Testing IWC Retrieval Methods Using Radar and Ancillary Measurements with In Situ Data

29 Pages·2008·5.79 MB·English

JANUARY2008 HEYMSFIELD ET AL. 135
Testing IWC Retrieval Methods Using Radar and Ancillary Measurements with In Situ Data
ANDREW J. HEYMSFIELD,* ALAIN PROTAT,(cid:1) RICHARD T. AUSTIN,# DOMINIQUE BOUNIOL,(cid:1)
ROBINJ.HOGAN,@ JULIEN DELANOË,@ HAJIME OKAMOTO,& KAORI SATO,&
(cid:1)(cid:1)
GERD-JAN VAN ZADELHOFF,** DAVID P. DONOVAN,** AND ZHIEN WANG
*NationalCenterforAtmosphericResearch,##Boulder,Colorado
(cid:1)Centred’E´tudedesEnvironnementsTerrestreetPlanétaires,Vélizy,France
#DepartmentofAtmosphericScience,ColoradoStateUniversity,FortCollins,Colorado
@DepartmentofMeteorology,ReadingUniversity,Reading,UnitedKingdom
&CenterforAtmosphericandOceanicStudies,TohokuUniversity,Sendai,Japan
**KoninklijkNederlandsMeteorologischInstituut,DeBilt,Netherlands
(cid:1)(cid:1)
DepartmentofAtmosphericSciences,UniversityofWyoming,Laramie,Wyoming
(Manuscriptreceived3October2006,infinalform26April2007)
ABSTRACT
Verticalprofilesoficewatercontent(IWC)cannowbederivedgloballyfromspacebornecloudsatellite
radar(CloudSat)data.IntegratingthesedatawithCloud-AerosolLidarandInfraredPathfinderSatellite
Observation(CALIPSO)datamayfurtherincreaseaccuracy.EvaluationsoftheaccuracyofIWCretrieved
fromradaraloneandtogetherwithothermeasurementsarenowessential.Aforwardmodelemploying
aircraftLagrangianspiraldescentsthroughmid-andlow-latitudeicecloudsisusedtoestimateprofilesof
whatalidarandconventionalandDopplerradarwouldsense.RadarreflectivityZ andDopplerfallspeed
e
atmultiplewavelengthsandextinctioninvisiblewavelengthswerederivedfromparticlesizedistributions
and shape data, constrained by IWC that were measured directly in most instances. These data were
providedtoeightteamsthattogethercover10retrievalmethods.Almost3400verticallydistributedpoints
from19cloudswereused.Approximatecloudopticaldepthsrangedfrombelow1tomorethan50.The
teamsreturnedretrievalIWCprofilesthatwereevaluatedinsevendifferentwaystoidentifytheamount
andsourcesoferrors.Themean(median)ratiooftheretrieved-to-measuredIWCwas1.15(1.03)(cid:2)0.66
forallteams,1.08(1.00)(cid:2)0.60forthoseemployingalidar–radarapproach,and1.27(1.12)(cid:2)0.78forthe
standard CloudSat radar–visible optical depth algorithm for Z (cid:3) (cid:4)28 dBZ. The ratios for the groups
e e
employingthelidar–radarapproachandtheradar–visibleopticaldepthalgorithmmaybelowerbyasmuch
as25%becauseofuncertaintiesintheextinctioninsmalliceparticlesprovidedtothegroups.Retrievals
fromfuturespaceborneradarusingreflectivity–Dopplerfallspeedsshowconsiderablepromise.Alidar–
radarapproach,asappliedtomeasurementsfromCALIPSOandCloudSat,isusefulonlyinanarrowrange
oficewaterpaths(IWP)(40(cid:5)IWP(cid:5)100gm(cid:4)2).BecauseoftheuseoftheRayleighapproximationat
highreflectivitiesinsomeofthealgorithmsanddifferencesinthewaynonsphericalparticlesandMieeffects
areconsidered,IWCretrievalsinregionsofradarreflectivityat94GHzexceedingabout5dBZ aresubject
e
touncertaintiesof(cid:2)50%.
