5.2 THE EFFECT OF TURBULENCE ON CLOUD MICROSTRUCTURE, PRECIPITATION FORMATION AND THE ORGANISATION OF STRATOCUMULUS AND SHALLOW CUMULUS CONVECTION

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1 5.2 THE EFFECT OF TURBULENCE ON CLOUD MICROSTRUCTURE, PRECIPITATION FORMATION AND THE ORGANISATION OF STRATOCUMULUS AND SHALLOW CUMULUS CONVECTION Charmaine N. Franklin * Centre for Australian Weather and Climate Researh A partnership etween CSIRO and the Australian Bureau of Meteorology, Aspendale, Vitoria, Australia 1. INTRODUCTION The effet of aerosols on louds remains one of the largest soures of unertainty in limate studies and many of the omplex aerosol-loud interations are assoiated with loud mirophysial proesses. To enale greater onfidene in limate projetions one of the proesses that requires a quantitative analysis is the seond indiret aerosol effet, whih is the effet from enhaned aerosol onentrations in louds suppressing drizzle and prolonging loud lifetimes (Alreht 1989). To e ale to quantify this effet with any real ertainty, the loud mirophysial proesses must e aurately represented in gloal limate models (GCMs), in partiular the autoonversion proess, whih desries the ollision and oalesene of small loud droplets to form larger raindrops. Rotstayn and Liu (2005) demonstrated how hanging autoonversion shemes in a GCM ould derease the gloally averaged seond indiret aerosol effet y 60%, highlighting the need for inreased understanding and a more aurate parameterization of autoonversion. In louds where the temperature does not reah freezing, it is the proess of ollision and oalesene that allows drops to grow to a size large enough to fall out of a loud as rain. Oservations of droplet growth tend to show a faster evolution and roader drop size distriution ompared to the theoretially alulated drop spetra, where the equations are applied to a randomly distriuted population of drops whose motion is governed y gravitational foring. Several physial effets have een suggested to play an important role in the redution of the growth times, inluding entrainment and mixing of dry air, turulene and the role of giant loud ondensation nulei (e.g. Beard and Ohs 1993). Turulene inreases the ollision rate of droplets in at least three ways: y hanging the droplet veloities and the spatial distriution of the droplets (e.g. Franklin et al. 2005), and y hanging the ollision and oalesene effiienies etween droplets. Although the effet of turulene on loud droplet ollision-oalesene rates is yet to e quantified y oservations, reent modeling studies have shown that turulene an inrease the ollision rates of droplets y several times the purely gravitational rate (Franklin et al. 2005, 2007; Wang et al. 2005; Pinsky et al. 2006). Franklin et al. (2007) performed diret numerial simulations (DNS) of droplets within turulent *Corresponding author address: Charmaine N. Franklin, CSIRO Marine and Atmospheri Researh, Private Bag No. 1, Aspendale, Vitoria, 3195, Australia; harmaine.franklin@siro.au flow fields and developed empirially derived equations that desrie the turulent ollision kernel for droplet pairs, where the larger droplet is within the radius range of µm and the eddy dissipation rate of turulent kineti energy (TKE) is etween 100 and 1500 m 2 s -3. These turulent ollision kernels were used in solutions of the stohasti olletion equation (SCE) y Franklin (2008) to develop model-ased empirial doulemoment parameterizations of the effet of autoonversion, aretion and self-olletion on the rain and loud water mixing ratios and the rain and loud drop numer onentrations. Parameterizations using oth turulent and non-turulent ollision kernels were developed. The SCE was solved for liquid water ontents in the range of g kg -1, loud droplet numer onentrations up to 500 drops m -3 and relative dispersion oeffiients of the initial drop size distriution etween 0.25 and 0.4. The initial drop size distriution was a Gamma funtion and the separation radius that determined the point at whih a loud droplet eomes a raindrop was 40 µm. Using the SCE results for suh a road range of drop size distriutions gives the resulting