ALBERT

Description: With a cloud parcel model we investigated how cloud processing and cloud evaporation modify the size distribution and the Angström exponent of an aerosol population. Cloud processing causes a decrease in particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dispersion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is subject to other influences. It is very sensitive for relative humidity, especially between 95% and 100%. In addition, kinetic limitations delay droplet evaporation during cloud dissipation, which hampers a direct relation between the Angström exponent and the relative humidity. Consequently, a direct interpretation of the Angström exponent in terms of aerosol properties that play a role in aerosol-cloud interactions, such as the fine mode fraction, is rather complex.

Description: With a cloud parcel model we investigate how cloud processing and cloud evaporation modify the size distribution and the Angström exponent of an aerosol population. Our study provides a new explanation for the observed variability of the aerosol optical thickness and Angström exponent in the vicinity of clouds. Cloud processing causes a decrease of aerosol particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dispersion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is very sensitive for changes in relative humidity during cloud evaporation, especially between 90% and 100%. In addition, kinetic limitations delay evaporation of relatively large cloud drops, especially in clean and mildly polluted environments where the coarse mode fraction is relatively large. This hampers a direct relation between the aerosol optical thickness, the Angström exponent and the ambient relative humidity, which may severely complicate interpretation of these parameters in terms of aerosol properties, such as the fine mode fraction.

Description: To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterised by an Atlantic Meridional Overturning Circulation (MOC) with a deep water formation at northern latitudes and the absence of such a deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the Atlantic as a whole is a basin with net evaporation, while the Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed, but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is relatively weak but intermittently strong deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model the stability properties of the Atlantic MOC on a retrograde rotating earth are also investigated, showing a similar regime of multiple equilibria as in the present-day case. These results indicate that the present-day asymmetry in surface freshwater flux is not the most important factor setting the Atlantic-Pacific salinity difference and, thereby, the asymmetry in the global MOC.

Description: To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterized by an Atlantic Meridional Overturning Circulation (MOC) with deep water formation at northern latitudes and the absence of such deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the North Atlantic is a basin with net evaporation, while the North Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is strong and highly variable deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model also the stability properties of the Atlantic MOC on a retrograde rotating earth are investigated, showing a similar regime of multiple equilibria as in the present-day case. These results demonstrate that the present-day asymmetry in surface freshwater flux is not a crucial factor for the Atlantic-Pacific asymmetry in the global MOC.