

AND DIVERGENCE
Pacific (85W- 180W)

30

1,1•est(solid).
C•trl•.(short-dashed).
East(long-dashed)
i.S.
I

[

I

[

I

'. I

]/'

JJA

•,

•

10

•

o

!

'

I

]

I

]/[

I

]

I

zL*LL: _?_"*

[.I

'

'"''"

•$ON

o.o
-20

o

•

-30

-4

-7.0

-2

0

2

-•

0

•

-•

0

•

-•

0

•

4

-2

0

2

4

Divergence

301

-14.0
0

6

12

18

24

6

Atlantic (55W- 10E)IDJt I [ I • /[ I i i x I] l, •-22.--. 2012• •oHour (LST)1800 - 0600 LST Wind & Divergence Difference. JJA.......
:..,•:,-..•.•:..•:..-.:::..-.:::•:•:.:•.,•,
ß•:. ::,::.•.........• •::•:?•:•..•.•::.•::;:•.•.,,.•..r•.•..:•. •,•::•:•:::••• • • • .......
• .............
•:•.......
:;•:•

ß.•........:•**.,•..................

.' .i.•?•

-20

-30
-4

-2

0

2

-2

0

2

0

Divergence

Figure 12. Mean seasonal
surfacedivergence(10-•s-•, dashedcurve)and 1030LST divergenceanomaly(reconstructedfrom the diurnalharmonic)relativeto

thedailymean(10-• s-•, solidcurve)zonallyaveraged
-140

-110

-80

-50

overthe tropicalPacificfrom 85øWto 180øW(top) andthe Atlantic from 55øWto 15øE(bottom).

LongiLude (deg.)

Figure 11.

(top) Mean surface wind divergenceanomalies(10-• s-•) duringJJA averagedat eachlocal time overthe western(westof 100øW,solidcurve),
central (100ø-85øW, short-dashedcurve), and eastern (east of 85øW, long-dashedcurve) United States.
(bottom) Mean 1800 LST minus 0600 LST differenceof surfacewinds (maximumarrow length - 2.0 m/s)

and divergence(rangingfrom -1.0 to +0.5 x 10-•s-i, shaded(white) areashavenegative(positive)di-ALlanLic,

1.85

0.20

'

1.65

,,

(not shown).
4.

Discussion

air temperatures(Figure 16). Associatedwith higher
surfacetemperaturesduring the day, sensibleheat from

4.'
,/.

'•-X

"

'

,' ,
\

,

-2.•o

41,/

\

• " "/, iA[ [ I [

[

[

-

[

[ I [

A 40W-20W
A 3.5S-7.5N• •,, x ,'

'-. /

x ,.'

". /

o.oo

-o.2o

30W-5E

2.20
2.00

0.00
-0.20

-3.oo

The diurnalharmonicof wind speed(Figure 6) is approximately in phase with the diurnal cycle of surface

"

" "/

1.45

vergence)for JJA overNorth America.

of the wind components. Seasonal variations of the
global mean amplitudes of the wave modes are small

DJF

2.05 [ I I I ' •k • I ' I [ I ] j• ' I ] [ 0.40

0.20-' ,t.
_ -' '... /,\.." "..

\

1.80

0.00

-0.20

the surfacepeaksin the afternoon, resulting in increased
t.6o [ I • I t I t I • I • , • I [ I [ -0.40
18
0
6
12
18
24
6
12
18
24
instability in the planetary boundary layer. This allows
Time (LST)
more downward turbulent mixing of momentum during
the day than at night and contributesto the maximum Figure 13. Diurnal evolution of the mean DJF diverwind speedin the afternoon. On the other hand, sen- gence(thicksolidcurve)anddivergenceanomalies(relby the fitted diursible heating from the surfacecan also generatespatial ative to the daily mean)represented
curve)andsemidiurnal(short-dashedgradients in surface pressurefields and thus stronger nal (long-dashedaveraged
overthreelatitudinalbands
winds during the day. The large land-seadifferencein curve)harmonics
in the tropical Atlantic. The thin solid curve is the sum
the amplitudes of S•, which is also seen in the diur_.

