Johnny A. Johannessen

Dedicated gravity field missions—principles and aims

Abstract Current knowledge of the Earths gravity field and its geoid, as derived from various observing techniques and sources, is incomplete. Within a reasonable time, substantial improvement can only come by exploiting new approaches based on satellite gravity observation methods. For this purpose three satellite missions will be realised, starting with CHAMP in 2000, followed by GRACE in 2002 and GOCE in 2004. Typical for all three missions is their extremely low and (almost) polar orbit, continuous and three-dimensional tracking by GPS and their ability to separate non-gravitational from gravitational signal parts. A further amplification of the gravity signal is achieved by inter-satellite tracking between two low orbiters in the case of GRACE and by gravity gradiometry in the case of GOCE. The rationale of GOCE will be discussed in more detail. The missions have a wide range of applications in solid Earth physics, oceanography, ice research, climatology, geodesy and sea level research.

Wind vector retrieval using ERS-1 synthetic aperture radar imagery

An automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ERS-1 SAR imagery. The wind direction comes from the orientation of low frequency, linear signatures in the SAR imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer. The wind direction thus has inherently a 180/spl deg/ ambiguity since only a single SAR image is used. Wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and /spl sigma//sub 0/ values to invert radar cross section models. The authors show that: 1) on average the direction of the roll vortices signatures is approximately 11/spl deg/ to the right of the surface wind direction and can be used to estimate the surface wind direction to within /spl plusmn/19/spl deg/ and 2) utilizing these estimated wind directions from the SAR imagery subsequently improves wind speed estimation, generating errors of approximately /spl plusmn/1.2 m/s, for ERS-1 SAR data collected during the Norwegian Continental Shelf Experiment in 1991.

On radar imaging of current features: 1. Model and comparison with observations

[1] A new radar imaging model of ocean current features is proposed. The simulated normalized radar cross section (NRCS) takes into account scattering from ‘‘regular’’ surfaces (by means of resonant Bragg scattering and specular reflections) and scattering from breaking waves. The description of background wind waves and their transformation in nonuniform medium is based on solution of the wave action conservation equation. Wave breaking plays a key role in the radar imaging model. Breaking waves scatter radio waves (thus directly contributing to the NRCS), provide energy dissipation in wind waves (thus defining the wave spectrum of intermediate scale waves), and generate short surface waves (thus affecting Bragg scattering). Surface current, surfactants accumulated in the convergence zone, and varying wind field are considered as the main sources for the NRCS manifestations of current features. The latter source can result from transformation of atmospheric boundary layer over the sea surface temperature front. It is shown that modulation of wave breaking significantly influences both radar returns and short wind waves. In the range of short gravity waves related to Ku- X-, and C-bands, the modulation of Bragg waves through wave breaking is the governing mechanism. The model is tested against well-controlled experiments including JOWIP, SARSEX, and CoastWatch-95. A reasonably good agreement between model and observations is obtained.

Mesoscale eddies are oases for higher trophic marine life.

Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.

Coastal ocean fronts and eddies imaged with ERS 1 synthetic aperture radar

ERS 1 C band synthetic aperture radar (SAR) data were collected during the Norwegian Continental Shelf Experiment (NORCSEX) both in November 1991 during the ERS 1 commissioning phase and at different seasons in 1992 and 1993. Characteristic SAR image expressions are observed in relation to perturbation of the surface current- short wave interaction across the Norwegian Coastal Current front for winds less than 10-12 rn s -1. In situ measurements document the existence of alternating zones of convergence and divergence coexisting with a strong near-surface current shear of nearly 4f (where f is the Coriolis parameter) across a distance of a few kilometers. Under calm to moderate winds, i.e., 4-7 rn s -1, characteristic expressions of upper ocean circulation features also include the manifestation of eddies through the presence of surface film which damps the Bragg waves. Comparison of a near-coincident National Oceanic and Atmospheric Administration advanced very high resolution radiometer image and an ERS 1 SAR image supports the interpretation that surface current fronts are imaged by SAR. In combination with an SAR image simulation model, the relative quantitative importance of shear, convergence, and divergence along the front is examined. Although the model formulation is simple and the absolute magnitude of the perturbations is uncertain, the study shows that the SAR images can sometimes be used to interpret frontal dynamics, including growth and decay of meanders.

