Comparative Analysis of the Measurement Accuracy of Three Wind Measurement Equipment

1 Introduction

Among many atmospheric factors, wind field data is one of the most intuitive and valuable data for analyzing weather evolution. Based on the evolution of wind field, we can understand some important physical processes of the occurrence and development of small and medium-scale severe convective weather. Therefore, the research on atmospheric wind field is receiving more and more attention.

With the development of atmospheric detection technology, the equipment used to detect atmospheric wind fields at home and abroad has also increased. At present, the instruments used to detect high-altitude atmospheric wind fields in China mainly include radiosondes, wind energy gradient towers, new generation Doppler weather radars, wind profiler radars, and micro-sonde aircraft. Due to the differences in the working principles and wind field inversion methods of various detection equipment, there must be differences in the accuracy of the measured atmospheric wind fields. High-precision atmospheric wind field data will play a very important role in improving the accuracy of short-term forecasts. Therefore, we compare the measurement accuracy of various wind measurement data to understand some of the causes of errors and uncertainties, so as to give full play to the role of wind profiler products in short-term weather forecasts and severe convective weather warning forecasts.

In addition, according to the future plan of the China Meteorological Administration, a professional wind energy resource observation network consisting of 400 70-120-meter wind observation towers is being built, and a wind profile radar network is also planned to be established in China. Aircraft detection of wind fields has also become a development trend. The completion of these detection equipment can truly make up for the lack of spatial density and time density of my country's conventional high-altitude detection station network. At the same time, the detection accuracy of these detection equipment has also become a matter of great concern to everyone. Therefore, the Meteorological Detection Center of the China Meteorological Administration organized this experiment to compare and analyze the measurement accuracy of the three detection instruments, which can not only serve as a basis for scientific research, but also provide theoretical support for radar network deployment and other work.

2 Principles of various detection instruments

2.1 Wind profiler radar and its wind field inversion method

Wind profiler radar is also called wind profiler. As the name implies, it provides the wind profile. Turbulence of various scales exists in the atmosphere at any time. The uneven changes in the refractive index caused by them will scatter radio waves. Turbulence exists in the airflow and moves with it, so turbulence can be used as a tracer of wind. Wind profiler radar uses clear sky turbulence as a detection target and uses the scattering effect of atmospheric turbulence on radar electromagnetic waves to detect atmospheric wind fields.

Since wind is a vector, the radar antenna is required to have three or five orthogonal beams. By measuring the Doppler frequency shift caused by the wind speed in each beam direction, the horizontal wind direction, wind speed and vertical airflow speed at the height of the echo signal can be calculated under certain assumptions (assuming uniform and isotropic turbulence). In one measurement, the wind profile radar can obtain the horizontal wind speed and direction profiles and vertical airflow profiles at different altitudes at the same time.

The wind field inversion method based on wind profiler radar detection data often uses the velocity-azimuth display method (Velocity Azimuth Display), called the VAD method.

2.2 Microsonde and its wind field inversion method

The microsonde is a meteorological aircraft specially used for atmospheric detection experiments. It has automatic navigation and autopilot functions. It uses GPS navigation to complete the flight of the scheduled route under the control of the automatic control system, and transmits the trajectory of the aircraft and other information to the ground in real time. The microcomputer of the ground receiving system displays the latitude, longitude and altitude of the aircraft. The wind field inversion methods of the microsonde include the horizontal airspeed zeroing wind measurement method and the analytical wind measurement method. This paper adopts the horizontal airspeed zeroing wind measurement method.

The horizontal airspeed zero wind measurement method uses the characteristic that the micro unmanned aircraft can hover in a very small radius, so that the aircraft hovers on the horizontal plane. After a circle, the aircraft returns to the same point relative to the air, and the sum of the horizontal airspeed vectors is zero. The horizontal horizontal wind speed is equal to the aircraft's horizontal ground speed. When the aircraft is flying in the air, the movement relative to the ground is called the ground speed, and the movement relative to the air is called the airspeed. The relationship between airspeed (Vg), ground speed (Va) and wind speed (V) is:

It can be seen from this that the wind speed obtained in this way is completely consistent with the physical meaning of daily business wind measurement. [page]

