Dry Boundary Layer Research Articles

Sensitivity of Simulated Conditions to Different Parameterization Choices Over Complex Terrain in Central Chile

This study evaluates the performance of fourteen high-resolution WRF runs with different combinations of parameterizations in simulating the atmospheric conditions over the complex terrain of central Chile during austral winter and spring. We focus on the validation of results for coastal, interior valleys, and mountainous areas independently, and also present an in-depth analysis of two synoptic-scale events that occurred during the study period: a frontal system and a cut-off low. The performance of the simulations decreases from the coast to higher altitudes, even though the differences are not very clear between the coast and interior valleys for 10 m wind speeds and precipitation. The simulated vertical profiles show a warmer and drier boundary layer and a cooler and moister free atmosphere than observed. The choice of the land-surface model has the largest positive impact on near-surface variables with the five-layer thermal diffusion scheme showing the smallest errors. Precipitation is more sensitive to the choice of cumulus parameterizations, with the simplified Arakawa–Schubert scheme generally providing the best performance for absolute errors. When examining the performance of the model simulating rain/no-rain events for different thresholds, also the cumulus parameterizations better represented the false alarm ratio (FAR) and the bias score (BS). However, the Morrison microphysics scheme resulted in the best critical success index (CSI), while the probability of detection (POD) was better in the simulation without analysis nudging. Overall, these results provide guidance to other researchers and help to identify the best WRF configuration for a specific research or operational goal.

The sensitivity of a mid-latitude maritime stratocumulus cloud to surface fluxes

Surface fluxes of stratocumulus clouds over mid-latitude oceans have larger variability and bigger Bowen ratios than those over subtropical oceans. This study investigates how mid-latitude maritime stratocumulus clouds respond to surface fluxes and what the underlying mechanisms are. A closed-cellular stratocumulus cloud over mid-latitude North Pacific Ocean is simulated with large eddy simulation. A control case and a reduced-flux case (with surface fluxes 50% reduced) are compared. Liquid water path (LWP) and turbulent kinetic energy (TKE) decrease as surface fluxes are reduced. It is revealed that reducing surface fluxes leads to a cooler and drier boundary layer, which directly leads to a higher cloud base. The weaker turbulence near the cloud top leads to less entrainment, resulting in a lower cloud top. Therefore, the cloud is 21% thinner with a smaller LWP in the reduced-flux case. Quadrant analysis shows that TKE production processes including surface heating, in-cloud latent heating, and cloud-top evaporative cooling are all weaker in the reduced-flux case. Meanwhile, cloud-top entrainment heating and overshooting of air parcels into the inversion, as TKE consumption processes, are also weaker in the reduced-flux case. These TKE production and consumption processes counteract with each other, leading to quite different pictures of turbulence transport in the two cases. In the subcloud layer, the control case is dominated by surface heating (indicating a turbulent heating effect), while the reduced-flux case is dominated by evaporative cooling in the downdraft region (indicating a turbulent cooling effect). In the cloud layer, the difference of heat flux between the two cases is mainly due to the difference in latent heating. At cloud top, the two cases have similar heat flux.

Revisiting the Land‐Ocean Contrasts in Deep Convective Cloud Intensity Using Global Satellite Observations

AbstractTropical convection tends to be more intense over land than ocean, but why? Numerous previous studies have investigated the causes of this difference. This paper revisits this question using CloudSat data and focuses on interconnecting various environmental parameters and cloud properties, which have often been examined in a piecemeal way in the past. Our analysis shows that if convection is treated as a process by which potential energy (convective available potential energy) is converted to kinetic energy (vertical velocity), then the conversion is more efficient over land than ocean. A key factor that affects this conversion efficiency is the lifting condensation level (LCL). Higher LCLs over land give rise to broader dry boundary layer thermals that transition to wider deep convective cores. Wider cores, in turn, are better protected from the dilution by entrainment, thus leading to stronger updrafts. This study highlights the importance of the dry stage of convection.

Why Do Eastern North Pacific Hurricanes Intensify More and Faster than Their Western-Counterpart Typhoons with Less Ocean Energy?