parameterizations greater statistial meaning and appliaility. The two suites of warm rain parameterizations, turulent and non-turulent, allow the investigation of the effet of turulene on the mirophysial proesses and the resulting feedaks in atmospheri models. These effets are explored in this work for stratoumulus and shallow umulus onvetion ases. 2. EXPERIMENT DESIGN The doule-moment warm rain mirophysis parameterizations of Franklin (2008) have een implemented in the UCLA large eddy simulation (LES) model. The turulent autoonversion equation has een modified to the following form, whih gives a etter representation of the DNS data at higher loud water ontents q t q r auto = ( ) R λ N 3 1 tan 2 ( R ) ( R R ) where q r and q are the rain and loud water ontents (kg m -3 ), N is the loud droplet onentration (m -3 ) and Rλ is the Taylor mirosale Reynolds numer of the flow field (see Franklin 2008 for information on relevant Reynolds numers for DNS). The LES ode is desried in detail in Stevens et al. (2005) and solves prognosti equations for the three veloity omponents, λ λ λ

2 the total water mixing ratio, the liquid water equivalent potential temperature and the mass and numer onentration of rain. The mass of loud water is defined impliitly due to the dependene of the liquid water potential temperature on the total ondensate, and the loud droplet numer onentration is a fixed parameter. The numerial solution of the loud proesses, inluding droplet sedimentation, is desried in Savi-Jovi and Stevens (2008). 2.1 Desription of the Shallow Cumulus Convetion Case - RICO The domain size of these experiments is 13.2 km square and 5 km deep, with grid spaing of 100 m in the horizontal and 40 m in the vertial. The time step is variale and is hosen as to keep the Courant numer etween 0.65 and The initial and oundary onditions and large sale forings are taken from the GEWEX Cloud Systems Study (GCSS) interomparison ase ( The average oserved loud droplet numer onentration during RICO was 70 m -3, and that numer has een used for the ontrol simulations. The length of the simulations for this ase are 24 hours and the profile statistis are taken as averages over the last 4 hours. After the initial spin up, the model produes numerous shallow preipitating onvetive louds as shown in Figure 1a. The louds typially extend up to 2400 m, have loud ases at around 600 m and tend to e 1-2 km in horizontal extent (see Fig. 2). Figure 1. a) Plan view of RICO liquid water at 840 m and ) ross-setion through a typial loud showing liquid water and ontour lines of the dissipation rate of TKE (ontour levels are 100, 500, 1000, 1500 m 2 s -3 ). 2.2 Desription of the Stratoumulus Case DYCOMS II The horizontal domain and grid spaing for this ase study are 6.6 km and 50 m respetively, while the vertial domain is 2 km and the grid spaing varies from 5 m at the surfae and the inversion to 80 m at the model top. The large-sale forings are taken form the GCSS interomparison study doumented in Akerman et al. (2009). The ontrol ase uses the oserved average loud droplet numer onentration of 55 m -3. The model is run for 6 hours and the profile statistis are alulated over the final 4 hours. This ase onsists of a noturnal stratoumulus under a dry inversion with emedded pokets of heavy drizzling open ellular onvetion. Typial liquid and rain water ross setions are shown in Figure 2. a a Figure 2. Cross setions of the liquid and rain water for the DYCOMS II ase. Contour lines of the TKE dissipation rate are 10 and 100 m 2 s RESULTS 3.1 Shallow Cumulus Convetion - RICO The turulent mirophysis parameterizations are applied in the regions of the louds where the dissipation rates of TKE are etween 100 and 1500 m 2

