Topoclimatic characteristics of Zahlé (Eastern Lebanon): Thermal breezes and urban heat island phenomenon – Preliminary results

This work examines the topoclimatic characteristics of Zahlé, specifically thermal breezes and the Urban Heat Island (UHI) phenomenon. Three-hourly data from 1994 to 2021 from conventional weather stations shows a frequency of breezes in the summer season of 74%. In Rayak, mountain and valley breezes have been noticed alternating between day and night, channeled by the topography of the Bekaa Valley. In Houch El Oumaraa, valley-sea breezes dominate during the day and mountain breezes occur at night. These breezes affect the distribution of air temperature and humidity, creating an average thermal contrast of 4.3°C between the city and its surrounding countryside at night. Data from weather stations implanted in 2022 and car surveys at a fine scale (half hourly data) level has confirmed the previous findings: During nighttime, the mountain breeze dominates and brings cool and moist air from the mountains into the valley at a low speed (<1 m/s). During the daytime, a valley breeze dominates and can reach speeds of up to 4 m/s. Additionally, a sea breeze with faster speeds is detected. The car surveys indicate a temperature variation of 9 °C between the valley and the surrounding urban areas during early morning.


Introduction
Thermal breezes with well-defined mechanisms (Braham 1979); (Prtenjak and Grisogono 2007); (Fernández-Camacho et al. 2010); (Zhang and Wang 2021) are winds of thermal origin and local scale whose horizontal extension can vary from a few hundred meters to several tens of kilometers (Carrega 1992).These breezes with low to moderate speeds (<11 m/s) are favorable to high concentrations of air pollutants (Dahech 2007).They also affect thermal comfort by advection of cool, moist air (Pooler 1963).The impact of thermal breezes on weather and air quality varies according to topoclimatic conditions and city size.First, certain topographic conditions are favorable to the stagnation of cool air near the surface and the occurrence of a thermal inversion favorable to the degradation of air quality (Michelot and Carrega 2014).Second, human activities and the artificialization of surfaces in urban areas contribute to the formation of an urban heat island (UHI), which is a positive thermal difference from the surrounding countryside.It is a well-documented phenomenon (Weston 1988), (Papanastasiou and Kittas 2012), (Acero et al. 2013).The UHI effect is caused by the combination of factors such as urban morphology, land cover and land use, anthropogenic heat release, and meteorological conditions.These factors lead to a warming of the urban atmosphere, resulting in higher urban air temperatures than surrounding rural areas.In fact, compared to the more vegetated countryside, the city, where impervious and low albedo surfaces dominate, stores more heat during the day and releases it at night in the form of terrestrial radiation (Sundborg 1950).This additional heat in cities becomes a concern during heat waves and could have significant health impacts (Bourque 2000).In the current context of climate change, The Intergovernmental Panel on Climate Change (IPCC) (Masson-Delmotte et al. 2021) predicts a sharp increase in these extreme situations in the short and medium term.
The combination of UHI and climate change is expected to have a significant impact on urban areas, particularly in terms of urban comfort (Füssel et al. 2012).Urban residents may experience increased heat stress, which can lead to a variety of negative health outcomes such as heat stroke and heat exhaustion (Lowe 2016).
UHIs are triggered by radiative weather when the sky is clear and the synoptic wind is weak.These conditions are very frequent in the Mediterranean as in Lebanon.
This country occupies a transition zone between the tropical deserts to the south and temperate Europe to the north.It is an area that is influenced by the prevailing westerlies: in summer, this westerly flow contracts northwards and gives way to high atmospheric pressure caused by the subsidence of the Hadley cell, which leads to atmospheric dryness and stability (Traboulsi 1981).At high altitudes, a high geopotential prevails: the subtropical anticyclone covers the whole region, the air is warm, subsident and stable (Blanchet 1979).These stable summer atmospheric conditions are ideal for the development of thermal breezes and UHI.
Moreover, Lebanon is affected by the westerly winds that hit the western chain, rising on its western slopes (Mounts of: Makmel, Mnaitra, Sannine, Knaisseh, Barouk and Niha) (Fig. 1), then descending on the eastern slopes.The Bekaa is downwind: the air masses lose much of their moisture on the western slopes of the western chain and suffer the foehn effect on the way down (Traboulsi 2010).This phenomenon may contribute to the warming of the city of Zahlé and intensifies the effect of the UHI.Many authors have studied the foehn effect (chinook in American regions), several investigations have helped to understand the important process that takes place (Cook and Topil 1952).
In the present work, we study the characteristics of these thermal breezes and UHI as two topoclimatic phenomena in the region of Zahlé in eastern Lebanon.This area, which has not yet been studied from a topoclimatic point of view, is marked by unique topographical features that make it vulnerable to climatic phenomena with opposite characteristics (breezes, thermal inversions, UHI...).Crossing tens of meters from the plain to the slopes of the western range makes us feel the rapid change in temperature, humidity and wind.These phenomena can be beneficial for agriculture, land use and water resources, but at the same time they can be detrimental by accumulating pollutants in the lower atmosphere.This study aims to show spatiotemporal distribution of wind, atmospheric temperature and humidity at multiscale method.In this work we mobilize both car survey measurements, a network of weather stations and satellite data from Aster TIR and Landsat 7.This approach is original for this study area, indeed the few studies on the topoclimate in Lebanon are applied in the city of Beirut (Sakr 2012).

