Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5// Author(s). This work is distriuted under the Cretive Commons Attriution. License. Atmospheric Chemistry nd Physics Effect of hygroscopic seeding on wrm rin clouds numericl study using hyrid cloud microphysicl model N. Ku nd M. Murkmi Reserch Institute for Glol Chnge, Jpn Agency for Mrine-Erth Science nd Technology (JAMSTEC), Yokohm, Jpn Meteorologicl Reserch Institute (MRI), Tsuku, Jpn Received: 8 Octoer 9 Pulished in Atmos. Chem. Phys. Discuss.: Novemer 9 Revised: 8 Mrch Accepted: Mrch Pulished: 9 April Astrct. The effect of hygroscopic seeding on wrm rin clouds ws exmined using hyrid cloud microphysicl model comining Lgrngin Cloud Condenstion Nuclei (CCN) ctivtion model, semi-lgrngin droplet growth model, nd n Eulerin sptil model for dvection nd sedimenttion of droplets. This hyrid cloud microphysicl model ccurtely estimted the effects of CCN on cloud microstructure nd suggested the following conclusions for moderte continentl ir mss (n ir mss with lrge numer of ckground CCN). () Seeding cn hsten the onset of surfce rinfll nd increse the ccumulted mount of surfce rinfll if the mount nd rdius of seeding prticles re pproprite. () The optiml rdius of monodisperse prticles to increse rinfll ecomes lrger with the increse in the totl mss of seeding prticles. () Seeding with slt micro-powder cn hsten the onset of surfce rinfll nd increse the ccumulted mount of surfce rinfll if the mount of seeding prticles is sufficient. (4) Seeding y hygroscopic flre decreses rinfll in the cse of lrge updrft velocity (shllow convective cloud) nd increses rinfll slightly in the cse of smll updrft velocity (strtiform cloud). (5) Seeding with hygroscopic flres including ultrgint prticles (r>5 µm) hstens the onset of surfce rinfll ut my not significntly increse the ccumulted surfce rinfll mount. (6) Hygroscopic seeding increses surfce rinfll y two kinds of effects: the competition effect y Correspondence to: N. Ku (ku@jmstec.go.jp) which lrge solule prticles prevent the ctivtion of smller prticles nd the rindrop emryo effect in which gint solule prticles cn immeditely ecome rindrop emryos. In some cses, one of the effects works, nd in other cses, oth effects work, depending on the updrft velocity nd the mount nd size of seeding prticles. Introduction Hygroscopic seeding to promote wter droplet colescence y introducing ppropritely sized slt prticles, spryed wter droplets, or sline solution into clouds hs long een known nd used (Bowen, 95; Bisws nd Dennis, 97; Cotton, 98; Murty et l., ). In clssic hygroscopic seeding pproches, lrge hygroscopic prticles t lest µm in dimeter re used to provide rindrop emryos. This method requires tht huge mount of seeding mteril e dispersed y ircrft. Drwcks of this method include its inconvenience for prcticl use, low cost-effectiveness, nd possile dverse effects of slt on the environment. Hygroscopic seeding with flres, which produces slt prticles. to µm in dimeter, hs een used to ugment precipittion from summertime convective clouds in South Afric nd Mexico, with promising results indicted y rdr-estimted rinfll (Mther et l., 997; WMO, ). The flre method hs een widely used in numer of countries nd regions ecuse of its convenience for field opertions nd the ovementioned promising experimentl results. However, results regrding seeding effects re Pulished y Copernicus Pulictions on ehlf of the Europen Geosciences Union.
6 N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds inconclusive due to the lck of understnding of the physicl processes leding to increses in rdr-estimted rinfll. To investigte the effects of hygroscopic seeding on cloud nd precipittion, mny studies hve pplied numericl models. Reisin et l. (996) simulted hygroscopic seeding of n xisymmetric convective cloud nd showed tht seeding hd drmtic effect on rinfll. They determined the cloud droplet numer y Cloud Condenstion Nuclei (CCN) ctivtion spectr nd distriuted the droplets over size ins ccording to gmm or exponentil function. However, y this method, the effect of hygroscopic seeding on the initil size distriution of cloud droplets could not e shown clerly. Tzivion et l. (994) pplied n xisymmetric convective cloud model with detiled tretment of wrm cloud microphysics to estimte the effect of hygroscopic seeding, which ws represented y dded wter droplets. Yin et l. () conducted numericl experiments to evlute the role of hygroscopic flre seeding using two-dimensionl (- D) sl-symmetric non-hydrosttic cloud model with detiled microphysicl scheme. They found tht seeding with the full prticle spectrum from flres could increse the rinfll mount in continentl clouds hving CCN concentrtions more thn 5 cm (ctive t % supersturtion). Teller nd Levin (6) lso crried out numericl experiments using the Tel-Aviv University two-dimensionl numericl cloud model with detiled tretment of cloud microphysics. Their results showed tht gint CCN enhnced the totl precipittion on the ground in polluted clouds. However, in the models of Tzivion et l. (994), Yin et l. (), nd Teller nd Levin (6), grid sizes rnged from 5 m in the verticl direction, which is not smll enough to estimte the mximum supersturtion tht significntly ffects CCN ctivtion. To precisely estimte the ctivtion of CCN, n Eulerin sptil frmework with very smll grid size or Lgrngin prticle frmework s prcel model is needed (Ku nd Fujiyoshi, 6). Cooper et l. (997) nd Cro et l. () investigted the effect of flre hygroscopic seeding using prcel model with precise microphysicl model. Their clcultions suggested tht rin formtion vi the collision-colescence process cn e ccelerted significntly y dding hygroscopic prticles. Segl et l. (4, 7) investigted the effect of hygroscopic seeding on wrm rin using -in cloud spectrl prcel model. Their simultions showed tht use of commercil hygroscopic flres incresed rindrop production in cloud prcels in which the nturl wrm rin process ws inefficient. They lso found tht the optimum seeding prticle rdius tht provided the mximum rindrop production under given mss of seeding regent vried from.5 to.5 µm nd slightly depended on the totl regent mss s well s the dynmic properties of cloud prcels. In ddition, they found tht the min effect of lrge solule erosols, which re ctivted t supersturtion <.4% nd elong