Publication Date

2012-12-07

Availability

Open access

Embargo Period

2012-12-07

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2012-11-12

First Committee Member

Bruce A. Albrecht

Second Committee Member

Benjamin P. Kirtman

Third Committee Member

Brian E. Mapes

Fourth Committee Member

Paquita Zuidema

Fifth Committee Member

Sonia M. Kreidenweis

Abstract

This dissertation includes an overview of aerosol, cloud, and precipitation properties associated with shallow marine cumulus clouds observed during the Barbados Aerosol Cloud Experiment (BACEX, March-April 2010) and a discussion of their interactions. The principal observing platform for the experiment was the Cooperative Institute for Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter (TO) research aircraft that was equipped with aerosol, cloud, and precipitation probes, standard meteorological instruments, and a up-looking cloud radar. The temporal variations and vertical distributions of aerosols observed on the 15 flights show a wide range of aerosol conditions that include the most intense African dust event observed at the Barbados surface site during all of 2010. An average CCN varied from 50 cm-3 to 800 cm-3 at super-saturation of 0.6 %, for example. The 10-day backward trajectories show that three distinctive air masses (originality of air mass as well as the vertical structure) dominate over the Eastern Caribbean (e.g., typical maritime air mass, Saharan Air Layer (SAL), Middle latitude dry air) with characteristic aerosol vertical structures. Many clouds in various phases of growth during BACEX are sampled. The maximum cloud depth observed is about less than 3 km and in most of the clouds is less than 1 km. Two types of precipitation features were observed for the shallow marine cumulus clouds with different impacts on boundary layer. In one, precipitation shafts are observed to emanate from the cloud base with evaporation in the sub-cloud layer (stabilize the sub-cloud layer). In the other, precipitation shafts emanate mainly near the cloud top on the downshear side of the cloud and evaporate in the cloud layer, leading to destabilizing the cloud layer and providing moisture to the layer. Only 42-44 % of clouds sampled were purely non-precipitating throughout the clouds; the remainder of the clouds showed precipitation somewhere in the cloud, predominantly closer to the cloud top. The relationship between aerosol (Na), cloud droplets (Nd), and precipitation rates (R) is addressed to explore aerosol-cloud-precipitation interactions. A robust increase in Nd with increase in aerosol concentrations is documented. Further, a strong linear relation between sub-cloud CCN and cloud-base Nd is observed in updrafts. The sensitivity of Nd to changes in vertical velocity perturbations w´ (i.e., dlnNd /dlnw´), is greater in the regimes of high aerosol concentrations, suggesting a slight increase in updrafts (or w´) in polluted conditions can lead to greater increases in Nd. Suppression of precipitation with aerosol is a common feature during BACEX. To quantify this decrease of precipitation toward higher aerosol concentration, the sensitivity of precipitation to changes in aerosol (i.e., precipitation susceptibility S0 ) is examined. S0 exhibits three regimes and peaks at intermediate range of cloud thickness. Further, the removal of Nd , due to the rain (wet scavenging), makes susceptibility stronger overall. In addition to the aerosol feeding clouds from the sub-cloud layer, small cumuli can alter the aerosol properties of their immediate environment through cloud and precipitation processes. In the warm cumuli studied, the depletion of aerosols near cloud field (so-called cloud halos/shell regimes) are notable, and the reduction of aerosols is more significant in precipitating clouds compared with non-and/or light-precipitating clouds. The modification of boundary layer aerosol by cloud processes is also explored. The comparisons of the thermodynamic structures observed over Africa with those at Barbados indicate that layers below the SAL are moistened by surface fluxes and convective processes as the air masses are advected across the Atlantic over 7-10 days.

Keywords

aerosol effects on cloud; cloud effects on aerosol; Saharan Air Layer (SAL); shallow marine cumuli; precipitation susceptibility; passive tracer (chaff)

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