1. Introduction surfaceandbycontrollingthelossofthermalenergyto
space. Because of their height in the atmosphere, ice
Clouds cover approximately 60% of the earth’s sur-
cloudshaveadominanteffectonlongwaveforcingand
face,stronglyinfluencingitsenergybudgetbycontrol-
on the earth’s net radiation budget (Hartmann et al.
ling the amount of solar radiation reaching the earth’s
1992). Details of the ice microphysics—including the
icewatercontent(IWC),icewaterpath(IWP),extinc-
##TheNationalCenterforAtmosphericResearchissponsored tioncoefficientinvisiblewavelengths((cid:6)),andicepar-
bytheNationalScienceFoundation. ticle shape—significantly affect ice cloud radiative
properties.
Cloud satellite radar (CloudSat), with an onboard
Corresponding author address: Andrew J. Heymsfield, 3450
millimeter-wavelength (94.05 GHz) radar, and the
MitchellLane,Boulder,CO80301.
E-mail:[email protected] Cloud-Aerosol Lidar and Infrared Pathfinder Satellite
DOI:10.1175/2007JAMC1606.1
©2008AmericanMeteorologicalSociety
JAM2605
136 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME47
Observation (CALIPSO) satellite, with a dual wave- however, excellent results were obtained when used
length(0.532and1.064(cid:7)m)anddualpolarizationlidar withancillarylidarorradiometricmeasurements.Mace
system, present new opportunities to characterize the et al. (2005) used a statistical approach to compare
microphysical properties of ice clouds on a global Moderate Resolution Imaging Spectroradiometer
scale.CloudSatprovidesdataenablinginvestigatorsto (MODIS) overpasses of cirrus with ground-based re-
quantitatively evaluate the relationship between verti- mote sensing observations. Using retrievals of cloud
calprofilesofcloudicewatercontentandcloudradia- properties, it was found that there was a positive cor-
tive properties and to utilize these results to improve relation in the effective particle size, the optical thick-
the representation of ice clouds in climate models ness, and the IWP between the satellite- and ground-
(Stephens et al. 2002). Because radar provides a mea- basedobservations,althoughthereweresometimessig-
surement of the equivalent radar reflectivity Z , it is nificant biases. Hogan et al. (2006b) utilized realistic
e
necessary to develop methods to convert Z to IWC. 95-GHz radar and 355-nm lidar backscatter profiles
e
Early methods for this conversion used relationships simulatedfromaircraft-measuredsizespectratoevalu-
betweenIWCandZ fromiceparticlesizespectramea- ate the correction of lidar signals for extinction by ice
e
surementscollectedinsitu(e.g.,Heymsfield1977)and cloud, a potential source of error in the retrieval of
atthesurface(Sassen1987).AspointedoutbyAtlaset IWC from the lidar–radar approach.
al. (1995), there is no universal IWC–Z relationship Although there are in situ validation activities
e
becauseoflargescatterandsystematicshiftsinparticle plannedforCloudSatandCALIPSO,thereareanum-
size from day to day and cloud to cloud. For that rea- ber of issues beyond the retrievals that can lead to er-
son, recently developed techniques have retrieved the rors in the retrieved IWC. The lidar–radar approach,
IWC using more than radar reflectivity from single ra- combining coincident measurements synergistically,
dar alone. These techniques include the use of radar may provide IWC better than could be derived from
combinedwithcollocatedlidardata(Intrierietal.1993; radar alone. However, there are issues related to con-
Wang and Sassen 2002a), Z and cloud optical depth verting lidar backscatter to extinction, although re-
e
derivedfromanIRradiometer(Matrosovetal.1998), centlydevelopedalgorithmstoderivevisibleextinction
Z and cloud visible optical depth (Benedetti et al. profiles are relatively insensitive to the details of ice
e
2003), Z measured at two frequencies (Hogan and Il- microphysics, lidar backscatter-to-extinction ratio, and
e
lingworth 1999), Z and cloud radiance derived from lidarcalibration(Hoganetal.2006b).Multiplescatter-
e
atmospheric emitted radiance measurements to derive ing is an additional problem, as are attenuation of
layer-averageIWCforthincirrus(Maceetal.1998),Z CloudSat’sradarbeamwhentheradarreflectivitiesex-
e
and Doppler fall speed (Matrosov et al. 2002; Mace et ceed (cid:8)5 dBZ , spatial averaging scales from space-
e
al.2002;Delanoëetal.2007;SatoandOkamoto2006), borneradar,thedifficultyincollocatinganaircraft,ra-
and Z and temperature (Liu and Illingworth 2000; dar,andanappropriatecloud,andthelargedifferences
e
Hogan et al. 2006a; Protat et al. 2007). in radar beam volume and the sample volume of an
Ifthegoalofspaceborneradaristoprovidevertical IWC measurement probe.