to the corse erosol mode, ws to form rindrop emryos, reveling the emryo effect, not the competition effect (wherey lrge solule prticles prevent ctivtion of smller or less solule prticles). However, estimtion of surfce rinfll using the prcel model ppers to e difficult. The competition effect ws emphsized y Ghn et l. (998) nd O Dowd et l. (999) nd discussed in Feingold nd Sieert (9). Approprite numer concentrtions of seeding prticles compete with the ckground CCN for ville excess wter vpor nd consequently lower the mximum supersturtion, decrese the numer concentrtions of ctivted cloud droplets, nd increse the droplet sizes, which enhnces collision-colescence mong cloud droplets nd ccelertes the formtion of rindrop emryos. It is importnt to study wht condition gives the emryo effect, the competition effect, or oth of them for estimtion of the effect of hygroscopic seeding. The purpose of the present study ws to quntittively evlute the effect of hygroscopic seeding on surfce rinfll from shllow wrm rin clouds using hyrid cloud microphysicl model tht incorportes Lgrngin CCN ctivtion model, semi-lgrngin droplet growth model, nd n Eulerin sptil model for dvection nd sedimenttion of droplets (Ku nd Fujiyoshi, 6). This model cn simulte the CCN ctivtion process precisely so tht the effect of slight chnge in the initil cloud droplet size distriution due to hygroscopic seeding cn e evluted in detil. The model cn lso ccurtely clculte the consequent droplet growth through condenstion nd collision-colescence s well s dvection, size sorting, nd sedimenttion of drops in clouds, producing relile estimte of the seeding effect on surfce precipittion. Becuse kinemtic flow field is ssumed in this simple model, dynmicl feedcks ssocited with microphysicl chnges due to vrying CCN cnnot e simulted. These effects will e studied y instlling our hyrid cloud microphysicl model into nonhydrosttic cloud model in future reserch. The simultions were performed for shllow convective nd strtiform clouds with moderte continentl ckground CCN. Seeding prticles used in the present simultions hd monodisperse, single log-norml (slt micro-powder) nd doule log-norml (hygroscopic flre) size distriutions. Model description The hyrid microphysicl cloud model ws developed to ccurtely estimte the numer concentrtion nd size distriution of cloud droplets nd the effect of CCN on cloud microstructures (Ku nd Fujiyoshi, 6). The ctivtion of CCN nd initil condenstionl growth re computed in Lgrngin prticle frmework using prcel model. The solute effect of CCN is tken into ccount even fter the ctivtion. Becuse the mximum supersturtion experienced y n ir prcel is estimted ccurtely, the numer of cloud droplets tht cn e ctivted is lso estimted ccurtely. This method precludes numericl diffusion of the droplet size Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//
N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds 7 distriution. A time step of.5 s is dopted for the prcel model to clculte CCN ctivtion nd the consequent condenstionl growth of droplets. This hyrid cloud microphysicl model lso uses two-moment in method sed on tht of Chen nd Lm (994) in -D grid model to estimte condenstion nd colescence with semi-lgrngin frmework nd to estimte sedimenttion nd dvection with n Eulerin sptil frmework. The cloud droplet size distriution estimted y the prcel model is used s the initil cloud droplet size distriution for the two-moment in method. This method for giving the initil cloud droplet size distriution seems to e preferle to previously used methods in which ctivted droplets were dded to the first in (Morrison nd Growski, 7; Growski nd Wng, 9) or distriuted to ins ssuming some size distriution shpes (Tzivion et l., 994; Reisin et l., 996). Detils of the model hve een reported y Ku nd Fujiyoshi (6). The present study mde the following improvements to the hyrid microphysicl cloud model of Ku nd Fujiyoshi (6). To properly estimte multi-colescence in one time step, two schemes re used. One is generl stochstic colescence scheme for rre lucky colescence etween droplets, nd the other is continuous colescence scheme for frequent colescence of lrge drop nd numerous smll droplets (numerous smll droplets re evenly shred y lrge drops) following the method reported in the doctorl disserttion of Jen-Ping Chen (99). We distinguish rre lucky colescence nd frequent colescence using the predicted frequency of collision in one time step. If the predicted frequency of collision etween one prticle in the i-th in nd smller prticles in the k-th, (k+)-th,..., nd i-th ins in one time step is or less, generl stochstic colescence scheme is used to clculte the growth of prticles in the i-th in y colescence with prticles in the k- th, (k+)-th,..., nd i-th ins. If the predicted frequency of collision of one prticle in the i-th in nd smller prticles in the first, second,..., nd i-th ins in one time step is lrger thn, continuous colescence scheme is used to clculte the growth of prticles in the i-th in y colescence with prticles in the first, second,..., nd (k )-th ins (see Appendix for detils). If only the generl stochstic colescence scheme is used, very short time step such s. s is needed to void underestimtion of colescence growth cused y the underestimtion of multiple colescences. This method using oth continuous nd stochstic schemes with time step of. s leds to the sme results s the method using only the stochstic scheme with time step of. s, s shown in the Appendix (Fig. A). However, in this study,.5 s time step is dopted for the in method considering other conditions. We use 7 ins to express rnge of rdii (from µm to 4 mm) for ctivted cloud droplets nd rindrops. In ddition, we dopt the colescence efficiency proposed y Seifert et l. (5) nd rekup scheme sed on tht of Feingold et l. (988) to estimte the collision-rekup of rindrops. Collision-colescence nd collision-rekup re clculted seprtely. In the clcultion of rekup, ll collisions re treted s stochstic colescence. This is oviously contrdictory tretment, s continuous nd stochstic schemes re used in the clcultion of collision-colescence. In the clcultion of rekup, using only stochstic scheme seems to cuse some underestimtion of the efficiency of rekup following multi-colescence in one time step. However, the proility of rekup