JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105,NO. D10, PAGES 12,207-12,219,MAY 27, 2000
Radiative heating rates and direct radiative forcing
by mineral dust in cloudyatmosphericconditions
Ana L/a Quijano,Irina N. Sokolik,andO. Brian Toon
Atmospheric
andOceanicSciences
Department
andLaboratory
for Atmospheric
andSpacePhysics,
Universityof
Colorado, Boulder
Abstract.We exploreseveralissuesrelevantto assessments
of solarandinfraredradiativeeffects
dueto mineralaerosols.One issueis the importanceof the verticaldistributionof dustfor
calculationsof dustradiativeheatingrates.Anotherissueis therole thatcloudsmay play in
augmenting
the radiativeforcingby dust.We alsoexploretheimportanceof thecompositionof
mineralaerosolsby employingspectralopticalpropertiesfor dustthatcomesfrom two different
regionsof the globe,the SaharanandAfghandeserts.A combinedlongwaveandshortwave
radiativetransfermodel was usedto determinethe instantaneous
radiativeforcing in the
atmosphere,
radiativefluxesat thesurface,andradiativeheatingratesby airbornemineralaerosols
for clear-skyand cloudyatmospheric
conditions.Extensivecalculationswith our modelshowthat
increasingdustloadingresultsin increasingbothsolarheatingratesandinfraredcoolingrates.
However, the net instantaneous
ratesduringthe day are alwayspositive,yielding net radiative
heatingof the dustlayer.With similaratmospheric
conditionsanddustloading,Saharandust
causeslargerheatingratesthanAfghandust.The magnitudes
of the Saharandustheatingratescan
easilybe 25% largerthanAfghandustheatingratesat high Sunanglesandoverbrightsurfaces.
Also, Saharandustyieldsmorepositivevaluesof TOA (top of the atmosphere)
radiativeforcing
thanAfghandust;andfor a diurnalaverage,thiscanleadto a changeof signof the TOA radiative
forcingfrom negativeto positivejust due to mineralogicalcomposition.Cloudssignificantly
influencethe directradiativeimpactof dustdependingon cloudaltitudeandopticaldepth.
Moreover,this influenceis stronglydependenton Sunpositionand surfacealbedo.
observationsof the effect of desert aerosolson heating rates are
1. Introduction
Recent studies have pointed out that there are large uncertainties in the assessments
of radiative impact by mineral dust on
regionaland global scales[Sokolik andToon, 1996; Tegen et al.,
1996]. Here we addressan issue that has not previously been
examined in detail: the importanceof a variable dust vertical
distribution in cloud-free and in overcast atmospheresto evaluations of the radiative heating rates by mineral aerosols. In
addition, we investigate the role that clouds may play in
augmenting the radiative forcing by dust at the top of the
atmosphereand at the surface.
Much attentionhasbeen recently paid to quantifyingthe radiative forcingby aerosol.However,the radiativeheating/coolingby
dustmustbe taken into accountto predictadequatelythe overall
impactof aerosolon weatherand climatebecauseheating/cooling
by dust alters atmosphericdynamics and thermodynamics.For
instance,the presenceof dust may causea stabilizing effect on
the temperature lapse rate. Karyampudi and Carlson [1988]
showed that radiative heating by Saharan dust contributesto
maintaininga war.mer and deeper Saharanair layer (SAL) over
the ocean,to enhancingthe strengthof the midlevel easterlyjet,
and to reducingthe convectionwithin the equatorialzone. On the
mesoscale,dust radiative heatingratescan affect the evolutionof
a dust storm leading to strongersurfacefrontogenesis[Chen et
al., 1994]. Consequently,knowledgeof radiativeheatingratesby
mineral aerosols may be decisive to better predictions of the
dynamics associatedwith dust transport.Unfortunately, direct
Copyright
2000bytheAmerican
Geophysical
Union.
scarceand complex [Fouquartet al., 1986]. Models remain a
powerfultool to studyradiativeheatingratesby dust,but they
requirethe appropriateinput parameterssuchas aerosoloptical
propertiesand vertical distributionat a given location.The goal
of this paper is to present a comprehensiveanalysis of
instantaneousdust radiative fluxes and heating rates based on
radiative
transfer calculations.
Previous studies have already explored radiative effects of
dust to various extents (e.g., Carlson and Benjamin, 1979;
Sokolik and Golitsyn, 1993; Tegen et al., 1996; Ackerman, 1997;
Liao and Seinfeld, 1998; Claquin et al., 1999a]. In Table 1 we
compare a few major studiesand presentwork indicating the
radiative effects addressedand the assumptionsutilized in each
work. Carlson and Benjamin [1979] used a simplified radiative
transfer model to determine
the effects of Saharan dust on the
radiative fluxes and heating/coolingrates in the atmosphere.
However,their studieswere limited by a singleverticalprofile of
dustdistribution.Sokolik and Golitsyn [1993] demonstratedthat
the vertical
distribution
is crucial for calculations
of radiative
heating/coolingratesby dustover bright surfaces(suchas desert)
by varying the height of the dust layer. In the presentstudywe
considerthe vertical location of the dust layer to be one of the
most important parametersfor our sensitivitystudy. We also
analyzehow the solar,infrared,and net heatingratesrespondto
various dust loadings,surfacealbedos,and Sun positions.We
focus on modelinginstantaneous
radiativefluxes becausethey
are important to obtain correct time averagingof the radiative
effects of dust as it is transportedfrom its sourcein the desert
toward the ocean. A proper criterion for selectionof adequate
spatialand temporalscalesfor averagingthe dustpropertiesstill
Papernumber2000JD900047.
remains an unresolved issue. Also instantaneous radiative fluxes
0148-0227/00/2000JD900047 $09.00
are requiredfor interpretationof the radiationmeasurements.
