Using VOCALS REx C-130 aircraft measurements to understand aerosol-cloud-precipitation interactions in marine stratocumulus
Terai, Christopher Ryutaro
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The high albedo and weak greenhouse effect of marine stratocumulus provide a strong cooling effect on the regional climate. The concentrations of aerosols, particularly those large enough to act as cloud condensation nuclei, can influence the properties of marine stratocumulus by modifying the cloud droplet size distributions and influencing drizzle production. This dissertation mainly presents results inferred from aircraft measurements taken aboard the NSF/NCAR C-130 research flights flown during the VOCALS Regional Experiment (Oct. to Nov. 2008). These measurements are used to shed light on the interaction between the marine stratocumulus and aerosol particles. Satellite retrievals and simple models are also used to identify and simulate the various processes that modulate aerosol concentrations and drizzle formation. The dissertation is organized into the following three sections: <bold>Characterizing pocket of open cells (POC)</bold> The aerosol, cloud, precipitation, and boundary layer properties from five POCs are studied using measurements from the C-130 aircraft. We found that POCs are observed in a wide range of conditions (inversion heights, surface wind speeds, surface and free tropospheric conditions, and background accumulation-mode aerosol concentrations). Despite the wide range of background conditions that POCs are observed under, they all exhibit low cloud droplet number concentrations, higher column maximum precipitation rates, and a narrow range of accumulation-mode aerosol concentrations in the subcloud layer. In all POCs, an ultraclean layer exists below the inversion, where accumulation-model aerosol concentrations are < 5 cm<super>-3</super>. All POCs are also populated by two types of clouds: deeper, cumuliform, active clouds and thin, stratiform, quiescent clouds. <bold>Estimating the microphysical process rates in POCs</bold> Using the same C-130 aircraft data, the collision-coalescence rates in POCs are estimated to quantify the various budgets of cloud droplet number concentration, drizzle number concentration, cloud water mixing ratio, and drizzle water mixing ratio. The rates in POC clouds are compared to rates in the surrounding overcast clouds. Accretion is found to be the largest contributor to both the loss of cloud droplet number concentration and the conversion of cloud water into drizzle water. Although decreases in cloud droplet number concentration due to accretion are not always stronger in POC compared to the surrounding overcast region, because of the lower cloud droplet number concentrations, the timescales of cleaning out cloud droplets is shorter in the POC. A parcel model with bin microphysics is employed to determine whether collision coalescence processes can explain the high concentration of drizzle drops with radii < 100 &mu m in quiescent clouds of POCs. Collision-coalescence of drops and condensation due to typical updraft speeds found in POCs are unable to explain the high concentration of drizzle drops. <bold>Comparing the precipitation susceptibility of marine stratocumulus from satellite retrievals and a diagnostic precipitation model</bold> A combined MODIS and CloudSat satellite dataset of coincident cloud and drizzle properties and a diagnostic precipitation model with stochastically perturbed microphysics are used to calculate the precipitation susceptibility in marine stratocumulus. The precipitation susceptibility quantifies the fractional decrease in precipitation rate due to a fractional increase in cloud droplet number concentration, such that a positive susceptibility means a suppression of precipitation due to increasing precipitation rate. Precipitation susceptibility are found to be positive in both satellite and model precipitation. Both satellite and model precipitation also show decreases in the susceptibility of probability of precipitation (POP) with increasing LWP, where the values largely agree between the two. Model sensitivities to parameter choices and regional differences in the satellite data are explored. Finally, the connection between susceptibility values and collision-coalescence processes (autoconversion and accretion) in the model are investigated. Although previous studies found that increasing ratio between accretion and autoconversion (<italic>A<sub>c</sub></italic>/<italic>A<sub>u</sub></italic>) explained decreases in susceptibilities, the modeling results show that this is not always the case.
- Atmospheric sciences