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1.
The forcing mechanisms for Antarctic coastal polynyas and the thermodynamic effects of existing polynyas are studied by means of an air-sea-ice interaction experiment in the Weddell Sea in October and November 1986.Coastal polynyas develop in close relationship to the ice motion and form most rapidly with offshore ice motion. Narrow polynyas occur frequently on the lee side of headlands and with strong curvature of the coastline. From the momentum balance of drifting sea ice, a forcing diagram is constructed, which relates ice motion to the surface-layer wind vector v
z
and to the geostrophic ocean current vector c
g
. In agreement with the data, wind forcing dominates when the wind speed at a height of 3 m exceeds the geostrophic current velocity by a factor of at least 33. This condition within the ocean regime of the Antarctic coastal current usually is fulfilled for wind speeds above 5 m/s at a height of 3 m.Based on a nonlinear parameter estimation technique, optimum parameters for free ice drift are calculated. Including a drift dependent geostrophic current in the ice/water drag yields a maximum of explained variance (91%) of ice velocity.The turbulent heat exchange between sea ice and polynya surfaces is derived from surface-layer wind and temperature data, from temperature changes of the air mass along its trajectory and from an application of the resistance laws for the atmospheric PBL. The turbulent heat flux averaged over all randomly distributed observations in coastal polynyas is 143 W/m2. This value is significantly different over pack ice and shelf ice surfaces, where downward fluxes prevail. The large variances of turbulent fluxes can be explained by variable wind speeds and air temperatures. The heat fluxes are also affected by cloud feedback processes and vary in time due to the formation of new ice at the polynya surface.Maximum turbulent fluxes of more than 400 W/m2 result from strong winds and low air temperatures. The heat exchange is similarly intense in a narrow zone close to the ice front, when under weak wind conditions, a local circulation develops and cold air associated with strong surface inversions over the shelf ice is heated above the open water. 相似文献
2.
W. Gorter J. H. van Angelen J. T. M. Lenaerts M. R. van den Broeke 《Climate Dynamics》2014,42(5-6):1595-1611
The present and twenty-first century near-surface wind climate of Greenland is presented using output from the regional atmospheric climate model RACMO2. The modelled wind variability and wind distribution compare favourably to observations from three automatic weather stations in the ablation zone of southwest Greenland. The Weibull shape parameter is used to classify the wind climate. High values (κ > 4) are found in northern Greenland, indicative of uniform winds and a dominant katabatic forcing, while lower values (κ < 3) are found over the ocean and southern Greenland, where the synoptic forcing dominates. Very high values of the shape parameter are found over concave topography where confluence strengthens the katabatic circulation, while very low values are found in a narrow band along the coast due to barrier winds. To simulate the future (2081–2098) wind climate RACMO2 was forced with the HadGEM2-ES general circulation model using a scenario of mid-range radiative forcing of +4.5 W m?2 by 2100. For the future simulated climate, the near-surface potential temperature deficit reduces in all seasons in regions where the surface temperature is below the freezing point, indicating a reduction in strength of the near-surface temperature inversion layer. This leads to a wind speed reduction over the central ice sheet where katabatic forcing dominates, and a wind speed increase over steep coastal topography due to counteracting effects of thermal and katabatic forcing. Thermally forced winds over the seasonally sea ice covered region of the Greenland Sea are reduced by up to 2.5 m s?1. 相似文献
3.
The atmospheric conditions that lead to strong offshore surface winds in Southern California, commonly referred to as Santa
Ana winds, are investigated using the North American Regional Reanalysis and a 12-year, 6-km resolution regional climate simulation
of Southern California. We first construct an index to characterize Santa Ana events based on offshore wind strength. This
index is then used to identify the average synoptic conditions associated with Santa Ana events—a high pressure anomaly over
the Great Basin. This pressure anomaly causes offshore geostrophic winds roughly perpendicular to the region’s mountain ranges,
which in turn cause surface flow as the offshore momentum is transferred to the surface. We find, however, that there are
large variations in the synoptic conditions during Santa Ana conditions, and that there are many days with strong offshore
flow and weak synoptic forcing. This is due to local thermodynamic forcing that also causes strong offshore surface flow:
a large temperature gradient between the cold desert surface and the warm ocean air at the same altitude creates an offshore
pressure gradient at that altitude, in turn causing katabatic-like offshore flow in a thin layer near the surface. We quantify
the contribution of “synoptic” and “local thermodynamic” mechanisms using a bivariate linear regression model, and find that,
unless synoptic conditions force strongly onshore winds, the local thermodynamic forcing is the primary control on Santa Ana
variability. 相似文献
4.