profiles of IWC and IWP for use in evaluating and In this study, we perform a detailed evaluation of
improvingtherepresentationofcloudsinclimatemod- IWC retrieval methods, using test datasets derived
els,itisnecessarytoassesstheaccuracyandlimitations from in situ microphysical measurements. The data
of the retrievals. Ground-based remote sensing mea- providedtoeightteamsincludedalloftheinformation
surements have been used in conjunction with in situ needed for their retrieval methods: vertical profiles of
observations to evaluate retrieved IWC (Matrosov et Z , (cid:6), temperature, and Doppler fall speed. However,
e
al. 1995; Wang and Sassen 2002a). The evaluations re- the IWCs were not provided to the teams. Their re-
lied on in situ measurements of particle size distribu- trieved IWCs were then compared with the measured
tions(PSD)andestimatesoficeparticlemassandwere values. This approach is not subject to the lidar and
based on samples from one or two midlatitude cirrus radarissuesraisedabove,whichobviouslywouldaddto
clouds.TheIWCsderivedinthiswayareaccurateonly theerror.Insection2,wedescribethetestdatasetand
toafactorof2,sothattheevaluationsarenotconclu- themethodology.Insection3,weevaluatetheresults,
sive. Sassen et al. (2002) used a cloud model with ex- andinsection4,theprincipalfindingsaresummarized.
plicit microphysics to test algorithms for retrieving cir-
2. Data overview and products provided to
rus cloud IWC from millimeter-wavelength radar re-
contributors
flectivitymeasurements.TheyfoundthatradarZ -only
e
approaches suffer from significant problems related to This section provides an overview of the tempera-
basic temperature-dependent cirrus cloud processes; turesandmicrophysicalpropertiesencounteredduring
JANUARY2008 HEYMSFIELD ET AL. 137
the cloud penetrations on 19 days used for this study (2007a,hereinafterH07a),theirFig.3(caseA4,here)].
and describes the methodology used to derive IWC, Onecasetotherightofthestar-shapedsymbolisfrom
radar reflectivity, and extinction estimates provided to the ARM 2000 IOP (A4). Case A4 took place at
the participants. warmer temperatures than the other midlatitude cases
Nineteen cloud penetrations from four field cam- and had more complex particle habits, largely com-
paigns in low and midlatitudes constituted the micro- posed of spatial-type ice crystals including bullet ro-
physicaldatasetusedinthisstudy.Fifteenpenetrations settes.
werefromaircraftLagrangianspiraldescentswherethe Four Lagrangian spirals were conducted in anvils/
aircraftdescendedat(cid:8)1ms(cid:4)1whiledriftinginaspiral convective outflow regions during the Cirrus Regional
with the wind.1 Study of Tropical Anvils and Cirrus Layers Florida-
There was one aircraft spiral up through the cloud Area Cirrus Experiment (CRYSTAL–FACE, herein-
layer.Therewerealsothreeballoon-borneascents.The after CF). The first three cases (CF1–3) covered the
microphysics during the penetrations are treated as regionfromcloudtoptobase(CF1wasaspiralascent).
thoughtheyrepresenttheverticaldistributionofcloud A fourth case (CF4) began near (cid:4)10°C and is consid-
microphysics through the layer, while recognizing that ereddowntothemeltinglayer.Constituenticeparticle
theaircraftsamplesportionsofdifferenticesourcere- habits for the four spirals were complex, often aggre-
gionsduringeachloopofaspiraldescentorascentand gatedandinvariousstagesofriming(Heymsfieldetal.
