3.1. Annual and Monthly Fog Events
The daily observational dataset consists of a daily report of fog occurrences even if a single instantaneous occurrence with few minutes’ duration occurs. Table 2
shows the number of daily fog occurrences in the three airports between 2002 and 2018, and the decadal average. The yearly average percentage of the days with at least one METAR fog report is 9.68% at Porto, 5.15% at Lisbon and 0.97% at Faro. This latter airport is discarded from the analysis since fog seldom occurs.
The annual cycle of the monthly mean frequency of fog is plotted in Figure 4
a,b and shows that fog occurrences are more frequent at Porto during the summer while, at Lisbon, the fog is a winter event. The annual cycles at both locations are completely different, with almost all the events in Lisbon occurring in the extended winter, while the maximal monthly frequencies of fog at Porto (a mere 300 km north of Lisbon) occur in summer. Moreover, at Porto, 70.3% of the daily occurrences are between June and September, and only 49.3% between November and February at Lisbon (Figure 4
a). However, the relative frequency of the monthly distribution at Lisbon (Figure 4
b) shows 84.5% of the fog occurrences, against the 61.7% at Porto, in the respective periods. Therefore, more fog occurrences in each day are observed at Lisbon compared to Porto. Fog is a more daily recurrent phenomenon at Lisbon than at Porto, despite being less frequent.
3.2. Diurnal Cycle and Duration
In general, at Porto, the formation of fog occurs before the sunrise (Figure 5
a). In the period June–August, there are some cases starting after sunrise, and the late-night formation of fog occurs between July and September. At Lisbon, fog is only observed between September and April. Fog forms mostly late at night or early in the morning, between 0300 and 0900 UTC (Figure 5
The lower panels (c) and (d) depict the distribution of the fog events duration time, up to 12 h. Despite the seasonality of the fog, the wider duration range is centred in August at Porto and in December at Lisbon.
At first inspection, the fog events at Porto and Lisbon are driven by distinct local advections. Figure 6
shows the prevailing wind direction and the cumulative frequency of the fog events’ length. The predominant fog events had a duration up to six hours in both airports, being associated to southerly wind at Porto (Figure 6
a) and to East-NorthEeast (ENE) wind at Lisbon (Figure 6
b). Regarding the longer events, ten events were longer than 12 h and last up to 24 h. At Porto, they were driven by 2 ms−1
SW flow, where the average wind direction is 245° and the median is 280°. At Lisbon, eleven events exceeded 12 h of duration, and eight of them lasted up to 36 h, and a single event was slightly longer than 52 h. All these events were driven by 1.5 ms−1
ENE flow, where the average wind direction was 60° and the median was 80°.
3.3. Fog Types Classification
The classification algorithm applied to Porto and Lisbon datasets classified 96.9% of the fog events at Porto and 98.9% at Lisbon. The frequency and the percentage of occurrence of each fog type are presented in Table 3
The predominant fog type at both airports is the cloud base lowering type, with 57.7% of the events at Porto and 66.9% at Lisbon, followed by the advection (27.8% and 24.0%), precipitation (6.7% and 5.3%) and radiation types (4.7% and 2.7%). For none of the fog events was the evaporation type identified, and the classification was inconclusive for 3.5% of the fog events at Porto and 1.1% at Lisbon. It should be noted that the ADV type, as the second most frequent type of fog at Lisbon, is suggested to be the mechanism favouring the formation of fog by the study of Teixeira and Miranda as a result of simulations generated from a 1D conceptual model.
The classification algorithm provides different mechanisms to estimate the fog of cloud-base lowering type: one associated to winds higher than 1.5 ms−1, which might be related to the conveyed low clouds laid at surface due to orography, and the other associated to wind intensities lower than 1.5 ms−1, which might be related to turbulent mixing due to the cloud top cooling. Below 1.5 ms−1, the number of the fog events from the most frequent type (CBL), at both airports, is 343 events at Porto, and 190 at Lisbon, and above 1.5 ms−1 windspeed events number 60 at Porto, and 61 at Lisbon.
The fog monthly distribution according to fog types presented in Figure 7
shows the relative frequency of the different types, where the prominence of the CBL type is evident at both airports, followed by the ADV and RAD types, the last one of which is very close to the PREC type. At Porto (Figure 7
a), CBL is the prevalent type during spring and CBL and ADV frequencies increase during the summer. Although a significant decrease of the frequency of the ADV type occurs in September, the frequency of CBL has only a slight decrease relative to August. The fog events in September are then merely the cloud-base lowering type. At Lisbon (Figure 7
a), CBL is clearly the predominant fog type, significantly more frequent than the ADV and RAD types.
3.4. Large to Local Scale Circulation Analysis
In this section, the fog results are analysed in the context of the large to local scale atmospheric circulation. Local advection is represented by the observed wind at the airports of Porto and Lisbon, whereas the large atmospheric circulation is defined, in each case, by one of the 10 circulation weather types presented in Section 2.2
shows that fog at Porto and Lisbon is favoured by different weather types. Fog at Porto is favoured in N flows and anticyclonic conditions, and clearly disfavoured in E, SE, S, SW, W, and cyclonic flows. Among the types of fog, the CBL type reveals an expressive occurrence associated to both NE flow and anticyclonic conditions as well. The NE flow is driven by a pressure perturbation (inverted trough) over the Iberian Peninsula under similar stable conditions as the anticyclonic weather type. At Lisbon, the more favoured flow regimes are anticyclonic and SW, where the CBL type stands out as well from the other types of fog. The anticyclonic pattern is not only associated to stable conditions and light winds, but also is characterized by a slight pressure perturbation giving to the circulation a smooth easterly component, favouring the advection from the Tagus estuary. The SW flow is also responsible for the moist air advection driven by the valley, from the sea towards to the airport plateau. As a simple point, 33.5% of the global fog occurrences at Faro were favoured in NE flow and anticyclonic conditions (not shown). However, since no statistics were further developed, the fog type classification was not performed at Faro airport.
