DTIC ADA530337: Parameterization of Middle Atmospheric Water Vapor Photochemistry for High-Altitude NWP and Data Assimilation

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Atmos. Chem. Phys.,8,7519–7532,2008
Atmospheric
www.atmos-chem-phys.net/8/7519/2008/
Chemistry
©Author(s)2008. Thisworkisdistributedunder
theCreativeCommonsAttribution3.0License. and Physics
Parameterization of middle atmospheric water vapor
photochemistry for high-altitude NWP and data assimilation
J.P.McCormack1,K.W.Hoppel2,andD.E.Siskind1
1SpaceScienceDivision,NavalResearchLaboratory,WashingtonDC,USA
2RemoteSensingDivision,NavalResearchLaboratory,WashingtonDC,USA
Received: 4June2008–PublishedinAtmos. Chem. Phys.Discuss.: 23July2008
Revised: 4November2008–Accepted: 17November2008–Published: 16December2008
Abstract. This paper describes CHEM2D-H2O, a new pa- abundancecontrolstheavailabilityofoddhydrogenspecies
rameterization of H O photochemical production and loss for catalytic ozone loss. In addition, the emission of long-
2
based on the CHEM2D photochemical-transport model of wave(terrestrial)radiationtospacebywatervaporisanim-
the middle atmosphere. This parameterization accounts for portant cooling process in the middle atmosphere. The rel-
the altitude, latitude, and seasonal variations in the photo- atively long photochemical lifetime of middle atmospheric
chemicalsourcesandsinksofwatervaporoverthepressure watervaporalsomakesitausefultracerforstudyingthedy-
region from 100–0.001hPa (∼16–90km altitude). A series namics of this region. Finally, the abundance of middle at-
of free-running NOGAPS-ALPHA forecast model simula- mosphericwatervaporisanimportantfactorcontrollingthe
tionsoffersapreliminaryassessmentofCHEM2D-H2Oper- formation of both polar stratospheric clouds in winter and
formance over the June 2007 period. Results indicate that polar mesospheric (or noctilucent) clouds near the summer
the CHEM2D-H2O parameterization improves global 10- mesopause, the former being important for heterogeneous
dayforecastsofuppermesosphericwatervaporcomparedto ozone loss. In more practical terms, water vapor is a fun-
forecasts using an existing one-dimensional (altitude only) damentalprognosticvariableinthedynamicalcoresofmost
parameterization. Most of the improvement is seen at high numericalweatherprediction(NWP)models. Fortheserea-
winterlatitudeswheretheone-dimensionalparameterization sons, NWP and data assimilation (DA) systems whose top
specifies photolytic H O loss year round despite the lack levelsextendintotheupperstratosphereandmesospherere-
2
of sunlight in winter. The new CHEM2D-H2O parameter- quire an accurate description of the photochemical sources
ization should provide a better representation of the down- andsinksofwatervapor.
welling of dry mesospheric air into the stratospheric polar
In general, operational requirements for timely forecasts
vortex in operational analyses that do not assimilate middle
prevent NWP systems from performing fully coupled pho-
atmosphericH Omeasurements.
2 tochemicalmodelcalculationsbecausetheyaretoocompu-
tationallyintensive. Onesolutiontothisproblemisemploy
parameterizationsbasedonlinearizationoftherelevantpho-
tochemical processes (see, e.g. Cariolle and De`que´, 1986).
1 Introduction
For a review of the development of linearized photochem-
istryparameterizations,seeMcCormacketal.(2006).
Althoughthemiddleatmosphere(15–100kmaltitude)isex-
tremely dry when compared to the troposphere, detailed Thispaperdescribesanewlinearizedgas-phasewaterva-
knowledge of the water vapor distribution in this region is por photochemistry parameterization that is based on the
importantforanumberofreasons.Forexample,watervapor CHEM2D zonally averaged photochemical-transport model
ofthe middle atmosphere. CHEM2D hasbeensuccessfully
used to develop fast, accurate parameterizations of strato-
Correspondenceto: J.P.McCormack spheric ozone photochemistry (McCormack et al., 2004,
([email protected]) 2006). MacKenzie and Harwood (2004) implemented this
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Parameterization of middle atmospheric water vapor photochemistry for
5b. GRANT NUMBER
high-altitude NWP and data assimilation
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14. ABSTRACT
This paper describes CHEM2D-H2O, a new parameterization of H2O photochemical production and loss
based on the CHEM2D photochemical-transport model of the middle atmosphere. This parameterization
accounts for the altitude, latitude, and seasonal variations in the photochemical sources and sinks of water
vapor over the pressure region from 100?0.001 hPa (16?90 km altitude). A series of free-running
NOGAPS-ALPHA forecast model simulations offers a preliminary assessment of CHEM2D-H2O
performance over the June 2007 period. Results indicate that the CHEM2D-H2O parameterization
improves global 10- day forecasts of upper mesospheric water vapor compared to forecasts using an
existing one-dimensional (altitude only) parameterization. Most of the improvement is seen at high winter
latitudes where the one-dimensional parameterization specifies photolytic H2O loss year round despite the
lack of sunlight in winter. The new CHEM2D-H2O parameterization should provide a better
representation of the downwelling of dry mesospheric air into the stratospheric polar vortex in operational
analyses that do not assimilate middle atmospheric H2O measurements.
