Characterization of the marine‐air intrusion Marinada in the eastern Ebro sub‐basin

The eastern Ebro basin is composed of an extensive irrigated plain, surrounded by rainfed slopes and wooden mountain ranges and open to the west to the agricultural western Ebro basin. The sea breeze generated at the coast is able to surmount the Catalan prelitoral range through its lowest heights, reaching the basin by its easternmost part. It is a well‐known feature in the region, called Marinada. A network of Automatic Weather Stations is used here to analyse a period of 19 years (2003–2021). A filtering procedure is developed which selects the events when the Marinada is present, based on detecting clear sky, weak wind conditions and the wind direction from the coast in the afternoon. The analysis of these days show that the Marinada propagates along the basin in the afternoon meanwhile observations of the specific humidity show a sudden increase as the temperature cools down, resulting on a cold and humid advection. It is also found that the timing of the arrival of the Marinada depends on the mesoscale/synoptical circulations already present in the region (westerlies or a thermal low).


| INTRODUCTION
The meteorological circulations in heterogeneous complex terrain are difficult to characterize due to the low representativeness of the observations at one particular point (Cuxart et al., 2019;Orlowsky & Seneviratne, 2014;Sim o et al., 2019).One source of spatial heterogeneity is the presence of well-defined thermally driven winds that occupy certain areas with very specific spatial limits, such as the land-sea breezes or the mountain-plain circulations.Some examples of the former are the interaction between the strong northerly wind Mistral along the Rhône Valley and the coastal sea breeze, which can delay breeze initiation or even prevent its occurrence (Bastin et al., 2006), similarly to what happens with the arrival of the Bora wind in the Adriatic coast (Telisman-Prtenjak et al., 2010).
When significant topography exists near the coastline, the interaction between the sea breeze and upslope winds strongly conditions the veering of the wind and its intensity in coastal regions, as shown in southern Europe by Arrillaga et al. (2016) and Grau et al. (2021).At a larger spatial scale, depending on the intensity of the thermal gradient between the land and the sea, the characteristics of the sea breeze front vary, as found in The Netherlands (Arrillaga et al., 2020).In the Iberian Peninsula there have been numerous studies devoted to the sea breeze with several regions inland (Kottmeier et al., 2000;Pérez-Landa et al., 2007;Redaño et al., 1991;Soler et al., 2011) which have raised also the subject of the interaction of the sea breeze with the Iberian thermal low in the warmest summer days.
Inland, topography is one of the main generators of thermally driven circulations.The thermal differences between the mountain valleys and the mountain slopes as well as those with the neighbouring plains condition the local circulations nearby the mountainous areas in the absence of well-defined synoptic pressure gradients, as shown recently in the northern Pyrenees, either showing the nocturnal progression of the gravity flows over the Garonne plain (Jiménez & Cuxart, 2014), or documenting the existence of exit jets from the Pyrenean valleys onto the foothills (Jiménez et al., 2019).Even moderate slopes extending over long distances may explain the observed diurnal cycle of local winds, as it is the case in the eastern Ebro basin, where a gentle slope 70 km long generates persistent daytime upslope flows and night-time downslope flows (Cuxart et al., 2012;Martínez et al., 2008).
Precisely in the eastern Ebro sub-basin all the phenomena just described take place.It is mostly a plain area over a gentle slope ascending towards the east, surrounded by mountain ranges of moderate altitude at a close range (up to 1600 m above sea level (asl) in the pre-Pyrenees) and the Mediterranean Sea at the other side of the southern mountain range (ranging between 700 and 1000 m asl).In this region the topography completely conditions the local circulations and the sea breeze manages to reach the area in the warm months of the year.In this work it is specifically intended to describe the events of sea breeze arrival on the area, the locally well-known wind called Marinada, which refreshes the hot summer late afternoons in the region the days that it is reported.The main difference with other inland sea breeze penetrations in the Iberian Peninsula is that here the phenomenon takes place surpassing a massive mountain range (the Catalan precoastal range) instead of following valleys towards inland.The Marinada is a well-known regime for the habitants of the region and an accurate statistical description of their main features is still missing.
The objective of this work is to statistically describe the main features of the Marinada events using 19 yearlong time series from weather stations of the Catalan Meteorological Service (SMC) to characterize the time of its arrival over the area and to provide elements for its identification.Afterwards the main findings are contrasted with the detailed observations gathered during the Spatial Observation Period (SOP) of the experimental campaign organized by the international project Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment (LIAISE; Boone et al., 2021), which took place in the area during the second half of July 2021.
The Marinada displays very different characteristics in the eastern Ebro sub-basin, with large variations found within distances of a few kilometres.In this work it is intended to describe these variations statistically and try to relate them to the physiographic characteristics of the area, which include large extensions of irrigated lands in the lower plain and rainfed slopes that induce mesoscale circulations with which the sea breeze interacts.
Regarding the organization of the flow at lower levels, previous studies in the eastern Ebro sub-basin have shown that westerlies are common in this region and when they weaken, locally-generated circulations prevail (Martínez et al., 2008).Besides, thermal gradients between the irrigated and rainfed-slope regions enhance the formation of this local circulations (Cuxart et al., 2012).In this work the main features of the Marinada when reaches the region are further explored to improve the understanding of the complexity of the lowlevel circulations.

