Wind characteristics over Polana Izerska measured by meteorological instruments

During the measurement campaign in Polana Izerska, two 1-day measurement sessions for meteorological background and vertical structure of the atmosphere were held. The first session (7 July 2015) covered the time from 08:15 to 18:30 UTC (10:15–20:30 local time). The second session was held on 8 July 2015 and covered the period from 06:05 to 14:10 UTC (08:05–16:10 local time). Both sessions covered the time of measurements held with use of UAV and provided relevant reference data. The meteorological measurements were done using of two automatic weather stations MetPak Pro produced by Gill Instruments. The stations enable an automatic registration of air temperature (T) and humidity (RH) as well as air pressure (P). MetPak Pro stations are also equipped with an integrated acoustic anemometer that allows to measure horizontal wind velocity (V) and wind direction (WD). Measurement characteristics of all sensors built in the MetPak Pro station are shown in Table ​Table33.

Measurement characteristics of the MetPak Pro sensors (Gill Instruments 2015)

Both stations were located in the southwestern part of Polana Izerska, in a field covered by grass with the distance of approximately 30 m to the nearest trees, the height of which did not exceed 5 m. The first weather station was placed at a height of 2.5 m a.g.l., while the second one was placed at the top of a mast (10 m a.g.l). The detailed locations of both the stations are presented in Fig. ​Fig.1c.1c. Time resolution of all meteorological measurements (T, RH, P, V, WD) was 1 min. In addition to the session-targeted MetPak Pro weather, there exists a permanent weather station located in the southern part of Polana Izerska (Fig. ​(Fig.1c).1c). The station is owned and maintained by Świeradów Forest Inspectorate, and it measures numerous weather characteristics at the height of 2 m a.g.l. with a temporal resolution of 12 min.

The measurements of vertical structure of the near-ground atmosphere were performed using a mobile Doppler SODAR (SOnic Detection And Ranging), manufactured by ELAT. The instrument provided continuous information on vertical component of wind velocity (Z) in a 350-m profile (the effective height of the measurement is limited by the power of signal, 3.8 kHz) during the sessions. Due to the 15 m “start zone,” the real height of the profile is 365 m a.g.l. (15 m of “start zone” and 350 m of the entire profile). Vertical resolution of the measurements was of 2 m and time resolution was equal to 4 s. The precision of measurement of Z component was of 0.12 ms⁻¹, with range measurements from −15 to 15 ms⁻¹.

The change of general advection pattern observed during the measurement campaign had changed the airflow dynamics between the first and second day of the campaign. On 7 July 2015, during the entire measurement period, turbulent air movements of convective genesis were predominant. This was indicated by the observed convective Cu clouds development as well as confirmed by the registered SODAR data. Therefore, for most of the measurement period over Polana Izerska upward air movements dominated, which in total accounted for 59.5% cases and were most intensive between 9:00 and 15:00 UTC (Table ​(Table4,4, Fig. ​Fig.5).5). In that time range, the upward movements accounted for more than 60% and mean hourly Z values for the whole SODAR measurement profile were above 0. In the evening (16:00–18:00 UTC), gradual blanking of convection was observed. As a consequence, the frequency of descending air movements increased and mean vertical air velocity (Z) was significantly below 0 (Table ​(Table44).

Characteristics of horizontal (V) and vertical (Z) components of wind velocity in Polana Izerska during measurements campaign on 7–8 July 2015; V component—results from anemometer (10 m a.g.l.); Z component—results from SODAR measurements (averaged frequencies from whole vertical profile)

Averaged results of SODAR measurements of vertical wind velocity Z during the first day of the measurement campaign (07/07/2015, 08:15–18:30 UTC)

During the second day of measurements (8 July 2015), an increase in horizontal air velocity was observed, and the dominant (55%) airflow direction was SW (Fig. ​(Fig.6).6). The changes in synoptic situation were also reflected in vertical velocities measured using SODAR. Convection was reduced in comparison with the first day. This is confirmed both by visual cloudiness observations and SODAR data (Table ​(Table4,4, Fig. ​Fig.7).7). Descending air movements were predominant (59% cases), and the mean hourly values of Z were below 0 and covered the range between −0.5 and −0.7 ms⁻¹. The meteorological conditions described above indicate that on the first day of measurements, advective features of airflow were strongly disturbed by local conditions, connected mainly with convection development at the southern slopes of surrounding mountains during the day and accompanying intensification of turbulent air movements that resulted in higher variation of horizontal airflow directions and high share of ascending air movements. On the second day, due to limited convection, airflow occurred to be driven mainly by synoptic advective conditions.

Frequency of wind direction at Mt. Śnieżka and in Polana Izerska during the measurement campaign

Averaged results of SODAR measurements of vertical velocity Z during the second day of the measurement campaign (08/07/2015, 06:05–14:10 UTC)

The analysis of wind conditions in Polana Izerska should also consider that the observed wind direction differed from the general western air advection assumed for the entire area (Fig. ​(Fig.6).6). During the measurement period W–SSW and W–WNW wind directions were observed at Śnieżka, on the first and second day, respectively. In Polana Izerska, the wind direction was SSW–S during the first day and SW during the second day. The observed deviation of wind direction (45–65° towards the South) from the general direction was probably caused by local conditions connected with the role of terrain relief and land cover. The wind field deformation was caused by friction in the near ground level. However, it could also be driven by local deformation of airflow in the lowering of the Wysoki Grzbiet ridge, between Mt. Świeradowiec and Mt. Podmokła. In the case of the W–WNW advection, the pass may arrange the airflow, partially blocked from the east by Mt. Podmokła.