Presentation
In-Situ Aircraft Observations of Ice Supersaturation and Cirrus Clouds in Global Field Studies
American Geophysical Union Fall Meeting 2012
(2012)
Abstract
Clouds play important roles in the Earth's climate and weather system, and the net forcing of all clouds results in a cooling effect on the Earth's surface. However, clouds remain one of the largest uncertainties in climate models. The IPCC AR4 report shows that both the magnitude and sign of the changes in cloud radiative forcing in response to anthropogenic aerosols are highly uncertain. Cirrus clouds are a type of ice clouds that occur at 235-185K with a net warming effect on the Earth surface. Cirrus cloud formation requires ice supersaturation (ISS), i.e., relative humidity with respect to ice (RHi) greater than 100%. Because ISS is critically related to the ice nucleation processes, it is also an indicator of any changes of ice nucleation and cirrus cloud formation. Here we use the in-situ 1 Hz aircraft observations by the Vertical Cavity Surface Emitting Laser (VCSEL) hygrometer on board the NSF Gulfstream-V research aircraft to analyze the differences of ISS distribution between the Northern and Southern Hemispheres (NH and SH). Our dataset is based on five deployments of the NSF Hiaper Pole-to-Pole Observations (HIPPO) Global field campaigns, including nine Pole-to-Pole transects from the year of 2009 to 2011, extending from 87°N to 67°S, covering four seasons, and the Stratosphere-Troposphere Analyses of Regional Transport (START08) campaign over North America region in April-June 2008. The flight track was mostly over the mid-Pacific Ocean, and also parts of the North America and Australia. We found that the frequency of ISS is much higher in NH than SH for the clear-sky conditions, while the in-cloud conditions show no significant difference between the two hemispheres. Our conclusion is in sharp contrast to the previous aircraft observations which concluded that the SH has higher frequency of ISS for clear-sky conditions based on two flight campaigns at Prestwick, Scotland (55°N) and Punta Arenas, Chile (55°S). We propose a method to separate cirrus cloud formation into three stages: 1) clear-sky ice supersaturated regions (ISSRs), 2) ISSRs with ice crystals, and 3) subsaturated cirrus clouds. Analyses of the maximum RHi inside the clear-sky ISSRs show a wider range of RHi distributions in NH than SH for the same lengths of ISSRs. These findings demonstrate a large difference in cirrus cloud formation processes between the two hemispheres, indicating different ice nucleation thresholds between the two hemispheres. The difference in nucleation thresholds could be influenced by the different aerosol loading in the two hemispheres in terms of number concentration, size and chemical contents, but direct linkage between the ISS distributions, the aerosol properties, and the representativeness of the flight tracks need more investigation.
Disciplines
- Climate and
- Meteorology
Publication Date
December 5, 2012
Citation Information
Minghui Diao and M. A. Zondlo. "In-Situ Aircraft Observations of Ice Supersaturation and Cirrus Clouds in Global Field Studies" American Geophysical Union Fall Meeting 2012 (2012) Available at: http://works.bepress.com/minghui-diao/19/