| The eastern Ebro river sub-basin
The Ebro basin in the northeastern Iberian Peninsula is a large basin, almost closed and triangularly shaped, about 400 km long in the northwest-southeast axis and 150 km wide at the basis at the southeast, limited by mountain ranges at all sides, the river flowing out to the Mediterranean Sea through a narrow long gorge (see Figure 1).It is mainly an agricultural area that combines rainfed and irrigated cultures, the latter especially in summer, taking advantage of the water coming from the Pyrenees.
The eastern Ebro sub-basin comprises essentially the lower basin of the Segre tributary, mainly at the east of the city of Lleida, in Catalonia (see Figure 1).In its centre there is a low flat area (about 200 m asl).Its natural vegetation would be perennial oaks and bushes, however currently the terrain is almost completely used for agriculture, most of which is irrigated in summer, consisting in fruit orchards, alfalfa, corn and other cereals.Surface irrigation allows to keep the salinity of the upper soil low.Water from irrigation comes from the dams in the river Segre and its tributaries to the north of the area of interest, in which water from the Pyrenees is stored.
To the east about 70 km separe the lower Segre River at 100 m asl to the limit of the basin at La Panadella at 710 m asl (labelled as XA in Figure 1).Between La Panadella and Tàrrega (335 m asl, labelled as C7 in Figure 1), 20 km distant, there is a slope angle of 1 , whereas further downslope between Tàrrega and the lower Segre the slope is only about one third as steep (0.3 ).To the south slopes are shorter (10 km) and steeper (3 ) until 700 m asl.To the north there is a gentle upward inclination of 0.3 for 20 km, until the first slopes of the pre-Pyrenees are found, much steeper, facing south and therefore very well illuminated during the day with the corresponding upslope flows.Due to the small amounts of precipitation in summer, these slopes are mainly arid in the warm months and covered with rainfed cultures (vineyards, almond and olive trees).Recently there is a progressive implementation of drip irrigation for some tree species (vineyards, almond and pistachio) as new canals are being built.
The whole area is surrounded (except to the west) by wooden mountain ranges, covered mostly by white pine and perennial oaks, corresponding to the Catalan precoastal range to the south and east and the pre-Pyrenees to the north, with slopes of the order of 4 .The rivers from the Pyrenees, namely the Segre and its tributaries, flow in the north-northeast-south-southwest direction and channelize the flow as they cross a number of east-west-oriented pre-Pyrenean ranges.
Even if the sub-basin is open to the west, we consider that its limit is found at the Cinca tributary, where the slope coming from east of 0.7 ends and at the other side there is a gentler slope from west coming from the upper Ebro basin.This terrain configuration is expected to favour the convergence of these two downslope flows at night over the Cinca basin.

