Comparison of Beijing MST Radar and Radiosonde Horizontal Wind Measurements

Mesosphere-stratosphere-troposphere (MST) radar can be used to continuously monitor the vertical and horizontal components of atmospheric wind at high spatial and temporal resolutions. Since the first MST radar system was developed in 1974, this technique has played an important part in studies of the structure and dynamics of the atmosphere (Woodman and Guillen, 1974).

The first MST radar system in China began making preliminary observations in November 1993. It was developed by LAGEO (the Key Laboratory of Middle Atmosphere and Global Environment Observation) at the Institute of Atmospheric Physics, Chinese Academy of Sciences (Lu and Li, 1996; Li and Lu, 1998). MST radar techniques have developed rapidly in China over recent years. Two MST radar systems have been constructed and are operated as part of the Chinese Meridian Project. This project consists of diverse ground-based remote sensing facilities aligned near 120^°E for monitoring and forecasting the spatial environment (including the middle and upper atmosphere) (Wang, 2010). One of the radar systems is the Wuhan Atmosphere Radio Exploration MST radar, which is operated by the Ionosphere Laboratory of Wuhan University and is located in Chongyang, Hubei Province, China (29^°31'58"N, 114^°8'8"E). The other is the Beijing MST radar system.

The Beijing MST radar system was also designed and constructed by LAGEO. The first system-integrated test was completed in May 2011 and the system was accepted at the end of July 2011. After one month of joint tests from 21 September 2011 to 20 October 2011, it was put into routine operation. The Beijing MST radar system is located at Xianghe Atmosphere Observatory of the Institute of Atmospheric Physics, Hebei Province, China (39^°45'14.40"N, 116^°59'24.00"E). The Beijing MST radar system uses an all-solid-state digital array pulse Doppler system. The operating frequency is 50 1 MHz with a peak power of 172.8 kW. The radar antenna system, which occupies an area of 10⁴ m², is a phased array of 24× 24 three-element Yagi antenna. This radar system actually can provide wind and turbulence measurements in the height ranges of 3-25 km and 60-90 km, and even above (Tian and Lu, 2016). It can realize atmospheric observations of the three-dimensional wind field and turbulence in these height ranges.

Before the application of these data, however, it is necessary to investigate the reliability and accuracy of the Beijing MST radar wind measurements. Much relevant research has been carried out from the beginning of the development of the MST radar system. To estimate the accuracy of radar wind measurements, comparisons have generally been made with radiosonde measurements, which have been adopted as reference standards. Table 1 lists some of the studies associated with comparisons of horizontal wind characteristics between UHF/VHF radar and radiosondes.

Table 1 shows that comparisons have been made between measurements from radar systems and both radiosondes launched from the radar site and radiosondes at distances of a few tens of kilometers. The results of these studies have generally shown good agreement between the radar and radiosonde measurements of horizontal winds. The discrepancies have been attributed to a combination of intrinsic systematic and random errors in both instruments, differences in measurement principles and the simultaneous comparison of winds sampled at different temporal and spatial scales (Belu et al., 2001). The radar measurements give an Eulerian estimate of the wind vector integrated with respect to volume and time over all altitudes quasi-simultaneously and at a given location, whereas the radiosonde gives a Lagrangian value at various altitudes, at different times and at different positions (Luce et al., 2001).

Most previous studies in this area have compared zonal winds and meridional winds as a function of height or height region. Comparisons of wind speed and wind direction are relatively rare. Only a few studies have investigated the influence of the real-time relative distance between the radiosonde and the radar system on the horizontal wind difference measured by the two instruments (Engler et al., 2008). (Luce et al., 2001) found that the differences between the winds measured by radar systems and radiosondes increase with increasing distance. (Thomas et al., 1997) compared data from balloon trajectories within 20^° of the radar site direction and with a horizontal separation of up to 18 km. They found that the magnitude of the improvement in the coefficient suggested only a small influence from the spatial separation of the two sets of measurements.

We report here a detailed comparison of the horizontal wind speed, wind direction, zonal wind and meridional wind within 3-25 km height measured by the Beijing MST radar system and a radiosonde launched about 40 km west of the radar site during the year 2012. Comparisons were made between 427 profiles from each instrument and 15 210 data pairs were compared. By considering a combination of characteristics of the variations in horizontal winds, the temporal and spatial scales of the synoptic systems, and the drift of the radiosonde balloons, detailed comparisons were made as a function of profiles, heights and seasons to investigate the consistency of the measurements of horizontal winds by the Beijing MST radar system and the radiosonde, and to explain the different levels of consistency.

