Surface meteorological observations, associated with gust fronts produced by thunderstorm outflows over Tehran, an area surrounded by mountains, have been analyzed. Distinctive features are sudden drop in air temperat...Surface meteorological observations, associated with gust fronts produced by thunderstorm outflows over Tehran, an area surrounded by mountains, have been analyzed. Distinctive features are sudden drop in air temperature, up to 10°C, sharp increase in wind speed, up to 30 m s?1, with wind shift, to northwesterly, pressure jump, up to 4 hPa, humidity increase, up to 40%, and rain after some 20 min. Gust fronts which often occur in spring time, have a typical thickness of about 1.5 km and produce vertical wind shear of the order of 10?2s?1. Although these features seem to be common for most of the events, their intensities differ from one event to another, indicating that the gust fronts may occur in different sizes and shapes. Apart from a dominant effect on the formation of the original thunderstorms, topography appears to break up the frontal structure of the gust fronts. The internal Rossby radius of deformation for these flows is small enough (～ 100 km) for rotational effects to be minor. A laboratory model of the gust front (gravity current) also shows that it initially has a distinctive head with a turbulent wake, and can be broken up by topography. It is shown that when the environment is stratified, turbulence due to lobes and clefts instabilities near the nose of the current is suppressed. When the ground is rough, these instabilities are highly amplified and the internal Froude number of the flow is reduced. The bottom slope in the presence of rough topography leads to the break up of the current head and produces a broad and highly non-uniform head, recognized in the density signals. Key words Frontal structure - Gravity current - Meteorological data - Outflows展开更多
In order to study the characteristic features of the frontal motion over mountains, a frontal model is designed inthis paper. The analytical solution of the model with the assumption of semigeostrophic approximation a...In order to study the characteristic features of the frontal motion over mountains, a frontal model is designed inthis paper. The analytical solution of the model with the assumption of semigeostrophic approximation and no frichon is obtained and compared with numerical solution without the assumption. It assumes that the front is a free surface betWeen two nuid layers with different densihes. It also assumes that the mountain and the front are indefinitelylong. Therefore, the motion of such a front can be determined by the one-dimensional shallow water equationsystem. By making a series of experiments, we confirm the rule further that front can be retarded when climbing themountain and accelerated when going down the mountains. We also found some interesting characteristics of frontalstructure influenced by topography in addihon.展开更多
Crustal seismicity in northwestern Mendoza Province in Argentina, corresponding to the transition zone between the Chilean-Pampean flat subduction zone（26.5-33.5°S） and the Southern Central Andes normal subduct...Crustal seismicity in northwestern Mendoza Province in Argentina, corresponding to the transition zone between the Chilean-Pampean flat subduction zone（26.5-33.5°S） and the Southern Central Andes normal subduction zone to the south, is studied in order to i） identify its relationship with the mapped structure, ii）determine deformational mechanisms and iii） constrain the geometry of the fold and thrust belt in the lower crust. Through this, we aim to determine which are the structures that contribute to Andean construction, east of the Frontal Cordillera in Argentina and at the western Principal Cordillera in Chile. Data from a temporary local seismic network are reprocessed in order to achieve a precise location of hypocenters and, whenever possible, to build focal mechanisms. Results are interpreted and compared with previous seismic studies and structural models. Analyzed seismicity is grouped around the eastern front of Frontal Cordillera, with hypocenters mainly at depths of 25-40 km. Contrastingly, earthquakes in the Principal Cordillera to the west are located at the axial Andean sector and Chilean slope, with depths shallower than 15 km. Obtained focal mechanisms indicate mainly strike-slip displacements, left lateral at Frontal Cordillera and right lateral at Principal Cordillera. Based on these observations, new possible structural models are proposed, where seismogenic sources could be either associated with inherited basement structures from the Cuyania-Chilenia suture; or correspond to deep-blind thrusts linked with a deeper-than-previously-assumed decollement that could be shared between Frontal Cordillera and western Precordillera. This deeper decollement would coincide in turn with the one determined from receiver function analysis for the eastern Sierras Pampeanas in previous works, potentially implying a common decollement all through the fold and thrust belt configuration. Apart from this, a new interpretation of seismogenic structures in Principal Cordillera near the Argentina-Chile boundary is provided.展开更多
South China suffered an extremely severe disaster caused by low-temperature and ice-snow during the last 20 days of January 2008. The freezing rain and snow are climatologically related to the South China quasistation...South China suffered an extremely severe disaster caused by low-temperature and ice-snow during the last 20 days of January 2008. The freezing rain and snow are climatologically related to the South China quasistationary front （SCQSF） in mid winter. The three-dimensional structure of the SCQSF was examined based on the weather charts, the sounding profiles, and the vertical cross-sections using the method of classical synoptic analysis.展开更多
