Chapter 11 CLOUD DYNAMICS AND CHEMISTRY

Transcription

1 Chapter 11 CLOUD DYNAMICS AND CHEMISTRY Shawn J. Roselle * and Francs S. Bnkowsk ** Atmospherc Modelng Dvson Natonal Exposure Research Laboratory U.S. Envronmental Protecton Agency Research Trangle Park, North Carolna ABSTRACT Chapter 11 descrbes the cloud module that s currently ncorporated nto CMAQ. Ths module smulates the physcal and chemcal processes of clouds that are mportant n ar qualty smulatons. Clouds affect pollutant concentratons by vertcal-convectve mxng, scavengng, aqueous chemstry, and removal by wet deposton. The CMAQ cloud module ncludes parameterzatons for sub-grd convectve precptatng and non-precptatng clouds and grdscale resolved clouds. Cloud effects on both gas-phase speces and aerosols are smulated by the cloud module. * On assgnment from the Natonal Oceanc and Atmospherc Admnstraton, U.S. Department of Commerce. Correspondng author address: Shawn J. Roselle, MD-80, Research Trangle Park, NC E-mal: sjr@hpcc.epa.gov ** On assgnment from the Natonal Oceanc and Atmospherc Admnstraton, U.S. Department of Commerce.

2 11.0 CLOUD DYNAMICS AND CHEMISTRY 11.1 Background Clouds play an mportant role n boundary layer meteorology and ar qualty. Convectve clouds transport pollutants vertcally, allowng an exchange of ar between the boundary layer and the free troposphere. Cloud droplets formed by heterogeneous nucleaton on aerosols grow nto ran droplets through condensaton, collson, and coalescence. Clouds and precptaton scavenge pollutants from the ar. Once nsde the cloud or ran water, some compounds dssocate nto ons and/or react wth one another through aqueous chemstry (.e., cloud chemstry s an mportant process n the oxdaton of sulfur doxde to sulfate). Another mportant role for clouds s the removal of pollutants trapped n ran water and ts deposton onto the ground. Clouds can also affect gas-phase chemstry by attenuatng solar radaton below the cloud base whch has a sgnfcant mpact on the photolyss reactons. The Models-3/CMAQ cloud model ncorporates many of these cloud processes. The model ncludes parameterzatons for several types of clouds, ncludng sub-grd convectve clouds (precptatng and non-precptatng) and grd-scale resolved clouds. It ncludes an aqueous chemstry model for sulfur, and ncludes a smple mechansm for scavengng Model Descrpton The cloud model can be dvded nto two man components, ncludng the sub-grd cloud model (sub) and the resolved cloud model (res). For large horzontal grd resolutons, the grd sze wll be larger than the sze of a typcal convectve cloud, requrng a parameterzaton for these sub-grd clouds. The sub-grd cloud scheme smulates convectve precptatng and nonprecptatng clouds. The second component of the cloud model consders clouds whch occupy the entre grd cell and have been resolved by the meteorologcal model. The rate of change n pollutant concentratons ( ) due to cloud processes s gven by: m m m m = + (11-1) sub res The terms on the rght-hand sde of Equaton 11-1 are solved separately at dfferent tmes. The nfluence of sub-grd clouds are nsttuted once an hour whle the resolved clouds mpact concentratons every synchronzaton tmestep. Each subcomponent of the cloud model s descrbed n detal below Subgrd Convectve Cloud Scheme m sub ( mxng scav aqchem wetdep) ~ f,,, (11-2) 11-1