3 Wind profile measurement comparison test and data comparison analysis

In order to compare and analyze the measurement accuracy of wind profiler radar, wind energy gradient tower and microsonde, the Meteorological Detection Center of China Meteorological Administration organized relevant personnel to conduct comparative tests at Xilinhot National Climate Observatory in Inner Mongolia on September 24, 2008. The detection equipment used in this test were 1290M mobile wind profiler radar (Airda3000) of Beijing Airda Electronic Equipment Co., Ltd., 100m wind energy gradient tower of the climate observatory and microsonde developed by Jiangxi Meteorological Science Research Institute. According to the weather forecast, the weather conditions at Xilinhot National Climate Observatory on the day of the test were "sunny, southwest wind 4-5, temperature 4-12°C", the weather system was relatively stable, and it met the meteorological conditions required for the test. In order to meet the test conditions of stable weather system in the detection area as much as possible, the three detection equipment were relatively close to each other during the test. The wind profiler radar is located at 116°19′49.7″E, 44°08′2.4″N, with an altitude of 1107m and a detection altitude of 50m-3400m; the wind energy gradient tower is located at 116°18′44.9″E, 44°08′32.4″N, with an altitude of 1160m and a detection altitude of 2m-100m; the micro-unmanned aircraft command point is located at 116°19′54.8″E, 44°07′42.1″N, and the detection altitude is manually controlled.

3.1 Comparison of low-level wind measurement data from wind gradient towers and wind profiler radars

According to the altitude of the detection equipment and its detection height, the wind gradient tower can obtain the minute average wind speed at an altitude of 1162m, and the minute average wind speed and minute average wind direction at altitudes of 1164m, 1170m, 1180m, 1190m, 1210m, 1230m, and 1260m respectively; while the wind profiler radar has a low-level altitude resolution of 50m, so it can only obtain the minute wind speed and wind direction at altitudes of 1157m, 1207m, and 1257m.

This paper first compares the average wind speed of 1162m/min from the wind energy gradient tower and the wind speed of 1157m/min from the wind profiler radar. A total of 326 sets of detection data at the corresponding time are selected. The wind speed difference is obtained by subtracting the wind speed measured by the wind profiler radar from the wind speed measured by the wind energy gradient tower. The specific comparison results are shown in Figure 1:

Fig.1 Distribution of the difference between the average wind speed at 1162 m/min on the wind energy gradient tower and the wind speed at 1157 m/min on the wind profiler radar

As can be seen from Figure 1, the wind speed difference between the two is relatively uniform, and the wind speed value measured by the wind gradient tower is slightly larger than that measured by the wind profile radar. According to calculations, the average wind speed of the wind gradient tower is 8.2m/s, while the average wind speed of the wind profile radar is 7m/s. The average difference between the wind speeds is 0.37m/s, and the standard deviation is 1m/s (see Table 1).

In addition, this paper compares the average wind speed and wind direction of the wind energy gradient tower at 1210m with the wind profiler radar at 1207m, and the average wind speed and wind direction of the wind energy gradient tower at 1260m with the wind profiler radar at 1257m, respectively, and selects 193, 165, 138, and 146 groups of detection data at the corresponding time. The wind speed difference in Figures 2 and 4 is obtained by subtracting the wind speed value measured by the wind profiler radar from the wind speed value measured by the wind energy gradient tower, while the wind direction difference in Figures 3 and 5 is obtained by subtracting the wind direction value measured by the wind profiler radar from the wind direction value measured by the wind energy gradient tower. The specific comparison results are shown in Figures 2, 3, 4, and 5.

Figures 2 and 4 show that the deviation distribution between the average wind speed at 1210m and 1260m of the wind energy gradient tower and the minute wind speed at 1207m and 1257m of the wind profiler radar is relatively uniform, and the wind speed value measured by the wind energy gradient tower is significantly larger than the wind speed value measured by the wind profiler radar. After calculation, the average difference in wind speed between the two is 1.85m/s and 1.76m/s respectively; the standard deviation is 0.82m/s and 0.83m/s respectively.

From the comparison of wind directions in Figures 3 and 5, the overall trend of wind direction deviation tends to be consistent, and the first half of Figure 5 shows that the wind directions of the two measurement methods are almost the same, while the wind directions of the two methods have changed significantly in the second half, and the wind direction difference has also increased. According to calculations, the average difference in wind direction between the two methods is 11° at around 1210m and 6.6° at around 1260m; the corresponding standard deviations are 12.7° and 11.1° respectively (see Table 1).