文摘Surface meteorological observations, associated with gust fronts produced by thunderstorm outflows over Tehran, an area surrounded by mountains, have been analyzed. Distinctive features are sudden drop in air temperature, up to 10°C, sharp increase in wind speed, up to 30 m s?1, with wind shift, to northwesterly, pressure jump, up to 4 hPa, humidity increase, up to 40%, and rain after some 20 min. Gust fronts which often occur in spring time, have a typical thickness of about 1.5 km and produce vertical wind shear of the order of 10?2s?1. Although these features seem to be common for most of the events, their intensities differ from one event to another, indicating that the gust fronts may occur in different sizes and shapes. Apart from a dominant effect on the formation of the original thunderstorms, topography appears to break up the frontal structure of the gust fronts. The internal Rossby radius of deformation for these flows is small enough (～ 100 km) for rotational effects to be minor. A laboratory model of the gust front (gravity current) also shows that it initially has a distinctive head with a turbulent wake, and can be broken up by topography. It is shown that when the environment is stratified, turbulence due to lobes and clefts instabilities near the nose of the current is suppressed. When the ground is rough, these instabilities are highly amplified and the internal Froude number of the flow is reduced. The bottom slope in the presence of rough topography leads to the break up of the current head and produces a broad and highly non-uniform head, recognized in the density signals. Key words Frontal structure - Gravity current - Meteorological data - Outflows
文摘In order to study the characteristic features of the frontal motion over mountains, a frontal model is designed inthis paper. The analytical solution of the model with the assumption of semigeostrophic approximation and no frichon is obtained and compared with numerical solution without the assumption. It assumes that the front is a free surface betWeen two nuid layers with different densihes. It also assumes that the mountain and the front are indefinitelylong. Therefore, the motion of such a front can be determined by the one-dimensional shallow water equationsystem. By making a series of experiments, we confirm the rule further that front can be retarded when climbing themountain and accelerated when going down the mountains. We also found some interesting characteristics of frontalstructure influenced by topography in addihon.
基金supported by"Proyecto Unidad Ejecutora IDEAN:Evolución geológica de los Andes y su impacto económico y ambiental"
文摘Crustal seismicity in northwestern Mendoza Province in Argentina, corresponding to the transition zone between the Chilean-Pampean flat subduction zone（26.5-33.5°S） and the Southern Central Andes normal subduction zone to the south, is studied in order to i） identify its relationship with the mapped structure, ii）determine deformational mechanisms and iii） constrain the geometry of the fold and thrust belt in the lower crust. Through this, we aim to determine which are the structures that contribute to Andean construction, east of the Frontal Cordillera in Argentina and at the western Principal Cordillera in Chile. Data from a temporary local seismic network are reprocessed in order to achieve a precise location of hypocenters and, whenever possible, to build focal mechanisms. Results are interpreted and compared with previous seismic studies and structural models. Analyzed seismicity is grouped around the eastern front of Frontal Cordillera, with hypocenters mainly at depths of 25-40 km. Contrastingly, earthquakes in the Principal Cordillera to the west are located at the axial Andean sector and Chilean slope, with depths shallower than 15 km. Obtained focal mechanisms indicate mainly strike-slip displacements, left lateral at Frontal Cordillera and right lateral at Principal Cordillera. Based on these observations, new possible structural models are proposed, where seismogenic sources could be either associated with inherited basement structures from the Cuyania-Chilenia suture; or correspond to deep-blind thrusts linked with a deeper-than-previously-assumed decollement that could be shared between Frontal Cordillera and western Precordillera. This deeper decollement would coincide in turn with the one determined from receiver function analysis for the eastern Sierras Pampeanas in previous works, potentially implying a common decollement all through the fold and thrust belt configuration. Apart from this, a new interpretation of seismogenic structures in Principal Cordillera near the Argentina-Chile boundary is provided.
基金the Key Project of the National Scientific and Technological Supporting Program of the 11th Five-Year Plan of China and the Open Project of the State Key Laboratory of Severe Weather of Chinese Academy of Meteorological Sciences in 2007.
文摘South China suffered an extremely severe disaster caused by low-temperature and ice-snow during the last 20 days of January 2008. The freezing rain and snow are climatologically related to the South China quasistationary front （SCQSF） in mid winter. The three-dimensional structure of the SCQSF was examined based on the weather charts, the sounding profiles, and the vertical cross-sections using the method of classical synoptic analysis.