3 The current sub-grd cloud scheme n CMAQ was derved from the dagnostc cloud model n RADM verson 2.6 (Denns et al., 1993; Walcek and Taylor, 1986; Chang, et al., 1987; Chang, et al., 1990). Seaman (1998) noted that most convectve parameterzatons are based on the assumpton that the area of an updraft s small compared to the area of the grd cell and most parameterzatons can be used at grd resolutons a small as 12 km (Wang and Seaman, 1997). In CMAQ, subgrd clouds are consdered only for horzontal grd resolutons on the order of 12 km or more. Seaman (1998) also ponted to a study by Wesman et al. (1997) that showed that explct cloud models can resolve convecton for resoluton fner than 5 km. Wthn CMAQ, for resolutons of 4 km or less, vertcal convecton s assumed to be resolved at the grd level; therefore, the resolved cloud model wll be the only cloud scheme used at small grd scales. The effects of sub-grd clouds on grd-averaged concentratons are parameterzed by modelng the mxng, scavengng, aqueous chemstry, and wet deposton of a representatve cloud wthn the grd cell. For all sub-grd clouds, a 1-hour lfetme (- ) has been assumed. Sub-grd clouds can be ether precptatng or non-precptatng, and the non-precptatng subgrd clouds are further categorzed as pure far weather (PFW) clouds and non-precptatng clouds coexstng (CNP) wth precptatng clouds. The subgrd cloud model determnes f precptatng or nonprecptatng clouds exst over each grd cell. Precptatng clouds are smulated when the meteorologcal preprocessor (currently the Mesoscale Model verson 5 or MM5, Grell et al., 1994) ndcates precptaton from ts convectve cloud model. The CMAQ mplementaton dffers from the RADM n that only the convectve precptaton amounts from MM5 are used to drve the subgrd precptatng cloud. RADM used the total precptaton (convectve and nonconvectve precptaton) to drve the subgrd cloud model. In CMAQ, the nonconvectve precptaton s used n the resolved cloud model. Nonprecptatng clouds are modeled f the mosture and temperature profles support the development of a cloud (Denns et al., 1993). Nonprecptatng clouds are modeled only when the relatve humdty of the source level s above 70% and the calculated cloud base s below 1500 m for PFW clouds or 3000 m for CNP clouds. For both precptatng and nonprecptatng cloud types, the geometry of the cloud (base, top, and spatal extent) are determned next. The cloud base s calculated by lftng a parcel of ar from the cloud source level (the level between the surface and 650 mb wth the hghest equvalent potental temperature) to the lftng condensaton level (LCL). The cloud top calculaton depends upon the cloud type and atmospherc stablty. For precptatng clouds n unstable condtons, the cloud top s found by followng the most adabatc lapse rate from the cloud base up to the level where t becomes cooler than the surroundng envronment. For precptatng clouds under stable condtons, the cloud top s set as the frst layer above the cloud base n whch the relatve humdty falls below 65%, but lmted to less than the 600 mb heght. For nonprecptatng clouds, further restrctons are placed on the cloud top. The cloud top calculaton for nonprecptatng clouds uses the same relatve humdty crteron as the precptatng clouds, but the cloud top s allowed to extend up to 500 mb for CNP clouds and only to 1500 m for PFW clouds. If the atmosphere s unstable, the nonprecptatng cloud top may be reduced n heght f the parcel method calculated a lower cloud top. The fractonal cloud coverage calculatons depend on cloud type and have been 11-2

