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Transport and Scavenging of Soluble Gases in a Deep Convective Cloud Transport and scavenging of soluble gases in a deep convective cloud The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Mari, Céline, Daniel J. Jacob, and Peter Bechtold. 2000. “Transport and Scavenging of Soluble Gases in a Deep Convective Cloud.” Journal of Geophysical Research 105 (D17): 22255. doi:10.1029/2000jd900211. Published Version doi:10.1029/2000JD900211 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:14117823 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D17, PAGES 22,255-22,267, SEPTEMBER 16, 2000 Transport and scavenging of soluble gases in a deep convective cloud Cdine Marl Laboratoired'A6rologie, UMR CNRS/Universit6Paul Sabatier,Toulouse, France Daniel J. Jacob Division of Engineeringand Applied Science,Harvard University, Cambridge, Massachusetts Peter Bechtold Laboratoire d'A6rologie,UMR CNRS/Universitd Paul Sabatier, Toulouse,France Abstract. A one-dimensionalentraining/detraining plume model is used to examine the transport and scavengingof soluble gasesin tropical deep convection. The model is applied to a continental system observedover Brazil during the Trace and AtmosphericChemistry Near the Equator-Atlantic (TRACE-A) TRACE-A aircraft campaign with outflows extending from 7 to 16 km altitude. Six gases are simulated: CO (inert tracer), CH3OOH, CH20, H202, HNO3 , and SO2. Observed(simulated) convective enhancement factors (CEF) at 7-12 km altitude, representingthe ratios of postconvectiveto preconvectivemixing ratios, are 2.4 (1.9) for CO, 11 (9.5) for CH3OOH, 2.9 (3.1) for CH20, 1.9 (1.2) for H202, and 0.8 (0.4) for HNO3. Simulated scavengingefficiencies in the convectivecolumn are 5% for CH3OOH, 23% for CH20, 66% for H202, 77% for HNO3, and 28% for SO2. The large CEF for CH3OOH reflects its low solubility and its boundary layer enrichment relative to the upper troposphere. The Henry's law constant for CH20 puts it at the threshold for etficient scavenging. Scavengingof SO2 is limited by the rate of aqueousphase reaction with H•O•, as H•O2 is itself efficiently scavengedby Henry's law equilibrium; efficient scavengingof SO2 requiresunusually high cloud water pH (pH > 6) to enablefast aqueousphase oxidation by Oa. Both HNO3 and HaOa are efficientlyscavenged in the lower (warm) part of the cloud, but H202 is released as the cloud freezes due to low retention efficiency during timing. Significant scavengingof H202 still takes place by cocondensationwith ice in the glaciated cloud but is less efficient than in the warm cloud. Inefficient scavengingof H202 in glaciated clouds may explain the observation,in TRACE-A and elsewhere, that HaO2 is enhanced in deep convective outflows while HNOa is depleted. Model results indicate little direct transfer of air from the boundary layer to the cloud anvil in the convective plume, because of low-level detrainment in the warm cloud and high-level entrainment in the glaciated cloud. We find instead a convectiveladder effect where midlevel outflow during the growing phase of the storm is teentrained into the convective plume as the storm matures. 1. Introduction thought due to the convectivetransport of radical pre- cursorssuch as acetone,peroxides and aldehydes[Sin#h Deep convection in the tropics is a critical mecha- et al., 1995; McKeen et al., 1997; Jae#l• et al., 1997; nisIn for transfer of air from the lower to the upper M•iller and Brasseur,1999]. Most of the NOx in the troposphere[Cotton et al., 1995]with many important upper troposphere is thought to originate from deep implicationsfor atmosphericchemistry [Prather and Ja- convective transport of NOx and nitrate reservoirs pro- cob,1997]. Recentstudies show that the upper tropo- ducedby surfacesources and lightning[Lamarque et al., sphere is more photochemically active than previously 1996; Jacob et al., 1996; Pickering et al., 1998; Jaegld et al., 19981. Deep convectivepumping of dimethyl- Copyright 2000 by the American GeophysicalUnion. sulfide and SO2 may drive efficient nucleation of sul- Paper number 2000JD900211. fate aerosolin convectiveoutflow [Chatfieldet Crutzen, 0148-0227/00/2000JD900211509.00 1984; Rodhe, 1999]. Considerableprecipitation takes 22,255 22,256 MARI ET AL.: TRANSPORT AND SCAVENGING OF SOLUBLE SPECIES place in associationwith deep convectionupdrafts, and an unresolved issue is the degree to which precipita- tion scavengingsuppresses the convectivecloud venting of water-solublegases [Andronache et al., 1999; Floss- mann and Wobrock,1996; Barth, 1994]. We present here an analysis of this issue. Our approachis based on a one-dimensional(l-D) entraining/detrainingplume