Transient Forcing Research Articles

Current response at the shelf break to transient wind forcing

An analysis of current meter and hydrographic data collected from an array of moorings situated at the shelf break off Nova Scotia is presented. In particular, the focus is on the ocean response to moderate (10 to 20m s−1) transient winds of two or more days duration blowing parallel to the local bathymetry. Under these conditions, upwelling occurs at the shelf break from depths of 400 m and with peak vertical velocities of about 2 mm s−1. The upwelling appears to be confined to within 10 km of the slope. Acceleration, Coriolis, and pressure‐gradient terms are important for the along‐ and cross‐slope momentum balances. Local topographic variations contribute to the anomalously large bottom currents on the shoreward side of the shelf break. Both coastal sea level and the shelf break circulation respond coherently to synoptic wind forcing, but quantitative agreement with two‐layer shelf models is poor.

An Interpretation of Sudden Warmings In Terms of Potential vorticity

A simple and concise interpretation of stratospheric sudden warmings is offered in terms Of the transient changes in the potential vorticity pattern. The warming is viewed as a manifestation of the reversal of the mean (zonally averaged) relative potential vorticity distribution over a deep layer of the stratosphere and mesosphere. The reversals of the mean zonal flow and of the latitudinal gradient of the mean temperature are regarded merely as accompanying features of the phenomenon. The linchpin of our interpretation is a particularly simple derived relationship between the wave potential vorticity and the mean relative potential vorticity of a quasi-geostrophic flow. This relationship points to the possibility of inducing a reversal of the mean relative potential vorticity distribution by purely transient wave forcing of the stratosphere by tropospheric flow. Furthermore, it indicates that the duration and amplitude of the wave-forcing, together with the transmission properties of the mean zonal flow structure to upward propagating planetary waves, will be factors that regulate the occurrence of these reversal events. These assertions are further examined and illustrated in a series of experiments undertaken with a severely truncated spectral quasi-geostrophic β-plane model.

Use of swept sine wave in physiological systems analysis.

The use of a swept sine wave as a transient forcing function for measuring the frequency response characteristics of a physiological system is suggested. The spectral characteristics of a linear frequency swept sine wave are illuistrated and it is shown that, with relatively short sweep sine wave excitations, one can obtain almost rectangular modulus spectra with relatively good frequency selectivity. Such a transient method may be of potential use in physiological systems analysis.

Generalized Representation of Generator Excitation Power Requirements

Generalized relations for generator excitation power requirements are given for steady-state excitation, transient field forcing, and power damping operations. These relations give the excitation power in per-unit, on the generator MVA base, as a function of the generator per-unit reactances and field time constant. The effect upon power damping of drawing the required excitation power from the main generator shaft is also investigated. It is found that, in general, the effect of powering the exciter from the main generator is to reduce the amount of damping that can be provided by excitation control. However, in the case of a topical conventional generator, the resultant reduction in damping is usually so slight (about 2%) as to be negligible.

Spectrally shaped transient forcing functions for frequency response testing

The transfer characteristics of dynamic force generating systems for frequency response measurements on mechanical systems often cause the generated force spectrum to differ in shape from the input voltage spectrum. By considering the spectral characteristics of the linear rapid frequency sweep, it is shown that good estimates of the modulus spectra of swept sinewaves with varying sweep rates may be calculated. It is shown how swept sinewave voltages of constant amplitude may be generated with an appropriately predetermined sweep rate to allow the transfer characteristics of the force generating system to be compensated for, thus producing a spectrally shaped transient forcing function at the output of the force generator. The application of the technique in vibration testing is demonstrated.

On the hydrographic response to transient meteorological disturbances

The response to transient forcing in a hydrographic system like the Baltic is considered. We find that under quite general conditions the baroclinic response is confined to a narrow coastal region while the response in the main part of the basin is essentially barotropic. The response is analysed quantitatively under somewhat more stringent conditions using a boundary layer type approach. We find that the barotropic large scale low frequency motions follow contours of constant depth, a feature which might be accessible to empirical verification. The baroclinic coastal response is found to be confined to a region of width 5–10 km (in the Baltic). If the dissipation is sufficiently weak, internal Kelvin waves travelling in a counter clockwise direction may be excited in the coastal region. DOI: 10.1111/j.2153-3490.1972.tb01545.x

On the hydrographic response to transient meteorological disturbances

The response to transient forcing in a hydrographic system like the Baltic is considered. We find that under quite general conditions the baroclinic response is confined to a narrow coastal region while the response in the main part of the basin is essentially barotropic. The response is analysed quantitatively under somewhat more stringent conditions using a boundary layer type approach. We find that the barotropic large scale low frequency motions follow contours of constant depth, a feature which might be accessible to empirical verification. The baroclinic coastal response is found to be confined to a region of width 5–10 km (in the Baltic). If the dissipation is sufficiently weak, internal Kelvin waves travelling in a counter clockwise direction may be excited in the coastal region.