Chien Wang Massachusetts Institute of Technology A Close Look at the Aerosol-Cloud Interaction in Tropical Deep Convection

Why Does Aerosol Matter to Clouds Saturation requirement to form new particles through homogeneous homo-molecular nucleation in the atmosphere: S > 3.5 Note: Typically, in-cloud S < 1.01 Lowering the required S 1)Mixed vapor of 2 (binary) or 3 (ternary) species – hetero-molecular homogeneous nucleation: mainly aerosol nucleation 2)Existing surface – heterogeneous nucleation on insoluble (with small contact angle) or soluble aerosols (ion factor as well)

Heterogeneous nucleation of droplets and ice crystals Four ice nucleation modes: Heterogeneous deposition Immerse Condensation-freezing Contact H 2 O(g) INIce crystal cloud droplet Water droplet nucleation: Hygroscopic aerosols acting as nuclei. Note that it is existing aerosol NOT molecular collision efficiency that determines the nucleation rate

water vapor CCN IN cloud droplet ice crystal rain drop snowflake graupel hailstone nucleation condensation/ evaporation riming/ freezing melting precipitation collection/ coagulation/ conversion Aerosol and Cloud microphysical processes

Scavenging: nucleation & impaction Production: evaporation (recycling)

Aerosol-Cloud Interaction Aerosols Clouds Aerosol Indirect Effect Cloud Indirect Effect(?!) Radiation and Its Impact on Radiation

A Three-Dimensional Cloud-Resolving Model Radiation:  -four-stream including ice cloud Radiation:  -four-stream including ice cloud Cloud Properties: winds, T, P, Qv, lightning 7 Hydrometeors (Q & N) 40+ microphysical conversions Cloud Properties: winds, T, P, Qv, lightning 7 Hydrometeors (Q & N) 40+ microphysical conversions Chemistry: Species: 25g+16c,r+7i Reactions: 35g + 21eq + 32aq + 7h Chemistry: Species: 25g+16c,r+7i Reactions: 35g + 21eq + 32aq + 7h Environment: large-scale forcings and input fluxes Environment: large-scale forcings and input fluxes Aerosols: N of CCN, IN or Multiple mode multi-moment model Aerosols: N of CCN, IN or Multiple mode multi-moment model 1 2 1a2 54a 4b 3b 6a 1b 3a 6b References: Wang and Chang, 1993; Wang et al., 1995; Wang and Prinn, 2000; Wang 2005; Ekman et al., 2004; 2006

 How does tropical deep convection respond to the 1) increase of CCN concentration; 2) change of aerosol chemical composition; and 3) modified aerosol properties at different altitudes?  What are the chemical and physical consequences of aerosol effect on convection? Research Issues Model and Simulations  CEPEX March 8 soundings; 200  100  50 grids with 2  2  0.5 km resolution; 4 hours simulation; supporting runs with 1.0 – 0.25 km horizontal and 250 – 50 m vertical resolutions.  Prognostic CCN (hygroscopic, Aitken or accumulation mode) and IN (water-insoluble); activation of CCN: N = CS k  No “external sources”  90 runs with 30 initial concentrations of CCN, CCN0 from 100 to 5500/cc with a increment of 200/cc; also 50 and 6000/cc; different autoconversion. A 3-D CRM Study (Wang 2005a&b; JGR)

The Response of Cloud Particle Number Concentrations to the Increase of Initial CCN Concentration Note: All runs use the same initial IN profile.

Effective Radius of Hydrometeors vs. Initial CCN Concentration

Total Precipitation vs. Initial CCN Concentration Maximum Coverage of Cloud vs. Initial CCN Concentration

Budget of Water: Supply and consumption of water vapor increases with CCN0; precipitation efficiency varies little with CCN0

3100/cc 700/cc 100/cc 700/cc 100/cc 60 min. 90 min. Surfaces of Updraft  5m/s (Brown) or Downdraft  2 m/s (Blue)

Correlations of microphysical conversions and precipitation: The importance of riming Budget of rain: The dominant role of Ice-phase microphysics

Cloud-Area-Mean Cloud Shortwave Forcing vs. Initial CCN Concentration Domain-Mean Cloud Shortwave Forcing vs. Initial CCN Concentration Radiative Effects

Water vapor redistribution by the modeled convection

Efficiency of vertical transport of gaseous species from the lower to upper troposphere

Scavenging efficiency of a fast soluble gas Y (Cohan et al., 1999): Here, the dilution factor of a species X: CO is used as the reference species in the modeled case with β = 82-88%.

Influence of the Modeled Cloud on Gaseous Chemistry: Total condensed water (0.1 g/kg surfaces in yellow color) and NO 2 (g)/NO(g) ratio (2.0 surfaces in blue color), all from CCN0 = 100/cm 3 run

Redistribution of OH Radicals

Influence of the Modeled Cloud on Heterogeneous Chemistry O 3 (s)  2 pptm (blue), CH 3 OOH(s)  2 pptm (green), and HNO 3 (s)  1 pptm (brown); all from CCN0=100/cm3 run.

Summary CCN CDNC CD r e LWC W Freezing Level & Q Warm Q R Qi Cld area Precip Q I Subl. Total Q R Soluble  Tracers  Likely changing color for continental cases Radiative forcing UT reactions LT photolysis

Important Points:  Many properties of modeled tropical deep convection DOES NOT respond MONOTONICALLY to the change in CCN0.  Aerosol effect could be more substantial in clean environment (low CCN0 cases).  Dynamics AND microphysics are equally important in determining the response of convective cloud to CCN0.  Ice-phase microphysics plays an important role in precipitation formation and development of modeled cloud.  Some conclusions drawn from this study perhaps can be only applied to the specific cloud type.

Importance of Including Prognostic Aerosol Properties in the Model: Results of a Size-Resolving Aerosol Model (Ekman et al., 2004) Note: Observed. max. value (>7nm) in anvil: 2.5·10 4 Modeled max. value (>6nm) in anvil: 5.5 ·10 4 Altitude (km) Horizontal distance (km) Aitken mode (6nm<d<30nm) Concentration (100 cm -3 ) Acc. mode aerosol (d>30nm) Altitude (km) Horizontal distance (km) Concentration (cm -3 )