Abstract
Abstract. We have extensively analysed the interdependence between cloud optical depth, droplet effective radius, liquid water path (LWP) and geometric thickness for stratiform warm clouds using ground-based observations. In particular, this analysis uses cloud optical depths retrieved from untapped solar background signals that are previously unwanted and need to be removed in most lidar applications. Combining these new optical depth retrievals with radar and microwave observations at the Atmospheric Radiation Measurement (ARM) Climate Research Facility in Oklahoma during 2005–2007, we have found that LWP and geometric thickness increase and follow a power-law relationship with cloud optical depth regardless of the presence of drizzle; LWP and geometric thickness in drizzling clouds can be generally 20–40% and at least 10% higher than those in non-drizzling clouds, respectively. In contrast, droplet effective radius shows a negative correlation with optical depth in drizzling clouds and a positive correlation in non-drizzling clouds, where, for large optical depths, it asymptotes to 10 μm. This asymptotic behaviour in non-drizzling clouds is found in both the droplet effective radius and optical depth, making it possible to use simple thresholds of optical depth, droplet size, or a combination of these two variables for drizzle delineation. This paper demonstrates a new way to enhance ground-based cloud observations and drizzle delineations using existing lidar networks.
Highlights
The response of global mean surface temperature to emissions of greenhouse gases from human activities remains highly uncertain (e.g. Hawkins and Sutton, 2009)
Ground-based observations for mid-latitude continental clouds are primarily provided by the Atmospheric Radiation Measurement (ARM) Climate Research Facility (Stokes and Schwartz, 1994), the NASA Aerosol Robotic Network (AERONET; Holben et al, 1998), the European project Cloudnet (Illingworth et al, 2007) and its descendant ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network)
New method Yes where coefficient A is 920 μm and the critical value r∗ varies with cloud optical depth τ
Summary
The response of global mean surface temperature to emissions of greenhouse gases from human activities remains highly uncertain (e.g. Hawkins and Sutton, 2009). To improve representations of cloud properties and their interactions with radiation and water budget in models, sustained efforts have been made to observe and study marine low-topped clouds Similar efforts have not been made for mid-latitude continental stratus and stratocumulus clouds, despite their strong links to local weather and climate (Del Genio and Wolf, 2000; Kollias et al, 2007), and their high occurrences compared to other cloud types over land (Sassen and Wang, 2008). Ground-based observations for mid-latitude continental clouds are primarily provided by the Atmospheric Radiation Measurement (ARM) Climate Research Facility (Stokes and Schwartz, 1994), the NASA Aerosol Robotic Network (AERONET; Holben et al, 1998), the European project Cloudnet (Illingworth et al, 2007) and its descendant ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network). At the ARM Oklahoma site, low stratiform clouds have been investigated in a variety of studies, from short-period field campaigns along with airborne and/or spaceborne measurements
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