Strong wind refers to the wind with a wind force level of more than 6.[3]The strong wind speed has 2-3 frequency gusts superimposed, especially the gusts every 3-5 minutes are the most regular.During strong winds, even duringboundary layerThe systematic vertical air flow in the upper and lower layers is reversed. The strong downdraft is below 120m, and the strong updraft is at least 280m away.
Wind has both size and direction. Therefore, wind forecast includes wind speed and direction.The wind speed is usually expressed by several levels of wind, and the level of wind is determined according to the impact of wind on ground objects.
Meteorologically, the wind power is generally divided into twelve grades.Level 0 wind is also called calm;Level 2 wind is called light wind, the leaves make a slight sound, and the human face feels windy;Level 4 wind is gentle, and the twigs of the tree shake, which can blow up dust and paper on the ground;Level 6 wind is called strong wind, big branches shake, wires whir, and it is difficult to walk with umbrellas;Grade 8 wind is called gale, and the twigs of trees can be broken, so people have great resistance when walking against the wind;Level 10 wind is called gale, which is rare on land. Trees can be pulled up and buildings are seriously damaged;Typhoon is the wind with force above 12（hurricane）, extremely destructive, rarely seen on land.In the weather forecast, we often hear words such as "north wind force 4 to 5". At this time, the wind force refers to the average wind force;If you hear“gustIn terms of "level 7", the gust refers to the wind whose speed varies from big to small, and the wind at this time refers to the wind when it is big.
Characteristic analysis
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The characteristics of wind conditions in the near surface layer involve engineering wind resistance, wind energy development and utilization, and pollutant diffusion, and are also important carriers of water vapor, heat, and momentum transport in the formation, development, and extinction of weather processes.Due to strong windatmospheric boundary layerThe structure and characteristics are obviously different from the characteristics of light wind conditions, so the key factors of wind characteristics concerned by different fields and purposes are also different.In order to meet the needs of disaster prevention and wind resistance, experts and scholars in the field of meteorology and engineering pay more and more attention to the study of strong wind boundary layer, and have made a number of meaningful research results. Relevant Chinese codes also put forward specific methods for the calculation of parameters involved in engineering wind resistance（Ministry of Construction of the People's Republic of China, 2006), but due to the limitation of measured data, most of the research results and the reference formulas and methods given in the current specifications are more suitable for uniform terrain, and less for complex mountain wind conditions.Based on the observation of wind conditions in the complex mountain area, the research focuses on the analysis of strong wind characteristics in the near ground layer concerned by engineering wind resistance under this terrain condition.
Data source and description
(1) Survey setup and environment description
① The observation setting is required for the construction of a bridge across a canyon in the mountainous area of southwest Guizhou. Two anemometers (10m and 60m high) are set at the proposed bridge sites at both ends of the canyon, of which a set of mechanical (cup) anemometers is set at 10m tower (Tower B), and a set of mechanical (cup) anemometers is set at 60m tower (Tower A) according to the gradient observation method (observation level is 10, 20, 30, 40, 60m)The gradient observation of the average wind field is carried out by the wind measuring instrument with the data sampling frequency of 1Hz;A R3-50 ultrasonic anemometer produced by Gill Company in the UK is set at the 30m level (about the same height as the bridge deck). This instrument is widely used to measure the wind condition of structural wind engineering. The instrument can automatically give a discriminant code while outputting data to identify the validity of the observation data.
The instrument uses an ambient temperature of - 40-60 ℃, a horizontal wind speed range of ± 45m/s, a measurement accuracy of<± 1%, and a maximum dynamic response frequency of 50Hz. This observation uses a sampling frequency of 10Hz to collect three-dimensional instantaneous wind speed data.