following the collision etween lrge drop nd numerous smll droplets is very smll. Hence the method in this study cn provide quite ccurte results. Numericl experiments The kinemtic frmework of this study is sed on tht used y Szumowski et l. (998) to test the wrm rin microphysicl model. The kinemtic cloud model prescries n evolving flow nd performs -D dvection of temperture nd wter vriles (domin: 9 km km, dx nd dz: 5 m, dt: s). The flow pttern shows low-level convergence, upper-level divergence, nd nrrow updrft locted in the center of the domin. The mgnitude, verticl structure, width, nd tilt of the flow through the centrl updrft re ll prescried using simple nlyticl functions. This kinemtic frmework with microphysicl scheme predicts temporl nd sptil evolution of wter vpor, hydrometeors, nd potentil temperture explicitly y using the prescried flow field nd initil nd oundry conditions of wter vpor content nd potentil temperture. The dvection scheme is modified version of tht of Smolrkiewicz (984). The ulk microphysicl scheme incorported in Szumowski s originl model is replced with our hyrid microphysicl model (Ku nd Fujiyoshi, 6). This simple model cnnot estimte the effect of rinfll-induced drg on dynmics. The effect of chnge in drg cused y differences in CCN will e studied in future work. However, the model cn estimte the effects of CCN on the cloud microstructure nd rindrop formtion. Therefore, this model is suitle for estimting the effect of hygroscopic seeding on wrm rin formtion. This model tkes the effect of updrft velocity into ccount, ut it cnnot tke the effect of geogrphic loction into ccount. These effects cnnot e neglected nd will e studied y instlling our hyrid cloud microphysicl model into non-hydrosttic cloud model in the future. Figure shows the initil stte of potentil temperture nd the mixing rtio of wter vpor. Figure presents the time evolution of updrft velocity ner the cloud center for the shllow convective cloud cse () nd for the strtiform cloud cse (). Seeding is ssumed to e crried out from n irplne under the cloud se in this study. Other methods (t the cloud top or in the cloud) will e considered in future study. Our preliminry numericl experiments using the hyrid microphysicl cloud model suggested tht lter timing of seeding leds to smller effect on precipittion. Therefore, www.tmos-chem-phys.net//5// Atmos. Chem. Phys.,, 5 5,
8 N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds ) itude (k km) 94 6 4 8 6 Potentil temperture (K) Mixing rtio of wter vpor (g kg - ) Updr rft velo ocity ( m / sec 8 6 4 cloud rises seeding Fig.. Initil stte of the () potentil temperture (K) nd () mixing rtio of wter vpor (g kg ). 4 the hygroscopic seeding is ssumed to egin 5 min fter cloud initition. Seeding durtions re min for the shllow convective cloud cse nd 95 min for the strtiform cloud cse. These durtions result in the sme seeded volumes in the clouds or the sme totl mounts of seeding prticles for the two cloud types. Figure shows the wind field t the time of pek updrft velocity for the shllow convective cloud cse () nd the strtiform cloud cse (). Ku nd Tked (98), Cooper et l. (997), Feingold et l. (999), nd Sleey nd Cotton (4) showed tht gint CCN hve the gretest effect on the precipittion efficiency of wrm rin clouds in cses with numerous smll ckground CCN. When low concentrtions of smll CCN re present, dding gint CCN results in slight decrese in rinfll, suggesting tht lmost ll rinwter is produced y condenstion onto smll CCN. On the other hnd, when high concentrtions of smll CCN re present, dding gint CCN leds to modest increse in rinfll mount, suggesting tht rinwter is produced minly from condenstion onto gint CCN nd smll cloud droplets cught y lrge droplets condensed on gint CCN (Ku nd Fujiyoshi, 6). Preliminry numericl experiments using our hyrid microphysicl cloud model (not shown here) lso suggested tht hygroscopic seeding cnnot increse wrm rin when the numer concentrtion of ckground CCN is low (the size distriution of mritime ckground CCN used in preliminry numericl experiments is shown y the purple line in Fig. 4). Therefore, it is ssumed tht CCN for the non-seeded cse (reference cse) consist of high concentrtions of smll prticles like continentl cse or polluted cse. To clerly estimte the role of seeding prticles, the numer concentrtion of lrge CCN is ssumed to e very smll (e.g., the numer concentrtions re. e 5 cm for CCN lrger thn µm in rdius nd 5. e 7 cm for CCN lrger thn 5 µm in rdius). The chemicl composition of these CCN is ssumed to e NCl. The CCN size distriution for the non-seeded cse (ckground CCN) is shown y the red line in Fig. 4. ) Updr rft velo ocity ( m / sec..8.6.4. cloud rises seeding 6 9 5 6 Fig.. Time evolution of updrft velocity ner the center of the cloud for the shllow convective cloud cse () nd for the strtiform cloud cse (). We use 8 clsses to express rnge of rdii (from.9 to 9 µm) for ckground CCN. Figure 5 shows cloud wter in the non-seeded cse for convective cloud t 5 min ( min fter cloud initition) nd strtiform cloud t 8 min ( min fter cloud initition).. Hygroscopic seeding with monodisperse prticles To estimte the most efficient rdius nd mount of seeding prticles, monodisperse NCl prticles re used s seeding prticles. Numer concentrtions of seeding prticles under the cloud se for cses, including five different rdii nd four different totl msses of seeding prticles, re shown in Tle. Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//
