Wind Direction in Moscow

Transcription

1 ISSN , Russian Meteorology and Hydrology, 2015, Vol. 40, No. 10, pp Allerton Press, Inc., Original Russian Text M.A. Lokoshchenko, 2015, published in Meteorologiya i Gidrologiya, 2015, No. 10, pp Wind Direction in Moscow M. A. Lokoshchenko Lomonosov Moscow State University, GSP-1, Vorob ovy Gory, Moscow, Russia, loko@geogr.msu.ru Received March 5, 2015 Abstract Discussed are the data on wind direction in the air layer from 40 to 500 m over Moscow for the period of The data was obtained with the MODOS sodar installed in Lomonosov Moscow State University. It is demonstrated that this wind direction has a stable southwestern mode in ten years on average. The western component of this mode strengthens in spring and summer (the southsouthwestern direction is registered more rarely, and the west-southwestern one, more frequently). The winds of northern and eastern directions are observed much more rarely than those of southern and western directions. The rarest wind direction over Moscow is the north-northeastern one. However, under conditions of the anticyclonic weather (in the zone of blocking anticyclones in summer or the Siberian high in winter), the prevailing wind direction can have the eastern component even during several weeks on average. The northern wind is most often registered in summer, and the eastern wind, in spring. The average right wind shear in the air layer of m is 20. The surface air layer with the quasiconstant wind direction is not more than 40 m high in two of three cases per month on average; its height reaches 60 or 80 m in several months. DOI: /S Keywords: Acoustic sounding, wind direction, Ekman layer, right wind shear, surface layer height, Moscow 1. INTRODUCTION The measurements of wind direction in the operational mode are essential for solving many problems such as updating the weather forecasts, providing the weather services for the aviation, monitoring the conditions of the potential accumulation of harmful impurities in the air basin over big cities, etc. The longterm data on wind direction are needed for the building climatology (orientation of projected industrial objects relative to the populated areas as well as to separate buildings), wind-power engineering, aviation (the construction of runways along the prevailing wind direction), and many other applied areas. The data on wind direction at different altitudes can be obtained using different methods of measurement. The data on wind direction at the height of 10 or 15 m above the underlying surface are provided by the ground-based network of weather stations using the M-63 anemorumbometers or their versions, the new sensors produced in the USA and installed at MKS automatic weather stations which have been implemented by Roshydromet in recent years; and traditional Wild weathercock as reserve instruments [10]. It should be mentioned that the data of wind direction observations obtained from some ground-based stations (especially urban ones) are not enough reliable due to the buildings on the surrounding territory and due to wind shadows caused by separate constructions and trees located close to the station. This may result in the bias in the estimates of wind direction occurrence frequency in the different sectors of the horizon. The data on wind direction measured automatically at high-altitude measuring complexes (in the center of the European part of Russia: at the meteorological tower in Obninsk and at the Ostankino Tower in Moscow) are available up to the height of m. Traditional mechanical anemometers (propeller-type in Obninsk and cup-type in Ostankino) or acoustic anemometers are used at these measurement sites. Wind direction in the air layer at the height up to km is measured based on the displacement of radiosondes or balloons which can be considered almost lag-free floats in the wind field. The AVK radars which were implemented at the USSR upper-air sounding network in the late 1980s jointly with the MRZ radiosondes, enabled measuring the profiles of major meteorological parameters (including wind direction) with high resolution (100 m) within the lower one-kilometer layer. The direct measurements of wind direction above 30 km are rarely feasible by means of the launch of weather rockets (with consequent observations of the 639