Table 2 lists the final specifications of Beijing MST radar system. Radiosonde winds were obtained from the GTS1 type digital radiosonde launched from Beijing Meteorological Observatory (39.80^°N, 116.47^°E), station index number 54511. The spatial separation between the radiosonde launch site and the Beijing MST radar system was about 40 km. The horizontal winds were obtained by tracking the position of the balloon using L-band radar. The raw data were sampled at 1 s intervals, resulting in an uneven height resolution. The raw data were processed using a linear interpolation to give the same height resolution as the MST radar system.

After considering the detection height of the MST radar system and the radiosonde, the height range 3-25 km was compared using the middle mode of the MST radar system. The middle mode of the MST radar system was operated twice every hour at 10 and 40 minutes past the hour. The radiosonde was launched twice a day at about 0715 and 1915 local standard time (LST), except during the flood season (from June to August) when additional radiosondes were launched at 1315 LST. Using the radiosonde launched at 0715 LST as an example, by about 0725 LST the balloon had usually risen to 3 km and the balloon could reach 25 km altitude by around 0830 LST; that is, the balloon ascended from 3 to 25 km in about one hour. We therefore selected the averaged MST radar middle mode observations at 0740 and 0810 LST as one profile to compare with the data profile of the radiosonde launched at 0715 LST. In the same way, the averaged data at 1940 and 2010 LST (1340 and 1410 LST) of the MST radar middle mode observations were compared with the data for the radiosondes launched at 1915 (1315) LST.

Radiosonde and MST radar measurements of the horizontal winds during 2012 were compared. Taking into account simultaneous observations and the restricted height coverage of certain balloons, comparisons were made between 427 profiles from each instrument. Thirty-nine height levels were compared for each profile and the number of profiles compared in each month was different. There were more comparison profiles during the summer, as there were more launches of the radiosonde in this season. As a result of missing data from one or both instruments in the upper height levels——due to low SNRs for MST radar and measurement errors for radiosonde (Weber and Wuertz, 1990)——the number of comparable points in the 427 profiles obtained from each instrument was 15 210. Therefore, the dataset consisted of 15 210 measurement pairs from the two instruments.

Wild data points were removed. First, we calculated the five-point center moving average for each profile as the reference profile. Then, the differences between the points in the original profile and the reference profile were computed. When the difference between the points was out of the range of Av-3σ to Av+3σ (Av and σ were the average and standard deviation of the differences between the points in the original profile and the reference profile, respectively), the corresponding point was identified as a "wild point" and was removed.

Comparisons between the horizontal wind speed (ws), horizontal wind direction (wd), the zonal wind (u) and the meridional wind (v) were analyzed statistically. To estimate the consistency of the wind measurements, the correlation coefficient, mean difference, mean absolute difference, standard deviation of difference and standard deviation of absolute difference were calculated.

Before discussing the statistical analysis, we first show several examples of the comparisons made. Figure 1 shows five comparisons of ws, wd, u and v measurements taken from observations in January, April, July and October 2012. The agreements were generally very good in each example.

Figure 1.  Vertical profiles of ws, wd, u and v (corresponding to panels in the first, second, third and fourth columns, respectively) measured by the Beijing MST radar system (blue lines) and the radiosonde (red lines) at 2300 UTC 5 January 2012, 2300 UTC 23 April 2012, 1100 UTC 2 July 2012, 1100 UTC 19 July 2012, and 1100 UTC 30 October 2012.

We start the preliminary comparison from the basic characteristics of the time-height plots of ws, wd, u and v for the year 2012.

Figure 2 shows that the spatial and temporal distributions of ws, wd, u and v obtained from the MST radar system and the radiosondes were generally highly consistent. The horizontal winds were weaker throughout the height range 3-25 km from June to August, but were stronger in January and February (Figs. 2a and b). Apart from during the summer, the horizontal wind directions were mainly in the range 225^°-315^° at heights of 3-25 km. In June, July and August, the wind directions showed a significant shift in the vicinity of 18 km height, turning from low-level westerly winds into high-level easterly winds (Figs. 2c and d). This is clearly seen in the time-height distribution plots of u (Figs. 2e and f). In addition, westerly winds were observed in the lower layers and easterly winds in the upper layers in May and September at around 20 km height. The zonal wind speed in winter can reach >60 m s^-1. The distribution of v is more complex. The winds in early July were mainly northerly, whereas southerly winds dominated in late July. As August proceeded, southerly and northerly winds appeared alternately in the height range 5-15 km (Figs. 2g and h).