N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds 9 m) k 4 5 6 7 8 9 X m s - m s - Fig.. The wind field t the time of pek updrft velocity for the shllow convective cloud cse () t 5 min nd for the strtiform cse () t 9 min). dn /d dlogr (cm - ).. Bckground Micro-powder Flre mritime Tle. Numer concentrtions (cm ) of seeding prticles under the cloud se in cses including five different rdii nd four different totl mounts of seeding prticles. The reference totl mount of seeding prticles (rtio =) is.8 4 g m. Rtio of totl mount of seeding prticles Rdius of seeding prticles..5.5 µm.5 µm 5 5 5 5. µm.5 5.6.5.5.5 µm... 5. µm.5.5.5.5... Rdius ( m) Fig. 4. Size distriutions of the ckground CCN (red line), slt micro-powder (green line), nd hygroscopic flre prticles (lue line). Totl numer concentrtion of ckground CCN (r>. µm) is cm. Those of seeding prticles shown y size distriutions in green nd lue re 9 cm nd 97 cm, respectively. Mritime ckground CCN, which ws used in preliminry numericl experiments, is lso shown in this figure. The totl numer concentrtion of mritime ckground CCN (r>. µm) is 4 cm... Convective cloud cse This section presents results of seeding the shllow convective cloud (Figs. nd ) with monodisperse prticles. Figure 6 shows the cloud droplet size distriutions t m ove the se of the cloud center (updrft velocity is pproximtely 4 ms ) t.5 min (9 s fter the strt of seeding) for the non-seeded cse nd seeded cses with the reference totl mss of seeding prticles ( rtio = in Tle ). The rdii of seeding prticles in ech seeding cse re.5,.5,.,.5, or 5. µm. Droplets condensed on seeding prticles stnd out for ech seeded cse. The results lso show tht seeding with smll prticles (.5 nd.5 µm in rdius) decreses the mode rdius of cloud droplets condensed on ckground CCN. On the other hnd, seeding with lrge prticles (.,.5, or 5. µm) does not significntly ffect the mode rdius of cloud droplets condensed on ckground CCN. Figure 7 shows temporl chnge in ccumulted surfce rinfll verged over the domin. The lck solid line in ech pnel indictes ccumulted rinfll for the nonseeded cse. For the seeded cses, seeding prticles hve rdii of.5 µm (red line),.5 µm (green line), µm (lue line),.5 µm (purple line), or 5 µm (ornge line). Four totl mounts of seeding prticles re exmined, with rtios of. (),.5 (),. (c), nd (d). Tle presents the numer concentrtions of seeding prticles under the cloud se. For reference, lck roken line in ech pnel shows ccumulted surfce rinfll from non-seeded mritime cloud (the size distriution of the mritime ckground CCN is shown y the purple line in Fig. 4). A lrger mount of seeding prticles.5 µm in rdius leds to less surfce rinfll. Seeding with prticles.5 µm in rdius increses surfce rinfll if sufficient mount of seeding prticles is used (5 cm, Fig. 7c), ut decreses the rinfll if the seeding mount is too lrge (5 cm, Fig. 7d). A lrge mount of lrge seeding prticles hstens the onset of surfce rinfll nd increses rinfll mount. With pproprite hygroscopic www.tmos-chem-phys.net//5// Atmos. Chem. Phys.,, 5 5,
4 N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds Convective cloud 5 min. Strtiform cloud 8 min. 6 9 X Fig. 5. Cloud wter (g kg ) in the non-seeded cse. () Convective cloud t 5 min ( min fter cloud initition). () Strtiform cloud t 8 min ( min fter cloud initition). seeding, the mount of surfce rinfll from moderte continentl cloud cn ecome similr to tht from non-seeded mritime cloud. Tle shows the rtios of seeded-cse to non-seeded-cse ccumulted rinfll verged over the domin t 6 min. Rtios representing decrese re given in lue, nd those indicting increse re shown in red. Rtios of seeded-cse to non-seeded-cse (665 cm ) cloud droplet numer concentrtions t 5 m ove the se of the cloud center t.5 min re lso shown in prentheses in Tle. These results indicte tht seeding cn increse rinfll if the size nd mount of seeding prticles re pproprite to sufficiently decrese the cloud droplet numer concentrtion (cses enclosed y green lines in Tle ). This effect is the competition effect, y which fewer lrge (here, rdii of.5 to.5 µm) solule prticles prevent the ctivtion of numerous smller prticles. Sufficient numers of gint prticles tht cn immeditely ecome rindrop emryos (here, lrger thn.5 µm) lso increse rinfll (cses enclosed y the ornge line in Tle ). This is the rindrop emryo effect. In the seeded cse with the lrgest mount of.5- µm prticles, the rinfll mount increses ecuse of oth the competition effect nd the rindrop emryo effect..5-µm prticles hve neither the competition effect nor the emryo effect ecuse they re not ig enough to ecome rindrop emryo nd to compete with the ckground CCN for excess wter vpor effectively. The optiml rdius (written in oldfce in Tle ) to increse rinfll ecomes lrger with the increse in the totl mss of seeding prticles. Yin et l. () investigted the effect of gint CCN on precipittion in convective clouds y numericl study. Becuse their study included ice-phse, microphysicl processes re more complex thn our study. However our results dn / dr ( cm dn / dr ( cm e+8 e+6 e+4 e+ e+.5 m 5.5 m. m.5 m 5 5. mm e- e-4 5 5 5 Rdius ( m) e+8 e+6 e+4 e+ e+.5 m 5.5 m. m.5 m 5 5. mm e- e-4 4 5 6 Rdius ( m) Fig. 6. Cloud droplet size distriutions t m ove the se of the cloud center for the shllow convective cloud cse (updrft velocity is out 4 ms ) t.5 min (), nd for the strtiform cloud cse (updrft velocity is out. ms ) t 68.5 min (). The lck line depicts the droplet size distriution for the non-seeded cse. For seeded cses, monodisperse slt prticles with five different rdii re seeded under the cloud se:.5 µm (red line),.5 µm (green line),. µm (lue line),.5 µm (pink line), nd 5. µm (light lue line). For seeding prticles of ech size, the totl mss is the sme s the reference totl mss ( rtio = in Tle ). do not contrdict their results. The condition (ckground CCN, dded gint CCN, nd updrft velocity) nd results of the seeded cse with the lrgest mount of.5-µm prticles in Tle is similr to CN cse in Tle in their pper. Comprison of two cses shows tht the decrese in cloud droplet numer y seeding in our study is lrger thn their study nd the increse in ccumulted rinfll verged over the domin y seeding in our study is smller thn the increse in mx. ccumulted rin y dding gint CCN in their study. Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//