3 a Figure 3. RICO loud properties for the simulations that use the turulent and non-turulent mirophysis parameterizations of Franklin (2008) and the mirophysis sheme of Seifert and Beheng (2001). s -3, with the higher dissipation rates assoiated with faster onversion rates from loud to rain water (see Franklin 2008). In the RICO ase the range of dissipation rates for whih the turulent mirophysis sheme is valid is enountered in extensive regions of the louds, with the highest dissipation rates ourring near the loud tops (see Fig. 1). These inreased autoonversion, aretion and self-olletion rates inrease the rain water mixing ratio of the louds as ompared to the simulation where the non-turulent parameterization is used as shown in Figure 3a. The results using the well known Seifert and Beheng (2001) (SB) sheme are inluded as a measure of onfidene for the new shemes of Franklin (2008), however, due to the very different nature of the shemes a omparison is eyond the sope of this paper and rather the fous is on the differenes etween the turulent and non-turulent results. Even though the rain water ontents are signifiantly inreased when the turulent mirophysis effets are inluded, the loud frations do not differ muh. The largest differene in loud fration ours in the levels aove 1000 m, where more loud water in the turulent ase generates greater loud frations (see Figures 3 and ). The simulation using the turulent mirophysis parameterization has on average greater loud water throughout the loud, however, the perentage inrease in the amount of rain water produed in this simulation ompared to the ase using the non-turulent mirophysis is far more than the inrease in the loud liquid water ontents. When the loud droplet numer onentration (CDNC) is redued, the preipitation rates are inreased for all shemes. Reduing the droplet numer onentration in the nonturulent mirophysis ase from 70 drops m -3 to 40 m -3 produes an average rainwater amount that is still less than that of the turulent mirophysis ontrol ase (not shown). The peak rainwater amount at 2000 m in the non-turulent ase with 40 drops m -3 is aout 60% of the turulent mirophysis result with 70 drops m -3. Figure 4a shows that the evaporation of rain water is greatly enhaned in the turulent mirophysis simulation due to an inrease in oth rain water and rain drop numer. The average vertially integrated TKE from the simulation using the turulent mirophysis is less than that of the non-turulent ase in the upper loud levels aove 2000m (see Fig. 4), however, in the lower levels, partiularly elow loud ase, the TKE from the turulent ase is greater than the non-turulent ase. The inreased TKE in the suloud layer of the turulene runs reflets the greater horizontal variaility assoiated with the enhaned evaporation of preipitation destailizing the levels elow the loud, as shown in Figure 4. In the turulent mirophysis simulation the redued TKE in the upper regions of the loud is aused y the inreased latent heating assoiated with the inreased loud and rain water in this ase. This inrease in the latent heating ompared to the non-turulent mirophysis simulation redues the entrainment and the uoyany prodution of TKE (see Fig. 4d). The redued uoyany and entrainment in the upper levels of the loud in the turulent ase ause a redution in the variane of the vertial motion as shown in Figure 4d. The updrafts within the louds in the turulent ase are stronger in the upper levels due to the inreased latent heating assoiated with the larger generation of rain and loud liquid water.

4 a d e f Figure 4. RICO loud and dynamial properties for the simulations that use the turulent and non-turulent mirophysis parameterisations of Franklin (2008) and Seifert and Beheng (2001). 3.2 Stratoumulus DYCOMS II Similar to the shallow onvetion ase, the dissipation rates of TKE are maximal in the upper levels of the stratoumulus loud layer, however, for this ase the dissipation rates are muh weaker. There are only small regions at the top of the loud where the dissipation rate reahes 100 m 2 s -3 (see Fig. 3) and, therefore, where the onversion rates etween loud and rain water will e aelerated y turulene effets. These small regions though do make a differene to the preipitation flux oth in the loud layer and the suloud layer as shown in Figure 5a, while the loud frations remain unhanged (see Fig. 5). For this ase the mirophysis sheme of Khairoutdinov and Kogan (2000) has een used as a omparison for the new shemes, however, the results from the K&K sheme will not e disussed in this paper as the fous is on the turulene effets.