12,207
12,208
QUIJANO ET AL.: RADIATIVE IMPACT BY DUST
The interaction of dust, clouds, and radiation is another focus
of this studysincethe interleavingof cloudyand dustyair masses
is a common situation. Often, satellites cannot detect clouds
whenthey are locatedbeneathor betweendustlayers.Liao and
Seinfeld[1998] have donea studyon the effect of thick cloudson
direct aerosolforcing. They examined,using a one-dimensional
radiative transfermodel, the sensitivityof dust radiative forcing
to its physicaland opticalproperties,the verticaldistributionof
dust in the atmosphere,surfacealbedo, and to the presenceor
+
+
+
+
absenceof clouds. However, their study does not addressthe
radiative heating rates by dust. We perform a similar
investigationexcept focusingmore on heating/coolingrates. In
our study we do not use partial derivativesto investigatethe
radiativeforcing sensitivities.Suchan approachassumesthat the
forcing dependslinearly on the parametersconsidered.Because
of somelarge nonlineardependencies
with Sun position(as will
be shown in this study), we deal exclusivelywith the absolute
values of radiative effects.
-.I-
i
i
-.I-
-,I--
I
i
+
+
+
+
+
+
+
+
+
In this paper we also discussthe importanceof the compositionof airbornemineral aerosolsfor the assessment
of their
radiative effects at both solar and infrared wavelengths.The
predominantavailable work on airbornemineral aerosolstakes
into accountthe size distributionof the particlesbut not their
varying refractiveindices [e.g., Tegen and Fung, 1994]. Recent
studies by Sokolik and Toon [1999] demonstratedthat dust
compositionis so fundamentalthat it must be includedin the
assessmentof the radiative forcing by mineral dust. A first
attempt to characterizethe mineralogical compositionof dust
sourceson the global scaleis donein a new paperby Claquinet
al. [1999b]. Becauseavailable data are extremely limited, it still
remains a complex problem to incorporatethe compositioninto
optical models of dust which originatesfrom various sources.
Therefore to evaluate the importanceof the compositionin the
calculations of the radiative heating rates, we consider two
differentmodelsof dust spectralopticalpropertiesreferredto as
Saharandustand Afghan dust [Sokolik et al., 1998].
The paperis organizedas follows:first, we describedustand
cloud scenariosand optical propertiesused in the radiative
transfer model (sections 2 and 3). In section 4 we discuss the
impact of mineral dust on heating rates in the absenceand
presenceof clouds.Next, we analyzeTOA radiativeforcingand
downwellingradiativefluxesat the surfacefor the samedustand
cloudscenarios.Finally, we cometo the conclusions.
2. Description of Dust and Cloud Scenarios
The sensitivityof dust net heatingrates and radiativeforcing
to the absenceor presenceof cloudsis investigatedwith a singlecolumn
radiative
transfer
code
based
on
the
correlated-k
distributiontechniqueincorporatedinto a two-streammodel for
inhomogeneousmultiple-scatteringatmospheres[Mlawer et al.,
1997; Sokolik et al., 1998; R. W. Bergstromet al., An improved
radiative transfer model for climate calculations, submitted to
Journalof GeophysicalResearch,1999 (hereinafterreferredto as
B99)]. To perform the analysis,we select midlatitudesummer
vertical profiles of temperature,pressure,ozone, and H20 vapor
mixing ratios, which are representativefor arid and semiarid
regions.The atmosphereis divided into 33 layersfrom sealevel
to 100 km. Our main interestin this paperis focusedon the first 6
km of the tropospherewhere airbornedustis usuallyfound.
Several observations[Rao et al., 1988; Jankowiak and Tanr6,
1992] have shownthat visible wavelengthopticaldepthscan be
aslargeas0.6 up to 2 over the AtlanticOceanandArabianSeaat
times with high dust loading. In remote ocean areas the total
aerosol optical thickness is usually less than 0.1. It is also
QUIJANO ET AL.: RADIATIVE IMPACT BY DUST
12,209
Altitude
Backgroundconditions
of Midlatitude
Summer
4 Km
Clear sky or
dust
cloud
dust
Fourth layer
3 Km_
dust
dust
dust
Third layer
dust
Clear sky or
dust
Second layer
2 Km _
cloud
I Km _
dust
Clear sky or
dust
cloud
"Clear
"Clear
"Clear
or Cloud
or Cloud
or Cloud
above"
between"
below"
First layer
Figure 1 Schematic
description
of thelayerverticaldistribution
in themodel.
observedby airborne Sun photometry[Schmid et al., 1998],
balloonborneaerosol profilers [Dulac and Chazette, 1998],
ground-basedlidars, and satellite observations[Hamonou and
Chazette, 1998; Chazette et al., 1998] that the dust vertical
distributionis a multilayerstructureoften mixed with clouds.In
choicesallow us to study the dust radiative effects at different
times of the day and over different surfacesas the airborne
mineral aerosolstravel from their sourceregion toward the sea
shore and over the ocean.
addition,the processedLITE (Lidar in space Technology 3. Spectral Optical Properties
Experiment)data provideinformationon the verticalstructureof
cloudsand aerosolsindicatingmixeddustand clouds[Winker et
al., 1996].
The model calculates shortwave fluxes in fourteen solar bands
and longwave fluxes in sixteen terrestrial bands (B99). The
Althoughwe are not awareof any statistics
of cloud-freeand
spectralopticalpropertiesneededfor a two-streammodel are the
cloudyconditionswhen a dustplumeis present,we can select extinction coefficient, single-scatteringalbedo, and asymmetry
several scenarios of dust and cloud vertical distributions that are
parameter,which are each depictedas a functionof wavelength
representative
of observations.
In Figure 1, three possibledust for Saharanand Afghan dust, and for marine stratuscloud in
verticaldistributions
in a cloud-freeor cloudyatmosphere
are Figures2a-2c, respectively.
shown.In all thesecaseswe considerrrfineralaerosolspectral
We usedtwo modelsfor dust spectralopticalpropertiesin the
propertiesto be constantwith height and chooseseveraldust solar and infrared regions.Sokolik et al. [1998] referredto them
loadingscenarios(qJdust
(0.5 gm) = 0.25, 0.5, 1, and 2). For the as Saharandust and Afghan dust. Both models have the same
cloudyatmosphere
we assumethe presence
of a homogeneousparticlesize distribution(lognormalwith effectiveradiusro = 0.5
marine stratusdeck with either an optical depth at visible gm and standarddeviation • = 2), which representsthe longwavelengths(0.5 gm) of 1 (thin cloud)or 10 (thickcloud).