Abstract Tropical cyclones operate as heat engines, deriving energy from the thermodynamic disequilibrium between ocean surfaces and atmosphere. Available energy for the cyclones comes primarily from upper-ocean heat content. Here, we show that eastern North Pacific hurricanes reach a given intensity 15% faster on average than western North Pacific typhoons despite having half the available ocean heat content. Eastern North Pacific hurricanes also intensify on average 16% more with a given ocean energy (i.e., air–sea enthalpy flux) than western North Pacific typhoons. As efficient intensifiers, eastern Pacific hurricanes remain small during their intensification period, tend to stay at lower latitudes, and are affected by relatively lower vertical wind shear, a colder troposphere, and a drier boundary layer. Despite a shallower warm upper-ocean layer in the eastern North Pacific, average hurricane-induced sea surface cooling there is only slightly larger than in the western North Pacific due to the opposing influences of stronger density stratification, smaller size, and related wave-interaction effects. In contrast, western North Pacific typhoons encounter a more favorable oceanic environment for development, but several factors cause typhoons to greatly increase their size during intensification, resulting in a slow and inefficient intensification process. These findings on tropical cyclones’ basin-dependent characteristics contribute toward a better understanding of TC intensification.

Impacts of Urban Expansion on the Diurnal Variations of Summer Monsoon Precipitation Over the South China Coast

AbstractThis study investigates the possible relationship between changes in monsoonal (May–June) rainfall diurnal variations over the south China coast and urban surface processes using hourly rain gage data during 1981–2014 and semi‐idealized numerical simulations. The continuous landward penetration of regional climatic rainband observed prior to urbanization is blocked by the city. The morning precipitation initiated at the coast “jumps” to the downstream area of city in the late afternoon, causing a 2 hr delay of the rainfall peak time. Simulations clearly reproduce this change by turning off/on the urban land use, and demonstrate a direct connection between the change of rainfall diurnal variations with a magnitude‐enhancing but slower‐penetrating sea breeze as well as the thermodynamic conditions modified by the city. Individual impacts of sensible heating, evapotranspiration, and friction from urban surface are examined using various sensitivity experiments. Urban sensible heat is found to act as the energy source to enhance the sea breeze and induce the asymmetric low‐level urban inflow that blocks the penetration of sea breeze and changes the rainfall location. Evapotranspiration and latent heat flux suppressed by the impervious urban surface causes a drier boundary layer and lower CAPE over the city. The “jumping” of the climatic rainband may result from the competing interaction of the dynamic and thermodynamic factors. Urban friction has little impact on the daytime rainfall, but could slow down seaward wind speed and enhance coastal convergence in the early morning if no strong urban heat island is present.

Understanding model diversity in future precipitation projections for South America

Precipitation patterns are expected to change in the future climate, affecting humans through a number of factors. Global climate models (GCM) are our best tools for projecting large-scale changes in climate, but they cannot make reliable projections locally. To abate this problem, we have downscaled three GCMs with the Weather Research and Forecasting (WRF) model to 50 km horizontal resolution over South America, and 10 km resolution for central Chile, Peru and southern Brazil. Historical simulations for years 1996–2005 generally compare well to precipitation observations and reanalyses. Future simulations for central Chile show reductions in annual precipitation and increases in the number of dry days at the end-of-the-century for a high greenhouse gas emission scenario, regardless of resolution and GCM boundary conditions used. However, future projections for Peru and southern Brazil are more uncertain, and simulations show that increasing the model resolution can switch the sign of precipitation projections. Differences in future precipitation changes between global/regional and high resolution (10 km) are only mildly influenced by the orography resolution, but linked to the convection parameterization, reflected in very different changes in dry static energy flux divergence, vertical velocity and boundary layer height. Our findings imply that using results directly from GCMs, and even from coarse-resolution (50 km) regional models, may give incorrect conclusions about regional-scale precipitation projections. While climate modelling at convection-permitting scales is computationally costly, we show that coarse-resolution regional simulations using a scale-aware convection parameterization, instead of a more conventional scheme, better mirror fine-resolution precipitation projections.

Impact of middle atmospheric humidity on boundary layer turbulence and clouds

An attempt is made to connect the moisture present above the boundary layer and its association with the cloud development and boundary layer turbulence using Large Eddy Simulation (LES) and aircraft observations. The dry boundary layer (BL) with shallow clouds observed during Cloud-Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the arid Indian peninsula is investigated in this study. The constant altitude aircraft observations at different elevations are used to verify LES derived fluxes and variances. LES simulations with the drier conditions above BL resulted in deeper, warmer and drier BL with an enhanced boundary layer turbulent kinetic energy. A dynamic feedback from dry air intrusion into the BL resulted in energetic BL eddies in the dry conditions and a doubling of moisture exchange coefficients. The liquid water content in cloud updrafts rather than in downdrafts were depleted significantly in dry conditions. LES sensitivity indicates that the middle atmospheric water vapor alone could influence the shallow to deep cumulus cloud transitions in the monsoon regime in a dramatic way, by changing the population of cumulus, congestus or deep cumulus clouds. A 30% drying above the BL could drastically reduce the liquid water path and cloud albedo by 10–15%.

