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Seoung Soo Lee - One of the best experts on this subject based on the ideXlab platform.

  • mid latitude mixed phase Stratocumulus Clouds and their interactions with aerosols how ice processes affect microphysical dynamic and thermodynamic development in those Clouds and interactions
    Atmospheric Chemistry and Physics, 2021
    Co-Authors: Seoung Soo Lee, Manguttathil Gopalakrishnan Manoj, Mohammad Kamruzzaman, Hyungjun Kim, Nobuyuki Utsumi, Jianping Guo
    Abstract:

    Abstract. Mid-latitude mixed-phase Stratocumulus Clouds and their interactions with aerosols remain poorly understood. This study examines the roles of ice processes in those Clouds and interactions using a large-eddy simulation (LES) framework. Cloud mass becomes much lower in the presence of ice processes and the Wegener-Bergeron-Findeisen (WBF) mechanism in the mixed-phase Clouds as compared to that in warm Clouds. This is because while the WBF mechanism enhances the evaporation of droplets, the low concentration of aerosols as ice nuclei (IN) and cloud ice number concentration (CINC) prevent the efficient deposition of water vapor whose mass is contributed by the evaporation. In the mixed-phase Clouds, the increasing concentration of aerosols that act as cloud condensation nuclei (CCN) decreases cloud mass by increasing the evaporation of droplets through the WBF mechanism and decreasing the intensity of updrafts. In contrast to this, in the warm Clouds, the absence of the WBF mechanism makes the increase in the evaporation of droplets inefficient, eventually enabling cloud mass to increase with the increasing concentration of aerosols as CCN. Here, the results show that when there is an increasing concentration of aerosols that act as IN, the deposition of water vapor is more efficient than when there is the increasing concentration of aerosols as CCN, which in turn enables cloud mass to increase in the mixed-phase Clouds.

  • Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin Stratocumulus Clouds to preindustrial and present-day aerosol conditions
    Atmospheric Chemistry and Physics, 2010
    Co-Authors: Seoung Soo Lee, Joyce E. Penner
    Abstract:

    Abstract. The response of a case of thin, warm marine-boundary-layer (MBL) Clouds to preindustrial (PI) and present-day (PD) conditions is simulated by a cloud-system resolving model (CSRM). Here, both the aerosol conditions and environmental conditions match those of a general circulation model (GCM). The environmental conditions are characterized by the initial condition and the large-scale forcings of humidity and temperature, as well as the surface fluxes. The response of the CSRM is compared to that simulated by the GCM. The percentage increase of liquid-water path (LWP) due to a change from the PI to PD conditions is ~3 times larger in the CSRM than that in the GCM due to the formation of cumulus Clouds. The formation of cumulus Clouds is controlled by a larger increase in the surface latent-heat (LH) flux in the PD environment than in the PI environment rather than by the change in aerosols. However, the aerosol increase from the PI to PD level determines the LWP response in the Stratocumulus Clouds, while the impacts of changes in environmental conditions are negligible for Stratocumulus Clouds. The conversion of cloud liquid to rain through autoconversion and accretion plays a negligible role in the CSRM in the response to aerosols, whereas it plays a role that is as important as condensation in the GCM. Also, it is notable that the explicit simulation of microphysics in the CSRM leads to a smaller LWP in the CSRM than that in the GCM using heavily parameterized microphysics for Stratocumulus Clouds. The smaller LWP in the CSRM is closer to an observed LWP than the LWP in the GCM for Stratocumulus Clouds. Supplementary simulations show that increasing aerosols increase the sensitivity of the cloud responses to the PI and PD environmental conditions. They also show that aerosol effects on Clouds depend on the cloud type. The LWP of warm cumulus Clouds is more sensitive to aerosols than that of Stratocumulus Clouds.