theballoonsdrifthorizontallyastheyascend.Thispo- 2004, their Fig. 4).
tential problem should, however, in principle not im- Figure 1b shows estimated radar reflectivities at
pacttheresultsofourcomparisonssincetheinstrument three radar frequencies—13, 35, and 95 GHz—as de-
observables are simulated from the observations. rived from calculations using the particle size distribu-
Figure1asummarizesthetemperaturesencountered tions. Condensed water content measurements were
duringthecloudpenetrations.Eachpointrepresentsa used to constrain the ice particle mass estimates asso-
5-saircraftora25-mballoon-borneaverageandvalues ciated with the PSD. The methodology used to calcu-
are arranged chronologically unless altered for clarity late these reflectivities is elaborated upon below. The
where noted. The penetrations to the left of the star- synoptically generated clouds have radar reflectivities
shapedsymbolarefromcoldtemperature,midlatitude, primarily below 0 dBZ whereas the convectively gen-
e
synopticallygeneratediceclouds,evidentlyformedpri- erated ones are almost all above 0 dBZ . For the cold,
e
marilythroughsynopticforcingatcirrusaltitudes.The synopticallygeneratedcloudcases,thereflectivitiesare
first eight cases are aircraft Lagrangian-type spiral de- about the same at the lower and higher frequencies,
scents from the First International Satellite Cloud Cli- signifying negligible Mie (non-Rayleigh) scattering.
matology Project (ISCCP) Regional Experiment Conversely,therearenumerousMiescatteringperiods
(FIRE) I, 1986 (F1–8) and three additional ones are for the convectively generated clouds sampled.
from the Atmospheric Radiation Measurement Pro- Figure 2 shows the visible optical depths ((cid:9)) esti-
gram (ARM) 2000 intensive observing period (IOP) matedforthevariouscloudssampled.The(cid:9)arefound
(A1–3). Three are from balloonborne ice crystal repli- byintegratingtheextinctionderivedfromthePSDand
cator ascents through cloud layers from FIRE II, 1991 cross-sectionalareasestimatedfromtheparticleprobes
(FR1–3). The constituent ice particles of the synopti- [forwardscatteringspectrometerprobes(FSSP)(cid:1)2D],
callygeneratedcirrusarepredominantlybulletrosettes asinH07a,fromthecloudtoptobase(exceptforcase
in various stages of development, and aggregates CF4,whichisincludedbutdidnotsampletocloudtop).
thereof[seeHeymsfieldandIaquinta(2000),theirFig. The effects of ice shattering on the inlet of the 2D-C
15 (case FR1, here); Heymsfield and Miloshevich probe(Fieldetal.2006)havebeentakenintoaccount.
(2003), their Fig. 4 (case A2, here); Heymsfield et al. TheIWPareshownasafunctionof(cid:9).Moredetailsof
(2004), their Fig. 4 (case A1, here); Heymsfield et al. howtheIWCwerederivedaregivenbelow.Thecases
studiedincludeallcoldcloudtypesgivenintheISCCP
classification as demarcated in the figure.
1Lagrangian spiral descent is not ideal for characterizing the Figures 3–5 provide an overview of the vertical dis-
instantaneousverticaldistributionofcloudproperties.However, tributionofIWCasafunctionoftemperaturefor18of
if a source region near cloud top is relatively steady state and the19cloudprofiles.(SpiralA4isnotplottedsoasto
verticalwindshearisnotappreciable,themicrophysicalproper- reducethenumberoffigures.)Notethefluctuationsin
ties sampled downward through the cloud layer can be used to
theIWCduringtheaircraftdescents(e.g.,Fig.3b):this
approximatetheverticalstructure.Unfortunately,asourceregion
is due to the aircraft spiraling in and out of generating
isusuallysmallinthehorizontalsothatfluctuationsinthemicro-
physicalpropertiescanbeexpectedduringeachloopofaspiral. cells and is an unavoidable issue with aircraft spirals
138 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME47
FIG.1.(a)Measuredtemperaturesand(b)derivedradarreflectivitiesfor19verticalprofiles
throughicecloudsinfourfieldcampaigns.Eachaircraft“point”representsdatafrom5sof
aircraftsamplingorabout750mofhorizontalpath.Eachballoonbornepointrepresents25m
vertically.In(a),initialtemperatureforeachprofileisshownwithopensquares,theflight
identificationisshownbelowsquares.Atthebottomof(a),filledcirclesshowflightsfrom
FIREI(F),filledsquaresfromARM2000IOP(A),exceptwherethestar-shapedsymbol
represents three ascents from balloonborne replicator during FIRE II (FR), and filled tri-
anglesfromCF.In(b),radarreflectivitiesarefor95GHz(blue),35GHz(green),and9.6GHz
(yellow).
