Simulation of marine stratocumulus: effect of precipitation parameterization and sensitivity to droplet number concentration

1998 ◽  
Vol 89 (1) ◽  
pp. 75-107 ◽  
Author(s):  
L. Delobbe
Keyword(s):  
Number Concentration ◽  
Marine Stratocumulus

Quantifying how model assumptions affect aerosol-cloud interactions in large-eddy simulations of warm stratocumulus clouds

2021 ◽  
Author(s):  
Matthias Schwarz ◽  
Julien Savre ◽  
Annica Ekman
Keyword(s):  
Number Concentration ◽  
Large Eddy Simulations ◽  
Radiation Balance ◽  
Marine Stratocumulus ◽  
Aerosol Cloud ◽  
Reference Simulation ◽  
Model Aerosol ◽  
Large Eddy ◽  
Stratocumulus Clouds ◽  
Climate Interactions

<p>Subtropical low-level marine stratocumulus clouds effectively reflect downwelling shortwave radiation while having a small effect on outgoing longwave radiation. Hence, they impose a strong negative net radiative effect on the Earth’s radiation balance. The optical and microphysical properties of these clouds are susceptible to anthropogenic changes in aerosol abundance. Although these aerosol-cloud-climate interactions (ACI) are generally explicitly treated in state-of-the-art Earth System Models (ESMs), they are accountable for large uncertainties in current climate projections.</p><p>Here, we present preliminary work where we exploit Large-Eddy-Simulations (LES) of warm stratocumulus clouds to identify and constrain processes and model assumptions that affect the response of cloud droplet number concentration (N<sub>d</sub>) to changes in aerosol number concentration (N<sub>a</sub>). Our results are based on simulations with the MISU-MIT Cloud-Aerosol (MIMICA, Savre et al., 2014) LES, which has two-moment bulk microphysics (Seifert and Beheng, 2001) and a two-moment aerosol scheme (Ekman et al., 2006). The reference simulation is based on observations made during the Dynamics and Chemistry of Marine Stratocumulus Field Study (DYCOMS-II, Stevens et al., 2003) which were used extensively during previous LES studies (e.g., Ackerman et al., 2009).</p><p>Starting from the reference simulation, we conduct sensitivity experiments to examine how the susceptibility (β=dln(N<sub>d</sub>)/dln(N<sub>a</sub>)) changes depending on different model setups. We run the model with fixed and interactive aerosol concentrations, with and without saturation adjustment, with different aerosol populations, and with different model parameter choices. Our early results suggest that β is sensitive to these choices and can vary roughly between 0.6 to 0.9 depending on the setup. The overall purpose of our study is to guide future model developments and evaluations concerning aerosol-cloud-climate interactions.  </p><p> </p><p><strong>References</strong></p><p>Ackerman, A. S., vanZanten, M. C., Stevens, B., Savic-Jovcic, V., Bretherton, C. S., Chlond, A., et al. (2009). Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer. Monthly Weather Review, 137(3), 1083–1110. https://doi.org/10.1175/2008MWR2582.1</p><p>Ekman, A. M. L., Wang, C., Ström, J., & Krejci, R. (2006). Explicit Simulation of Aerosol Physics in a Cloud-Resolving Model: Aerosol Transport and Processing in the Free Troposphere. Journal of the Atmospheric Sciences, 63(2), 682–696. https://doi.org/10.1175/JAS3645.1</p><p>Savre, J., Ekman, A. M. L., & Svensson, G. (2014). Technical note: Introduction to MIMICA, a large-eddy simulation solver for cloudy planetary boundary layers. Journal of Advances in Modeling Earth Systems, 6(3), 630–649. https://doi.org/10.1002/2013MS000292</p><p>Stevens, B., Lenschow, D. H., Vali, G., Gerber, H., Bandy, A., Blomquist, B., et al. (2003). Dynamics and Chemistry of Marine Stratocumulus—DYCOMS-II. Bulletin of the American Meteorological Society, 84(5), 579–594. https://doi.org/10.1175/BAMS-84-5-579</p>

Retrieval of Cloud Properties for Partly Cloudy Imager Pixels

2005 ◽  
Vol 22 (1) ◽  
pp. 3-17 ◽  
Author(s):  
James A. Coakley ◽  
Michael A. Friedman ◽  
William R. Tahnk
Keyword(s):  
Optical Depth ◽  
Cloud Cover ◽  
Tropical Rainfall Measuring Mission ◽  
Number Concentration ◽  
Threshold Method ◽  
Marine Stratocumulus ◽  
Cloud Properties ◽  
Retrieval Scheme ◽  
Column Number ◽  
Pixel Scale

