Hydraulic Regimes of Flow over Mountains during Severe Downslope Windstorms: Novorossiysk Bora,Novaya Zemlya Bora,and Pevek Yuzhak

Common features of the flow behavior over mountains within the hydraulic jump model are identified based on an analysis of 36 episodes of severe winds in the regions of Novorossiysk, Pevek, and Novaya Zemlya. In all these episodes, the incoming flow is characterized by a strong inversion layer at altitudes of 0.5–1.5 km and, in the case of bora, by a critical level in the wind profile in the middle troposphere, which creates conditions for a weakened dynamic interaction between the low-level air flowing over mountains and the upper layers of the atmosphere. The wind-speed increase on the lee slope is caused by the transition of the incoming flow from the subcritical to supercritical state. In this case, the velocity amplitude increases with increasing inversion intensity. Model estimates of wind-speed increase are in good agreement with observations at lee-side weather stations for episodes with a strong elevated inversion.     

SWAN模型模拟风浪场侧边界失真范围研究

鉴于SWAN模型存在着不能有效地模拟固壁边界附近风浪场的缺点,即在边界附近所模拟的波要素存在失真的现象,研究了在不同水深、风速和风向情况下模型侧边界附近波要素的失真范围,并对计算结果进行了详细的分析。结果表明水深、风速和风向对于侧边界附近波要素的失真范围具有不同的影响,即在风速一定的情况下,失真范围随着水深的增大而增大;水深一定的情况下,失真范围随着风速的增大而减小、随着风向的增大而增大。在利用SWAN模型模拟计算近岸或内陆湖泊风浪场时,必须采取适当的措施以减少实际计算域侧边界附近计算结果的失真范围。     

The impact of the Black Sea on the local thermal and baric processes in the atmosphere

Relying on Shuleikin’s theory, defining the relationship between the elements of the temperature and air pressure fields, the paper provides numerical characteristics of the cyclone evolving into a hurricane on 15 November 1992. Calculated is the trade wind-induced circulation component for the western section of the Black Sea during the period of autumn-winter-time cooling of waters. The paper examines specific features of the north-west storms, Crimean bora, and breeze-induced circulation in the region. Translated by Vladimir A. Puchkin.     

Fine structure of a Greenland reverse tip jet: a numerical simulation

Reverse tip jets are strong low-level winds with easterly component that form near the southern tip of Greenland. In the present study, a reverse tip jet case which occurred from 21 to 22 December 2000 was examined to clarify its fine structure using a numerical model with a horizontal resolution of 3 km. The reverse tip jet, showing the supergeostrophic wind speed with a maximum wind speed in excess of 45 m s⁻¹, extended from the east coast of Greenland to the west of Cape Farewell with anticyclonic curvature. A cloud free region coincided with the jet indicated that there was a mesoscale downdraft. Along the eastern edge of the jet, a banded cloud formed between the upstream easterly wind and the colder northerly wind that is a part of the jet and is located along the east coast. This cloud was associated with large gradients in surface wind speed, temperature, moisture, and heat flux. A maximum surface total heat flux of 300 W m⁻² coincided with the location of the jet. It is suggested that the orographic deflection by Greenland's large-scale topography as well as small-scale downslope winds behind mountains with fiords causes the reverse tip jet.     

A review of recent advances in understanding the meso- and microscale properties of the severe Bora wind

A gusty downslope windstorm that blows at the eastern Adriatic coast is called bora. Similar winds exist at many other places on virtually all continents. Related hourly mean wind speeds surpassing 20 m s⁻¹, with gusts reaching up to 50 or even 70 m s⁻¹, in the coastal mountain lee areas are common (hurricane speeds). There has been substantial progress in bora observations and measurements, understanding, modelling and its more detailed prediction during the last 25 yr. It was generally thought before that bora was a falling, mostly thermodynamically driven wind; however, (severe) bora is primarily governed by mountain wave breaking. Understandings of bora interactions and influences on other processes have taken place as well, most notably in the air-sea interaction, but are not completed yet. The overall progress mentioned would not be possible without airborne data, non-linear theory and advances in computational techniques, most notably mesoscale numerical models. Some gaps in bora knowledge are also indicated, for example, dynamical transition from weak to moderate to strong to severe bora flows, where the latter are the main subject here, and vice versa. Moreover, the role of the boundary layer and waves on the upwind side of the bora evolution and the consequent lee side flow structures are inadequately understood; this is especially so for bora at the southern Adriatic coast. The focus here is on stronger bora flows at the NE Adriatic coast.     