  • Impact of solar radiation on aerosol-cloud interactions in thin Stratocumulus Clouds
    2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner
    Abstract:

    Abstract. This study examines the role of solar radiation in the effect of aerosols on liquid-water path (LWP) in thin, marine Stratocumulus Clouds with LWP of ~50 g m −2 or less by performing four sets of simulations with different solar radiation. Each set is composed of a simulation with present-day (PD) aerosols and a simulation with preindustrial (PI) aerosols. As solar radiation increases, decoupling within the marine boundary layer (MBL) becomes stronger, leading to less condensation and less LWP and thus the absence of the surface precipitation. This enables the evaporation of rain to affect the cloud-base instability. As rain evaporation increases due to more conversion of cloud liquid to rain in the PI case, the cloud-base instability increases and thus updrafts increase which leads to larger LWP in the PI case than in the PD case. In the cases with no surface precipitation, when solar radiation decreases and thus decoupling becomes weaker, rain evaporation and cloud-base instability become larger, which increases the LWP more with PI aerosols than with PD aerosols. As solar radiation decreases further, condensation and, thus, the LWP increase, which leads to the presence of the surface precipitation. This stabilizes the entire MBL and thus prevents the interactions that cause the evaporation of rain to enhance the cloud-base instability. In cases with the surface precipitation, the in-cloud interactions among cloud droplet number concentration (CDNC), supersaturation, and updrafts play an important role in the effect of aerosols on the LWP; these in-cloud interactions produce larger LWP with the PD aerosols than with the PI aerosols. In a case with lower solar radiation and with surface precipitation, weaker decoupling induces stronger in-cloud interactions, which results in larger increases in LWP with PD aerosols compared to PI aerosols than that in a case with higher solar radiation. The results of this study demonstrate that solar radiation can act as an important environmental factor by inducing a large variation in the LWP and by changing the sign of aerosol effects on the LWP of thin Stratocumulus Clouds. Hence, the effect of solar radiation on decoupling and thus on the feedbacks between microphysics and dynamics needs to be included in climate models for a better prediction of the effect of aerosols on Clouds and thus climate.

  • Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin Stratocumulus Clouds to preindustrial and present-day aerosol conditions
    2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner
    Abstract:

    Abstract. The response of a case of thin, warm marine-boundary-layer (MBL) Clouds to preindustrial (PI) and present-day (PD) conditions is simulated by a cloud-system resolving model (CSRM). Here, both the aerosol conditions and environmental conditions match those of a general circulation model (GCM). The environmental conditions are characterized by the initial condition and the large-scale forcings of humidity and temperature, as well as the surface fluxes. The response of the CSRM is compared to that simulated by GCM. The percentage increase of liquid-water path (LWP) due to a change from the PI to PD conditions is ~3 times larger in the CSRM than that in the GCM due to the formation of cumulus Clouds. The formation of cumulus Clouds is controlled by a larger increase in the surface latent-heat (LH) flux in the PD environment than in the PI environment rather than by the change in aerosols. However, the aerosol increase from the PI to PD level determines the LWP response in the Stratocumulus Clouds, while the impacts of changes in environmental conditions are negligible for Stratocumulus Clouds. The conversion of cloud liquid to rain through autoconversion and accretion plays a negligible role in the CSRM in the response to aerosols, whereas it plays a role that is as important as condensation in the GCM. Supplementary simulations show that increasing aerosols increase the sensitivity of the cloud responses to the PI and PD environmental conditions and that aerosol effects on Clouds depend on the cloud type; the liquid water path (LWP) of warm cumulus Clouds is more sensitive to aerosols than the LWP of Stratocumulus Clouds.

  • comparison of a global climate model simulation to a cloud system resolving model simulation for long term thin Stratocumulus Clouds
    Atmospheric Chemistry and Physics, 2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner, Minghuai Wang
    Abstract:

    Abstract. A case of thin, warm marine-boundary-layer (MBL) Clouds is simulated by a cloud-system resolving model (CSRM) and is compared to the same case of Clouds simulated by a general circulation model (GCM). In this study, the simulation by the CSRM adopts higher resolutions which are generally used in large-eddy simulations (LES) and more advanced microphysics as compared to those by the GCM, enabling the CSRM-simulation to act as a benchmark to assess the simulation by the GCM. Explicitly simulated interactions among the surface latent heat (LH) fluxes, buoyancy fluxes, and cloud-top entrainment lead to the deepening-warming decoupling and thereby the transition from stratiform Clouds to cumulus Clouds in the CSRM. However, in the simulation by the GCM, these interactions are not resolved and thus the transition to cumulus Clouds is not simulated. This leads to substantial differences in liquid water content (LWC) and radiation between simulations by the CSRM and the GCM. When Stratocumulus Clouds are dominant prior to the transition to cumulus Clouds, interactions between supersaturation and cloud droplet number concentration (CDNC) (controlling condensation) and those between rain evaporation and cloud-base instability (controlling cloud dynamics and thereby condensation) determine LWC and thus the radiation budget in the simulation by the CSRM. These interactions result in smaller condensation and thus smaller LWC and reflected solar radiation by Clouds in the simulation by the CSRM than in the simulation by the GCM where these interactions are not resolved. The resolved interactions (associated with condensation and the transition to cumulus Clouds) lead to better agreement between the CSRM-simulation and observation than that between the GCM-simulation and observation.