lived accumulationmode of airbornedustbut with very different
The essentialvariablesin the dustradiativeimpactdueto the refractive indices. Modeling dust radiative effects relying on
presenceof clouds are whether the cloud is located under, over, refractive indices measuredfor bulk dust sampleshas serious
or betweenthe dustlayers,and cloud thickness."Clear or cloud limitations[Sokolik and Toon, 1999]. Unfortunately,thereare no
above"represents
clear sky or cloudsabovedust.Very often, data on the compositionof dust collected in Tajikistan. Our
cirrus clouds are found lying well above the dust, but for spectralrefractiveindex for this region comesfrom bulk samples
simplicity,we havetakena stratuscloudto lie just abovethedust relatively close to the source.In this comparativestudy we use
layer. "Clear or cloud below" is found when rrfineral aerosols the refractive indices of Pattersonand Gillete [1981] for Saharan
froma duststormarriveat theseashoreandareupliftedby either dustalso measuredfor bulk samplesdespitethe available data on
a cloud-free
or anovercast
marineboundary
layer.Finally,"clear dust compositionfor this region. The differencesin refractive
or cloud between"representsclear sky or clouds that form index betweenthesetwo dustsamplesare dictatedby differences
betweendustlayers.
in mineralogical characteristics. We know, as a first
The surface
albedoof theserepresentative
scenarios,
A,, is approximation,that Saharandust collectedin Barbadoshas more
equalto 0.3 for deserts
and0.05for oceans
at all wavelengths
and hematitethan Afghan dust collectedin Tajikistan.Hematiteis a
solar zenith angles. The instantaneousradiative fluxes are reddishmineral composedof iron oxide (Fe203) that strongly
calculated
for differentSunpositions,
mainlyfor highSunangles absorbsradiationin the solarand infrared.A differentproportion
(•'go = 0.8) and low Sun angles(~go= 0.25), thoughwe of hematite can change significantlythe imaginary refractive
performedsome calculationsover a full diurnal cycle. These index of dust, which in turn deterurinesits optical and radiative
12,210
QUIJANOET AL.' RADIATIVEIMPACTBY DUST
'•
1.2
•-
1
Afhgan
coefficients
is located
in theatmospheric
window
(8-12gm),
a
whereSaharar.
dustabsorbs
andemitsmoreinfraredradiation
•uid water
thanAfghandust(Figure2b). Finally,thereare no crucial
o 0.8
differences
between
theasymmetry
parameters
of dustmodels
._
•
0.6
(Figure 2c).
aran dust
Liquidwateroptical
properties
areincluded
inFigure
2 since
wearegoing
tostudy
thechanges
in dustradiative
impact
dueto
'o
0A
t
._.N
-•
0.2
o
0
thepresence
of a marinestratus
cloud.We havetakenits size
distribution
tobelognormal
withro= 5 gmandc•= 2 based
on
,
(urn)
0.1
tOO Liou [1992].
1.2
o
In the nexttwo sections
we presentcalculations
fromour
modelof thedustandcloudradiative
impact
asa function
of dust
optical depth, layer vertical distribution,mineral aerosol
1
•
0.8
.//••
ø""•aran
dust
....
•
0.6
•
0.4
composition,
surface
albedo,
andSunposition.
Wehave
tokeep
inmind
thatalthough
solar
andnetradiative
impact
depend
onall
thesevariables,
infrared
radiative
effects
onlydepend
onthe
vertical
location,
loading,
andcomposition
ofdustlayers
andon
ater
Afhgandust
thepresence
ofclouds
intheatmosphere.
.• 0.2
o
4. RadiativeHeatingRatesof Dustin the
o.1
1
A.(•m)
10
100 Presence and Absence of Clouds
1.2
Therateof change
of temperature
in a layerduetoradiative
heating/cooling
iscalled
radiative
heating/cooling
rate,whichis
Liquidwater
1
defined as
0.8
0.6
dT g ' VFsolar+iR
0.4
dt
-
Afhgan
dust
,,•......
0.2•
,
(1)
CpßdP
where
gistheacceleration
ofgravity,
C•,isthespecific
heat
capacity,
and]7Fsolar+iR
istheradiative
fluxF, divergence
overa
Saharan dust
0,
layerof pressuredifferencedP, whichwe defineas
0.1
lOO
Figure2 Spectra
of (a) normalized
extinction
coefficientVF = (F(downwelling(,[,),
P) - F(upwelling(•),
P))
Kent/Kent(0.5
gm), (b)single-scattering
albedo
too,
and(c)
-(F(,•,P +dP)- F(•',P +dP)).
asymmetry
parameter
g calculated
for Saharan,
Afghan
dust
models,andmarinestratuscloud.
Since
thereareveryfewmeasurements
of radiative
heating
effects
[Sokolik
andToon,1999].In thewavelength
range
from rateswith a finer verticalresolutionthan 1 km, we choosea
0.2 to 0.5 gm, the single-scattering
albedoof Saharan
dustis
muchsmallerthan it is for Afghandust(Figure2b). The
modelverticalresolutionof 1 km at the lower altitudes.The
smoothprofilesof the heatingratesdrawnbetweenthe actual
dominant
difference
between
Saharan
andAfghan
dustextinction calculation
pointsareapproximate.
7
• IR
NE'
II
SOLAR
!
o,
$::
, ,
i
i
ß
lo ,l
,
-3
-2
-1
0
1
2
3
4
5
6
7
HeatingRate(K/day)
Figure
3. Solar
(long-dashed
lines),
infrared
(short-dashed
lines),
and
net
(solid
lines)
radiative
heating
rates
ofSaharan
and
Afghan
dust
and
forclear
sky,
over
the
desert
(As
=0.3)
atgo
=0.8;
squares
are
forclear
sky;
triangles
are
forSaharan
dust;
diamonds
are
forAfghan
dust;
"clear
above"
and
Xaast
(0.5
gm)
=0.5.