Development of a Single-Column Model in RegCM4 and Its Preliminary Application for Evaluating PBL Schemes in Simulating the Dry Convection Boundary Layer

Development of a Single-Column Model in RegCM4 and Its Preliminary Application for Evaluating PBL Schemes in Simulating the Dry Convection Boundary Layer

Interbasin Differences in the Median and Variability of Tropical Cyclone MPI in the Northern Hemisphere

AbstractUsing the 1980–2017 6‐hourly best‐track tropical cyclone data and global reanalysis data, we studied the interbasin differences in the median and variability of tropical cyclone maximum potential intensity (MPI) as a function of sea surface temperature (SST) in the North Atlantic (NA), eastern North Pacific (ENP), western North Pacific (WNP), and North Indian Ocean (NI). Results show that the MPI median increases by 4.8, 7.7, 6.4, and 4.4 m s−1 per degree increase in SST in the NA, ENP, WNP, and NI, respectively. The MPI is the largest in the NI at SST between 27 °C and 28.5 °C and in the ENP at SST above 28.5 °C, while is the smallest at SST below 29.5 °C in the WNP. The environmental factors that contribute to such interbasin differences were compared among basins. The greatest MPI in the ENP is largely contributed by the colder troposphere and drier boundary layer. The warmer troposphere and wetter boundary layer are responsible for the smallest MPI in the WNP. In the NA, the warmer outflow layer reduces the thermodynamic efficiency and partly offsets the positive contributions by the colder troposphere and drier boundary layer, resulting in the moderate MPI. In the WNP, the variability in MPI decreases with increasing SST across all four basins and is the largest. The large variability at low SSTs is largely contributed by the variability in air temperature, which includes both the air‐sea temperature difference and the outflow layer temperature, while the variability at relatively high SSTs is dominantly contributed by boundary layer moisture.

Climatology and Analysis of High-Impact, Low Predictive Skill Severe Weather Events in the Northeast United States

Abstract This study identifies high-impact severe weather events with poor predictive skill over the northeast United States using Storm Prediction Center (SPC) convective outlooks. The objectives are to build a climatology of high-impact, low predictive skill events between 1980 and 2013 and investigate the differences in the synoptic-scale environment and severe weather parameters between severe weather events with low predictive skill and high predictive skill. Event-centered composite analyses, performed using the National Centers for Environmental Prediction Climate Forecast System Reanalysis and the North American Regional Reanalysis, suggest low predictive skill events occur significantly more often in low-shear environments. Additionally, a plurality of low probability of detection (POD), high-impact events occurred in low-shear, high-CAPE environments. Statistical analysis of low-shear, high-CAPE environments suggests high downdraft CAPE (DCAPE) and relatively dry lower levels of the atmosphere are associated with widespread severe weather events. DCAPE and dry boundary layer air may contribute to severe wind gusts through strong negative buoyancy and enhanced evaporative cooling of descending saturated parcels.

Effects of the surface heterogeneities on the local climate of a fragmented landscape in Amazonia using a tile approach in the Eta/Noah‐MP model

This study evaluates the use of tiles to estimate the effects of surface heterogeneities in simulations of local climate in a landscape‐fragmented region of Amazonia. Three experiments using the Eta/Noah‐MP model were designed as follows: two experiments with a spatial resolution of 5 km with and without the use of the tile approach. The third experiment used a finer spatial resolution of 2 km without the use of the tile approach (high‐resolution experiment). Simulations were carried out for the rainy and dry seasons, which correspond to the months of March and September 2003, respectively. In general, the magnitude of surface fluxes is significantly affected by the introduction of tiles. In the dry season, the use of tiles in the simulation increases the magnitude of precipitation (about 7 mm day−1), skin temperature (about 4 °C), sensible heat flux (about 80 W m−2) and surface albedo (about 0.7). It reduces the latent heat flux (about 30 W m−2) and net radiation (about 40 W m−2). On the other hand, in the rainy season, the model produces excessive precipitation. This may have partly masked the signs of the land fragmentation impact. The tile experiment generates a warmer and drier boundary layer during the dry season. The impact of the surface heterogeneity representation is higher in the dry season than in the rainy season. The results suggest that the degree of impact of the subgrid process representation on the local climate is related to the spatial scale of the fragmentation. The use of the tile approach improves the representation of the effects of landscape heterogeneity on the spatial distribution of surface flux variability in fragmented areas. Comparison against measured data from flux towers in the region show that the model can simulate diurnal and seasonal variations in the local fluxes, despite the biases.

