A Framework for Aerosol-Cloud Interactions Monitoring

Abstract:

A broad range of strategies have been used to study Aerosol-Cloud Interactions (ACI). However, the wide scope of methods and scales used makes it difficult to quantitatively compare result from different studies.

In this paper, we propose a method of aerosol-cloud interaction monitoring based on widely available remote sensing instruments and easily applicable at many different observatories. This method provides a way of identifying cases where a change in the aerosol environment causes a change in the cloud. In this scheme we attempt to use (as far as possible) the observed signal from lidar and radar. For an aerosol proxy we use the attenuated backscatter (sensitive to aerosol concentration) and to obtain information about changes in the cloud we use the radar reflectivity factor (sensitive to cloud droplet size and concentration). Assuming a positive dependence between the number concentration of cloud droplets and the number concentration of aerosol we expect that an increase of the attenuated backscatter coefficient will correspond to a small increase of the radar reflectivity factor (due to the increase of cloud droplets concentration). However, the slope of this correlation will vary. A number of factors, such as meteorology or cloud drop microphysical properties, can influence changes in a cloud. For that reason we put a constraint on the liquid water content using liquid water path information from microwave radiometers. This limitation ensures that the variability in the cloud will be primarily due to changes in microphysical properties associated with the variation in aerosols. Further, we limit the cases only to non-precipitating, low-level stratiform and stratocumulus clouds without drizzle. 

Although this method is based on a synergy of instruments, we use widely available systems for an efficient evaluation of the aerosol influence on the cloud. The main advantages of this scheme are the use of direct observables from widely spread remote sensing instruments and the lack of assumptions about the microphysical properties of the clouds. We plan to implement this framework over the cloud profiling sites of the ACTRIS network in Europe to enable monitoring of the ACI close to real-time. We believe that obtaining data in the same format over multiple regions will result in a better understanding of the ACI.