Figure 2.  Time-height plots of ws, wd, u and v for the MST radar system and the radiosonde. The time period was 2012 and included 427 profiles for both instruments. The negative values in panels (e, f) represent an easterly wind and the positive values indicate a westerly wind. In panels (g, h), negative (positive) values represent a northerly (southerly) wind.

In addition to the detailed characteristics of the time-height distribution, the wind characteristics also showed obvious seasonal variations. We therefore carried out further analyses of the seasonal mean distribution of ws, wd, u and v with height for both instruments (Fig. 3).

Figure 3a shows that the average horizontal wind speed was at a maximum in winter, followed by spring and autumn, and at a minimum in summer. The height range of the maximum wind speed in each season was about 10-15 km. The seasonal mean wind speeds measured by the radiosonde in this height range were larger than the MST radar observations in each season.

Figure 3.  Seasonal mean distribution with height of (a) ws, (b) wd, (c) u and (d) v for the MST radar system and radiosondes. The green, red, blue and pink curves represent spring (March, April, May), summer (June, July, August), autumn (September, October, November) and winter (December, January, February), respectively. The solid lines represent the MST radar and the dotted lines represent the radiosondes.

Figure 4.  Monthly mean distribution with height of ws, wd, u and v for the MST radar system (a, b, e and f) and the radiosondes (c, d, g and h). Green, red, blue and pink curves represent spring, summer, autumn and winter, respectively. The dashed lines, solid lines and dotted lines correspond to the first, second and third month in each season.

Figure 5.  Scatterplots of ws, wd, u and v measured by the Beijing MST radar system versus 427 radiosondes launched 40 km from the MST radar site in 2012 in the height range of 3-25 km. The total number of data pairs was 15 210. The red line is the linear least-squares fit quantified by the fitting equation at the top of each panel.

Figure 3b shows that the horizontal wind directions clearly shifted from west to east during summer in the height range 18-22 km. Figure 3c clearly shows the shift in direction from a westerly to an easterly wind and that the wind speed was <5 m s^-1 from 18 to 22 km. The meridional winds were light with speeds close to 0 m s^-1 at 18-22 km in summer (Fig. 3d). This meets the definition of a quasi-zero wind layer at about 20 km; the zonal winds turn from westerly below this layer to easterly above this layer and the meridional winds are all light at this time. The height range 18-22 km in summer is therefore referred to as a quasi-zero wind layer.

Figure 4 shows the analysis of the monthly mean ws, wd, u and v distributions with height. The trends in the monthly mean profiles for ws, wd, u and v measured by the Beijing MST radar system and the radiosonde were basically the same. Figures 4b and d show the wind shift in each month more specifically. In May and September, the months of transition from spring to summer and summer to autumn, there was a wind shift in the height range 18-22 km. The wind direction during June to September also changed rapidly with height at about 5 km altitude.

After the preliminary analysis of the wind characteristics measured by the MST radar system and the radiosondes above, the seasonal and monthly characteristics of the horizontal winds were also obtained. Next, comparisons were undertaken in a statistical sense, and made contrastive analysis as a whole. Figures 5 and 6 show scatterplots and the linear least-squares fits of ws, wd, u and v. We then calculated the correlation coefficient, mean difference, mean absolute difference, standard deviation of difference and standard deviation of absolute difference for ws, wd, u and v measured by the two instruments, and show the results below.

3.2.1. Scatterplots and linear least-squares fits

Figure 5 shows that the slopes of the least-squares fit lines for ws, wd, u and v were 0.94, 0.89, 0.96 and 0.83, respectively. Good consistencies were found for u and ws. However, the distribution of scatter points for wd was widely dispersed (Fig. 5b) when we considered whether the wind speed affected the consistency of the wind direction. Figure 6 therefore shows a scatterplot of the wind direction at different wind speeds and its least-squares fit.