Aerosol impacts on radiative and microphysical properties of clouds and precipitation formation

Through modifying the number concentration and size of cloud droplets, aerosols have intricate impacts on radiative and microphysical properties of clouds, which together influence precipitation processes. Aerosol-cloud interactions for a mid-latitude convective cloud system are investigated using a two-moment aerosol-aware bulk microphysical scheme implemented into the Weather Research and Forecasting (WRF) model. Three sensitivity experiments with initial identical dynamic and thermodynamic conditions, but different cloud-nucleating aerosol concentrations were conducted. Increased aerosol number concentration has resulted in more numerous cloud droplets of overall smaller sizes, through which the optical properties of clouds have been changed. While the shortwave cloud forcing is significantly increased in more polluted experiments, changes in the aerosol number concentration have negligible impacts on the longwave cloud forcing. For the first time, it is found that polluted clouds have higher cloud base heights, the feature that is caused by more surface cooling due to a higher shortwave cloud forcing, as well as a drier boundary layer in the polluted experiment compared to the clean. The polluted experiment was also associated with a higher liquid water content (LWC), caused by an increase in the number of condensation of water vapor due to higher concentration of hygroscopic aerosols acting as condensation nuclei. The domain-averaged accumulated precipitation is little changed under both polluted and clean atmosphere. Nevertheless, changes in the rate of precipitation are identified, such that under polluted atmosphere light rain is reduced, while both moderate and heavy rain are intensified, confirming the fact that if an ample influx of water vapor exists, an increment of hygroscopic aerosols can increase the amount of precipitation.

Interactions between cumulus convection and its environment as revealed by the MC3E sounding array

This study attempts to understand interactions between midlatitude convective systems and their environments through a heat and moisture budget analysis using the sounding data collected from the Midlatitude Continental Convective Clouds Experiment (MC3E) in central Oklahoma. Distinct large-scale structures and diabatic heating and drying profiles are presented for cases of weaker and elevated thunderstorms as well as intense squall line and supercell thunderstorm events during the campaign. The elevated cell events were nocturnal convective systems occurring in an environment having low convective available potential energy (CAPE) and a very dry boundary layer. In contrast, deeper convective events happened during the morning into early afternoon within an environment associated with large CAPE and a near-saturated boundary layer. As the systems reached maturity, the diagnosed diabatic heating in the latter deep convective cases was much stronger and of greater vertical extent than the former. Both groups showed considerable diabatic cooling in the lower troposphere, associated with the evaporation of precipitation and low-level clouds. The horizontal advection of moisture also played a dominant role in moistening the lower troposphere, particularly for the deeper convective events, wherein the near surface southeasterly flow allows persistent low-level moisture return from the Gulf of Mexico to support convection. The moisture convergence often was present before these systems develop, suggesting a strong correlation between the large-scale moisture convergence and convection. Sensitivity tests indicated that the uncertainty in the surface precipitation and the size of analysis domain mainly affected the magnitude of these analyzed fields rather than their vertical structures.

Properties of a Simulated Convective Boundary Layer in an Idealized Supercell Thunderstorm Environment

Abstract Nearly all previous numerical simulations of supercell thunderstorms have neglected surface fluxes of heat, moisture, and momentum. This choice precludes horizontal inhomogeneities associated with dry boundary layer convection in the near-storm environment. As part of a broader study on how mature supercell thunderstorms are affected by a convective boundary layer (CBL) with quasi-two-dimensional features (i.e., boundary layer rolls), this paper documents the methods used to develop a realistic CBL in an idealized environment supportive of supercells. The evolution and characteristics of the modeled CBL, including the horizontal variability of thermodynamic and kinematic quantities known to affect supercell evolution, are presented. The simulated rolls result in periodic bands of perturbations in temperature, moisture, convective available potential energy (CAPE), vertical wind shear, and storm-relative helicity (SRH). Vertical vorticity is shown to arise within the boundary layer through the tilting of ambient horizontal vorticity associated with the background shear by vertical velocity perturbations in the turbulent CBL. Sensitivity tests suggest that 200-m horizontal grid spacing is adequate to represent rolls using a large-eddy simulation (LES) approach.

