Joint Air-Sea Interaction Experiment Research Articles

Determination of the Rate of Dissipation of Turbulent Energy from Simultaneous Temperature and Velocity Shear Microstructure Measurements

Abstract Spectra of turbulence have been examined for both temperature gradient and velocity shear. The data for this comparison are 10–15 m segments of vertical microstructure profiles (at depths of 5–100 m) obtained during the 1978 Joint Air Sea Interaction experiment (JASIN). From the simultaneous measurement of them two microstructure quantities, the universal spectral constant q (the least principal rate of strain of the velocity spectrum) has been determined to be 3.7 ± 1.5. As well, the dissipation rate has been calculated from the high-wavenumber cut-off of the temperature microstructure spectra (ϵB) and from velocity shear (ϵSH). For a range of values from 8 × 10−9 to 5 × 10−7 m2 s−3 these two measures, ϵB and ϵSH, agree to within a factor of 2 on average. And finally, estimates of ξθ (temperature dissipation rate), ϵ and mean temperature gradient have been used to estimate a mixing efficiency, Γ = 0.24.

Determinations by Seasat of atmospheric water and synoptic fronts

The Seasat Scanning Multichannel Microwave Radiometer (SMMR)1,2 determined the total atmospheric water vapour, qt, over 600-km wide swaths with a resolution of 54 km. Using radiosonde data from the Joint Air–Sea Interaction experiment, JASIN3, we show here that the SMMR qt distributions can be used to detect the position of atmospheric fronts in the lower troposphere. Unlike visible and IR radiometry these SMMR determinations are not hampered by extensive cirrus or by lack of frontal cloud. Advantages of the SMMR over previous passive microwave instruments on satellites such as Nimbus 5 and 6 (refs 4–9) are: the use of more channels, allowing better discrimination between the effects of liquid water, water vapour and sea state; and improved spatial resolution. The SMMR performance has been evaluated at several workshops10–14. Our results15 show that the SMMR qt determinations have similar accuracy to in situ radiosonde measurements.

Evaluation of the Seasat wind scatterometer

Surface wind velocities have been derived from backscatter measurements of the ocean surface made by a satellite-borne, microwave sensor. Comparisons with high-quality surface-based measurements obtained during the Joint Air-Sea Interaction experiment are described. The accuracy of the scatterometer winds at this mid-latitude site, + or 1.6 m/s in speed and + or - 18 deg in direction, for winds between 3 and 16 m/s is within the design specification.

A comparison of SEASAT 1 altimeter measurements of wave height with measurements made by a pitch‐roll buoy

Measurements of significant wave height made by a pitch‐roll buoy during the Joint Air‐Sea Interaction Experiment (JASIN) are compared with measurements made at the same time by the SEASAT 1 radar altimeter as processed by the Jet Propulsion Laboratory SEASAT algorithms. Comparisons are made for eight occasions during which the significant wave height lay between 0.7 and 2.3 m. The radar altimeter is found to underestimate slightly the wave heights when compared with the pitch‐roll buoy. The mean ratio of the measurements is 0.96±0.04, and the standard deviation of individual comparisons about this mean is 0.10.

Determination of Surface Stress by Seasat-SASS: A Case Study with JASIN Data

The values of sea surface stress determined with the dissipation method and those determined with a surface-layer model from observations on F.S. Meteor during the Joint Air-Sea Interaction (JASIN) Experiment are compared with the backscatter coefficients measured by the scatterometer SASS on the satellite Seasat. This study demonstrates that SASS can be used to determine surface stress directly as well as wind speed. The quality of the surface observations used in the calibration of the retrieval algorithms, however, is important. This sample of measurements disagrees with the predictions by the existing wind retrieval algorithm under non-neutral conditions and the discrepancies depend on atmospheric stability.

Seasat and Jasin

Abstract Microwave measurements on board Seasat during its 3 months of operation allow estimation of several important oceanographic and meteorological quantities to be made, including surface wind and wave fields. Since it was an experimental system comparison with conventional data is required. This note briefly reviews the measurements and describes some preliminary work in which high-quality data from the Joint Air-Sea Interaction experiment have been used to assist in the validation of each of the sensors.

Remote acoustic sensing of internal wave fields in the North Atlantic

A towed multifrequency acoustic backscattering system was used during the joint air-sea interaction experiment (JASIN) to detect the short period internal wave field which existed in the experiment area. The experiment was conducted from July–September 1978 in an area centered at 12.5°W 59°N. The data to be presented were obtained during the second phase of the experiment, which lasted from the latter part of August to mid-September. Graphic acoustic records indicating the nature of the internal wave field which was detected and three-dimensional plots revealing the depth variability of the mixed layer base as a function of time during two 24-h box tows will be presented. A possible mixing event was detected and will be discussed. [Work supported by NORDA.]

The Joint Air-Sea Interaction Experiment—JASIN 1978

In June the first of 17 ships and aircraft left their home bases to participate in the Joint Air-Sea Interaction (JASIN) Experiment. This paper summarizes the scientific and operational plans for the field experiment.

Near-surface current measurements

Near-surface current measurements made in the JASIN '72 (Joint Air-Sea Interaction) experiment have been intercompared. There is close agreement between the measurements of a 10-m VACM (Vector averaging current meter) under a toroid moored in 2500 m of water and of 12-m drifters tracked from a ship: differences were only 2 cm s −1 in currents ranging from 10 to 30 cm s −1. Comparisons of the 10-m VACM and a 12-m Aanderaa meter revealed that the Aanderaa vane failed to follow the frequent reversals associated with the surface gravity wave field and hence the instrument biased currents high (by factors of between 1 and 10). A model demonstrated this failing: it is suggested that VACM measurements also bias high (by 10%) but no sure evidence could be found.