Study area
The caza1 of Zahlé covers 425 km 2 , it is located in a valley (the Bekaa Valley) situated between two mountain ranges: the western range (Mount Lebanon) and the eastern range (Anti Lebanon).The topography is uneven with altitudes ranging from 870 m in the valley to 1920 m in the high  1).There are various economic activities, but agriculture is predominant.Nevertheless, there is industrial activity in the valley and quarries on the slopes.The main settlements are in Zahlé, Taalabaya, Chtaura and Barr Elias.The region has a Mediterranean climate with presence of two well-defined periods: a dry and hot period that predominates from May to September, and a wet and mild period from October to April (Fig. 2).

Weather data
The data for this study primarily comes from two fixed meteorological stations in the study area: Houch El Oumaraa (33.81667 N, 35.85 E) and Rayak (33.86667N, 36 E) (as shown in Fig. 2).Houch El Oumaraa is situated in an urban setting on the foothills of the Mount Lebanon range, while Rayak is located in a sub-urban environment in the Bekaa Valley.Both stations sit at a similar altitude of approximately 900 m.We select wind data from 1994 to 2021 for Houch El Oumaraa and from 1994 to 2009 for Rayak for the summer season (June, July, and August).These data are derived from NOAA website: https:// www.ncei.noaa.gov.We find this period to be long enough to study the topoclimate of the Zahlé valley.Furthermore, we obtain recorded hourly and tri-hourly data of wind (wind speed, wind direction, sea level pressure), from 1994 to 2021 from the NOAA website for two additional stations outside the study area (Fig. 1): the Beirut airport station and the Dahr El Baidar station.The Beirut airport station is located on the coast in Beirut, while the Dahr El Baidar station is situated in a mountainous location at an altitude of 1570 m.The Bekaa valley lies to the East of these two stations.For these stations, we set filters to automatically select breezes by excluding all extreme lowpressure situations (sea level pressure ≤1008 hpa), limiting wind speeds to 11 m/s, and requiring alternation of directions between day and night breezes, based on the work of (Neumann and Mahrer 1971), (Simpson 1994), (Planchon and Cautenet 1997), (Borne et al. 1998) and (Dahech et al. 2005).Day and night hours are set according to the sunrise and sunset times.In Lebanon's summer season, sunrise occurs at approximately 6:30 am, and sunset takes place at around 8 pm (local time).Consequently, the timeframe from 7 am to 8 pm is considered daytime, and the duration from 8 pm to 7 am is considered nighttime.In addition to the wind data, we also use surface and upper air pressure (geopotential 500 hPa) and dew point.The fix weather stations mentioned above use cup anemometers for wind data and high-quality sensors for temperature and dew point, with a margin of error of 0.5°C for atmospheric temperature, 1°C for dew point, 3° for wind direction and 0.5 m/s for wind speed.
We use anemograms and wind roses to understand the mechanism and characteristics of breezes.We set up three Davis Vantage Vue weather stations in Bar Elias (at 872 m elevation), Zahlé city (at 913 m elevation), and a slope in the city (at 1050 m elevation) (Fig. 3) to study topoclimatic phenomena on a fine temporal scale.Half-hourly data of wind direction and speed, atmospheric temperature, dew point, and barometric pressure are obtained from the Davis stations, with a data uncertainty of 0.5 m/s for wind speed, 3° for wind direction, 0.5 °C for temperature, and 1°C for dew point according to manufacturer (Vue_Spec.Pdf n.d.).A cup anemometer and high-quality sensors for temperature, relative humidity, dew point, wind speed, and wind direction are installed in these weather stations.We possess data in half-hour intervals for the Davis stations during the summer months of 2022 (June, July, and August) which demonstrate the alternating breezes.
Additionally, we conduct car survey measurements of air temperature and relative humidity that traverse several settlements in the valley, vegetated areas, and agricultural fields (as shown in Fig. 3).Our measurements focus on the type of radiative time and are conducted 6 AM, lasting an average of one hour.To conduct the car survey measurements, we utilize a portable humidity and temperature meter called "Smart Sensor".This device has a resolution of 1°C for air temperature and 5% for relative humidity.Its response time for temperature is 3 seconds and for relative humidity is 30 seconds.The dew point is obtained by application of this equation (Lawrence 2005 We determine the thermal difference between the city and countryside by comparing the minimum summer temperatures at the Houch El Oumaraa and Rayak weather stations.

