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Fabrikant A.L. Spiesberger J.L. Silivra A.A. Hurlburt E.E. 《Oceanic Engineering, IEEE Journal of》1998,23(1):20-25
An acoustic tomography simulation is carried out in the eastern North Pacific ocean to assess whether climate trends are better detected and mapped with mobile or fixed receivers. In both cases, acoustic signals from two stationary sources are transmitted to ten receivers. Natural variability of the sound-speed field is simulated with the Naval Research Laboratory (NRL) layered-ocean model. A sequential Kalman-Bucy filter is used to estimate the sound speed field, where the a priori error covariance matrix of the parameters is estimated from the NRL model. A spatially homogeneous climate trend is added to the NRL fluctuations of sound speed, but the trend is not parameterized in the Kalman filter. Acoustic travel times are computed between the sources and receivers by combining sound speeds from the NRL model with those from the unparameterized climate trend. The effects of the unparameterized climate trend are projected onto parameters which eventually drift beyond acceptable limits. At that time, the unparameterized trend is detected. Mobile and fixed receivers detect the trend at about the same time. At detection time, however, maps from fixed receivers are less accurate because some of the unparameterized climate trend is projected onto tile spatially varying harmonics of the sound-speed field. With mobile receivers, the synthetic apertures suppress the projection onto these harmonics. Instead, the unparametrized trend is correctly projected onto the spatially homogeneous portion of the parameterized sound-speed field 相似文献
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Silivra A.A. Spiesberger J.L. Fabrikant A.L. Hurlburt H.E. 《Oceanic Engineering, IEEE Journal of》1997,22(1):143-150
A basin-scale acoustic tomography simulation is carried out for the northeast Pacific Ocean to determine the accuracy with which time must be kept at the sources when clocks at the receivers are accurate. A sequential Kalman filter is used to estimate sound-speed fluctuations and clock errors. Sound-speed fluctuations in the simulated ocean are estimated from an eddy-resolving hydrodynamic model of the Pacific forced by realistic wind fields at daily resolution from 1981-1993. The model output resembles features associated with El Nino and the Southern Oscillation, as well as many other features of the ocean's circulation. Using a Rossby-wave resolving acoustic array of four fixed sources and twenty drifting receivers, the authors find that the percentage of the modeled ocean's sound-speed variance accounted for with tomography is 92% at 400-km resolution, regardless of the accuracy of the clocks. Clocks which drift up to hundreds of seconds of error or more for a year do not degrade tomographic images of the model ocean. Tomographic reconstructions of the sound-speed field are insensitive to clock error primarily because of the wide variety of distances between the receivers from each source. Every receiver “sees” the same clock error from each source, regardless of section length, but the sound-speed fluctuations in the modeled ocean cannot yield travel times which lead to systematic changes in travel time that are independent of section length. The Kalman filter is thus able to map the sound-speed field accurately in the presence of large errors at the source's clocks 相似文献
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Mapping climatic temperature changes in the ocean with acoustictomography: navigational requirements
Spiesberger J.L. Fabrikant A.L. Silivra A.A. Hurlburt H.E. 《Oceanic Engineering, IEEE Journal of》1997,22(1):128-142
In eddy-resolving hydrodynamic models, first-mode baroclinic Rossby waves linked to El Nino/Southern Oscillation are the dominant features which change basin-wide temperatures below the seasonal thermocline in the northeast Pacific at periods less than a decade. Simulations are carried out in which Rossby waves are mapped using acoustic tomography. Based on the model which propagated these waves, a Kalman filter is used to map temperature signals for a year. The modeled data are taken from a dense network of acoustic tomography sections. At 300-m depth, where the temperature perturbations associated with Rossby waves are about ±1°C, 80% to 90% of the model variance is accounted for with tomographic estimates. The corresponding standard deviations of the estimates are less than 0.1°C at 400-km resolution. About 80% of the model variance is accounted for with tomography when the navigational errors of the sources and receivers are as poor as one kilometer. Consequently, it may be unnecessary to accurately navigate actual tomographic instruments to map climate change. Modeling results are insensitive to: 1) a reduction in data due to a significant number of instruments which fail; 2) whether the instruments are mobile or fixed; 3) the detailed trajectories of mobile receivers; 4) the shape of the a priori spectrum of ocean fluctuations; 5) the corrections to the acoustic travel-time biases; and 6) the errors in the sound-speed algorithm. In basin-scale arrays, the modeled variance of acoustic travel time depends on the horizontal wavenumber of temperature as k-5.5. Because sound has little sensitivity to small wavelengths, modeled Rossby waves can be mapped in a day from a few sources and of order ten receivers. The results only depend on the model having large scales in space and time 相似文献
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Spiesberger J.L. Bushong P.J. Metzger K. Jr. Birdsall T.G. 《Oceanic Engineering, IEEE Journal of》1989,14(1):108-119
Continuous acoustic transmission (133 Hz, 60-ms resolution) between a bottom-mounted source near Oahu, Hawaii, and a bottom-mounted receiver at 4000-km range near the coast of northern California was recorded to learn how to measure precisely the travel time so that basin-scale fluctuations in the Pacific can be detected. Daily incoherent averages of some of the multipaths exhibited stability during this period. The standard deviation of the travel time of the resolved peaks in the daily incoherent averages is about 30 ms. An acoustic method, based on cross-correlation, is derived to estimate the change in the average acoustic phase (travel time) to a precision of about 0.018 cycles (135 μs) every 2 min. Travel-time estimates based on the cross-correlator reduce the aberrations due to internal waves by about 19 dB in comparison with CW transmissions. The new travel-time estimator is applied to the measurements to examine some of the fluctuations of the Pacific 相似文献
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John L. Spiesberger Harley E. Hurlburt Mark Johnson Mark Keller Steven Meyers Jim O'Brien 《Dynamics of Atmospheres and Oceans》1998,26(4):209-240
An interpretation is made of interannual changes in acoustic travel time between Oahu and seven receivers at distances of 3000–4000 km. Measurements were made in late 1983, and over two 5-month intervals between 1987 and 1989. Previous publications demonstrated that these changes stem from variations in temperature. Two hydrodynamic ocean models are used to identify plausible oceanic features that could cause these variations. They are from the Naval Research Laboratory and the Florida State University at (1/8)° and (1/6)° resolution, respectively, and are forced with different interannual wind sets for more than a decade. Modelled El Niño's and La Niña's generate poleward travelling Kelvin waves on the eastern boundary of the Pacific. These excite Rossby waves that propagate westward at mid-latitudes. Rossby waves are the dominant model features which affect the modelled acoustic travel times, and hence section-averaged temperatures in the eastern North Pacific. These waves yield travel times whose standard deviations and rates of changes are similar to the measurements. In the observations, some sections separated by less than 500 km exhibit trends in heat content with opposite signs. Similar variability can be explained with modelled Rossby waves. Model wavelengths less than 500 km, eddies, and seasonal cycles induced by seasonal winds yield travel times that are two orders of magnitude too small to account for the data. 相似文献
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