2 640 LOKOSHCHENKO Fig. 1. The comparison of hourly mean values of wind direction in the air layer from 290 to 310 m from the MODOS sodar data and at the height of 301 m from the data of the propeller-type anemometer at the meteorological mast in Obninsk on October 12 November 11, The heavy black line 1 is the linear trend (y = 1.02x 7.06; R 2 = 0.964); the red line 2 is the parabolic trend; the thin black line 3 is the one-to-one data fit. horizontal displacement of the cloud of chaffs, falling spheres, or head of the rocket at the descending branch of its parachute flight) or using the data of observations of the movement of noctilucent clouds. Instruments for the remote sensing, namely, Doppler radiosondes, Doppler sodars (acoustic radars) or Doppler lidars hold a special place in the measurements. In the majority of cases the altitude range of their measurements holds an intermediate position between the data of high-altitude constructions and radiosondes, i.e., from 300 m to several kilometers. An advantage of these methods is that measurements can be carried out in the continuous automatic mode. Sodars have a relatively small altitude range (to m on average) as compared with other remote-sensing instruments but are notable for very high spatial resolution (10 20 m) [4, 8]. Besides, sodar data on the wind direction are more reliable as compared with data from the ground-based network because usually there are no vertical obstacles in the layer above the zone of silence of sodars (20 40 m, as a rule). 2. LONG-TERM DATA ON WIND DIRECTION OBTAINED AT MSU The acoustic sounding of the atmosphere in the Meteorological Observatory of the Department of Geography (The Faculty of Meteorology and Climatology) of Lomonosov Moscow State University (MSU) started in There it became round-the-clock and long-term for the first time in the former USSR. The METEK MODOS Doppler sodar (Germany) has been used since This is the first serial sodar in Russia. Its working frequency is 2 khz, the altitude range is from 40 to 500 m, and the spatial resolution is 20 m. The data on wind profiles are available every 10 minutes on average. The long-term data on wind speed from this sodar were published in [7, 18, 19]. The objective of the present paper is to generalize separately the results of wind direction measurements in the air layer up to 500 m over Moscow which were collected during 10 years. On October 12 November 11, 2008 the MODOS sodar operated in Obninsk at the distance of 200 m from the Taifun Scientific Industrial Association meteorological tower [16]. The obtained data on wind speed at that period were analyzed by the author in [7]. In the present paper let us consider data on wind direction. Figure 1 presents the comparison of the results of measurements provided by the sodar for every 10 minutes on average within the limits of the pulse scattering volume from 290 to 310 m and by the M-47 anemometer installed at the tower at the height of 301 m. It is clear that the obtained readings form two dense areas concentrated close to the one-to-one fit line, and the gap between them is from 30 to 110 because the wind of eastern and northeastern directions was not observed during the experiment. The coefficient of the linear correlation of the data was equal to 0.98 and the coefficient in the equation of linear regression was equal to 1.00 with the accuracy to one hunderdth. The statistical relation is close to linear that is corroborated by the parabolic trend almost coinciding with the linear one (the parabolic correlation coefficient is also equal to 0.98). Thus, the experiment proved the high degree of reliability of the sodar data on wind direction.

3 WIND DIRECTION IN MOSCOW 641 Fig. 2. The wind rose over Moscow in the air layer from 40 to 500 m for the period from November 11, 2004 to December 31, The values of occurrence frequency in separate segments are presented in the form of color scale. The MODOS sodar data on wind direction over Moscow for the period from November 2004 to March 2008 were presented for the first time in [18]. Later the data till August 2008 [19], February 2009 [8], and March 2009 [14] were presented. The wind direction under special conditions of abnormal heat was considered in [6]. The methodology of the analysis of these data is simpler than that of the analysis of the wind speed data because the errors typical of wind speed measurements (systematic bias in the estimates towards one or another direction) are absent in wind direction measurements [7]. It is obvious from physical considerations that at the high altitude the sodar registers the strong wind more often; at the same time, it registers any strong wind equally often regardless of its direction. The pointing of the MODOS sodar antenna system in the cardinal directions was carried out at the beginning of its operation in November 2014 using the usual compass. Therefore, all data on the wind direction are presented below not taking into account the magnetic declination, with the zero reading in the direction of the North Magnetic Pole. The position of the antenna system has been kept constant since then; therefore, the 10-year wind direction data series is homogeneous with the accuracy to 1 (the error in the installation of the antenna platform in the former position after the sodar was returned from Obninsk where the experiment was conducted, in 2008). Figure 2 presents the summary wind rose for the whole ten-year ( ) period of measurements and for the