Frontal Air-Sea Interaction Experiment Research Articles

The Time-Varying Calibration of an Airborne Lyman-α Hygrometer

Abstract Calibration of an airborne Lyman-α hygrometer against simultaneous measurements of humidity from a dewpoint hygrometer shows that Lyman-α biases drift with time. Analyses of low-level flight data from four days during the 1986 Frontal Air–Sea Interaction Experiment indicate that Lyman-α gains are constant for each Right (∼ 3 h duration) but different on each day. If not explicitly accounted for, the time-varying bias can introduce a significant error in calculated humidities from the Lyman-α hygrometer.

Dependence of the microwave radar cross section on ocean surface variables: Comparison of measurements and theory using data from the Frontal Air‐Sea Interaction Experiment

The purpose of this investigation was to study the ability of theoretical radar cross section (RCS) models to predict the absolute magnitude of the ocean radar cross section under a wide variety of sea and atmospheric conditions. The dependence of the RCS on wind stress (as opposed to wind speed) was also studied. An extensive amount of experimental data was acquired during the Frontal Air‐Sea Interaction Experiment (FASINEX). This consisted of RCS data from the NASA‐Jet Propulsion Laboratory Ku band scatterometer mounted on a C130 aircraft (10 separate flights), as well as a wide variety of atmospheric measurements (including stress) and sea conditions. Measurements across an ocean front demonstrated that the vertical polarization (V‐pol) and horizontal polarization (H‐pol) radar cross section were more strongly dependent on wind stress than on wind magnitude. Current theoretical models for the RCS, based on stress, were tested with this data. In situations where the Bragg scattering theory does not agree with the measured radar cross section (magnitude and angle dependence), revisions are hypothesized and evaluated. For example, the V‐pol theory worked well in most cases studied, while the H‐pol theory was usually too low by about a factor of 2 at incidence angles of 50° and 60°.

A case study of wave‐current interaction near an oceanic front

High resolution directional wave spectrum data were obtained from two NASA airborne radars during the Frontal Air‐Sea Interaction Experiment (FASINEX) in February 1986. The observations show a significant change in the wave number spectrum across the front. On the basis of surveys from a towed sensor and on satellite imagery, the front location and current field are estimated. A numerical model is developed for the wave‐current interaction and is used to model the wave refraction across the frontal current. A parametric study is performed to demonstrate the effects of current meandering. The main consequence of meandering is the formation of caustics and shadow zone regions in which the wave energy is significantly enhanced or reduced. Spectral simulation along the aircraft track reveals a reduction of more than 60% in wave energy in the shadow zone; this is consistent with the observations.

The Marine Boundary Layer in the Vicinity of an Ocean Front

Abstract Aircraft observations obtained during the Frontal Air–Sea Interaction Experiment (FASINEX) are used to investigate the structure of the marine atmospheric boundary layer in the vicinity of an ocean front. A quasi-stationary sea surface temperature (SST) discontinuity of 2°C was maintained across the frontal zone throughout the duration of the experiment The primary response of the atmosphere to changes in the SST was observed in the surface-related turbulence fluxes. In the case of warm air flowing over cold water, the boundary layer appears to develop an internal boundary layer (IBL) in response to the sudden change in the sea surface temperature. The organized updrafts and downdrafts within this layer collapse with entrainment–detrainment processes in these cells dominating the turbulence statistics. The IBL grows in response to the wind shear in this layer, although the surface shear stress is much smaller on the colder side of the front than on the warm. The depth of the IBL, and, in the abse...

Atmospheric turbulence structure in the vicinity of an oceanic front

Fast response data taken aboard the National Oceanic and Atmospheric Administration WP‐3D aircraft are used to determine the structure of atmospheric boundary layer turbulence on either side of a well‐developed sea surface temperature front southwest of Bermuda. The data were taken on February 17, 1986, as part of the Frontal Air‐Sea Interaction Experiment (FASINEX). A broad region of low‐humidity air extending from 15 km to 35 km south of the front is probably due to the presence of a frontally induced secondary circulation. Evidence for a secondary flow is found in both the time series of atmospheric variables and the statistics obtained from conditionally sampled updrafts and downdrafts in the transect across the SST front. Larger sea‐air temperature and humidity differences on the warm (south) side of the front give rise to surface layer sensible and latent heat and buoyancy fluxes that are larger than those on the cold side. Turbulence structure appears to be influenced as much by the presence of strong wind shear at the top of the boundary layer as by differing conditions at the surface on either side of the front. A larger rate of entrainment on the warm side of the front is indicated by the greater influence of low‐momentum air from the overlying shear layer on updrafts in the upper part of the mixed layer, as well as the more frequent overturning of cool/moist updrafts and warm/dry downdrafts. It is conjectured that the larger entrainment rate is due to the interaction between the inversion layer and more energetic updrafts produced by greater surface forcing on the warm side of the front.

