1. Introduction. Three Different Behaviors of Liquid Water Path of Water Clouds in Aerosol-Cloud Interactions

Transcription

1 Three Different Behaviors of Liquid Water Path of Water Clouds in Aerosol-Cloud Interations Qingyuan Han 1, William B. Rossow 2, Jian Zeng 1, and Ronald Welh 1 1 University of Alabama in Huntsville Huntsville, AL 2 NASA/Goddard Institute for Spae Studies New York, NY Abstrat Estimates of the indiret aerosol effet in GCMs assume either that loud liquid water path is onstant (Twomey effet) or inreases with inreased droplet number onentration (drizzle-suppression or Albreht effet). On the other hand, if loud thermodynamis and dynamis are onsidered, loud liquid water path may also derease with inreasing droplet number onentration, whih has been predited by model alulations and observed in ship-trak and urban influene studies. This study examines the different hanges of loud liquid water path assoiated with hanges of loud droplet number onentration. Satellite data (January, April, July and Otober 1987) are used to determine the loud liquid water sensitivity, defined as the ratio of hanges of liquid water path and hanges of olumn droplet number onentration. The results of a global survey for water louds (loud top temperature >273K, optial thikness 1 τ<15) reveal all three behaviors of loud liquid water path with aerosol hanges: inreasing, approximately onstant, or dereasing as loud olumn number onentration inreases. We find that (1) in about one third of the ases, predominantly in warmer loations or seasons, the loud liquid water sensitivity is negative and the regional and seasonal variations of the negative liquid water sensitivity are onsistent with other observations, (2) in about one third of the ases, a minus one third (-1/3) power law relation between effetive droplet radius and olumn number onentration is found, onsistent with a nearly onstant loud water path, and (3) in the remaining one third of the ases, the loud liquid water sensitivity is positive. These results support the suggestion that it is possible for an inrease of loud droplet number onentration to both redue loud droplet size and enhane evaporation just below loud base, whih deouples the loud from the boundary layer in warmer loations, dereasing water supply from surfae and reduing loud liquid water. Our results also suggest that the urrent evaluations of the negative aerosol indiret foring by GCMs, whih are based on either the Twomey or Albreht effets, may be overestimated in magnitude. 1. Introdution Aerosol radiative forings, both diret and indiret, are the most unertain atmospheri forings of limate hange. Between them, the aerosol indiret foring, whih is related to the loud radiative property hanges through loud-aerosol interations, is the most unertain (IPCC 1996). The importane of the aerosol indiret effet is inreased by the suggestion that it is the most likely explanation for the observed derease of the diurnal temperature yle (Hansen et al., 1997). Signifiant progress has been made in reent years Corresponding author address: Dr. Qingyuan Han, University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL han@nsst.uah.edu to evaluate the aerosol indiret effet by using prognosti equations for liquid water ontent and loud droplet number onentration in global limate models (e.g., Del Genio et al., 1996; Lohmann et al., 1999; Rotstayn, 1999; Ghan et al., 2001; Menon et al., 2000). These physially-based GCMs are more reliable in prediting hanges in limate beause they are not tuned to parameterizations that may only be valid under urrent limate onditions. However, the results of these models are quite different beause loud droplet number onentrations and loud liquid water ontents are alulated differently. To redue the differenes in global model results, and thus the unertainties in estimations of the aerosol indiret effet, global satellite observations of loud and aerosol properties and their relationships are ruially needed. During the first phase of GACP (Global Aerosol Climatology Projet), new variables and their relationships have been retrieved from satellite