② Description of observation environment
Figure 1 Topography of the Observation Environment on the Project Site
The 60m gradient anemometer tower (Tower A) is located on a small hill at an altitude of 876m in the north of the bridge site (Figure 1). Its northwest to southeast side is a canyon with a depth of more than 300 meters. The canyon runs from northwest to southeast. There are mountains with an altitude of more than 900 m from the northeast to the east of the observation tower, and the mountain on the north side is slightly lower than the altitude of the observation tower.The anemometer tower is surrounded by dozens of cm high low sparse shrubs;The 10m tower (Tower B) is located on the hill at an altitude of 864m to the south of the bridge site. There are also low and sparse shrubs around the anemometer tower.
(2) Refer to the description of weather station
The long-term meteorological station adjacent to the project location is Qinglong County Meteorological Station, which is about 12km away from the project location. The observation site is 1552m above sea level. Wind observation data began in 1961, and hourly wind measurement data is available at the same time as the project site observation.
(3) Wind speed sample screening
The meteorological discipline has clear regulations on the classification of wind speed, that is, the wind with an average wind speed of ≥ 17m/s in 10min is called "gale", but the concept and measurement of "gale" in other fields such as daily life, wind power generation and engineering wind resistance vary with the affected objects, generally referring to the condition of relatively high wind speed. For the convenience of description,Here, the wind that meets the screening conditions in this paper is generally called "strong wind".
The microstructure of the near surface wind (including vertical variation, gusty and pulsating characteristics), to a large extent, determines the safety design and investment cost of structures.A large number of observation facts have proved that the microstructure of strong wind and light wind conditions in the near surface layer is very different. Since engineering wind resistance mainly focuses on strong wind conditions, in order to avoid confusion and error caused by the difference between the characteristics of wind conditions in the near surface layer under light wind conditions and strong wind, for the needs of engineering wind resistance research, strong wind samples (or processes) should be screened from the observation data.
Table 1 Classification of Atmospheric Boundary Layer Stability by Pasquill Method
It is generally believed that most strong wind processes are neutral stratification, and the applicable conditions of the exponential or logarithmic formula described by the wind profile recommended by the current national regulations are also neutral stratification, so the neutral stratification is taken as the basic condition for screening strong wind samples.
It can be seen from the stability classification standard of Pasquill atmospheric boundary layer (Table 1) that the atmosphere is stable (E, F) and unstable (AB. C), the corresponding wind speed is small. When the 10m high average wind speed is>6m/s, the atmospheric stratification is C (weakly unstable) only when strong solar radiation occurs, and other conditions are neutral stratification. By analogy, when the 10m high average wind speed is>6m/s, the near ground layer can generally meet the requirements of neutral atmospheric stratification.Considering the local strong wind climate and the characteristics of field observation data in Guizhou, and meeting the sample length required for research, the 60m high average wind speed ≥ 9m/s of Tower A observed in the average field is selected as the strong wind analysis sample.The frequency of occurrence of average wind speed ≥ 9m/s in the year of field observation (12 months) of this level is 1.2%;Since there are few effective data for pulsation field observation, in order to meet the sample length required for the study, a case with 30m high average wind speed of tower A ≥ 7m/s is selected as the analysis sample for the study.The frequency of average wind speed ≥ 7m/s at this altitude in the observation year is 3.4%.
Influence of terrain on average wind field
Table 2 Wind Direction Frequency of Qinglong Meteorological Station and Site Wind Measuring Points
The influence mechanism of mountainous terrain on wind field is relatively complex. On the one hand, the local rise and fall of air flow caused by uneven solar radiation received, and on the other hand, the direction and speed of low-level air flow are changed due to the undulation of terrain. For engineering wind resistance, the main concern is the change of wind field characteristics at strong wind speed.
