May 1, 1998 - and implications for cirrus ice nucleation. Andrew J. Heymsfield, Larry M. Miloshevich, and Cynthia Twohy. National Center for Atmospheric ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 9, PAGES 1343-1346, MAY 1, 1998
Upper-tropospheric relative humidity observations and implications for cirrus ice nucleation Andrew J. Heymsfield, Larry M. Miloshevich, and Cynthia Twohy National Center for Atmospheric Research,Boulder, CO Glen
Sachse
NASA Langley ResearchCenter, Hampton, VA Samuel Oltmans
NOAA Climate Monitoring and DiagnosticsLaboratory, Boulder, CO
Abstract.
Relative humidity (RH) measurementsgree of ice-supersaturationmust have been present in
acquired in orographic wave cloud and cirrus environments are used to investigate the temperaturedependent RH required to nucleate ice crystMs
in the upper troposphere,RH,•c(T).
High ice-
supersaturationsin clear air- conduciveto the maintenanceof aircraft contraillsyet below RH,•½ and therefore insufficient
for cirrus formation-
are not uncom-
the clear air to maintain the contraill,but a higher icesupersaturationmust be requiredto produceice crystals from the ambient aerosolpopulation.
Heymsfieldand Miloshevich[1995,hereafter•HM"] usedmeasurements in orographicwavecloudand cirrus
environments fromFIRE-II (the First ISCCP Research Experiment,PhaseII) to placeupper and lower lim-
mon. Earlier findings are supported that RH,• in its on RH,• in the temperature range-35 to -55øC. mid-latitude, continental environments decreasesfrom Orographicwavecloudsare well-suitedfor studyingice water-saturationat temperaturesabove-39øC to 75% nucleation becausethe airflow is relatively laminar and RH at-55øC. Uncertainty in determiningRH,• below the RH increasescontinuouslyvia vertical lifting toward -55øC results in part from size detection limitations of RH,• at the upwind cloud boundary. Furthermore, the microphysicalinstrumentation, but analysisof data ice crystalsin wavecloudsare carried downwindrather from the SUCCESS experiment indicates that RH,•,• than falling through the ascendingair as in cirrus, albelow -55øC is between 70 and 88%. A small amount lowing the ice nucleationconditionsto be sampledreof data acquiredoff-shoresuggeststhe possibilitythat peatedly[He•trnsfield andMiloshevich, 1993].The peak RH,• may also depend on properties of the aerosols. RH observedduring penetrationsinto the upwind edge of wave clouds occurs when the vapor depletion rate from growth of recently-nucleatedice crystalsis equal to the vapor supplyrate from coolingdue to vertical ve1. Introduction
locitiesof severa•m s-x, and represents an upperlimit
Uncertaintiesin the radiation and moisture budgets in climate modelsresult in part from inadequateknowledgeof cirrus microphysicalpropertiesand the thermodynamic conditionsrequired to nucleatecirrus ice cryst&Is. This study examinesthe temperature-dependent relative humidity required to nucleate ice crystals in
the uppertroposphere, RH,•,c(T). This conditionde-
to RH,• sinceice crystalsmust have nucleatedprior to the peak and therefore at a lower RH. HM found that
thesepeak RH values- their empirical curve-
varied from water-saturation at temperatures
above-39øCto 75% RH (with respectto water) at55øC. ttM Mso determined a lower bound on RH,•,•
in
the temperaturerange-35to-47øC- about10%below
termines cirrus initiation and frequency of occurrence, influencescirrusmicrophysicaland radiative properties, and therebyinfluencescirrus effectson climate. Ice-supersaturation in the upper tropospherehaslong been inferredfrom the observationof long-lastinga•r-
the RHh,• curve-
based on the maadmum RH mea-
but only recently has instrumentation been capable of accurately measuringice-supersaturationfrom aircraft at cold temperatures. The contraillshownin Fig. I was produced in ascendingair upwind of an orographic wave cloud by the NASA DC-8 aircraft dur-
colder temperatures. Most data presentedin this paper representmid-latitude, continental, non-convective
sured in "clear air" in a non-orographicenvironment during FIRE-II. In this article we use RH and microphysicaldata from balloon launchesnear Boulder, Colorado and from the FIRE-II and SUCCESS experiments
craftcontrailsin otherwise clearair [e.g.,Brewer,1946], to furtherexploreboundson RH,•c(T), particularlyat
ing SUCCESS(the "Subsonicaircraft: Contrailland
environments.