By comparing Fig. 5b and Fig. 6, it is obvious that most of the more dispersed points in Fig. 5b correspond to wind speeds <5 m s^-1. When the wind speed was between 5 and 10 m s^-1, the slope of the least-squares fit line for wind direction increased to 0.95. The slope reached 0.97 at wind speeds >10 m s^-1. We therefore concluded that there was poor consistency between the wind directions obtained using the MST radar and radiosonde measurements at smaller wind speeds. It is understandable that in a situation of lower wind speeds, any small error in both u and v will reflect a larger error in wind direction.

3.2.2. Results of statistical comparison

Table 3 shows the results of the statistical comparison made using the correlation coefficient, mean difference (MST radar minus radiosonde), mean absolute difference, standard deviation of difference and standard deviation of the absolute difference for values of ws, wd, u and v obtained by the MST radar system and the radiosondes.

The correlation coefficient for u (0.98) was the highest, followed by ws (0.97) and wd (0.92); the correlation coefficient for v (0.90) was the lowest, but was still fairly high. The values of ws and u showed better agreement than those of wd and v.

The absolute differences for ws, u and v were all about 2.5 m s^-1. The mean difference and standard deviation of the difference for wd were 0.77^° and 24.86^°, respectively.

Figure 6.  Scatterplots of wind direction at different wind speeds. Green points indicate ws ≤ 5 m s^-1 and the green line is the least-squares fitted line. Red points represent 5< ws≤ 10 m s^-1 and the red line is the least-squares fitted line. Blue points represent ws≤ 10 m s^-1 and the yellow line is the least-squares fitted line. The number of points (npt) and fitting equations are given.

Figure 7.  (a) Probability distribution of correlation coefficients for ws, wd, u and v profiles measured by the Beijing MST radar system and the radiosondes at intervals of 0.1. (b) Probability distribution of mean difference for ws, wd, u and v profiles measured by the Beijing MST radar system and the radiosondes at intervals of 1 m s^-1 for ws, u and v (bottom axis) and 5^° for wd (top axis and red bar chart). Comparisons were made between 427 profiles from each instrument. The mean differences between profiles were calculated from the values of each profile measured by the Beijing MST radar system minus the corresponding profile values of the radiosonde, and then taking the average as the mean difference for this profile pair.

Figure 8.  Correlation coefficient, mean difference and standard deviation of difference (stdd) for ws, wd, u and v observed by the MST radar system and the radiosondes as a function of height for each season and during the year 2012. Columns from left to right represent the correlation coefficient, mean difference (MST radar minus radiosonde) and the standard deviation of the difference of the two instrument measurements, respectively. Rows from top to bottom indicate ws, wd, u and v in sequence. Curves in each panel with different colors represent different seasons and the whole year.

The mean differences for u and v were -0.32 and -0.25 m s^-1, respectively. This is close to the results of a previous study, in which values of -0.58 m s^-1 for u and -0.78 m s^-1 for v were observed in a study comparing the Indian Gadanki MST radar system with measurements from a radiosonde launched about 90 km from the radar site using unedited data in the height range 3.6-18 km (Kishore et al., 2000). Other previously reported results of the mean difference include values of -0.37 m s^-1 for u and 0.1 m s^-1 for v (Weber et al., 1990) and -0.66 m s^-1 for u and -0.71 m s^-1 for v (Weber and Wuertz, 1990); the detailed comparison information is given in Table 1.

The standard deviations of the difference for u and v were 3.33 and 3.58 m s^-1, respectively. This is also close to, and in the range of, the results of previous studies. For example: 3.65 m s^-1 for u and 3.06 m s^-1 for v with 657 data pairs covering the height range 1.83-16.73 km (Weber et al., 1990); 2.98 m s^-1 for u and 3.02 m s^-1 for v with 6480 data pairs covering the height range 3.6-18 km (Kishore et al., 2000); and 4.60 m s^-1 for u and 4.67 m s^-1 for v with 18,173 data pairs covering height range 3-10 km (Weber and Wuertz, 1990).

The standard deviation of the difference for ws (3.37 m s^-1) was smaller than the 3.6 m s^-1 observed in a previous comparison study between the ALWIN MST radar system and radiosonde (launched from the radar site) measurements in the height range 2-8 km reported by (Engler et al., 2008). However, it was larger than the 2.6 m s^-1 observed in the comparison study between the Japanese MU radar system and radiosonde (launched 30 km east of the radar site) measurements in the height range 2-20 km (Luce et al., 2001). The value is in the range 2-4 m s^-1 observed in the comparison study between the CLOVAR VHF radar system and radiosonde (launched within 400 m of the radar site) measurements in the height range 2-7 km (Belu et al., 2001).