Synoptic-scale atmospheric circulation patterns drive wind forcing of dynamic and thermodynamic processes in Arctic sea ice. Synoptic typing and compositing are common techniques used to identify a limited number of prevailing weather classifications that govern a region's climate. This work investigates atmospheric circulation patterns (surface to 250?hPa) for the southern Beaufort Sea and corresponding surface wind regimes within each synoptic type. Significant changes (p?<?0.05) in relative frequencies of a number of synoptic types were attributed to declining summer sea ice. Corresponding upper-level circulation anomalies show increasingly meridional atmospheric circulation. Synoptic Types 9 and 11 were identified as key October-November-December circulation features that represent deepening of the Aleutian low with concomitant strengthening of pressure gradients over the southern Beaufort Sea. Classification of coastal-based wind observations shows a shift towards increased easterly wind forcing. A case study of surface wind data from the CCGS Amundsen (2009–2011) provided a direct example of the surface wind regime within the marginal ice zone within each synoptic type during a period of reduced Arctic sea-ice cover. 相似文献
5.
Dong-Hyuk Kim Hwa-Woon Lee Soon-Hwan Lee 《Asia-Pacific Journal of Atmospheric Sciences》2010,46(4):393-403
The wind distribution over the Korean Peninsula was analyzed using numerically optimized wind data to reduce the uncertainties in estimating the wind resources. The simulated data were validated by a comparison with surface wind observations and three statistical indexes. According to the simulated surface winds, mesoscale circulation, such as land-sea breeze and mountain-valley winds affect the wind characteristics of the hub height at coastal and inland regions. However, the prevailing winds are strongly associated with the synoptic forcing at the island and mountainous regions, not the regional circulation. On the other hand, the atmospheric stability definitely affects the strength of the daytime and nocturnal wind speed at a hub height. Overall, there was a significant difference between the numerical and logarithmic method to estimate the wind energy at hub height. Moreover, the discrepancy in the wind density estimated using the two methods becomes clear over inland and mountainous areas. 相似文献
6.
An ice breeze mechanism for boundary-layer jets 总被引：1，自引：0，他引：1
The existence of a low-level (z=~1000 m) jet adjacent to a sea-ice boundary is investigated with a two-dimensional numerical model. A thermally-direct ice breeze circulation is induced by specifying an ice-sea surface temperature gradient, with the mean geostrophic wind parallel to the ice edge. Pressure changes associated with over-water mixed-layer development create an increase in geostrophic velocity that accounts for most of the increase in wind speed. A change in initial geostrophic wind direction has significant effects on location and intensity of the low-level jet; geostrophic winds parallel to the ice edge result in stronger jets than occur with cross-ice geostrophic winds. An inertial oscillation simulated by the model in 1-D makes a negligible contribution to the low-level jet. 相似文献
7.
Günther Heinemann 《Boundary-Layer Meteorology》1999,93(1):75-116
The aircraft-based experiment KABEG97 (Katabatic wind and boundary-layer front experiment around Greenland) was performed in April/May 1997. During the experiment, surface stations were installed at five positions on the ice sheet and in the tundra near Kangerlussuaq, West Greenland. A total of nine katabatic wind flights were performed during quite different synoptic situations and surface conditions, and low-level jets with wind speeds up to 25m s-1 were measured under strong synoptic forcing of the katabatic wind system. The KABEG data represent a unique data set for the investigation of katabatic winds. For the first time, high-resolution and accurate aircraft measurements can be used to investigate the three-dimensional structure of the katabatic wind system for a variety of synoptic situations.Surface station data show that a pronounced daily cycle of the near-surface wind is present for almost all days due to the nighttime development of the katabatic wind. In a detailed case study the stably-stratified boundary layer over the ice and the complex boundary-layer structure in the transition zone ice/tundra are investigated. The katabatic wind system is found to extend about 10 km over the tundra area and is associated with strong wind convergence and gravity waves. The investigation of the boundary-layer dynamics using the concept of a two-layer katabatic wind model yields the results that the katabatic flow is always a shooting flow and that the pure katabatic force is the main driving mechanism for the flow regime, although a considerable influence of the large-scale synoptic forcing is found as well. 相似文献
8.