Joyce E. Penner - One of the best experts on this subject based on the ideXlab platform.

  • Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin Stratocumulus Clouds to preindustrial and present-day aerosol conditions
    Atmospheric Chemistry and Physics, 2010
    Co-Authors: Seoung Soo Lee, Joyce E. Penner
    Abstract:

    Abstract. The response of a case of thin, warm marine-boundary-layer (MBL) Clouds to preindustrial (PI) and present-day (PD) conditions is simulated by a cloud-system resolving model (CSRM). Here, both the aerosol conditions and environmental conditions match those of a general circulation model (GCM). The environmental conditions are characterized by the initial condition and the large-scale forcings of humidity and temperature, as well as the surface fluxes. The response of the CSRM is compared to that simulated by the GCM. The percentage increase of liquid-water path (LWP) due to a change from the PI to PD conditions is ~3 times larger in the CSRM than that in the GCM due to the formation of cumulus Clouds. The formation of cumulus Clouds is controlled by a larger increase in the surface latent-heat (LH) flux in the PD environment than in the PI environment rather than by the change in aerosols. However, the aerosol increase from the PI to PD level determines the LWP response in the Stratocumulus Clouds, while the impacts of changes in environmental conditions are negligible for Stratocumulus Clouds. The conversion of cloud liquid to rain through autoconversion and accretion plays a negligible role in the CSRM in the response to aerosols, whereas it plays a role that is as important as condensation in the GCM. Also, it is notable that the explicit simulation of microphysics in the CSRM leads to a smaller LWP in the CSRM than that in the GCM using heavily parameterized microphysics for Stratocumulus Clouds. The smaller LWP in the CSRM is closer to an observed LWP than the LWP in the GCM for Stratocumulus Clouds. Supplementary simulations show that increasing aerosols increase the sensitivity of the cloud responses to the PI and PD environmental conditions. They also show that aerosol effects on Clouds depend on the cloud type. The LWP of warm cumulus Clouds is more sensitive to aerosols than that of Stratocumulus Clouds.

  • Impact of solar radiation on aerosol-cloud interactions in thin Stratocumulus Clouds
    2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner
    Abstract:

    Abstract. This study examines the role of solar radiation in the effect of aerosols on liquid-water path (LWP) in thin, marine Stratocumulus Clouds with LWP of ~50 g m −2 or less by performing four sets of simulations with different solar radiation. Each set is composed of a simulation with present-day (PD) aerosols and a simulation with preindustrial (PI) aerosols. As solar radiation increases, decoupling within the marine boundary layer (MBL) becomes stronger, leading to less condensation and less LWP and thus the absence of the surface precipitation. This enables the evaporation of rain to affect the cloud-base instability. As rain evaporation increases due to more conversion of cloud liquid to rain in the PI case, the cloud-base instability increases and thus updrafts increase which leads to larger LWP in the PI case than in the PD case. In the cases with no surface precipitation, when solar radiation decreases and thus decoupling becomes weaker, rain evaporation and cloud-base instability become larger, which increases the LWP more with PI aerosols than with PD aerosols. As solar radiation decreases further, condensation and, thus, the LWP increase, which leads to the presence of the surface precipitation. This stabilizes the entire MBL and thus prevents the interactions that cause the evaporation of rain to enhance the cloud-base instability. In cases with the surface precipitation, the in-cloud interactions among cloud droplet number concentration (CDNC), supersaturation, and updrafts play an important role in the effect of aerosols on the LWP; these in-cloud interactions produce larger LWP with the PD aerosols than with the PI aerosols. In a case with lower solar radiation and with surface precipitation, weaker decoupling induces stronger in-cloud interactions, which results in larger increases in LWP with PD aerosols compared to PI aerosols than that in a case with higher solar radiation. The results of this study demonstrate that solar radiation can act as an important environmental factor by inducing a large variation in the LWP and by changing the sign of aerosol effects on the LWP of thin Stratocumulus Clouds. Hence, the effect of solar radiation on decoupling and thus on the feedbacks between microphysics and dynamics needs to be included in climate models for a better prediction of the effect of aerosols on Clouds and thus climate.

  • Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin Stratocumulus Clouds to preindustrial and present-day aerosol conditions
    2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner
    Abstract:

    Abstract. The response of a case of thin, warm marine-boundary-layer (MBL) Clouds to preindustrial (PI) and present-day (PD) conditions is simulated by a cloud-system resolving model (CSRM). Here, both the aerosol conditions and environmental conditions match those of a general circulation model (GCM). The environmental conditions are characterized by the initial condition and the large-scale forcings of humidity and temperature, as well as the surface fluxes. The response of the CSRM is compared to that simulated by GCM. The percentage increase of liquid-water path (LWP) due to a change from the PI to PD conditions is ~3 times larger in the CSRM than that in the GCM due to the formation of cumulus Clouds. The formation of cumulus Clouds is controlled by a larger increase in the surface latent-heat (LH) flux in the PD environment than in the PI environment rather than by the change in aerosols. However, the aerosol increase from the PI to PD level determines the LWP response in the Stratocumulus Clouds, while the impacts of changes in environmental conditions are negligible for Stratocumulus Clouds. The conversion of cloud liquid to rain through autoconversion and accretion plays a negligible role in the CSRM in the response to aerosols, whereas it plays a role that is as important as condensation in the GCM. Supplementary simulations show that increasing aerosols increase the sensitivity of the cloud responses to the PI and PD environmental conditions and that aerosol effects on Clouds depend on the cloud type; the liquid water path (LWP) of warm cumulus Clouds is more sensitive to aerosols than the LWP of Stratocumulus Clouds.

  • comparison of a global climate model simulation to a cloud system resolving model simulation for long term thin Stratocumulus Clouds
    Atmospheric Chemistry and Physics, 2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner, Minghuai Wang
    Abstract:

    Abstract. A case of thin, warm marine-boundary-layer (MBL) Clouds is simulated by a cloud-system resolving model (CSRM) and is compared to the same case of Clouds simulated by a general circulation model (GCM). In this study, the simulation by the CSRM adopts higher resolutions which are generally used in large-eddy simulations (LES) and more advanced microphysics as compared to those by the GCM, enabling the CSRM-simulation to act as a benchmark to assess the simulation by the GCM. Explicitly simulated interactions among the surface latent heat (LH) fluxes, buoyancy fluxes, and cloud-top entrainment lead to the deepening-warming decoupling and thereby the transition from stratiform Clouds to cumulus Clouds in the CSRM. However, in the simulation by the GCM, these interactions are not resolved and thus the transition to cumulus Clouds is not simulated. This leads to substantial differences in liquid water content (LWC) and radiation between simulations by the CSRM and the GCM. When Stratocumulus Clouds are dominant prior to the transition to cumulus Clouds, interactions between supersaturation and cloud droplet number concentration (CDNC) (controlling condensation) and those between rain evaporation and cloud-base instability (controlling cloud dynamics and thereby condensation) determine LWC and thus the radiation budget in the simulation by the CSRM. These interactions result in smaller condensation and thus smaller LWC and reflected solar radiation by Clouds in the simulation by the CSRM than in the simulation by the GCM where these interactions are not resolved. The resolved interactions (associated with condensation and the transition to cumulus Clouds) lead to better agreement between the CSRM-simulation and observation than that between the GCM-simulation and observation.

  • aerosol effects on liquid water path of thin Stratocumulus Clouds
    Journal of Geophysical Research, 2009
    Co-Authors: Joyce E. Penner, Stephen M Saleeby
    Abstract:

    three cases of thin warm Stratocumulus Clouds with LWP < 50 g m �2 . We use a Cloudsystem resolving model coupled with a double-moment representation of cloud microphysics. Intensified interactions among the cloud droplet number concentration, condensation, and dynamics at high aerosol play a critical role in the LWP responses to aerosol increases. Increased aerosols lead to increased CDNC, providing the increased surface area of droplets where water vapor condenses. This increases condensation, and thus condensational heating, to produce stronger updrafts, leading to an increased LWP with increased aerosols in two of the cases where precipitation reaches the surface. In a case with no surface precipitation, LWP decreases with increases in aerosols. In this case, most of precipitation evaporates just below the cloud base. With decreases in aerosols, precipitation increases and leads to increasing evaporation of precipitation, thereby increasing instability around the cloud base. This leads to increased updrafts, and thus condensation, from which increased LWP results.

