Uwe Schimpf

Estimation of Air-Sea Gas and Heat Fluxes from Infrared Imagery Based on Near Surface Turbulence Models

Water surface infrared images were obtained during the GASEX2001 experiment in the South Equatorial Pacific waters and during the laboratory experiment in the AEOLOTRON wind wave tank at University of Heidelberg in October 2004. The infrared imagery during these experiments reveals coexistence of roller type turbulence and intermittent breaking events. Previous interpretations of the infrared images relied on the surface renewal model, in which the water surface is assumed to be occasionally renewed by bursts of turbulent eddies reaching the water surface. A new complementary model (eddy renewal model) based on stationary and spatially periodic turbulent eddies is developed to reinterpret the infrared images of near surface turbulence. The model predicts warm elongated patches bounded by cold streaks aligned with mean wind, being consistent with field and laboratory infrared images. The model yields bulk temperature estimates and mean heat flux estimates that are very close to those based on the surface renewal model.

The Influence of Intermittency on Air-Water Gas Transfer Measurements

This paper theoretically investigates the influence of intermittency on determining average transfer velocities using different measuring techniques. It is shown that all measuring techniques can significantly be biased by intermittency. Mass balance and eddy correlation measurements are only biased when the concentration difference between the air and the water is spatially or temporally inhomogeneous over the measurement interval. Mean transfer velocities calculated either from mean boundary layer thicknesses or from thermographic techniques, which compute the mean transfer velocity either from concentration differences of from time constants, are biased toward lower values. The effects can be large and a simple stochastic bimodal model is used to estimate the effect.

Infrared imaging: a novel tool to investigate the influence of surface slicks on air-sea gas transfer

The influence of surface films on air-sea gas exchange at low and moderate wind speeds is investigated. Observations were made in the small Heidelberg circular wind-wave facility and in coastal and offshore waters south of Cape Cod, New England. The passive controlled flux technique was used to investigate the micro turbulence very near the water surface, which controls the rate of transfer of momentum, heat, and mass across the air-sea interface. The analysis of infrared image sequences allows the estimation of the net heat flux at the water surface, the skin-bulk temperature difference across the thermal sublayer and thus the heat transfer velocity. Using Schmidt number scaling, estimates of the gas transfer velocity are obtained. Experimental evidence shows that increased surface film concentrations suppress near surface turbulence and thus decrease the gas exchange compared to a slick-free ocean interface. If a surfactant is present, turbulent mixing is dampened and direct renewal of the surface is inhibited. A surface slick changes the hydrodynamic boundary conditions in that the length scales of near surface turbulence controlling air sea gas exchange are modified. The micro-scale temperature fluctuations at the water surface indicate that at low wind speeds the transport process is dominated by large-scale turbulence, whereas at higher wind speeds the smallest observed scales dominate the transport.

Measuring important parameters for air-sea heat exchange

The heat transfer between the ocean and the atmosphere is one of the most important parameters governing the global climate. Important parameters include the heat transfer velocity and the net heat flux as well as parameters of the underlying transport model. However, the net heat flux is hard to measure since processes take place in the thermal boundary layer, that is the topmost layer of the ocean less than 1 mm thick. Current techniques rely on three independent measurements of the constituent fluxes, the sensible heat flux, latent heat flux and radiative flux. They depend on indirect measurements of meteorological parameters and rely on a combination of data from different sensors using a number of heuristic assumptions. High relative errors and the need for long temporal averaging reduce the practicability of these techniques. In this paper a novel technique is presented that circumvents these drawbacks by directly measuring the net heat flux across the air-water interface with a single low-NETD infrared camera. A newly developed digital image processing technique allows to simultaneously estimating the surface velocity field and parameters of the temporal temperature change. In particular, this technique allows estimating the total derivative of the temperature with respect to time from a sequence of infrared images, together with error bounds on the estimates. This derivative can be used to compute the heat flux density and the heat transfer velocity, as well as the probability density function of the underlying surface renewal model. It is also possible to estimate the bulk-skin temperature difference given rise to by the net heat flux. Our technique has been successfully used in both laboratory measurements in the Heidelberg Aeolotron, as well as in field measurements in the equatorial pacific during the NOAA GasExII experiment this spring. The data show that heat flux measurements to an accuracy of better than 5% on a time scale of seconds are feasible.

Air-sea gas transfer and micro turbulence at the ocean surface using infrared image processing

To obtain an insight into the transfer process at the air-water interface new techniques for the quantitative investigation of the gas exchange have been developed. The controlled flux technique, CFT (Jahne et al. 1989) uses heat as a proxy tracer for gases to measure the air-sea gas transfer velocity with a high spatial and temporal resolution. The results of a field cruise and a laboratory study are discussed and compared with a model (Schimpf et al.) predicting the sea surface temperature distribution based on surface renewal (Danckwerts 1970). The sea surface temperature fluctuations associated with the interplay of diffusive and turbulent transport give direct insight into the mechanisms of gas transfer. Using infrared image processing the spatial structure of the micro turbulence at the ocean surface is analyzed.

Measurements of the air-sea gas transfer and its mechanisms by active and passive thermography

In order to reliably measure air-sea gas transfer velocities in the field with a high spatial and temporal resolution a new technique has been developed called the controlled flux technique, CFT. The current implementation splits up into two independent techniques using active and passive thermography, respectively. The CFT field instrument has been successfully used during two research cruises along the California Pacific coast (MBL/CoOP, 1995) and on the North Atlantic (CoOP, 1997). In addition to in-situ gas transfer rates, thermography of the ocean surface gives direct insight into the mechanisms of gas transfer. It has been shown that surface renewal dominates the transfer processes even at low wind speeds.

On the investigations of micro turbulence at the water surface using infrared imaging

Laboratory studies in the circular Heidelberg wind wave flume and a field study during the 1997 CoOP (Coastal Ocean Processes) experiment in the Atlantic Ocean have been conducted to investigate the relationship between air-sea gas transfer and the micro-scale temperature fluctuations at the water surface. The controlled flux technique (CFT) uses heat as a proxy tracer for gases to measure the gas transfer velocity. The gas transfer rates derived from the experiments agree with the wind speed dependence of the Wanninkhof relationship. Surface renewal events were observed at different spatial scales and at all wind speeds conditions. The investigations suggest that surface renewal is the dominant process in effecting transport across the aqueous boundary layer. Furthermore, infrared image sequences of the water surface allow a detailed study of near surface turbulence with respect to scale and orientation. The micro-scale temperature fluctuations associated with the turbulent transfer at the interface show the same behavior in the field and laboratory. At low wind speed the large scales are dominant, whereas at high wind speeds the small scales dominate the transport at the interface.