印度洋海温异常对西北太平洋台风的影响及机制研究

基于近40 a NCEP/NCAR再分析月平均高度场、风场、涡度场、垂直速度场以及NOAA重构的海面温度(sea surface temperature,SST)资料和美国联合台风预警中心(Joint Typhoon Warning Center,JTWC)热带气旋最佳路径资料,利用合成分析方法,研究了前期春季及同期夏季印度洋海面温度同夏季西北太平洋台风活动的关系。结果表明:1)前期春季印度洋海温异常(sea surface temperature anomaly,SSTA)尤其是关键区位于赤道偏北印度洋和西南印度洋地区对西北太平洋台风活动具有显著的影响,春季印度洋海温异常偏暖年,后期夏季,110°～180°E的经向垂直环流表现为异常下沉气流,对应风场的低层低频风辐散、高层辐合的形势,这种环流形势使得低层水汽无法向上输送,对流层中层水汽异常偏少,纬向风垂直切变偏大,从而夏季西北太平洋台风频数偏少、强度偏弱,而异常偏冷年份则正好相反。2)春季印度洋异常暖年,西北太平洋副热带高压加强、西伸;而春季印度洋异常冷年,后期夏季西北太平洋副热带高压减弱、东退,这可能是引起夏季西北太平洋台风变化的另一原因。     

Variability of sea surface temperature in the Japan Sea and its relationship to the wind-curl field

The variability of sea surface temperature (SST) in the Japan Sea is investigated using the complex EOF analysis of daily data produced at Tohoku University, Japan (New Generation SST; 2002–2006). The relationship with the wind field is investigated from the daily NCEP/NCAR reanalysis data with a 1° spatial resolution. Anomalies in the SST (SSTAs) are calculated by subtracting the basin-average annual variation estimated as a leading mode of temperature. The leading mode of an SSTA represents a adjustment to the annual mean variation, most significant in December in the zone of subtropical waters entering the sea through the Korean Strait and in the northwestern sea, over which a cyclonic wind curl develops in the cold period. The semiannual variability mode is identified, which is characterized by the largest temperature increase (decrease) in the western branch of the subarctic front (in the Tatar Strait), which lags by two months behind the semiannual changes in wind curl over the sea. An episodic SSTA movement is detected in the northern part of the sea, which moves from east to west along the western branch of the Tsushima Warm Current with a speed corresponding in magnitude to an advective scale.     

Surface current patterns in front of the Venice Lagoon

Surface patterns of the low-frequency current in a 20 × 30 km region in front of the Venice Lagoon were analysed from a 13-month-long HF radar data set. Surface circulation was related to prevalent wind regimes in the area and to the tidal flow through the lagoon inlet. Three different categories of wind-forcing were defined: bora (NE wind), sirocco (SE wind), and finally the category containing all other wind directions and calms (winds lower than 3 m/sec). Mean flow and vorticity spatial distributions were discussed for different wind conditions. The coastal area about 5 km wide is characterized by a flow field with maximum vorticity. Outside the coastal boundary layer the interior flow is part of the Adriatic basin-wide cyclonic circulation. Two counter-rotating vortices of the dimension of about 4–5 km were evidenced in the average flow field in front of the lagoon inlet (Malamocco inlet) in all situations except for the bora. The vortex-pair is probably associated with the tidal flow through the inlet. The bora wind induces a strong southward coastal jet detached from the coast by about 5 km homogenizing the flow and eliminating the dipole. The average coastal flow pattern in calm wind conditions was also analysed as a function of the strong inflow/outflow (currents in the inlet > 0.7 m s⁻¹) from the lagoon inlet. In both cases the vorticity pattern is similar, with the negative vorticity to the left of the inlet and positive to the right looking seaward.     