Evaluating and understanding top of the atmosphere cloud radiative effects in Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) Coupled Model Intercomparison Project Phase 5 (CMIP5) models using satellite observations

In this study, the annual mean climatology of top of the atmosphere (TOA) shortwave and longwave cloud radiative effects in 12 Atmospheric Model Intercomparison Project (AMIP)‐type simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) is evaluated and investigated using satellite‐based observations, with a focus on the tropics. Results show that the CMIP5 AMIPs simulate large‐scale regional mean TOA radiative fluxes and cloud radiative forcings (CRFs) well but produce considerably less cloud amount, particularly in the middle and lower troposphere. The good model simulations in tropical means, with multimodel mean biases of ‐3.6 W/m2 for shortwave CRF and ‐1.0 W/m2 for longwave CRF, are, however, a result of compensating errors over different dynamical regimes. Over the Maritime Continent, most of the models simulate moderately less high‐cloud fraction, leading to weaker shortwave cooling and longwave warming and a larger net cooling. Over subtropical strong subsidence regimes, most of the CMIP5 models strongly underestimate stratocumulus cloud amount and show considerably weaker local shortwave CRF. Over the transitional trade cumulus regimes, a notable feature is that while at varying amplitudes, most of the CMIP5 models consistently simulate a deeper and drier boundary layer, more moist free troposphere, and more high clouds and, consequently, overestimate shortwave cooling and longwave warming effects there. While most of the CMIP5 models show the same sign as the multimodel mean, there are substantial model spreads, particularly over the tropical deep convective and subtropical strong subsidence regimes. Representing clouds and their TOA radiative effects remains a challenge in the CMIP5 models.

Spectral width of premonsoon and monsoon clouds over Indo‐Gangetic valley

The combined effect of humidity and aerosol on cloud droplet spectral width (σ) in continental monsoon clouds is a topic of significant relevance for precipitation and radiation budgets over monsoon regions. The droplet spectral width in polluted, dry premonsoon conditions and moist monsoon conditions observed near the Himalayan Foothills region during Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) is the focus of this study. Here σis small in premonsoon clouds developing from dry boundary layers. This is attributed to numerous aerosol particles and the absence/suppression of collision‐coalescence during premonsoon. For polluted and dry premonsoon clouds,σ is constant with height. In contrast to premonsoon clouds, σ in monsoon clouds increases with height irrespective of whether they are polluted or clean. The mean radius of polluted monsoon clouds is half that of clean monsoon clouds. In monsoon clouds, both mean radius and σ decreased with total cloud droplet number concentration (CDNC). The spectral widths of premonsoon clouds were independent of total droplet number concentrations, but both σ and mean radius decreased with small droplet (diameter < 20 μm) number concentrations in the diluted part of the cloud. Observational evidence is provided for the formation of large droplets in the adiabatic regions of monsoon clouds. The number concentration of small droplets is found to decrease in the diluted cloud volumes that may be characterized by various spectral widths or mean droplet radii.

2005年8月15日関東に発生した短時間強雨をもたらした大気安定度に対する境界層パラメタリゼーションの感度分析

Sensitivity of boundary layer parameterizations to atmospheric stability preceding heavy rainfall is investigated using the 5th-generation NCAR Mesoscale Model (MM5). Four turbulence parameterizations [MRF, Blackadar, Gayno-Seaman and Eta] and four surface flux schemes [Carlson-Boland, Garratt, Zilitinkevich and Janjic] are selected for evaluation. The Carlson-Boland surface flux scheme produces deep convection and causes hot and dry boundary layer regardless of the turbulence parameterizations. The combination of the MRF turbulence parameterizations and the Garratt surface flux scheme well reproduce local heavy rainfall which observed at Tokyo on 15 August 2005.

