Aerosol-Clouds-Precipitation-Climate Initiative (ACPC) Focus on Houston: Large Response of Deep Convection Microstructure, Hydrometeors and Electrification to Urban Pollution

The uncertainties about interactions between aerosols, clouds, precipitation, and climate (ACPC) are a fundamental limitation to our ability to understand past climate change and to project future warming reliably. The ACPC initiative aims at a better scientific understanding of these interactions at a fundamental level. The goal is to identify, disentangle, and quantify signals of impacts of aerosol perturbations on clouds, precipitation, and radiation, taking into account adjustments and feedback processes by synergistically exploiting observations and models across scales.

To advance these goals, ACPC is focusing on the summer deep convection in the Houston area, where relatively pristine clouds from the Gulf meet the Houston urban and industrial air pollution. The ongoing availability of coverage by polarimetric radar and lightning mapping array, along with satellite retrievals of CCN, allows documenting aerosol effects on deep convective clouds. Some of the results so far show that:

(1) Added CCN to deep convective clouds delays the arrival of the main precipitation shaft to the ground by up to 20 minutes.

(2) Added CCN invigorates the convection until near CCN= 1000 cm^-3, after which saturation occurs.

(3) Increasing CCN from ~400 to an optimum of ~1000 cm^-3 increases lightning activity by an order of magnitude. Further increase of CCN decreases lightning.

(4) The role of CAPE is mainly to make deep convection possible. Once the clouds exist, the CCN concentrations appear to dominate the clouds vigor and electrification, within the observed range of CAPE.