15. SUBJECT TERMS
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7520 J.P.McCormacketal.: ParameterizedH Ophotochemistry
2
Fig.1.VerticalprofilesofthedominantCHEM2Dmodellossrates(moleculescm−3s−1)at5◦Non15Junefor(a)CH4and(b)H2O.
type of water vapor photochemistry parameterization in a etal.(2003). CHEM2Dextendsfrompoletopolewithgrid
general circulation model to study trends in middle atmo- pointsspacedevery4.8◦inlatitude;theverticaldomaincon-
spherichumidity. Hereweapplyasimilarapproachtopro- sistsof88pressurelevelsfromthesurfacetop=6×10−5hPa
vide a suitable representation of middle atmospheric water (∼116km)spacedevery∼1.3km.
vaporforoperationalNWP/DAsystems. CHEM2DcomputesthesolarUVheatingabove40kmex-
This new water vapor photochemistry parameterization, plicitlyusingthesamespectraldistributionofsolarUVand
designated CHEM2D-H2O, has been recently implemented absorptioncrosssectionsasinthemodelphotolysiscalcula-
in the high-altitude version of the Navy Operational Global tions. Detailsofthephotolysiscalculationscanbefoundin
Atmospheric Prediction System (NOGAPS-ALPHA). Here Summersetal.(1997)andreferencestherein. Modeled
we present a description of CHEM2D-H2O as well as re- O absorptionintheSchumann-Rungebandsisbasedon
2
sults from a series of NOGAPS-ALPHA forecast model theparameterizationofMinschwaneretal.(1993). H Oab-
2
simulations designed to evaluate CHEM2D-H2O and com- sorption is treated as in Siskind et al. (1994). O and H O
2 2
pareitsperformancetoasimplerone-dimensional(1-D)wa- absorption at Lyman-α is based on the method of Lewis
ter vapor photochemistry scheme currently used in opera- et al. (1983). The effects of CH absorption at Lyman-α
4
tional NWP/DA systems. The ultimate goal of this work on H O are not considered. Solar cycle variations in irra-
2
is to provide global simulations of mesospheric water va- diance are specified from 1200–8000 A˚based on the mea-
por accurate enough to identify the physical processes gov- surements of Lean et al. (1997). The CHEM2D photoly-
erning polar mesospheric cloud (PMC) formation near the sis rates reported here are for solar minimum conditions.
summer mesopause. Section 2 gives a general overview The Lyman-α irradiance at solar minimum was taken to be
of the CHEM2D model and middle atmospheric water va- 3×1011photonscm−1s−1.
por photochemistry. Section 3 describes the implementa-
Watervapor(H O)inthemiddleatmosphereisproduced
2
tionofCHEM2D-H2OinNOGAPS-ALPHA.Section4ex-
directly through oxidation of stratospheric methane (CH )
4
amines middle atmospheric water vapor fields from a se-
by the hydroxyl radical (OH+CH →H O+CH ) and in-
4 2 3
riesofNOGAPS-ALPHAforecastmodelsimulationstoas-
directly through a series of reactions involving the methyl
sesstheperformanceofCHEM2D-H2O.Section5summa-
radical (CH ). The net effect is that approximately two
3
rizes these results and outlines future research applications
H OmoleculesareproducedforeachCH moleculelostin
2 4
ofCHEM2D-H2O.
the stratosphere (e.g., LeTexier et al., 1988, and references
therein).Consequently,thetotalnumberdensityofhydrogen
throughoutmuchofthestratosphereQ=2[CH ]+[H O]isa
4 2
2 H OphotochemistryintheCHEM2Dmodel
2 constant (neglecting the relatively small amount of molecu-
lar hydrogen, H ). This relationship can be used to express
2
CHEM2D is a zonally averaged (2-D) global model that stratospheric H2O production in terms of H2O abundance,
features a fully self-consistent treatment of radiative, pho- whichisadvantageoussinceiteliminatestheneedforaprog-
tochemical, and dynamical processes of the middle atmo- nosticCH4variable.
sphere (see, e.g., McCormack et al., 2006; McCormack et Figure 1a plots vertical profiles of individual CHEM2D
al.,2007). Themodelphotochemistryaccountsforreactions CH lossrates(moleculescm−3s−1)at5◦Non15June. Re-
4
among54differentspeciesusingreactionratesfromSander actions (5) and (6) listed in Fig. 1a are the key reactions
Atmos. Chem. Phys.,8,7519–7532,2008 www.atmos-chem-phys.net/8/7519/2008/
J.P.McCormacketal.: ParameterizedH Ophotochemistry 7521
2
Fig.2. Pressure-latitudeplotsofCHEM2DCH4photochemicallifetimes(indays)for(a)15Juneand(b)15December,andofCHEM2D
H2Olifetimesfor(c)15Juneand(d)15December.Shadingin(a)and(b)indicatesregionwhereCH4lossisprimarilythroughphotolysis.