Interaction between Net Shortwave Flux and Sea Surface Temperature

Abstract Ocean surface shortwave irradiance estimates, From GOES satellite data computed using the model of Gautier and Frouin (1985), are compared to in situ measurements from research vessels and buoys during the frontal air-sea interaction experiment (FASINEX). They reveal that the satellite method overestimates percentage cloudiness during fractional cloud cover and large satellite viewing angles. An empirical relationship, based on physical constraints, is developed to correct for the overestimation of percentage cloud cover under these conditions. Subsequent comparisons of the corrected satellite estimates with in situ measurements show a root-mean-square difference of 10% of the daily mean values, with a mean difference between satellite and in situ data of 1–10 W m−2. From fields of corrected satellite estimates a cloudiness parameter, called the equivalent cloud amount, is used to examine the influences and feedbacks between the clouds and the sea surface temperatures (SST's). Correlations betwee...

Ocean Radar Backscatter Relationship with Near-Surface Winds: A Case Study during FASINEX

A case study of the ocean radar backscatter dependence on near-surface wind and wind stress is presented using the data obtained on February 18, 1986 during the Frontal Air-Sea Interaction Experiment. The particular wind-wave conditions and their variations across a sharp sea surface temperature front are described. The small change in wind speed across the front cannot account for the large change in wind stress implying significant changes in the drag coefficient and surface roughness length. The results strengthen the hypothesis that radar backscatter is closely correlated to wind stress, and therefore, could be used for remote sensing of the wind stress itself over the global oceans.

Small-scale structures in the north-west Atlantic sub-tropical front

Mixing in the ocean thermocline is thought to occur in part by random superposition of internal waves, leading to occasional episodes of shear instability1. Statistical models of the resulting mixing have been formulated2,3 which are roughly consistent with the few available direct estimates of average mixing rates4–6. However, clear evidence of the direct association between individual mixing patches and wave instability has been lacking7. Here we report the use of a new fine- and microstructure profiler during the Frontal Air–Sea Interaction Experiment (FASINEX)8 to investigate mixing processes in the sub-tropical front of the north-west Atlantic in great detail. The novel combination of fine-scale and microstructure sensors on one vehicle provided sufficient resol-ution to reveal quantitative evidence of shear instability in the thermocline. The strongest sub-surface mixing was found in association with small-vertical-scale internal waves that may have been influenced by the velocity and density structure of the front.

Frontal surveys with a towed profiling conductivity /temperature/depth measurement package (SeaSoar)

In April 1986, RV Oceanus participated in the Frontal Air-Sea Interaction Experiment (FASINEX1) in the region 27-29 N, 68-71 W. The objective was to survey a pronounced oceanic front2–4 and its effect on the oceanic and atmospheric boundary layers. Although moorings have yet to be recovered, and much of the data analysis is as yet only preliminary, it is possible to report here the typical density structure of a front as derived from SeaSoar5,6 (Fig. 1a) observations, which were analysed within a few hours of collection. Previously unreported detail is revealed, including subducted pycnostads (areas of weak vertical density gradient) with anomalous temperature/salinity signature, splitting of the frontal jet and three-dimensional structure not apparent in the surface temperature field.

The Frontal Air-Sea Interaction Experiment (FASINEX); Part II: Experimental Plan

The Frontal Air-Sea Interaction Experiment (FASINEX) is a study of the response of the upper ocean to atmospheric forcing in the vicinity of an oceanic front in the subtropical convergence zone southwest of Bermuda, the response of the lower atmosphere in that vicinity to the oceanic front, and the associated two-way interaction between ocean and atmosphere. FASINEX is planned for the winter and spring of 1985-1986 with an intensive period in February and March 1986 in the vicinity of 27 deg N, 70 deg W, where sea-surface temperature fronts are climatologically common. Measurements will be made from buoys, ships, aircraft, and spacecraft. A previous article gave a brief history of FASINEX and presented its scientific goals. This article describes the FASINEX experimental plan.

The Frontal Air-Sea Interaction Experiment (FASINEX); Part I: Background and Scientific Objectives

Abstract The Frontal Air-Sea Interaction Experiment (FASINEX) is a study of the response of the upper ocean to atmospheric forcing in the vicinity of an oceanic front in the subtropical convergence zone southwest of Bermuda, the response of the lower atmosphere in that vicinity to the oceanic front, and the response of the associated two-way interaction between ocean and atmosphere. FASINEX is planned for the winter and spying of 1985/86 with an intensive period in February and March 1986 in the vicinity of 27°N, 70°W, where sea-surface-temperature fronts are climatologically common. Measurements will be made from buoys, ships, aircraft, and spacecraft. This article gives a brief history of FASINEX and presents its scientific goals. An article in next month's BULLETIN will describe the experimental plan.