!

of the two harmonics.
The averaging
wasdoneat eachnal harmonicof surfacepressure[Dai and Wang,1999], UTC time, but the resultsare plottedagainstthe lois consistentwith the stronger diurnal cycle in surface cal time at the centerof the zonesas definedby the
turbulent mixing and larger sensibleheating over land longitudes
andlatitudesindicatedon eachpanel.

DAI AND DESER: DIURNAL VARIATIONS IN SURFACE WINDS AND DIVERGENCE
Pacific ( 160E- 130W),

ANN

describedaboveis a primary causefor the diurnal phasepattern of surfacedivergenceoverthe westernand eastern United States. Dai et al. [1999]suggestthat thisdiurnal cycleof divergencecouldbe a major factor controlling the diurnal timing of summer convectiveprecipitation over the United States.
The diurnal cycleof surfaceconvergenceis generally

,. o.
0.o

•////•• • •• •.o••consistentwith the diurnal cycle of convective precipitation which tends to have a nighttime or an earlyo.s ••i•Z•••••-••o•//•/•
0

•
5

10

15

20

31,12125

morningmaximumovermanyoceanicregions[Kraus,
1961; Janowiaket al., 1994; Changet al., 1995]anda late afternoonmaximumover land areas[Dai et al.,1999]. However,Plate I showsthat there are excep-

tions to this generaltendency. For example, surface
wind convergenceoccursfrom about midnight to 1000
Figure 14. Annual mean diurnal cycle of the zon- LST overthe centralUnitedStates(Figure11) (whichis
allyaveraged
surface
winddivergence(10-? s-i) acrossthe Pacific (160øE-130øW) as a functionof latitude roughlyconsistentwith the early morningprecipitation
based on the estimated diurnal harmonic of the 1976- maximumthere) and in the late afternoonand evening
1995period. The averagingwasdoneat eachlocal time. (1800-2100LST) overthe centralPacificfrom 10øNto
Positive(negative,hatched)valuesindicatedivergence25øN (Figure 14).
(convergence)
relativeto the daily mean.
Gray and colleagues[Gray and Jacobsen,
1977;Foltz
and Gray, 1979]have suggested
that low-leveldivergence(convergence)
shouldbe a maximumat the endof the nightoverrelativelyclear-sky(cloudy)areasdue
than over ocean, although the same mechanismsmay to the fact that atmosphericradiative coolingat night iswork over ocean as well.
largeroverclear-skythan cloudyareas.The phasepatThe land-oceanphasedifferencesin the diurnal cycle tern of the diurnal harmonic of surfacedivergenceoverof surfacewind divergence(Plate I and Figure 10) is the central Pacific from 10øN-25øN to 25øN-45øN andanalogousto the land-oceandifferencesin mean summer overthe equatorialITCZ regionis correlatedwith cloud
and winter divergencefields (cf. section3.2 and Fig- coverover these regions(Figure 17) in a manner thature 8). During the daytime, land areaswarm up more appears to broadly support the radiative cooling hyrapidly than the oceans,which resultsin much higher pothesis. However, the afternoon maximum in surface
surfacetemperatures over land than over ocean in sum- wind convergence
overthe southernPacificconvergencemer and the smallest land-ocean temperature differ- zone(.-•15ø-27øS)seemsto be inconsistent
with the raencein winterby the afternoon(1400-1600 LST) (Fig- diative coolingmechanism. Further study is neededto
ure 16). During the night, radiativecoolingdecreases understandthe diurnal phasedifferencesin surfacewind
surfacetemperaturesfaster over land than over ocean, divergencebetween the North and the South Pacific.
Local Time (hr)which results in substantially lower temperatures overland than over ocean in summer and the largest landocean temperature difference in winter around dawn