Contoursaredrawnat3,5,10,30,50,100,300,500,and1000days.
responsible for stratospheric H O production. Above the where [CH ] and [H O] denote CHEM2D CH and H O
2 4 2 4 2
0.1hPa level, CH loss occurs through photolysis and does abundances (moleculescm−3) and L is the loss rate
4 i
notproduceH Odirectly. (moleculescm−3s−1)fortheindividualreactionsinFig.1.
2
As a result total hydrogen Q is not conserved in the Figures 2a and b show that the shortest CH lifetimes
4
mesosphere. Middle atmospheric H2O is destroyed pri- (<2days) are in the summer mesopause region where pho-
marily through photolysis in the mesosphere by solar UV tolysis is the dominant loss mechanism. A secondary mini-
and Lyman-α radiation, yielding a number of different by- muminτ isseennearthesummerpolarstratopausedue
CH4
products depending on the energy of the incoming radia- to rapid CH loss via reaction with chlorine (Reaction 4 in
4
tion (Steif et al., 1975). H2O is lost in the stratosphere Fig.1).
through reaction with atomic oxygen. Figure 1b plots in- The shaded regions in Fig. 2 indicate where CH loss is
4
dividualCHEM2DverticalprofilesofH2Olossratesat5◦N dominatedbyphotolysis,aprocessthatdoesnotdirectlypro-
on 15 June. The loss rates for the photolysis Reactions 1– duce H O. The unshaded regions of Fig. 2a and b indicate
2
3inFig.1barebasedonestimatedquantumyieldsof0.78, whereCH lossproducesH OinthestratosphereviaReac-
4 2
0.10, and 0.12, respectively. These values of the quantum tions5and6asinFig1a.
yieldsweredeterminedfromasynthesisofavailablelabora- Valuesofτ forlevelsbelow0.1hPaexceed50days,in-
CH4
torymeasurements(Steifetal.,1975;Harichetal.,2000). dicatingthatstratosphericH Oproductionisslowcompared
2
Photochemical loss rates are commonly used to infer an to typical transport timescales. As a result, including the
effectivephotochemicallifetime,whichisaconvenientway effects of this process in typical NWP/DA systems issuing
toquantifytherelevanttimescalesforphotochemistryrela- 5- to 10-day forecasts is not crucial provided that accurate
tivetootherphysicalprocessessuchasadvection. Figure2 stratospheric humidity data are being regularly assimilated
plots CHEM2D photochemical lifetimes of CH4 and H2O globally. In the absence of such data, or when conducting
(τCH4 andτH2O respectively)forJuneandDecembercondi- longer free-running model simulations for seasonal predic-
tions throughout the middle atmosphere. The lifetimes are tion or climate simulations, the contribution of CH oxida-
4
computedfromthesumoftheindividuallossratesas tiontoH Oproductionbecomesmoresignificant.
2
τ = [CH4] , τ = [H2O] (1) Fig. 2c and d show that values of τH2O in the summer
CH4 P6 L H2O P4 L hemisphere are less than 5 days above the 0.01hPa level,
i=1 i i=1 i
indicating that H O photolysis is an important effect for
2
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7522 J.P.McCormacketal.: ParameterizedH Ophotochemistry
2
Fig.3. Pressure-latitudeplotsofnetCHEM2Dmiddleatmospherewatervaporphotochemicaltendency(P−L)H2O for(a)15Juneand
(b)15Decemberinpartspermillion(volume)permonth. Contoursaredrawnat±0.1,±0.3,±1,±3,and±10ppmvpermonth. Dashed
contoursdenotenegativevalues.
∂r
accurate medium-range forecasts in this region. Values of =k (r −r)−k r (2)
1 Q 2
τ near the mesopause increase rapidly poleward of 50◦ ∂t
H2O
latitudeinwinterastheamountofsunlightdiminishes. where r is the local water vapor mixing ratio and r is the
Q
To highlight the latitude, altitude, and seasonal depen- equivalenttotalhydrogenmixingratio.
dences of H2O photochemistry in the middle atmosphere, Thecoefficientsk1 andk2 aredeterminedfromanalytical
Fig. 3 plots the net CHEM2D photochemical tendency for fits to quoted values of τ in the stratosphere and τ
CH4 H2O
water vapor, (P−L)H2O, in parts per million by volume in the mesosphere, respectively, at various altitudes from
(ppmv) per month as a function of latitude and pressure for BrasseurandSolomon(1986).