Satellite data
The LST (Land Surface Temperature) variable reflects the amount of radiation emitted from the surface and sub surface of the earth, and the exchange of energy between the earth surface and atmosphere (Weng et al. 2019).Remote sensing in the TIR (Thermal infrared) region provides an opportunity to obtain information about this variable.Using a Landsat 7 ETM+ night time satellite image dated June 10, 2002, with a spatial resolution of 60 m for thermal band 6, we spatialize the surface thermal field.The image, taken at 21:13 local time, approximately two hours after sunset, undergoes processing using ERDAS software.We follow three steps to obtain the surface temperature: 1. Conversion from DN (digital number of pixel) to radiance.This is based on the following equation (Markham and Barker 1987): Q dn is the gray value of an analysed pixel, Lmin (λ) and Lmax (λ) are the minimum and maximum spectral radiances detected for Q dn = 0 and Q dn = 255 respectively.

Calculation of the proportion of vegetation Pv
Aster-TIR We use Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images to spatialize the surface thermal field on September 27 2010, June 21 2015, June 10 2019, July 4 2020, and June 5 2021.We process the TIR (Thermal infrared) 11 band, with a spatial resolution of 90 m, at the wavelength of 8.475-8.825μm (Abrams et al. 2015).We derive the brightness temperatures by applying the Planck formula as detailed in previous work of (Jiménez-Muñoz et al. 2010) and (Oguz 2015).Satellite data can be found at this website: www.earth explo rer.usgs.gov.
Table 1 displays all the data that has been described in the methodology.

Rayak
In Rayak, the anabatic flow is oriented from the directions between South and West (Fig. 4 a), with a dominant SW direction.It originates in the Bekaa valley and is channelled towards the NE by its topography.Its dominant speed is between 5 and 8 m/s.At night, the wind rose (Fig. 4 b) shows a dominant flow coming from the NE, with a dominant speed between 1 and 3 m/s.This is a katabatic flow channelled by the topography of the valley, which descends from the high altitudes by a siphoning effect: cold and dense air flows gently down the talwegs and replaces the valley air warmed during the day.As a rule, winds flow upslope, up valley, and from the plain to the mountain massif during daytime.During night-time, they flow downslope, downvalley, and from the mountain massif to the plain (Whiteman 2000).The alternation between these two breezes occurs almost daily, which indicates the high frequency of the phenomenon in summer.
The anemogram (Fig. 5), shows that the anabatic flow originates in the morning around 6 am 2 about two hours after sunrise.At this time, the air in the valley starts to warm up due to solar radiation, and the upward movement of the air gradually increases as the temperature rises throughout the day.During this phase, the wind direction changes from NE to SW (40 to 220 degrees) and the average wind speed reaches its maximum value at 4 pm, rising to 6 m/s.This breeze lasts until 8 pm.The wind speed starts to decrease after 4 pm, the solar rays have a steeper angle towards the ground.At 9 pm, the night thermal inversion starts, the wind direction is progressively deviated to the north and speeds continue to decrease.During the day time, the more pronounced warming of the slopes, due to their orientation and inclination to the valley floor, causes upward movements (anabatic breezes).This movement is reversed during the first hours of the night.As the slopes cool down more rapidly, colder and denser air moves down the slopes (katabatic breezes) and is channelled through the valley (Fig. 7).