Upwelling: Oceanic Structure at the Edge of the Arctic Ice Pack in Winter

Observations taken on an expedition into the Arctic Ocean north of Spitsbergen indicated the existence of a region of wind-driven upwelling along the edge of the ice pack. Models underestimate the 12-kilometer width of the upwelling region.

On the Use of Doppler Shift for Sea Surface Wind Retrieval From SAR

The synthetic aperture radar (SAR) Doppler centroid has been used to estimate the scatter line-of-sight radar velocity. In weak to moderate ocean surface current environment, the SAR Doppler centroid is dominated by the directionality and strength of wave-induced ocean surface displacements. In this paper, we show how this sea state signature can be used to improve surface wind retrieval from SAR. Doppler shifts of C-band radar return signals from the ocean are thoroughly investigated by colocating wind measurements from the ASCAT scatterometer with Doppler centroid anomalies retrieved from Envisat ASAR. An empirical geophysical model function (CDOP) is derived, predicting Doppler shifts at both VV and HH polarization as function of wind speed, radar incidence angle, and wind direction with respect to radar look direction. This function is used into a Bayesian inversion scheme in combination with wind from a priori forecast model and the normalized radar cross section (NRCS). The benefit of Doppler for SAR wind retrieval is shown in complex meteorological situations such as atmospheric fronts or low pressure systems. Using in situ information, validation reveals that this method helps to improve the wind direction retrieval. Uncertainty of the calibration of Doppler shift from Envisat ASAR hampers the inversion scheme in cases where NRCS and model wind are accurate and in close agreement. The method is however very promising with respect of future SAR missions, in particular Sentinel-1, where the Doppler centroid anomaly will be more robustly retrieved.

Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations

Previous analysis of Advanced Synthetic Aperture Radar (ASAR) signals collected by ESAs Envisat has demonstrated a very valuable source of high-resolution information, namely, the line-of-sight velocity of the moving ocean surface. This velocity is estimated from a Doppler frequency shift, consistently extracted within the ASAR scenes. The Doppler shift results from the combined action of near surface wind on shorter waves, longer wave motion, wave breaking and surface current. Both kinematic and dynamic properties of the moving ocean surface roughness can therefore be derived from the ASAR observations. The observations are compared to simulations using a radar imaging model extended to include a Doppler shift module. The results are promising. Comparisons to coincident altimetry data suggest that regular account of this combined information would advance the use of SAR in quantitative studies of ocean currents.

An ERS 1 synthetic aperture radar image of atmospheric lee waves

An ERS 1 synthetic aperture radar (SAR) image of the island Hopen shows a distinct 7.6-km wavelength wave phenomenon near the island. This wave phenomenon is interpreted as the surface imprint on open water of atmospheric lee waves. The pattern is visible in the SAR image, since the lee waves modulate the horizontal wind speed near the ocean surface which, in turn, modulates the surface roughness and the radar cross section. The physical setting for the observation is presented and discussed. The lower bound on horizontal wind speed modulation is estimated to range from 3±2 ms−1 (for the wind speed minima) to 12±2 ms−1 (for the wind speed maxima) based upon the observed radar cross-section modulation and the ERS 1 scatterometer wind retrieval model CMOD4. The wavelength and wind speed modulation are consistent with linear lee wave model predications. The model uses an atmosphere with an exponential profile of the Scorer parameter (ratio of buoyancy frequency to wind speed) to represent a shallow, ground-based inversion layer observed at Bear Island and a bell-shaped barrier to represent the forcing effects of Hopen.

Slick detection in SAR images

A study of oil slicks and their look-alikes in SAR imagery is performed. Slicks are classified according to characteristic image expressions, backscatter profiles and gradients, geographical occurrence and weather limitations. A conceptual model for distinguishing oil slicks from other slick expressions is proposed. The model perform best when auxiliary data is available. The radar backscatter cross section and the radar contrast or damping ratio, defined as the ratio of the backscattered radar power from slick free and slick covered surfaces, are calculated for the different types of slick surfaces. Finally, a comparison between experimental results from a laboratory study and the SAR data is performed.<<ETX>>