15Juneand15December,whereP isthenetproductionrate Thecoefficientsk andk varywithaltitudeandarecon-
1 2
and L is the net loss rate. In the stratosphere, (P−L)H2O stant with latitude and season. It is assumed that rQ has a
is weakly positive as a result of relatively slow H2O pro- constant value of 6.8ppmv based on the results of Randel
duction through CH4 oxidation in the absence of water va- et al. (1998). Figure 4 plots CHEM2D values of rQ show-
por photolysis at these altitudes (see Figs. 1 and 2). In the ingthisassumptionholdsformostofthestratosphereexcept
mesosphere,(P−L)H2Oismostlynegativeduetophotolytic winterpolarregionswheredownwardtransportofdrymeso-
loss, with the largest loss rates occurring near the summer sphericairoccurs(LeTexieretal.,1988;Randeletal.,1998).
mesopause region. The positive values of (P−L)H2O in Thus the ECMWF scheme provides a reasonable 1-D pho-
the winter hemisphere near 80km poleward of 50◦ latitude tochemical constraint on global stratospheric H O, but may
2
are the result of enhanced H2O production via the reaction overestimatestratosphericH2Oproductioninregionswhere
OH+HO2 → H2O+O2. This enhancement is due to pole- conservationofrQbreaksdown.
wardtransportofoddhydrogenfromsunlitlatitudesintopo-
To compare this 1D approach to the CHEM2D-H2O pa-
lar night, where it becomes long-lived in the upper meso-
rameterization, Fig. 5 plots the vertical profile of the com-
sphere(BrasseurandSolomon,1986). binedphotochemicallifetime,(k +k )−1 fromtheECMWF
1 2
The values of τ , τ , and (P−L) presented in
CH4 H2O H2O scheme (ECMWF, 2006) along with CHEM2D values of
this section, which vary with latitude, altitude, and month, τ and τ over the equator for each month of the year.
CH4 H2O
serve as the basis for the new CHEM2D-H2O parameteri- Only CHEM2D values of τ below the 0.1hPa level are
CH4
zation. CHEM2D-H O differs from the water vapor photo-
2 plotted in Fig. 5, since it is in this region where CH loss
4
chemistry parameterization currently used in the European
leadstoH Oproduction. AlsoplottedinFig.5arevaluesof
2
CentreforMediumRangeWeatherForecasts(ECMWF)In-
thecombinedCHEM2Dlifetime
tegrated Forecast system (IFS) (Untch and Simmons, 1999;
1
ECMWF,2006;Feistetal.,2007),whichparameterizespro- τ∗ = (3)
duction via stratospheric CH oxidation and loss via meso- τ−1 +τ−1
4 CH4 H2O
spheric photolysis as a function of altitude only, neglecting
foreachmonth,whicharedeterminedfromtheplottedvalues
possiblelatitudeandseasonaldependences. Thismethodex-
ofτ andτ inFig.5.
pressesthewatervaporphotochemicaltendencyas CH4 H2O
The quantity τ∗ provides a single, concise description of
thetimescaleforH Ophotochemistrythroughoutthemiddle
2
atmospherethatvarieswithlatitudeandseasonanalogousto
Atmos. Chem. Phys.,8,7519–7532,2008 www.atmos-chem-phys.net/8/7519/2008/
J.P.McCormacketal.: ParameterizedH Ophotochemistry 7523
2
Fig.4.Pressure-latitudeplotsoftotalhydrogenrQ(inppmv)fromtheCHEM2Dmodelfor(a)15Juneand(b)15December.Contoursare
drawnevery1ppmv.
Fig.5. Comparisonofcombinedphotochemicallifetime(days)in Fig.6. Comparisonofcombinedphotochemicallifetime(days)in
the ECMWF water vapor photochemistry parameterization (black the ECMWF water vapor photochemistry parameterization (black
curve)withCHEM2DlifetimesτH2O (purplecurves),τCH4 (gold curve) with values of the CHEM2D effective lifetime τ∗ (red
curves), τ∗ (red curves) for all 12 months over the equator. The curves)at70◦Nonthe15thdayofMarch, June, September, and
CHEM2DτCH4 profileisonlyplottedatlevelswhereCH4 oxida- December.
tionisthedominantlossprocess(c.f.,Fig.1andunshadedregion
inFig.2).
The CHEM2D model results presented in this section
demonstrate that the net middle atmospheric H O photo-
2
the 1-D profile of (k +k )−1 in the ECMWF scheme. The chemicaltendencyexhibitspronouncedlatitudeandseasonal
1 2
overlappingcurvesinFig.5indicatethatthereisvirtuallyno variationsthatshouldbeaccountedforinNWP/DAsystems
seasonal dependence in the H O production and loss terms extendingintothemesosphere. TheCHEM2D-H2Oparam-
2
atlowlatitudes,inagreementwithFigs.2and3. Asaresult, eterization described in the following section accounts for
CHEM2D values of τ∗ for all 12 months closely match the thesevariations.
