In the period from 1967 to 2010, weather was the primary cause of the growing percentage of annual aircraft accidents. Most of the accidents attributed to weather conditions occur in latitudes between 12° and 38° N/S [1
]. Moreover, adverse winds and turbulence have had a large contribution to aviation weather-related accidents [2
]. Previous studies have shown that mountain wave mechanisms are an important source of turbulence, contributing to numerous aircraft incidents and accidents. For instance, mountain waves have been associated with extreme clear-air turbulence at cruise levels, causing severe structural damages to aircraft [3
] or leading to several seriously injured passengers [4
]. In the lower troposphere (below 3 km), mountain wave turbulence can be particularly hazardous and was the probable cause of a fatal light aircraft crash on 31 July 2007 in Australia [5
]. On 18 November 2008, a commercial aircraft experienced severe turbulence when descending over the south-eastern coast of Iceland. This incident was associated with a large amplitude lee wave that caused an atmospheric rotor and strong downslope winds [6
]. On 20 December 2008, during the take-off phase at the Denver International Airport, a Boeing 737 deviated off the side of the runway due to severe crosswinds (approximately 37 kt) caused by lee waves (Keller et al. [7
]). Mountain waves were also the probable cause of a hard landing event in the Pico (Azores) aerodrome (Maruhashi et al. [8
Several studies have characterized the flow response to an isolated circular mountain in terms of dimensionless numbers, such as the Froude number, Fr = U/Nhm
], where N is the Brunt–Väisälä frequency, hm
the mountain height and U the upstream wind speed. Other studies have considered the inverse of Fr, also known as non-dimensional mountain height, M [13
]. These studies showed that for Fr << 1 (M >> 1) upstream stagnation and flow splitting occurs and a wake may form leeward of the obstacle. Upstream stagnation is defined as an abrupt slowing of the airflow on the windward side of a mountain. Flow stagnation occurs due to the positive upstream pressure anomaly associated with the disturbance and occurs at height zs on the upstream slope. Below zs, the flow splits and circumvents the obstacle rather than flowing over it, while above zs the flow surpasses the obstacle, phenomena known as flow splitting [14
]. The wake region can be identified by a flow deceleration or by the presence of vortices (Smith [15
]). These vortices can have either a quasi-steady nature [9
] or an unstable pattern, in which vortices of alternating signs are periodically shed downstream to form a Kármán vortex street [17
]. Schär and Smith [10
] showed that wake vortices could form in association with hydraulic jumps, when the upstream Froude number is below 1 and the inversion layer lies at or below the mountain top. Other studies [11
] suggested that for a low Froude number, lee vortex formation might be associated with wave breaking (breakdown of laminar flow). Moreover, the occurrence of wave breaking is more probable for wider (elliptic) mountains [14
]. Wave breaking has a turbulent nature and may produce downslope windstorms [3
], which can be hazardous to aviation. Therefore, it is very important to predict the favourable conditions for their occurrence.
The Madeira archipelago is located in the subtropical eastern North Atlantic, lying between the 32.4° N and 33.2° N parallels and centred along the 16.8° W meridian (Figure 1
). The island of Madeira, the largest of the archipelago, has a ca. 54 km mountain range, WNW-ESE oriented, with an NNE-SSW length of ca. 22 km. The island has a complex terrain, rising steeply from the coast and peaking at an altitude of 1862 m (Pico Ruivo).
The weather in Madeira is strongly influenced by the position and strength of the semi-permanent subtropical anticyclone over the North Atlantic (known as the Azores high). Spring sets the beginning of the rapid expansion, strengthening, and northward migration of the Azores high, which continues until summer (Davis et al. [21
]). This intensification leads to the reinforcement of the subsidence in the Macaronesia region, contributing to the strengthening of the inversion layer and to the lowering of its base height during these months [22
]. Due to this fact, at Funchal (main city in Madeira), the inversion layer is frequently below the height of the Madeira mountain range (Grubišić, et al. [23
]). Furthermore, from April to August, the predominant incoming flow to Madeira is from North to Northeast, driven by the eastern branch of the Azores high. Therefore, Madeira Island is frequently oriented nearly perpendicular to the incoming airflow. The combination of the aforementioned conditions is indeed prone to wake formation [23
The Madeira International Airport (MIA) lies on the island’s south-eastern coast and, due to its location, is often affected by turbulence associated with the island wake. For instance, during the first 100 days of 2018, approximately 130 h, 550 aircraft movements and 80,000 passengers were affected, due to adverse wind conditions. Since tourism is the main Madeira’s economic activity and the bulk of tourists visiting Madeira arrive through the airport, these air-traffic restrictions have a large negative impact on the island’s economy.
