2.1 Measurement site

The measurement site, located in central Portugal near the Spanish border, was selected because of the unique terrain feature of two nearly parallel ridges with a valley-to-ridge-top elevation difference of about 200 m and a ridge-to-ridge distance of 1.4 km (Fig. 1). The ridges run from the northwest to the southeast and are hereafter referred to as SW and NE ridges, respectively. The terrain roughness is characterized by a patchy vegetation of pine and eucalyptus trees with heights of up to 15 m. A detailed site description can be found in Vasiljević et al. (2017).

Table 1Positions of instruments used in this study. Easting and northing are in the PT-TM06/ETRS89 coordinate system and elevation is above sea level. Elevation of the masts (first two rows) refers to the mast base and elevation of the WindScanners (remaining rows) refers to the scanner head position. LRWS indicates long-range WindScanners.

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2.2 WindScanner measurements

We deployed six long-range WindScanners, scanning Doppler lidars (Vasiljevic et al., 2016), on top of the ridges at the Perdigão site. All scanners performed range–height indicator (RHI) scans along three transects which are almost perpendicular to the ridges. The transects are orientated along the SW–NE direction (Fig. 1; see Table 2 for the exact directions). The scans with a maximal range of 3 km covered the area within the valley. Outside the valley, the scans covered areas to a maximum range of 3 km and up to 1600 m above ground level. Elevation profiles along the transects are shown in Fig. 2. We measured continuously during the upwards movement of the scanners with an averaging elevation angle of 0.75^∘ over a range of 25^∘. Range gates were placed every 15 m, starting at a range of 100 m, extending to 3000 m. With range gates, we refer to the time intervals used for determining the wind speed from the back-scattered light. These time intervals correspond to spatial intervals along the line of sight (LOS) for which the LOS wind speed is evaluated. They translate into a weighting function along the LOS, which in this case has a full-width half maximum of approximately 30 m. The RHI scans were carried out twice per hour for 10 min by all lidars. During one 10 min period, 23 scans were performed; a single scan took about 24 s to complete. We averaged the data over each 10 min period, after filtering the data by the carrier-to-noise ratio (CNR $<-\mathrm{24}$ dB) and removing hard target influences. To ensure that the pointing accuracy of a minimum of 0.05^∘ is maintained during the entire measurement period, the WindScanner's leveling was checked and the known hard targets were mapped consistently through the measurement campaign (Vasiljević et al., 2017).

For the analysis, we selected periods within two 40^∘ wind direction sectors centered at a line perpendicular to the ridge (214–254 and 34–74^∘). The direction is measured by the 100 m sonic anemometer on the mast located on the southwest ridge. In total, 1619 10 min periods of lidar measurements are available for the analysis in these sectors (Fig. 3).

Figure 2Elevation profiles of measurement transects. The transect colors correspond to Fig. 1. The vertical scale is exaggerated by a factor of 2. The origin (range of 0) refers to the positions of WindScanners 105, 102 and 106 for transects 1, 2 and 3, respectively. The coordinates of the transects are available as CSV files in the Supplement.

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Figure 3Distribution of the 1613 available 10 min periods with wind directions perpendicular to the ridges (214–254 and 34–74^∘). The observational period lasted for 68 days starting on 8 April and ending on 14 June 2017. The intensive observation period of the campaign started on day 24 (1 May 2017). The availability data can be found in the Supplement.

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Table 2Parameters describing the measurement transects. Peak height gives the highest points at the southwest (SW) and northeast (NE) ridges, respectively. The average terrain elevation is measured along the transects in between the peaks. The maximum height of the ridges relative to the terrain height at the transect before and after the ridges is given by h_SW and h_NE. A_SW and A_NE is the half width at half height of the faces inside the valley.