The effect of the high frequency (synoptic) variability of wind and heat fluxes upon the surface ocean off south-central Chile (west coast of South America) is investigated using a regional ocean model. We focus our analysis in austral summer, when the regional wind experiences significant day-to-day variability superimposed on a mean, upwelling favorable flow. To evaluate the nature and magnitude of these effects, we performed three identical simulations except for the surface forcing: the climatological run, with long-term monthly mean wind-stresses and heat fluxes; the wind-synoptic run, with daily wind stresses and climatological heat fluxes; and the full-synoptic run, with daily wind-stresses and daily fluxes. The mean currents and surface geostrophic EKE fields show no major differences between simulations, and agree well with those observed in this ocean area. Nevertheless, substantially more ageostrophic EKE is found in the simulations which include synoptic variability of wind-stresses, impacting the total surface EKE and diffusivities, particularly south of Punta Lavapie (37° S), where the lack of major currents implies low levels of geostrophic EKE. Summer mean SSTs are similar in all simulations and agree with observations, but SST variability along the coast is larger in the runs including wind-stress synoptic variability, suggesting a rather linear response of the ocean to cycles of southerly wind strengthening and relaxation. We found that coastal SST variability does not change significantly in the first tenths of kilometers from the shore when including daily heat fluxes, highlighting the prominent role of wind-driven upwelling cycles. In contrast, in the offshore region situated beyond the 50 km coastal strip, it is necessary to include synoptic variability in the heat fluxes to account for a realistic SST variability. 相似文献
9.
The 2018 Winter Olympic and Paralympic Games will be held in Pyeongchang, Korea, during February and March. We examined the near surface winds and wind gusts along the sloping surface at two outdoor venues in Pyeongchang during February and March using surface wind data. The outdoor venues are located in a complex, mountainous terrain, and hence the near-surface winds form intricate patterns due to the interplay between large-scale and locally forced winds. During February and March, the dominant wind at the ridge level is westerly; however, a significant wind direction change is observed along the sloping surface at the venues. The winds on the sloping surface are also influenced by thermal forcing,showing increased upslope flow during daytime. When neutral air flows over the hill, the windward and leeward flows show a significantly different behavior. A higher correlation of the wind speed between upper-and lower-level stations is shown in the windward region compared with the leeward region. The strong synoptic wind, small width of the ridge, and steep leeward ridge slope angle provide favorable conditions for flow separation at the leeward foot of the ridge. The gust factor increases with decreasing surface elevation and is larger during daytime than nighttime. A significantly large gust factor is also observed in the leeward region. 相似文献
10.
Local Circulation Diurnal Patterns and Their Relationship with Large-Scale Flows in a Coastal Area of the Tyrrhenian Sea 总被引：1，自引：0，他引：1
Igor Petenko Giangiuseppe Mastrantonio Angelo Viola Stefania Argentini Lucia Coniglio Paolo Monti Giovanni Leuzzi 《Boundary-Layer Meteorology》2011,139(2):353-366
In order to characterise the local low-level circulation in the Tyrrhenian Sea coastal area near Rome, the wind field observed
by conventional anemometers, Doppler sodar, and rawinsonde has been analysed. The prevailing diurnal behaviour of wind speed
and direction as a function of season was highlighted, and the existence of two different patterns of the local circulation,
mainly due to land and sea breezes and to the drainage flow from the mouth of the Tiber valley, revealed. The comparison between
the low-level circulation and synoptic flow allowed us to determine the influence of the large-scale flow on nocturnal currents
that are observed at the Pratica di Mare site and the way that wind direction evolves during the day. Numerical simulations
are consistent with experimental data and depict the main features of the low-level wind field in the area. 相似文献
11.
12.
Shupo Ma Libo Zhou Han Zou Meigen Zhang Peng Li 《Meteorology and Atmospheric Physics》2013,120(1-2):45-51
Observations and model simulations were conducted in a typical Himalayan valley to investigate the role of snow/ice cover in the formation of the local diurnal wind. An unusual local circulation was observed in the Himalayas with a strong down-valley flow dominant from noon to midnight, greatly differing from those in other mountainous regions. Two experiments with snow/ice cover included/excluded were performed using the Regional Atmospheric Modeling System (RAMS) to reconstruct the Himalayan circulation, and to reveal the role of snow/ice in this circulation. The results show that the wind system in the Himalayas is composed of both glacier winds driven by the snow/ice cover and classical mountain-valley winds. In particular, the glacier winds establish the distinctive feature of the Himalayan local circulation, i.e., the strong down-valley flow in the afternoon. 相似文献
13.