4 descrbed thoroughly by Denns et al. (1993). For precptatng clouds, the model uses a parameterzaton smlar to approach of Kuo (1974). The fractonal coverage parameterzaton for the nonprecptatng clouds s based on relatve humdty. The convectve cloud smulated by the sub-grd cloud model s consdered to be composed of ar transported vertcally-from below the cloud, entraned from above the cloud (for precptatng clouds), and entraned from the sdes of the cloud. Concentratons of pollutants for each layer of the cloud are calculated by: down ( ) ( ) up m ( z) = f ent f sde m + f sde m ( z) f ent m (11-3) where f sde s the fracton of entranng ar orgnatng from the sde of the cloud. For nonprecptatng clouds, no entranment of ar from above the cloud s allowed and therefore f sde =1. The entranment, f ent, s calculated by teratvely solvng conservaton and thermodynamc equatons (Denns et al., 1993; Chang et al, 1990,1987; and Walcek and Taylor, up m down m 1986). The terms and represent the above and below cloud concentratons, respectvely. Once the cloud volume has been determned, vertcally-averaged cloud temperature, pressure, lqud water content, total water content, and pollutant concentratons, are computed wth lqud water content (W c ) as the weghtng functon (gves the most weght to the layers wth the hghest lqud water content). Thus, the average pollutant concentratons wthn the cloud are calculated by: m = zctop m ( z) W ( z) dz zcbase zctop W zcbase c c ( z) dz (11-4) Wth the averages over the cloud volume, the processes of scavengng, aqueous chemstry, and wet deposton are consdered. The fnal step n cloud mxng s the reapportonng of mass back nto ndvdual layers. Ths s accomplshed usng cloud fractonal coverage, ntal n-cloud concentratons, fnal n-cloud concentraton, and the ntal vertcal concentraton profle. For precptatng clouds, the average pollutant concentraton for the grd cell wthn the cloud layers s computed by: m m ( z, t 0 + τ ) = m ( z, t0) ( t0 + τ ) cfrac + m ( z, t 0) 1 cfrac m ( t0) [ ] (11-5) 11-3

5 where cfrac s the fractonal cloud coverage. There are varatons on ths equaton for below cloud, above cloud, and for nonprecptatng clouds Scavengng and Wet Deposton Pollutant scavengng s calculated by two methods, dependng upon whether the pollutant partcpates n the cloud water chemstry and on the lqud water content. (1) For those pollutants that are absorbed nto the cloud water and partcpate n the cloud chemstry (and provded that the lqud water content s > 0.01 g/m 3 ), the amount of scavengng depends on Henry's law constants, dssocaton constants, and cloud water ph. (2) For pollutants whch do not partcpate n aqueous chemstry (or for all water-soluble pollutants when the lqud water content s below 0.01 g/m 3 ), the model uses the Henry's Law equlbrum equaton to calculate endng concentraton and deposton amounts. The rate of change for n-cloud concentratons (m ) for each pollutant () followng the cloud tmescale (- ) s gven by: m scav = m e ατ τ 1 (11-6) where s the scavengng coeffcent for the pollutant. For subgrd convectve clouds, - s 1 hour and for grd resolved clouds t s equal to the CMAQ s synchronzaton tmestep. For gases, the scavengng coeffcent s gven by: 1 α = TWF (11-7) τwashout 1+ H where H s the Henry s Law coeffcent for the pollutant, TWF s the total water fracton gven by: TWF H O = ρ 2 (11-8) W TRT where ' H2O s the densty of water, W T s the mean total water content (kg/m 3 ), R s the Unversal gas constant, and T s the n-cloud ar temperature (K). The washout tme, - washout represents the amount of tme requred to remove all of the water from the cloud volume at the specfed precptaton rate (P r ), and s gven by: Here, z s the cloud thckness. τ washout W T z = ρ P H2O 11-4 r (11-9)