5 a d e f Figure 5. DYCOMS II loud and dynamial properties for the simulations that use the turulent and non-turulent mirophysis parameterisations of Franklin (2008) and the mirophysis sheme of Khairoutdinov and Kogan (2000). The inreased rain water in the turulent mirophysis simulation is assoiated with a greater numer of rain drops and larger evaporation rates of rain water, partiularly at loud ase (see Figure 5). As for the shallow onvetion ase, the inreased evaporation leads to greater variaility and higher TKE in the turulent simulation as shown in Figures 5d and e. In this ase the stronger irulations our oth in the suloud layer and at the top of the loud layer. The enhaned rain water in the turulent mirophysis simulation has a positive feedak in this ase, with more rain produing more evaporation of drizzle drops at loud ase, whih destailizes the suloud layer and leads to stronger irulations and TKE. The oservations for this ase showed that the vertial winds were negatively skewed just aove

6 loud ase (Akerman et al. 2009) and the simulation with the turulent mirophysis produes a loser math with nearly equal strength etween updrafts and downdrafts at this height (see Fig. 5f). a 4. AEROSOL EFFECTS - AVERAGED CLOUD PROPERTIES AT DIFFERENT CLOUD DROPLET CONCENTRATIONS Four simulations of the stratoumulus ase were performed with eah of the non-turulent and turulent mirophysis parameterizations. The simulations differ in the presried CDNC and reveal how the loud properties hange with hanges in aerosol loading as manifested in hanges of loud droplet numer. Figure 6 shows the average loud properties over the last 4 hours of the DYCOMS II simulations. The loud fration inreases monotonially for oth the non-turulent and turulent ases as the CDNC is inreased (see Fig. 6a). There is a strong relationship etween inreasing loud fration and dereasing rain water path as the CDNC is inreased. This result for a stratoumulus loud agrees with the oneptual model that greater aerosol loading suppresses preipitation formation and leads to larger loud frations. For the CDNC values explored herein the non-turulent mirophysis simulations demonstrate that stratoumulus louds typial of this ase study inrease the amount of loud water and redue the rain water ontent when there is an inrease in loud droplet numer, therefore, they show a positive seond aerosol indiret effet (Fig. 6). While this is also true for the lowest three CDNC used in this study for the turulent mirophysis, for the maximum onentration of 200 drops m -3 the turulent simulation shows a redution in oth the rain and liquid water paths. Other studies have also shown a non-monotoni inrease in LWP with inreasing aerosol onentrations and suggest that there is a limit to the degree of liquid water that an uild up in stratoumulus louds. The redued rain water leading to a redued liquid water path in the turulent simulation with highest CDNC shows a negative seond aerosol indiret effet. Figure 6 shows that there is an inrease in the loud ase heights as loud droplet numers are inreased and preipitation is dereased. The loud ase lowers in regions of preipitation due to the preipitation hanges affeting the thermodynami state of the suloud layer. This is also shown y the lower and more variale loud frations of the turulene simulations, suggesting that the evaporation of the enhaned preipitation plays an important role in reorganizing the irulations. The TKE inreases with CDNC in all simulations exept for the turulent ase with highest CDNC as shown in Figure 6d. d Figure 6. Average DYCOMS II loud and dynamial properties for speified CDNC.

7 Figure 7 shows the effets of inreasing the CDNC in the RICO simulations. In these shallow umulus onvetion ases the liquid water path inreases as the rain water path inreases (see Fig 7a), whih is the opposite of the stratoumulus ase of DYCOMS II. Inreased CDNC results in redued rainwater in oth ases, ut in the RICO ases this also results in redued liquid water paths. The inreased CDNC will tend to slow the ollisionoalesene proess, enhane evaporation and redue the drop fall speeds. The result of this and the susequent feedaks in these small louds is to redue the liquid water path as well as the amount of preipitation. All ases, therefore, simulate a negative seond aerosol indiret effet exept for the highest CDNC using the non-turulent mirophysis sheme, whih shows a small inrease in liquid water path. The hange in average loud fration for all simulations is small and generally less than 1%. Figure 7. Average RICO loud and dynamial properties for speified CDNC. The TKE response to inreased CDNC is shown in Figure 7. The non-turulent mirophysis simulations tend to show an inrease in vertially averaged TKE as CDNC inreases, while the turulent mirophysis ases show a derease in TKE for the three higher CDNC ases. The redution in TKE and rain water for the turulent mirophysis ases may e due to the negative feedak that the a enhaned preipitation has on the entrainment and uoyany prodution of TKE as disussed previously. 