Impact of Increased Aerosol Load on Mid-latitude Cirrus Clouds

Tuesday, 20 March 2018
Iriarte (Hotel Botanico)
Amit K Pandit, Physical Research Laboratory, Physics, Ahmedabad, India, Jean-Paul Vernier, Science Systems and Applications, Inc., Hampton, United States, Harish Gadhavi, Physical Research Laboratory, Ahmedabad, India, Thomas Duncan Fairlie, NASA Langley Research Center, Hampton, United States and Achuthan Jayaraman, National Atmospheric Research Laboratory, Gadanki, Tirupati, India
Abstract:
The sulfate aerosol load in the upper troposphere and lower stratosphere region has increased by a factor of 2-3 compared to the background levels due to several tropical volcanic eruptions since 2005. This increase is observed mostly over the Northern Hemisphere mid-latitudes in contrast to Southern hemisphere mid-latitudes where the sulfate aerosol concentrations remained close to background level. Several studies suggest the potential of stratospheric sulfate aerosols in modifying the microphysical and radiative properties of cirrus clouds by shifting the freezing mechanism from homogeneous to heterogeneous freezing. This results into optical thinner clouds with fewer and larger ice-crystals. However, robust observational evidences for the same are not available yet.

A recent study has reported an 8% decrease in cirrus cloud reflectance over Northern hemisphere mid-latitudes during 2001-2011 which was linked to increase in sulfate aerosol concentration. Cirrus reflectance in that study was derived from Moderate Resolution Imaging Spectroradiometer (MODIS) sensors onboard Aqua and Terra satellites which are not sensitive enough in detecting thin cirrus clouds, especially during the presence of multi-layered clouds. Our study investigates the role of down-welling sulfur aerosols from lowermost stratosphere to upper troposphere in modifying the cirrus cloud microphysical (ice-crystal number concentration, effective diameter and ice-water content) and optical properties (optical thickness, depolarization ratio and color ratio) by using collocated observations of cirrus clouds from space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and Infrared Imaging Radiometer (IIR) onboard Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) over Northern hemisphere mid-latitudes since 2006. Properties of cirrus clouds over Southern Hemisphere mid-latitudes are also compared with those of Northern Hemisphere mid-latitudes.

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