V.H.S. Khoo

Bio: V.H.S. Khoo is an academic researcher from National University of Singapore. The author has contributed to research in topics: Radar imaging & Atmospheric model. The author has an hindex of 1, co-authored 2 publications receiving 21 citations.

An ERS-1 synthetic aperture radar image of a tropical squall line compared with weather radar data

Abstract: A radar image acquired by the C-band synthetic aperture radar (SAR) aboard the European Remote Sensing satellite ERS-2 over the coastal waters south of Singapore showing radar signatures of a strong tropical squall line ("Sumatra Squall") is compared with coincident and collocated weather radar data. Squall line features such as the gust front, areas of updraft convergence, and rain areas are identified. Possible attenuation effects from the rain drops in the atmosphere under very heavy rain (rain rate >100 mm/h) is suggested. In addition, the possibility of extracting the associated geophysical parameters, i.e., rain rate and wind speed from SAR imagery is investigated. The rain rate is estimated from the attenuation signature in the SAR image. Comparison between the estimated rain rate and weather radar rain rate shows consistency. Wind speed associated with the squall line is estimated based on the CMOD4 wind scatterometer model. The estimated wind speed pattern appears to be in agreement with the observed squall line structure. Possible errors in the wind estimation due to effects of rain are suggested.

Recent Observing System Experiments on the impact of ERS scatterometer wind data on numerical weather simulations of cold surge

Abstract: This study examines the impact of scatterometer wind data from the ERS satellite on the performance of a numerical weather prediction (NWP) model in simulating the planetary-scale characteristics of cold surge during the northern winter. A comprehensive set of Observing System Experiments (OSEs) were carried out using the Meteorological Service Singapore's (MSS) data assimilation and forecasting system. The period chosen for the OSEs was during a cold surge which occurred from 13 Nov 1995 to 23 Nov 1995. Two experiments, with ten day forecasts from 00 UTC, were carried out every day for this period: (1) control experiment, which uses both the conventional weather observations and the ERS scatterometer wind data, and (2) without ERS data experiment, which uses only the conventional weather observations. The comparison of the results from these experiments allows the authors to assess the impact of ERS scatterometer wind data on the data assimilation and forecasting system of MSS in simulating the characteristics of cold surge. Overall, the impacts on the performance of the NWP model in simulating the planetary-scale characteristics of cold surges are neutral albeit an observed small positive impact on the lower and upper level forecast wind fields during the developing stage of the surge. The authors' expect the impact of the scatterometer wind data in numerical forecasts to be more significant from the improvement in the wind scatterometer model, the assimilation method and the resolution of the NWP model.

Tropical Cyclone Morphology from Spaceborne Synthetic Aperture Radar

Abstract: In 2008, the Canadian Space Agency sponsored the Radarsat Hurricane Applications Project (RHAP), for researching new developments in the application of Radarsat-1 synthetic aperture radar (SAR) data and innovative mapping approaches to better understand the dynamics of tropical cyclone genesis, morphology, and movement. Although tropical cyclones can be detected by many remote sensors, SAR can yield high-resolution (subkilometer) and low-level storm information that cannot be seen below the clouds by other sensors. In addition to the wind field and tropical cyclone eye information, structures associated with atmospheric processes can also be detected by SAR. We have acquired 161 Radarsat-1 SAR images through RHAP between 2001 and 2007. Among these, 73 images show clear tropical cyclone eye structure. In addition, we also acquired 10 images from the European Space Agency's Envisat SAR between 2004 and 2010. Both Atlantic hurricanes and Pacific typhoons are included. In this study, we analyze these 83 (73 R...

On the Discrimination of Radar Signatures of Atmospheric Gravity Waves and Oceanic Internal Waves on Synthetic Aperture Radar Images of the Sea Surface

Abstract: Synthetic aperture radar (SAR) images acquired over the ocean frequently show sea wave-like patterns that have wavelengths well above those of ocean surface waves and that are sea surface signatures of oceanic internal waves (OIWs) or of atmospheric gravity waves (AGWs). However, it is often difficult to decide whether they result from the first or the second kind of waves, which has led many investigators to misinterpret SAR images of the sea surface. Based on solitary wave and radar imaging theories of AGWs and OIWs, we present criteria that help distinguish between them. However, there are cases where these criteria, which are based solely on the shape and structure of the features visible on the SAR images, yield ambiguous results. In these cases, one must resort to additional information on the generation of AGWs and OIWs, which are listed in this paper.

Simultaneous Observations and Analysis of Severe Storms Using Polarimetric X-Band SAR and Ground-Based Weather Radar

Abstract: Recent advances in synthetic aperture radar (SAR) technology have revived meteorological applications with this type of radar. SARs are designed for surface imaging, but now that several X-band multipolarization SAR satellites are in orbit, the attenuation and backscatter caused by precipitation can be better studied. The results presented here demonstrate some of the possibilities by analyzing observations from dual-polarization (HH, VV) TerraSAR-X (TSX) acquisitions over central Florida surrounding severe storms in August 2008. Simultaneous to the SAR acquisitions, WSR-88D ground weather radars in Melbourne and Tampa Bay, FL, collected reflectivity and radial velocity data; the observed strong precipitation cells from convective storms are colocated with severe attenuation in the corresponding SAR images. The observations from SAR measurements are explained quantitatively by converting ground radar reflectivity into spaceborne radar attenuation via a theoretical model. In addition, polarization analysis comparing the SAR image to two additional TSX acquisitions 11 days apart and without rain provides an indication of storm-induced propagation effects on X-band SAR. Specifically, the copolar ratio Z_dr and the copolar correlation differences exhibit behavior that is better explained by the precipitation impact versus surface changes. Multiple regions with varying ground cover, including urban, and storm characteristics are analyzed to highlight the complexity of meteorological research using SAR while revealing a potential use of the technology to investigate the storm structure.