Characteristics and influencing factors of frontal upwelling in the Yellow Sea in summer

Frontal upwelling is an important phenomenon in summer in the Yellow Sea (YS) and plays an essential role in the distribution of nutrients and biological species. In this paper, a three-dimensional hydrodynamic model is applied to investigate the characteristics and influencing factors of frontal upwelling in the YS. The results show that the strength and distribution of frontal upwelling are largely dependent on the topography and bottom temperature fronts. The frontal upwelling in the YS is stronger and narrower near the eastern coast than near the western coast due to the steeper shelf slope. Moreover, external forcings, such as the meridional wind speed and air temperature in summer and the air temperature in the preceding winter and spring, have certain influences on the strength of frontal upwelling. An increase in air temperature in the previous winter and spring weakens the frontal upwelling in summer; in contrast, an increase in air temperature in summer strengthens the frontal upwelling. When the southerly wind in summer increases, the upwelling intensifies in the western YS and weakens in the eastern YS. The air temperature influences the strength of upwelling by changing the baroclinicity in the frontal region. Furthermore, the meridional wind speed in summer affects frontal upwelling via Ekman pumping.     

Co-variation of the surface wind speed and the sea surface temperature over mesoscale eddies in the Gulf Stream region:momentum vertical mixing aspect

The co-variation of surface wind speed and sea surface temperature (SST) over the Gulf Stream frontal region is investigated using high-resolution satellite measurements and atmospheric reanalysis data. Results show that the pattern of positive SST-surface wind speed correlations is anchored by strong SST gradient and marine atmospheric boundary layer (MABL) height front, with active warm and cold-ocean eddies around. The MABL has an obvious transitional structure along the strong SST front, with greater (lesser) heights over the north (south) side. The significant positive SST-surface wind-speed perturbation correlations are mostly found over both strong warm and cold eddies. The surface wind speed increases (decreases) about 0.32 (0.41) m/s and the MABL elevates (drops) approximate 55 (54) m per 1℃ of SST perturbation induced by warm (cold) eddies. The response of the surface wind speed to SST perturbations over the mesoscale eddies is mainly attributed to the momentum vertical mixing in the MABL, which is confirmed by the linear relationships between the downwind (crosswind) SST gradient and wind divergence (curl).     

Influence of asymmetric wind stress curl on the general ocean circulation using an eddy-resolving quasi-geostrophic model

On the general ocean circulation forced by the asymmetric wind stress curl, the role of the eddies which are detached from the western boundary current is studied using an eddy-resolving two-layered quasi-geostrophic numerical model with free-slip boundary condition. An ideal sinusoidal function is used as the wind stress curl, and amplitude is assumed to be larger over the southern basin than over the northern one. In contrast with the antisymmetric wind forcing, in the asymmetric wind stress case, the subtropical western boundary current overshoots to the north from the zero wind stress curl line. As the asymmetricity of the wind forcing becomes larger, the separation point of the time mean field is located further north. The eddies generated in the region of the subtropical recirculation are advected northward by the western boundary current and they are detached from subtropical gyre. The release of these eddies to the north basin leads to weaken the subtropical recirculation system. From the analysis of the potential vorticity budgets, in the asymmetric case, it is shown that detached eddies play an important role in transporting the negative vorticity which is excessively inputted into the southern basin, to the northern basin, in addition to the terms which transport vorticity in the antisymmetric case, i.e., the vorticity transport by the meander of the jet. Under the free-slip boundary, more than a quarter of that excess vorticity is transported by those detached eddies in some cases.     

Seasonal variation in the three-dimensional structures of coastal thermal front off western Guangdong

The seasonal structure and dynamic mechanism of oceanic surface thermal fronts(STFs) along the western Guangdong coast over the northern South China Sea shelf were analyzed using in situ observational data, remote sensing data, and numerical simulations. Both in situ and satellite observations show that the coastal thermal front exhibits substantial seasonal variability, being strongest in winter when it has the greatest extent and strongest sea surface temperature gradient. The winter coastal thermal front begins to appear in November and disappears after the following April. Although runoff water is more plentiful in summer, the front is weak in the western part of Guangdong. The frontal intensity has a significant positive correlation with the coastal wind speed,while the change of temperature gradient after September lags somewhat relative to the alongshore wind. The numerical simulation results accurately reflect the seasonal variation and annual cycle characteristics of the frontal structure in the simulated area. Based on vertical cross-section data, the different frontal lifecycles of the two sides of the Zhujiang(Pearl) River Estuary are analyzed.     