Yefim L Kogan - One of the best experts on this subject based on the ideXlab platform.

  • Large-Eddy Simulation of Air Parcels in Stratocumulus Clouds: Time Scales and Spatial Variability.
    Journal of the Atmospheric Sciences, 2006
    Co-Authors: Yefim L Kogan
    Abstract:

    Abstract Large ensembles of air parcel trajectories driven by the (large-eddy simulation) LES-generated velocity fields from simulations of Stratocumulus Clouds were analyzed, focusing on statistics of air parcel in-cloud time scales, as well as their spatial variability. In the case of a drizzling Stratocumulus cloud the in-cloud residence time is 2–5 times longer than the characteristic cloud eddy turnover time. About 70% of all air parcels cycle in the cloud more than 2 times and about 50% more than 3 times, thus indicating that air cycling is an essential feature of drizzling Stratocumulus cloud dynamics. The extent of cycling is different in the case of nondrizzling Stratocumulus cloud, where mean in-cloud time scales are on the order of eddy turnover time. Evidently air cycling in cloud depends on boundary layer stability and flow circulation; the latter is affected by cooling of evaporating drizzle and heating by solar radiation. Results show significant inhomogeneity of in-cloud time scales, which...

  • spectral dependence of radiative horizontal transport in Stratocumulus Clouds and its effect on near ir absorption
    Journal of Geophysical Research, 2002
    Co-Authors: Evgueni I Kassianov, Yefim L Kogan
    Abstract:

    [1] The spectral dependence of the radiative horizontal transport (the horizontal transport of radiative energy) E and its effect on the accuracy of spectral and broadband absorption retrieval in the near-infrared (IR) wavelength range was investigated using a large-eddy simulation (LES) cloud model with explicit microphysics and a three-dimensional Monte Carlo radiative transfer model. Two typical types of marine Clouds representing inhomogeneous overcast and broken Stratocumulus Clouds have been simulated. We demonstrate that (1) the basic statistics (e.g., variance and correlation function) of the horizontal transport are wavelength-dependent and (2) the estimates of spectral and broadband absorption with a given accuracy (e.g., rmse ∼4%) may require the use of different spatial resolutions.

  • Midlatitude Aerosol-Cloud-Radiation Feedbacks Mechanisms in Marine Stratocumulus Clouds
    1998
    Co-Authors: Yefim L Kogan
    Abstract:

    Abstract : The development and improvement of cloud microphysical and radiative parameterizations for use in mesoscale models. Investigation of marine Stratocumulus Clouds microphysics and radiative processes using the CIMMS LES model with explicit microphysics and radiation. The data from FIRE II/ASTEX and MAST field experiments will be used to validate the model and to improve our understanding of the interactions between the microphysical, radiative, and thermodynamical processes.

Stephen M Saleeby - One of the best experts on this subject based on the ideXlab platform.

  • aerosol effects on liquid water path of thin Stratocumulus Clouds
    Journal of Geophysical Research, 2009
    Co-Authors: Joyce E. Penner, Stephen M Saleeby
    Abstract:

    three cases of thin warm Stratocumulus Clouds with LWP < 50 g m �2 . We use a Cloudsystem resolving model coupled with a double-moment representation of cloud microphysics. Intensified interactions among the cloud droplet number concentration, condensation, and dynamics at high aerosol play a critical role in the LWP responses to aerosol increases. Increased aerosols lead to increased CDNC, providing the increased surface area of droplets where water vapor condenses. This increases condensation, and thus condensational heating, to produce stronger updrafts, leading to an increased LWP with increased aerosols in two of the cases where precipitation reaches the surface. In a case with no surface precipitation, LWP decreases with increases in aerosols. In this case, most of precipitation evaporates just below the cloud base. With decreases in aerosols, precipitation increases and leads to increasing evaporation of precipitation, thereby increasing instability around the cloud base. This leads to increased updrafts, and thus condensation, from which increased LWP results.

