Jingsong Yang

A Backscattering Model of Rainfall Over Rough Sea Surface for Synthetic Aperture Radar

Spaceborne high-resolution synthetic aperture radar (SAR) is a potential powerful tool for rainfall pattern and intensity observations over the sea surface. However, many interesting rain-related phenomena revealed by SAR images are still not fully understood due to poor theoretical modeling of the rain-wind-wave interactions. This paper attempts to develop a physics-based radiative transfer model to capture the scattering behavior of rainfall over a rough sea surface. Raindrops are modeled as Rayleigh scattering nonspherical particles, whereas the rain-induced rough surface is described by the Log-Gaussian ring-wave spectrum. The model is validated against both empirical models and measurements. A case study of collocated Envisat ASAR data and NEXRAD rain data is presented to demonstrate the performance of the newly developed model. Finally, numerical simulation results suggest that rain-related scattering becomes significant as compared with wind-related scattering when the frequency is above C-band, whereas the raindrop volumetric scattering becomes significant above X-band.

Study of the propagation direction of the internal waves in the South China Sea using satellite images

Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.

Comparison of Typhoon Centers From SAR and IR Images and Those From Best Track Data Sets

This paper compares the typhoon centers from the tropical cyclone best track (BT) data sets of three meteorological agencies and those from synthetic aperture radar (SAR) and infrared (IR) images. First, we carried out algorithm comparison using two newly developed algorithms and one existing wavelet-based algorithm, which were used to extract typhoon eyes in six SAR images and two IR images. These case studies showed that the extracted eyes by the three algorithms are consistent with each other. The differences among them are relatively small. However, there is a systematic difference between the extracted centers and the typhoon centers from the three BT data sets, which were interpolated to the imaging times first. We then compared the typhoon centers determined from 25 SAR and 43 IR images with those from the three BT data sets to investigate the performance of the latter at the sea surface and at the cloud top, respectively. We found that the typhoon centers from the three BT data sets are generally closer to the locations extracted from the SAR images showing sea-surface imprints of the typhoons than those from the IR images showing cloud-top structures of the typhoons.

Coastally trapped atmospheric gravity waves on SAR, AVHRR and MODIS images

Alternative dark–bright patterns on two ENVISAT Advanced Synthetic Aperture Radar (ASAR) images of the east coast of the Korean Peninsula acquired on 18 and 19 May 2004 are interpreted as atmospheric gravity waves (AGWs) on the basis of simultaneous multi‐satellite observations and atmospheric gravity wave theory. The AGWs appeared in the form of a wave packet containing several waves located between 50 and 200 km offshore. The wavelengths were ranging from 13 to 20 km. The lengths of AGW crests were from 20 to 150 km. An NOAA‐17 pass was received about 30 min after the ASAR pass. Its channel 4 infrared (IR) image clearly shows wave‐like moisture patterns. However, the sea surface temperature (SST) image derived after applying nonlinear calibration and split‐window atmospheric correction shows no wave patterns. A daytime true‐colour MODIS image taken about 14 h later still shows a cloud band of same AGWs, indicating the lifespan of the standing AGWs can be over half a day. Although oceanic internal waves (IWs) may also cause similar wave patterns imaged by spaceborne SAR as they modulate the ocean surface roughness, we provide evidence to eliminate this possibility in this case. The characteristics of satellite observed AGWs are in good agreement with these simulated by a linear coastal AGW model.

Internal Wave Packet Characterization from SAR Images Using Empirical Mode Decomposition (EMD)

This paper addresses the problem of the characterization of oceanic internal wave packets from SAR images. Empirical Mode Decomposition (EMD) provides a new strategy for extracting internal wave signatures, and offers the precision, flexibility and accuracy which are proved by Huang et al. in 1998. The EMD is employed in a scale-space analysis of SAR image profiles to locate and characterize the wave parameters of internal waves. First, the EMD is applied for the nonlinear internal wave temporal series of synthetic aperture radar (SAR) decomposition. Second, a criterion based on the theory that the max normalized deflection stands for the largest energy is introduced to detect the internal wave component. The results show that EMD is an excellent tool to detect internal waves against background noise, and to estimate, with a good degree of precision, soliton wavelengths from SAR ocean image profiles.