whole sounding range from 40 to 500 m. Its computation using the MODOS software enables obtaining the uniform graphic data both on the wind direction and wind speed. The distribution of wind speed values for different wind directions is presented in the form of concentric circles with different radia. The occurrence frequency in the separate sectors can be judged by the color scale. The whole range of occurrence frequency (from the rarest to the most frequent values) corresponds to the gradual change of light purple into dark blue and black; the white color means the absence of data with the prescribed wind speed and wind direction. As clear from Fig. 2, the southwestern (225 to 235 ) moderate (with the speed from 5 to 10 m/s) wind is most frequently (black color) observed in the lower half-kilometer layer over Moscow. Taking into account the eastern (positive) magnetic declination for Moscow ( ), the more accurate boundaries of this southwestern mode are from 235 to 245. This is the principal mode in the wind direction distribution and it prevails absolutely within the whole wind speed range including the highest values from 20 to 25 m/s (the cases of record high wind speed equal to 30 m/s and more that are described in [7], are single and not manifested in the total sample with the prescribed lower limit of the scale). The rarest direction is northeastern for the light wind (<5 m/s) and northern for the moderate wind. The cases of strong wind for these directions were observed very rarely. The qualitatively similar annual wind rose with the strongly pronounced prevalence of southwestern direction was registered in all years of sounding. Thus, the principal southwestern mode is unconditionally stable in time. The differences in the conditions in separate years boil down to the additional features of distributions, namely, to the existence of a wider or narrower principal mode, secondary modes, and higher or lower occurrence frequency of values in different sectors. For example, in 2010, when the winter was very cold and the summer was abnormally hot, the average annual occurrence frequency of southeastern and eastern wind direction turned out to be much higher than in other years.

4 642 LOKOSHCHENKO The occurrence frequency (%) of wind direction in different seasons and for the year as a whole from the MSU sodar data in the air layer from 40 to 500 m for Wind direction Winter Spring Summer Autumn Year North North-northeast Northeast East-northeast East East-southeast Southeast South-southeast South South-southwest Southwest West-southwest West West-northwest Northwest North-northwest The table presents the estimates of occurrence frequency of different wind directions including the intermediate ones for different seasons and for the year as a whole (~ separate readings for every 10 minutes on average). The data only for the first four years (from November 11, 2004 to August 31, 2008) are generalized here; however, taking into account the temporal stability of occurrence frequency estimates, these data are also representative for the whole period of sounding. Such table with the values only for the principal directions and only for the year as a whole was published in [18] in its preliminary form and in [19] in its final form. Since the MSU area is the edge of Teplyi Stan Upland and is open and relatively flat, the presented estimates of occurrence frequency for separate wind directions are quite reliable. As clear from the table data, the distribution of occurrence frequency of wind directions corroborates the well-known climatologic features of large-scale circulation in the center of the European part of Russia. The southwestern wind direction is really most often observed here [12]. According to our data, the comparatively high average annual occurrence frequency (more than 6.25% that would correspond to the uniform distribution for all 16 wind directions in case of the symmetric wind rose) is typical of the wide sector from the southern to west-northwestern directions. The winds of west-southwestern and southwestern directions are observed most often: their average occurrence frequency exceeds even 12.5%, i.e., the doubled value for the case of the uniform distribution. It should be noted that the occurrence frequency of these wind directions within the annual course is maximum (about 15%) in autumn and winter that is associated with the fact that at that period Moscow is frequently (since late autumn) located on the southeastern periphery of the Icelandic low. The similarity of autumn and winter conditions should be noted, the estimates of occurrence frequency of different wind directions in these seasons almost coincide. On the contrary, the southern and southwestern wind is registered less frequently in summer and spring and the western direction becomes more frequent. As to eastern and northern winds, in spring and in summer their occurrence