Mean Surface Height Research Articles

Laser scanning tomography of localised nerve fibre layer defects

AIMSRetinal nerve fibre layer photography is a well established method to qualitatively document early structural changes which might be induced by primary open angle glaucoma. The aim was to analyse...

Estimation of dynamic ocean topography in the Gulf Stream area using the Hotine formula and altimetry data

Two modifications of the Hotine formula using the truncation theory and marine gravity disturbances with altimetry data are developed and used to compute a marine gravimetric geoid in the Gulf Stream area. The purpose of the geoid computation from marine gravity information is to derive the absolute dynamic ocean topography based on the best estimate of the mean surface height from recent altimetry missions such as Geosat, ERS-1, and Topex. This paper also tries to overcome difficulties of using Fast Fourier Transformation (FFT) techniques to the geoid computation when the Hotine kernel is modified according to the truncation theory. The derived absolute dynamic ocean topography is compared with that from global circulation models such as POCM4B and POP96. The RMS difference between altimetry-derived and global circulation model dynamic ocean topography is at the level of 25cm. The corresponding mean difference for POCM4B and POP96 is only a few centimeters. This study also shows that the POP96 model is in slightly better agreement with the results derived from the Hotine formula and altimetry data than POCM4B in the Gulf Stream area. In addition, Hotine formula with modification (II) gives the better agreement with the results from the two global circulation models than the other techniques discussed in this paper.

Animal evaluation of Aries HD perennial ryegrass selected for high digestibility

Aries HD perennial ryegrass (Lolium perenne L.) has been selected for increased digestibility in summer and early autumn. It is claimed to be the first commercial perennial ryegrass selected specifically for improved digestibility. The relative performance of weaned Romney lambs over summer was compared using Yatsyn 1 perennial ryegrass as a control cultivar. There were 3 replicates of each cultivar of 0.33 ha initially stocked with 8 lambs/plot (24 lambs/ha). Swards were continuously stocked with lambs with a variable stocking rate to maintain a mean sward surface height of 6 cm. Lambs grazing Aries HD plots consistently achieved higher liveweight gains over the experiment. Liveweight gain was higher on Aries HD than Yatsyn 1 during summer dry conditions 24/1/96 to 4/3/96 (50 vs. 31 ± 7.8 g/ day, P

Recirculation and seasonal change of the Kuroshio from altimetry observations

Surface height fluctuations across an eastward flowing boundary current, such as the Gulf Stream and the Kuroshio, can be caused by changes both in the upstream inflow and in the neighboring recirculation gyres. Clarifying these causes is important in understanding the time‐dependent nature of the boundary currents and their relationship to the surface wind and buoyancy forcing. Using satellite altimetry data to identify these causes, however, requires mean surface height field information, which is not readily available from the altimetry observation owing to lack of accurate geoid data. The present study presents a method to estimate the mean surface height profile across a boundary current system (including recirculation gyres) by combining altimetrically measured residual height data and historical hydrographic data. Applying this method to the Kuroshio revealed that the absolute surface height profiles thus estimated agree well with the results from quarterly, in‐situ hydrographic observations. By separating the signals of the surface height fluctuations into those of the eastward flowing jet and the recirculation gyres, we found that the seasonal cycle in the surface transport of the Kuroshio, which has a maximum in July and August, is primarily due to the seasonal change in the intensity of the recirculation gyre south of the Kuroshio.

Mapping the mean sea surface elevation field from satellite altimetry data using optimal interpolation

Abstract Optimal interpolation technique is applied to satellite altimetry data in order to recover a sea surface elevation signal from the data, which includes a large satellite radial orbit error, also to make a map of temporal mean sea surface dynamic topography (SSDT, the mean surface height relative to the geoid). The method is applied to Seasat data for a study area southeast of Japan, for which a fairly precise gravimetric geoid is available. Estimated mean elevation field relative to the best available geoid qualitatively shows existence of the Kuroshio in a limited local area close to Honshu, Japan. But for the whole of the area studied, the elevation field is much more rugged than expected mean SSDT and appears to include a relatively large geoid error; namely, the mapped mean sea surface elevation field cannot describe the mean SSDT field. Instead, correction of the provided geoid is evaluated by this altimetrically estimated elevation field together with the mean SSDT estimated from hydrograph...

Mean flow and variability in the Kuroshio Extension from Geosat altimetry data

Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian‐shaped and by successively fitting this synthetic current's height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi‐stationary meanders, and its decrease thereafter is mainly due to large‐scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large‐scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low‐latitude Pacific Ocean.

Step response and spatial resolution of an optical heterodyne profiling instrument

An optical heterodyne instrument for measuring profiles of smooth surfaces has been developed. The instrument is intended for measuring very small deviations from the mean surface height but is demonstrated to be capable of measuring step heights of several hundred nanometers. The step response and the spatial resolution of the instrument are calculated and experimentally verified.

Ocean surface height‐slope probability density function from SEASAT altimeter echo

This paper describes new methods for the interpretation of microwave scatter from the sea. Roughsurface scattering theory has led to an integral equation for the echo voltage in terms of the joint heightslope probability density function (pdf). We have developed methods for the inversion of this equation that do not require the assumption of a model for either the ocean surface or the transmitted pulse. In addition they are far more efficient than traditional grid‐search methods for model fitting. We have applied our methods to SEASAT data taken over Hurricane Fico. Statistical data analysis was performed to give the data covariance matrix and the correlation coefficient between successive waveforms and to trace the propagation of error through the inversion process to give statistical uncertainties in the derived parameters. The usual assumption of Rayleigh statistics is shown to be violated by the data, since the number of degrees of freedom increases linearly with power, independent of sea state; we conclude that this is an instrumental effect. We find that the waveforms are significantly correlated over distances of several kilometers at high sea states and suggest coherence of long ocean waves over these distances as the cause. We obtained waveheight higher‐order parameters by least squares fitting of a Gram‐Charlier model for probability densities to the calculated values. For averages over 160 km of sea, we obtain a mean surface height bias correction, significant waveheight, and height skewness parameter with standard deviations of 4, 0.7, and 15%. In addition to this height bias correction easily removed from the data, we identify another sea‐state‐dependent height bias that can exceed 50 cm. That this exists in the data is proof that one must employ the joint height‐slope pdf in the scattering model, and not merely the height pdf as conventionally used.

Southern Hemisphere Forecast Experiments with a One-Level Spectral Model

Abstract A one-level primitive equation spectral model has been initialized with hemispheric 500 mb geopotential height and vorticity fields using the Southern Hemisphere GARP data sets for the period 3–12 November 1969 inclusive. Experiments performed to choose values of the external parameters of the model showed that resolution up to wavenumber 15 and a mean free surface height of 5.6 km were suitable for 48 h prognoses. The distributions with respect to both latitude and planetary wavenumber in the mean square error were determined for the model prognoses. Over the mid-latitude range 30°S to 60°S and over the wavenumber range 3 to 9, 48 h prognoses proved consistently better than persistence.