2 2 observations inluding near-global surveys of the relationship between loud albedo and effetive radius (Han et al., 1998a), loud olumn number onentration (Han et al., 1998b), and loud olumn suseptibility (Han et al. 2000). Some of these results have been used for omparisons with model preditions. For example, in the study reported by Han et al. (1998a), results of a near-global survey reveal that loud albedo and droplet radius are positively orrelated for most optially thin louds (τ<15) and negatively orrelated for most optially thik louds (τ>15), where τ is referred to λ = 0.6 µm. Suh a relationship ompares favorably with the behavior exhibited by several GCMs (e.g., Lohmann et al., 1999; Ghan et al., 2001). Nevertheless, the estimated aerosol indiret effet (-1.7 W/m 2 ) from the MIRAGE model (Ghan et al., 2001) is muh larger than that (-0.4 W/m 2 ) estimated by Lohmann et al. (1999) using the ECHAM model, even though the loud liquid water ontent hanges due to the aerosol effet are smaller in the MIRAGE than in the ECHAM model. This indiates that more detailed quantitative omparisons inluding relationships among different parameters and their variations are needed. Cloud mirophysis shemes in most GCMs inlude at least two variables: loud droplet number onentration and loud liquid water ontent (e.g., Del Genio et al., 1996; Lohmann et al., 1999; Ghan et al., 1997; Rotstayn, 1999; Menon et al., 2002) with droplet size inferred from these two. Inreases in loud droplet number onentration, N, are a diret indiation of the aerosol-loud interation, onsidered the driving fore of the indiret effet. This has been suggested by observations during the past several deades (e.g., Warner and Twomey, 1967; Fitzgerald and Spyers-Duran, 1973; Eagan et al., 1974; Alkezweeny et al., 1993; Hudson and Svensson, 1995). The loud liquid water ontent is the basi parameter for alulating loud proesses, espeially radiation and preipitation. Therefore, model estimates of the aerosol indiret effet inludes two links: one is to model the relation between loud droplet number onentration and aerosol onentrations (e.g., Hudson et al., 2000 and referenes therein) and the other is to predit the loud liquid water ontent with hanging loud droplet number onentrations (e.g., Durkee et al., 2000 and referenes therein). Following earlier investigations (for a review, see Twomey, 1993), most studies have been foused on the first link, produing empirial relations between aerosol onentrations and loud droplet number onentrations (e.g., Jones et al., 1994, 1999; Bouher and Lohmann, 1995, Jones and Slingo, 1996; Rotstayn, 1999) and physially-based aerosol ativation relations (e.g., Ghan et al., 1997; Lohmann et al., 1999). The intention of this study is to investigate the seond link, i.e., to examine the hanges of loud liquid water assoiated with hanges of loud droplet number onentration. Based on a onsideration of loud mirophysis, Albreht et al. (1989) proposed that inreased droplet number onentration leads to smaller droplet sizes that make preipitation formation more diffiult produing larger water ontents. This idea is supported by observations showing inreased liquid water path and suppressed drizzle in ship traks (Radke et al., 1989, Ferek et al., 2000) and in smoke plumes (Rosenfeld et al., 1999). However, model studies with a more omplete treatment of the interations of loud dynamis, thermodynamis and radiation show that, even though drizzle is suppressed, ooling just below loud base is enhaned beause the smaller (and more numerous) loud droplets evaporate more rapidly. This ooling ats together with the radiative heating of the loud base to suppress turbulent mixing, deoupling the loud from the rest of the boundary layer and reduing the supply of water vapor and of the loud liquid water. In partiular, Akerman et al. (1995) show that these hanges inrease the amplitude of the diurnal yle of loud water ontent beause the Albreht effet operates at night to inrease loud water ontent but that this is overwhelmed during the day beause the evaporative ooling reinfores the tendeny for the loud layer to deouple from the rest of the boundary layer. Dereased liquid water ontents with inreased droplet number onentration are also supported by observations of ship traks (e.g., Platnik et al., 2000, Akerman et al., 2000) and of urban influenes on loud properties (Fitzgerald and Spyers-Duran, 1973). In urrent GCMs, the response of loud liquid water to hanges in droplet number onentration is through the influene of droplet number on the autoonversion of loud water to rain, i.e., larger droplet onentration will either derease the autoonversion rate of loud droplets (e.g., Beheng, 1994; Lohmann and Feihter, 1997) or inrease the ritial threshold for autoonversion to start (e.g., Rotstayn, 1999). These mehanisms lead to a general