| Climatological features
The dynamical climatology of the Ebro valley (Jiménez et al., 2009;Jorba et al., 2004;Mir o et al., 2020;Peña et al., 2011) indicates a dominance of the westerly flows, oriented along its axis, that may generate the strong Cierzo wind (Masson & Bougeault, 1996;Ortega et al., 2022) in the central and southern Ebro basin, but not affecting much the eastern sub-basin.Precipitation (Cortesi et al., 2014;Turco & Llasat, 2011) is mostly associated to three types of situations: (i) frontal passages between fall and spring, (ii) Mediterranean low-pressure systems bringing water from the eastern quadrantoccasionally providing heavy rainfall events in fall due to the warm sea waters-and (iii) air-mass thunderstorms and convective systems mostly in summer.
The average annual precipitation varies between more than 700 mm in the immediate surrounding ranges (and up to more than 1000 mm in the Pyrenees) to 400 mm in the lower plain, yielding semi-arid conditions in summer (Martínez et al., 2007).In summer the usual small amount of precipitation causes the need of extensive irrigation for agriculture, which in turn requires the monitoring of evapotranspiration as well as an adequate water management (Bellvert et al., 2015;Castellví et al., 2001).
The known features at mesoscale in the sub-basin include (i) gravity flows developing over the rainfed slopes, (ii) flows from narrow river valleys coming from the Pyrenees (as in Jiménez and Cuxart, 2014), (iii) cold air pools in the bottom area with fog in winter, (iv) baroclinic low-level jets due to thermal gradients between the slopes and the flat areas (Cuxart et al., 2012), (v) the arrival in summer of the sea breeze as it flows over the precoastal range, (vi) the interaction of the westerly flows with the basin topography and (vii) the possible effect of the irrigated areas as generators or modifiers of the within-basin circulations.
The interaction between the surface and the atmosphere depends very much on the state of the soil-vegetation systems, showing significant differences between crop types at the local scale and between dry and wet areas at the mesoscale.Therefore the effect of local and mesoscale advection must be considered in both the sensible and latent heat fluxes (Cuxart et al., 2016;Sim o et al., 2019), a subject of intense research currently (Mauder et al., 2020).

| Data from automatic weather stations
The network of automatic weather stations (AWS) from SMC in the eastern Ebro sub-basin comprises about 40 stations (Servei Meteorològic de Catalunya, 2011), separated a typical distance of about 15 km, with varying density depending on the zone (see Figure 1b).Most of the AWS have the wind measured at 10 m above ground level (agl) although some of them measure it at 2 m agl as they were set with agro-meteorological purposes (see Table 1).For all cases, temperature and humidity are observed at 1.5 m agl.For some locations, there are also measurements of the solar radiation.
The selected series for the analysis are those that can provide at least 19 years of hourly observations .
For some specific issues we may use stations with shorter time series.Stations are analysed for the lower irrigated area, the surrounding slopes, the mountain ranges and the western limit area (see the region of interest in Figure 1b).Statistics are produced for the whole series and monthly and annual averages are computed to characterize the mean patterns.The complete list of AWS from the SMC network used in this work is in Table 1 and their locations are shown in Figure 1b.

| Observations in Els Plans during the LIAISE experimental field campaign
The LIAISE field campaign took place in 2021 in the area of this study, which has a semi-arid climate, with the aim to explore the surface-atmosphere dynamics in a large irrigated area and its mesoscale interactions with the surrounding rainfed areas (Boone et al., 2021).Observations at three well-instrumented sites lasted from late spring to early autumn, and a special observing period (SOP) with additional measurements took place during the second fortnight of July, when the surface contrast between irrigated and natural surfaces is maximal.
In the current study, atmospheric observations from SOP at the rainfed site of Els Plans are taken to deeply analyse the features of Marinada during LIAISE.This station was located a few kilometres to the southwest of Tàrrega (C7) and to the northwest of Sant Martí de Riucorb (WL), respectively (see locations in Figure 1b).Analysed data from this site includes wind speed and direction at 3 m agl, together with air temperature and humidity at 2.5 m agl.Data recorded were 10-minute averages from a sampling rate of 1 s.These observations are taken in section 4.2 to further analyse the Marinada.