6 The accumulaton mode and coarse mode aerosols are assumed to be completely absorbed by the cloud and ran water. Therefore, the scavengng coeffcents for these two aerosol modes are smply a functon of the washout tme: 1 α = τ washout (11-10) The Atken mode aerosols are treated as ntersttal aerosol and are slowly absorbed nto the cloud/ran water. Ths process s dscussed n detal n the aerosol chapter (Chapter 10). The wet deposton algorthms n CMAQ were taken from the RADM (Chang et al., 1987). In the current mplementaton, deposton s accumulated over 1-hour ncrements before beng wrtten to the output fle. The wet deposton amount of chemcal speces (wdep ) depends upon the precptaton rate (P r ) and the cloud water concentraton (m ): τ wdep = m Pr dt 0 (11-11) Deposton amounts are accumulated for each of the modeled speces, but the user specfed whch speces are wrtten to the output fle. Ths s handled n the Program Control Processor (see Chapter 15) Aqueous Chemstry The aqueous chemstry model evolved from the orgnal RADM model (Chang et al., 1987; and Walcek and Taylor, 1986). The model consders the absorpton of chemcal compounds nto the cloud water; the amount that gas-phase speces absorb nto the cloud water depends on thermodynamc equlbrum, whle accumulaton-mode aerosols are consdered to have been the nucleaton partcles for cloud droplet formaton and are 100% absorbed nto the cloud water. Then the model calculates the dssocaton of compounds nto ons, oxdaton of S(IV) to S(VI), and wet deposton. The speces that partcpate n the aqueous chemstry are gven n Table Ths verson of the aqueous chemstry model dffers from Walcek s scheme n that t tracks contrbutons from gases and aerosols separately. It also consders the scavengng of ntersttal aerosols, and t allows for varable-length cloud tme scales. 11-5

7 Table Lst of Speces Consdered n the Aqueous Chemstry Model Gases SO 2 HNO 3 N 2 O 5 CO 2 NH 3 Aerosols = SO 4 (Atken & accumulaton) + NH 4 (Atken & accumulaton) - NO 3 (Atken, accumulaton, & coarse) Organcs (Atken & accumulaton) Prmary (Atken, accumulaton, & coarse) H 2 O 2 CaCO 3 O 3 MgCO 3 formc acd NaCl methyl hydrogen peroxde Fe 3+ peroxy acetc acd Mn 2+ H 2 SO Resolved Cloud Scheme KCl [ ] () () () () W z = Q z + Q z ρ z (11-12) C C R Number (Atken, accumulaton, & coarse) At any grd resoluton, clouds may be resolved by the MM5, whch could nclude stratus, cumulus, or crrus type clouds. The resolved clouds have been smulated by the MM5 to cover the entre grd cell. No addtonal cloud dynamcs are consdered n CMAQ for ths cloud type snce any convecton and/or mxng would have been resolved and consdered n the vertcal wnd felds provded by MM5. A resolved cloud horzontally covers the entre grd cell and vertcally extends over the whole depth of the layer, thus and are equvalent n resolved clouds. These clouds are actvated n MM5 when the humdty s hgh enough for water vapor to condense, and then MM5 computes cloud and ran water amounts accordng to any of several mcrophyscal submodels. Usng the total of the condensed cloud water and ran water reported by MM5, the CMAQ resolved cloud model then consders scavengng, aqueous chemstry, and wet deposton. The average lqud water content W c n a model layer (z) for the resolved cloud s gven by: where Q C (z) s the cloud water mxng rato (kg/kg), Q R (z) s the ran water mxng rato (kg/kg), '(z) s the ar densty (kg/m 3 ). Precptaton amounts for resolved cloud layers, P r (z), are derved m m 11-6

8 usng the MM5 non-convectve precptaton amounts (R n ), apportoned nto ndvdual model layers wth the vertcal profle of condensed lqud water as follows: () P z = R r n W Wc () z () c z dz (11-13) Once quanttes for precptaton rate, lqud water content, etc. have been calculated, then the scavengng, aqueous chemstry, and wet deposton are solved usng the same procedures as n the subgrd clouds. m res m m = + (11-14) scav aqchem Several assumptons have been made n the current mplementaton of the resolved cloud model. (1) The lfetme of the resolved cloud computatons vares based on the synchronzaton tmestep of CMAQ. (2) Followng the method of operator splttng, the effect of the resolved clouds on pollutant concentratons occurs at the end of the cloud lfetme, thus no exchange between layers s permtted durng the cloud lfe-cycle. (3) The pollutants, cloud water, and ran water are unformly dstrbuted wthn the grd cell. Because of the separaton of MM5 from CMAQ, we do not have the nformaton to do precptaton fluxes. Even f a complete cloud precptaton model was developed wthn CMAQ, there s no guarantee that t would be consstent wth what was done n MM Conclusons One of the concepts for Models-3 was that multple modules may exst for each physcal process of the ar qualty model. The mplementaton descrbed here s the frst module avalable for modelng cloud physcs and chemstry. Other subgrd cloud models (.e., the Kan-Frtsch (1990, 1993) and Betts-Mller (1986)) are under consderaton and may be ncluded as optonal modules for CMAQ. In addton, a more detaled resolved cloud model s under development whch wll nclude a mcrophyscal submodel for followng the evoluton of the cloud (.e., cloud droplet formaton, growth of ran droplets, and descent through model layers to the ground). It wll also consder resolved cloud lfetmes whch extend beyond the CMAQ synchronzaton tmestep, thus mantanng the partton between gas and aqueous-phase pollutants durng the gas-phase chemstry calculatons. The current mplementaton of the cloud model n CMAQ wll be evaluated usng avalable datasets and wll be used as a reference for evaluatng future cloud modules for CMAQ. 11-7