throughcloudlayers.TheIWCfallintherange0.001– wouldseethroughthedepthofthesecloudlayers.(The
0.3gm(cid:4)3.TheexceptionsarecasesCF2andCF4,with methodusedtoderiveZ isdiscussedattheendofthis
e
IWC (cid:8) 1 g m(cid:4)3. section and in appendix A.) Few points had reflectivi-
The vertical bars on the sides of each panel in Figs. ties above 0 dBZ , indicated by dark boxes on the left
e
3–5 provide information on the penetration depth of vertical bars.
vertically pointing lidar (right) and radar (left) for the The method to calculate the IWC and dBZ is best
e
cloud layers (see legend, Fig. 4i). Given that a lidar shown by first using examples from ARM IOP spiral
beamisoccultedatanopticaldepthofapproximately3 descentsA1–A3.Figures3a–cplottheIWCsmeasured
(Kinneetal.1992),thelayersinFigs.3a–ccanbefully directlybyacounterflowvirtualimpactor(CVI)probe
penetrated by upward (ground based) and downward (Twohy et al. 1997). Also shown in the panels (red
(e.g., spaceborne) lidar. Similarly, the CloudSat 94- symbols) are points representing the IWCs derived
GHz radar, with a detection threshold of (cid:4)28 dBZ , from the PSD (FSSP (cid:1) 2D probes), with mass–
e
Fig 1live4/C
JANUARY2008 HEYMSFIELD ET AL. 139
minority of the periods during the spirals except for 5
March,theIWCfellbelowtheCVIdetectionthreshold
(number of instances, Table 1) and were subsequently
derived from the m(D) relationships together with the
PSD.
AlsonotefromTable1thattheIWCinFSSP(small
particle)sizesamountedto12%–17%ofthetotalIWC.
The IWC in small particle sizes was considered in the
derivationofthem(D)relationships,andalthoughthe
precise value of the IWC in small particles is not well
known, the error resulting from the addition of the
FSSPIWCforthosecasesfallingbelowtheCVIdetec-
tionthresholdandfortheFIREIcasesisatmost15%.
NotethattwocasesA1andA4didnothavesupporting
FSSPdata.Thiswouldleadtoanunderestimateofthe
FIG.2.IWPasafunctionofopticaldepthfor19cloudprofiles. IWC for those periods when the IWC fell below the
For ARM and CF observations, IWP are primarily from direct CVI’s detection threshold.
measurements,andfortheothercases,fromtheparticleprobes. Particle habits were predominantly bullet rosettes
Extinctionisderivedfromtheparticleprobes.
and rosette aggregates for ARM cases A1–3. For that
reason, we can use the same values for a(T) and b
dimensionalrelationshipsscaledtoagreewiththemea- found for the ARM cases to estimate the IWC for the
sured IWC when the IWC is above the CVI detection remainingmidlatitude,synopticallygeneratedicecloud
thresholdofabout0.01gm(cid:4)3(seeH07a).Themass–di- layers (cases F1–F8 and FR1–3). From Table 1, given
mensional relationships (m (cid:10) aDb) derived from the that the particle habits from the FIRE cases are the
scaling process are used directly to produce reliable same as for the ARM cases, we can expect that the
IWCs only when the measured values fall below the applicationofthem(D)relationshipstotheFIREIand
CVI’sdetectionthreshold,asevidencedbyapplication II datasets will produce a mean error of (cid:2)10%.