inrease in loud liquid water ontent with inreasing droplet number (e.g., Ghan et al., 2001). Although evaporation and its influene on droplet sizes are onsidered in a few GCMs (e.g., Lohmann et al., 1999), its influene on thermodynamis and the feedbak on loud liquid water is diffiult to parameterize partially due to the oarse vertial resolution in GCMs (Del Genio, 2000, personal ommuniation). The questions are: what is the general behavior of loud liquid water in response to inreased droplet number onentrations and what are its temporal and spatial variations? If loud liquid water inreases with inreased droplet number in the majority of louds, then the onsideration of loud mirophysis is good enough

3 3 and we are onfident about the responses of loud liquid water (and thus loud optial properties) to aerosol-loud interations. If this is not the ase, then more effort has to be made to inlude the diffiult but important effets of loud dynamis and thermodynamis in models for an aurate estimation of the aerosol indiret effet. The purpose of this study is to answer the above questions using satellite observations. The onept of the loud liquid water sensitivity is defined in setion 2. The satellite data used in this study are desribed in setion 3. Results and onlusions are presented in setion 4 and 5, respetively. 2. Cloud liquid water sensitivity We start with a definition that makes the omparison between results of model predition and satellite observation more preise. Sine observations show that hanges in loud geometrial thikness during aerosol-loud interations annot be ignored (e.g., Hobbs et al., 1970, Akerman et al., 2000), onsistent with model preditions (Pinus and Baker, 1994; Akerman et al., 1993), olumn-integrated values of loud droplet number onentration, N, and liquid water ontent, LWP, are more appropriate in desribing this relationship to avoid assumptions of onstant geometrial thikness of the louds. Satellite remote sensing an provide estimates of these olumnintegrated parameters, i.e., olumn droplet number onentration (Han et al., 1998b), N = N h (1) and liquid water path (e.g., Greenwald et al., 1995, Han et al., 1994) LWP = lw h (2) where h is the loud geometrial thikness. The form of (1) and (2) assumes vertial uniformity; in the more general ase the satellite retrieval represents the vertial integrals of N and LWC. The relation between LWP and N is (Han et al., 1998b) N 3 LWP = πρ 3 4 w re (1 b)(1 2 b) where b is the effetive variane in a gamma size distribution. We define the loud water sensitivity as LWP δ= (3) N when loud thikness h is a onstant, δ= LWC/ N. Note that this definition is similar to the definition of loud olumn suseptibility (Han et al. 2000), in whih α (hanges in loud spherial albedo) is replaed by LWP (hanges in loud liquid water path). The reason that we do not use the term suseptibility here is that it means apt to or the potential to be affeted by and therefore is determined by properties of individual louds as first proposed by Twomey (1991). However, aerosol-loud interations are not only determined by the properties of louds and aerosols, they are also determined by the onditions of environment suh as thikness of boundary layer (e.g., Durkee et al., 2000). This is the reason that ship traks are not found in many louds with high suseptibilities (e.g., Platnik and Twomey, 1994; Coakley et al., 2000). In our approah, the loud water sensitivity, δ, is derived using the least-squares linear regression to determine the slope of LWP and N for all water louds within a 2.5 o x2.5 o grid box during eah one month period. Therefore, the derived value desribes what atually happened, whih is determined not only by loud proesses, but also by the ondition of environments. In this sense, the terminology loud olumn suseptibility used in Han et al. (2000) is misleading: it should be modified to loud albedo sensitivity when it was derived based on monthly data from a grid box. Liquid water sensitivity represents the hange of liquid water path orrelated with hanges in olumn droplet number onentration, whih is affeted by the total water availability: louds in a moist environment (e.g., maritime) tend to have larger liquid water sensitivity than those in a dry environment (e.g., ontinental). To this end, we normalize the liquid water sensitivity for different environments to isolate better the effet of aerosol-loud interation; the relative loud water sensitivity is defined as LWP / LWP ln( LWP) β = N / N ln( N ) (4) The hanges in LWP are aused by two fators, i.e., hanges in volumetri mean droplet radius ( r ) and hanges in the olumn number onentration (N ): dln( LWP) = 3dln( r) + dln( N ) If the relation between effetive radius and volume average radius is used (e.g., Martin et al., 1994), kr = r (5) 3 3 e

