Cloud Droplet Growth by Condensation and Aggregation EPM Stratocumulus and Arctic Stratocumulus

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1 Cloud Droplet Growth by Condensation and Aggregation EPM Stratocumulus and Arctic Stratocumulus US Department of Energy, ARM

2 Typical EPMS Characteristics Altitude: ~ 2 km Depth: 1-2 km Typically cooling clouds due to contribution to shortwave albedo Strong diurnal cycle Published by AAAS J M Creamean et al. Science 2013;339:

3 Ship-based Observations of the Diurnal Cycle of Southeast Pacific Marine Stratocumulus Clouds and Precipitation Aim to resolve complex diurnal processes that define EPMS Using meteorological data from ship-based study Provides several cloud-droplet study inputs: Burleyson, C. et al. Ship-Based Observations of the Diurnal Cycle of Southeast Pacific Marine Stratocumulus Clouds and Precipitation (2013). doi: /jas-d

4 The Sensitivity of Springtime Arctic Mixed-Phase Stratocumulus Clouds to Surface Layer and Cloud-Top Inversion Layer Moisture Sources Explores relation of radiative properties to moisture source (inversions, ice, presence of liquid cloud water above cloud top) Using observations collected during field campaign in Barrow, Alaska McFarquhar et al Provides several cloud-droplet study inputs: De, I. J. S. The Sensitivity of Springtime Arctic Mixed-Phase Stratocumulus Clouds to SurfaceLayer and Cloud-Top Inversion-Layer Moisture Sources (2013). doi: /jasd

5 Drizzle and likelihood of precipitation Figure 3 from Low cloud precipitation climatology in the southeastern Pacific marine stratocumulus region using CloudSat Anita D Rapp et al 2013 Environ. Res. Lett doi: / /8/1/014027

6 EPMS Model K: Heat Conduction Term D: Diffusivity term L: Latent Heat ρl: Liquid Water Density ρ: Ice Crystal Density R: Gas Constant e: Saturation Pressure S: Supersaturation κ: Thermal Conductivity Dv: Water Vapor Diffusivity C: Morphological multiplier

7 Modeling Assumptions Temperature in EPMS Does not vary much spatially T < 5 Saturation Pressure Varies with Temperature Variance is ignored due to small spatial range Thermal conductivity and Water Vapor Diffusivity Small changes with temperature Other condensing vapors ignored due to small availability Morphology of droplet Assumed Spherical Upwelling and Downwelling negligible Water Vapor Mass Mixing Ratio Assumed constant Ice Crystal Morphology Assumed Spherical Ice Crystal Density Assumed Constant

8 Arctic Stratus Model R: Snowflake Radius r: Aggregate water vapor radius u T : Terminal Velocity n: Particle Size Distribution E: Collection Efficiency w l : Liquid Water Mixing Ratio ρ l : Snowflake Density ρ a : Air Density c p : Specific Heat L li :Latent heat Γ d : Dry adiabatic lapse rate Γ s : Saturated adiabatic lapse rate u z : Upwelling velocity

9 Arctic Stratus Modeling Assumptions Terminal Velocities Approximated difference of 1 m/s Collection Efficiency Assumed perfect Upwelling and Downwelling ignored, though not negligible Snowball Density Assumed incompressible Size Distribution Negligible at large radius Use Forward model to describe discrepancy

10 Results Model evaluated growth rates of cloud droplets in EPMS and AS clouds Final radius held constant Also, the growth rates of snowflakes due to aggregation

11 Final droplet size held constant in EPMS cloud

12 Results of snowflake growth Growth depends on both mixing ratio (amount of water vapor) and initial size

13 Final droplet size held constant in AS cloud (258 K) Usually takes 15 to 30 minutes for snow crystals. Growth solely depends on S values

14 Example of seeding in AS clouds

15 Discussion Results display relationship of growth rate with supersaturation and radius increase. T and e s held constant Overall, condensational growth (EPMS) occurs ~5 to 10 times faster than accretion growth (AS) AS clouds do not depend on T Model results have implications for geoengineering Cloud seeding Creating heavier droplets (larger CCN) But what about the indirect effect (Twomey effect)?