The results in this study cover a larger height range of 3-25 km with 15 210 data pairs. Given that the SNR of the MST radar system in the upper heights is lower, the uncertainty of the data for horizontal winds obtained at particular heights is increased. Therefore, considering that we made the comparisons over a larger height range, our standard deviation of the difference for u, v and ws was relatively good. The statistical analysis was followed by further comparative analyses with respect to the profiles and as a function of height.

The correlation coefficient and mean difference of the profiles for ws, wd, u and v measured by the Beijing MST radar system and radiosonde were analyzed to investigate the consistency of the profiles, and the probability distributions are shown in Fig. 7.

Figure 7a shows that the correlation coefficients for the ws and u profiles in the range 0.9-1 represented 88% and 91% of the total, respectively. For wd, correlation coefficients in the ranges 0.9-1 and 0.8-0.9 had percentages of 50% and 25%. The correlation coefficients in the range 0.9-1 for the v profiles accounted for 22%. The percentage of correlation coefficients for the v profiles in the ranges 0.8-0.9 and 0.7-0.8 were 25% and 21%, respectively.

Figure 7b shows that the mean difference in the ws, u and v profiles in the range -1 to 0 m s^-1 accounted for the highest proportion, with values of 42%, 43% and 38%, respectively. The second highest percentage of the mean differences for the ws, u and v profiles were in the range 0-1 m s^-1, with values of 28%, 32% and 28%, respectively. The mean difference in the wd profiles was mainly in the range 0-5^°, with a percentage of 46%. A total of 22% of the wd profiles had a mean difference in the range -5^° to 0^°.

These analyses showed a good agreement between profiles. The profiles for u showed the highest consistency, followed by those for ws, which were only a little lower than u, then wd; the profiles for v showed the lowest consistency. These results agree with the overall statistical analyses and may be related to the fact that the values of v were small and the sign changed along the profiles. With respect to the wind direction, this may be related to the shift in the wind directions in a certain month and at a certain height because the fact that the two instruments take samples at different times and in different spaces may lead to a larger difference. Further detailed in-depth analyses were therefore carried out.

Comparisons were made as a function of height for each season and over the whole year with respect to the correlation coefficient, mean difference and standard deviation of difference for ws, wd, u and v observed by the Beijing MST radar system and the radiosondes (Fig. 8).

Figure 8a shows that the whole year correlation coefficients of ws at all heights were >0.9. At heights above 18 km, however, the correlation coefficients of ws were relatively lower in the summer. This is explained by the existence of the quasi-zero wind layer around 18-22 km during summer with low mean wind speeds above 18 km (Fig. 3a). In addition, the SNR at heights above 18 km was also small in summer (Fig. 9a). The rate of acquisition of MST radar data and the percentage of comparison data pairs of the two instruments above 18 km were also lower in summer than in other seasons (Fig. 9b). The rate of data acquisition is defined as the percentage of effective detection points in a total detection time during a certain period of time. The percentage of comparison data pairs for the two instruments represents the percentage of data pairs available for comparison in the whole set of data pairs.

Figure 8b shows that the annual average difference in ws between the Beijing MST radar system and the radiosondes was about -2 to 2 m s^-1 at each height. At around 19 km height, the annual mean difference was at a maximum of 2 m s^-1. The seasonal mean profiles showed that the mean differences were large at a height of 18-20 km in spring and summer, which was related to the presence of the quasi-zero wind layer. The mean difference was -2 m s^-1 at about 12 km because the height region of 10-12 km corresponded to the area of maximum wind speed. The mean difference was also about -2 m s^-1 at a height of 6 km and the winter mean profile showed large absolute values around 5 km as a result of the wind speed increasing rapidly near 5 km height in winter (Fig. 3a).

Figure 9.  (a) Seasonal and annual average SNR of MST radar at each height. Green, red, blue and pink curves represent the spring, summer, autumn and winter average, respectively. The black curve indicates the whole year average. (b) Seasonal and annual average of the Beijing MST radar (dotted lines) and radiosonde (dashed lines) rates of data acquisition and the percentage of comparison data pairs of the two instruments (solid lines) at every height. The green, red, blue, pink and black curves represent spring, summer, autumn, winter and the whole year average, respectively.