4 4 then the expression beomes dln( LWP) = 3dln( r ) + dln( k) + dln( N ) (6) e It is lear that the effet of hange in k is not independent from hanges in LWP; it is part of the effet of hanges in r and thus part of the hanges in LWP. Therefore, by estimating hanges in LWP, the effet of hanges in k is already inluded. One relationship losely related to the liquid water sensitivity that is often used in models (e.g., Del Genio et al., 1996) is the relation between effetive droplet radius, r e, and volume number onentration, N, : re 1/3 N or d(ln r e ) 1 d(ln N ) = 3 based on some observations (e.g., Stephens, 1978). For this relation to be valid, the liquid water ontent, effetive variane, and loud thikness have to be independent of N. There are airraft measurements that either agree with or violate the above important relation (e.g., Akerman et al., 2000) and no global statistis available to verify it. Although it is diffiult to diretly verify this relation using urrent satellite data, it is possible to hek the slightly different relation r e or N γ d(ln re ) d(ln N ) = γ (7) This relation is useful for verifying model results beause N is the produt of other two model parameters: volume number onentration (N) and loud thikness (h). In ase of zero liquid water sensitivity and negligible hanges in k, the value γ would be 1/3. 3. Method and Data The data used are the near-global datasets of loud properties inluding loud optial thikness, effetive radius, liquid water path and olumn number onentrations for January, April, July and Otober 1987 developed using ISCCP data (Han et al., 1994, Han et al., 1998b). The original ISCCP analysis separates loudy and lear image pixels (area about 4 x 1 km 2 sampled to a spaing of about 30 km) and retrieves loud optial thikness and top temperature (T ) from radianes measured by AVHRR at wavelengths of µm (Channel 1) and µm (Channel 4), assuming r e = 10 µm. The analysis uses the NOAA TIROS Operational Vertial Sounder (TOVS) produts to speify atmospheri temperature, humidity and ozone abundane and also retrieves the surfae temperature (T s ). The ISCCP analysis is extended by retrieving r e from AVHRR radianes at wavelengths of µm (Channel 3) and revising the values of τ to be onsistent for louds with T 273 K (Han et al., 1994, 1995). Only liquid water louds are onsidered in this study beause 90% of the tropospheri aerosols are distributed below 3 km altitude (Griggs, 1983). Moreover, aerosol effets on ie louds may be different than on liquid water louds. The radianes are modeled as funtions of illumination/viewing geometry by inluding the effets of Lambertian refletion/emission from the surfae (the oean refletane is anisotropi, see Rossow et al., 1989), absorption/emission by H 2 O, CO 2, O 3, O 2, N 2 O, CH 4, and N 2 with the orrelated k-distribution method (Lais and Oinas 1991), Rayleigh sattering by the atmosphere and Mie sattering/absorption by horizontally homogeneous loud layers using a 12-Gauss point doubling/adding method. The droplet size distribution is assumed to be the gamma-distribution. Error soures are disussed and validation studies are reported in Han et al. (1994, 1995). Note that the satellite-measured radiation is only sensitive to the droplet sizes in the topmost part of the louds; therefore, the values of LWP obtained by this analysis may be biased if r e at loud top is systematially different from the vertially averaged value (Nakajima et al., 1991). For non-preipitating louds (LWP 150 g/m 2 ), the results of this method agree well with ground based mirowave radiometer measurements (Han et al., 1995). Lin and Rossow (1994, 1996) show exellent agreement of mirowave (from SSM/I) determinations of LWP over the global oean with those obtained from the ISCCP results, assuming 10 µm droplets. Greenwald et al. (1997) ompare mirowave retrievals of LWP from SSM/I and from GOES-8 over the Paifi Oean and they found RMS differenes between these two independent retrievals is as low as kg m -2 for overast senes. The two parameters used to derive liquid water sensitivity, LWP and N, are obtained from r e and τ by (Han et al., 1995), and (Han et al., 1998b) N 2 LWP = r τρ (8) 3 e w τ = 2 2πr e(1- b)(1-2b) (9) where b is effetive variane of loud droplet size distribution. The value of b is taken as in the retrieval of N, equivalent to a k value in Eq. (6) 0.495, whih is smaller than the range of 0.67 to 0.80 as suggested by Martin et al. (1994) in order to offset the