  • Aerosol effects on liquid‐water path of thin Stratocumulus Clouds
    Journal of Geophysical Research, 2009
    Co-Authors: Seoung Soo Lee, Joyce E. Penner, Stephen M Saleeby
    Abstract:

    three cases of thin warm Stratocumulus Clouds with LWP < 50 g m �2 . We use a Cloudsystem resolving model coupled with a double-moment representation of cloud microphysics. Intensified interactions among the cloud droplet number concentration, condensation, and dynamics at high aerosol play a critical role in the LWP responses to aerosol increases. Increased aerosols lead to increased CDNC, providing the increased surface area of droplets where water vapor condenses. This increases condensation, and thus condensational heating, to produce stronger updrafts, leading to an increased LWP with increased aerosols in two of the cases where precipitation reaches the surface. In a case with no surface precipitation, LWP decreases with increases in aerosols. In this case, most of precipitation evaporates just below the cloud base. With decreases in aerosols, precipitation increases and leads to increasing evaporation of precipitation, thereby increasing instability around the cloud base. This leads to increased updrafts, and thus condensation, from which increased LWP results.

Graham Feingold - One of the best experts on this subject based on the ideXlab platform.

  • Network approach to patterns in Stratocumulus Clouds
    Proceedings of the National Academy of Sciences of the United States of America, 2017
    Co-Authors: Franziska Glassmeier, Graham Feingold
    Abstract:

    Stratocumulus Clouds (Sc) have a significant impact on the amount of sunlight reflected back to space, with important implications for Earth's climate. Representing Sc and their radiative impact is one of the largest challenges for global climate models. Sc fields self-organize into cellular patterns and thus lend themselves to analysis and quantification in terms of natural cellular networks. Based on large-eddy simulations of Sc fields, we present a first analysis of the geometric structure and self-organization of Sc patterns from this network perspective. Our network analysis shows that the Sc pattern is scale-invariant as a consequence of entropy maximization that is known as Lewis's Law (scaling parameter: 0.16) and is largely independent of the Sc regime (cloud-free vs. cloudy cell centers). Cells are, on average, hexagonal with a neighbor number variance of about 2, and larger cells tend to be surrounded by smaller cells, as described by an Aboav-Weaire parameter of 0.9. The network structure is neither completely random nor characteristic of natural convection. Instead, it emerges from Sc-specific versions of cell division and cell merging that are shaped by cell expansion. This is shown with a heuristic model of network dynamics that incorporates our physical understanding of cloud processes.

  • wind speed response of marine non precipitating Stratocumulus Clouds over a diurnal cycle in cloud system resolving simulations
    Atmospheric Chemistry and Physics, 2016
    Co-Authors: Graham Feingold, J Kazil, Takanobu Yamaguchi
    Abstract:

    Abstract. Observed and projected trends in large-scale wind speed over the oceans prompt the question: how do marine Stratocumulus Clouds and their radiative properties respond to changes in large-scale wind speed? Wind speed drives the surface fluxes of sensible heat, moisture, and momentum and thereby acts on cloud liquid water path (LWP) and cloud radiative properties. We present an investigation of the dynamical response of non-precipitating, overcast marine Stratocumulus Clouds to different wind speeds over the course of a diurnal cycle, all else equal. In cloud-system resolving simulations, we find that higher wind speed leads to faster boundary layer growth and stronger entrainment. The dynamical driver is enhanced buoyant production of turbulence kinetic energy (TKE) from latent heat release in cloud updrafts. LWP is enhanced during the night and in the morning at higher wind speed, and more strongly suppressed later in the day. Wind speed hence accentuates the diurnal LWP cycle by expanding the morning–afternoon contrast. The higher LWP at higher wind speed does not, however, enhance cloud top cooling because in Clouds with LWP ⪆ 50 g m−2, longwave emissions are insensitive to LWP. This leads to the general conclusion that in sufficiently thick Stratocumulus Clouds, additional boundary layer growth and entrainment due to a boundary layer moistening arises by stronger production of TKE from latent heat release in cloud updrafts, rather than from enhanced longwave cooling. We find that large-scale wind modulates boundary layer decoupling. At nighttime and at low wind speed during daytime, it enhances decoupling in part by faster boundary layer growth and stronger entrainment and in part because shear from large-scale wind in the sub-cloud layer hinders vertical moisture transport between the surface and cloud base. With increasing wind speed, however, in decoupled daytime conditions, shear-driven circulation due to large-scale wind takes over from buoyancy-driven circulation in transporting moisture from the surface to cloud base and thereby reduces decoupling and helps maintain LWP. The total (shortwave + longwave) cloud radiative effect (CRE) responds to changes in LWP and cloud fraction, and higher wind speed translates to a stronger diurnally averaged total CRE. However, the sensitivity of the diurnally averaged total CRE to wind speed decreases with increasing wind speed.

