2 resultados para Continental precipitation annual amplitude

em Archimer: Archive de l'Institut francais de recherche pour l'exploitation de la mer


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The TOPEX/POSEIDON mission offers the first opportunity to observe rain cells over the ocean by a dual-frequency radar altimeter (TOPEX) and simultaneously observe their natural radiative properties by a three-frequency radiometer (TOPEX microwave radiometer (TMR)). This work is a feasibility study aimed at understanding the capability and potential of the active/passive TOPEX/TMR system for oceanic rainfall detection. On the basis of past experiences in rain flagging, a joint TOPEX/TMR rain probability index is proposed. This index integrates several advantages of the two sensors and provides a more reliable rain estimate than the radiometer alone. One year's TOPEX/TMR TMR data are used to test the performance of the index. The resulting rain frequency statistics show quantitative agreement with those obtained from the Comprehensive Ocean-Atmosphere Data Set (COADS) in the Intertropical Convergence Zone (ITCZ), while qualitative agreement is found for other regions of the world ocean. A recent finding that the latitudinal frequency of precipitation over the Southern Ocean increases steadily toward the Antarctic continent is confirmed by our result. Annual and seasonal precipitation maps are derived from the index. Notable features revealed include an overall similarity in rainfall pattern from the Pacific, the Atlantic, and the Indian Oceans and a general phase reversal between the two hemispheres, as well as a number of regional anomalies in terms of rain intensity. Comparisons with simultaneous Global Precipitation Climatology Project (GPCP) multisatellite precipitation rate and COADS rain climatology suggest that systematic differences also exist. One example is that the maximum rainfall in the ITCZ of the Indian Ocean appears to be more intensive and concentrated in our result compared to that of the GPCP. Another example is that the annual precipitation produced by TOPEX/TMR is constantly higher than those from GPCP and COADS in the extratropical regions of the northern hemisphere, especially in the northwest Pacific Ocean. Analyses of the seasonal variations of prominent rainy and dry zones in the tropics and subtropics show various behaviors such as systematic migration, expansion and contraction, merging and breakup, and pure intensity variations, The seasonality of regional features is largely influenced by local atmospheric events such as monsoon, storm, or snow activities. The results of this study suggest that TOPEX and its follow-on may serve as a complementary sensor to the special sensor microwave/imager in observing global oceanic precipitation.

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The Arctic continental margin contains large amounts of methane in the form of methane hydrates. The west Svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone. The presence of bottom simulating reflectors (BSR) on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area. Resistivity obtained from marine controlled source electromagnetic (CSEM) data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments. We acquired two CSEM lines in the west Svalbard continental slope, extending from the edge of the continental shelf (250 m water depth) to water depths of around 800 m. High resistivities (5-12 Ωm) observed above the BSR support the presence of gas hydrate in water depths greater than 600 m. High resistivities (3-4 Ωm) at 390-600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone (GHSZ) of the continental slope. In addition, high resistivities (4-8 Ωm) landward of the GHSZ are coincident with high-amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas. Pore space saturation estimates using a connectivity equation suggest 20-50% hydrate within the lower slope sediments and less than 12% within the upper slope sediments. A free gas zone beneath the GHSZ (10-20% gas saturation) is connected to the high free gas saturated (10-45%) area at the edge of the continental shelf, where most of the seeps are observed. This evidence supports the presence of lateral free gas migration beneath the GHSZ towards the continental shelf.