Figure 10.  Seasonal and annual (a) mean longitude and (b) latitude of radiosonde balloon position, and (c) the mean distance between the radiosonde balloon and Beijing MST radar site at each height. The orange lines in panels (a, b) represent the scope of MST radar detection and the vertical lines show the longitude and latitude just above the radar site. The red crosses show the longitude and latitude of the radiosonde balloon launch sites.

The annual mean difference was positive above 15 km, but negative below 15 km. This means that the wind speeds measured by the Beijing MST radar system were larger than the radiosonde observations above 15 km and smaller below 15 km. Figure 10a clearly shows that the annual mean position of the radiosondes is west of the Beijing MST radar system below 14 km, and east of the MST radar system above 14 km. The annual mean distance between the MST radar system and the radiosonde was smallest at a height of about 13 km (Fig. 10c), which corresponded to an annual mean difference of about 1 m s^-1.

Figure 8c shows that the annual average of the standard deviation of the difference between the value of ws measured by the Beijing MST radar system and the radiosonde was about 2.5 m s^-1 in the height range 3-10 km, except at a height of around 5-6 km, where it was 3 m s^-1 on account of the rapid increase in wind speed in winter. It was about 3-4.2 m s^-1 at 10-15 km and at a maximum near 12 km, corresponding to the height region for the maximum wind speed. The annual average standard deviation of difference was about 2.8 m s^-1 in the height range 15-23 km.

The mean difference and standard deviation of difference for ws in winter were -1 to 1 m s^-1 and about 3 m s^-1, respectively, in the height range 10-20 km (Figs. 8b and c). However, Fig. 10a shows that the mean position of the radiosonde balloon above 10 km was east of the MST radar site and that it drifted far away at greater heights. The mean distance between the radiosonde balloon and the MST radar site was 5 km at a height of 10 km; this distance was 40 km at a height of 15 km and 70 km at a height of 20 km (Fig. 10c). This indicated that the distance between the MST radar site and the radiosonde balloon did not affect the consistency of ws measured by the two instruments in winter, and that it was related to the characteristics of the horizontal wind in winter. Figures 3 and 4 show that the wind direction remained at about 270^° and that the mean wind speed was >20 m s^-1 in winter at a height of 10-20 km. Because the weather systems in winter usually have long temporal and large spatial scales with a homogeneous horizontal distribution, the two instruments were within the same relatively homogeneous region of the weather system despite the large distance between them, leading to a good agreement in the measurements. Similar results were obtained for the zonal wind u as for ws (Figs. 8g-i).

Figure 8d shows that the correlation coefficient of the whole year for wd measured by the MST radar system and radiosondes was almost >0.9 at each height except near 18 and 5 km, which had the lowest value of 0.8. The correlation coefficient in autumn was lower around 5 km, which was related to the rapid change in wind direction with height near 5 km in autumn (Fig. 3b). Figures 8e and f show that the annual mean difference and standard deviation of difference for wd measured by the two instruments were larger at heights of around 18 and 5 km as a result of the rapid change in wind direction with height during summer, and in the transition months from spring to summer and from summer to autumn, at these heights.

Figure 8j shows that the correlation coefficients for v for the whole year were about 0.9 at heights below 15 km, then decreased rapidly in the height range 15-20 km, and were about 0.6 above 20 km. This was related to the absolute values of v decreasing with height and approaching 0 m s^-1 (Fig. 3d). Figure 8k shows that the annual mean difference in v at all heights was about -2 to 1 m s^-1. The mean difference in v measured by the MST radar system and radiosonde were positive at heights below 10 km and negative above 10 km height. At the height of about 19 km, the mean difference in v in summer was larger, which was related to the change in wind direction. Figure 8l shows that the standard deviation of the difference in v obtained by the two instruments was larger at heights of around 12 and 18 km. This was attributed to the alternation of southerly and northerly wind directions from late July to August in the height range 12-14 km (Figs. 2g and h), and the change in wind direction near 18 km during summer and the related transition month.

