Showing papers by "Quanan Zheng published in 2008"

Sub-mesoscale ocean vortex trains in the Luzon Strait

Abstract: [1] This study uses the Argos satellite-tracked surface drifter trajectory data and ENVISAT (European satellite) ASAR (advanced synthetic aperture radar) images to illustrate the ocean vortex trains (OVT) in the Luzon Strait. Two cases that occurred in the northwest of Babuyan Island are observed. The first train of three cyclonic vortices showed up on drifter trajectories from 20° to 20.5°N and from 120° to 121°E, and the second, consisting of five pairs of cyclonic-anticyclonic vortices, occurred on the upstream side of the first one from 19.5° to 20.0°N and from 121.0° to 122.0°E and showed up on the ASAR images acquired on 19 November 2006. The total length of the vortex train axis reaches about 250 km. All vortices propagate northwestward (∼315° TN). The mean angular velocity is (2.07 ± 0.18) × 10−5 s−1. Theoretical models of ocean vortex radar image derived from radar imaging theories are used to extract dynamical parameters from ASAR imagery signatures, which include the distance between two consecutive vortices and that between two rows of vortices of (22.6 ± 1.9) km and (8.2 ± 1.2) km, respectively, the maximum rotational velocity radius as 4.70 km, and the vortex rotational angular velocity 3 × 10−5s−1. Dynamical analyses give the mean velocity of the current of 0.65 ms−1, and the propagation velocity of the vortex 0.58 ms−1. The vortex shedding rate is estimated as 2.57 × 10−5 s−1. The Reynolds number is estimated as 50 to 500. For the individual vortex and the vortex train, the Rossby numbers are O(0.4), and O(0.5), respectively, implying that both vortex and vortex train observed in the Luzon Strait have a sub-mesoscale nature. This study also reveals a strong current with an average surface current velocity of around 0.7 ms−1 and the direction of around 315° TN flowing directly from the Pacific to SCS passing through the southern Luzon Strait. The mean flow velocity can be calculated using methods developed in this study and OVT dynamical models. This information may provide more insight to the circulation systems in the area including the origin of Kuroshio.

An overview on SAR measurements of sea surface wind

Abstract: Studies show that synthetic aperture radar (SAR) has the capability of providing high-resolution (sub-kilometer) sea surface wind fields. This is very useful for applications where knowledge of the sea surface wind at fine scales is crucial. This paper aims to review the latest work on sea surface wind field retrieval using SAR images. As shown, many different approaches have been developed for retrieving wind speed and wind direction. However, much more work will be required to fully exploit the SAR data for improving the retrieval accuracy of high-resolution winds and for producing wind products in an operational sense.

Estimating amplitudes of internal waves using satellite ocean colour imagery of the South China Sea

Abstract: A method is presented for estimating the amplitudes of internal waves (IWs) in ocean colour imagery. The method is tested in the northern South China Sea (SCS) because the environmental conditions there are such that IWs are generated frequently and can therefore be observed easily. A relationship between satellite-derived chlorophyll concentration (chl) and the amplitudes of IWs is obtained. Using a deep chlorophyll maximum (DCM) model, the amplitude at the light penetration depth can be estimated. From theoretical models, the amplitude of an IW at the depth where the maximum vertical isopycnal displacement appears is also derived. The results indicate that the amplitudes of IWs in the northern SCS determined by the methods developed in this study are comparable with historical regional data.

A statistic‐thermodynamic model for the DOM degradation in the estuary

Abstract: [1] This study aims to clarify the role of dissolved salts playing in the degradation process of terrestrial dissolved organic matter (DOM) at a scale of molecular movement. The molecular thermal movement is perpetual motion. In a multi-molecular system, this random motion also causes collision between the molecules. Seawater is a multi-molecular system consisting from water, salt, and terrestrial DOM molecules. This study attributes the DOM degradation in the estuary to the inelastic collision of DOM molecule with charged salt ions. From statistic-thermodynamic theories of molecular collision, the DOM degradation model and the DOM distribution model are derived. The models are validated by the field observations and satellite data. Thus, we conclude that the inelastic collision between the terrestrial DOM molecules and dissolved salt ions in seawater is a decisive dynamic mechanism for rapid loss of terrestrial DOM.

SAR Measurement of Ocean Surface Wind Using A Physics Model

Abstract: Results of ocean surface wind speed retrieval from C-band ENVISAT ASAR images using a physics wind model are shown The physics model is based on the radar backscatter theory in which the radar cross section is calculated considering the contribution of both Bragg scattering or resonance and specular reflection from the sea surface The wind speeds retrieved from VV- or HH-polarized ASAR images using the physics model were compared to both buoy measurements in Hong Kong coastal waters and that retrieved using the empirical C-band algorithms (CMOD4, CMOD5, CMOD_IRR2) The results show the feasibility of the physics model for ocean surface wind speed retrieval from both VV and HH-polarized ASAR images at moderate wind condition