(Figure 16). Associatedwith this diurnal cyclein theland-oceandifferenceof surfaceair temperatures,which
is primarily due to land temperature changes,there
should be a continental-scalediurnal atmospheric cir-Marinestratuscloud cover off the westernbound-

aries of the subtropical Americas and Africa has beenfoundto exhibit substantialdiurnal variations(amplitude - 3-10%, strongestin summer), with maxima
around0300-0500 LST [Warren et al., 1988; Rozendaal et al., 1995].It hasbeensuggested
that absorptionof solar radiation within the clouds(which causesen-

culation(superposedon top of the daily mean circula- trainment of dry air from abovethe boundary layer thattion) in whichsurfaceair risesand converges
overland dissipates
the stratusduringthe day) and advectionof
areas and sinks and divergesover the oceansin the af- moist boundary layer air from an upwind region more
ternoon and early evening,and the oppositein the earlymorning. The land-oceanphasedifferencein the diurnal
cycle of surface divergenceis consistentwith the existence of such a solar-driven

diurnalcirculation

betweenthe continents and the oceanswith a mean amplitudeof .-•2x 10-7 s-• (Figure10).

favorable to cloud formation could be responsiblefor
the early morning maximum of marine stratus clouds[Rozendaalet al., 1995].Plate I showsthat the diurnal
maximumof convergence(divergence)is in phasewith
the early morningmaximum(late afternoonminimum)of stratus cloudsover the aboveregions. Since surface
Figure 11 showsthat much of the air mass associ- convergence(witha diurnalamplitudeof .-•3-4 x 10-7
ated with the diurnal cycle of surfacewind divergence s-i) enhances
the moisturefluxintothe marineboundover the western and eastern United States comes fromary layer, it may favor stratus cloud formation. The
the adjacentoceans(includingthe Gulf of Mexico)with relative importance of diurnal convergenceversusdiura small contribution
from the central United States.
nal cloud top heating for the observeddiurnal cycle ofThis suggeststhat the diurnal land-ocean circulation marine stratus clouds remains to be assessed.

31,122DAI AND DESER:DIURNAL VARIATIONSIN SURFACEWINDS AND DIVERGENCE
0.5

S1, ANN

0.4

Zonal

Wind

0.4

0.3

0.3

0.2

0.2

0.1

0.1

S2, ANN

Zonal

Wind

4

8

0.0

0.0

1

-- S1, ANN

3

5

7

9

11

MerldionalWind

0.4

--

-6

-4

-2

0

S2, ANN

2

6

10

12

MeridionalWind ---

0.3

--

9

-8

--

-7

-5

-3

-1

1

3

5

7

9

0.2

11

-8

-6

-4

-2

0

2

4

6

8

10

12

0.5
0.4

eed--• .4

_

0.3

.3

0.2

.2

S2,ANNWindSpeed---

0.1
0.0

-9

-7

-5

-3

-1

1

3

5

7

9

-8

11

-6

-4

-2

0

2

4

6

8

10

12

10

12

0.3

0.2

S1, ANN

Divergence

S2, ANN

-

__

0.0
-9

-7

-5

-3

-1

Wave

1

Number

3

5

7

9

s

Divergence

0.2

__

11

0.1
-8

-6

-4

-2

Wave

0

4

6

Number2s

8

Figure 15. Globally(50øS-70øN)
averaged
amplitudes
ofthediurnal(leftcolumn)
andsemidiurnal(rightcolumn)
cyclesat various
zonalwavenumbers
formeanannualsurface
windcomponents,wind speed,and divergence.

5. Summary
Diurnaland semidiurnal

variations in surface winds

wind speedand the wind componentsin all the seasons. The diurnal cycle accountsfor 50-70% of theand wind divergence
over'the globe(50øS-70øN)have dailyvarianceoverlandand30-40% of the dailyvaribeendocumentedusing3-hourlywind observations
from anceoverocean,while the semidiurnalcycleexplains
about 15-25% of the daily varianceovermost of theapproximately10,000 stationsand COADS marine re-ports during 1976-1997. The main results are summa- globe.
The diurnal harmonic of surface wind speed peaks
rized as follows:

Surfacewinds have a strong diurnal cycle over land aroundearly afternoon(1400 LST) overmostland arareas(strongestover high terrain suchas the Tibetan eas and around 1200-1400 LST over the oceans,which
Plateau, the westernUnited States,and the Andesand is approximatelyin phasewith the diurnalcycleof surthat the increasedin summer)with an amplitudeof 0.6-1.1 m/s for wind faceair temperatures.It is suggested
speedand 0.5-0.7 m/s for the zonal and meridional downward turbulent mixing of momentum in the afwind components. Over the oceansthe amplitude of ternoon, which resultsfrom increasedinstability of thethe diurnal cyclesis about 0.3-0.4 m/s for both the planetary boundary layer due to solar heating at thewind speed and the wind componentsand vary little surface, could be a major mechanismfor the diurnal
from winter to summer. The semidiurnal cycle in sur- cycleof wind speed;tidal variationsin surfacepressure
face winds is weaker than the diurnal cycle and has an and temperature could also contribute to the diurnal
amplitudeof 0.2-0.3 m/s overboth land and oceanfor cycleof surfacewinds(largelyoverland).

