It is now recognized that drizzle is a common feature of marine stratocumulus clouds and that it has an important impact on the heat, water, and aerosol budgets of cloud decks. Additionally, analyses of mesoscale organization in marine stratocumulus clouds (typically on a scale of 5 - 100 km) show that drizzle is the key process linking mesoscale organization and cloud microphysical processes. The formation of pockets of open cells (POCs), which is believed to result from drizzle-aerosol-cloud interactions, results in large changes in cloud-fraction and albedo - properties of cloud decks that are significant for the climate. While the formation of POCs is the most dramatic example of these linkages, it is only one of many manifestations of drizzle-aerosol-cloud interactions that determine the characteristics of the stratocumulus layer. As such, there is a pressing need for observational studies that provide quantitative estimates of drizzle rates and of processes directly linked to drizzle (such as the removal of aerosol particles) and for a comprehensive depiction of the kinematic, thermodynamic, and cloud-microphysical structure within different types of mesoscale organization; these can be used to investigate drizzle-cloud structure linkages and for comparison with structures obtained from numerical simulations.
This project will contribute to the quantitative and qualitative needs by deploying a suite of instruments on the NSF C-130 aircraft during VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment). Measurements that will include: vertical cross-sections of radar reflectivity and Doppler velocity, measurements of cloud-base height, cloud condensation nuclei (CCN), aerosol, and hydrometeor size spectra. Reflectivity cross-sections and vertical-plane velocity fields from the radar will be used to identify and characterize mesoscale structures within the stratocumulus-topped boundary layer and, through the use of a conditional sampling approach, to develop a composite depiction of mesoscale structures. Radar cross-sections will also provide a context for other measurements made onboard the NSF C-130.
Expected outcomes include: quantitative estimates, which will be incorporated into integrated VOCLALS-REx datasets, of: precipitation rate (cloud-base and sea surface), cloud-base and cloud-top heights, sub-cloud aerosol and CCN spectra, and the rate of CCN removal due to drizzle; an assessment of probable mechanisms underlying the transition from unbroken to broken marine stratocumulus will be made using the composite depictions of the mesoscale structures.
Two broader impacts are expected: First, in collaboration with other VOCALS investigators, integrated datasets from VOCALS-REx will be compiled and made available to the atmospheric science community. In conjunction with other VOCALS-REx investigators, parameterizations of cloud microphysical processes in terms of cloud macrophysical characteristics will be developed. Second, expertise in the synergistic use of radar, lidar and in situ observations will be developed through graduate student training.
Project Outcomes This grant supported the deployment of the Wyoming Cloud Lidar (WCL) and a Cloud Condensation Nuclei (CCN) counter on the NSF/NCAR C130 during the VOCALS Regional Experiment (VOCALS-REx) and the subsequent analysis of data from the WCL, Wyoming Cloud Radar (WCR), CCN counters, and pre- and post project calibration of the Passive Cavity Aerosol Spectrometer (PCASP) which provides a proxy of the CCN concentration at 1 Hz. VOCALS-REx was an international project to investigate marine stratocumulus in the South-Eastern Pacific. Among the key goals of VOCALS-REx was investigating the interplay between aerosol (both natural and anthropogenic), cloud properties, and drizzle. Given the significant net cooling that marine stratocumulus decks, such as that in the South-Eastern Pacific, exert on global climate, understanding the processes that control the albedo, extent, and fractional coverage of these cloud decks is crucial to improving our understanding of global climate. Cloud radars such as the WCR are ideal for investigating the extent, amount, and organization of drizzle due to their disproportionate sensitivity to large (precipitation-sized) particles. In addition, height-resolved remote sensing measurements, such as those from the WCR and WCL provide key context for the in situ measurements made onboard the C130. As such, a dataset containing a variety of quantities derived from the WCR and WCL data in combination with in situ measurements from onboard the C-130 has been developed. This dataset, referred to as the WCR-WCL Integrated Dataset, is intended to facilitate use of WCR and WCL data both for comparison with results from numerical simulations and to provide auxiliary information and broader context for those using other in situ measurements from the C130. Key quantities included in this dataset include: cloud top and cloud base height, indicators of drizzle and cloud, and radar reflectivity (a proxy for precipitation rate) at cloud base and several fixed heights. This dataset is available alongside other VOCALS-REx datasets through the master data list: http://data.eol.ucar.edu/master_list/?project=VOCALS Key outcomes from our research include the development of a technique for retrieval of cloud droplet number concentration using extinction retrieved from the WCL data which are compared to cloud droplet number concentrations predicted from measured CCN concentrations and updraft velocities. This complements traditional cloud droplet number concentration studies in which CCN data from sub-cloud flight legs are used in conjunction with in-cloud measurements of droplet concentration, thereby resulting in a separation between the in-cloud and sub-cloud legs. Another study, currently nearing completion, has investigated the effect of cloud processes, specifically entrainment and drizzle on cloud properties. This analysis, which forms the core of Jayson Stemmler’s M.S. thesis, complements previous work focusing on the impact of aerosol on cloud properties. In addition, doctoral thesis work, conducted by Gökhan Sever, is examining the effect of drizzle on CCN activation