(1) Influence of terrain on wind direction
Figure 2 Dominant Wind Direction Affected by Topography
① The dominant wind direction is Qinglong Meteorological Station, with an altitude of 1552m, which can better represent the average wind conditions in a large local area.It can be seen from the frequency of wind directions of the station over the years (Table 2 and Figure 2a) that the dominant wind direction in local winter is from north to northeast, and the south wind is dominant in spring, summer and autumn;From the distribution of dominant wind directions in each month (Table 3), it can be found that the dominant wind direction in 10 months (March December) of the year is southerly.The dominant wind direction of Tower B and Tower A (10m, 30m and 60m with wind direction observation) at the bridge site observation point in the observation year is southeast (SE), and the dominant wind direction (SE) is more stable with the increase of observation height.From Tower A 60rllWind Rose(Figure 2b) It can be seen visually that the wind direction distribution characteristics are obviously different from those of the local long-term meteorological station: the statistical results of the 60m high wind direction data of Tower A show that the most frequent wind direction in each month of the year is southeast (SE) (Table 3), while the main wind direction of Qinglong meteorological station is northeast outward in January, and the other 10 months are southerly;The statistical results also show that the 60m high wind direction frequency of Tower A is 47% in the ESE SSE sector, 27% in the WNW NNW sector, that is, the wind direction frequency along the canyon reaches 74% (Figure 2b).It can be seen that due to the influence of the canyon terrain from southeast to northwest, the local southerly wind at the bridge site survey station mostly turns to southeast wind, and the northeasterly wind mostly turns to northwest wind.
Figure 3 Distribution of Maximum Wind Speed and Direction Affected by Topography
Table 3 Maximum Wind Direction and Frequency of Two Stations in Each Month
② Maximum wind speed and direction
Figure 3 shows the distribution characteristics of the maximum wind speed and direction at the local long-term meteorological station and the project location. It can be seen that the wind direction of the maximum wind speed of the long-term meteorological station over the years is mainly distributed in the S-SW direction (Figure 3a), while the wind direction of the strong wind (10 min average wind speed ≥ 9m/s) during the one-year observation at the 60m project location of Tower A is more concentrated in the SE direction, with a frequency of 84% (Figure 3b).
Influence of topography on fluctuating wind field
Figure 4 Measurement example of turbulent fluctuating wind speed in typical strong wind process
The forcing effect of mountainous terrain on the wind field also causes the change of its fluctuating wind characteristics.According to the three-dimensional anemometer set at 30m level of Tower A (equivalent to the height of the bridge deck), the impact of terrain on the fluctuating wind field under strong wind (10min wind speed ≥ 7m/s) is analyzed.Figure 4 shows a measured example of a typical strong wind process wind speed. The observation time is April 5, 2006, the sampling period is 20 minutes long, the process average wind speed is 10.8m/s, and the process maximum gust (0.1s) wind speed is 14.3m/s.It can be seen from the figure that under the complex terrain conditions of the valley, the turbulent fluctuating wind speed has a complex time change process, and the obvious strong gusty wind characteristics can also be seen.
research conclusion
The following preliminary results are obtained through calculation and analysis of field wind measurement data in complex mountains:
(1) The main topographic feature affecting the local wind field of the project is the deep canyon in the northwest southeast direction. Its existence completely changes the local low-level wind field, changing the direction of the dominant wind direction and the maximum wind speed. The vertical distribution of wind speed becomes more complex, even under the conditions of neutral atmospheric stratification (strong wind),The vertical profile of wind does not satisfy the power exponential distribution form at all.
(2) The angle of attack of wind in complex mountainous areas may be far greater than 5 °, and the angle of attack of strong wind in different wind directions will vary greatly. Some directions have positive angles of attack and some directions have negative angles of attack, and the difference between the two directions can reach 20 °.
(3) The turbulence intensity in different wind directions is obviously different, but the turbulence intensity along the canyon direction is small;The turbulence intensity ratio of wind in different directions in the three-dimensional direction is also quite different, and does not meet the ratio given in the current "Design Guide".
(4) The turbulence integral space scale is relatively large, of which the longitudinal value is 20% - 60% larger, the transverse value can be more than 3 times larger in some special wind directions, and the vertical value is generally about an order of magnitude larger than the flat terrain.
