James G. Hudson — Desert Research Institute
Stephen R Noble — Desert Research Institute
Category
Warm low clouds, including aerosol interactions
Description
Mean cumulative drizzle drop concentrations (Nd) larger than various threshold diameters for various modal ratings denoted by the legend colors 1-8 (1 is most bimodal; 8 is most unimodal). In MASE stratus there is more drizzle in clouds grown on unimodal CCN (high ratings). In ICE-T cumuli there is more drizzle in clouds grown on bimodal CCN (low ratings). Similar results for drizzle LWC.
Bimodal spectra observed by DRI CCN spectrometers in two aircraft campaigns with different cloud types showed opposite relationships with droplet concentrations (Nc). Clouds associated with bimodal CCN had higher Nc than clouds associated with unimodal CCN in MASE stratus clouds whereas clouds associated with bimodal CCN in ICE-T cumuli had lower Nc than clouds associated with unimodal CCN. There are commensurate contrasts of droplet mean diameter and spectral width. Drizzle relationships with CCN modality are correspondingly opposite: order of magnitude fewer drizzle drops and lower drizzle liquid water content in clouds closest to bimodal CCN than clouds closest to unimodal CCN in stratus, but in cumuli order of magnitude more drizzle in clouds closest to bimodal CCN compared to clouds closest to unimodal CCN (Figure). Moreover, when observed spectral modalities from extreme bimodal to extreme unimodal are sorted on an eightfold scale, drizzle amounts in clouds closest to CCN of each modality group follow in the same order; i.e., mean drizzle amounts for each modality group are in the same bimodality/unimodality order for all eight groups in both projects. Therefore, the main cause of bimodal CCN in stratus clouds, chemical transformations of trace gases dissolved within droplets, produce an additional indirect aerosol effect (IAE) on these most climatically important clouds; higher droplet concentrations and precipitation suppression. Anthropogenic contributions to sulfur dioxide, ozone, hydrogen peroxide and nitrogen oxides that generate chemical cloud processing are yet another IAE. For cumuli where coalescence processing dominates, opposite cloud processing effects ensue, lower droplet concentrations and precipitation enhancement associated with more bimodal CCN.
Surface aerosol modality at SGP was correlated with cloud fraction, cloud base altitude, liquid water path, vertical wind and precipitation. When these correlations were time adjusted to account for aerosol movement from clouds to the surface, clouds were consistently implicated to be the source of aerosol bimodality. Downdrafts, especially those associated with precipitation, were the most prolific conveyors of bimodal aerosol.Because of around-the-clock year-round coverage surface measurements provide more data more economically than aircraft campaigns. Thus, aerosol and remote sensing measurements at ARM sites can be used to advance understanding of cloud processing.