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3.1 Atmospheric stability

To define atmospheric stability, we calculated the gradient Richardson number at the mast located on the northeast ridge (Fig. 1). The SW ridge mast could not be used due to missing temperature measurements for a major part of the observational periods. The gradient Richardson number is defined as in Stull (2012):

where Θ is the mean temperature measured at 100 m, $\partial \stackrel{\mathrm{‾}}{\mathrm{\Theta }}$ the potential temperature difference between 10 and 100 m, g=9.81 m s⁻² is the gravitational acceleration, and u and v are the two horizontal components of the mean wind vector. The potential temperature is approximated by $\stackrel{\mathrm{‾}}{\mathrm{\Theta }}\approx T+(g/C_{\mathrm{p}})\cdot z$, where g∕C_p = 0.0098 K m⁻¹ (Stull, 2012). In order to reduce the effect of terrain-induced flow on the stability estimates, we calculate the difference between the wind speed measured by the 100 m sonic and at ground level, 0 m, where the wind speed is assumed to be zero. A similar approach has been used by Burns et al. (2011). All calculations are based on 30 min averages. In total, we could assess the atmospheric stability for 82 % of the 10 min periods due to missing temperature data from the meteorological mast.

Moreover, the Brunt–Väisälä frequency, which is defined as

is estimated from the mast measurements for stably stratified conditions. We use again the potential temperature difference between the 10 and 100 m levels at the mast on the northeast ridge.

Figure 4Detection of recirculation zones at the second transect (lateral distance of 0 is the position of WindScanner 102), view from the southeast. (a) Schematic of the overlay of the two RHI scans at the transect. Blue and red areas are corresponding to the RHI scans and the grey area shows the terrain. (b) Lidar measurements along the transect and detected recirculation zone for one 10 min period with Ri_G=0.016. Positive LOS velocities indicate flow towards the northeast (from left to right in the plot). The terrain is plotted in grey and the contour line defining the recirculation zone is plotted in black inside the velocity field. Time is in UTC.

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3.2 Detection of recirculation zones

We develop a method to identify recirculation zones in measurements taken by lidars along the measurement transects (Fig. 5). This method is based on measurements by two lidars located on the same transect. By using two lidars, the area covered by the scans within the valley can be maximized. The RHI scans provide LOS wind speed measurements in a polar coordinate system with the lidar at the origin. Consequently, two lidars which are measuring in the same vertical plane will provide data in polar coordinate systems with different origins. Therefore, the LOS wind speeds measured by the lidars are interpolated to a Cartesian coordinate system with the abscissa orientated along the measurement transect and a grid size of 10 m in both the vertical and horizontal directions. In overlap regions, only measurements of the lidar further downwind are used. Additional regions that are sampled by the second lidar upwind are filled in a second step. This procedure prevents discontinuities between the two scans from interfering with estimates within the recirculation zone (Fig. 4a). We do not attempt to combine LOS velocities measured by the lidars to get the true horizontal wind along the measurement transect, since the elevation angles are too small to measure the vertical component precisely. Moreover, the assumption of zero vertical wind speed, as often applied in flat terrain, is not valid in complex terrain.

Having the measurements combined, the zero contour line of the velocity field behind the upwind ridge is determined using linear interpolation. The area bounded by this contour and the terrain beneath describes the dimensions of the recirculation zone (Fig. 4). Recirculation is defined to be presented for reverse flow speeds greater than 0.5 m s⁻¹. Also, the length of these zones is required to exceed five grid cells or 50 m in the horizontal dimension. To increase the robustness of this method, we require that at least one valid measurement is available below the contour line. A potential disadvantage is that recirculation appearing close to the ground cannot be captured since measurement range gates of the lidars that reach into the ground or vegetation must be discarded due to the hard target returns.

Figure 5Flow diagram of recirculation zone detection process. Recirculation is detected for periods that are flagged with “1”; no recirculation is detected for periods flagged with “0”.