Tle. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 6 min. Seeding of shllow convective cloud is crried out with monodisperse N. Ku nd M. Murkmi: slt prticles Effect of of different hygroscopic sizes nd seeding totl msses. on wrm Rtios rinin clouds lue nd red indicte significnt 4 decrese nd increse, respectively. Rtios of seeded-cse to non-seeded cse (665 cm - ) Tle. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 6 min. Seeding of shllow cloud droplet numer concentrtions t 5 m ove the se of the cloud center t.5 min convective cloud is crried out with monodisperse slt prticles of different sizes nd totl msses. Rtios in lue nd red indicte significnt decrese nd increse, respectively. re shown in Rtios prentheses. of seeded-cse to non-seeded cse (665 cm ) cloud droplet numer concentrtions t 5 m ove the se of the cloud center t.5 min re shown in prentheses. Rtio of totl mount of seeding prticles Rdius of seeding prticles..5.5 m. (.9).7 (.66). (.). (5.).5 m. (.97). (.87). (.56). (.8). m. (.99). (.94). (.88). (.7).5 m. (.). (.98). (.97).5 (.7) 5. m. (.). (.99). (.99). (.85) mm) omin ( in the d verged infll v ted R Accumul mm) omin ( in the d verged infll v ted R...8.6.4. non-seeded.5 m.5 m m.5 m 5 m Rtio of totl mss of seeding prticles:. 5 5 4 45 5 55 6...8.6.4. Rtio of totl mss of seeding prticles:. c...8.6.4. Rtio of totl t mss of seeding prticles:.5 5 5 4 45 5 55 6...8.6.4. Rtio of totl t mss of seeding prticles: 4 d Accumul 5 5 4 45 5 55 6 5 5 4 45 5 55 6 Fig. 7. Temporl chnge of ccumulted surfce rinfll verged over the domin for the shllow convective cloud cse. In ech pnel, the lck solid line indictes ccumulted surfce rinfll for the non-seeded cse. For seeded cses, monodisperse slt prticles with five different rdii re seeded under the cloud se:.5 µm (red line),.5 µm (green line), µm (lue line),.5 µm (purple line), nd 5 µm (ornge line). Rtios of the totl mounts of seeding prticles to the reference re. (),.5 (),. (c), nd (d). Numer concentrtions of seeding prticles re shown in Tle. For reference, ccumulted surfce rinfll from non-seeded mritime cloud is lso shown in ech pnel y lck roken line. www.tmos-chem-phys.net//5// Atmos. Chem. Phys.,, 5 5,
Tle. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 6 min. Seeding of strtiform cloud is crried out with monodisperse slt 4 prticles of different sizes N. nd Ku totl nd msses. M. Murkmi: Rtios in lue Effect nd red of hygroscopic indicte significnt seeding on wrm rin clouds decrese nd increse, respectively. Rtios of seeded-cse to non-seeded cse (8 cm - ) Tle. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 6 min. Seeding of strtiform cloud droplet numer concentrtions t m ove the se of the cloud center t 6 min cloud is crried out with monodisperse slt prticles of different sizes nd totl msses. Rtios in lue nd red indicte significnt decrese nd increse, respectively. re Rtios shown in ofprentheses. seeded-cse to non-seeded cse (8 cm ) cloud droplet numer concentrtions t m ove the se of the cloud center t 6 min re shown in prentheses. Rtio of totl mount of seeding prticles Rdius of seeding prticles..5.5 m. (.6).7 (.95).4 (6.8). (4.).5 m. (.8). (.94). (.).5 (.8). m. (.84). (.69). (.7). (.7).5 m. (.9). (.7). (.55). (.48) 5. m. (.97). (.79). (.6). (.4) mm) omin ( in the d verged infll v ted R...8.6.4. non-seeded d.5 m.5 m m 5.5 m 5 m Rtio of totl mss of seeding prticles:....8.6.4. Rtio of totl mss of seeding prticles:.5 Accumul 9 4 5 6 9 4 5 6 omin ( mm).. c Rtio of totl mss of seeding prticles:... d Rtio of totl mss of seeding prticles: in the d.8.8 verged infll v ted R.6.4..6.4. 5 Accumul 9 4 5 6 9 4 5 6 Fig. 8. Sme s Fig. 7 except for strtiform cloud... Strtiform cloud cse This section exmines the seeding of strtiform cloud (Figs. nd ) with monodisperse hygroscopic prticles. Figure 6 shows the cloud droplet size distriution t m ove the se of the cloud center, s in Fig. 6; however, in this cse the ir prcel tkes longer to rech m from the cloud se ecuse of the smll updrft velocity. Therefore lrge droplets, produced y condenstionl growth of lrge seeding prticles (here,.,.5, nd 5. µm in rdius) nd susequent colescence cn e seen. Figure 8 shows tht lrger mount of.5-µm rdius seeding prticles leds to smller mount of rinfll. For reference, the ccumulted surfce rinfll from non-seeded mritime cloud is lso Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//
N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds 4 Tle 4. Rtios of seeded-cse to non-seeded cse ccumulted surfce rinfll verged over the domin t 6 min. Seeding of shllow convective cloud is conducted with slt micro-powder of different concentrtions. A rtio indicting increse is colored red. Rtios of seeded-cse to non-seeded-cse (665 cm ) cloud droplet numer concentrtions t 5 m ove the se of the cloud center t.5 min re shown in prentheses. Numer concentrtion of 9. 45 9 8 seeding prticles (cm ) Totl mss of seeding 5 4 4 4 4 prticles (g m ) Rtio of ccumulted. (.99). (.95). (.88). (.56) rinfll to non-seeding cse shown in ech pnel y lck roken line. Tle gives the rtios of seeded-cse to non-seeded-cse ccumulted rinfll verged over the domin t 6 min. Rtios of the cloud droplet numer concentrtion t m ove the se of the cloud center t 6 min for the seeded cse to tht for the non-seeded cse (8 cm ) re lso shown in prentheses in Tle. The results shown in Fig. 8 nd Tle re similr to those in Fig. 7 nd Tle. However, we cn see tht the competition effect of gint CCN (here, from to 5 µm in rdius) occurs in more cses (cses enclosed y green lines in Tle ) thn in Tle ecuse low updrft velocities llow the low numer concentrtions of ctivted droplets to grow lrger nd deplete more excess wter vpor, leding to n enhnced competition effect nd simultneously, the rindrop emryo effect to some extent. Note tht decrese in the cloud droplet numer concentrtion does not lwys led to remrkle increse in ccumulted surfce rinfll. In the cloud with low updrft velocity, the numer concentrtions of cloud droplets re smll enough (cloud droplet sizes re lrge enough) to produce rindrops effectively even for the non-seeded cse. Therefore, the difference in ccumulted surfce rinfll mount etween seeded nd non-seeded cses is less thn expected from the rtio of cloud droplet numer concentrtions..5-µm prticles hve neither the competition effect nor the emryo effect even in strtiform cloud cse s well s in convective cloud cse (Sect...).. Hygroscopic seeding with slt micro-powder We now investigte seeding with slt micro-powder (Oshiomicron, produced y Ako Ksei, Co., Ako, Jpn). These seeding prticles re mde of NCl, nd their size distriution is pproximted y log-norml distriution. The size distriution of slt micro-powder in this study is ssumed to e log-norml with mode rdius of.5 µm nd totl concentrtion of 9 cm, s shown y the green line in Fig. 4. However, slt micro-powder with mode rdius s smll s.4 µm cn e mnufctured. dn / dr ( cm dn / dr ( cm e+8 e+6 e+4 e+ e+ e- 9 cm - 45 cm - 9 cm - 8 cm - e-4 5 5 5 Rdius ( m) e+8 e+6 9 cm - 45 cm - e+4 e+ e+ e- 9 cm - 8 cm - e-4 4 5 6 Rdius ( m) Fig. 9. Cloud droplet size distriutions t m ove the