QUIJANO ET AL.: RADIATIVE IMPACT BY DUST
12,211
also showsthat under similar atmosphericconditionsand dust
loading, Saharandust causeslarger heating rates than Afghan
dust. This greaterheating is because,in the wavelengthrange
from 0.2 to 0.5 gm, the single-scattering
albedoof Saharandust
is much smallerthan it is for Afghan dust, so the Saharandust
has greater solar absorption (Figure 2b). In Figure 4 we
5
E
•,4
performeda diurnalaverageat 30øNduringsummerof the net
2
0.5
•.5
•-•,4
_,= 3
< 2
-'
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
DiurnalAveraged
Net HeatingRate(K/day)
Figure4. Diurnally
averaged
netradiative
heating
rates(30øN,
summer)at (a) As =0.2 and (b) As =0.05; squaresare for clear
sky; trianglesare for Saharandust;diamonds
are for Afghan
dust;"clearabove"and 1Jdust
(0.5 gm) = 0.5.
As depictedin Figure3, thepeakof solarheatingandinfrared
coolingrelativeto clear-skyconditionsoccursin the upperdust
layer due to solar absorption and terrestrial emission,
respectively.For the solarzenithangleshown,mineralaerosols
producea significantnetheatingwheretheyarelocated.Figure3
radiative heating rates for Saharanand Afghan dust distributed
over a homogeneous
verticalprofile in the lowest3 km with As=
0.2 (,Figure4a) andAs= 0.05 (Figure4b). A surfacealbedoof 0.2
is used as a rough averageof the different surfacesthat Saharan
dust might encounter while traveling from its African source
toward the seashoreor Afghan dustwhile crossingthe mountain
chainthat separatesAfghanistanfrom Tajikistan.The differences
in diurnal averagedheating rates between Saharanand Afghan
dust remain to be very important (maximum differences-0.24
K/d at As=0.2 and 0.19 K/d at A• = 0.05 for 1Jdust
(0.5 gm) = 0.5).
Net radiative heating rates increase with dust loading, as
Figure 5 showsfor Saharandust.This sensitivityto the amountof
dust has a strongdependenceon solar zenith angle and a much
weaker dependenceon surface albedo, especially at low Sun
angles. Moreover, the increasein heating as we increasedust
opticaldepthis linear, as a first approximation,only at high Sun
angles.
Figure 6 showsheating rates as a functionof three different
dustverticaldistributionsrepresented
in Figure 1. Solarradiative
heating rates (dT/dt) are directly proportionalto the amountof
sunlight absorbed by the layer ( VFsola0 and inversely
proportionalto the atmosphericpressurechange(dP) in going
from height z to height z + dz. When a dustlayer is uplifted, dP
decreases rapidly while VFsola
r remains almost constant.
Thereforethe solar heating rate increases(Figure 6a). However,
infrared heating rates remain almost constant(Figure 6b). The
constantinfraredcoolingis due to a balancebetweentwo factors:
as the altitudeincreases,dP decreases
rapidly. On the otherhand,
temperaturein the tropospheredecreaseswith altitude and so
doesthe infrared emissivity;thereforecoolingin the infraredby
uplifted mineral aerosolsis less efficient, and the absolutevalue
of VFm decreases.
Hencethe increasein net heatingratesas dust
7
•, 2
1
0
1
2
3
4
5
6
7
8
9
10
11 12
-2
-1
0
1
-2
-1
0
1
2
3
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
Net HeatingRate (K/day)
9
10 11
12
2
Net HeatingRate (K/day)
Figure 5. Net radiativeheatingratesat (a) As=0.3, lao= 0.8; (b) As=0.3, go= 0.25; (c) As=0.05, go= 0.8;
and (d) As=0.05, go= 0.25 for varyingopticaldepthsof Saharandust;dashedline is for clearsky;squares
are for 1;dust(0.5
gm) = 0.25; trianglesare for 1Jdust(0.5
gm) = 0.5; diamondsare for 1Jdust(0.5
gm) = 1; circles
are for 1;dust(0.5
gm) = 2; "clearabove."
12,212
QUIJANO ET AL.: RADIATIVE IMPACT BY DUST
7
E 4
_---2
ß
01
2
4
6
Solar HeatingRate (K/day)
7
0
-2.5
i
i
i
-2
-1.5
-1
i
-0.5
0
IR Heating Rate (K/day)
averages[Alpert et al, 1998] of radiativeheatingratesdue to dust
have limited applications.
According to Quijano et al. [1997] a marine stratuscloud
altersthe magnitudeof dust heatingratesdependingon both its
altitude and optical depth (Figure 8). In general, a large net
cooling occursin the region where the cloud is located. When
dust is below a marine stratuscloud, the net cooling at the cloud
increases(Figure 8a) becauseof a balancebetweenthe infrared
emissionof aerosolsand cloud (not shown). In the "cloud below"
case (Figure 8c), the solar radiation reflected from the cloud
increasesas the cloud gets thicker and, as a result,the radiation
heatsdust layers that are above, much as a bright surfacedoes.
For the samereason,heatingratesincreasein dustlayers above
thecloudin the "cloudbetween"case(Figure8b).
Figure 9 showsthe profile of solar, infrared, and net heating
rateswhen a cloud is over dust at high Sun angles.For high Sun
the presenceof a cloud layer can increaseor decreasethe net
heating rates by mineral aerosolsdependingon cloud optical
thickness.A more careful scrutiny reveals the reasonsfor this
behavior.An opticallythick cloud reducesthe solarflux below it
and henceheatingratesfall within the dust sincesolar energyis
absorbedand reflectedby the cloud itself. The thin cloud does
not attenuatemuch solar radiation, so dust solar heating rates
changelittle. Conversely,dust infrared cooling declinesbecause
of enhanced downward infrared flux emitted from the cloud aloft.
This increaseor decreaseof net heatingratesby mineralaerosols
dependingon cloudoptical thicknessdoesnot happenat low Sun
anglesfor overlying clouds (Figure 9). Then, the presenceof a
cloudlayer over dustcausesa decreaseof solarheatingratesand
infrared cooling rates suchthat the net heatingratesby mineral
7 I
6I
0
2
4
6
8
Net HeatingRate (K/day)
Figure 6. Radiative heating rates of (a) solar, (b) infrared, and
(c) net by dust over the desertat go = 0.8 for three considered
dust vertical distributions; squares are for "clear above";
diamondsare for "clearbetween";trianglesare for "clearbelow";
Saharandust,X•ust
(0.5 gm) = 0.5.
•' 4 -•
•:
2
0
1
2
3
4
7
x
risesis dueentirelyto thesolarpartof thespectrum.
We alsofind
that with any dust vertical distribution,the maximumsolar
heatingandinfraredcoolingoccuralwaysin theupperdustlayer.