Abstract Retrievals of cloud properties from satellite imagery often invoke the assumption that the fields of view are overcast when cloud-contaminated, even though a significant fraction are only partially cloud-covered. The overcast assumption leads to biases in the retrieved cloud properties: cloud amounts and droplet effective radii are typically overestimated, while visible optical depths, cloud altitudes, cloud liquid water amounts, and column droplet number concentrations are typically underestimated. In order to estimate these biases, a retrieval scheme was developed to obtain the properties of clouds for partially covered imager fields of view. The partly cloudy pixel retrieval scheme is applicable to single-layered cloud systems and invokes the assumption that clouds that only partially cover a field of view are at the same altitude as nearby clouds from the same layer that completely cover imager pixels. The properties of the retrieval are illustrated through its application to 2-km Visible and Infrared Scanner (VIRS) data from the Tropical Rainfall Measuring Mission (TRMM) for a marine stratocumulus scene. The scene was chosen because the cloud properties are typical of such systems based on an analysis of VIRS data for February and March 1998. Comparisons of properties for clouds in partly cloudy pixels and those for clouds in nearby overcast pixels reveal that the optical depths and droplet effective radii are generally smaller for the clouds in the partly cloudy pixels. In addition, for pixel-scale cloud fractions between 0.2 and 0.8, optical depth, droplet effective radius, and column droplet number concentration decrease slowly with decreasing cloud cover fraction. The changes are only about 20%–30%, while cloud cover fraction changes by 80%. For comparison, changes in optical depth and column number concentration retrieved using a threshold method decrease by 80%–90%. As long as the cloud cover in partly cloudy pixels is greater than about 0.1, uncertainties in the estimates of the cloud altitudes and of the radiances for the cloud-free portions of the fields of view give rise to uncertainties in the retrieved cloud properties that are comparable to the uncertainties in the properties retrieved for overcast pixels.

scholarly journals Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud

2012 ◽  
Vol 12 (5) ◽  
pp. 2459-2468 ◽  
Author(s):  
F. Ditas ◽  
R. A. Shaw ◽  
H. Siebert ◽  
M. Simmel ◽  
B. Wehner ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Liquid Water Content ◽  
Cloud Microphysics ◽  
Number Concentration ◽  
Cloud Base ◽  
Size Distributions ◽  
Marine Stratocumulus ◽  
Model Following ◽  
Critical Supersaturation ◽  
Kohler Theory

Abstract. This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435±87 cm−3 and compares well with the observed median droplet number concentration of Nd = 464 cm−3. Furthermore, a 50% activation diameter of Dp50≈115 nm was derived, which was linked to a critical supersaturation Scrit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σS' = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S') are calculated based on highly-resolved thermodynamic data showing a standard deviation of S' ranging within 0.1%≤σS'≤0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of S' with σS'=0.1% at cloud base and a decreasing σS' with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.

scholarly journals A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of earth's large marine stratocumulus decks

2011 ◽  
Vol 11 (10) ◽  
pp. 28663-28687 ◽  
Author(s):  
D. A. Hegg ◽  
D. S. Covert ◽  
H. H. Jonsson ◽  
R. K. Woods
Keyword(s):  
Cloud Condensation Nuclei ◽  
Number Concentration ◽  
Mode Number ◽  
Simple Relationship ◽  
Cloud Drop ◽  
Marine Stratocumulus ◽  
Accumulation Mode ◽  
The Pacific ◽  
Drop Number ◽  
Climate Studies

Abstract. Aircraft-based measurements of cloud condensation nuclei (CCN), accumulation mode and Aitken mode number concentrations, cloud drop number concentration (CDNC), and selected ancillary measurements are presented for the three large, semi-permanent marine stratocumulus decks of the earth (in the Pacific offshore of California and Chile and in the Atlantic offshore of Namibia). Based on these data, a simple linear relationship between CDNC and the accumulation mode number concentration (AMNC) is derived via regression. The R2 value for this regression is 0.90, higher than those found for CDNC-CCN linear regressions. Explanations of the relatively favorable CDNC-AMNC relationship and its utility for climate studies are discussed.

scholarly journals A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of Earth's large marine stratocumulus decks

2012 ◽  
Vol 12 (3) ◽  
pp. 1229-1238 ◽  
Author(s):  
D. A. Hegg ◽  
D. S. Covert ◽  
H. H. Jonsson ◽  
R. K. Woods
Keyword(s):  
Cloud Condensation Nuclei ◽  
Number Concentration ◽  
Mode Number ◽  
Simple Relationship ◽  
Cloud Drop ◽  
Marine Stratocumulus ◽  
Accumulation Mode ◽  
The Pacific ◽  
Drop Number ◽  
Climate Studies