1Dscheme’sk−1andk−1profile.
1 2
At higher latitudes, however, seasonal variations in τ
H2O
and τ are both quite large. As a result, τ∗ also exhibits 3 CHEM2D-H2OinNOGAPS-ALPHA
CH4
large seasonal variations at higher latitudes. To illustrate
This section describes the CHEM2D-H2O parameteriza-
thispoint,Figure6comparesvaluesof(k +k )−1 fromthe
1 2 tionanditsimplementationinNOGAPS-ALPHA.First,we
ECMWFschemewithvaluesofτ∗at70◦NforMarch,June,
briefly describe the NOGAPS-ALPHA forecast model and
September,andDecember.
data assimilation components. We then provide a detailed
description of the CHEM2D-H2O scheme as it is currently
usedinNOGAPS-ALPHA.
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7524 J.P.McCormacketal.: ParameterizedH Ophotochemistry
2
3.1 NOGAPS-ALPHAdescription Sounding of the Atmosphere Using Broadband Radiome-
try (SABER) instrument on the Thermosphere Ionosphere
The high-altitude NOGAPS-ALPHA NWP/DA system Mesosphere Energetics and Dynamics (TIMED) satellite
(Hoppeletal.,2008)iscapableofassimilatingmiddleatmo- (Kutepov et al., 2006), up to the 0.002hPa level, (E2008).
sphere constituent measurements such as H2O profiles ob- Above this level, model fields are initialized with climatol-
tainedfromtheNASAEOSAuraMicrowaveLimbSounder ogyasinMcCormacketal.(2006).Thealtituderesolutionof
(MLS) (Eckermann et al., 2008, referred to hereafter as the MLS water vapor retrievals decreases significantly with
E2008). The68-level(L68)NOGAPS-ALPHAglobalspec- increasingaltitude(Lambertetal.,2007). AGaussianaver-
tral forecast model (GSFM) initializes and advects spe- agingkernelintheverticaldirectionwithafull-widthathalf
cific humidity q at all levels from the surface to its top at maximum value of 2 km is applied to the assimilated MLS
5×10−4hPa. The q fields are initialized using traditional watervaporprofiles. Thiswasdonetoconstraintheanalysis
meteorological analyses from the surface up to the 200hPa tothelowresolutionMLSprofilesratherthanriskintroduc-
level. Above this level, the humidity fields can be initial- inganyspuriousverticalstructureintheH Ofieldsthatmay
2
izedusingeitherassimilatedwatervapormeasurementsfora ariseindata-poorregions.Thebackgrounderrorvariancefor
specificdateorazonalmonthlymeanclimatologybasedon watervaporintheupperatmospherewassetlargeenoughto
measurementsfromtheUpperAtmosphericResearchSatel- ensurethattheanalysiswashighlyconstrainedtotheobser-
lite(UARS)HalogenOccultation(HALOE)andMLSinstru- vations, producing a zonal mean distribution that is nearly
ments (Randel et al., 1998; Grooß and Russell, 2005), de- identicaltotheMLSdata. Forfurtherdetailsoftheassimila-
pendingontheapplication. tionsystem,seeHoppeletal.(2008).
ParameterizationsformoistphysicsinNOGAPS-ALPHA A comprehensive evaluation of the NOGAPS middle at-
areidenticaltothoseusedintheoperationalversionofNO- mospheric data assimilation system, including comparisons
GAPS(HoganandRosmond,1991). Theseincludeshallow with independent observations, is currently underway. The
cumulus mixing (Tiedtke, 1984), deep cumulus convection purposeofthispaperistointroducetheCHEM2D-H2Opho-
(Peng et al., 2004), and convective, stratiform, and bound- tochemistry parameterization and evaluate its performance
ary layer cloud formation and precipitation (Slingo, 1987; based on NOGAPS-ALPHA forecast model simulations of
Teixeira and Hogan, 2002). For a more complete descrip- middle atmospheric water vapor. The following section de-
tion of NOGAPS-ALPHA model physics, see McCormack scribestheimplementationofCHEM2D-H2OinNOGAPS-
et al. (2006) and E2008. Since they are designed primarily ALPHA.
fortroposphericapplications,themodel’smoistphysicsrou-
tines are only employed from the surface up to the 50hPa 3.2 TheCHEM2D-H2Oparameterization
level (∼20 km). In the stratosphere and mesosphere, pa-
rameterizedwatervaporphotochemicalproductionandloss TheCHEM2D-H2OparameterizationinNOGAPS-ALPHA
constraintheNOGAPS-ALPHAq fields. Withoutthispho- expresses the local time rate of change of the H2O mixing
tochemical constraint, upwelling of stratospheric air into ratio r as the difference between the zonally averaged pro-
the mesosphere would produce unrealistically high values duction and loss rates computed with the CHEM2D model
of mesospheric humidity over forecast periods of 2–3 days. (seeFig.3):
This would pose a problem for accurate forecasts of meso-
∂r
spherichumidity,especiallyinregionswherehumiditymea- =(P −L)H2O. (4)
∂t
surementsarenotassimilated.