6
Figures7a and7b showSaharan
andAfghandustnetheating
rates at two different surface albedos and solar zenith angles,
respectively.
In this analysis,net heatingrateshavethe same
behavioras solarheatingratessinceinfraredfluxesin ourmodel
are not sensitiveto the variation in either of the two parameters.
Heatingratesdue to mineralaerosolsover stronglyreflective
surfaces
increaserelativeto heatingratesoverdarksurfaces
since
more solar flux is reflectedfrom the ground(see Figure 7a).
-1
0
1
2
3
4
5
Net HeatingRate (K/day)
Figure7b showsdustnetradiativeeffectsat highandlow Sun Figure 7. Net radiativeheatingrates(a) overA• =0.3 andA•
are for
angleswhenthe aerosollayerlies over a desert.Heatingrates =0.05 at go = 0.8 for SaharanandAfghandust;squares
decline for low Sun (/.to= 0.25) becausefewer photonsare
Saharan dust over the desert; diamonds are for Saharan dust over
available to heat dustlayers.
It is clear from the last three setsof graphsthat heatingrates
the ocean;trianglesare for Afghandustoverthe desert;crosses
are for Afghandust over the ocean(b) at two differentSun
dueto mineralaerosols
experience
largevariations
asdusttravels
from its sourceto otherregions.Also, the fact that aftersunset
thelongwave
contribution
stillpersists
is important
sincethennet
heatingbecomes
infraredcoolingduringthenight.Therefore
the
diurnal averages[Carson and Benjamin,1979] or monthly
positions
overthe desertfor SaharanandAfghandust;squares
are for Saharandust at high Sun angles;diamondsare for
Saharandustat low Sun angles;trianglesare for Afghandustat
highSunangles;crosses
arefor Afghandustat low Sunangles;
"clearbelow" and X•u•t(0.5 gm)= 0.5.
QUIJANO ET AL.: RADIATIVE IMPACT BY DUST
7
Z•VTOA
= -(Fsolar+IR
(•'TOA'
P'dusty)
- Fsolar+IR
0'TOA'
P'clear)),
12,213
(2)
for topof the atmosphere
(TOA) radiativeforcing,ZIFToA,
which
is the differencein valueof netupwellingflux (Fsolar+
IR)in clear
anddustyatmospheric
conditions,
respectively.
We also define the downwellingfluxes differenceat the
surfacezlFsFc as
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
AFsF
C= Fsolar+
m(,I,sFc,
P'dusty)
7
- Fsolar+iR
(•SFC,
P,clear),
6
(3)
zlFsFc is the differencein net downwellingfluxes(Fsolar+
IR)
("dusty"minus"clear"conditions)at the surface(SFC). When
eitherZlFTo^or zlFs[:½
is negative,aerosols
producea cooling
effect either at TOA or at the surface.
The magnitude
of thedirectaerosol
radiativeforcingdepends
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
•
stronglyon the dustloadingand opticalcharacteristics,
surface
albedo,andsolarzenithangle[SokolikandToon, 1996].As we
seein Figure10a,dustcausesa positiveradiativeforcingover
bright surfacesat high Sun anglesfor "clearabove"becauseat
this particularSun positionthe mineralaerosollayershavea
loweralbedothanthesand-covered
surface
below.Thispositive
forcing,whichincreases
withdustopticaldepth,happens
for all
cloudless
cases
regardless
of d•everticaldistributions.
A negative
forcingoccursover dark surfaces
at bothlow and high Sun
angles.Mineralaerosols
causea negativeradiativeforcingover
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
bright surfacesat low Sun angles(Figure 10b) when thereis
more dust scatteringthan dust absorption.However, as we
increasethe dustopticaldepth,dustabsorption
increases,
andfor
Saharandust,radiativeforcingbecomes
positiveat highdust
opticaldepths.Saharan
dustcauses
a largerpositiveforcingthan
Figure 8. Net radiative heating rates by dust and cloud over Afghandustover brightsurfaces
and high Sun angles,and a
Net HeatingRate (K/day)
oceanat go = 0.8 for (a) "clear or cloud above,"(b) "clear or
cloud between," and (c) "clear or cloud below"; diamonds are for
smaller negative forcing when surfacesare darker and Sun
positionis low. This largerpositiveforcingfor Saharandustis
"onlythin cloud";trianglesare for "onlythick cloud";squaresare dueto thehighersolarabsorption
of Saharan
dustascompared
to
for only dust;soliddiamondsare for "dustand thin cloud";solid Afghan dust. As in the case of radiativeheatingrates the
trianglesare for "dustand thick cloud"; Saharandust and Tdust sensitivity
of TOA radiativeforcingto dustopticaldepthis more
(0.5 gm)= 1.
linearat highSunanglesthanat low Sunangles.
However,only
in thecaseof low surfacealbedoandhighSunangle(As= 0.05,
kto
= 0.8) canwe considerthedependence
betweenTOA radiative
forcinganddustopticaldepthto be nearlylinearfor optical
aerosolsremain almostconstant(seeFigure 9f). Becauseof the depthslessthan 2.
slant paths, both optically thin and thick clouds cause the
The presence of dust in the atmospheredecreases
underlyingdust to have heatingrates similar to those in dust
downwellingradiativefluxesat the surfacealmostlinearlywith
layers which have no cloud abovethem.
opticaldepthin all cloudless
cases(Figure10c)for the
Overall,ourcalculations
of radiativeheatingratesdemonstrate increasing
This decrease
doesnot depend
that the differencesin magnitudedue to dustcomposition
are mineraldustmodelsconsidered.
much on surfacealbedo,especiallyat low Sun angles.Since
very important.Also, the variationsin magnitudeof net heating
Saharandust absorbsmore solarradiationthan Afghan dust,
ratesasdustopticaldepthincreases
havea strongdependence
on
larger decreasesof downwellingradiativefluxes at the surface
solarzenithangle.The profilesof solarandinfraredheatingrates
for Saharandustare expected.However,we seethat this decline
are significantlyalteredby changingthe dustverticaldistribution
sincethemaximumof solarandinfraredcoolingoccursalwaysin
theupperdustlayers.In addition,a marinestratuscloudhaslarge
effectson the magnitudes
of dustheatingratesdepending
on its
(-10 W/m2at'c(0.5
gm)=0.5)onlyhappens
athighSunangles.