Houch El Oumaraa
During the night, Houch El Oumaraa is subject to the effect of mountain breezes flows due to its location in a complex topography.Flows between SW and NNW are observed (Fig. 8).The Houch El Oumaraa area is influenced by daytime flows with a dominant wind speed between 3 and 5 m/s, with directions from S to NNW with an overwhelming dominance of W. Flows from the SSW, SW and WSW are also noticeable (Fig. 9).The southern directions (S, SW) indicate a valley breeze prevailing from 8 am until 12 pm (see anemogram of Houch El Oumaraa Fig. 14 b) with a maximum speed of 8 m/s.The westerly flow is a combination between synoptic wind and sea breeze (SB) that hits the coast during the day, it can travel up to tens of kilometres (Carrega 1992) to reach the station site through the valleys of the western Mount Lebanon range (Nahr Beirut, Nahr El Mot, Hammana, Falougha, Tarchich) (Fig. 15).As the synoptic wind strengthens, detecting the presence of the sea breeze becomes more difficult (Gahmberg et al. 2010).
To investigate this flow, it seems necessary to explore its trajectory.For this reason, the data from the coastal station located at Beirut airport were analysed first, followed by the data from the Dahr El Baidar mountain station, and finally the data from Houch El Oumaraa station (Fig. 10).
In Beirut, the westerly flow exhibits two primary directions: southwest (SW) and west-southwest (WSW).These directions indicate the path of the synoptic wind along the coastline.The sea breeze is influenced by the prevailing flow direction.On average, this flow blows at a speed ranging from 3 to 8 m/s, occasionally reaching 10 m/s.At nighttime, there is a gentle land breeze blowing from the south, eastsoutheast, and east, with relatively low velocity (Fig. 11).
The anemogram shows that the sea breeze sets in around 7 am in Beirut.At this time, air with high humidity starts to enter the coast.On average, this breeze continues until around 9:00 pm.The onset of the land breeze occurs later, around 10:00 pm, when the wind direction shifts towards the south (Fig. 12).At this altitude, this surface wind flow is influenced by the synoptic wind.When comparing this wind rose with the Beirut Airport Station wind rose (Fig. 11), it is observed that the synoptic flow at this station primarily exhibits W and WNW directions.In contrast, at Beirut station, the prevailing flow direction is southwest (SW).This disparity can be attributed to the channeling effect caused by the topography of Dahr el Baidar and the western mountain range.The dominant speed is between 5 and 8 m/s, and the speeds between 8 and 11 m/s represent 20%.
The morning peak of dew point temperature in Beirut and Houch El Oumaraa occurs at different times and is associated with changes in wind direction and speed.In Beirut, the peak happens at 9 am, one hour after the sea breeze sets in, with an increase of 4 °C as indicated by the shift in wind direction from 150° (SSE) to 240° (WSW) and the increase in wind speed from 4 to 5 m/s between 6 and 8 am (red circles in Fig. 14a).In Houch El Oumaraa, the dew point rise gradually during the day time and a peak occurs at 2 pm (Fig 14b).
This approach allowed us to follow the trajectory of the westerlies, which originate on the coastline around 8 am, it then enters the continent by progressing eastwards.It hits the Mount Lebanon and is channelled by the topography of the valleys to finally reach the Houch El Oumaraa station around 12 pm (Fig. 15).