Significant wave height estimation using azimuth cutoff of C-band RADARSAT-2 single-polarization SAR images

This paper proposes two simple models, look-up table (LUT) model and empirical model, to directly retrieve significant wave height (Hs) using synthetic aperture radar (SAR) azimuth cutoff (λc). Both models aim at C-band VV, HH, VH, and HV single-polarization SAR images. The LUT model relates Hs to λc, while the empirical model relates Hs to both λc and SAR range-to-velocity (β). The LUT model coefficients are derived by simulation under different sea states and observation conditions, which depend on incidence angle (θ), wave direction (dw), and β but are independent of polarization. The empirical model coefficients are obtained by fitting the collocated data, which only depend on polarization. To fit empirical model coefficients and validate the two models, C-band RADARSAT-2 fine quad-polarization (VV+HH+VH+HV) single-look complex (SLC) SAR images and collocated buoy data are collected. Retrieved Hs, using Yang model and the two models proposed in this paper from four kinds of polarization SAR data, are compared with buoy Hs. Results show that both LUT and empirical models have the capacity of retrieving Hs from C-band RADARSAT-2 co-polarization SAR data, while Yang model is not suitable for these kinds of SAR data. Moreover, the empirical model is also valid for cross-polarization SAR data showing clear ocean wave stripes.

Nonlinear internal wave amplitude remote sensing from SAR imagery

The amplitude of internal waves is very difficult to retrieve from satellite. In this paper, a method is given to estimate the amplitude of nonlinear internal waves from synthetic aperture radar (SAR) imagery. It is assumed that the observed groups of nonlinear internal wave packets on SAR imagery are generated by local semidiurnal tides. The mean distance between the leading crest of two successive wave packets has been used to derive the group velocity of the nonlinear internal waves. The amplitude of nonlinear internal waves has been calculated from a model which consists of the KdV equation, action balance equation and Bragg scattering model. Case studies in China Seas show reasonable results.

Observations of typhoon eye using SAR and IR sensors

In this review, recent studies on the observations of typhoon eyes by images acquired by multiple sensors, including synthetic aperture radar (SAR), and infrared (IR) radiometer, are first summarized. Large horizontal distances between typhoon eyes on the ocean surface by SAR and those on the cloud top by IR sensors have been demonstrated; these have previously been ignored but should not be ignored in typhoon forecasts and numerical simulations. Then, based on nine published typhoon cases, the horizontal shifts and vertical tilt angles from the cloud-top typhoon eye locations by IR sensors on board the Feng-Yun 2 (FY-2) and Multi Functional Transport Satellite (MTSAT) to those at sea surface by SAR are further estimated. This shift difference between different sensors raises an issue on project distortion and navigation system errors for FY-2 and MTSAT satellites, which are of concern to both space agencies and data users. Finally, issues for current ongoing study and future research related to typhoon eyes are discussed, including rainband tracking between sensors for local wind speeds.

Data fusion of significant wave height from HY-2A and other satellite altimeters §

The HY-2A (launched on 2011/8/16) is the first microwave marine remote sensing satellite from HY-2 satellite series developed by China. It includes a dual-frequency altimeter in Ku and C-bands. In this paper, (1) significant wave height (SWH) data from HY-2A altimeter are corrected by using collocated NDBC (U.S. National Data Buoy Centre) buoy data and are compared with Jason-1/2 data; (2) data fusion of SWH from the combination of HY-2A altimeter and other satellite altimeters (Jason-1/2 and Envisat) is given and the results are compared one another; (3) the merged SWH data from HY-2A, Jason-1/2 and Envisat are used to analyze the characteristics of SWH in China Seas and adjacent waters. Comparisons show that the results are good and HY-2A altimetry data work well with other satellite altimetry data.

Wave effects on the retrieved wind field from the advanced scatterometer (ASCAT)

To improve retrieval accuracy, this paper studies wave effects on retrieved wind field from a scatterometer. First, the advanced scatterometer (ASCAT) data and buoy data of the National Data Buoy Center (NDBC) are collocated. Buoy wind speed is converted into neutral wind at 10 m height. Then, ASCAT data are compared with the buoy data for the wind speed and direction. Subsequently, the errors between the ASCAT and the buoy wind as a function of each wave parameter are used to analyze the wave effects. Wave parameters include dominant wave period (dpd), significant wave height (swh), average wave period (apd) and the angle between the dominant wave direction (dwd) and the wind direction. Collocated data are divided into sub-datasets according to the different intervals of each wave parameter. A root mean square error (RMSE) for the wind speed and a mean absolute error (MAE) for the wind direction are calculated from the sub-datasets, which are considered as the function of wave parameters. Finally, optimal wave conditions on wind retrieved from the ASCAT are determined based on the error analyses. The results show the ocean wave parameters have correlative relationships with the RMSE of the retrieved wind speed and the MAE of the retrieved wind direction. The optimal wave conditions are presented in terms of dpd, swh, apd and angle.