model (P. Bechtoldet al., A mass flux convectionscheme for regional and global models, submitted to Quartely Journal of the Royal Meteorological Society, 2000; hereinafter Bechtold et al., submitted manuscript, 2000) that has been de- signedto providea deepconvective parameterization for mesoscaleand global-scalemodels. This model allows us to identify the key parameters controlling the ver- tical redistribution and scavengingof chemical species in a deep convectivecloud. We examine here the fate Grid cell of six different gasescovering a range of solubility in water: (1) CO as an insolubletracer of boundarylayer Figure 1. Schematicof the one-dimensionalconvec- pollution, (2) CH20, H20•, CH3OOH as precursorsfor tive cloud model with updraft and downdraft regions the HOx (HOx - OH + peroxy radicals)family in the (solidarrows), entrainment (open arrows with dashed uppertroposphere, (3) HNO3 as a reservoirfor NOx ? tails),detrainment (open arrows with dash-dottedtails) and (4) SO• as the precursorfor sulfateaerosols. and compensationallarge-scale subsidence (thick solid We focus our analysis on a tropical continental con- arrows) vective caseobserved during the Transport and Atmo- sphericChemistry Near the Equator-Atlantic(TRACE- A) aircraft experimentin September-October1992. dence and includes entrainment and detrainment pro- TRACE-A was designedto determine the role of biomass cesses(Figure 1). burning in causingthe large-scaleenhancement of tro- In the present application the model is initialized with posphericozone observed over the south Atlantic basin vertical profiles of temperature, water vapor and hori- in austral spring [Fishman et al., 1996]. Two flights zontal winds from the European Centre for Medium- over eastern Brazil examined the convective pumping Range Weather Forecasts(ECMWF) analysisat grid of biomass burning pollution to the upper troposphere. resolution 0.5 ø x 0.5ø. Convection is triggered by con- Of particular interest is flight 6, which provided vertical vective instability and large-scaleconvergence. The net profiles of gasesfor both preconvectiveand postconvec- convectiveavailable potential energy(CAPE) is usedto tive conditions[Pickering ct al., 1996]. We usethe data determine the updraft and downdraft air mass fluxes. from flight 6 to constrainand evaluate our simulationof At least 90% of the initial CAPE in the soundingmust convectivetransport and scavengingof chemicalspecies. be removed by the calculated convective fluxes. En- trainment and detrainment refer to mass exchangebe- tweenthe updraft and the environment(Figure 1). The 2. Model Description environment is defined as the grid scaleof the ECMWF 2.1. One-Dimensional Entraining/Detraining analysis and is also viewed here as the scale of the pre- convective aircraft observations. Plume Model Entrainment and detrainment rates are computed fol- A one-dimensional entraining/detraining plume lowingthe schemeof Kain and Fritsch[1990] as a func- model for wet convection(Bechtold et al., submitted tion of the buoyancy characteristicsof individual up- manuscript,2000) is usedfor this study. This modelhas draft subparcels. The mixing between clouds and their beendesigned to providea parameterizationof wet con- environments occurs very near the periphery of clouds vectionin the mesoscalemodel Meso-NH ILaroreet al., and generatessubparcels containing various proportions 1998].Subgrid-scale fluxes through convective updrafts of clear and cloudy air. Positively buoyant subparcels and mixing with the environmentthrough entrainment follow the cloudy updraft, entraining air from the en- and detrainment are diagnosedin terms of grid-scale vironment, while negatively buoyant subparcelsdetrain thermodynamicand dynamicvariables [Bechtold et al., to the environment. 2000;also Bechtold et al., submittedmanuscript, 2000]. At eachmodel level, the rate 5Mr (kgs-1), at which The formulation essentiallyfollows the schemeof Kain environmental air flows into the mixing region at the and Fritsch[1990] which is an advancedversion of the periphery of the updraft is expressedby Fritsch and Chappell[1980] scheme. The model pa- rameterizes a convectiveupdraft plume balanced by a convectivedowndraft plume and by large-scalesubsi- - 0.2 &, MARI ET AL.' TRANSPORT AND SCAVENGING OF SOLUBLE SPECIES 22,257 where R0 (1500 m) and Muo (kg s-•) are the updraft where C is the mixing ratio in the environmental air. radius and the massflux at cloud base, and 5z (m) is Entrainment is a critical process as it reduces the the vertical thickness of the level. The mass in this buoyancyof the cloudy air parcel and thus limits the mixing region
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