frequency is much higher than in winter and autumn. The eastern and southeastern wind directions are most often observed in spring whereas the northern direction, in summer. This is an effect of the meridional flows going along the periphery of stationary anticyclones which are formed rather often in the European part of Russia in summer under conditions of westerlies weakening. As known, at the middle latitudes in spring and summer the intensity of westerlies is lower as a whole than during the cold season due to decrease in the interlatitudinal temperature gradient. As applied to the spring months, this conclusion is also proved by increase in the occurrence frequency of the eastern wind. We can only admire B.P. Alisov for the accuracy of his formulation that absolutely agrees with our data: the Atlantic entries weaken in spring, the wind direction becomes unstable, and the eastern constituent arises [1]. Nevertheless, as clear from the table, the eastern and northern winds over Moscow are the rarest during the year on average. The north-northeast direction has the lowest occurrence frequency: less than 3%. The prevalence of southeastern wind direction in winter is corroborated by the similar conclusion made in [1, 3]. In summer

5 WIND DIRECTION IN MOSCOW 643 the western component strengthens and the wind with the direction from southwestern to western is registered (the mode falls on the west-southwestern direction). This result agrees with the data of [1] ( as the summer begins, the wind direction becomes mainly western ) but does not corroborate the thesis from [3] on the prevalence of the wind of northwestern direction over Moscow in summer. It should be noted that taking into account the general right wind shear in the layer of m (as compared with the ground-based data), in the summer months the northern component in the principal mode is not pronounced all the more so. As mentioned, the principal southwestern mode in the distribution of wind direction is stable in time and is almost always manifested in the case of averaging for separate months. At the same time, the type of the distribution can be qualitatively different on separate days or even in separate months depending on the synoptic conditions. For example, severe frost was registered on January 16 24, 2006: the average daily air temperature in Moscow during that period since January 17 amounted from 16.7 to 28.0 C, and in the morning on January 18 the temperature dropped to 30.1 C (the only case in recent 10 years). The ultrapolar invasion of the continental Arctic air from North Siberia to the area of Moscow took place at that time along the periphery of the anticyclone with the center over the Kara Sea. As clear from Fig. 3a, the intensive cold advection was manifested in the weekly wind rose (for the sample of separate values of wind direction for every 10 minutes on average) in the form of the strongly pronounced northeastern mode. No thaw days were registered in Moscow from December 29, 2009 to February 23, 2010 (57 days, values). All that time the capital was mainly situated on the southwestern periphery of the Siberian high that defined the presence of the principal southeastern mode instead of the usual southwestern mode (Fig. 3b). Finally, the average daily air temperature did not drop below the normal for any separate day from June 20 to August 19, 2010 [6]. Under these conventional timeframes of the period of catastrophic heat, readings of wind direction were obtained. It is clear that the wind rose (Fig. 3c) was characterized by the only east-southeastern mode at that time. The closeness to the center of the blocking anticyclone defined the low values of wind speed: the modal interval of wind speed was from 0 to 5 m/s whereas it embraced the values from 5 to 10 m/s in two other examples as well as in the summary long-term wind rose. 3. AVERAGE VALUE OF VERTICAL WIND SHEAR Now let us consider the vertical variations of wind direction. The layerwise wind roses for the eight-year period of measurements are presented in Fig. 4, and the respective values of occurrence frequency (%) in separate sectors with the step of 10 within the range from 180 to 280 are the following: Wind direction, degree Sounding height, m (5 10 m/s) (10 15 m/s) It is clear that both wind roses are characterized by the same principal mode that is displaced clockwise as the height increases: from the southwest direction (with the maximum at 230 ) in the layer from 40 to 200 m to the west-southwestern direction (with the maximum at 250 ) in the overlying layer from 220 to 500 m. This displacement equal to about 20 between the conventional heights of 120 and 360 m (the middles of both layers) indicates the quite natural right wind shear in the Ekman layer. As known, the left wind shear that is usually associated with the effects of thermal wind, is registered much more rarely than the right one; this indicates the general increase in the Coriolis force with height within the atmospheric boundary layer. The similar result was obtained both from the data of balloon sounding in Pavlovsk (the right wind shear by 24 on average in the air layer up to 2 km) [5] and from the data of measurements at the meteorological tower in Obninsk [11]. There the right wind shear by about 20 averaged for several years was observed at the height of 265 m as compared with the surface wind direction (from 19 to 25 depending on the azimuth sector). The very insignificant occurrence frequency of light wind (with the speed below 5 m/s) in the air layer above 220 m is certainly not surprising