| Diurnal cycle of the organization of the flow at lower levels
The wind roses during daytime (averaged from 1200 to 1500 UTC) and night-time (from 0000 to 0300 UTC) in El Poal (V8, bottom of the sub-basin, see location in Figure 1b) are shown in Figure 2.This AWS is taken as an example of the organization of the flow at lower levels.Winds are from the western sector during the central hours of the day (Figure 2a), corresponding to the general westward upslope direction of the sub-basin as well as the main synoptic wind direction in midlatitudes.Instead, during night-time winds are weaker and from the opposite sector (east, Figure 2b), corresponding to the downslope circulations.
To obtain a simultaneous view of the wind at the subbasin scale, in Figure 3 the wind roses for the period 2003-2021 are shown during the central hours of the day (averaged from 1200 to 1500 UTC) and the night (from 0000 to 0300 UTC) for a number of AWS (see Table 1 and Figure 1b).In the daytime westerlies (Figure 3a) are found at almost every AWS corresponding both to weak westerly flows and to upslope flows under low pressure-gradient conditions.
At night (Figure 3b) easterly flows dominate, adapting to the slope direction of each AWS, converging to the central bottom plain.Since all nights are included in the analysis, wind roses show other directions corresponding to the situations with larger-scale winds that prevent the formation of the locally-generated flows.Winds are weaker than during daytime and westerlies are less frequent, more important for stations to the west of the sub-basin (as for Gimenells VH and Vilanova de Segrià VM; Figure 1).Instead in the stations more to the east, at the centre of the bottom plain (Golmés WC and El Poal V8), easterlies prevail as the downslope flows from this sector dominate.
At 1700 UTC (considering all the analysed years), winds are from the downslope sector in July (Figure 3c), specially for the AWS at the east, centre and south of the basin, with wind intensities larger than the nocturnal circulations (Figure 3b).This circulation cannot be related to a thermal origin because it is reported 3 h before sunset when the nocturnal cooling is not initiated (the slopes are westerly oriented).It is related to the well-known wind in the region called Marinada that is reported in the afternoon during the warm months of the year.
Figure 4a shows the temporal evolution of the percentage of hourly observations in July for each wind sector for an AWS in the centre of the bottom plain (El Poal, V8).There the westerly flows prevail until 1600-1700 UTC (2-3 h before sunset), showing then a turn to southeast lasting about 4 h which is attributed to the Marinada, and finally the easterly downslope flows are established at approximately 2000-2100 UTC (1-2 h after sunset).
In an AWS at the foothills of the southern mountain range (Sant Martí de Riucorb, WL), a similar pattern takes place during the daytime, but the veering at 1700 UTC will stay along the night (Figure 4b).This is because at this location the direction of the Marinada and of the downslope flows are the same and the separation between both regimes must be done looking at other variables such as the wind speed or the air moisture content, as it will be described in the next section.
To check that the wind is from maritime origin, the same plots are made for Constantí (VQ) and Prades (XR, local slope westerly oriented), placed at the coast and at the top of the Catalan coastal mountain range, respectively (see locations in Figure 1b).From sunrise to sunset, winds are from the southern sector in Constantí (VQ; Figure 4c), corresponding to the direction of the sea breeze, while they have an opposite direction during the nocturnal hours.Instead, winds are from the western sector during the morning in Prades (XR; Figure 4d), corresponding to the upslope wind circulations, that turn towards the southeast in the afternoon related to the propagation of the sea breeze front inland.The evidences that the sea breeze surmounts this mountain range to reach the Ebro sub-basin are further explored in the next section.
To perform the filtering procedure, we define first an insolation deficit index as , where Q t is the daily average theoretical insolation at the top of the atmosphere (TOA) and Q e is the daily average insolation measured by the AWS, both taken at the same latitude and day of the year.This index was used in Martínez et al. (2008) to select nights with stably stratified conditions (to allow the local circulations to develop) and in Conangla et al. (2018) to study the features of the cold air pooling generated in the bottom of the valley.
To select the Marinada events, we will combine here the condition of mostly clear skies (Q d <0:4) with a dynamical condition, similar to the one developed by  (2021) (for the island of Mallorca in the Mediterranean sea) using a coastal AWS, inspired by Borne et al. (1998).
The consecutive conditions applied are: • Mostly clear skies shall precede the event (Q d <0:4, which allows the presence of shallow cumulus).
• The wind blows from the expected sea breeze directions, i.e., 70-180 , during at least 3 h before sunset.• The veering to these wind directions must take place after 1200 UTC to minimize other synoptic or mesoscale effects (see more details about this condition in section 4.2).• The above conditions occur simultaneously in the three nearby AWS, to eliminate local effects.
In the area where the three selected AWS are located, downslope flows come roughly from the same direction as the sea breeze.Since there is generally good insolation in summer until sunset, proper downslope flows cannot start until well into the night-time.In winter the criteria given above may allow downslope flows to be selected mistakenly as sea breeze arrivals, as solar irradiation in winter is weak.As documented in Maz on et al. ( 2015), sea breeze is a frequent regime in the Catalan coast during daytime from March to October.Therefore we will consider that the Marinada cases are properly selected only for this period of the year.
The results of the described selection procedure are shown in Figure 5, where the number of sea breeze arrivals per month and year is given.As anticipated, most of the events take place between May and September, with the maximal occurrence in July and August.During the 2003-2021 period the number of days per year fulfilling the conditions lies between 50 and 80.The maximum occurrences take place for very hot summers (see reports in https://www.meteo.cat/).There is a good correlation (r 2 = 0.7, statistically significant) between the number of sea breeze days and the monthly average maximum temperature at El Poal (V8), pointing to an increase of the number of Marinada events as the annual average regional temperature rises.Instead, rainy summers reduce the number of events, as it was the case for years 2013 and 2014.From now on, the Marinada events are inspected for July because is the month when this regime is more frequent and the method applied to select these events is more accurate.