A Diagnostic for Evaluating the Representation of Turbulence in Atmospheric Models at the Kilometric Scale

Abstract Turbulence is well represented by atmospheric models at very fine grid sizes, from 10 to 100 m, for which turbulent movements are mainly resolved, and by atmospheric models with grid sizes greater than 2 km, for which those movements are entirely parameterized. But what happens at intermediate scales, Wyngaard’s so-called terra incognita? Here an original method is presented that provides a new diagnostic by calculating the subgrid and resolved parts of five variables at different scales: turbulent kinetic energy (TKE), heat and moisture fluxes, and potential temperature and mixing ratio variances. They are established at intermediate scales for dry and cumulus-topped convective boundary layers. The similarity theorem allows the determination of the dimensionless variables of the problem. When the subgrid and resolved parts are studied, a new dimensionless variable, the dimensionless mesh size , needs to be added to the Deardorff free convective scaling variables, where h is the boundary layer height and hc is the height of the cloud layer. Similarity functions for the subgrid and resolved parts are assumed to be the product of the similarity function of the total (subgrid plus resolved) variables and a “partial” similarity function that depends only on . In order to determine the partial similarity function form, large-eddy simulations (LES) of five dry and cloudy convective boundary layers are used. The resolved and subgrid parts of the variables at coarser grid sizes are then deduced from the LES fields. The evolution of the subgrid and resolved parts in the boundary layer with is as follows: fine grids mainly resolve variables. As the mesh becomes coarser, more eddies are subgrid. Finally, for very large meshes, turbulence is entirely subgrid. A scale therefore exists for which the subgrid and resolved parts are equal. This is obtained for in the case of TKE, 0.4 for the potential temperature variance, and 0.8 for the mixing ratio variance, indicating that the velocity structures are smaller than those for the potential temperature, which are smaller than those for the mixing ratio. Furthermore, boundary layers capped by convective clouds have structures larger than dry boundary layer ones as displayed by the scaling in the partial similarity functions. This new diagnostic gives a reference for evaluating current and future parameterizations at kilometric scales. As an illustration, the parameterizations of a mesoscale model are eventually evaluated at intermediate scales. In its standard version, the model produces too many resolved movements, as the turbulence scheme does not sufficiently represent the impact of the subgrid thermal. This is not true when a mass-flux scheme is introduced. However in this case, a completely subgrid thermal is modeled leading to an overestimation of the subgrid part.

Analysis of Air Flow and Turbulent Fluxes Features of Night Fog

The air flow and turbulent fluxes features during the radiation fog formed on the dawn of 17 October 2007 is discussed in order to study the mechanism of an unexpected night fog, based on the meteorological and turbulent data obtained from the 250 m height tower in Tianjin, as well as the NCEP reanalysis data and other observational data. The results show that the lower layer easterly flow coming from the south region of the Northeast cold high pressure led to remarkable temperature fall and humidity value increase in the daytime prior to the fog formation, which quickly turned the dry boundary layer to be moist. The vapor transfer indicated that the vapor of the radiation fog was provided by the easterly advection from Bohai Sea, not from local area. Turbulent vapor fluxes increased ten times as that before the fog. The horizontal vapor fluxes transported against the wind direction, which led to the escape of water vapor from Tianjin city and the dissipation of fog.

Manipulating marine stratocumulus cloud amount and albedo: a process-modelling study of aerosol-cloud-precipitation interactions in response to injection of cloud condensation nuclei

Abstract. We use a cloud-system-resolving model to study marine-cloud brightening. We examine how injected aerosol particles that act as cloud condensation nuclei (CCN) are transported within the marine boundary layer and how the additional particles in clouds impact cloud microphysical processes, and feedback on dynamics. Results show that the effectiveness of cloud brightening depends strongly on meteorological and background aerosol conditions. Cloud albedo enhancement is very effective in a weakly precipitating boundary layer and in CCN-limited conditions preceded by heavy and/or persistent precipitation. The additional CCN help sustain cloud water by weakening the precipitation substantially in the former case and preventing the boundary layer from collapse in the latter. For a given amount of injected CCN, the injection method (i.e., number and distribution of sprayers) is critical to the spatial distribution of these CCN. Both the areal coverage and the number concentration of injected particles are key players but neither one always emerges as more important than the other. The same amount of injected material is much less effective in either strongly precipitating clouds or polluted clouds, and it is ineffective in a relatively dry boundary layer that supports clouds of low liquid water path. In the polluted case and "dry" case, the CCN injection increases drop number concentration but lowers supersaturation and liquid water path. As a result, the cloud experiences very weak albedo enhancement, regardless of the injection method.