The Madeira atmospheric wake has been previously documented [23
]. However, to the authors’ best knowledge, the impact of this wake to the airport operations has not been addressed. Besides, the link between synoptic-scale weather patterns and wind regimes at Madeira Island and, in particular at the MIA, was poorly addressed. At the Portuguese Weather Service (Portuguese Institute for Sea and Atmosphere, IPMA, Lisbon, Portugal), the Application of Research to Operations at Mesoscale (AROME) model is routinely used by forecasters. Therefore, the assessment of the performance of AROME wind forecasts is of great importance.
This study has four goals. The first is to diagnose the take-off and landing restrictions at Madeira airport due to wind limitations. The second is to identify relationships between these operating limitations and synoptic-scale regimes. The third objective is to illustrate the interaction between the synoptic-scale forcing and the orographic effects during one particular event of adverse winds at the MIA (case study). The final goal is to assess the agreement between AROME forecasts and observations of wind at 10 m-height on the Madeira island, primarily at the MIA.
The remainder of this paper is organized as follows. A brief description of the observations, the AROME model and the weather type methodology is presented in Section 2
. The wind regime in Madeira Island, the temporal variability of the take-off and landing restrictions at Madeira airport due to adverse winds, as well as its connection with weather types, is analysed in Section 3
. An episode of adverse winds at the MIA is analysed in Section 3.3
. The objective verification of wind forecasts of the AROME model is also presented in Section 3.4
. Finally, the summary and conclusions are drawn in Section 4
4. Summary and Conclusions
The landing and take-off operations at the Madeira International Airport (MIA) have operating mandatory limitations related to adverse wind conditions. These conditions are less frequent and persistent in winter than in summer. The present study established a link between these adverse winds at the MIA and the synoptic-scale circulation, using a weather type (WT) classification, which allows identifying twenty WTs.
During the extended winter period (October–March), the relative frequencies of occurrence of the 20 WTs reveal a nearly uniform distribution, being the cyclonic regimes were more frequent than in summer. On the other hand, during the extended summer period (April–September), five anticyclonic WTs are predominant, cumulatively representing nearly 70% of days. In summer, nearly 60% of the landing restriction events are associated with three of these WTs (WT14, WT11 and WT1). However, for the days with WT14, only 19% led to adverse winds at MIA. For the other WTs, this frequency decreases to 15% and 12%, respectively, for WT11 and WT1. These three regimes reflect the presence of the Azores high with a vertically shallow structure, despite some variations in their strength and position. These WTs induce NNE/NE winds near the surface over the Madeira Island, with the wind backing up to 700 hPa. This backing is maximum (nearly 90 degrees) for WT11.
The prevalence of NNE/NE inflow over Madeira favours the occurrence of an upstream flow stagnation in its northern (windward) side, explaining the prevalence of weak winds in this region. Concurrently, the flow splitting phenomenon leads to a wind intensification on the western and eastern regions (lateral flank shocks), confirmed by the highest wind records, and the development of a lee wake. Since the MIA lies on the island’s south-eastern coast, the wind conditions at the MIA are strongly influenced by the position of the lee wake and/or eastern flank shock.
The most severe and longer-lasting wind conditions at the MIA, with a higher frequency of gusts above 40 kt, arises under the influence of WT13. This regime reveals an intensification and eastward migration of the Azores high, with respect to its climatological position. Another three WTs were identified as responsible for adverse wind conditions at the MIA, namely WT18, WT19 and WT6. The high-pressure systems in these regimes reveal a strengthening, and either a north-eastward (WT18) or eastward shift relative to the average conditions. Furthermore, these four WTs have an important common characteristic, i.e., the maximum 700 hPa or 500 hPa geopotential height is positioned nearly above the maximum at the MSLP level. This indicates that the anticyclonic development extends to the mid-troposphere and, consequently, induces upstream anomalously strong NNE/NE winds near and above the mountaintop level. These conditions in the presence of strong stable stratification, which is fostered by strong anticyclones, favour the development of strong downslope winds [37
The temporal evolution of adverse wind conditions at the MIA in recent years was also analysed. During the extended summer, in the 2015–2018 period, the number of days affected by these limitations has more than doubled compared to previous years (2010–2014). This growth was mostly driven by the reinforcement and eastward/north-eastward expansion of the Azores anticyclone, identifiable by the anomalously high frequency of the corresponding WTs. For instance, in 2018, the abnormal frequency of occurrence of WT14 has the greatest contribution to trigger adverse winds at the MIA. In 2015, the most adverse WTs (WT13, WT6, WT18, WT19) totalized 12 days (7 above average). On the other hand, the WT13 was absent in 2011, 2012 and 2014, when the adverse wind events were less frequent than after 2014.