9 11.4 References Betts, A.K. and M.J. Mller, A new convectve adjustment scheme. Part II: Sngle column tests usng GATE wave, BOMEX, ATEX, and arctc ar- mass data sets. Quarterly J. Roy. Meteor. Soc., 112, Chang, J.S., R.A. Brost, I.S.A. Isaksen, S. Madronch, P. Mddleton, W.R. Stockwell, and C.J. Walcek, A three-dmensonal Euleran acd deposton model: Physcal concepts and formaton. J. Geophys. Res., 92, Chang, J.S., P.B. Mddleton, W.R. Stockwell, C.J. Walcek, J.E. Plem, H.H. Lansford, F.S. Bnkowsk, S. Madronch, N.L. Seaman, D.R. Stauffer, D. Byun, J.N. McHenry, P.J. Samson, H. Hass., The regonal acd deposton model and engneerng model, Acdc Deposton: State of Scence and Technology, Report 4, Natonal Acd Precptaton Assessment Program. Denns, R.L., J.N. McHenry, W.R. Barchet, F.S. Bnkowsk, and D.W. Byun, Correctng RADM s sulfate underpredcton: Dscovery and correcton of model errors and testng the correctons through comparsons aganst feld data, Atmos. Envron., 26A(6), Grell, G.A., J. Dudha, and D.R. Stauffer, A descrpton of the ffth generaton Penn State/NCAR mesoscale model (MM5). NCAR Tech. Note NCAR/TN-398+STR, 138 pp. Kan, J.S. and J.M. Frtsch, A one-dmensonal entranng/detranng plume model and ts applcaton n convectve parameterzaton. J. Atmos. Sc, 47, Kan, J.S. and J.M. Frtsch, Convectve parameterzaton for mesoscale models: The Kan- Frtsch scheme. The Representaton of Cumulus Convecton n Numercal Models, Meteor. Monogr., 46, Amer. Meteor. Soc., Kuo, H.L., Further studes of the parameterzaton of the nfluence of cumulus convecton on large-scale flow, J. Atmos. Sc., 31, Seaman, N.L., Meteorologcal Modelng for Ar-Qualty Assessments: A NARSTO Revew Paper, Submtted to Atmos. Envron. Walcek, C.J. and G.R. Taylor, A theoretcal method for computng vertcal dstrbutons of acdty and sulfate producton wthn cumulus clouds, J. Atmos. Sc 43, Wang, W. and N.L. Seaman, A comparson study of convectve parameterzaton schemes n a mesoscale model. Mon. Wea. Rev., 125, Wesman, L.M., W.C. Skamarock and J.B. Klemp, The resoluton dependence of explctly modeled convectve systems, Mon. Wea. Rev., 125,

10 Ths chapter s taken from Scence Algorthms of the EPA Models-3 Communty Multscale Ar Qualty (CMAQ) Modelng System, edted by D. W. Byun and J. K. S. Chng,