2.
Observed
Ice Nucleation
Balloon-borne
Conditions
Cloud Effects Special Study" experiment). Some de-
2.1.
Measurements
Copyright1998by theAmericanGeophysical Union.
Ice-supersaturations of 20-30% have been observed in clear air and in regionsof low ice crysta•concentra-
Papernumber98GL01089.
are shown in Fig.
0094-8534/98/98GL-01089505.00
NOAA balloon-bornecryogenichygrometer[Oltmans
tion near the tops of cirrus [HM]. RH measurements
1343
2 from monthly launchesof the
1344
HEYMSFIELD
ET AL.: RELATIVE
HUMIDITY
FOR ICE NUCLEATION
IN CIRRUS
FIRE-II
Sabreliner
100' kl'•l•r•-/•i• ' .................. . ß .... ' "
•
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•
40
,
, • ....
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wavecloudnear Boulder,Coloradoon 2 May 1996.
:":: • •' •'
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Figure 1. Contrailproduced upwind of anorographic
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Photocourtesy ofDanBreed(NCAR/MMM).
• 60 and Hoffman, 1995]. The highestRH pointsbelow40øC are consistentwith RHh,, within the instrument's
estimatedaccuracyof 5-10%. Althoughtheselaunches were preferentially conducted in dear-air conditions,
somelaunchesare knownto havepassedthroughcirrus and thereforethe data doesnot necessarily representRH,,½. However,Fig. 2 demonstrates that high ice-supersaturationsare not uncommon. 2.2.
•
--,---
•'
40
,
--,:.'...t., ,'•"! '
20
.
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....
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[igute •. • me•u•ements f•om a c•yogenichyg•omete• on t•e NOA• Sab•e•e• •c•af[ du•ing •I•II, shown• • func[ion of tempe•a[u•e. Panel A: clea•-• data • dete•m•ed f•om 2D-C me•u•ements; Panel B'
FIRE-II
Clear-airRH data from FIRE-II (not previouslyreportedby HM) areshownin Fig. 3A, wherein thiscase •-cloud da•a. Sold •ne • RHea; d•hed •ne • ice"clear air" meansan absenceof ice crystalslarger than the 50/•m detectionthresholdof a Particle Measuring Systems2D-C probe for at least two secondsprior to and after an accepteddata point. Since the detection threshold of the 2D-C probe is so large, data points that lie abovethe RHh, curve were examinedusingsimultaneousparticle collectiondata from a Video Ice
salutation.
the VIPS, or the imagesshowedextremelylow ice crystad concentrations. The accuracy of the NCAR cryo-
genic hygrometerusedto make the RH measurements is 4-1øCin the frost-pointtemperature,or about 10% in RH at upper-tropospheric air temperatures[HM]. (vIPS) The highestclear-airRH data pointsare 5-7% above 1996].Thesepointswereeitherverifiedto be freeof ice crystalslargerthan the 5-10 tzmdetectionthresholdof RHa, over the temperature range for which RHa, is defined. The RH at-62øC in Fig. 3A reaches78%, therefore RH,•½ is not given by extrapolation of the
Marshall, Colorado Soundings
.... i.... i .... i.... i.... i.,, ,,I,,,•
100
:._./•.....:. -'-.'..-•.:::?'!" f•'•','-.•.'•-"•',' •
'-"•: -_..-'-:t.'" '"':"'-' ':.--'.' •:•:':':•"
80
'-•;.;'' -')"---:".-J-'--,'•'":-:"" "•:'K'::•'•-' ;•'"•,"•'. ,•: :.• -..:,,....•.
. ---.. ß. - .: .•½-• .•,:4• •-..• '.
•. ,•,, ;;-..".,..