| Statistical features of the days influenced by the Marinada
For the analysed period 2003-2021, Table 2 shows that 290 and 76 days (18% and 25% of the Marinada events) during the whole year and in July, respectively, take place in two or more consecutive days.The fact indicates the persistence of the synoptic situation, either with westerly flows or related to the presence of a thermal low in the centre of the Ebro Valley.Recent results from Ventura et al. (2023) show that during summer (JJA for 1951(JJA for -2020) ) nearly 50% of the days the synoptical situation is dominated by weak pressure gradient conditions and about 25% is influenced by anticyclonic situations.These synoptical features favour the presence of thermally-driven circulations in the region, in agreement with results from Table 2.
To inspect what kind of meteorological regime is dominant when sea breeze events occur in July, Figure 6 shows the relation between the hour of its arrival at the AWS of Tàrrega (C7) and Cervera (C8) using the wind speed and the wind direction in the morning (here taken at 1000 UTC).Westerlies in the morning prevail as they represent 78% of the cases.Early Marinada events (before 1600 UTC) correspond to weak westerlies, whereas stronger westerly winds correlate with later events that usually occur after 1600 UTC.The wind is from the southern sector in the morning 19% of the times, usually corresponding to varying synoptic or mesoscale patterns, as there is no clear relation between wind speed and the time of the initiation of the Marinada, and a similar deduction can be made for the few cases (3%) when the wind is from the northern sector.The events that have moderate wind speeds from the southern sector at 1000 UTC and veer early in the day may correspond to well-defined thermal low events that allow for morning entrances of the marine wind into the valley.These cases will be described at the end of the next section.
Figure 7 shows the temporal evolution of the averaged 2 m temperature, humidity and wind speed for the Marinada and no-Marinada events observed in Tàrrega (C7) during July of the period 2003-2021.For the Marinada events, the maximum temperature is the warmest (Figure 7a) and the cooling rate in the afternoon is larger (2 CÁh −1 , between 1600 and 2000 UTC) than for the rest of cases (about 1.5 CÁh −1 during the same period).This is related to the fact that the Marinada is a cold advection (air from the sea) that enhances the already present air cooling in the afternoon, once it propagates through the basin.The most noticeable signature of the presence of Marinada is in the specific humidity.Figure 7b shows that a sudden increase is reported in the afternoon for the Marinada events (about 2 gÁkg −1 Áh −1 , 1600-2000 UTC), supporting the idea of the maritime origin of this air mass that reaches the sub-basin.Instead, for the no-Marinada events, it is nearly constant with no clear diurnal cycle.Finally, the averages show that the wind speed is stronger in the afternoon for the Marinada events (between 1600 and 2000 UTC; Figure 7c) than for the rest.Instead, the no-Marinada events are dominated by synoptic pressure gradients.
It is important to recall that the mean temporal evolutions in Figure 7 are valid for this site (Tàrrega, C7) located at the rainfed slopes and the increase of humidity due to the arrival of the Marinada is larger than in the bottom irrigated areas, as it will be discussed in the next section.