Sea breeze investigation
The above results reflect an examination of data collected at three-hour intervals over the course of several years and months.However, due to incomplete data, it is difficult to accurately monitor the sea breeze day per day.Therefore, it is essential to analyze high-resolution data from the Davis stations that were installed in the summer of 2022.In summer, at ground level, the entire Middle East experiences low pressures, a fairly deep depression is centered over Northwest India and extends through an isobaric valley over the Persian Gulf, Mesopotamia, northern Syria, and often even reaching Cyprus (Persian Trough).At the same time, the upper levels of the atmosphere are dominated by a subtropical high-pressure system that spans the entire Mediterranean region (Blanchet 1965), (Tyrlis et al. 2013).
In June 2022, we documented a total of 10 days characterized by sea breezes, followed by 11 days in July and 6 days in August (Fig. 16).To determine the timing of the sea breeze (SB) onset, we simultaneously examined four parameters: a shift in wind direction towards westerly directions, a decrease in atmospheric temperature, an increase in dew point level, and a rise in wind speed.Typically, in Zahlé, these observations occur after 2 pm.The average duration of SB events recorded by the installed Davis stations ranged between 30 minutes and 4 hours.Analysis of the data collected from the three Davis stations in the Zahlé area revealed that the occurrence of SB is noticeable during periods of weak synoptic wind, attributed to a gentle pressure gradient.However, as the synoptic wind intensifies in the eastern Mediterranean region, detecting the SB becomes more challenging (Azorin-Molina and Chen 2009), (Gahmberg et al. 2010).
We selected 2 specific days as an example of 2 different situations: 4 July and 8 July 2022.direction from E to S, accompanied by an increase in wind speed from 0.5 m/s at 9:30 am to 2.4 m/s at 12:00 pm.The valley breeze persisted until 12:30 pm.From 1:30 pm to 7:00 pm, the prevailing wind was dominated by the synoptic wind with a W direction.Upon the arrival of the westerly wind (combination of sea breeze, mountain breeze and synoptic wind), we observed a rise in the dew point, which increased from 9 to 14 °C.Additionally, there was a significant surge in the wind speed, escalating from 0.9 to 2.2 m/s.Between 3 and 4 pm, we  noticed an additional elevation in the dew point level from 14 °C at 2:30 pm, to 17.5 °C at 3:30 pm, along with an escalation in wind speed to 3.15 m/s.Additionally, there was a decrease in atmospheric temperature observed (from 32°C at 2:30 pm to 30.2°C on 4:00 pm).These observations indicate the arrival of a sea breeze reinforced by the synoptic at this time.The duration of the SB is about 4 hours (from 3 to 7 pm).Due to the intricate topography in the surrounding area, the mountain breeze was effectively established during the night, characterized by a gentle wind speed.
At the valley sub urban site (Fig. 1, Fig. 17b), the arrival of the sea breeze is not easily discernible.The W-WNW wind took over and prevailed from 12:00 pm to 8:00 pm.We noticed an elevation of the dew point and the wind speed at 3 pm with a decrease in atmospheric temperature which indicates a SB arrival.During the night, the mountain breeze was established, characterized by low wind speeds.
At the hillside Davis station located at an altitude of 1050 m on a hill around Zahlé city (Fig. 1, Fig. 17c), we have observed the presence of the valley breeze.It occurred from 9:00 am to 1:30 pm and was characterized by S direction.Throughout this period, the wind speed gradually increased from 1 m/s to 3.6 m/s.Following the valley breeze, the NW wind took control from 2:00 pm to 8:00 pm.After its arrival, we noted significant changes in atmospheric conditions.The dew point experienced an increase, starting at 9°C and reaching 17 °C at 3:30 pm.Additionally, there was a temperature drop of 2°C, as the temperature decreased from 30°C at 2:30 pm to 28°C at 4:00 pm.Furthermore, the wind speed intensified from 2.7 m/s at 2:00 pm to 5 m/s at 4:00 pm.This indicates the SB arrival at 3 pm.During the night, the mountain breeze was established, prevailing with NNW direction.The maximum wind speed observed during this period was 1.3 m/s.
On this particular day in Beirut Airport Station, the sea breeze established at 8 am.It was characterized by an initial shift in direction from ESE to SSW 8 am, gradually progressing throughout the day towards SW.Additionally, there was a noticeable increase in both dew point (from 19 to 23 °C) and wind speed between 6 and 8 am (from 1.5 to 3.6 m/s) (Fig. 17d).
We have conducted an examination of the surface barometric pressure in the region, we observed the presence of a Persian trough system over the gulf region and extended to East Mediterranean region (Fig. 18a).The wind chart at 850 hPa shows winds with NW flow direction, reaching speeds of 18 km/h along the coastline of Lebanon (Fig. 18b).
• July 2022: Throughout the day, the synoptic wind was weak and the breezes were well established.At the urban station in Zahlé (Fig. 19a), a mountain breeze occurred during the night with low speeds, lasting until 7 am.The valley breeze emerged at 8:00 am, accompanied by higher wind speeds, and prevailed until 1:30 pm.At 2 pm, there was a shift in wind direction towards the west, coinciding with a significant increase in the dew point from 4°C at 1:30 pm to 13°C at 2 pm, indicating a substantial rise in air humidity.The wind speed intensified during these events, peaking at a maximum value of 2.2 m/s between 1 and 2:30 pm.Between 3 and 3:30 pm, the atmospheric temperature decreased by 0.5 °C.These observations suggest the arrival of a SB at 2 pm, which persisted until 4:30 pm.After 4:30 pm, a valley breeze reestablished itself.The valley station (Fig 19b) experienced the establishment of a valley breeze at 7:30 am, with a prevailing direction ranging from SE to SSW.This breeze persisted until 1:30 pm.At 2:30 pm, the SB emerged, characterized by a shift in the wind direction to WNW and a significant increase in the dew point from 6 °C at 1 pm to 16 °C at 2 pm.The wind speed intensified, reaching 2.9 m/s at 2 pm.During time of the SB, the temperature dropped by 1 °C between 1 and 4:30 pm.The WNW direction of the SB lasted until 7:30 pm at this station.
The hillside station (Fig. 19 c) experienced the establishment of valley breeze at 9 am, which continued until 2 pm.At this time, the SB arrived, accompanied by a shift in the wind direction to the NW and a significant increase in the dew point, reaching 13 °C at 2 pm (from 5 to 13°C).The wind speed intensified, reaching a maximum of 3.1 m/s, while the temperature dropped by 1°C between 2:30 pm and 3 pm.The NW direction of the SB at this station can be attributed to its channeling effect caused by the topography of the Berdawni valley.
At Beirut Airport station (Fig. 19 d), the arrival of the SB occurred at 8 am, coinciding with a shift in the wind direction to the SSW.Along with this change, there was a significant increase in both the dew point and the wind speed.
During this day, the SB was detected at the all 3 weather stations located in the Bekaa valley and Zahlé.
On this day, there was a prevailing weak low-pressure system in the Persian Gulf region and EM region (Fig. 20a).The synoptic wind is weak due to mild pressure gradients.The Persian trough reaches the Eastern Mediterranean and its effect is not limited to the Persian Gulf.The wind chart at 850 hPa shows winds, characterized by NW flow direction, reaching speeds of 6 km/h along the coastline of Lebanon (Fig. 20b).
During the penetration of the SB, the rapid increase in wind speed and humidity happens at varying times on different days: occasionally before the shift towards the west, at other times after it, and sometimes coinciding with the westward shift.It is important to mention that the SB is mostly observed and has longer duration at the two stations: the urban station at an elevation of 913 meters and the sub-urban station at 872 meters.This is likely due to the topography of these stations, which are situated in a valley.This topography causes the breeze to descend rapidly from the mountains into the valley.The SB duration may vary between stations according to their location.The earlier observed time for detecting the SB is 2:00 pm.