6 644 LOKOSHCHENKO Fig. 3. The average wind roses for the periods of weather anomalies. (a) Severe frost on January 16 23, 2006; (b) durable frost on December 29, 2009 February 23, 2010; (c) catastrophic heat on June 20 August 19, Fig. 4. The wind roses in the layer (a) from 40 to 200 m and (b) from 220 to 500 m for the period from November 11, 2004 to December 31, THE HEIGHT OF SURFACE AIR LAYER The occurrence frequency of various wind directions is traditionally presented in the form of the wind rose. The wind direction profiles in the direction height coordinates are used more rarely due to the need in taking account of the cyclicity of the wind in the values of its direction. In [15], the author gave the examples of such monthly mean profiles for the first time. In [17], the author presented the data generalized for the first six years of observations. As clear from Fig. 5, the natural right wind shear in the Ekman layer

7 WIND DIRECTION IN MOSCOW 645 Fig. 5. The examples of monthly mean wind direction profiles over Moscow from the MODOS sodar data. (a) August 2008; (b) July 2009; (c) November Confidence intervals were computed with the significance level of 5%. at the height of not less than 200 m is usually manifested in wind direction profiles. However, in some cases (Figs. 5b and 5c), the wind direction in the lower part of profiles coincides with the accuracy up to 1. This can be explained by the insignificant impact of the Coriolis force causing the right wind shear as compared with other forces affecting an air particle, namely, the friction and pressure gradient. As known, the surface layer is that characterized by the constancy of flows with height. However, the surface air layer is also the layer with the similar wind direction [2, 9]. Hence, the analysis of wind direction profiles makes it possible to determine independently the height of the surface air layer H above which all forces are of the same order of magnitude. In some works H is estimated in wide limits: from 50 to 100 m and, in some cases, from to m [9, 13]. In [2], the value of m is given as the estimate of the height of the air layer with the constant wind direction. It can be supposed that if the considerable wind shear starts already at the first level of sodar data (Fig. 5a), H 40 m (the upper boundary of the zone of silence of the sodar within which measurements are impossible). According to the data presented in [17], H 40 m approximately in two of three cases (in 39 of 56 considered months of sounding). In the rest of 17 cases, the monthly mean value of H was usually equal to 60 m and sometimes even to 80 m. The different form of profiles in Fig. 5 (the presence or absence of the lower layer with the same direction) is evidently explained by the effects of the synoptic conditions, first of all, by the different direction and speed of thermal wind in separate months. In accordance with the proposed simple criterion, the average height of the surface air layer over Moscow is <50 m in any case. Under assumption on the very high value of H = 40 m in all months, when this height is masked by the zone of silence, this height is equal to 48 m that is the wittingly overestimated value. Assuming that H = 30 m in all these months, the average height of the surface air layer for the whole sample of considered months is equal to 42 m. Thus, the analysis of sodar data with the high resolution enables assessing the height of the surface air layer as a layer with the quasiconstant wind direction. The more accurate result can be obtained with the small sodar (mini sodar) having the smaller zone of silence. Apart from monthly mean estimates, wind direction profiles were additionally computed at different hours of the day for three summer months of 2009 on average. It turned out that the maximum values of H in the diurnal course (up to m) were registered in the middle of the day (from 12:00 to 15:00) and the minimum ones ( 40 m), in the late evening and at night [17]. The obtained result is in correlation with the classic ideas concerning the effects of thermal stratification on the height of the surface air layer; the result also indirectly corroborates the reliability of the estimates of this height based on the sign of the beginning of the stable vertical wind shear. 5. CONCLUSIONS 1. Sodar data on wind direction are notable for the high degree of reliability that is corroborated by the comparison with contact measurements. 2. The prevailing wind directions over Moscow in the layer from 40 to 500 m both for the year as a whole and in all seasons are west-southwestern and southwestern; the role of the western component