| The humidity as an indicator of the arrival of the Marinada
As it is shown in the previous section, once the Marinada reaches the region a wind veering towards the south-eastern sector (corresponding to the downslope direction for the eastern mountain slopes of the sub-basin) is reported in the afternoon, together with an increase in the wind speed.The Marinada is a maritime air mass that surmounts the Catalan coastal mountain range and reaches the sub-basin, resulting in a cold and humid advection (see Figure 7).
In order to study how the Marinada travels through the sub-basin, the fast change in the values of the specific humidity of the air indicates the time when the Marinada reaches a station in the sub-basin.To proceed, the detection of a clear increase of specific humidity in a period less than an hour between 1200 UTC and the sunset is inspected for all the AWS used (Figure 1) during the period 2003-2021 taking the Marinada events selected by the filter.It is considered that the Marinada has reached the location once this sudden change is reported.
The statistical analysis for the ensemble of the selected Marinada events in July allows to explore the soundness of the above preliminary interpretation made with few stations.Figure 8 displays some relevant averaged features of the Marinada arrival and its propagation through the basin during the studied period.Since the increase of specific humidity seems to be a clear signal of the change of air mass (as in Bastin et al., 2005), in Figure 8a the corresponding increase at the different stations is shown, ranging between 1 and 2 gÁkg −1 .
The averaged time of this increase is displayed in Figure 8b, clearly illustrating the sea breeze front progression from east to west, as the flow surmounts the southern mountain range at 1530 UTC and reaches the slopes at the north side of the sub-basin after 1700 UTC, compatible with front speed of 5-6 mÁs −1 .The averaged temperature change as the sea breeze reaches an AWS nears −2 C during 1 h (Figure 8c), showing a cold advection despite the adiabatic heating as it travels downhill.Once the Marinada reaches the site, the wind speed slightly increases (Figure 8d), depending on the already present circulations at each site.It is worth mentioning a progression of the sea breeze along the Ebro river gorges from the south, pointing to a double entrance of the maritime air in the sub-basin, surrounding the main mountain range through the two lowest passes.
The sudden change in the sub-basin dynamics is clearly seen in Figure 9 where the wind roses before and after the arrival of the Marinada at each AWS are shown, as the upvalley flow is completely substituted by the south-east winds in the whole area.Determining the ending time of the sea breeze regime seems possible for each particular AWS as described before, but we still miss a single parameter to analyse it statistically, as the situation following the change of regime is very different at each AWS (as pointed in Figure 4).(21 July, 2021).The same in (c) but for a case (20 July, 2021) that the veering of the wind is reported early in the morning and this case is not selected by the filter (labelled as "Ma*" in Figure 10).For all cases, there is the temporal evolution for wind speed and direction, temperature and specific humidity at some locations through the basin: WL (taken to select the Marinada events) and V8 (at the bottom plain) and in Els Plans (one of the main sites of the LIASE experimental field campaign).The prevailing wind regime in the area corresponds to synoptic westerly flows, while under high-pressure systems basin-scale winds dominate.The latter consists also in westerlies during the day, corresponding to upslope flows, while easterly downslope flows happen at night.During the SOP diurnal westerlies dominated except for a time interval of extreme heat (20-24 July) when a thermal low centred in the Ebro basin imposed a southeasterly regime most of the day.The daily evolutions of wind, temperature and humidity for the entire SOP at different sites over the basin can be found in Data S1, Supporting Information.
Between 15 and 18 July synoptical westerly flows dominated and the sea breeze arrived between 1 and 2 h before sunset and lasted well after it, as seen in Figure 11a for 15 July when inspecting the wind veering and the increase of specific humidity.The late arrival of the sea breeze may be related to the opposing effect of the upslope flows at the southern slopes of the sub-basin during the central hours of the day.This event is well captured by the filter.
As the temperature raises and a thermal low develops over the Ebro basin (20-24 July), westerlies weaken first and disappear later, allowing an earlier arrival and stop of the sea breeze.In 21 July (Figure 11b) the arrival of the Marinada overrules the weak westerlies much earlier than the standard westerlies case, shortly after 1200 UTC, event also being selected by the filter.However, in the case of 20 July (Figure 11c), the westerlies are even weaker and the Marinada reaches the area well before 1200 UTC, with the consequence that the filter does not retain this particular case.Besides, the increase in the specific humidity when the Marinada reaches the bottom parts of the basin (V8) is not as clear as for the rest of the events classified as "Ma" and statistically described in Figure 8.
In order to see how often the Marinada arrivals take place before 1200 UTC, the number of such events has been counted for the entire 2003-2021 series.Figure 12 indicates that the number of early events is around 10 per year-mostly between June and Augustcompared to roughly 65-70 days per year taking place after 1200 UTC (Figure 5).Two of such events happen during the LIAISE SOP and are indicated with "Ma*" in Figure 10.These cases correspond to an intense thermal low in the Ebro basin, with temperatures above 30 C in the morning and above 35 C in the afternoon, allowing to infer that these early Marinada events are linked to high-pressure systems with very hot temperatures developing intense thermal lows over the Iberian Peninsula.
To better describe the thermal low conditions, aeronautical meteorological reports at the airports in Zaragoza (central and bottom Ebro basin) and Reus (at the coast, south to Barcelona and close to the exit of the Ebro river) are taken (see locations in Figure 1a).It is found that the pressure gradient (computed as the pressure at sea level in Zaragoza minus the one in Reus) is always negative at 1500 UTC under thermal low conditions (on average −1.7 hPa during July for the studied period 2003-2021) whereas it is nearly zero for the rest of Marinada events (on average −0.2 hPa).Thus, under thermal low conditions, low pressures are reported in the centre of the Ebro valley with warm temperatures during the early morning that favour ascent motions, the entrance of maritime air in the basin and the propagation of the Marinada towards the studied region.
In the particular case of July 23, the southeasterly flow is of synoptic origin and it is unclear if a sea breeze event also takes place.Afterwards westerlies prevail again but the Marinada events are not clearly defined, as temperatures are relatively fresh inland (below 30 C) and there are very often clouds over the area, including a rain event before sunset on 26 July.Nevertheless, 25 July is still selected as a Marinada event as it fulfils all the requested criteria by the filter.
The analysis shows that, in the warm months, it is common to observe the arrival of the Marinada on the sub-basin, up to 70 days per year or 33% of the warm days, usually during a number of consecutive days.This intrusion takes place typically between 1500 and 1600 UTC, although the timing is conditioned by the strength of the westerlies before its arrival or by to the presence of a thermal low in the Ebro valley due to warm temperatures in summer.Particularly, westerlies delay the arrival of the Marinada (getting closer to sunset) meanwhile the presence of a thermal low enhances the Marinada arrival, due to the ascending motions related to the solar heating in the warmest parts of the basin.Under these conditions, the arrival of the Marinada can be close to noon, or even earlier for the hot summer days.
For both situations, in July the front propagates on average at a typical rate of 5 mÁs −1 towards north.Its arrival implies a clear increase of the specific humidity (1-2 gÁkg −1 ) and a decrease of temperature of about 2 K. Observations indicate that the Marinada is a cold and humid air mass (of maritime origin) that travels through the basin interacting with the already generated circulations and local features.
Results show that it is important to consider the influence of the Marinada to study the organization of the flow at lower levels, particularly in summer, when the LIAISE experimental field campaign took place in 2021.
U R E 1 (a) Topography of the Ebro basin and a zoom in (b) covering the region of interest, including the network of AWS from SMC.The location of the stations used to select the days when the coastal sea breeze reaches the region are shown in red.Names and coordinates of the AWS are found in Table 1 and the star indicates the location of the Els Plans site during the LIAISE experimental field campaign.The labels of ZAZ and REU indicate the locations of Zaragoza and Reus airports, respectively.In (b) the thick bold line represents the topography contour at 300 m asl [Colour figure can be viewed at wileyonlinelibrary.com] Wind roses computed in El Poal (V8, see location in Figure 1) for the studied period during (2003-2018): (a) daytime (from 1200 to 1500 UTC) and (b) night-time (from 0000 to 0300 UTC) [Colour figure can be viewed at wileyonlinelibrary.com] Masouleh et al. (2019) (for the coast of Australia) or by Grau et al.