Thermal difference between urban and sub urban areas
A comparative study of the minimum air temperature (Tn) in the two stations -urban (Houch El Oumaraa) and sub urban (Rayak) -was performed in order to calculate the intensity of the UHI following the example of several previous studies such as (Cantat 2004), (Oke and Cleugh 1987).
The daily minima recorded in the two stations (Fig. 21) show the effect of the urban fabric.Indeed, the Houch El Oumaraa station, located in the city, records a difference in average temperature of 4.3°C compared to the rural station.This relatively high difference can be explained by the frequency of weather conditions conducive to the installation of an intense UHI at night (clear skies and very low or calm winds).The UHI usually reaches its highest intensity  The car survey measurements of atmospheric temperature and dew point temperature on 12-6-2022 at 6:00 am (before sunrise) (Fig. 23) show the thermal difference and between urban and rural areas.
In a radiative weather condition during the night, advection occurs, whereby cool and moist air is brought into the valley by the mountain and land breezes.As a result, there is a significant temperature difference between the valley and the surrounding settlements, with the valley being up to 9 °C cooler, as shown in Figure 23.Additionally, several factors contribute to the higher dew point experienced in the valley during the early morning compared to the surrounding areas.Firstly, the absence of the westerly flow effect on the western slopes during the night leads to decreased air humidity levels.Secondly, the katabatic breeze transports moist air into the valley, creating a cold and humid environment that contributes to the increase in humidity levels.Finally, irrigating crops in the valley at night also adds to the rise in humidity levels.