DAI AND DESER: DIURNAL

VARIATIONS

1•25-60N Land (solid line) vs Ocean (short-broken) Taa•_ , , I ' ' • ' ' • ' r--• , , • , , .•-5

IN SURFACE WINDS

AND DIVERGENCE

over the oceans and in the afternoon

31,123over most land ar-eas.

Surfacewind divergencepeaks around 0600 LST in
the western and eastern United States and around 2000

LST in the central United States. This alternatingfeature is evident in all the seasonsand resemblesthe pattern in the mean divergencefield. It is also consistent
with the phasechangesacrossthe United Statesin summer precipitation[Dai et al., 1999].
-7

0

•

•

I

6

•

•

I

12

•

•

I

18

•

•

I

24

•

•J

6

•

•

-25

12

Over the central Pacific, the diurnal anomaly of surface wind divergence peaks around 0600-0800 LST
from 10øN to 25øN and about

• •xx

•24

,

JJA-

4

,/

12 hours later from 25øNto 45øN. This phase pattern is correlated with cloud
cover in a mannerthatis consistentwiththe radiative

coolinghypothesis[Gray and Jacobsen,
1977;Foltz andGray,1979]for the diurnalcyclein oceanicrainfalland
convection.

Over the tropical Atlantic and eastern and central
portionsof the tropical Pacific, there existsa zonally co-herentpattern with maximumconvergence(divergence)
north (south) of the equator around0900-1200 LST.
15

0

•

•

I

6

•

•

I

•

•

I

•

•

I

•

•

I

•

•

--5

With the exception of the Pacific in JJA, the latitudinal structure of the divergenceanomaly at 1030 LST is
Observedmean surfaceair temperature generally similar to the profile of the mean daily diver12

18

24

6

2

Hour (LST)

Figure 16.

(øC) duringDJF (to•) and JJA (bottom)averaged
at gence,indicativeof an enhancement(-•10%) of the local
eachlocM time overthe land (solidcurve)and oceanic Atlantic and Pacific Hadley cells at 1030 LST relative(short-dashedcurve) areasfrom 25øN to 60øN. The to the daily mean.
long-dashedcurve is the land minusoceandifference Diurnal anomaliestend to be convergent(divergent)of surfaceair temperature(onthe right ordinate).The in the early morning (late afternoonto evening)over
temperaturedata are from the samedata sets as forsurfacewinds and have a coveragesimilar to Figure 1.

Surface wind divergencealso has a stronger diurnal
cycle over land, especiallyover high terrain and during
summer, than over ocean, with an amplitude ranging
from 2.0 x 10-7 s-1 over ocean to about 3.0-5.5 x 10-7

subtropical oceanic regions off the western boundaries
of Americas and Africa. This is in phase with the diurnal cycle of stratus cloud cover over these regions. It
is suggestedthat the diurnal cycle of large-scaledivergencemay contribute to the diurnal variations of marine
stratuscloud cover.

The daily anomaly of •;urface wind speed is dominated by westward propagating wave number I mode,s-1 overland. The semidiurnalcyclehasan amplitude which is the mainof 1.0-2.0 x 10-7 s-1 over both land and ocean in
all the seasons. The percentagesof the daily varianceexplainedby the two harmonicsare comparableto thosezonal wave o• the diurnal

harmonic.

For the zonal and merid•onal wind componentsand surface wind divergence,both the wave number I mode ofthe diurnal

harmonic

and the wave number

2 mode offor surface winds.