(5) Within the frequency domain range (0.1-0.5Hz) sensitive to structures such as bridges, the turbulence spectral density values in each direction are significantly different. In the longitudinal direction, the values in different wind directions can differ by 8 times, and the values in the transverse and vertical directions can differ by 6 times. However, the turbulence spectral density values in either direction are 1-2 orders of magnitude smaller than the typhoon center.
It should be noted that although the 1-year observation data can basically show the general strong wind characteristics of the local area, it is often difficult to "capture" the local small probability (or extreme) strong wind process through short-term observation. Therefore, the proposed large-scale project also needs to be based on the analysis results from field observation, combined with relevant wind tunnel simulation tests,To determine and further verify some special design parameters;The near surface wind profile of complex terrain generally does not conform to the power exponent or logarithmic distribution. Further research is needed to find out the mathematical model (or simulation scheme) that conforms to the various "abnormal" profile shapes studied and analyzed in actual measurement, so as to objectively describe the near surface wind profile of complex terrain.[1]
application
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Mesocyclone is a velocity derived product of WSR-88D radar, which represents the small-scale vortex associated with strong updraft in convective storms.The concept was first introduced by the National Severe Storm Laboratory of the United States to forecast tornadoes and other disastrous weather.According to statistics, 30%~50% of mesocyclones can produce tornadoes, and about 90% of mesocyclones are associated with local strong winds or hail.In China, tornado weather is relatively rare.Taking Shanghai as an example, in the six years from 1998 to 2003, only five tornadoes can be found in the meteorological records, while in other weather with destructive gales, at least half of them are accompanied by mesocyclones. This does not include threshold setting, and the situation that mesocyclones caused by distance folding are not recognized.Therefore, the research hopes to understand the relationship between mesocyclone and tornado, thunderstorm, gale and other disastrous weather by studying the algorithm of mesocyclone and its occurrence and development process, and to obtain some information from it to use mesocyclone products to forecast local strong wind (greater than 17m · s-1）Reference information for.
Definition and algorithm of mesocyclone
(1) Definition of mesocyclone
The mesocyclone can be simulated as a Rankine combined vortex, that is, within the mesocyclone core, the tangential velocity is proportional to the vortex radius;Outside the mesocyclone core, the tangential velocity is inversely proportional to the vortex radius and decreases with the increase of the radius.According to the statistical results of mesocyclone detection data in Oklahoma, the small-scale vortex meeting the following criteria is a mesocyclone:
① Diameter of nuclear area (maximum flow velocity VinAnd maximum discharge speed VoutDistance) ≤ 9km, rotation speed Vr=(Vout+Vin）/2 ≥ the set threshold value, which can be based on its rotation speed and the relative radar range of strong mesocyclone, medium cyclone, weak mesocyclone and weak shear.
② Vertical extension thickness>3km less.
③ The duration of meeting the above two categories of indicators is at least two individual scans.
In conclusion, strong shear, vertical extension to a certain height and duration are three necessary criteria for identifying mesocyclone.
(2) Introduction to mesocyclone algorithm
Firstly, the average radial velocity data is searched to find the range database adjacent to the azimuth with the Doppler velocity value increasing continuously, which is combined into the mode vector;Measure the speed change from the maximum negative value to the maximum positive value and the distance between these values, calculate the angular momentum (distance X △ speed) and tangential shear (△ speed) according to these two quantities, and compare them withangular momentumThe threshold value is compared with the shear threshold value, and the mode vector that exceeds the threshold value is processed in the next step;The mode vectors on the same elevation angle are combined according to their spatial proximity (less than the radial distance threshold (TRD) and the azimuth threshold (TMA)), and compared with the minimum number threshold (TPV) of mode vectors required to determine a 2D feature, to check whether they are the same 2D feature;If the distance between the center of a two-dimensional feature and the radar is less than the distance threshold (TRA), and the ratio of its radial length to its tangential length is between the minimum proportional threshold (TRM) and the maximum proportional threshold (TRM), then the two-dimensional feature is considered symmetrical.On the other hand, if the distance between the two-dimensional feature center and the radar is greater than the distance threshold, and the ratio of its radial length to its azimuth length is between the minimum scale threshold (TRF) of long-range storms and the maximum scale threshold (TFR) of long-range storms, then the two-dimensional feature is also considered symmetric, otherwise it is classified as asymmetric;The 2D features whose height is lower than the feature height threshold (TFM) are vertically correlated.Compare the 2D features on the first elevation with those on the second elevation. If the center point of the smaller 2D features is vertically located in the larger 2D feature area, then the features are considered to be vertically related, thus judging that they constitute a 3D circulation.