西太平洋8708号台风海面风、浪结构及其关系的遥感研究

本文以Geosat卫星高度计1987年8月11日在西太平洋海域上的-上升轨道测得的风、浪资料为基础,统计分析了8708号台风影响下的海面风速和海浪特征.结果显示,此次台风影响下的海面风速和海浪波高的空间分布具有相对台风中心近似对称的结构特征,但在台风内区,台风移动方向的右方风速较左方风速增加较快,同时在台风外围,右方风速较左方风速衰减也较快;有效波高没有明显的内、外区结构,且左、右方波高随距离变化也呈不同的衰减率;风速与有效波高的关系在台风中心左右也呈现明显的不同;本文给出了台风的风速及波高随相对台风中心距离变化的经验关系式,以及合风风速与波高的经验关系式等.     

Circulation and heat budget of the northern Adriatic Sea (Italy) due to a Bora event in January 2001: a numerical model study

The Princeton Ocean Model was implemented to investigate the response of northern Adriatic Sea during the Bora event in January 2001 when strong wind and surface cooling was reported. The model has been run with realistic wind stress, surface heat flux and river runoffs forcings continuously from 1 January 1999 to 31 January 2001. The wind stress and surface heat flux was computed by the bulk parameterization, using the European Centre for Medium Range Weather Forecast analysis fields and the Comprehensive Ocean Atmosphere Data Set cloud data. All the freshwater sources along the Adriatic coastlines were represented by point or line source functions. Open boundary conditions in the Ionian Sea along a latitudinal boundary were nested within a large scale model of the Mediterranean Sea. The numerical study found that, before the Bora event of 13–17 January 2001, the water column of the northern Adriatic Sea was stratified by salinity, and the temperature was already cooler at the surface and over the shallower shelf region. The pre-Bora circulation of the northern Adriatic Sea was relatively weak and baroclinic with maximum surface currents occurred near the Italian coast. During the Bora event, the water column was well mixed in the most of coastal region of the northern Adriatic Sea. The atmospheric cooling produced colder water over the northern and western Adriatic Coast. The circulation of the northern Adriatic Sea was barotropic and dominantly wind driven, with maximum current speed of about 1 m s⁻¹. The numerical study also demonstrated that the Bora event decreased the heat content of the water column with an area averaged value of 205 W m⁻² over the shallow northern shelf. It was concluded that the heat budget of the northern Adriatic Sea during the Bora event was a balance between the surface heat loss, horizontal net heat inflow and resulting heat content decrease. The horizontal advection played a particularly important role in controlling the water temperature change over the shallower northern shelf.     

全球有效波高和风速的时空变化及相关关系研究

The climatology of significant wave height（SWH） and sea surface wind speed are matters of concern in the fields of both meteorology and oceanography because they are very important parameters for planning offshore structures and ship routings. The TOPEX/Poseidon altimeter, which collected data for about 13 years from September 1992 to October 2005, has measured SWHs and surface wind speeds over most of the world＇s oceans. In this paper, a study of the global spatiotemporal distributions and variations of SWH and sea surface wind speed was conducted using the TOPEX/Poseidon altimeter data set. The range and characteristics of the variations were analyzed quantitatively for the Pacific, Atlantic, and Indian oceans. Areas of rough waves and strong sea surface winds were localized precisely, and the correlation between SWH and sea surface wind speed analyzed.     