::•:.,.t..,: ..:.,.•.-•_':";':, ......'_-•. • ..•...ßo ß ß.... ..... •.t.o•.•.. ;•-...• ...-•...,. ~ •.-•._•.,..•.•, •., ß --:-,.., ,:.... .•._..;.'..:•_•;.......
wave cloud environments
because the vertical
velocities
ß • -g.•, ..... -• ,-¾•.'•. -.,,..:•_-. •........ •.-. ,•" ": ..... •,.•- ..... ,-•,-' • :.'•-.;. ., ß,.,..., ,.ß ß _..•,•, .•, ,7. ..•. .,•,• .:..,.
are lower and therefore the increase in RH while crys-
"•:• . --.g.. , •,,ß •:,. -.. o-_ . ., ..,.,•,. .. - • •.• ß • , t...~: ...... t-•\ :.. ß•,• ß'• '.•.ß .•..e_..?•,..,
data (Fig. 3B)demonstratethat the RH can increase
ß
....:,.,.=,
'.'"'.'-' •:c-:-' •,•
•.
-•. "•':,
-
-
•.-
..,
-:
.-.•.',•,,...•...•?•.•- ,,.,..:. :.•,.. •,• ,,..,..,-•.'.. '::-T ..... ....• ......'•"•"':."•",--•.:'•: ".-.:"•: •::, "' 0
-70
RHa, curve below-55øC. However, slow ice crystal growth rates at cold temperatures may have resulted in an erroneous"clear air" classificationof some highRH points sinceundetectableice crystals smaller than 5-10/•m may have recently nucleated. If so, the resulting overestimateof RH,•½ should be small relative to
' •-".:.,
-60
-,50
: -.- -•
-40
-50
T (øC)
-20
',,''. -':-",.'.:'
,•..,.....-.
tals grow to detectablesizesis less. The in-cloudcirrus substantially above RH,•½.
High in-cloud RH values
are morelikely to occurwhenthe verticalvelocity(and hencethe vaporsupplyrate) is high, or when the ice
.-•.,i•7•,
-10
•
ß
0
crystals are small or the temperature is low, both of which result in slowice crystal growth rates and therefore low vapor depletionrates.
Figure 2. RH measurementsfrom the NOAA cryo- 2.3. SUCCESS genichygrometerfor monthly balloonlaunchesbetween Figure 4 showsRH in clear air measuredby a diode 1991 and 1996 near Boulder, Colorado, shownas a function of temperature. Solid line is RH•, from HM; laser hygrometer(DLH) [Va•t et al., 1997]for three dashed line is ice-saturation. flightsduring SUCCESS,wherein this case"clearair"
HEYMSFIELD
ET AL.' RELATIVE
SUCCESS
Clear
Air
HUMIDITY
Data
IN CIRRUS
1345
highestRH regionsmay not havebeenpenetratedsince the aircraft wasrestrictedto fly primarily normal rather than parallel to the wind direction, althoughthe data were acquired immediately upwind of the visible cloud edgewherethe highestRH valueswould be expected. The orographicwaveenvironmentsampledon the lee sideof the Rocky Mountainson 2 May at temperatures
A: .50 April 1996
lOO
FOR ICE NUCLEATION
8O
between-57and-65øC (Fig. 4B) showedclear-airRH valuesapproachingwater-saturation,seeminglyinconsistent with
the FIRF_,-II
and balloon-borne
measure-
ments. As exploredin Section3, ice nucleation(and RH,,,c) likely precededthe peak RH becausethe ver-
2O
ticalvelocities are high(~2.5 m s-•) andicecrystal growthratesat -65øCare very slow(abouta factorof 7 slowerthan at -55øC),implyinga relativelyrapid in-
lOO
creasein RH aboveRH,•,c during the time when newlynucleatedice crystals grew to detectablesizes. The off-shore,non-waveenvironmentsampledoff the
8O
•' 60 rY
Californiacoaston 12 May (Fig. 4C) providesclearair data at low temperatures when the vertical veloci-
tieswerelowerthanin Fig. 4B (generally