Discussion
The Zahlé region experiences strong topoclimatic influences during summer due to its rugged terrain.This region encompasses various types of breezes such as orographic and sea breezes, as well as the urban heat island phenomenon.These thermal winds alter temperature distribution and have a cooling effect, which can impact the spatiotemporal variation of air quality.However, this work does not address the relationship between air pollution and breezes due to a lack of data.Future studies will examine this relationship by measuring fine particles and NO2 using portable sensors.
This study presents original results in a field that has been little or no studied at all.However, the temperature data used in this work are limited because of gaps in the meteorological data recorded by the conventional network.Moreover, these three-hourly data do not allow for an exact determination of the breeze's onset times, hence the need to establish fixed weather stations with a finer temporal resolution (30 minutes), and installed in locations more representative of the urban and sub urban environment of the Bekaa Valley.Three Davis-type meteorological stations are installed starting from January 2022.
Investigation of the arrival of the Mediterranean sea breeze to the Jordan valley has been made by many authors: (Alpert et al. 1982), (Lensky and Dayan 2012), (Naor et al. 2017), (Kunin et al. 2019).This breeze, which typically comes from the west, rises over hills and reaches Lake Kinneret, located in the northern region of the Jordan valley, around 1 pm -2 pm (local time) (Alpert et al. 1982).The lake Kinneret is approximately 45 km away from the Mediterranean coast.Similarly, the Houch el Oumaraa station is located in the Bekaa Valley, which is an extension of the great rift valley that formed the Jordan Valley (Ambraseys and Barazangi 1989).This station is situated about 35 km from the Mediterranean coast in a comparable location.Additionally, Helmis et al. 1994 have detected sea breeze flow on the top of Humettos mountain (1026 m height and 17 km away from the coast).Our results regarding the arrival of the sea breeze to the Davis stations are supported by the findings of these studies.
Additional studies, yet to be undertaken, could complement this work, such as vertical air measurements to study In radiative weather, he determined that the city center is significantly warmer than the suburbs, with a differential of around 5°C. Carrega 2013 studied the urban climate of a Mediterranean city, Nice, France and in January 1983, he observed an average difference of 4°C between the urban coastline and the rural Var plain.Besides, (Carrega and Viaux 2017) demonstrated the cooling effects of land and mountain breezes, which have a temperature-lowering effect through cold advections.

Conclusions
Thermal breezes have a significant impact on the Bekaa Valley in Zahlé.In Rayak, the valley breeze is prominent during the daytime, starting at 8 am, and flows up the slopes.Conversely, the mountain breeze, a katabatic breeze that brings cold and dense air down from the slopes to the valley, dominates the night.The topography of the valley shapes the direction of these breezes.
In Houch El Oumaraa, there is an alternation between valley-sea breezes during the day and mountain breezes at night, shaped by the valley's topography.The day starts with the valley breeze around 8 AM and lasts until 2 pm.The sea breeze was identified through the analysis of highresolution data collected by the Davis station installed in the Zahlé region.During its penetration an increase of wind speed is observed and the dew point rises.The upslope mountain flow and the SB carry the humidity 40 km over land.The complex topography of the western range contributes to the flow canalization.The 2 Davis stations: Zahlé urban station and Zahlé hillside stations are situated next to the Berdawni valley which amplifies the channeling of NW wind.
The comparison of air temperatures reveals a noticeable difference between the city and the countryside, averaging 4.3°C.The reinforcement of the thermal contrast is a result of various factors such as breezes and watering of agricultural areas that cool the area, while urbanization contribute to warming.
Nighttime satellite imagery displays a change in the thermal field, with the conurbations of Zahlé and Taalabaya being warmer than their surrounding countryside.Within the urbanized area, the surface temperature varies according to the land use, with the highest temperatures recorded in densely populated areas and the coolest in watered areas located in the valley.The satellite-estimated thermal difference between urban and sub urban environments is around 10°C.
The influence of topoclimatic phenomena in this region raises concerns about air quality and thermal comfort, particularly with the growth of industrial activities.Future studies aim to investigate how thermal breezes and UHIs impact air quality and thermal comfort in Zahlé.
Author's contribution The authors confirm contribution to the paper as follows: study conception and design: Rabih Zeinaldine, Salem Dahech; data collection: Rabih Zeinaldine; analysis and interpretation of results: Rabih Zeinaldine, Salem Dahech; draft manuscript preparation: Rabih Zeinaldine, Salem Dahech.All authors reviewed the results and approved the final version of the manuscript.

