Field Observations of Sea Spray, Gas Fluxes and Whitecaps (SEASAW)

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment


It is widely accepted that the activities of mankind are leading to changes in global climate; however, the extent of those changes is far from certain due to the complexity of the climate system and the number of interacting processes involved. A central process in all climate studies is the interaction of radiation / incoming solar (shortwave) radiation, and outgoing infra-red (longwave) radiation / with the atmosphere and in particular with clouds. Clouds present a large source of variability, and uncertainty, in the radiative balance due both to the variation in extent, location, and type of cloud, and to the strong variation in properties such as reflectivity with changes in the concentration and size distribution of cloud droplets or ice crystals. Marine stratocumulus clouds / extensive sheets of low level clouds / play a major role in the global radiation balance. The size and number of their cloud droplets depends strongly on the number of aerosol particles available for droplets to form on. Sea-salt aerosol are a major source of such condensation nuclei. The generation of sea salt aerosol occurs through evaporation of water droplets generated by bubble bursting and spray torn from wave tops by the wind. The size and number of droplets produced, and hence of the aerosol produced, varies greatly with conditions: wind speed, wave state, the presence of surface films produced by plankton, etc. In order to accurately represent marine clouds, and so get the radiation balance correct in climate models, we must first determine how much aerosol and of what size, is generated under any given conditions. There is considerable uncertainty in this, particularly for the smallest aerosol, which are most relevant to climate processes. This project will measure the amount of aerosol at different sizes generated near the surface and transported upwards into the atmosphere, along with the wind speed, wave size, white-capping, and heat and moisture transfers under a wide range of different conditions. Measurements of aerosol very close to the sea surface will enable aerosol generation events to be related directly to individual breaking waves. The results will be used to improve our understanding of aerosol generation, and ultimately the fidelity of cloud representation within climate models. Another major uncertainty in modelling the future climate is the rate at which CO2 is transferred between the atmosphere and the oceans. CO2 absorbs infra-red radiation; an increase in CO2 in the atmosphere due to the burning of fossil fuels means more infra-red radiation is absorbed, causing a warming of the atmosphere. The observed increase in CO2 in the atmosphere is less than might be expected given the amount of fuel burnt. This is due in large part to the absorption of CO2 by the oceans. Although CO2 is absorbed by the oceans as a whole, on regional scales the transfer of CO2 between the atmosphere and ocean can occur in either direction, depending upon the local concentrations of the gas in the air and water. The rate of the transfer depends also on the wind speed, bubble formation, sea-state, and surface films. As with aerosol production, there are large uncertainties in how the rate of transfer varies with conditions / by a factor of two or more under some conditions. Direct measurements of the transfer of CO2 between the atmosphere and ocean, along with those of the meteorological and ocean conditions, will be used to reduce the uncertainty in the parameterization of CO2 transfer. This will in turn allow improvements to long term climate models.


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Description First direct eddy covariance measurements of size resolved sea spray aerosol fluxes.

First demonstration of the impact of wave state on the sea spray aerosol flux - a wave roughness Reynolds number explains up to twice the variance in the flux measurements as wind speed alone. A new sea spray flux parameterization is developed in terms of the wave roughness Reynolds number.

Measurements of the production flux of sea spray per unit area of whitecap demonstrates that the size distribution varies with wave state, invalidating the assumed constant production flux used by many models.
Sectors Environment