oftheserelationshipstomorerecentdatasetswherethe AsshowninFigs.3d–fand4a–h,theprofilesofIWC
CVI threshold was (cid:8)0.001 g m(cid:4)3. H07a parameterizes for the synoptically generated cirrus exhibit consider-
the results of the scaling process. The coefficient a in able structure. The warmer temperature ice cloud lay-
them(D)relationshiphasbeenrepresentedintermsof ers were optically thick enough that a lidar beam was
temperaturea(T)andtheexponentbisevaluatedusing unabletopenetratethedepthofseveraloftheselayers
verticallypointingDopplerradarobservationsforcases (seeSassenetal.1994,whoreportedlidarobservations
A1, A4, and many other ARM observations (Heyms- fortheFIREcases).Thereflectivitiesalsoreach0dBZ
e
field et al. 2007b, hereinafter H07b). Temperatures in andabove,highlightedbythedarkverticalbarsonthe
thesethreecasesspannedtherangefrom(cid:4)50°to(cid:4)20°C left side of the panels, in some instances. For the
(andforacasenotshown,12March,to0°C).TheIWCs warmer case A4 (not plotted), where spatial-type ice
measured for the three cases are relatively constant crystals including bullet rosettes dominated, an a(T)
with temperature and fall primarily in the range from relationship appropriate for the range of temperatures
0.001 to several tenths of a gram per meter cubed. consideredwasdevelopedandevaluatedonthebasisof
Table 1 summarizes the application of the tempera- the four ARM cases.
ture-dependent mass–dimension relationships to the TheIWCsfromthefourCFcloudlayerssampledat
ARM 2000 spirals themselves. The median and mean temperaturesfrom(cid:4)45°to0°Cweremeasureddirectly
valuesoftheratior(cid:10)IWC(PSD)/IWC(measured)are ratherthaninferredfromanm(D)relationshipbecause
case dependent and are generally accurate to within the CVI detection threshold was usually exceeded
10%.Thegreaterdifferencenotedforthe9Marchcase (Table 1). We derived a(T) and b(T) in them(D) re-
is largely due to CVI measurement error. Sampling lationship and compared the IWC derived from the
during this case was in and out of generating cells and PSD with those measured. The ratios are nearly unity
theassociatedtrails.TheCVIexhibitshysteresis:water except for the 18 July case where there were no FSSP
vapor remains inside the instrument’s housing, leading PSD to include in the PSD estimate (Table 1).
to small underestimates within the IWC region and The ice clouds sampled during CF had large IWCs
greateroverestimateswhentheIWCdecreasesrapidly, and changed little in the vertical, indicating that they
as was the case on 9 March. Note also that for a small were primarily regions of fallout. In general, these
140 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME47
FIG.3.VerticaldistributionsofIWCfromLagrangianspiraldescentsduring(a)–(c)ARM2000IOPand(d)–(f)balloonborneice
crystalreplicatorascentsthroughcirrusduringFIREII.Spirals/ascentcodesusedinFig.1aareshownineachpanel.In(a)–(c),the
blacksolidlineshowsCVI-measuredIWC;reddotsshowthosederivedfromtheparticlesizedistributions,scaledaccordingtothe
measurements,andareusedexclusivelywhereIWCfallbelowtheCVIdetectionthreshold.LegendforverticalbarsisshowninFig.
4iandisdiscussedinthetext.
cloudswereopticallythick,inhibitinglidarpenetration For midlatitude clouds, we used the Mie scattering
into the cloud layer. The radar reflectivities above 0 estimatesforsphericalparticlesfromBohrenandHuff-
dBZ , as indicated by the bars on the left side of each man (1983). This method was chosen because the par-
e
panel in Fig. 5, extend throughout the layers. ticlessampledwerequasi-spherical:bulletrosetteswith
The participants received estimates of the radar re- some aggregates of rosettes. Using a discrete dipole
flectivities, derived from the PSD and m(D) relation- approximation for different crystal shapes, including
shipsforfrequenciesof9.6,35,and95GHz(seeFig.1b quasi-spherical ice particles, Okamoto (2002) found
and the discussion in appendix A). (A frequency of 95 that shape influences on Z are less than 2 dBZ for
e e
ratherthan94GHzisusedintheremainingpartofthis ensemble (PSD) volume-equivalent effective radii of
paper to conform to retrievals reported in the litera- less than 100 (cid:7)m. This criterion was satisfied for all
ture, although differences for frequencies of 94 and 95 of the midlatitude PSD. The dual-wavelength ratio,
GHz are negligible.) dWR (cid:10) dBZ (35 GHz)/dBZ (95 GHz), calculated
e e
Fig 3live4/C
JANUARY2008 HEYMSFIELD ET AL. 141
FIG.4.AsinFig.3,butforFIREILagrangianspiraldescents.NoIWCmeasurementsweremade;thereforeIWCareestimated
fromthePSD.Legendisshownin(i).