Allforecastmodelresultsinthisstudyemploytriangular Although specific humidity q is the NOGAPS-ALPHA
truncation of the first 79 wavenumbers (T79), with a hori- model’sprognosticvariable,forthesakeofconsistencywith
zontal grid spacing of 1.5◦ in longitude and latitude on the the photochemical parameterization in Eq. (2) we will use
quadraticGaussiangrid. Themodelusesthenon-orographic volumemixingratior asthemoisturevariableinthefollow-
gravity wave drag parameterization of Garcia et al. (2007) ingdiscussion. Modelmixingratior isrelatedtoq through
withthesamesettingsasinE2008.TheT79L68modelisini- therelation
tialized with analyzed winds, temperature, and constituents
M q
(e.g., O andH O)producedbytheNOGAPS-ALPHADA r = d (5)
3 2 M 1−q
w
component, which is described in detail by Hoppel et al.
(2008)andE2008. Fromthesurfacetothemid-stratosphere, whereM andM arethemolecularweightsofdryairand
d w
theseNOGAPS-ALPHAanalysesarebasedonassimilation H O,respectively.
2
of conventional meteorological data sets used by the opera- We assume that (P−L) is primarily a function of r.
H2O
tionalT239L30system. In the mesosphere, this assumption is justified because the
FortheJune2007periodstudiedhere,NOGAPS-ALPHA lossrateduetophotolysisisdirectlyproportionaltothelocal
assimilates Aura MLS temperature, O , and H O profiles H Omixingratio. Inthestratosphere,wheretheproduction
3 2 2
(Froidevaux et al., 2006), as well as temperatures from the ratesviamethaneoxidationdependonthelocalCH mixing
4
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J.P.McCormacketal.: ParameterizedH Ophotochemistry 7525
2
Fig.7.Pressure-latitudeplotsoftheeffectivephotochemicallifetimeτ∗(days)usedintheCHEM2D-H2Oparameterizationfor(a)15June
and(b)15December.
ratio,thisassumptionholdswherethelocalH Omixingra- whereτ∗ representsthecombinedtimescalesforH Opro-
2 2
tiocanbeexpressedintermsoftheCH mixingratiothrough ductionviaCH oxidationinthestratosphereandH Opho-
4 4 2
theapproximateconservationofr (seeFig.4). Wherethe tolysisinthemesosphere. CHEM2Dmodelvaluesofτ∗ are
Q
assumptionofr conservationbreaksdown,i.e.,inthewin- tabulated as a function of latitude, pressure and season and
Q
terpolarstratosphere,CHEM2Dvaluesofτ exceed100 theninterpolatedinspaceandtimetotheNOGAPS-ALPHA
CH4
days. As shown below, accounting for these latitude and forecast model grid. Figure 7 plots the latitude and altitude
seasonalvariationsinH Oproductionandlossensuresthat dependenceofτ∗forJuneandDecemberconditions.
2
theCHEM2D-H2Oparameterizationonlyintroducesphoto- The analytical ECMWF expression Eq. (7) assumes con-
chemicaltendenciesinregionswhereitsunderlyingassump- stant total hydrogen r =6.8ppmv at all altitudes. As illus-
Q
tionsarevalid. trated in Fig. 4, this assumption breaks down in the upper
The function (P−L) [r] can be approximated using stratosphereandmesosphere. Inaddition,CH oxidationbe-
H2O 4
a first-order Taylor series expansion about a reference state comesanegligiblesourceofH Ointhemesosphere. Thus,
2
suchthat for CHEM2D-H2O we instead derive (P−L)o values as
∂r(λ,∂φt,p,t) =(P−L)oH2O+ ∂(P −∂rL)H2O(cid:12)(cid:12)(cid:12)(cid:12) (cid:0)r −ro(cid:1)(6) aCHfuEnMcti2oDnmofoldaetilt,uedxea,mpprelesssuorfewahnidchsewaseorensdhiorewHc2ntOlyprfervoimoutshlye
o
inFig.3. Thisapproachenablesustocomputeequilibrium
where λ is longitude, φ is latitude, p is pressure, and “o”
rates without any explicit reference to total hydrogen, and
denotesthereferencestate.
thustoextendtheseratesintothemesospherewheretotalhy-
WeproceedbyassumingthatEq.(6)yieldsanequilibrium
drogenconservationbreaksdownandisdominatedbypho-
(reference) state ro that is the net balance between photo-
tolyticloss.
