Monitoring Aerosol ­ Cloud Interactions in Liquid Water Clouds

Research output: ThesisDissertation (TU Delft)

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Abstract

This thesis presents a new method for the continuous observation of aerosol-cloud interactions with ground-based remote sensing instruments. The described method is based on the measurements from UV lidar, radar and radiometer. All of those instruments are capable of obtaining continuous, high-resolution measurements. In order to facilitate its easy implementation to measuring sites the method is based on a standardized Cloudnet data format. The main goal is to monitor the change in the cloud droplet concentration, as obtained from the measurements by cloud radar and radiometer, to then compare it to the aerosol background below the cloud, represented by the attenuated backscatter measured by UV lidar. The response of the cloud to the aerosol background can best be measured when the amount of available water is kept constant. Hence the measurements from the radiometer, specifically the derived liquid water path (LWP), which is used to constrain the cloud response. Based on the value of the LWP, analyzed data is divided into bins and for each of these the relation between cloud droplet effective radius and integrated value of the attenuated backscatter are calculated. This metric is called ACIr and is used to describe the strength of the relation between the clouds microphysical properties and the aerosol background below the cloud.
The method was first tested and applied to pristine marine clouds as measured at the Graciosa Island in the Azores. The application was then extended to the Cabauw site located in the Netherlands. On both sites a decrease in the cloud size was observed in combination with a simultaneous increase of the aerosol loading below the cloud. This relation was particularly strong for a mid range of the LWP, between 40 and 60 gm-2 LWP for the cases from Azores and between 60 and 105 gm-2 for the cases from the Netherlands. These results indicate that the process of aerosol-cloud interactions is a predominant one only under those conditions where a mid amount of water is available. When the amount of available water is less than 40 gm-2 this process is harder to observe, due to the initial stage of cloud formation. In the case of LWP above 105 gm-2 other cloud processes, such as collision and coalescence, seem to be predominant. The results from the analysis of the Cabauw dataset, which was the more extensive dataset, also made clear that updraft within the cloud plays a significant role in invigorating aerosol particles into becoming cloud droplets. A possible extension of the presented method includes obtaining optical cloud extinction from the UV lidar measurements. The presented retrieval method can obtain very reliable results when compared to the simulated results. Hence the cloud optical extinction can be used as a proxy of the cloud properties and the described method of monitoring aerosol-cloud interactions can be applied to measurement sites where only UV lidar and radiometer are present. This thesis shows that ground-based remote sensing instruments used in synergy can efficiently and continuously monitor aerosol–cloud interactions.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Russchenberg, H.W.J., Supervisor
Award date21 Jan 2021
DOIs
Publication statusPublished - 2021

Keywords

  • aerosol
  • clouds
  • aerosolclouds interactions
  • remote sensing

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