Fig 4live4/C
142 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME47
FIG.5.AsinFig.3,butforCFspirals.LegendforverticalbarsisshowninFig.4i.Verticaldottedlinecorrespondstotheabscissa
oftherightedgeofplotsinFigs.3and4.
Fig 5live4/C
JANUARY2008 HEYMSFIELD ET AL. 143
TABLE1.Applicationofmass–dimensionrelationshipstoARM2000IOPandCF2002spirals.[FortheFIRE-I(casesF1–8)and
FIRE-IIReplicatorcases(FR1–3),therewerenodirectmeasurementsoftheIWC.]TheasteriskindicatesIWC(Meas)(cid:5)0.005gm(cid:4)3.
IWC(PSD)/IWC(Meas)
TotalNo. Pointsbelow
Date Median Mean Stddev ofpoints CVIthreshold* RatioIWC /IWC
FSSP meas
5Mar2000(A1) 1.07 1.08 0.16 231 121 Nodata
9Mar2000(A2) 0.88 0.89 0.26 601 38 0.17
13Mar2000(A3) 0.93 1.0 0.30 311 29 0.13
12Mar2000(A4) 0.93 1.0 0.30 216 44 Nodata
16Jul2002(CF1) 0.95 1.04 0.64 281 3 0.12
18Jul2002(CF2) 0.85 0.84 0.11 301 1 Nodata
25Jul2002(CF3) 1.02 1.09 0.29 151 1 0.11
26Jul2002(CF4) 0.99 1.04 0.26 311 1 0.12
from the midlatitude PSD, is generally less than a few The few large particles sampled by the particle
Z decibels (Figs. 6a and 1b), indicating that errors re- probesdominatethereflectivity.InH07a,wereporton
e
sulting from the Mie scattering effects are generally an evaluation of the influence of the statistics of the
negligible. The largest dWR values are for 12 March PSDonZ .Statisticalvariationoftheiceparticlecon-
e
2000 (spiral A4). In Fig. 6b, the measured dWR centrations in the largest sizes is used to evaluate the
sampled by a ground-based radar on 12 March 2000, effects on Z . There is essentially no net bias in the
e
includingtheperiodofthespiral(seeH07b),followthe calculated Z resulting from the statistics of the small
e
same distribution with Z and has approximately the sample, although in the retrievals this could lead to
e
same magnitudes as in the calculations. increases in the statistical uncertainty in IWC derived
An estimate of the magnitude of the error of the from spaceborne platforms.
derived midlatitude radar reflectivities given for the Theensemble,reflectivity-weightedfallspeedsV at
Z
evaluationscanbeestimatedbyusingthePSDfromthe thethreeradarwavelengthsandforRayleighscatterers
four ARM spirals. The masses of all ice particles were werecalculatedfromthemassesandterminalvelocities
increased by 10%, thereby increasing the IWC by an of the ice particles integrated across the PSD. H07b
equal amount, expressing a reasonable degree of un- describe the methodology and compare the estimated
certainty in the CVI measurements and in the FSSP- distribution of V with reflectivity with that measured
Z
sizedparticles.Thenetincreaseinradarreflectivitywas byverticallypointingDopplerradarontwoofthedays
only 0.65 (cid:2) 0.34 dBZ . On the basis of this sensitivity used in this algorithm evaluation (A2 and A4). In ap-
e
study and 1) because the midlatitude particles are pendix A, we discuss the impact of errors in our esti-
quasi-sphericalwithnegligibleMieeffects,2)themass– mates of V on the retrieved IWC.