14.
延庆-张家口地区复杂地形冬季山谷风特征分析 总被引：8，自引：4，他引：4
基于2016年12月—2017年2月和2017年12月—2018年2月两年冬季的近地面自动气象站逐时观测数据以及张家口探空数据分析延庆-张家口一带（包括张家口崇礼、赤城、海坨、小五台山区,延怀、怀涿、洋河、蔚县盆地以及北京延庆、昌平、怀柔部分平原地区）复杂地形的风场精细化时、空分布特征,揭示不同复杂地形下局地风场的时、空变化规律,加深对复杂地形动力、热力作用对近地面风场影响的认识,为冬季山区风场预报以及复杂地形数值模式改进提供参考。结果表明:晴朗小风天风持续性作为矢量平均风速和标量平均风速的比值,可以作为研究风场变化规律的重要参数。根据风持续性的日变化特征,可以将研究区域内所有站点分为10种类型,分别代表不同局地地形特征的影响,风持续与风向变化的相关也很强。研究区域主要有3种类型的地形风:斜坡风、峡谷风以及较大尺度的山区平原风。不同地形特征下的风场、风持续性存在明显不同的日变化特征,山风和谷风相互转化的时间也不同,山区最早,盆地次之,平原区最晚;山风时段持续时间较谷风时段长,风速小;晴朗小风天实测风反映了实际风场的特征,而排除环境背景风场,弱化地形动力作用后整个冬季的局地风作为理论山谷风,更能反映热力作用下的山谷风特征。 相似文献
15.
Miles G. McPhee 《Dynamics of Atmospheres and Oceans》1978,2(2):107-122
A simple model for simulating the motion of pack ice during periods of energetic inertial oscillation is developed by writing an integrated momentum equation for the ice and upper ocean driven by surface wind stress. Damping is provided by a term proportional to the component of mass transport parallel to wind stress, which is a measure of the departure from a steady-state balance of wind stress and Coriolis force. Oceanic boundary-layer transport is taken to be proportional to the square of the surface velocity on the basis of extensive current measurements made under drifting ice, providing a relationship between ice velocity and total transport. Ice drift velocities measured at AIDJEX stations during the summer of 1975 are simulated with some success using measured local winds. A simple superposition of waves generated at the corners of the triangular array of stations is considered and it is shown that at times the waves are coherent across the 150 km array and at other times there is considerable interference at lesser scales. The importance of the motions for the production of new ice is briefly discussed. 相似文献
16.
Summary The wind regime of the Lake Tekapo Basin is examined with reference to the interaction of multi-scale local, regional and synoptic circulations. Analysis of the historical wind direction record from Mt John identifies airflow from three principal directions to most frequently affect the study area. Both seasonal and diurnal trends in the frequency of each directional category are identified, which reflect the influence of local thermal forcings and seasonal changes in synoptic circulation on the Lake Tekapo windfield. Meteorological observations from a network of automatic weather stations and anemographs within the study area identified Lake Tekapo to generate its own circulation system, a lake/land breeze. This combines with the larger scale valley wind, which during ideal conditions continues after sunset in the upper reaches of the lake catchment. During light to moderate foehn northwesterly conditions, the combined lake breeze/valley wind system remains decoupled from the prevailing synoptic airstream. Towards evening when local thermal circulations weakened, a channelised foehn airstream often becomes dominant over the entire field area. Observations made during this investigation have a number of applied implications with respect to air pollution dispersion modelling and forecasting within alpine lake basins.With 7 Figures 相似文献
17.
