Ivan Astin

Salinity Assimilation Using S(T): Covariance Relationships

Assimilation of salinity into ocean and climate general circulation models is a very important problem. Argo data now provide far more salinity observations than ever before. In addition, a good analysis of salinity over time in ocean reanalyses can give important results for understanding climate change. Here it is shown from the historical ocean database that over large regions of the globe (mainly midlatitudes and lower latitudes) variance of salinity on an isotherm S(T ) is often less than variance measured at a particular depth S(z). It is also shown that the dominant temporal variations in S(T ) occur more slowly than variations in S(z), based on power spectra from the Bermuda time series. From ocean models it is shown that the horizontal spatial covariance of S(T ) often has larger scales than S(z). These observations suggest an assimilation method based on analyzing S(T ). An algorithm for assimilating salinity data on isotherms is then presented, and it is shown how this algorithm produces orthogonal salinity increments to those produced during the assimilation of temperature profiles. It is argued that the larger space and time scales can be used for the S(T ) assimilation, leading to better use of scarce salinity observations. Results of applying the salinity assimilation algorithm to a single analysis time within the ECMWF seasonal forecasting ocean model are also shown. The separate salinity increments coming from temperature and salinity data are identified, and the independence of these increments is demonstrated. Results of an ocean reanalysis with this method will appear in a future paper.

A survey of studies into errors in large scale space-time averages of rainfall, cloud cover, sea surface processes and the earth's radiation budget as derived from low earth orbit satellite instruments because of their incomplete temporal and spatial coverage

This survey considers those studies conducted into estimating errors in satellite derived large scale space-time means (of the order of 250 km by 250 km by a month) for rainfall, cloud cover, sea surface processes and the Earths radiation budget, resulting from their incomplete coverage of the space-time volume over which the mean is evaluated. Many of these studies have focused on estimating the errors in space-time means post satellite launch and compare mean data derived from such satellites with that from an independent data set. Pre-launch studies tend to involve computer simulations of a satellite overflying and sampling from an existing data set and hence the two approaches give values for sampling errors for specific cases. However, more generic sampling papers exist that allow the exact evaluation of sampling errors for any instrument or combination of instruments if their sampling characteristics and the auto-correlation of the parameter field are known. These generic and simulation techniques have been used together on the same data sets and are found to give very similar values for the sampling error and are presented. Also considered are studies in which data from several satellites, or satellite and ground based measurements are combined to improve estimates in the above means. This improvement being brought about not only by increased spatial and temporal coverage but also by a reduction in retrieval error.

Turbulent times in the northern polar ionosphere

A model is presented of the growth rate of turbulently generated irregularities in the electron concentration of northern polar cap plasma patches. The turbulence is generated by the short-term fluctuations in the electric field imposed on the polar cap ionosphere by electric field mapping from the magnetosphere. The model uses an ionospheric imaging algorithm to specify the state of the ionosphere throughout. The growth rates are used to estimate mean amplitudes for the irregularities, and these mean amplitudes are compared with observations of the scintillation indices S-4 and sigma(phi) by calculating the linear correlation coefficients between them. The scintillation data are recorded by GPS L1 band receivers stationed at high northern latitudes. A total of 13 days are analyzed, covering four separate magnetic storm periods. These results are compared with those from a similar model of the gradient drift instability (GDI) growth rate. Overall, the results show better correlation between the GDI process and the scintillation indices than for the turbulence process and the scintillation indices. Two storms, however, show approximately equally good correlations for both processes, indicating that there might be times when the turbulence process of irregularity formation on plasma patches may be the controlling one.

A general formalism for the distribution of the total length of a geophysical parameter along a finite transect

The distribution of the total length of a geophysical parameter along a finite linear transect is required to determine confidence intervals for its true mean fractional coverage. The distribution is analogous to the total time a queue is busy within a finite time interval. As posed as a queuing problem, this distribution has been the subject of study for the last 40 years using a variety of approaches. Using a heuristic approach, the general equation for the distribution is derived within a geophysical context. As an example, the general equation for the distribution is applied to an exponential field and validated against a published simulation of an exponential field of leads in ice floes. The formalism allows for exact confidence intervals to be derived, and its application to hypothesis testing and experimental design in remote sensing is discussed.

Map of low-frequency electromagnetic noise in the sky

The Earths natural electromagnetic environment is disturbed by anthropogenic electromagnetic noise. Here we report the first results from an electromagnetic noise survey of the sky. The locations of electromagnetic noise sources are mapped on the hemisphere above a distributed array of wideband receivers that operate in a small aperture configuration. It is found that the noise sources can be localized at elevation angles up to ∼60° in the sky, well above the horizon. The sky also exhibits zones with little or no noise that are found toward the local zenith and the southwest of the array. These results are obtained by a rigorous analysis of the residuals from the classic dispersion relation for electromagnetic waves using an array analysis of electric field measurements in the frequency range from ∼20 to 250 kHz. The observed locations of the noise sources enable detailed observations of ionospheric modification, for example, caused by particle precipitation and lightning discharges, while the observed exclusion zones enable the detection of weak natural electromagnetic emissions, for example, from streamers in transient luminous events above thunderclouds.

