Investigation of air-sea fluxes over the Southern Ocean using an eddy-covariance technique and parameterization using stability functions

© 2019 Sushma Reddy Chen ReddyThe Earth is an integrated system that consists of sub-systems that interact and influence each other. These interactions have an important influence on the understanding of weather and climate of the earth system. Air-sea interactions are one such interaction that affects the Earth's system — thus making it essential to understand the physical processes that affect the prediction and forecast of the weather and climate. The present state of art climate and numerical weather prediction models use bulk models which are based on Monin-Obukhov similarity theory and Charnock's relations to determine the fluxes across the air-sea interface. The COARE 3.5 model is the best performing model available, and it is seen that the model underestimates the fluxes at higher wind speeds. Hence, to avoid any assumptions and circular dependencies, we need to build a simple parameterization of coefficients of fluxes to determine fluxes. Eddy Covariance, the purest form of flux calculation, is used to develop the parameterization. Eddy covariance relies on high-frequency 3-D winds, which, on ships, are contaminated by platform motions. However, in the absence of reliable accelerometer data, or a failed collocated accelerometer, calculating these motions is difficult. Here, in this study, we studied if the ship's motion reference data can replace external collocated accelerometer data. We have characterized that for the anemometer mounted on the foremast of the R/V Investigator, and there is a lag of 1.4 sec in the ship's motion reference unit data. Hence, we can correct the wind speeds for platform motions using the ships' motion data after adjusting to the lag. The spectral speak due to the platform motions observed in the measured raw data by anemometer is removed after the corrections performed by the ship's data. Hence, achieving the redundancy of the external collocated accelerometer, GPS receiver, and heading sensors. The fluxes computed from the eddy covariance technique are used to get a simple parameterization to estimate fluxes. Here, we have developed the coefficients of drag, latent heat fluxes in terms of simple functions of Reynolds and bulk Richardson number, which are physically dependent on velocity and stability of the atmospheric boundary layer. The model proposed does not depend on any assumptions or does not have any circular dependencies. The coefficient of sensible heat flux could not be parameterized as we observed that there is no dependence on Reynolds number in the neutral, stable region. The proposed model is performing better compared to that of the COARE 3.5 model at higher wind speeds. Gas transfer across the air-sea interface is challenging to measure, and the existing relationships for the gas transfer velocity with wind speeds have a high variance at high wind speeds. It is essential to measure gas transfer velocities in the Southern Ocean as it is least sampled with the rough environment and high surface waves. It is estimated that the Southern Ocean is the largest sink of anthropogenic carbon dioxide, with about 40% of the total world ocean sink. Gas transfer velocities of CO_2 in the Southern Ocean are measured, and it is found that the results obtained are within the range that is reported by the previous researchers. However, there are no sufficient data points, and the variance in the data is high to get any conclusions from the results obtained