We present an analysis of data from a nearly 1-year measurement campaign performed at Høvsøre, Denmark, a coastal farmland area where the terrain is flat. Within the easterly sector upstream of the site, the terrain is nearly homogenous. This topography and conditions provide a good basis for the analysis of vertical wind-speed profiles under a wide range of atmospheric stability, turbulence, and forcing conditions. One of the objectives of the campaign was to serve as a benchmark for flow over flat terrain models. The observations consist of combined wind lidar and sonic anemometer measurements at a meteorological mast. The sonic measurements cover the first 100 m and the wind lidar measures above 100 m every 50 m in the vertical. Results of the analysis of observations of the horizontal wind-speed components in the range 10–1200 m and surface turbulence fluxes are illustrated in detail, combined with forcing conditions derived from mesoscale model simulations. Ten different cases are presented. The observed wind profiles approach well the simulated gradient and geostrophic winds close to the simulated boundary-layer height during both barotropic and baroclinic conditions, respectively, except for a low-level jet case, as expected. The simulated winds are also presented for completeness and show good agreement with the measurements, generally underpredicting the turning of the wind in both barotropic and baroclinic cases. 相似文献
18.
利用逐小时风速观测资料以及台风年鉴资料,分析了2008~2014年登陆我国大陆地区的51次热带气旋（TC）的地面风场分布特征,包括TC登陆期间大陆地面风场演变和大风分布特征、海岛站和内陆站的风速差异以及海拔对风力造成的影响等。结果表明:6级及以上大风主要发生在距离TC中心300 km内、TC强度达到台风（TY）以上时,并主要位于TC移动方向的右侧,尤其是右前象限;华南区TC风场分布主要由在此区域登陆的TC（Ⅰ类）造成,较大风速区包括广东西南部沿海、雷州半岛附近和海南西部沿岸;华东区TC风场分布主要由在此区域登陆的TC（Ⅱ类）造成,杭州湾出海口以及浙闽沿海是较大风速区;6级及以上大风广泛分布在华南和华东沿海,6~7级地面大风高频站主要位于杭州湾附近,8级及以上地面大风高频站点在杭州湾和福建沿海分布比广东西南部更为密集;TC登陆前后均可能造成大风,大风出现时间与站点至TC中心的距离密切相关;同等强度TC在海岛站造成的风速比陆地站更大,对高海拔站点造成的风力大于低海拔站点。本文研究结论对于TC大陆地面风场的预报具有一定参考价值。 相似文献
19.
北方麦收期间连阴雨天气环流特征 总被引：15，自引：0，他引：15
利用1980-2004年5月下旬至6月中旬北方麦收区30个代表站降水实况资料，连阴雨期间亚欧范围500hPa逐日形势图和500hPa高度平均图等，分析总结了近25年来北方麦收期间连阴雨的天气气候和环流形势特征；对北方麦收期间出现的连阴雨天气过程与环流形势和影响系统的关系进行初步探讨，确定连阴雨天气的概念模型。分析表明，阻塞高压形势且贝加尔湖附近伴有冷涡是造成连阴雨天气最主要的环流特征；在500hPa地转风υ场上，麦收区多处于南北风交界处；长连阴雨期间，850hPa东亚地区中低纬度盛行南风为主要特征。 相似文献
20.
Abstract Anemometer‐measured winds for the period 5–13 March 1994 were used to study the coherence of observed and forecast coastal winds along the mid‐Labrador shelf. The reliability of these variables in predicting the response of the ocean and ice to wind forcing is an important issue for ice forecasting in this area. Two anemometer‐equipped 2‐m ice beacons were deployed on pack ice north of Wolf Island and a third beacon was deployed on Grady Island. The results indicate that due to the influence of local topography, 10‐m winds observed at the meteorological station in Cartwright, Labrador provide a poor estimate (r2 = 0.2) of wind conditions over the offshore sea‐ice. In contrast, the σ = 1 level (～10 m) winds from the Canadian Meteorological Centre's Regional Finite Element (RFE) model provided a better correlation with anemometer beacon winds (0.90 for the 6‐hour forecast down to 0.45 at 36 hours). However, the RFE model overestimates the magnitude of the winds by 10–40%. The response of the ocean and ice cover to wind forcing was measured by an ocean bottom‐mounted acoustic Doppler current proþler (ADCP). Relative to the 2‐m beacon winds, the ice moved at 2.5% of wind magnitude and turned 0.6° to the left of the wind. The ocean response decreased with depth until it reached a constant value of 0.9% of the wind speed. The turning angle increased from 0.3° to the right of the wind at 3.5 m to 50° at the lowest level measured by the ADCP (73 m depth). Approximately 57% of the variance in the ocean currents at 3 m below the surface can be attributed to the 2‐m winds; at 73 m the explained variance decreases to 27%. 相似文献