Hypotheses have been proposed for decades about cloud condensation nuclei (CCN) aerosol effect on delaying the warm rain process, invigorating deep convective cloud vertical development, and enhancing mixed-phase process. Observational support has been only qualitative with mixed results due to lack of regional measurements of CCN, while simulations have not produced a robust consensus. Quantitative assessment of these relationships became possible with the advent of CCN retrievals from satellites; when combined with measurements by polarimetric radar and Lightning Mapping Array (LMA), tracking convective cells observed on radar and examining precipitation properties throughout the cells' life cycle has permitted the study of the impact of CCNs on cloud and precipitation characteristics.
We composited more than 2000 well-tracked cells in the Houston region and stratified them by CCN, convective available potential energy (CAPE) and urban/rural locations. The analyzed cell properties include reflectivity (Z), differential reflectivity (ZDR) and LMA data. The results show that added CCN to deep convective clouds delays the initiation of precipitation by up to 20 minutes. Added CCN invigorate the convection until saturation near CCN = 1000 cm^-3; increasing CCN from ~400 to an optimum of ~1000 cm^-3 increases lightning activity by an order of magnitude. A further increase of CCN decreases lightning rates. Adding CAPE enhances lightning only under low CCN <500 cm^-3. Urban area enhances lightning for the same CCN only under low CCN conditions. Urban heat island cannot explain this observation. In summary, CAPE is essential for the initiation of deep convection. It has been believed that CAPE and lightning are positively related. This is indeed the case when CAPE is low. But when CAPE is high, which means that deep convection is already in progress, aerosols dominate the lightning activity. These insights lead to refinement of the physical hypotheses which provide impetus for a field campaign in the Houston area.
- Zhang, Renyi University Distinguished Professor