At low Sunangles,Afghandusthasa slightlylargerdecrease
of
downwelling fluxes at the surface due to an enhanced
backscatteringof sunlight toward space. Hence the surface
altitude,its opticaldepth,andthe Sunposition.
energybudgetis decreased
by both absorptionin the dustlayer
andreflectionof sunlightbackto space.Whetherabsorption
or
5. Radiative Forcing of Dust in the Absenceand scatteringis most importantdependsmainly on dust optical
Presence of Clouds
propertiesand on solarzenithangle.Claquinet al. [1999a] found
thatunderduststormconditions
in theSahara,diurnallyaveraged
To assessthe direct solar and infraredradiativeforcing of downwellingnet fluxes at the desertsurfacecan increaserelative
mineral aerosolin a cloud-freeatmosphere,
we have employed to clear-skyconditions.
In thisparticularcase,thelongwavegain
the definitions
of energyby emissionof large dust particlesbecomesmore
12,214
QUIJANO ET AL.: RADIATIVE IMPACT BY DUST
2 3
..•
I
0
........
-10-8
0
-6
•
-2
0
2
4
6
8
10
-10
-8
-6
Solar Heating Rate (•day)
-4
-2
0
4
2
6
8
10
Solar Heating Rate (K/day)
b,e
6
0
-8
-6
-4
-2
0
2
4
6
8
10
InfraredHeating Rate (K/day)
7-•
c
E 5
a) 4
:33,
•:2
1
-10
-8
-6
-4
-2
0
2
4
6
8
10
-10
-8
Net Heating Rate (K/day)
-6
-4
-2
0
2,
4
6
8
10
Net Heating Rate (K/day)
Figure 9. Radiativeheatingratesoverthe desert(As=0.3) of (a) solar,(b) infrared,and (c) net at go= 0.8
and of (d) solar, (e) infrared,and (f) net at go = 0.25' squaresare for "clearabove,"diamondsare for "thin
cloudabove"'trianglesare for "thickcloudabove";Saharandustand1;dust
(0.5 gm) = 0.5.
0
0.5
1
1.5
0
2
8O
5
60
0
2
-5
-20
-10
-4o
-60
o.5
1
1.5
2
- 15
-80
100
-20
0
0
o.5
i
1
.....
• '
1.5
]
2
.....
0
O.5
1
1.5
0
-50
•'-100
-20
- 15O
-40
-200
-60
-250
-300
-80
-35O
-100
Dust opticaldepth
Dust opticaldepth
Figure10. Netradiative
forcing
atTOA(a,b) andchanges
in thenetdownwelling
radiative
fluxesatthe
surface
(c, d) for Saharan
(openicons)andAfghan(lust(solidicons);squares
arefor As=0.3,go= 0.8;
triangles
areforAs=0.3,go= 0.25'diamonds
areforAs=0.05,go= 0.8;circles
areforAs=0.05,go= 0.25;
"clear above."
2
QUIJANO ET AL.' RADIATIVE
IMPACT BY DUST
12,215
with a differencein magnitudeof a factorof 2 (Figure 12b). Over
the land the two dust modelshave a diurnal mean forcing with a
differentsign(+7.67W/m2 for Saharan
and-1.52W/m2 for
10
Afghan) (Figure 12b). Comparingthese values with previous
results[Liao and Seinfeld, 1998, Figure 5; Claquin et al., 1999,
Table 3] we find qualitative agreement between them. A
quantitativecomparisonis not possiblesincetheir inputsare not
exactlythe sameas in this paper.We alsolearnfrom Figure 12a
that over the land it is not clear how to interpret the physical
meaningof diurnal averagessince the sign of radiative forcing
changesduring the day. A shortertimescalefor averagingof dust
radiativeforcingis required.
The net TOA radiative forcing and the downwelling fluxes
difference at the surface due to dust m cloudy conditionsare
defined,respectively,as
-lO
-20
-•-30
LF_.-40
-5O
Z•t7TOA
=-(Fsolar+IR
(•'TOA
,P,dusty+cloudy)
- Fsolar+iR
(q'TOA
,P,clear)),
(5)
Z•FsFc
= Fsolar+IR
("•SFC'
P'dusty
+cloudy)
- Fsolar+iR
('•SFC,
P,clear
)ß
(6)
-60
-7O
Dust is often associated
with cloud,thoughwhen a cloudis
beneathor betweenthe dustlayers,it may not be detectedby
-80
satellite. Because of this interaction we based the reference of our
-90
flux calculations
on theclear-skyatmosphere.
To compare
model
resultswith satelliteobservations,
ourdefinitionof forcingis the
Dust optical depth
mostappropriate.
If, on the contrary,we werestudyingthe effect
Figure 11. Global diurnally averagednet radiative forcing at of cloudson direct dustradiativeforcing,the referenceof our
TOA for Saharan (open icons) and Afghan dust (solid icons); flux calculations
wouldhavebeenthe cloudysky [seeLiao and
squaresare for As =0.2; diamondsare for As =0.05; "clearabove" Seinfeld,1998], asthe followingequations
show:
and 'c0,•,t
(0.5 gin) = 0.5.
40
important
thanthe lossof sunlight
by absorption
and
backscattering.
Therefore
fromFigures
10cand10dwedonot
30
inferthatdownwelling
fluxesat thesurface
havea systematic '-'
decrease
for all possiblesituations.
20
lO
In Figure
11weshow
thedependence
oftheglobal
diurnal
average
ofTOAradiative
forcing
forSaharan
andAfghan
dust
withvarying
mineral
aerosol
opticaldepth.The diurnally
averaged
forcing
AiYistheonecalculated
byCess
[1985]
and
usedby LiaoandSeinfeld
[1998].
We change
theformula
o
-10
-20
-30
0
slightly
byadding
theinfrared
termseparately:
5
10
15
20
25
10
AF--Ne
t =-I•AFSolar
(Jr0)dJt0
+AFIR.
2
(4)
---•'-1o
Then,
onlySaharan
dust
over
theland(As
= 0.2)causes
apositive ---net radiative forcing.