5. CONCLUSIONS The use of new warm rain mirophysis parameterizations has allowed an investigation into the effets of turulene on loud mirophysial proesses in stratoumulus and shallow onvetive louds. Turulene had a greater effet on the simulated preipitation rates in the shallow onvetion ase where the larger TKE dissipation rates produed a more rapid onversion of loud water to rain water. The muh weaker dissipation rates in the stratoumulus ase, however, also showed a hange in the simulated preipitation when the effets of turulene on mirophysial proesses were inluded in the LES model. Both ases using the turulent mirophysis sheme produed greater evaporation rates of rain water, whih aused a hange in the thermodynamis of the suloud layer, destailizing the lower levels and enhaning the horizontal variaility and TKE in this region. The differene etween the two ases in the effet of the turulent mirophysis was at the upper levels of the louds. In the shallow onvetion ase the enhaned latent heating assoiated with the greater rain and loud liquid water redued the entrainment and uoyany prodution of TKE, therefore, produing a negative feedak to the enhaned preipitation formation assoiated with turulent effets. In ontrast, the stratoumulus ase showed a positive feedak with enhaned rainwater produing greater TKE in oth the suloud layer and in the upper loud region. Inluding the effets of turulene in the mirophysis parameterizations minimizes the need to artifiially redue CDNC in order to simulate oserved preipitation rates. Sensitivity studies where the CDNC was varied showed agreement with the oneptual model for lightly drizzling stratoumulus louds that greater aerosol loading, as manifested in greater CDNC, suppresses preipitation formation leading to larger loud frations and liquid water paths. This positive seond indiret aerosol effet was produed in all of the DYCOMS II simulations exept for the ase using the turulent mirophysis with the highest CDNC, whih showed that there may e a limit to the amount of liquid water that an uild up in this stratoumulus ase. The shallow onvetion ase of RICO produed a negative seond indiret aerosol effet in all ut one simulation. The inreased CDNC in the small onvetive louds redued the prodution of rainwater, enhaned the evaporation and led to a redution in the liquid water path. Aknowledgements This work was supported y the Australian Department of Climate Change. Bjorn Stevens from the Max Plank Institute for Meteorology and Veria Savi-Jovi from UCLA are thanked for providing the

8 UCLA LES model ode and the settings for the RICO and DYCOMS II experiments. Referenes Akerman, A.S., and oauthors, 2009: Large-eddy simulations of a drizzling, stratoumulus-topped marine oundary layer. Mon. Wea. Rev., 137, Alreht, B.A., 1989: Aerosols, loud mirophysis, and frational loudiness. Siene, 245, Beard, K.V. and H.T. Ohs, 1993: Warm-rain initiation: An overview of mirophysial mehanisms. J. Appl. Meteor., 32, Franklin, C.N., P.A. Vaillanourt, M.K. Yau, and P. Bartello, 2005: Collision rates of loud droplets in turulent flow. J. Atmos. Si., 62, Franklin, C.N., P.A. Vaillanourt, and M.K. Yau, 2007: Statistis and parameterizations of the effet of turulene on the geometri ollision kernel of loud droplets. J. Atmos. Si., 64, Franklin, C.N., 2008: A warm rain mirophysis parameterization that inludes the effets of turulene. J. Atmos. Si., 65, Khairoutdinov, M. and Y. Kogan, 2000: A new loud physis parameterization in a large-eddy simulation model of marine stratoumulus. J. Atmos. Si., 57, Pinsky, M.B., A.P. Khain, B. Grits and M. Shapiro, 2006: Collisions of small drops in a turulent flow. Part III: Relative droplet fluxes and swept volumes. J. Atmos. Si., 63, Rotstayn, L.D., and Y. Liu, 2005: A smaller gloal estimate of the seond indiret aerosol effet. Geophys. Res. Lett., 32, L05708, doi: /2004gl Saviv-Jovi, V. and B. Stevens, 2008: The struture and organization of preipitating stratoumulus. J. Atmos. Si., 65, Seifert, A., and K.D. Beheng, 2001: A doule-moment parameterization for simulating autoonversion, aretion and selfolletion. Atmos. Res., 59-60, Stevens, B., and oauthors, 2005: Evaluation of largeeddy simulations via oservations of noturnal marine stratoumulus. Mon. Wea. Rev., 133, Wang, L.-P., O. Ayala, S.E. Kasprzak and W.W. Graowski, 2005: Theoretial formulation of ollision rate and ollision effiieny of hydrodynamially interating loud droplets in turulent atmosphere. J. Atmos. Si., 62,