Array analysis of electromagnetic radiation from radio transmitters for submarine communication

The array analyses used for seismic and infrasound research are adapted and applied here to the electromagnetic radiation from radio transmitters for submarine communication. It is found that the array analysis enables a determination of the slowness and the arrival azimuth of the wave number vectors associated with the electromagnetic radiation. The array analysis is applied to measurements of ∼20–24 kHz radio waves from transmitters for submarine communication with an array of 10 radio receivers distributed over an area of ∼1 km ×1 km. The observed slowness of the observed wave number vectors range from ∼2.7 ns/m to ∼4.1 ns/m, and the deviations between the expected arrival azimuths and the observed arrival azimuths range from ∼−9.7° to ∼14.5°. The experimental results suggest that it is possible to determine the locations of radio sources from transient luminous events above thunderclouds with an array of radio receivers toward detailed investigations of the electromagnetic radiation from sprites.

Rapid variations in echo power maps of VHF radar backscatter from the lower atmosphere

Abstract For the first time, echo power maps of aspect-sensitive VHF backscatter are shown, with good time and spatial resolutions, for angles 0°–7° from zenith. Sequences of power maps show large changes in appearance over timescales of a few minutes and height intervals of a few hundred metres. Often, individual power maps are consistent with tilted and distorted specular-type scattering layers, rather than anisotropic turbulence, and the direction of maximum echo power is sometimes several degrees off-vertical. Nevertheless, after time-averaging the variable echo-power patterns, the average pattern can become almost circular and centred on zenith, as has been assumed in the past. Echo power maps measured in strong windshear beneath the jet stream show a skewing of the echo power distribution. However, some power maps in the lower stratosphere, despite stronger wind shear, appear more constrained and their maximum echo power remains closer to zenith.

Polar cap plasma patch primary linear instability growth rates compared

Four primary plasma instability processes have been proposed in the literature to explain the generation of phase scintillation associated with polar cap plasma patches. These are the gradient drift, current convective, and Kelvin-Helmholtz instabilities and a small-scale “turbulence” process. In this paper the range of possible values of the linear growth rates for each of these processes is explored using Dynamics Explorer 2 satellite observations. It is found that the inertial turbulence instability is the dominant process, followed by inertial gradient drift, collisional turbulence, and collisional shortwave current convective instabilities. The other processes, such as Kelvin-Helmholtz, collisional gradient drift, and inertial shortwave current convective instabilities, very rarely (<1% of the time) give rise to a growth rate exceeding 1/60, that is deemed to be significant (in publications) to give rise to GPS scintillation.

Multipath Propagation of Low Frequency Radio Waves Inferred from High Resolution Array Analysis

The low frequency radio sky shows the locations of electromagnetic radio sources with a characteristic dilution of precision. Here we report a thorough high resolution analysis of radio waves from low frequency (∼20-150 kHz) radio communication transmitters which are recorded with a small aperture array of radio receivers during the day. It is found that the observed dilution of precision results from the array geometry of the radio receivers, a birefringent wave propagation and the correlated multipath propagation of low frequency radio waves. The influence of the array geometry on the dilution of precision is reduced by taking into account the impulse response of the array. This procedure reveals for the very first time the splitting of one single radio source into two distinct source locations separated by ∼0.2°-1.9° which result from a birefringent wave propagation. The two locations are yet more clearly identified by using the polarity of the modulated wave number vectors of the radio waves. This polarity is also used to quantify the dilution of precision arising from correlated multipath propagation which is discriminated against wave number fluctuations arising from the timing accuracy of the radio receivers. It is found that ∼69% of the wave number variability is of natural origin and ∼31% originates from the timing accuracy of the receivers. The wave number variability from correlated multipath propagation results in a standard deviation ∼2-8% relative to the source location. This compact measurement of correlated multipath propagation is used to characterize the uncertainty of source locations in the radio sky. The identification of correlated multipath propagation strongly suggests the existence of very fast processes acting on time scales <1 ms in the D-region ionosphere with physically meaningful effects on low frequency radio wave propagation. This important result has implications for practical applications in that the observed multipath propagation enables the determination of natural limits for the accuracy of navigation and lightning location methods using low frequency radio waves.

Sampling Errors and Bias in Satellite-Derived Fractional Cloud Cover Estimates from Exponential and Deterministic Cloud Fields as a Consequence of Instrument Pixel Size and Number

Abstract The proposed European Space Agency’s cloud profiling radar Millimetre Active Cloud Structure Imaging Mission is a nadir-pointing radar with a 1-km footprint; it will need to integrate the received signal power for a reasonable amount of time (1.4–14 s) along track to detect a cloud. As a result, the radar will provide a set of long (10–100 km) narrow pixels that are registered either as cloudy or as clear, depending on how much cloud there is in each. These are thus likely to give a biased estimate of fractional cloud cover over a region because the radar will be unable to detect small clouds or gaps. For a nadir-pointing radar the clouds and gaps essentially form a 1D sequence that can be modeled by a general single-server queue. This model allows analytical expressions to be found for the bias and sampling error in specific cases. Such expressions are presented for the two extremes of the Erlang queuing model, that is, for an exponential and a deterministic queue. These are then compared to res...