-2o
• -3o
For a diurnalaveragethe infraredforcingbecomes
more
important
thanforinstantaneous
calculations
since
atnight
the
onlyradiative
contribution
is at 1Rwavelengths,
whichis
positive.
Asanillustration,
Figure
12shows
thediurnal
cycle
(24
-40
b
-50
0
hours)
of netradiative
forcing
forSaharan
dust(squares)
and
Afghan
dust
(diamonds)
atthesame
conditions
used
toperform
5
10
15
2o
25
Hours
the diurnalaverages
of radiativeheatingrates(30øN,summer Figure 12. Diurnalcycleof netradiativeforcing(30øN,summer)
season)(Figure4) [Sokolikand Toon, 1997]. In Figure12a, for Saharan(squares)and Afghan dust (diamonds)(a) over the
calculationsare shown for dust over land (A.,.= 0.2). Over the
land, and (b) over the ocean, "clear above" and Tdust
(0.5 gin) =
oceanbothdustmodelsgivea negativediurnalmeanforcingbut
0.5.
12,216
QUIJANOET AL.' RADIATIVEIMPACTBY DUST
Calculatedusingeq.(5)
Calculatedusingeq.(7)
(a)
60
Solar
dust + thin cloud
only dust
dust + thick cloud
IR
IR
Net
IR
2O
Net
---.-
Solar
Net
Solar
Net
-20
Solar
dust + thin cloud
dust + thick cloud
o
u•
-4o
-80
-100
(b)
200
Net
Solar
150
lOO
Net
Solar
Solar
Solar
Net
•o
Net
dust + thick cloud
o
dust + thin cloud
-5o
only dust
dust + thick cloud
dust + thin cloud
-lOO
•
(c)
Net
250]
Solar
2001
150
only dust
dust + thin cloud
Net
100
Net
Net
Solar
Solar
Net
50
IR
dust + thick cloud
dust + thin cloud
dust + thick cloud
Figure13. Solar,infrared,andnetradiative
forcingat TOA withSaharan
dustandcloudscalculated
by
equations
(5) and(7) (seetext);(a) overtheoceanat go= 0.25and1;dust
(0.5gm)= l' (b) overthedesert
at
go= 0.8 and'rdust
(0.5gm)= l' (c)overthedesert
atlao= 0.8and'rdust
(0.5gm)= 2; "clearorcloudbelow."
ZXFToA
=-(Fsolar+IR
(?TOA'
P'dusty
+cloudy)
- Fsolar+IR
(?TOA'
P'cloudy)),
(7)
radiativeforcingof dustin cloud-freeconditions
(Figure13a).
Thisincrease
of negativeradiativeforcinghappens
regardless
of
the vertical distribution of cloud and dust and is due to the
increasedsolarlossby absorption
and scattering
of the cloud.
Ovsr brightsurfaces
and at high Sun angles,the signof the
radiativeforcingchanges
frompositiveto negativeasthe cloud
(8)
opticaldepthincreases(Figure 13b). The solar part of the
spectrum
usuallyplaysa veryimportant
rolein thevalueof the
net radiativeforcing.However,when Saharandustis located
Figure13 illustrates
the differences
in TOA radiativeforcing abovean opticallythin or thickcloudandhasa 'r(0.5gm) >1.8,
calculated
by two approaches.
Accordingto Liao andSeinfeld's theTOA netradiativeforcingremainspositiveat highSunangles
definition(equation(7)), TOA forcingin thepresence
of a thick over bright surfaces(Figure 1l c), and the infraredradiative
to thenetresult.Dustoptical
cloudis positivefor all surfaces
sincedustis alwaysgoingto add forcinggivesthemajorcontribution
of 2 havebeenmeasured
overtheAtlanticOceanat times
a warmingeffectto a cloudyatmosphere.
In contrast,
according depths
et al., 1999].Also,it is possible
to our definition(equation(5)) the presence
of a cloudoverthe withhighdustloading[Chiapello
oceansincreasesthe overallnegativeradiativeforcingrelativeto to find an opticaldepthof 2 or evenlargeroveraridregionsin
Z•7SFC
- Fsolar+IR
(•'SFC'
P'dusty
+cloudy)
- Fsolar+IR
(•'SFC'
P'cloudy),
QUIJANO ET AL.' RADIATIVE IMPACT BY DUST
12,217
(a)
100
5O
Net
Solar
Net
Solar
0
o
-100
36.7
-150
-200
7.31
Thick cloud under the dust layer
-25o
Thick cloud over the dust layer
(b)
I.Lo=0.8,
onlydust
o
•=0.8,
go=0.25,
dust+ thickcloud
-5o
onlydust
•=0.25,
dust+ thickcloud
15.34
16.92
-1 oo -
-15o-2oo
-
-250
-
-300
-
-350
-
41.5
Figure 14. (a) Solar, infrared, and net radiativeforcing at TOA over the desertand go = 0.8 for two different
Vertical distributionswith thick clouds; Saharan and Afghan dust;IJdust
(0.5 gm) = 0.5. (b) Net radiative
forcing at TOA over oceanat high (go = 0.8) and low Sun angles(go = 0.25) for one verticaldistributionwith
only dust and dustplus thick cloud; Saharanand Afghan dust, "clearor cloudbetween,"and IJdust
(0.5 gm) =
1.
Africa [D'Almeida, 1987]. Although the size distributionof dust
from theseobservationsmay be different than the one we use for
our study (ro = 0.5 gm), we may still have the same changeof
sign of the TOA radiative forcing when dust is over a thick cloud
and hasa large opticaldepth.This is becausedustabsorptionand
forward scatteringat solar wavelengthsincreaseswith particle
size, and the positivelongwaveforcingdue to dustbecomesmore
importantfor largeparticles[Claquinet al., 1999a].
As Figure 14a shows,the differencein radiativeforcingdueto
Saharanand Afghan dust is greaterwhen the cloud is under the
dustlayer than when the cloud is over it. A lower cloud is able to
heat the dust layers by reflecting sunlight. Therefore more
photonsare available to be transmitted,scattered,and absorbed
by dust, and differencesbetween the two optical models are
greater.A similareffect occursif the surfacealbedois high and
we have only dust in the atmosphere.Then, the ground will
reflect more solar radiation, heat the dust, and the differences
between Saharanand Afghan mineral aerosolswill increaseas
well. On the other hand, a cloud over the dustlayer diminishes
a cloud-freecaseat high Sun angles.When the Sun is low on the
horizon,the slantopticaldepthat the upperlayersof dustis great
enoughto mask the cloud influence.
