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Graham D. Quartly

Abstract

The normalized backscatter, σ 0, observed by an altimeter at its Ku band is used to infer wind speed above the ocean surface, and for the dual-frequency TOPEX altimeter, algorithms exist to calculate wind stress and rainfall from simultaneous observations at its two frequencies (Ku band and C band). However, the creation and application of such algorithms rely on the long-term stability of an altimeter, or even coreferencing the values obtained by one altimeter to those of another. This paper proposes a method of monitoring an altimeter’s backscatter values using the constancy of the correlation between the values at its two different frequencies. Although the amplitude of the scattering by the ocean surface may vary by more than an order of magnitude, the coherency of its behavior at different spatial scales enables it to be used as a constant reference surface for dual-frequency altimetry. Using the observed close correlation between σ0C and σ0Ku for each TOPEX cycle, the drift in σ 0 calibration may be assessed through the shifts in the mean relationship. Application to the first 150 cycles of data from TOPEX side A shows good general agreement with the already applied corrections (derived from cycle averages of σ 0); however, these corrections have overcompensated slightly leaving a remnant drift of 0.03 dB yr−1. In a particular instance, the response of the altimeter can be seen to take a day to recover from an instrument shutdown. Initial investigations for TOPEX side B (which apart from the antenna is an identically constructed but separate instrument) confirm that dual-frequency methods can be used to cross-calibrate nonsimultaneous sensors.

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Graham D. Quartly

Abstract

Altimeter waveform data over the oceans are routinely processed to give sea surface height, h, significant wave height, H s, and normalized signal strength, σ 0 (from which wind speed is inferred). The onboard processing of TOPEX altimeter waveform data makes use of a series of different “gates” (averages of waveform bins) according to the wave height conditions being encountered. This formulation was to facilitate the rapid processing of data on board the satellite. Although the processing aims to make a seamless connection between the use of different gates as appropriate, there are various discrepancies in performance, which become manifest when the users are attempting to detect the fine differences associated with global environmental change. Here a brief overview is given of the various effects of the choice of gate index on the derived geophysical parameters. The H s distribution is shown to be affected by the transition between gate indices, and the C-band σ 0 values exhibit a step change there too. The positioning of waveforms within the reception window is also gate dependent, but the routine TOPEX ground processing compensates for that. Improved editing criteria (based on the standard deviations of h and H s) are proposed.

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Graham D. Quartly

Abstract

The predominant effect of rain on altimeter data is through the attenuation it causes of radar pulses propagating through it. Both the ERS-1 and TOPEX/Poseidon have recorded sharp decreases in the observed backscatter σ 0, which have been attributed to rain events. However, the structure of rain cells and the spatial variation of rain rate can be much smaller than the altimeter footprint over which σ 0 is calculated. Here an algorithm is derived to determine the rain rate and its spatial structure. This paper demonstrates that an unbiased estimator can be produced and shows the accuracy of its estimates through simulations of events of known magnitude and how the errors depend upon the accuracy of the assumptions used.

Although only yielding information over a very narrow swath, rain structure and rain-rate information derived from altimetry could greatly extend records of precipitation over many areas of the oceans where little or no direct estimates exist. Routine processing of altimeter waveform data offers the prospect of a large dataset of high-resolution sections of precipitation patterns. This may be used to investigate the small-scale spatial structure of storms or in conjunction with other sensors to improve estimates of the global precipitation field. The application to real altimeter waveform data, and its validation against independent estimates of rain rate, is the subject of a forthcoming paper.

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Graham D. Quartly and Meric A. Srokosz

Abstract

The retroflections of the East Madagascar Current and Agulhas Current are complex rapidly evolving systems, the latter controlling the passage of warm salty water from the Indian Ocean to the Atlantic. The TRMM Microwave Imager (TMI) provides frequent observations of sea surface temperature through clouds, allowing one to monitor the evolution of these systems. The authors develop a simple feature-tracking system that obviates the need for user intervention, and use its results to guide more focused studies. In the period 1997–99, westward progradation of the Agulhas retroflection (associated with ring shedding) is observed about eight times per year, agreeing with previous estimates from infrared data, and many rings move westward or northwestward. However, this behavior is seen to change in the 2000–01 time period, with the Agulhas retroflection occurring farther to the east. A few Natal pulses are seen, but cannot be linked conclusively to the spawning of rings due to TMI's limited latitudinal coverage. The majority of features originating at the East Madagascar retroflection appear to migrate southwestward. A new observation from the data is that, although the first northward meander of the Agulhas Return Current is constrained by bathymetry, its position does vary intermittently, remaining fixed in a given location for up to six months at a time. Southward propagation of features is noted along two ridges: although eddies have been found before along the eastern slope of the Mozambique Ridge, the new results for the Madagascar Ridge indicate an extra pathway for the eddies. Eddylike features are also found leading from the Agulhas Return Current back toward the Agulhas Current. The narrow “corridor” of these features suggests that it is controlled by the gyre recirculation in the southwest Indian Ocean.

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Trevor H. Guymer, Graham D. Quartly, and Meric A. Srokosz

Abstract

An investigation into a potentially important, but little-studied effect on altimeter data—rain contamination—has been carried out using ERS-1. The method involves identifying large changes in the radar backscatter coefficient and relating these to atmospheric liquid water estimates obtained from an onboard microwave radiometer. The latter is found to provide a useful means of distinguishing between wind and rain events. In general, the backscatter coefficient is reduced most when the liquid water content is high, and by an amount that is consistent with atmospheric attenuation at the radar frequency in use. However, some examples of enhanced backscatter were also observed indicating a possible reduction in surface roughness by the impact of raindrops on the ocean surface. Examination of return pulse shapes across significant rain events reveals behavior consistent with published theoretical work and shows how rain may lead to loss-of-lock in extreme conditions. The results of this study have implications for improved data quality flagging procedures and correction of ERS-1 altimeter winds.

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