8 646 LOKOSHCHENKO increases in summer. The moderate wind (from 5 to 10 m/s) with the direction from 235 to 245 is most frequently observed. The northern wind is most often observed in summer and the eastern wind, in spring. 3. Under conditions of blocking anticyclones impeding the westerlies, the type of the wind rose can be qualitatively different even at the scale of several weeks, namely, with the northeastern or southeastern mode becomes principal instead of the southwestern one. 4. The average right wind shear in the Ekman layer from 120 to 360 m is 20 on average. 5. According to sodar data, the height of the surface air layer as a layer with the similar monthly mean wind direction, exceeds the value of 40 m in 1 of 3 cases and, as a rule, is equal to 60 m or, in some cases, 80 m; in the rest of the months the average height of the surface layer in Moscow is below 50 m. ACKNOWLEDGMENTS The author heartily thanks V.G. Perepelkin for his assistance in the sounding, M.A. Novitskii and N.F. Mazurin for their assistance in conducting the experiment in Obninsk, and E.A. Yavlyaeva and N.G. Nikitina for their assistance in the data analysis. The research was partially supported by the Russian Foundation for Basic Research (grant ). REFERENCES 1. B. P. Alisov, Climate of the USSR (Moscow State Univ., Moscow, 1956) [in Russian]. 2. V. A. Belinskii, Dynamic Meteorology (Gosudarstvennoe Izd-vo Tekhniko-teoreticheskoi Literatury, Moscow, 1948) [in Russian]. 3. Climate of Russia, Ed. by N. V. Kobysheva (Gidrometeoizdat, St. Petersburg, 2001) [in Russian]. 4. N. P. Krasnenko, Acoustic Sounding of the Atmospheric Boundary Layer (IOM SO RAN, Tomsk, 2001) [in Russian]. 5. D. L. Laikhtman, Physics of the Atmospheric Boundary Layer (Gidrometeoizdat, Leningrad, 1961) [in Russian]. 6. M. A. Lokoshchenko, Catastrophic Heat of 2010 in Moscow from Data of Ground-based Meteorological Measurements, Izv. Akad. Nauk, Fiz. Atmos. Okeana, No. 5, 48 (2012) [Izv., Atmos. Oceanic Phys., No. 5, 48 (2012)]. 7. M. A. Lokoshchenko, Wind Regime in the Lower Atmosphere over Moscow from the Long-term Acoustic Sounding Data, Meteorol. Gidrol., No. 4 (2014) [Russ. Meteorol. Hydrol., No. 4, 39 (2014)]. 8. M. A. Lokoshchenko, Sodars and Their Application to Meteorology, Mir Izmerenii, No. 6 (2009) [in Russian]. 9. L. T. Matveev, Fundamentals of General Meteorology. Atmospheric Physics (Gidrometeoizdat, Leningrad, 1984) [in Russian]. 10. Handbook on Hydrometeorological Instruments and Installations (Gidrometeoizdat, Leningrad, 1971) [in Russian]. 11. Typical Characteristics of the Lower 300-Meter Atmospheric Layer from the Measurements at Meteorological Tower, Ed. by N. L. Byzova (Gidrometeoizdat, Moscow, 1982) [in Russian]. 12. S. P. Khromov, Fundamentals of Synoptic Meteorology (Gidrometeoizdat, Leningrad, 1948) [in Russian]. 13. S. P. Khromov and L. I. Mamontova, Meteorological Dictionary (Gidrometeoizdat, Leningrad, 1974) [in Russian]. 14. M. A. Lokoshchenko, Sodar Measurements of Wind Speed and Wind Direction above Big City (Moscow), in Proceedings of the Seventh International Conference on Urban Climate (ICUC-7) (Yokohama, Japan, 2009). 15. M. A. Lokoshchenko, Wind Structure of Lower Atmosphere above Moscow by the Sodar Data, in Proceedings of the 15th International Symposium for the Advancement of Boundary Layer Remote Sensing (ISARS) (Paris, France, 2010). 16. M. A. Lokoshchenko, N. F. Mazurin, M. A. Novitsky, et al., Results of Simultaneous Measurements of Wind Profiles by Sensors on High Mast and Sodars during Experiment in Obninsk, in Proceedings of the 15th International Symposium for the Advancement of Boundary Layer Remote Sensing (ISARS) (Paris, France, 2010). 17. M. A. Lokoshchenko and N. G. Nikitina, Profiles of Wind Direction and Studying of the Ground Air Layer Height by Use of the Sodar Sounding, in Extended Abstracts of Presentations from the 16th International Symposium for the Advancement of Boundary-layer Remote Sensing (Boulder, Colorado, USA, 2012). 18. M. A. Lokoshchenko and E. A. Yavlyaeva, Wind Profiles in Moscow City by the Sodar Data, in Proceedings of the 14th International Symposium for the Advancement of Boundary Layer Remote Sensing (14th ISARS), Riso National Laboratory, Denmark, IOP Conference Series: Earth and Environmental Science, No , Vol. 1 (IOP Publishing, Bristol and Philadelphia, 2008). 19. M. A. Lokoshchenko, E. A. Yavlyaeva, and H.-J. Kirtzel, Sodar Data about Wind Profiles in Moscow City, Meteorologische Zeitschrift, No. 3, 18 (2009).