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I G U R E 3 Map of the wind roses computed from the AWS used in this work (see locations in Figure 1b) for the analysed years (2003-2021) during: (a) daytime (from 1200 to 1500 UTC), (b) night-time (from 0000 to 0300 UTC) and (c) only in July at 1700 UTC.Colours of the wind roses as in Figure 2 [Colour figure can be viewed at wileyonlinelibrary.com] Temporal evolution of the percentages of the wind direction during July from 2003 to 2021 for (a) El Poal (V8), (b) Sant Marti de Riucorb (WL), (c) Constantí (VQ) and (d) Prades (XR).See locations in Figure 1b [Colour figure can be viewed at wileyonlinelibrary.com]F I G U R E 5 Occurrences of sea breeze arrivals in the eastern Ebro sub-basin during the studied period 2003-2021 [Colour figure can be viewed at wileyonlinelibrary.com] T A B L E 2 Number of events with sea breeze intrusions that occur in a given number of consecutive days taking the whole year and in July for the period 2003Relation between the hour of the initiation of the Marinada (wind veering towards the downslope direction) and the wind direction at 1000 UTC.Data from Tàrrega (C7) and Cervera (C8) are taken for the analysed period (2003-2021) during July.Colours indicate the ranges of the wind speed (WS) at 1000 UTC and the numbers indicate the events that fulfil to the wind ranges indicated [Colour figure can be viewed at wileyonlinelibrary.com]F I G U R E 7 Averaged time series for July over the period 2003-2021 taking the Marinada and no-Marinada events in Tàrrega (C7) for (a) 1.5 m temperature, (b) 1.5 m specific humidity and (c) 10 m wind speed [Colour figure can be viewed at wileyonlinelibrary.com]