5 5 effet of overestimate of r e by satellite retrievals (Han et al., 1998b). All of the individual pixel values are olleted for eah 2.5 x 2.5 map grid ell for eah month, representing both spatial variations at sales ~ km and daily variations over eah month. Only louds with loud top temperature warmer than 273 K were used in this study. To redue the possible effets of loud frational loud over on loud droplet radius (Han et al., 1995), only pixels with loud optial thikness larger than unity were inluded. Sine thinner louds are more apt to be influened by the aerosol indiret effet, only results of louds with 1 τ<15 are shown. Typially, about 100 samples per map grid ell per month are available; results are not reported if there are fewer than 10 samples. Confidene level of the regression results varies with different grid boxes. On average, orrelation oeffiient r=0.159 is signifiant at the 0.95 onfidene level. The liquid water sensitivity, δ, is derived by least squares linear regression between LWP and N values. The power γ in the power law relation of r e and N, whih is related to the relative liquid water sensitivity, β, by Eq. (5), is derived by least squares linear regression between ln(r e ) and ln(n ). 4. Results 4.1 Liquid Water Sensitivity Figure 1 is a near-global survey of the liquid water sensitivity in water louds for January, April, July and Otober Considering the whole range and Figure 1: Liquid water sensitivity of water louds for January, April, July and Otober The unit is in [g m -2 /3x10 6 m -2 ]. For a typial 300 m thikness of loud, 1 [g m -2 /3x10 6 m -2 ] orresponds to an inrease of loud liquid water path by 1g m -2 for a hange of loud droplet number onentration by 100 m -3. Figure 2: Histogram of the liquid water sensitivity for January, April, July and Otober appropriate details in spatial variations, the units used are [g m -2 /3x10 6 m -2 ]. For a typial 300 m thikness of loud (Wang et al., 2000), 1 [g m -2 /3x10 6 m -2 ] orresponds to an inrease of loud liquid water path by 1g m -2 for a hange of loud droplet number onentration by 100 m -3. Green and blue olors represent negative liquid water sensitivities and yellow and red olors stand for positive liquid water sensitivities. The mean and standard deviations of the liquid water sensitivity are: ± 19.6, 3.95 ± 24.4, 3.03 ± 25.8, and 2.34 ± 16.7 for January, April, July and Otober 1987, respetively. The most obvious feature is that negative liquid water sensitivities are by no means rare -- they are everywhere. For ontinental louds, most louds show neutral or slightly negative liquid water sensitivities. For maritime louds, there are areas with both large negative and large positive liquid water sensitivities with a strong seasonal dependene, i.e., negative liquid water sensitivity is more ommon in the summer hemisphere. If the negative liquid water sensitivity is aused by deoupling of boundary layer, then the above relation suggests that the deoupling happens more often in warm areas than old areas. This warm area deoupling is found by observations of four years of surfae remote sensing data from the ARM (Atmospheri Radiation Measurement) Cloud and Radiation Testbed site (Del Genio and Wolf, 2000). In an effort to explain the negative dependeny of loud optial thikness on surfae temperature, they found that the boundary layers are different for old and warm surfae temperatures: stratified and onvetive boundary layers are assoiated with old temperatures and mixed or deoupled boundary layers are assoiated with warm temperatures. Detailed analyses of boundary layer onditions show that while the deoupling of boundary layer is responsible for dereasing of loud liquid water and thinning of the

6 6 Table I. Perentage of loud liquid water sensitivity for different ranges δ= LWP/ N <0 δ 0 δ= LWP/ N >0 δ<-35-35<δ -25<δ -15<δ -5<δ 5<δ 15<δ 25<δ 35<δ 45<δ 95<δ <-25 <-15 <-5 <5 <15 <25 <35 <45 <95 Jan Apr Jul Ot loud layer, it is not related to surfae temperature (Del Genio and Wolf, 2000). In other words, warmer surfae temperature alone is not the ause of the deoupling of the boundary layer and the dereasing of loud liquid water path; other fators must play a role in this proess. The oinidene of negative liquid water Table II. Relative liquid water sensitivity, β, and γ values in relation r e ~N γ β= ln(lwp)/ ln(n ) <0 β 0 β= ln(lwp)/ ln(n ) >0 β<-70% -70%<β -50%<β -30%<β -10%<β<10% 10%<β 30%<β 50%<β β>70% <-50% <-30% <-10% <30% <50% <70% γ< <γ< -0.50<γ< -0.43<γ< -0.37<γ< -0.30<γ< -0.23<γ< -0.17<γ< -0.10<γ Jan Apr Jul Ot