chemicalproductionandloss,suchthatanydeviationsfrom
Adopting an approach similar to that for the CHEM2D
this state can be treated as small perturbations about that
ozone photochemistry parameterization, or CHEM2D-OPP
reference state. This follows the method used in linearized
(McCormacketal.,2006),thespecifichumidityphotochem-
ozonephotochemistryschemes,asreviewedbyMcCormack
icaltendencyinNOGAPS-ALPHAisappliedbyfirstdefin-
etal.(2006)andLahozetal.(2007).
ing a photochemical steady state value for the water vapor
Applying the expansion Eq. (6) to the ECMWF relation
mixingratio
Eq.(2),itisstraightforwardtoshowthat
rss =ro+(P −L)oτ∗ (10)
(P −L)oH2O =k1(cid:0)rQ−ro(cid:1)−k2ro, (7) sothattheH2Omixingratiotendencycanbeexpressedas
∂(P −L)o ∂r −(r −rss)
∂r H2O|o =−(k1+k2) (8) ∂t = τ∗ . (11)
Theupdatedmixingratiovalueiscomputedusingastandard
illustrating that this approach is mathematically consistent
backward-Eulersolutionoftheform
withthe1-DparameterizationinEq.(2).
" #
AsdiscussedinSect.2andillustratedinFigs.5and6,the 1t
CHEM2D-H2OanalogofEq.(8)is r(t +1t)=r(t)+[rss −r(t)] τ∗ (12)
1+ 1t
∂(P −L)o τ∗
H2O| =−(cid:0)τ∗(cid:1)−1 (9) andthenconvertedtospecifichumidityusingEq.(5).
o
∂r
www.atmos-chem-phys.net/8/7519/2008/ Atmos. Chem. Phys.,8,7519–7532,2008
7526 J.P.McCormacketal.: ParameterizedH Ophotochemistry
2
ppmv
Fig.8. Pressure-latitudeplotsofthezonalmeanreferencestateH2Omixingratioro (inppmv)forJunebasedon(a)thecombinedUARS
HALOE/MLSclimatologyusedinEXP1and(b)theAuraMLSclimatologyusedinEXP2.
Figure 8 compares these two ro distributions. The Aura
Table 1. H2O photochemistry parameterizations used in the
MLSH OmixingratiosinFig.8baregenerallyhigherthan
NOGAPS-ALPHAforecastmodelsimulations. 2
the UARS-based values in Fig. 8a throughout the Northern
Hemisphereextratropicaluppermesosphere.Theresultspre-
ExperimentName Description
sented in the following section demonstrate how these dif-
EXP1 CHEM2D-H2Owithro
ferences can affect 10-day forecasts of middle atmospheric
basedonUARSHALOE/MLS
H O.
EXP2 CHEM2D-H2Owithro 2
basedonAuraMLS
EXP3 ECMWF
4 Results
Table1liststhreesetsofNOGAPS-ALPHAforecastmodel
In theory, the reference state mixing ratio ro should cor- simulations, designated EXP1, EXP2, and EXP3, used to
respondtotheCHEM2Dmodelmixingratioatphotochem- evaluate the new CHEM2D-H2O parameterization. EXP1
ical equilibrium. In practice, values of ro(φ,p,t) are often employs CHEM2D-H2O with the UARS-based ro for the
specified using an observation-based climatology of middle month of June (see Fig. 8a). EXP2 employs CHEM2D-
atmospheric water vapor. Based on earlier experience with H2OwiththeAuraMLS-basedro forJune(Fig.8b). EXP3
linearizedozonephotochemistryparameterizations(McCor- usesthe1Dwatervaporphotochemistryparameterization(2)
mack et al., 2006; Geer et al., 2007; Coy et al., 2007), use currently used in the operational ECMWF IFS. These three
of an observation-based reference state ensures that the lin- setsofsimulationseachconsistoffive10-dayforecastsini-
earizedphotochemicaltendencytermswillnotproducelarge tialized on 5, 10, 15, 20, and 25 June 2007, encompassing
biasesbetweenthemodeledandassimilatedconstituentval- the period when PMC’s were observed in the Arctic region
ues,whichcannegativelyimpactforecastskill. Inthisstudy, (E2008). The EXP1, EXP2, and EXP3 simulations all use
we perform a similar evaluation of the CHEM2D-H2O pa- thesamesetsofinitialconditions.
rameterization’ssensitivitytotheassumedbackgroundstate We first examine the performance of CHEM2D-H2O us-
using two different ro distributions. The first is based on ingtheUARS-basedreferencestate(EXP1). Figure9aplots
monthly zonal mean MLS/HALOE climatology (Randel et thewatervapormixingratioinitialconditionsfor00:00UTC
al., 1998; Grooß and Russell, 2005) between 100–0.01hPa on 5 June 2007, which are based on the assimilation of
combined with CHEM2D model values above the 0.01hPa Aura MLS Version 2.2 (V2.2) H O profiles between 100–
2
level. The second combines June zonal mean Aura MLS 0.002hPa (∼16–90km) combined with CHEM2D model
H Omixingratiosaveragedovertheyears2005–2008upto valuesoftheH Oabovethe0.002hPalevel(seeE2008for
2 2
the 0.002hPa level with CHEM2D model values above this details). Note that EXP1, EXP2, and EXP3 simulations all
level. Solar UV fluxes in the CHEM2D model were set to usethesamesetsofinitialconditions.
solarminimumlevelsfollowingLeanetal.(1997)tomatch
June2007conditions.