Figure 2, 3 Deviation distribution of the average wind speed and direction at 1210m of the wind energy gradient tower and the minute wind speed and direction at 1207m of the wind profile radar

Figure 4, 5 Deviation distribution of the average wind speed and direction at 1260m of the wind energy gradient tower and the minute wind speed and direction at 1257m of the wind profile radar

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3.2 Comparison of wind measurement data from three detection devices: wind gradient tower, wind profile radar, and microsonde

This section uses the microsonde data obtained during the aircraft test from 13:30 to 15:00 on the afternoon of September 24, and selects the wind profiler radar data and wind energy gradient tower data at the same altitude layer at the corresponding time for comparison. The wind speed and wind direction comparison results of the three detection equipment are shown in Figures 6 and 7. A total of 30 groups of microsonde data and wind profiler radar data were selected for this comparison, while only 9 groups of data were included in the comparison due to height restrictions of the wind energy gradient tower. It can be calculated that the wind speeds measured by the sounding aircraft and the wind energy gradient tower are very similar, with an average difference of only 0.9m/s. However, the wind speed measured by the wind profiler radar is quite different, with an average difference of 3.4m/s.

From Figure 7 and the calculated data, we can see that the wind direction measured by the microsonde is very consistent with that measured by the wind profiler radar, with an average difference of only 0.47°. The average difference between the wind direction of the microunmanned sonde and the wind gradient tower is 7.1°.

Figure 6 Comparison of wind speed data measured by three detection devices at the same time and altitude

Figure 7 Comparison of wind direction data measured by three detection devices at the same time and altitude

3.3 Results Analysis

This paper uses two methods to compare and analyze the wind measurement data obtained by three detection devices. The analysis results show that the wind speed difference between the lowest layer of the wind profiler radar and the lowest layer of the wind energy gradient tower is small, only 0.37m/s. The difference between the second and third layers of the wind profiler radar and the corresponding layers of the wind energy gradient tower is slightly larger. In addition, as shown in Figure 6, the comparison of the detection data of the three detection devices between 1200m and 1400m shows that the wind speed measured by the microsonde is very different from the wind speed measured by the wind energy gradient tower, but the difference is large with the wind speed measured by the wind profiler radar, with an average difference of 3.4m/s. The main reason is that the wind energy gradient tower data entered into the comparison is relatively small. In addition, the wind measurement principles and wind field inversion methods of the two are too different, which is also an important reason for this result. The wind direction comparison in Figure 7 shows that the wind directions measured by the microsonde are consistent with those measured by the wind profiler radar and the wind energy gradient tower.

Based on the above comparison results, the main factors causing the differences in comparison are: (1) Differences in observation methods, that is, the detection principles of various detection equipment are different, the detection targets are also different, and the detection data must have errors; (2) Differences in data processing methods, that is, the methods of wind field inversion are very different. Wind profile radar wind measurement is the average value of wind at a certain altitude layer above it and within the detection range; while the wind energy gradient tower measures the wind at a specific location, which is an average of minute winds; the aircraft measures the average wind at the trajectory points of the aircraft during the flight cycle; (3) The quantity and quality of valid data. The data selected in this paper are all data after the maximum error is eliminated, so the data quality can be guaranteed. However, due to the limitations of conditions, the amount of data that can be used for the comparison of the three detection equipment at the same time is relatively small, which also affects the accuracy of the comparison results to a certain extent; (4) The simultaneity and co-location of the data. The better the simultaneity and co-location of the data, the more valuable the comparison results are. Since the weather system was relatively stable during this experiment, the distance between the detection equipment was very close, and the time resolution of each detection equipment was not large, the experimental conditions basically met the principles of simultaneity and co-location.

4 Conclusion

Through comparative analysis of the data obtained in this experiment, we can draw the following conclusions:

(1) Compared with the other two detection devices, the advantages of the wind energy gradient tower are stable and reliable measurement data and low cost, while its disadvantages are low detection height and small range.

(2) Compared with the other two detection devices, wind profiler radar detection is not only real-time, but also has a high detection altitude and a large range. Therefore, the use and deployment of wind profiler radars can improve the shortcomings of insufficient sounding data stations in my country, and can obtain real-time wind data, providing more timely and valuable reference for weather forecasts. From the comparative analysis, it can be seen that the data obtained by wind profiler radar is significantly smaller than that of the other two detection devices, mainly because wind profiler radar obtains the average value of wind within a certain range.

(3) Compared with the other two detection devices, the biggest advantage of the microsonde detection data is that the aircraft route can be manually controlled and the detection method is flexible. From the comparative analysis, it can be seen that the detection data of the microsonde is not much different from the observation data of the wind energy gradient tower, which is within the allowable error range. Therefore, the microsonde has a large application space. It can not only be used for high-altitude detection in sparsely populated remote areas, but also for high-altitude detection in special meteorological conditions or sudden meteorological events. Micro unmanned aircraft can become a convenient, economical and flexible low-altitude detection tool.