Abstract. Aircraft–based measurements of cloud condensation nuclei (CCN), accumulation mode and Aitken mode number concentrations, cloud drop number concentration (CDNC), and selected ancillary measurements are presented for the three large, semi-permanent marine stratocumulus decks of the earth (in the Pacific offshore of California and Chile and in the Atlantic offshore of Namibia). Based on these data, a simple linear relationship between CDNC and the accumulation mode number concentration (AMNC) is derived via regression. The slope of the regression is 0.72 ± 0.04 with an R2 of 0.90, higher than those found for CDNC-CCN linear regressions. Explanations of the relatively favorable CDNC-AMNC relationship and its utility for climate studies are discussed.

scholarly journals Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud

2011 ◽  
Vol 11 (11) ◽  
pp. 29777-29805 ◽  
Author(s):  
F. Ditas ◽  
R. A. Shaw ◽  
H. Siebert ◽  
M. Simmel ◽  
B. Wehner ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Liquid Water Content ◽  
Cloud Microphysics ◽  
Number Concentration ◽  
Cloud Base ◽  
Marine Stratocumulus ◽  
Model Following ◽  
Critical Supersaturation ◽  
Kohler Theory ◽  
Turbulence Parameters

Abstract. This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence parameters to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a spectral cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated to 435±87 cm−3 and compares well with the observed median droplet number concentration of Nd=456 cm−3. Furthermore, a 50% activation diameter of Dp50 ≈ 115 nm was derived, which was linked to a critical supersaturation Scrit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σS' =0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S') are calculated based on highly-resolved thermodynamic data showing a standard deviation of S' ranging within 0.1% ≤ σS' ≤ .3%. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a spectral cloud microphysical model. These results show highest fluctuations of S' with σS' =0.1% at cloud base and a decreasing σS' with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.

scholarly journals The Influence of Entrainment and Mixing Assumption on Aerosol–Cloud Interactions in Marine Stratocumulus

2009 ◽  
Vol 66 (5) ◽  
pp. 1450-1464 ◽  
Author(s):  
Adrian A. Hill ◽  
Graham Feingold ◽  
Hongli Jiang
Keyword(s):  
Optical Depth ◽  
Relative Sensitivity ◽  
Number Concentration ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Cloud Optical Depth ◽  
Aerosol Cloud ◽  
Aerosol Cloud Interactions ◽  
Entrainment Effect ◽  
The Impact

Abstract This study uses large-eddy simulation with bin microphysics to investigate the influence of entrainment and mixing on aerosol–cloud interactions in the context of idealized, nocturnal, nondrizzling marine stratocumulus (Sc). Of particular interest are (i) an evaporation–entrainment effect and a sedimentation–entrainment effect that result from increasing aerosol concentrations and (ii) the nature of mixing between clear and cloudy air, where homogeneous and extreme inhomogeneous mixing represent the bounding mixing types. Simulations are performed at low resolution (Δz = 20 m; Δx, y = 40 m) and high resolution (Δz = 10 m; Δx, y = 20 m). It is demonstrated that an increase in aerosol from clean conditions (100 cm−3) to polluted conditions (1000 cm−3) produces both an evaporation–entrainment and a sedimentation–entrainment effect, which couple to cause about a 10% decrease in liquid water path (LWP) when all warm microphysical processes are included. These dynamical effects are insensitive to both the resolutions tested and the mixing assumption. Regardless of resolution, assuming extreme inhomogeneous rather than homogeneous mixing results in a small reduction in cloud-averaged drop number concentration, a small increase in cloud drop effective radius, and ∼1% decrease in cloud optical depth. For the case presented, these small changes play a negligible role when compared to the impact of increasing aerosol and the associated entrainment effects. Finally, it is demonstrated that although increasing resolution causes an increase in LWP and number concentration, the relative sensitivity of cloud optical depth to changes in aerosol is unaffected by resolution.

scholarly journals TWOMEY EFFECT IN SUBTROPICAL STRATOCUMULUS CLOUDS FROM UV DEPOLARIZATION LIDAR

2018 ◽  
Vol 176 ◽  
pp. 05002
Author(s):  
Martin de Graaf ◽  
Jessica Brown ◽  
David Donovan
Keyword(s):  
Monte Carlo ◽  
Multiple Scattering ◽  
Cloud Droplet ◽  
Number Concentration ◽  
Inversion Method ◽  
African Continent ◽  
Marine Stratocumulus ◽  
Remote Island ◽  
Stratocumulus Clouds ◽  
Droplet Sizes

Marine stratocumulus clouds are important climate regulators, reflecting sunlight over a dark ocean background. A UV-depolarization lidar on Ascension, a small remote island in the south Atlantic, measured cloud droplet sizes and number concentration using an inversion method based on Monte Carlo (MC) modelling of multiple scattering in idealised semiadiabatic clouds. The droplet size and number concentration weremodulated due to smoke from the African continent, measured by the same instrument.

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