Funding
The research leading to these results has received funding from faculty of societies and humanities of the university "Paris Cité" in order to translate it into English.Special thanks to the faculty for providing the Davis weather stations and the equipment needed for car survey measurements.

Fig. 1
Fig. 1 Location, topography and land use, data sources: Google earth image, SRTM image of Lebanon 2004, Landsat 8 image dated July 2017, stereographic projection

Fig. 2
Fig. 2 Ombrothermic diagram between 1995 and 2015 with errors bars representing standard deviation of precipitation and temperature.Source: meteorological data from the Beirut airport weather station °C) and RH% is the relative humidity in percentage.

Fig. 3
Fig. 3 Car survey route and location of weather stations.Source of image: google earth The katabatic flow starts to set in during the night, characterised by northerly directions with a NE dominance.Calm is the dominant situation in the night phase.The katabatic flow, of very low speed (generally lower than 3 m/s), persists throughout the night until the arrival of the morning inversion and the onset of the anabatic flow around 6 am.Day (fig.6 a) and night (Fig. 6 b), the airflows are always channelled by the topography of the valley.
They represent mountain breezes coming from the surrounding mountains of the western range.The SW-SSW flows are the most likely a mountain breeze channeled by the topography of the Wadi el Doulm valley.The NE flow is weak.Low speeds characterize these flows, calm air represents 19% of the speeds.Other flows coming from the W, NW and N are also observed, they represent descending breezes from the eastern slopes of the western chain of Mount Lebanon.

Fig. 4 Fig. 5
Fig. 4 Summer wind speed roses in Rayak during the day (local time: from 8 am to 8 pm, a) and night (from 10 pm to 5 am, b).Tri-hourly data from the Rayak weather station between 1994 and 2009, source NOAA

Fig. 8 Fig. 9
Fig. 8 The night wind in Houch El Oumaraa represented on a topographic diagram (data from the station in Houch El Oumaraa from 1994 to 2021), local time: from 9 pm to 5 am, sources NOAA

• 4
July 2022: At the Zahlé urban station situated in the urban area of Zahlé city (Fig. 1, Fig. 17a), we observed the occurrence of the valley breeze.It commenced at 9:00 am (Local Time) and became well established by 10:30 am.This phenomenon was characterized by a shift in wind

Fig. 11
Fig. 11 Summer direction rose by speed bands in Beirut.Hourly data from the airport weather station between 1994 and 2021, source NOAA

Fig. 12
Fig. 12 Anemogram showing the time of onset of sea breeze in Beirut (hourly data from the airport weather station between 1994 and 2021, local time, source: NOAA).

Fig. 13
Fig. 13 a) Summer diurnal direction rose by speed bands recorded in Dahr El Baidar.Local tri-hourly data from the Dahr El Baidar weather station: from 8 am to 5 pm of the period from 1963 to 2009, b) Topography of Dahr El Baidar pass (Google earth, SRTM)

Fig. 16
Fig. 16 Sea breeze frequency and duration in the summer of 2022.Data from installed Davis weather stations

Fig. 17
Fig. 17 Anemograms with temperature and dew point charts in Zahlé Davis urban station (a), valley sub-urban Davis station (b), the hillside Davis station (c) and Beirut airport station (d) showing the breezes alternance, 4 July 2022

Fig. 19
Fig. 19 Anemograms with temperature and humidity charts in Zahlé Davis urban station (a), valley sub-urban Davis station (b), the hillside Davis station (c) and Beirut Airport station (d) showing the breezes alternance, on 8 July 2022

Fig. 21
Fig. 21 Evolution of daily minimum temperatures in the Rayak and Houch El Oumaraa stations during the summer season of 1999, 2001 and 2002 (the only data at our disposal and available on the NOAA website)

Fig. 23
Fig. 23 Spatial distribution of the atmospheric temperature and the dew point on 12 June 2022 at 6 am (local time)

Table 1
Summary of the data used in the study