利用SWAN模型计算的侧边界附近风浪场的变化特征

鉴于SWAN模型不能有效地模拟侧边界附近的风浪场,详细研究了在不同水深和风速情况下模型侧边界附近波要素,包括波高、周期、波向和波长的变化特征和失真范围。计算结果表明水深和风速的变化对于侧边界附近不同波要素的影响是不同的。在风速一定的情况下,失真范围随着水深的增大而增大。在水深一定的情况下,根据波高、周期和波长的相对误差所计算的失真范围与根据他们的绝对误差所计算的失真范围不同。随着风速的增大,根据相对误差所计算的失真范围减小;而根据绝对误差所计算的失真范围变化不大。随着风速的增大,波向的失真范围减小。研究了在水深变化的水域,包括太湖和淀山湖在内的侧边界附近的风浪场,结果表明如果计算范围不适当扩大确实会导致侧边界附近的风浪场失真。因此在利用SWAN模型模拟计算近岸或内陆湖泊风浪场时,需采取适当的措施以减小计算误差。     

Ocean temperature change around New Zealand over the last 36 years

Ocean temperature changes around New Zealand are estimated from satellite sea surface temperature (SST) products since 1981, two high resolution expendable bathythermograph transects (HRXBT) since 1986 and 1991, and Argo data since 2006. The datasets agree well where they overlap. Significant surface warming is found in subtropical waters. Greatest warming is east of Australia and in the central Pacific. All NZ coastal waters are warming, with strongest warming east of Wairarapa and weakest between East Cape and North Cape. Temperature changes are surface intensified, extending to ～200 m in the northeast and at least 850 m in the eastern Tasman. Significant interannual variability is coherent over a large area of ocean north of the Subtropical Front and modulates extreme events. NZ air temperatures are highly correlated at interannual timescales with SSTs over a broad region of ocean north of the Subtropical Front from the eastern Tasman to east of the dateline.     

降雨条件下全极化微波辐射计海面风场校正

为提高降雨条件下星载全极化微波辐射计海面风场精度,通过匹配WindSat海面风场和降雨率数据以及美国国家浮标中心浮标观测数据,得到18 996组匹配样本,深入分析了降雨对海面风场反演精度的严重影响,构建了风场校正模型。试验结果表明,降雨导致海面风速被严重高估,风向误差随着降雨率的增大而增大。校正后的风速精度在低风速段提升明显。无论降雨率多大,校正后风速精度均比校正前高。风速均方根误差由原来的2.9 m/s降低到了2.1 m/s,风向均方根误差由原来的26.9°降低到了26.3°。     

Climatic parameters of wind-field variability in the Black Sea region: Numerical reanalysis of regional atmospheric circulation

A reanalysis of atmospheric circulation in the Black Sea region is performed with a high spatial resolution of 25 × 25 km for the period from 1958 to 2001. Climatic wind speed fields are estimated, as are their spatial structure and seasonal variability. Mesoscale regions of cyclonic and anticyclonic speed vorticity, which are connected with edge effects and orography, are distinguished. To single out the monsoon mechanism in the annual votricity cycle, numerical experiments on the sensitivity that the regional atmospheric circulation has to the sea-surface temperature perturbations have been carried out. Large-scale regional peculiarities of the surface wind field are considered for different seasons. The vorticity of the surface wind speed and strength is assessed. Large annual average values of the vorticity are obtained which are comparable with the range of seasonal variability; they evidence the defining role that wind plays in the generation and seasonal variability, as well as the average cyclonic water circulation in the Black Sea.     

On the hydrodynamic correction of SEASAT altimeter data (Hudson bay area of Canada)

Abstract

The SEASAT Geophysical Data Record (GDR) file includes a number of corrections for instruments, atmospheric effects, coastal effects, and geophysical effects. However, the transient sea surface variation due to the ocean circulation and wind surge is not implemented. In this research an interactive numerical scheme, based on the vertically integrated hydrodynamic equations, is developed to make this correction. Since the exactness of the transient sea surface depends upon the exactness of the input meteorological data, a method is also developed for extracting surface wind speed and direction from weather charts for the interactive hydrodynamic numerical algorithm.

The application of the algorithm over the Hudson Bay of Canada demonstrates that this technique can easily be applied to any regional altimeter data over a water‐covered area. The resulting transient sea surface profiles and the comparisons between wind fields derived from the weather charts and altimeter inferred wind fields over Hudson Bay for revolution numbers 559, 564, and 574 are presented.