After analysing the large-scale setting favourable for fog, it is important to regard the local flow properties that lead to fog in both airports. The local advection analysis is based on the observed wind intensity and direction during fog, as shown in Figure 9
. The wind intensity is divided into six classes according to the characterization of the wind in the Section 2.3
At Porto (Figure 9
a), fog is more frequent in low-speed winds between 1 and 2 ms−1
from NW, and from the south between 2 and 3 ms−1
. In Lisbon (Figure 9
b), fog occurs mostly under low speed NE wind, the wind sector linked to the air advection from the Tagus estuary, which is very infrequent in summer and somewhat frequent in winter. At Porto, fog may arise at higher wind speed than in Lisbon, especially when it is associated with local SW and southerly wind. These local evidences are in line with the large-scale atmospheric flow given by the weather types S and SW, in the northern region of Portugal.
The wind roses shown in Figure 10
combine the wind direction with the frequency of occurrence of each fog type, which clearly indicates that fog is strongly constrained by local circulation. The fog types CBL and ADV are firmly present in the prevailing wind directions at both airports, slightly above 43% and 66% of the fog events at Porto and Lisbon, respectively. Despite the predominance of the CBL type in the fog events, the ADV is the most common fog type in the predominant wind directions at Porto, whereas at Lisbon the CBL remains as the typical fog type. Another interesting feature at Porto is the occurrence of the PREC type bounded to the S-SW quadrant while the RAD type is spread through several wind directions, and at Lisbon the RAD type is aligned to the prevalent direction of the CBL and ADV types (ENE). The PREC type is inexpressive to any wind direction.
In addition to the previous results, observed meteorological parameters are plotted to represent the behaviour of each fog type at both airports. One event of all fog types was selected for each location based on the visual inspection of the differences between the meteorological variables, as evidence of the primary mechanism. The selected criteria were the seasonal occurrence at each location, the weather type associated to the day of occurrence and the fog event must be consistent in terms of persistence (duration longer than two hours) and intensity (marked visibility reduction below 1000 m). Figure 11
allows a detailed analysis of these fog cases, concerning the horizontal visibility, the wind speed and direction, the temperature, the dewpoint, the present weather, and the relative humidity. The horizontal visibility plot is highlighted below 1000 m and the present weather plot indicates the synoptic code form between 40 and 49 (WMO, 2011) [16
] reported before and during the fog events. At Porto, the fog event of 2.5 h on 28 January 2009 is classified as PREC, the event of 14 August 2002 was ADV and 2 h long, the 2.65 h-long event of 28 August 2006 was CBL type and the event of 1 November 2011 was RAD type and 9.2 h long. At Lisbon, the two-hour fog event of 1 January 2009 was chosen as an example of the PREC type. The event of 22 February 2002, which was 4.8 h long is as an example of the ADV type, the event of 3.4 h on 14 January 2007 was CBL and the event of 3 January 2010 was RAD type and 3.8 h long.
The present weather plots for PREC type show that the fog is preceded by drizzle during the five hours prior to the onset at both locations. At Porto, the precipitation changes to rain three hours before the onset, increasing the liquid water at surface available to evaporate. Precipitation before onset is a distinct feature of the PREC type. Simultaneously, in both locations, the duration of the PREC event is shorter than the other types. The ADV type plots show evidence of two different mechanisms favouring the formation of fog. Mist is reported before the onset at Porto and at Lisbon, however, steady southerly wind at Porto along with the cooling and increase of relative humidity followed by saturation leads to local condensation and results in fog formation. At Lisbon, the saturated northeast inflow from the Tagus estuary and the unchanged temperature suggests a distant formation of fog and further advection. The CBL type is characterized by a sudden reduction of the horizontal visibility after four hours of mild wind conditions before the onset. Regarding the RAD type, the horizontal visibility reduces steeply though the wind is lighter, even almost calm.
The behaviour of each fog type may be interpreted by the deviation of the onset and dissipation from the sunrise and sunset, respectively, and from the duration, the horizontal visibility, and the intensity (minimum visibility). These are listed in Table 4
. In general, the formation of fog occurs before sunrise, where the RAD type forms earlier than the other types and dissipates after the sunrise alongside with the CBL type. Although less frequent than the CBL and the ADV types, the RAD type lasts longer in both locations: 3.6 h long at Porto and 3.8 h at Lisbon. According to the horizontal visibility values, the fog events at Lisbon are denser, except those associated to the PREC type. At Lisbon, the visibility median associated to the ADV type is 388.5 m against the 586.5 m at Porto. The CBL and RAD types reduces the visibility by 200 m more in half of the events than at Porto, roughly to 300 m, and the intensity of 25% of the CBL and the RAD events reduces the visibility to 150 m, against the 300 m of the visibility’s first quartile at Porto.
At Porto, the CBL type produces the stronger fog events with an average of 217.8 m of minimum visibility. At Lisbon, the minimum visibility average associated to the CBL type is 232.5 m.
In short, the results are as expected: fog requires weak winds, both synoptic and local, as observed in anticyclonic weather, and moist air is also present over the estuary or coastal region and persistent cooling, as at the end of the night.