According to Liao and Seinfeld [1998], in the absenceof
clouds,TOA and surfaceshortwaveforcingis not very sensitive
to the altitude of the dust layer. However, in the presenceof a
cloud layer TOA warmingis strongestwhen the dustlayer lies
abovethe cloud and surfacecooling is strongestwhen the dust
layer lies below the cloud. We find that this dependenceon the
vertical location of cloud becomes more important as dust
loading increases.In the absenceof clouds, there is a maximum
TOA warmingdifference
of 3, 4, and6 W/m2 between
"clear
below"and "clearabove"for a total dustopticaldepthat 0.5 gm
of 0.5, 1, and 2, respectively.However,in the presenceof a thick
cloudandfor the samecasesof dustloading,thisTOA maximum
warming difference between "cloud below" and "cloud above"
wouldbe 85, 140,and212 W/m2.
Downwelling fluxes at the surface are also sensitiveto the
vertical distributionof dust in the presenceof clouds.However,
on dustaltitudedoesnot changesignificantlyas
the differences
in radiativeforcingdueto a differentcomposition this dependence
of mineral aerosols because it both attenuates solar radiation
dust loadingincreases.The n'aximumcoolingdifferencein the
before it impingeson the aerosollayers and masks the dust presence of clouds between "cloud below" and "cloud above"
be50,59,and57 W/m2fora totaldustoptical
depth
at0.5
infraredemissionfrom beneath.Surprisingly,Figure 14b shows would
that a cloud between the dust layers increasesthe difference gm of 0.5, 1, and 2, respectively.
betweenthe SaharanandAfghandustradiativeforcingrelativeto
Further analyzingdownwellingfluxes at the surfacewe note
12,218
QUIJANe ET AL.' RADIATIVE IMPACT BY DUST
Table
2 Changes
intheSolar
+IRDownwelling
Fluxes
attheSurface
AFsFc
(W/m2);
"Clear
orCloud
Between"
and Xdust
(0.5 gm) = 0.5
OnlyDust
As= 0.3,go= 0.8
As= 0.3,go= 0.25
As= 0.05,go= 0.8
As= 0.05,go= 0.25
Saharan
Afghan
-88.93
-54.87
-95.42
-55.48
-75.69
-56.14
-87.3
-57.88
Dust+ ThinCloud
Dust+ ThickCloud
Saharan
Saharan
-104.46
-48.9
-123.07
-51.35
Afghan
-87.09
-45.86
-110.83
-49.43
-361.73
-77.47
-400.86
-82.76
Afghan
-344.57
-72.8
-388.51
-79.18
is strongly
fromTable2 thatthepresence
of a thinor a thickclouddecreases altitudeandopticaldepth.Moreover,thisinfluence
onSunposition
andsurface
albedo.
surfacefluxesat high Sunangles.However,it alsoshowsthata dependent
Althoughdiurnal averagesare an often-usedtool as a
thin cloudcausesa slightincreasein the surfacefluxesat low
of themeandailyeffectof atmospheric
aerosols,
they
Sunanglesrelativeto "onlydust"conditions.
For slantpathsa summary
thin cloud doesnot make a big differencein decreasing
solar can missessentialdetailsof how the dustaffectsthe atmospheredownwelling
fluxesrelativeto "onlydustconditions."
However, underlyingsurfacesystem.In particular,heatingratesdue to
experience
largevariations
asdusttravelsfrom
the increasein infrareddownwellingflux due to the presenceof mineralaerosols
its
source
to
other
regions.
Because
of
this,
to
performa diurnal
the cloudis largeanddoesnot dependon solarzenithangle.In
of heatingratesin thepresence
of dustlayerscanmiss
addition,Table 2 showsthat in the presenceof a cloudwe find a average
largerdecrease
of downwelling
radiative
fluxcsatthesurface
for
the development
of someatmospheric
dynamics
whoseeffects
the Saharandustcompared
to theAfghandustat low Sunangles. can be enhancedor reducedby the radiativeimpact of dust
on thetimeof theday.TeA radiative
forcingcould
This resultoccursbecauseof compensating
effectsbetweensolar depending
change
sign
during
the
day
due
to
variations
of
dust
loadingand
andinfraredabsorption
andscattering
by the aerosolandcloud
vertical distributionin the presenceand absenceof clouds.
layers.
In summary,
dustin a cloud-freeatmosphere
causes
a positive Diurnalaverageswould missthis variability.Thereforeit is
useful to look at instantaneous radiative effects of mineral
TOA (top of the atmosphere)
radiativeforcingover bright
aerosols.
Averagesof radiativeeffectsshouldbe shorterthana
surfaces
at highSunangles.Conversely,
mineralaerosols
causea
day to correctlyrepresent
the significant
changes
in signand
negativeradiativeforcingoverbrightsurfaces
at low Sunangles
magnitude
of
the
heating
and
forcing.
This
also
means
thatmore
and over dark surfaces.For a diurnalaverage,only Saharandust
observations
from the petXinent
aerosolregimesare neededto
over land (As= 0.2) has a positivenet radiativeforcing.In the
deriverealisticmodelsof the presenceof cloudsbelow,within,
presence
of cloudsTeA radiativeforcingchanges
frompositive and over the dust layers as they are transportedby the
to negativeasthe cloudopticaldepthincreases.
The differencein
radiativeforcingby dust from differentregionsis greaterwhen
the cloud is under the dust layer than when the cloud is over it.
The dependence
of radiativeforcingon cloudaltitudebecomes
moreimportantas the dustloadingincreases.
The presenceof a
cloud usually decreasessurfacefluxes relativeto "only dust"
conditions.A thin cloud at low Sun angles is an exception,
causinga slightincreasein the surfacefluxes.
6. Conclusions
atmosphericcirculation.
Acknowledgments.
Thisresearch
wassupported
by NASA
EeS-IDSgrantNAG 5-6504andeNR grantN00014-98-1-0121.
The authorswould like to thankLinh-ChanBrown for help with
final editing.
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