se of the cloud center for the shllow convective cloud cse (updrft velocity is out 4 ms ) t.5 min () nd for the strtiform cloud cse (updrft velocity is out. ms ) t 68.5 min (). The lck line indictes the droplet size distriution for the non-seeded cse. For seeded cses, different totl numer concentrtions of slt micro-powder re seeded under the cloud se: 9 cm (red line), 45 cm (green line), 9 cm (lue line), or 8 cm (purple line)... Shllow convective cloud cse For this simultion, we seed shllow convective cloud with the slt micro-powder. Figure 9 shows the cloud droplet size distriutions t m ove the se of the cloud center (updrft velocity is out 4 ms ) t.5 min (9 s fter the strt of seeding) for the non-seeded cse (lck line) nd seeded cses. For the seeded cses, numer concentrtions of slt micro-powder under the cloud se re 9 cm (red line), 45 cm (green line), 9 cm (lue line), nd 8 cm (purple line). The cloud droplets condensed on the slt micro-powder rnge from pproximtely 5 to µm in rdius, nd lrger mount of seeding prticles decreses the mode rdius nd numer of droplets condensed on ckground CCN. www.tmos-chem-phys.net//5// Atmos. Chem. Phys.,, 5 5,
44 N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds n the do omin (m mm) nfll ve erged i Ac ccumul ted Ri n the do omin (m mm) nfll ve erged i Ac ccumul ted Ri...8 8.6.4. cse 9cm - ; Numer concentrtion 45 cm - 9 cm - 8 cm - 5 5 4 45 5 55 6...8 8.6.4. cse 9cm - ; Numer concentrtion 45 cm - 9 cm - 8 cm - 9 4 5 6 Fig.. Temporl chnge in ccumulted surfce rinfll verged over the domin for the shllow convective cloud cse () nd strtiform cloud cse (). The lck line indictes ccumulted surfce rinfll for the non-seeded cse. For seeded cses, different totl numer concentrtions of slt micro-powder re seeded under the cloud se: 9 cm (red line), 45 cm (green line), 9 cm (lue line), or 8 cm (purple line). Figure shows temporl chnge in ccumulted rinfll verged over the domin, nd Tle 4 gives the rtios of ccumulted rinfll verged over the domin t 6 min for the seeded cse to tht for the non-seeded cse. Figure nd Tle 4 show tht seeding with slt micro-powder hstens the onset of surfce rinfll nd increses ccumulted rinfll due to the competition effect if there is sufficient numer of seeding prticles. The results shown in Tle 4 re similr to those for seeding prticles with rdius of.5 µm in Tle. Tle 5. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 6 min. Seeding of strtiform cloud is crried out with slt micro-powder of different concentrtions. Rtios of seeded-cse to non-seeded-cse (8 cm ) cloud droplet numer concentrtions t m ove the se of the cloud center t 6 min re shown in prentheses. Numer concentrtion of 9. 45 9 8 seeding prticles (cm ) Totl mss of seeding 5 4 4 4 4 prticles (g m ) Rtio of ccumulted. (.85). (.84). (.76). (.8) rinfll to non-seeding cse.. Strtiform cloud cse Seeding the strtiform cloud with slt micro-powder is crried out from under the cloud se. Figure 9 shows the cloud droplet size distriutions t m ove the se of the cloud center (updrft velocity is pproximtely. ms ) t 68.5 min (.5 min fter the strt of seeding) for the non-seeded cse nd seeded cses. The rdii of cloud droplets condensed on slt micro-powder rnge from out 8 to µm. Tle 5 lists ccumulted rinfll verged over the domin t 6 min. Figure nd Tle 5 show tht seeding with micro-powder cn hsten the onset of surfce rinfll nd increse the ccumulted mount of surfce rinfll from strtiform cloud, ut its effect is smller thn tht in the shllow convective cloud cse (Fig. nd Tle 4). Seeding with slt micro-powder cn decrese the numer concentrtions of cloud droplets to some extent. However, in the strtiform cloud with low updrft velocity, cloud droplet numer concentrtions re smll enough (cloud droplet sizes re sufficiently lrge) to produce rindrops effectively even for the non-seeded cse. Therefore, the differences in the ccumulted surfce rinfll mount etween seeded nd nonseeded cses re less remrkle thn for the shllow convective cloud in which the non-seeded cse did not efficiently produce rindrops.. Hygroscopic seeding y flre Field nd numericl experiments hve exmined severl kinds of flres. The lue line in Fig. 4 shows the size distriution of seeding prticles composed of CCl produced from urning flre sed on lortory mesurements for the ICE 7% flre produced y Ice Crystl Engineering (R. T. Bruintjes, personl communiction, 6). The size distriution without prticles lrger thn 5 µm in rdius is lso exmined to estimte the effect of hygroscopic seeding prticles with lrge rdius ut low numer concentrtion. This size distriution hs totl numer concentrtion of hygroscopic prticles of 97 cm. Here, we exmine three Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//
N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds 45 dn / dr ( cm dn / dr ( cm e+8 e+6 e+4 e+ 94 cm - 485 cm - 97 cm - e+ Convective e- Up to 5 m e-4 4 5 Rdius ( m) e+8 e+6 94 cm - 485 cm - e+4 97 cm - e+ e+ Strtiform e- Up to 5 m c e-4 4 5 6 7 Rdius ( m) dn / dr ( cm dn / dr ( cm e+8 e+6 e+4 e+ e+ Convective Up to 5 m e- e-4 4 5 Rdius ( m) e+8 e+6 e+4 e+ e+ Strtiform Up to 5 m e- d e-4 4 5 6 7 Rdius ( m) Fig.. Cloud droplet size distriutions t m ove the se of the cloud center for the shllow convective cloud cse (updrft velocity is out 4 ms ) t.5 min () nd () nd for the strtiform cloud cse (updrft velocity is out. ms ) t 68.5 min (c) nd (d). Numericl simultions re performed with flre prticles with rdii up to 5 µm () nd (c) or up to 5 µm () nd (d). The lck line indictes the size distriution for the non-seeded cse. For seeded cses, different totl numer concentrtions of flre prticles re seeded under the cloud se: 94 cm (red line), 485 cm (green line), nd 97 cm (lue line). numer concentrtions of prticles seeded under the cloud se (94 cm, 485 cm, nd 97 cm )... Shllow convective cloud cse The shllow convective cloud is seeded with flre prticles with rdii up to 5 µm or up to 5 µm. Figure nd show the cloud droplet size distriutions t m ove the se of the cloud center (updrft velocity is pproximtely 4 ms ) t.5 min (9 s fter the strt of seeding) for the non-seeded nd seeded cses. The lck line indictes the droplet size distriution for the non-seeded cse. For seeded cses, hygroscopic flre prticles with numer concentrtions of 94 cm (red line), 485 cm (green line), nd 97 