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4.1 Occurrence of recirculation

On average, recirculation occurs frequently, in 52 % of all 10 min time periods examined. This ratio changes for the individual transects. Along transect 1 (the furthest northwest, with the lowest point in the valley and the shortest distance between ridges), recirculation occurs most frequently 69 % of the time, while transect 2 observes recirculation 51 % of the time. Transect 3 (the furthest southeast, with the highest low point in the valley and the longest peak-to-peak distance) only observes recirculation in 32 % of the available periods. These variations of recirculation occurrence may be related to the transects' elevation profiles within the valley. The average elevation along transects 1 and 2 is the same, whereas the average elevation along transect 3 is 15 m higher (Table 2). Additionally, the elevation profile of transect 3 shows a hill at the center of the valley (Fig. 2). We assume that these features suppress the formation of recirculation along transect 3. Other terrain parameters and land use/land cover characteristics only vary insignificantly among the transects.

Figure 7Occurrence of recirculation depending on the mean wind speed measured by the 100 m sonic on the upstream mast. The histograms are separated by transect and wind direction.

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4.1.2 Dependence on mean wind direction

Recirculation occurs more often for southwesterly wind directions (Fig. 7). A possible explanation for this difference could be that the northeast face of the southwest ridge (the downwind face in southwesterly flow) has steep escarpments close to the ridge top and the average slope is slightly steeper compared to the southwest face of the northeast ridge (the downwind face in northeasterly flow). Both the higher steepness and the escarpments make flow separation more likely. At transect 1, recirculation occurs predominantly, especially for southwesterly flow. The ratio of periods with recirculation occurrence to no occurrence is balanced at transect 2, and at transect 3 recirculation can only be observed 32 % of the time. For all transects, the chance of recirculation is higher for southwesterly wind directions.

Figure 8Zero contour line (black line) defining the upper border of recirculation zones of the mean velocity field of all recirculation periods per transect and flow direction. The flow speed is positive in the direction of the mean flow field and the transect origins (lateral distance of 0) refer to the positions of WindScanners 105, 102 and 106 for transects 1, 2 and 3, respectively.

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4.1.3 Dependence on mean wind speed

More differences between the transects emerge when we group the data by wind speed. Transects 1 and 2 show a very high chance of recirculation to occur for wind speeds above 8 m s⁻¹ (Fig. 7). For wind speeds above 8 m s⁻¹, fewer cases of recirculation occur for northeasterly wind directions, whereas recirculation is present in almost all cases for southwesterly wind directions. Moreover, the wind speed bin of 6 m s⁻¹ shows interesting results in the inter-transect comparisons: at transect 1, recirculation dominates this bin, whereas a contrary trend can be observed at the other transects.

Figure 9Observational periods with recirculation (colored markers) and without recirculation (grey cross marks) under stratified conditions as a function of mean lee-side slope (h∕A) and Nh∕u. The round markers refer to recirculation at the southwest ridge and the square markers to northeast ridge. The colored diamond markers show the mean value of Nh∕u per transect during periods with recirculation and the grey markers the mean for periods without recirculation. The solid black line shows where $NA/u=\mathit{\pi }$.

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4.1.4 Characteristics under stratified conditions

The findings above show that recirculation occurs as expected, prevailingly under unstable atmospheric conditions, but occurrences under stably stratified conditions are not absent. In general, three types of behaviors can be expected when flow encounters an obstacle (e.g., a hill or a ridge): (i) complete attachment of the flow, where separation downstream is suppressed by strong stratification; (ii) downstream separation of flow direction directly behind the obstacle; and (iii) post-wave separation (Baines, 1998). The occurrence of these behaviors is sensitive to the downstream slope h∕A of the obstacle and Nh∕u. Steeper downstream slopes support the formation of flow separation behind an obstacle and stronger stratification and higher values (separation occurs when $NA/u<\mathit{\pi }$) of Nh∕u suppress separation, respectively. The relation of these mechanisms is well summarized in Fig. 5.8 in Baines (1998), and we present our findings in the same non-dimensional framework in Fig. 9. The observations made in Perdigão, recirculation and non-recirculation periods, are falling almost entirely in an area where $NA/u<\mathit{\pi }$ in which separation is expected. However, looking at the mean value for recirculation and non-recirculation periods, it becomes noticeable that the mean of non-recirculation periods of Nh∕u is higher consistently at all transects. The higher mean value of non-recirculation periods aligns with the expectations described above to find attached flow or post-wave separation (which is not necessarily detected by our detection method) for higher values of Nh∕u.