(3) Three classifications of mesocyclones
According to the characteristics of different stages of mesocyclone, it can be divided into three types: uncorrelated shear, three-dimensional shear and mesocyclone.If a 2D feature is symmetric but cannot be vertically related to another, it is considered as uncorrelated shear;If two or more 2D features are vertically related, but less than two 2D features are symmetrical, the 3D feature is called 3D shear;If more than two vertically related features are symmetrical, then this three-dimensional feature is called mesocyclone.
Statistical analysis of mesocyclone characteristic parameters
The most important characteristic values of the three categories of cyclones in recognition are rotation speed, minimum height of feature center, feature thickness, shear strength, maximum shear and its position, etc.Research on ShanghaiDoppler radar The characteristic parameters of 93 mesocyclone products detected from July 1998 to September 2003 were statistically analyzed, hoping to obtain some information related to forecasting strong winds, including 45 uncorrelated shear, 48 three-dimensional shear and mesocyclone.
Forecast reference information
Through the study of mesocyclone conceptual model and algorithm, combined with the above eigenvalue statistics and case analysis, it can be verified that mesocyclone products are related to some form of strong weather (big hail, disastrous wind or tornado), showing the existence and characteristics of those storm level eddies related to thunderstorms.Shanghai is dominated by weak shear and weak mesocyclone, and the probability of strong mesocyclone that may cause tornado is relatively small.Mesocyclone is closely related to the occurrence of disastrous weather such as thunderstorm and gale, so it is expected to be used to forecast mesoscale destructive gale.According to its intensity and classification, various radar products can be comprehensively used to determine the weather type, affected area, time, etc.
Forecast focus of disastrous strong wind:
(1) When the radar recognizes mesocyclone and three-dimensional shear, it can consider to forecast local gale, and pay attention to the phenomenon and area of obvious wind speed increase in wind field data.
(2) In the work of using mesocyclone to forecast mesoscale strong wind, the comprehensive use of R, SRM, STI, CM, VMP and other products will greatly help to improve the accuracy and timeliness of forecast. It is better to use these products as the warning products of mesoscale cyclone.Pay special attention to whether there is a strong echo center or a strong echo band (area) in the reflectivity factor R product, and pay close attention to those echo systems with strong intensity, large height gradient and fast moving speed.
(3) It can be known through analysis thatDoppler radar The relevant information of mesocyclone (or uncorrelated shear, three-dimensional shear) can be obtained directly, and the generation of mesocyclone and the change of eigenvalue can be known in advance.
(4) When measuring, it should be noted that if a mesocyclone in the middle layer develops towards the ground, its rotation speed and shear continue to strengthen, and its extension height increases, it may further develop into a tornado.
(5) The mesocyclone algorithm helps us identify tornadoes and strong winds. However, due to the discontinuity of radar scanning in time and space, the algorithm has some defects. R and SRM products should be checked at the same time to determine the existence of mesocyclone.
(6) For mesocyclones in their infancy and maturity stages, we should judge their current position and moving speed in combination with Doppler radar and storm track ST/products, and make predictions in a timely manner. For mesocyclones that are already in their demise stage, unless there are some environmental conditions that can regenerate them, they are generally no longer the focus of prediction.
(7) Check whether there is obvious convergence or shear of air flow in SRM products.If not, it indicates that the precipitation is caused by a large-scale weather system, and mesocyclone will not occur in general.[2]