cm (lue line) re seeded under the cloud se. The cloud droplets condensed on the seeding prticles rnge from pproximtely 5 to 8 µm (Fig. ) or 5 to 46 µm (Fig. ) in rdius. Furthermore, with lrger mounts of seeding prticles, the mode rdius of droplets decreses, nd the cloud droplet numer increses. Figure shows the temporl chnge of ccumulted rinfll verged over the domin. Tles 6 nd 7 give rtios of seeded-cse to non-seeded-cse ccumulted rinfll verged over the domin t 4 nd 6 min. The results in Fig. nd Tle 6 indicte tht seeding with flre prticles up to 5 µm in rdius increses cloud droplet numer concentrtions nd decreses surfce rinfll. The numerous smll prticles produced from urning flre suppress the condenstionl growth of cloud droplets, decrese the collisioncolescence efficiency of cloud droplets, nd consequently decrese surfce rinfll. The effect of smll seeding prticles incresing the numer of ctivted cloud droplets is dominnt over the competition effect of lrge seeding prticles (lrger thn.5 µm in rdius, in convective cloud cses) suppressing the ctivtion of smller CCN. Figure nd Tle 7 show tht seeding with flre prticles up to 5 µm in rdius lso increses cloud droplet numer concentrtions ut hstens the onset of surfce rinfll nd increses the ccumulted mount of surfce rinfll due to the rindrop emryo effect of gint nd ultr-gint CCN (lrger thn 5 µm in rdius, in convective cloud cses) t 4 min. However, it slightly decreses the ccumulted mount of surfce rinfll t 6 min. This indictes tht during the erly stge of rin formtion, rindrop emryos originting from CCN lrger thn 5 µm in rdius efficiently collect cloud droplets nd promote rindrop formtion. However, during the lte stge of rin formtion, when most of the rindrop emryos originting from CCN lrger thn 5 µm www.tmos-chem-phys.net//5// Atmos. Chem. Phys.,, 5 5,
46 N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds Tle 6. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 4 nd 6 min. Seeding of shllow convective cloud is crried out with flre prticles (up to 5 µm in rdius). A rtio indicting decrese is colored lue. Rtios of seeded-cse to non-seeded-cse (665 cm ) cloud droplet numer concentrtions t 5 m ove the se of the cloud center t.5 min re shown in prentheses. Numer concentrtion of 94 485 97 Seeding prticles (cm ) Totl mss of seeding 4.7 5. 4.4 4 prticles (g m ) Rtio of ccumulted rinfll to.8.5. non-seeding cse t 4 min. Rtio of ccumulted rinfll to. (.4).9 (.4).8 (.67) non-seeding cse t 6 min. Tle 7. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t 4 nd 6 min. Seeding of shllow convective cloud is crried out with flre prticles (up to 5 µm in rdius). A rtio indicting n increse is colored red. Rtios of seeded-cse to non-seeded-cse (665 cm ) cloud droplet numer concentrtions t 5 m ove the se of the cloud center t.5 min re shown in prentheses. Numer concentrtion of 94 485 97 Seeding prticles (cm ) Totl mss of seeding 9.4 5.4 4 4.7 4 prticles (g m ) Rtio of ccumulted rinfll to.7.. non-seeding cse t 4 min. Rtio of ccumulted rinfll to. (.4).9 (.).9 (.66) non-seeding cse t 6 min. domin (mm) infll v verged in the Ac ccumul ted R domin (mm) infll v verged in the Ac ccumul ted R. Flre ( r < 5 m). cse.8 8 94 cm - ; Numer concentrtion 485 cm - 97 cm -.6.4. 5 5 4 45 5 55 6. Flre ( r < 5 m). cse.8 8 94 cm - ; Numer concentrtion 485 cm - 97 cm -.6.4. 5 5 4 45 5 55 6 hve grown into rindrops nd fllen out of the cloud s precipittion, numerous cloud droplets with smller sizes re left in the cloud without efficiently producing rindrops... Strtiform cloud cse Next, seeding of strtiform cloud with flre prticles up to 5 µm or up to 5 µm in rdius is simulted. Figure c nd d show the cloud droplet size distriutions t m ove the se of the cloud center (updrft velocity is pproximtely. ms ) t 68.5 min (.5 min fter the strt of seeding) for the non-seeded nd seeded cses. As illustrted in these figures, cloud droplets condensed on seeding prticles rnge from 8 to 5 µm (Fig. c) or from 8 to 65 µm (Fig. d) in rdius, nd lrger mounts of seeding prticles led to smller mode rdii of droplets nd incresed cloud droplet numers. Figure shows the seeding effect on ccumulted surfce rinfll for hygroscopic flre prticles up to 5 µm () nd 5 µm () in rdius. Tles 8 nd 9 present rtios of seededcse to non-seeded-cse ccumulted rinfll verged over Fig.. Temporl chnge of ccumulted surfce rinfll verged over the domin. Shllow convective cloud is seeded with flre prticles up to 5 µm() or up to 5 µm() in rdius. The lck line indictes the ccumulted surfce rinfll for the non-seeded cse. For seeded cses, different totl numer concentrtions of flre prticles re seeded under the cloud se: 94 cm (red line), 485 cm (green line), or 97 cm (lue line). the domin t nd 6 min. Figure nd Tles 8 nd 9 show tht seeding with flre prticles up to 5 µm or 5 µm increses the cloud droplet numer, hstens the onset of surfce rinfll, nd increses ccumulted surfce rinfll for the first min, s in the convective cloud cses (Fig. nd Tles 6 nd 7). However, unlike the convective cloud cses, it does not decrese the ccumulted mount of rinfll t 6 min. Tle 8 nd Fig. show tht pproprite concentrtions of lrge nd gint CCN (smller thn 5 µm) produce lrge cloud droplets, ut not numerous smll droplets, nd Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//
N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds 47 Tle 8. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t nd 6 min. Seeding of strtiform cloud is crried out with flre prticles (up to 5 µm in rdius). A rtio indicting n increse is colored red. Rtios of seeded-cse to non-seeded-cse (8 cm ) cloud droplet numer concentrtions t m ove the se of the cloud center t 6 min re shown in prentheses. Numer concentrtion of 94 485 97 Seeding prticles (cm ) Totl mss of seeding 4.7 5. 4.4 4 prticles (g m ) Rtio of ccumulted rinfll to... non-seeding cse t min. Rtio of ccumulted rinfll to. (.99). (.98). (.7) non-seeding cse t 6 min. Tle 9. Rtios of seeded-cse to non-seeded-cse ccumulted surfce rinfll verged over the domin t nd 6 min. Seeding of strtiform cloud is crried out with flre prticles (up to 5 µm in rdius). A rtio indicting n increse is colored red. Rtios of seeded-cse to non-seeded-cse (8 cm ) cloud droplet numer concentrtions t m ove the se of the cloud center t 6 min re shown in prentheses. Numer concentrtion of 94 485 97 Seeding prticles (cm ) Totl mss of seeding 9.4 5.4 4 4.7 4 prticles (g m ) Rtio of ccumulted rinfll to..4.4 non-seeding cse t min. Rtio of ccumulted rinfll to. (.97). (.94). (.6) non-seeding cse t 6 min. domin (mm) infll v verged in the ted R ccumul Ac domin (mm) infll v verged in the Ac ccumul ted R. Flre ( r < 5 m). cse.8 8 94 cm - ; Numer concentrtion 485 cm - 97 cm -.6.4. 