  • effect of biomass burning on marine Stratocumulus Clouds off the california coast
    Atmospheric Chemistry and Physics, 2009
    Co-Authors: Graham Feingold, J Brioude, O R Cooper, M Trainer, S R Freitas, D Kowal, J K Ayers, E M Prins
    Abstract:

    Abstract. Aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system. Biomass burning aerosols are of special interest due to their radiative impact (direct and indirect effect) and their potential to increase in the future due to climate change. Combining data from Geostationary Operational Environmental Satellite (GOES) and MODerate-resolution Imaging Spectroradiometer (MODIS) with passive tracers from the FLEXPART Lagrangian Particle Dispersion Model, the impact of biomass burning aerosols on marine Stratocumulus Clouds has been examined in June and July of 2006–2008 off the California coast. Using a continental tracer, the indirect effect of biomass burning aerosols has been isolated by comparing the average cloud fraction and cloud albedo for different meteorological situations, and for clean versus polluted (in terms of biomass burning) continental air masses at 14:00 local time. Within a 500 km-wide band along the coast of California, biomass burning aerosols, which tend to reside above the marine boundary layer, increased the cloud fraction by 0.143, and the cloud albedo by 0.038. Absorbing aerosols located above the marine boundary layer lead to an increase of the lower tropospheric stability and a reduction in the vertical entrainment of dry air from above, leading to increased cloud formation. The combined effect was an indirect radiative forcing of −7.5% ±1.7% (cooling effect) of the outgoing radiative flux at the top of the atmosphere on average, with a bias due to meteorology of +0.9%. Further away from the coast, the biomass burning aerosols, which were located within the boundary layer, reduced the cloud fraction by 0.023 and the cloud albedo by 0.006, resulting in an indirect radiative forcing of +1.3% ±0.3% (warming effect) with a bias of +0.5%. These results underscore the dual role that absorbing aerosols play in cloud radiative forcing.

  • Turbulence, Condensation, and Liquid Water Transport in Numerically Simulated Nonprecipitating Stratocumulus Clouds.
    Journal of the Atmospheric Sciences, 2003
    Co-Authors: Shouping Wang, Qing Wang, Graham Feingold
    Abstract:

    Abstract Condensation and turbulent liquid water transport in Stratocumulus Clouds involve complicated interactions between turbulence dynamics and cloud microphysical processes, and play essential roles in defining the cloud structure. This work aims at understanding this dynamical–microphysical interaction and providing information necessary for parameterizations of the ensemble mean condensation rate and turbulent fluxes of liquid water variables in a coupled turbulence–microphysics model. The approach is to simulate nonprecipitating Stratocumulus Clouds with a coupled large eddy simulation and an explicit bin-microphysical model, and then perform a budget analysis for four liquid water variables: mean liquid water content, turbulent liquid water flux, mean cloud droplet number concentration, and the number density flux. The results show that the turbulence contribution to the mean condensation rate comes from covariance of the integral cloud droplet radius and supersaturation, which enhances condensat...

  • Simulations of marine Stratocumulus Clouds during ASTEX: Comparisons with radar radiometer measurements
    1994
    Co-Authors: Graham Feingold, A. S. Frisch, Bjorn Stevens, William R. Cotton
    Abstract:

    The role of marine Stratocumulus Clouds in modifying the Earth`s radiative budget has focused unprecedented attention on this cloud type and spawned major field experiments such as FIRE and ASTEX (Atlantic Stratocumulus Transition Experiment). Two major hypotheses have driven their research: the first proposes that enhanced sources of cloud condensation nuclei (CCN) will result in higher droplet concentrations and more reflective Clouds. The second follows from the first and proposes that larger concentrations of small droplets will suppress drizzle formation, and maintain higher liquid water paths.