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I G U R E 8 Averaged features of the arrival of the sea breeze in the studied region for July: (a) increment of the specific humidity; (b) time (in UTC) of the sea breeze arrival and (c) increment of temperature and (d) wind speed.The increments are computed from the differences of the field around 2 h close to the arrival of the sea breeze.Symbols indicate the length of the data used: the complete studied period (2003-2021, in circles) and shorter time series (data starting after 2003, in rhombus) [Colour figure can be viewed at wileyonlinelibrary.com]

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I G U R E 9 The same as Figure 3 but taking only data from July during 2003-2021 and with the wind roses computed for: (a) 1 h before and (b) 1 h after the sea breeze arrival at each AWS.Colours as in Figure 2 [Colour figure can be viewed at wileyonlinelibrary.com]

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I G U R E 1 0 Hourly evolution of wind (a) direction and (b) speed, (c) specific humidity and (d) air temperature close to the surface at the rainfed site Els Plans during the special observing period (SOP; 15-29 July, 2021) of the LIAISE field campaign.Grey areas show the nocturnal periods and the blue ones the intensive observing days during the campaign.Label "Ma" on top indicates the Marinada events detected by the filter whereas those Marinada event under the presence of a thermal low with the wind veering reported before 1200 UTC are indicated with "Ma*."Labels "Sy" and "Cl" indicate the days when the winds are strongly influenced by the synoptical conditions and when clouds are present, respectively.Vertical dashed lines indicate the hour of the arrival of the Marinada [Colour figure can be viewed at wileyonlinelibrary.com]Examples of Marinada events for a case with (a) predominance of westerlies (15 July, 2021) and (b) influenced by a thermal low

Figure 10
Figure10shows the temporal evolution of wind, air temperature and humidity at the rain-fed site of Els Plans during the special observing period (SOP, 15-29 July 2021) of the LIAISE field campaign.The days selected by the filter are indicated with "Ma."The prevailing wind regime in the area corresponds to synoptic westerly flows, while under high-pressure systems basin-scale winds dominate.The latter consists also in westerlies during the day, corresponding to upslope flows, while easterly downslope flows happen at night.During the SOP diurnal westerlies dominated except for a time interval of extreme heat (20-24 July) when a

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I G U R E 1 2 (a) Monthly averaged (2003-2021) number of days with Marinada under intense thermal low (classified as "Ma*" in Figure 10).The same in (b) but for the number of days per year [Colour figure can be viewed at wileyonlinelibrary.com] List of the automatic weather stations (AWS) from SMC presented in the current study T A B L E 1Note: Symbols (*) indicate the AWS used to select cases with intrusions of the sea breeze.These AWS are shown in Figure1in red.The rest of AWS are used to analyse the variability within the basin.An AWS code in bold indicates that wind measurements are taken at 2 m above ground level (agl), otherwise, they are at 10 m agl.Stations are classified according to their location in seven categories: West, Northern slopes, Plain, Southern slopes, Eastern slopes, Mountain, and River and others.
The vertical black lines indicate sunset and sunrise hours and the coloured vertical lines the arrival of the Marinada for each location [Colour figure can be viewed at wileyonlinelibrary.com] 4.2 | Marinada events during the LIAISE experimental field campaign