The operational AROME model was used to better understand the triggering mechanisms of the adverse winds at the MIA on a day under the influence of an intense Azores high (typical of WT13). During that day, several flights were cancelled and/or delayed and a missed approach event occurred. During this severe wind episode, there was a reasonable agreement between forecasts and observations of wind gust at the MIA. However, the model overestimates the wind gust when the wind speed is less than 10 kt.
According to the AROME forecasts, during this episode, the upstream conditions were characterized by a low-level temperature inversion and by a Froude number varying between 0.4 and 0.8. These conditions are prone to flow-splitting, wave breaking and lee wake development. Accordingly, AROME forecasts of wind at 10 m-height identify a stagnation area northwards of the island and a flow splitting signature, with a wind intensification on the island lateral (eastward/westward) flanks. The lee wake to the south of the island was also predicted by AROME. Due to its location, throughout the day, the MIA was exposed to either weak winds, as the wake is shifted eastwards, or strong winds, associated with the flow splitting. In addition, the period of strongest winds (with gusts above 40 kt) at the MIA coincides with the strengthening of upstream stagnation and the consequent reinforcement of the flow splitting. Moreover, during this period, the AROME predicted strong downslope winds (~ −2 m s−1
), reaching the vicinity of the Madeira airport, in association with large-amplitude mountain waves. These results suggest that the presence of downslope winds reaching low levels can be a useful predictor of severe wind conditions at the MIA. However, in order to validate this hypothesis, other episodes must be analysed and, more importantly, the predictions of vertical wind velocities should be compared to the measurements provided by remote sensing equipment, such as sodars [37
]. Therefore, it would be very useful to conduct a field campaign, ideally during the summer months.
Although not necessary for the development of downslope windstorms, the presence of a critical level or wave breaking tends to increase the likelihood and strength of windstorms [40
]. Moreover, the presence of critical levels induced by directional wind shear plays a crucial role in triggering mountain-wave turbulence [42
]. The occurrence of a strong temperature inversion also plays an important role in fostering the formation of downslope winds and rotors [34
]. Thus, in the future, it would be imperative to explore the possible connection between these phenomena and the strengthening of winds at the MIA. Thus, it would be necessary to perform a climatological study to characterize the upstream environment, taking into account several ingredients, such as the formation of critical levels and their heights, as well as temperature inversions and their strengths. For this purpose, radiosonde observations in the Porto Santo Island (northeastwards of Madeira) would be very valuable.
Lastly, this study presents an assessment of the AROME 10-m height wind forecasts on Madeira Island. This analysis was carried out for the 3-year period of 2015–2017, during the months of June to August, when wake episodes are more prevalent. On average, AROME overestimates the wind gust, with RMSE ranging between 1.6 m s−1
and 3.9 m s−1
(~7.6 kt). The exceptions occur in stations on the west and east edges, where the model underestimates the wind gust. At the MIA, the RMSE of wind gust varies between 2.8 m s−1
(RWY23) and 3.9 m s−1
(RWY05). Regarding the mean wind speed, the RMSE is 2.2–2.7 m s−1
. The wind direction errors are small (RMSE < 20°) in regions where moderate to strong winds prevail, namely in RWY23 and on the flanks of the island. Moreover, in these locations, the Pearson correlation exceeds 0.7 for wind speed and wind gust. On the other hand, in locations where weak winds are more frequent due to the upstream stagnation or to the wake influence, the RMSE for wind direction varies between 30° and 77°, whereas the correlations between observation and forecasts decrease significantly. The difficulty of NWP models to correctly predict weak winds is a widely recognized limitation [8
]. This limitation has a great impact in the area of the MIA, since this airport is located in a region where wind varies strongly over short distances, due to its proximity to the wake and the region of maximum wind speed (caused by the flow splitting). As the island’s very complex topography can significantly influence the precise location of these structures, higher resolutions of the NWP models may be critical to increase forecast accuracy. In the lower troposphere, turbulence and wind shear play a crucial role and can be potentially dangerous during landing and take-off operations. Therefore, in the future, it would be important to validate wind forecasts in the lower troposphere using data from Aircraft Meteorological Data Reports (AMDAR).