A195-02
Boundary Layer Cloud Controlling Factors in the Midlatitudes: Southern versus Northern Ocean Clouds
Tuesday, 15 December 2020: 10:04
Virtual
Catherine M Naud, Columbia University in the City of New York, Palisades, NY, United States, James F Booth, CUNY-City College, New York, United States, Alain Protat, Bureau of Meteorology, Melbourne, Australia, Katia Lamer, Brookhaven National Laboratory, Brookhaven, NY, United States, Roger Marchand, University of Washington, Department of Atmospheric Sciences, Seattle, WA, United States and Greg M McFarquhar, Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, OK, United States
Abstract:
Boundary layer clouds are ubiquitous over the oceans, and play an important role reflecting shortwave radiation. However, their evolution in a changing climate is unclear and current general circulation models (GCMs) still exhibit a number of biases in their representation. Subtropical cloud amount is found to strongly depend on changes in sea surface temperature and inversion strength: warmer surface temperatures work to diminish cloud amount (positive feedback), while the concomitant increase in inversion strength works to enhance it. This behavior, documented in observations, suggests a net positive feedback mechanism in subtropical low clouds that can be used to constrain and evaluate GCMs. However, with much stronger winds and active circulation, extratropical boundary layer clouds are not necessarily controlled in the same fashion, and their net feedback is not yet fully observationally constrained. Therefore, using observations and reanalysis, we explore a number of large-scale cloud controlling factors for their role in modulating low-level cloud properties in the midlatitudes.
Using observations from the DoE ARM fixed site in the eastern North Atlantic (ENA), along with observations from multiple campaigns recently conducted in the Southern oceans (MARCUS, MICRE and CAPRICORN), we examine the large-scale environmental factors that control boundary layer cloud base and top altitude. We find that for both the NH and SH sites the controlling factor that correlates best with cloud boundaries is a measure of lower tropospheric stability (first introduced to identify cold air outbreaks), the difference in potential temperature between the skin and 800 hPa. Furthermore, unlike in the subtropics, inversion or subsidence strength are found to be weaker controlling factors. Finally, we find noticeable differences between the two hemispheres in the overall structure of the boundary layer itself, suggesting that other controlling factors might be important in the extratropics.
To help generalize these results, we will present recent work conducted using satellite observations. In particular we will discuss which aspects of the environment can explain differences between the two hemispheres as well as between extratropical and subtropical clouds.