7 7 sensitivity in warmer seasons shown in the Figure 1 suggests a possible role for loud mirophysis. That is, inreased droplet number onentration leads to dereases of droplet size (whih is a global phenomena as will be shown later), hene to enhaned loud base ooling due to evaporation and to redued water supply from surfae due to a weakened oupling between louds and boundary layer. Figure 2 shows histograms of the perentage of louds for eah liquid water sensitivity ategory with its values listed in Table 1. On an annual average, loud liquid water sensitivities are negative about one third of the time and positive about a quarter of the time; these perentages vary somewhat with season. 4.2 Relative liquid water sensitivity Figure 3 is a near-global survey of the relative liquid water sensitivity. It is apparent that, unlike the nearneutral absolute liquid water sensitivities, the relative liquid water sensitivities are mostly negative over land, whih means that the relative hange in liquid water path is notably related to the relative hanges in olumn droplet number onentration even though the absolute hanges are small. The mean and standard deviations for the relative liquid water sensitivity are: ± 0.26, ± 0.29, ± 0.30, and.029 ± 0.22 for January, April, July and Otober 1987, respetively. Figure 4 show histograms of the perentage of louds for eah relative liquid water sensitivity ategory with its values listed in the Table 2. On an annual average, the relative liquid water sensitivities are negative about 40% of the times, while they are positive about 28% of the times, these perentages Figure 3: Relative liquid water sensitivity (β) and power γ in the relation r e ~N γ of water louds for January, April, July and Otober Figure 4: Histogram of the Relative liquid water sensitivity (β) of water louds for January, April, July and Otober vary somewhat with season. Figures 3 and 4 reveal that the effetive droplet radius and olumn droplet number onentration are always negatively orrelated, suggesting that enhaned droplet number onentration always leads to dereased droplet size, although to different degrees. Many field observations find that a (-1/3) power law is valid for relations between droplet radius and volume number onentrations but it was also notied that variations in loud layer thikness ouldn t be negleted (e.g., Durkee et al., 2000; Akerman et al., 2000). Our results show that in about one third of the ases the minus one third power law (-0.37<γ<-0.30) is valid even for droplet radius and olumn number onentrations, whih means that loud layer thikness variations do not seem dominant. 5. Disussions and Conlusions The response of loud liquid water path to olumn droplet number onentration hanges is an important part in estimating the aerosol indiret effet. In GCMs loud liquid water path has been parameterized either as onstant (Twomey Effet) or inreasing with inreasing droplet number onentrations due to suppression of drizzle (Albreht Effet). Although model studies and field observations suggest that there may be another response, i.e., loud liquid water ontent may be dereased with inreasing droplet number onentrations, the relative frequeny of this behavior has been unknown. This study examines the loud responses (for louds with top temperature > 273 K and optial thikness 1 τ<15) by retrieving the liquid water sensitivity on a near-global sale using satellite data and finds that more than in one third of the ases, the liquid water sensitivities are negative, i.e., loud liquid water path dereases with inreasing olumn number onentrations. Another finding of