9 4 5 6. Flre ( r < 5 m). cse.8 8 94 cm - ; Numer concentrtion 485 cm - 97 cm -.6.4. 9 4 5 6 enhnce the collision-clescence process, resulting in not only slightly erly onset of rinfll ut lso slight increse in ccumulted surfce rinfll even t 6 min. However, high concentrtions of lrge nd gint CCN increse the numer concentrtions of cloud droplets nd decrese their sizes, suppressing the collision-colescence process nd rindrop formtion. On the other hnd, s seen in Tle 9 nd Fig., gint nd ultr-gint CCN (up to 5 µm) produce rindrop emryos, hstening the onset of rinfll nd slightly incresing the ccumulted mount of surfce rinfll even t 6 min. The enhncement of rindrop formtion due to higher concentrtions of rindrop emryos is dominnt over the suppression of rindrop formtion due to numerous smll droplets. Fig.. Sme s Fig. except for strtiform cloud. 4 Conclusions This study exmined the effects of hygroscopic seeding on shllow wrm rin clouds using hyrid cloud microphysicl model comining Lgrngin, semi-lgrngin, nd Eulerin frmeworks. The hyrid cloud microphysicl model cn ccurtely estimte the effect of CCN on cloud microstructure. The simultion results suggest the following conclusions regrding moderte continentl ir msses.. Seeding cn hsten the onset of surfce rinfll nd increse the ccumulted mount of surfce rinfll if the rdius nd mount of seeding prticles re pproprite.. The optiml rdius to increse rinfll ecomes lrger with the increse in totl mount of seeding prticles.. Seeding with slt micro-powder cn hsten the onset of surfce rinfll nd increse the ccumulted mount www.tmos-chem-phys.net//5// Atmos. Chem. Phys.,, 5 5,
48 N. Ku nd M. Murkmi: Effect of hygroscopic seeding on wrm rin clouds c min. min. Micro powder 9 cm - min. Micro powder 8 cm - 6 9 X Fig. 4. Rdr reflectivity in shllow convective cloud t min for the non-seeded cse () nd for seeded cses with slt micropowder of different totl numer concentrtions: 9 cm () nd 8 cm (c). of surfce rinfll if the mount of seeding prticles is sufficient (out 4 e 4 g m ) nd the updrft velocity is lrge (shllow convective cloud). 4. Seeding with hygroscopic flre decreses rinfll in the cse of lrge updrft velocity (shllow convective cloud) nd slightly increses rinfll in the cse of smll updrft velocity (strtiform cloud). 5. Seeding with flre prticles including ultr-gint prticles (r>5 µm) hstens the onset of surfce rinfll. 6. Two effects of hygroscopic seeding increse surfce rinfll: the competition effect, wherey lrge solule prticles prevent the ctivtion of numerous smller prticles, nd the rindrop emryo effect of gint nd ultr-gint solule prticles tht cn immeditely ecome rindrop emryos. In some cses, one of the effects works, nd in other cses, oth effects work, depending on the updrft velocity nd the mount nd size of seeding prticles. Our results from hygroscopic flre seeding (4 nd 5) do not contrdict the results of Cooper et l. (997) nd Cro et l. (). Their results showed tht the ddition of hygroscopic prticles cn significntly ccelerte rin formtion through the wrm-rin process. They used prcel models to simulte the seeding effect of hygroscopic flre s producing wide rnge of prticle sizes including gint nd ultr-gint CCN. They derived their conclusions from cloud droplet size distriutions in prcels. Our results lso show tht seeding with flre prticles cn ffect the cloud droplet size distriution nd cn hsten the onset of surfce rinfll. However, our results derived from the two-dimensionl cloud model indicte tht ccumulted surfce rinfll does not increse considerly y hygroscopic flre seeding ecuse of the lrge numer of smll prticles produced from the urning flre. Generlly speking, prcel models cnnot properly express erly sedimenttion of rindrops nd the collection of cloud droplets y rindrops. This issue my explin why our results nd those of previous numericl studies using prcel models differ regrding the effectiveness of hygroscopic flre seeding for ugmenting ccumulted surfce precipittion. Bowen (95) nd Feingold et l. (996) emphsized the importnce of in-cloud dwell time (in-cloud residence time) for drizzle production. Their studies suggest tht optiml rdius to increse rinfll depends on updrft velocity ecuse lrger droplet in weker updrft leds to smller residence time. Comprison etween Tles nd cnnot show cler influence of residence time ecuse these tles show only ccumulted nd verged surfce rinfll. Detil nlysis (grid to grid, time to time) is needed to mention the importnce of in-cloud dwell time. Rdr-estimted rinfll is often used to ssess the effect of hygroscopic seeding in field experiments (e.g., Mther et l., 997). Figures 4 to 7 show rdr reflectivity in shllow convective cloud t nd min. These figures indicte tht seeding with micro-powder nd flre prticles increses rdr reflectivity. However, in our numericl results, hygroscopic flre seeding decreses rinfll in the cse of shllow convective cloud. Therefore, n increse in rdr reflectivity does not necessrily men n increse in the ccumulted surfce rinfll mount. Our results suggest, s do others (Ku nd Tked, 98; Cooper et l., 997; Feingold et l., 999; Sleey nd Cotton, 4), tht the effects of hygroscopic seeding cn vry with the ckground CCN numer concentrtions (not shown in this pper) nd updrft velocity t the cloud se (or cloud types) s well s the sizes nd mounts of seeding prticles nd the timing of seeding (not shown in this pper). Therefore, more oservtionl dt must e collected on ckground CCN numer concentrtions nd cloud-se updrft velocities. In this study, the chemicl composition of the ckground CCN is ssumed to e NCl. We lso performed experiments with CCN composed of (NH 4 ) SO 4. The numericl results showed tht the hygroscopicity wekens when the chemicl composition of CCN is chnged from NCl to (NH 4 ) SO 4, which decreses the numer concentrtion of ctivted droplets nd consequently increses the Atmos. Chem. Phys.,, 5 5, www.tmos-chem-phys.net//5//