Z
dimension relationships produce accurate IWC (Table The volume extinction coefficient in visible wave-
1),and3)thereisreasonablygoodagreementbetween lengths(cid:6)wassuppliedforthoseparticipantsemploying
the calculated and measured reflectivities (H07b), we a lidar–radar approach. The (cid:6) were estimated from
conclude that the Z derived from the forward model twice the total particle area per unit volume in sizes
e
are probably accurate to 1 dBZ . from FSSP (assuming the particles were spherical)
e
Non-Rayleigh effects are significant for the CF through to the largest particles measured by the imag-
clouds (Fig. 1b) that were dominated by aggregates of ingprobes(basedontheparticlecross-sectionalareas).
complexcrystalshapes.Itisreasonabletoassumethat Heymsfield et al. (2006) compare estimates of (cid:6)from
theCFparticlesthatdominatetheradarreflectivityare the particle probes with those measured by the cloud
horizontally aligned aggregates of aspect ratio (height integrating nephelometer (CIN) probe (Gerber et al.
to diameter) equal to 0.6 based on the observations of 2000) from CF. The (cid:6)from the CIN are, on average,
Magono and Nakamura (1965) and Hanesch (1999). about 2 times those derived from the particle probes.
Matrosovetal.(2005)havedevelopedaT-matrixscat- Heymsfield et al. (2006) provide reasons why the par-
teringmodelthatadequatelydescribestheradarpolar- ticleprobeestimatesarelikelytobemorereliablethan
ization backscattering properties of most nonspherical those from the CIN, although we are not confident
(oblate)atmospherichydrometeors,includingicecloud about the contribution from the FSSP. The potential
particles, pristine snowflakes, and raindrops. Our for- impact of errors in (cid:6)on the retrieved IWC can be es-
wardmodelusestheMatrosovetal.(2005)modelwith timated by assuming, for example, that (cid:6)are over- or
an assumed particle aspect ratio of 0.6. underestimated by 20% but that the radar reflectivity
144 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME47
FIG.6.Comparisonofdual-wavelengthratio[dBZe(35)/dBZe(95)GHz]asafunctionofZe
at35GHzderivedfrom(a)midlatitudePSDand(b)ARMSGP2000IOP,fromUniversity
ofMassachusettsradarson12Mar2000.
supplied to the investigators is exactly correct. Taking withverticalprofilesoftemperature(T),Z atfrequen-
e
the FIRE cases (F1–F8) as an example, the Donovan cies of 9.6, 35, and 95 GHz, reflectivity-weighted fall
andvanLammeren(2001)radar–lidarmethodpredicts speeds (V ) at these wavelengths (cid:6) and an optical
Z
an increase in the IWC of 1.178 (cid:2) 0.014 g m(cid:4)3 (for depth (cid:9)as derived from (cid:6)integrated downward from
(cid:6) (cid:10) (cid:1)20%) and a decrease of 0.85 (cid:2) (cid:4)0.01 g m(cid:4)3 cloud top and upward from cloud base. The vertical
((cid:6)(cid:10)(cid:4)20%).[Potentialsystematicoverestimatesinthe resolution of the profiles was (cid:8)5 m.
(cid:6)provided to the investigators resulting from shatter- Table 2 summarizes the IWC retrieval methods and
ingontheinletoftheFSSPprobe(Fieldetal.2003)are the publications that describe their methodology,
evaluated in appendix B and quantified in Table B1.] groupedbetweendarkhorizontallinesaccordingtothe
NotethatFSSPdatawereunavailableforcasesA1,A4, methodsused.Mostofthealgorithmsusegamma-type
and CF2. particle size distributions but several use normalized
PSD represented in terms of the melted equivalent di-
3. Results
ameter. The algorithms include the use of Z alone
e
Eightteams,somewithmorethanoneIWCretrieval [1(Z95)], Z and (cid:9)[2((cid:11)95_(cid:12)D)], Z and T [3(ZT)], Z
e e e
method,participatedinthisstudy.Theywereprovided and (cid:6)[4(LiRad)], lidar–radar approach), and Z and
e