Figure 10Maximum reverse flow speed divided by the inflow wind speed over the Richardson number. The color scaling shows the wind speed measured at the upstream mast 100 m above ground.

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Table 3Comparison of wind characteristics during the period of 16:30–16:40 UTC on 5 May 2017 measured by the sonic anemometers at 100 m height on the upwind SW and downwind NE ridges. Turbulence intensity is defined as $I={\mathit{\sigma }}_U{\stackrel{\mathrm{‾}}{U}}^{-\mathrm{1}}$, where $\stackrel{\mathrm{‾}}{U}$ is the mean wind speed and σ_U the standard deviation of U. Turbulent kinetic energy is calculated as $e=\frac{\mathrm{1}}{\mathrm{2}}\left[\stackrel{\mathrm{‾}}{{u^{\prime }}^{\mathrm{2}}}+\stackrel{\mathrm{‾}}{{v^{\prime }}^{\mathrm{2}}}+\stackrel{\mathrm{‾}}{{w^{\prime }}^{\mathrm{2}}}\right]$, where u^′, v^′ and w^′ are the fluctuating parts of the wind vector components over a 10 min average as measured by the sonic anemometers.

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Table 4Changes in the wind characteristics at the downwind ridge.

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4.2 Recirculation zone characteristics

The observed recirculation zones can be described by their shape in terms of extent and height and the magnitude of reverse flow speed. On average, the observed recirculation areas extend to 697 m in the horizontal for both wind directions, which is almost exactly half the average ridge-to-ridge distance at the transects. At the southeastern transect (transect 3, the longest transect with the least frequent recirculation), the average extent is 22 % smaller than the mean of all transects (Fig. 4). The average extent at the northwestern transect (transect 1, the shortest transect) is 9 % higher than the average extent. The average depth of observed recirculation zones is 157 m above the valley bottom height and changes insignificantly among the transects, whereas the maximum depth shows larger variation among the transects. We found that recirculation zones at transect 1 reach up to 300 m above the valley bottom, and 215 and 189 m at transects 2 and 3, respectively. Maximum reverse flow speeds of above 4 m s⁻¹ are observed. The median reverse flow speeds at the transects are 2.0, 1.5 and 1.4 m s⁻¹ for transects 1, 2 and 3, respectively. For all transects and both flow directions combined, a trend for higher reverse flow speeds for Richardson numbers close to zero is found (Fig. 10). For all transects and both flow directions combined, the relation of the reverse flow speeds to the Richardson number is presented in Fig. 10. The reverse flow speed measurements are divided by the inflow wind speed measured at the meteorological masts. For unstable atmospheric conditions (Ri<0), no clear trend for the relation of reverse wind speed and inflow wind speed can be observed. Ratios from less than 0.1 to more than 0.5 can be observed. During conditions with Ri>0, ratios are generally lower and appear to decrease with increasing Ri. For high wind speeds (greater than 12 m s⁻¹), ratios of less than 0.3 are observed and they occur, as expected, for small or slightly positive Richardson numbers.

4.3 Impact on potential turbine sites

Ridges and hills are advantageous sites for wind turbines, due to the terrain-induced speed-up (Hunt and Snyder, 1980). However, these stronger winds can come at a cost, due to higher turbulence levels and gustiness which increase loads and decrease the lifetime of turbines. In this section, we assess differences in wind characteristics at the downwind ridge for the entire dataset as well as a specific case study of the period shown in Fig. 4.