This study compared the horizontal winds measured by the newly constructed Beijing MST radar system at heights of 3-25 km with those measured by radiosondes launched about 40 km west from the radar site over the year 2012. A total of 427 profiles and 15 210 data pairs were compared. Very good agreement was found between horizontal wind measurements made by the Beijing MST radar system and the radiosondes.

The correlation coefficients for ws, wd, u and v were 0.97, 0.92, 0.98 and 0.90, respectively. The mean differences (absolute differences) between the measurements made by the Beijing MST radar system and the radiosonde were 0.77^° (12.02^°) and -0.44 (2.51), -0.32 (2.46) and -0.25 (2.54) m s^-1 for wd, ws, u and v, respectively. The standard deviation of difference (absolute difference) for wd, ws, u and v were 24.86^° (21.78^°) and 3.37 (2.28), 3.33 (2.27) and 3.58 (2.54) m s^-1, respectively. The values of these statistical parameters were within the range of results obtained in previous studies.

The wind field characteristics showed seasonal and monthly variations. The wind speed was highest in winter, followed by spring and autumn, while summer showed the lowest speeds seen at almost all heights between 3 and 25 km. The wind speed increased with height from 3 to about 12.5 km, then decreased, leading to higher wind speeds in the height range 10-15 km. There was a quasi-zero wind layer at heights of 18-22 km in summer, with low wind speeds and a large change in wind direction. A change in wind direction was also seen in May and September in the height range 18-22 km. The wind direction changed rapidly with height at about 5 km during summer and in September. A comparison of the seasonal and monthly mean profiles between the wind measurements from the MST radar system and the radiosonde showed good consistency, except for the height region with higher wind speeds and in the quasi-zero wind layer and areas where the winds changed direction.

A comparison of profiles showed good agreement, with the correlation coefficients for the ws, u and wd profiles of 0.9-1 comprising 88%, 91% and 50% of the total. The highest and second highest percentage of the mean difference for the ws, u and v (wd) profiles were in the range -1 to 0 m s^-1 (0^°-5^°) and 0-1 m s^-1 (-5^°-0^°).

The annual mean difference at all heights was in the range -2 to 2 m s^-1 for ws and u and in the range -2 to 1 m s^-1 for v. The annual standard deviation of the differences between the MST radar system and the radiosonde measurements for ws and u were in the range 2.5-3 m s^-1, apart from the height region 10-15 km, which had the largest value of about 4 m s^-1. Therefore, the comparisons as a function of height showed good agreement between the two instruments at most heights.

The discrepancy is a result of the characteristics of the variation in the wind field. In the height regions with larger wind speeds or a large velocity gradient and in the quasi-zero wind layer and areas with a change in wind direction, the consistency between the horizontal winds measured by the Beijing MST radar system and the radiosonde was relatively poor. The discrepancy also occurred when the SNR of the MST radar was small and when the rates of data acquisition for both the MST radar system and the radiosonde were lower, resulting in a lower number of data pairs to compare for the two instruments. This occurred at greater heights. It should be noted that, because the peak power of the Beijing MST radar system is 172.8 kW, the upper limit of good wind measurements is about 18 km. However, in spite of the lower data acquisition rate, the data obtained in the height range 18-25 km were also useful.

This study found that the differences between the MST radar system and the radiosonde horizontal wind measurements did not simply increase with the drift of the balloon, which would result in an increase in the real-time distance between the two instruments, but that it depended on the spatiotemporal scale and the horizontal homogeneity of the weather systems. The spatial scale of weather systems is usually small to medium during summer, with a short duration and poor horizontal homogeneity. Therefore, in spite of the small distance between the two instruments, the discrepancy was large. By contrast, the synoptic systems are larger in winter and have good horizontal homogeneity, so although the distances between the radar system and the radiosondes were larger, the differences between the horizontal wind measurements of the two instruments were small.

Another interesting result was that, below 15 km height, the radar system underestimated the wind speeds when the radiosonde balloon was to the west of the radar site; whereas, above 15 km height the radar system overestimated the wind speeds when the radiosonde balloon was east of the radar site.

These analyses of the horizontal winds measured by the Beijing MST radar system and the radiosondes illustrate the good performance and detection accuracy of the newly constructed Beijing MST radar system. It will play a vital part in supporting high-resolution temporal and spatial measurements in the troposphere and lower stratosphere. The MST radar system will be a unique observation facility for studies of atmospheric dynamics in addition to being an operational meteorological observation system with high temporal and vertical resolutions.