8 8 this study is that although loud droplet sizes always derease with enhaned olumn droplet number onentrations as expeted, for a majority of the ases, the quantitative relation between these r e and N does not suggest an invariant liquid water path during aerosol-loud interations. Regional and seasonal variations of the liquid water sensitivity show that most negative values are in the warm zone or summer hemisphere. This an be explained by the findings that the boundary layer is different in warm season from that in old season at the ARM Southern Great Plains site: well-mixed or deoupled boundary layers in summer and wellstratified boundary layers in winter (Del Genio and Wolf, 2000). They also found that the deoupled boundary layer is strongly assoiated with a dereased liquid water path but deoupling is not dependent on surfae temperature. Combined with their findings, our results suggest that the inreased droplet number onentration leads to dereased droplet size and enhaned evaporation just below loud base, whih auses the boundary layer deoupling in warm zones, onsistent with simulations of model studies (Akerman et al., 1995). We note that the pattern of retrieved liquid water sensitivity may inlude ontributions from louds formed in different air masses, whih is espeially true for areas lose to oastlines. For example, maritime louds with small droplet number onentration and ontinental louds with large droplet number onentration are often both found in ertain oast regions (e.g., Minnis et al., 1992; Twohy et al., 1995). Nevertheless, the negative liquid water sensitivity found in vast areas, inluding the remote oean areas and relatively lean southern hemisphere, suggests that enhaned droplet number onentration plays an important role in induing the deoupling of the boundary layer, reduing water vapor supply from the surfae and desiating loud liquid water. We also note that the results of this study should not be regarded as before and after aerosol-loud interations for individual louds, instead, the results are statistial in nature. This should not be a problem when used for omparison with GCM results beause loud properties predited by GCMs are also statistial in nature they are not speifi preditions for individual louds in a weather system. The results presented here are limited beause they are for daytime-only, in fat afternoon-only, so that the aerosol-related hanges in the louds that we observe may not be true of the morning or nighttime hanges. Although the day-time part of the loud hanges is most relevant to the albedo effet, we may not truly understand what is going on with marine boundary layer louds and aerosol effets on them until we have omprehensive observations overing the whole diurnal yle, as well as all synopti and seasonal variations. In addition, we are only able to orrelate observed systemati hanges in loud properties, not atually observe their variation in time; hene, to onfirm hypotheses of ause-and-effet will require supplementary in situ and ground-based measurements that atually resolve the loud hanges. However, the value of these results is to show that these relationships are not onstant but dynami in harater, varying with meteorologial regime. Aknowledgments. This researh is supported by NASA grants NAG5-7702, NCC8-200 and NAS Referenes Akerman, A. S., O. B. Toon, and P. V. Hobbs, 1993: Dissipation of marine stratiform louds and ollapse of the marine boundary layer due to the depletion of loud ondensation nulei by louds. Siene, 262, Akerman, A. S., O. B. Toon, and P. V. Hobbs, 1995: Numerial modeling of ship traks produed by injetions of loud ondensation nulei into marine stratiform louds. J. Geophys. Res., 100, Akerman, A. S., O. B. Toon, J. P. Taylor, D. W. Johnson, P. V. Hobbs, and R. J. Ferek, 2000: Effets of Aerosols on Cloud Albedo: Evaluation of Twomey s Parameterization of Cloud Suseptibility Using Measurements of Ship Traks. J. Atmos. Si., 57, Albreht, B. A., 1989: Aerosols, loud mirophysis and frational loudiness. Siene, 245, Alkezweeny, A. J., D. A. Burrows, and C. A. Grainger, 1993: Measurements of loud droplet-size distributions in polluted and unpolluted stratiform louds. J. Appl. Meteor., 32, Beheng, K. D., 1994: A parameterization of warm loud mirophysial onversion proesses, Atmos. Res., 33, Bouher, O., and U. Lohmann, 1995: The sulfate-ccnloud albedo effet: a sensitivity study with two general irulation models. Tellus, 47B, Coakley, J. A. Jr., P. A. Durkee, K. Nielsen, J. P. Taylor, S. Platnik, B. A. Albreht, D. Babb, F.-L. Chang, W. R. Tahnk, C. S. Bretherton, and P. V. Hobbs: 2000: The Appearane and Disappearane of Ship Traks on Large Spatial Sales. J. Atmos. Si., 57, Del Genio, A. D., M. S. Yao, W. Kovari, and K. K. W. Lo, 1996: A prognosti loud water parameterization for global limate models. J. Climate, 9, Del Genio, A. D., and A. B. Wolf, 2000: The temperature dependene of the liquid water path of low louds

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