Atmos. Chem. Phys.,8,7519–7532,2008 www.atmos-chem-phys.net/8/7519/2008/
J.P.McCormacketal.: ParameterizedH Ophotochemistry 7527
2
(a) 2007060500 : H2O (ppmv) : day =0.0 (b) 2007060500 : H2O (ppmv) : day=10.
(c) 2007061500 : H2O (ppmv) : ANL (d) 2007061500 : H2O (ppmv) : A-F
Fig.9. Pressure-latitudeplotsof(a)NOGAPS-ALPHAanalyzedzonalmeanH2Omixingratiosfor00:00UTC5June2007;(b)zonalmean
forecastmodelH2Omixingratiosatday10oftheEXP1modelsimulationinitialized00:00UTC5Juneandvalid00:00UTC15June2007
(c)zonalmeanNOGAPS-ALPHAanalyzedH2Oat00:00UTCon15June2007;(d)differencesinH2Omixingratiosbetweenthe10-day
forecast(F)in(b)andtheanalyzed(A)valuesin(c).Solidanddashedcontoursdenotepositiveandnegativedifferences,respectively.
Figure9bplotsthemodelH Omixingratiosonday10of lyzedH Oand10-dayforecastH O.After10daystheEXP1
2 2 2
the EXP1 simulation valid 15 June at 00:00 UTC. A com- model simulation underestimates the zonal mean H O mix-
2
parison of Fig. 9b with Fig. 9a shows that the major differ- ing ratio throughout the northern extratropical upper meso-
ences in NOGAPS-ALPHA zonal mean H O after 10 days spherebymorethan2ppmv. Thisisthesameregionwhere
2
are: (1) a decrease at all latitudes above the 0.01hPa level theUARS-basedandAuraMLS-basedrodistributionsdiffer
(∼82km); (2) an increase near 0.1hPa (∼65km) poleward substantially (Fig. 8), and where photolytic loss dominates
of50◦N;(3)adecreaseoverhighsouthernlatitudesbetween (Fig.3).
1.0–0.1hPa. The high latitude changes are consistent with
Asmentionedintheprevioussection,theperformanceof
upwelling (downwelling) of moist (dry) air in the summer
theCHEM2D-H2Oparameterizationcanbeaffectedbythe
(winter) polar mesosphere. The broad decrease above the
choice of the background reference state ro because it re-
0.01hPalevelistheresultoftheparameterizedphotolysisof
laxestheforecastmodelH Omixingratiosintheupperlev-
2
H O.
2 elstowardthereferencestatevaluero. Inpractice,thewater
vapor distribution used to represent ro should be chosen so
Figure 9c plots the zonal mean NOGAPS-ALPHA an-
as to avoid any systematic bias between the prognostic hu-
alyzed H O mixing ratios valid on 00:00 UTC 15 June
2
midity variable and the assimilated humidity fields used to
2007. This and all following plots of analyzed H O ex-
2
initializeandupdatetheforecastsystem(see,e.g.Coyetal.,
tend to 0.0025hPa, which is the closest vertical level to
2007). Systematically low values of ro, such as those seen
the0.002hPaupperlimitforscientificallyusefulMLSH O
2
inFig.8a,canleadtoanoverestimateoftheH Olossinthe
measurements (Froidevaux et al., 2006). Since NOGAPS- 2
northernsummeruppermesosphere. Thismayhelpexplain
ALPHA analyzed H O is completely constrained by the
2
whythe10-dayEXP1forecastunderestimatestheH Omix-
MLSmeasurements,theanalyzedzonalmeanvaluesarees- 2
ing ratio compared to the analyses (Fig. 9d) in the northern
sentially the same as a zonal mean estimated directly from
extratropicaluppermesosphere.
thedailyMLSdata. ComparisonoftheanalyzedH Owith
2
the 10-day fields from EXP1 (Fig. 9b) shows large differ- Toinvestigatethispossibility, weisolatetheeffectofpa-
encespolewardof30◦Nabovethe0.01hPalevel,wherethe rameterizedwatervaporphotochemistrybytakingthediffer-
analyzed values are much higher than the modeled values. encebetweenthemodelmixingratior andapassivehumid-
Figure9dplotsthedifferencesbetweenthezonalmeanana- ity tracer r , which uses the same initial conditions as r
pass
www.atmos-chem-phys.net/8/7519/2008/ Atmos. Chem. Phys.,8,7519–7532,2008