The diurnal cycle of surface divergencetends to beout of phase for many adjacent regions. In general,

land areas (exceptfor the innermostparts of the continents) such as Australia, easternAsia, westernand
eastern North America, the Indian subcontinent, andthe Mediterranean regiontend to have maximum diver-gence(minimumconvergence)arounddawn(0600-0800
LST), while the oceansadjacentto the continentshave
maximumdivergencein the evening(1700-1900 LST).

Annual•Cloud

Amount1983-91

ISCCP

Data

4o

'•J 30• •.o1oThis suggeststhat there exists a continental-scalediurnal circulation superposedon top of the daily mean
150E
170E
170W
150W
130W
l'10W
circulation in which surfaceair risesand convergesoverLongitude (deg.)
the continents and sinks and divergesover the adjaFigure 17. Satellite-observed
mean (1983-1991)ancent oceansin the afternoonand early evening,and the nual total cloudamount (%) overthe northernPacificopposite Occursin the early morning. The land-ocean derived from the International
Satellite Cloud Climaphase differenceof surfacedivergenceis also consistent tologyProject (ISCCP) data sets[Rossow
and$chi•er,with the notion that precipitationtendsto peak at night 1991].

31,124DAI AND DESER:DIURNAL

VARIATIONS

the semidiurnalharmonic are important, although zonal

wavecomponents(exceptfor wavenumber2) of the diurnal harmonicare generallystrongerthan thoseof the

IN SURFACE

WINDS

AND DIVERGENCE

Foltz, G. S., and W. M. Gray, Diurnal variation in the troposphere'senergybalance,J. Atmos. Sci., 36, 1450-1466,
1979.

Fu, R., A.D. DelGenio, and W. B. Rossow, Behavior ofsemidiurnal
harmonic.
deepconvectivecloudsin the tropical Pacific from ISCCP
The results of this study, especiallythose concerning
radiances, J. Clim., 3, 1129-1152, 1990.
the phase of surface wind divergenceover the tropical Gray, W. M., and R. W. Jacobson,Jr., Diurnal variation ofdeep cumulus convection,Mon. Weather Rev., 105, 1171Pacific and Southern Oceans,may contain large uncer-

1188, 1977.
tainties due to the poor sampling of data. The exact
Gutzler, D. S., and L. M. Hartten, Daily variability of lower
range of the uncertaintiesis difficult to obtain because tropospheric winds over the tropical western Pacific, J.
of the variable sampling resolution and number of reGeophys.Res., 100, 22,999-23,008, 1995.

ports. A valueof 4-1.5 hours (i.e., half of the sampling Haurwitz, B., and D. Cowley, The diurnal and semidiurnal barometric oscillations,global distribution and annualinterval) may be usedas a roughestimatefor the error

variation, Pure Appl. Geophys., 102, 193-222, 1973.
Heino, R., On the diurnal variation of wind in Finland, Tech.
the data sets(e.g., COADS is updatingthe data set for
Rep. 17, 21 pp., Finland Meteorol. Inst., Helsinki, 1978.
the 1990s.toincludemorehourly data from TAO buoys) Hendon, H. H., and K. Woodberry, The diurnal cycle ofshouldimprovethe resultspresentedhere. Analysesof
tropical convection, J. Geophys.Res., 98, 16,623-16,637.
the vertical structure of the diurnal variations of winds
Hering, W. S. and T. R. Borden Jr., Diurnal variations inthe summer wind field over the central United States, J.
[Pedder,1978;WhitemanandBian, 1996;Williamsand
Avery, 1996]and divergencein the troposphereshould Atmos. Sci., 19, 81-86, 1962.
Hsu, H.-H., and B. Hoskins, Tidal fluctuations as seen inprovide a more complete picture of the surface diurnal
ECMWF data, Q. J. R. Meteorok $oc., 115, 247-264,

in the phase discussedabove. Future enhancementsto

circulations

documented here.

Acknowledgments.
We thank J. M. Wallace and two
anonymous reviewers for their helpful comments. We are
grateful to Steve Worley for assistingus with the COADS
data set, Gregg Walters for his help with the GTS station
data set, and Dennis Shea for computer assistance. A. Dai
was supported by a NOAA Postdoctoral Program in Climate and Global Change fellowship, administeredby the
University Corporation for Atmospheric Research. The National Center for Atmospheric Researchis sponsoredby the
National

Science Foundation.

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