Intense precipitation events in the Central range of the Iberian Peninsula

Intense orographic precipitation associated with the Central range was analysed using data from 19 episodes, with the highest average values for the study area, of precipitation accumulated within 24 h, occurring between years 1958 to 2010. All events were associated with a south-westerly tropospheric flow, a low level jet, and high moisture 10 flux at low levels. The observed moisture flux was higher than 100 m g (s kg) and the dry and wet Froude numbers were greater than 1. The selected area to study this synoptic situation was Gredos, broad and high range, which is located in the eastern part of the Central mountain range and generates leeward “orographic shadow”. The effect of the Central range on the spatial distribution of precipitation on the Iberian Peninsula plateau results in a sharp increase in precipitation in the south of the Central mountain range, followed by a decrease to the north of this range. 15


Introduction
The factors affecting the development of precipitation are complex and the forecasting of precipitation is therefore difficult, particularly with the use of larger spatial and temporal resolutions (Llasat and Siccardi, 2010). Heavy precipitation is generally associated with a high moisture content, vertical movement, and static instability (Reale and 20 Lionello, 2013;Chen et al., 2013). There are many studies addressing cases of heavy precipitation. Thus, for example, Schwartz et al., (1990) analysed the evolution of the convective environment during episodes of heavy precipitation, and Fernández-Montes et al., (2014) or Garavaglia et al., (2014) studied the synoptic patterns typically associated with heavy precipitation.
Some authors have attempted to elucidate the key factors involved in the production of heavy precipitation or 25 intense orographic precipitation. These factors include precipitable water (Bližňák et al., 2014), static instability (Funk, 1991), moisture at low levels (Massari et al., 2014), waves upstream from the area of heavy precipitation (Yu et al., 2015), dynamically forced vertical movements associated with wind maxima in the high troposphere (Ma and Bosart, 1990).
Other articles have analysed moisture flux convergence at low levels (Banacos and Schultz, 2005), orographic lifting of conditionally unstable air masses , and the fact that the maximum precipitation rate mainly depends on the 30 ratio of mountain height to the level of free convection, the ridge/aspect ratio, and a parameter that measures the ratio of advective to convective time scales (Miglietta and Rotunno, 2009). Mountains can allow airflow or partially or completely blocks the flow. The Froude number (F) or the wet Froude number (F w ) were used to characterize the flow over mountains (e.g. Durram and Klemp, 1982;Colle, 2004;Chu and Lin, 2000;Chen and Lin, 2005). Values of Froude number lower than 1 (around 1) was found to be associated with blocked (partially blocked) flow, while values greater than 1 implied flow that pass over mountains.
Research efforts have also focused on the study of heavy precipitation episodes in different areas. For example, Chiao et al., (2004) or Foresti and Pozdnoukhov (2012) discuss the heavy orographic precipitation in the Alps; Lang and 5 Barros (2004) studied winter storms in the Central Himalayas, and Prat and Barros (2010) studied the spatial gradients and vertical structure of orographic precipitation in the southern Appalachians. Yu and Cheng (2008) analysed heavy orographic precipitation associated with typhoon Xangsane. Furthermore, other such examples can be found in the works of Grumm et al., (2002) and Schumacher and Johnson (2006) studying the USA; Chen et al., (2007) studying Taiwan;Teixeira and Prakki (2007) studying Brazil;Federico et al., (2008) and Buzzi et al., (2014) studying Italy;Stefanescu et 10 al., (2014) for Romania or Dasari and Salgado (2015) for Madeira Island (Portugal).
In the case of Spain, heavy precipitation episodes are characteristics of the weather, especially on and around the Mediterranean coast and during late summer and autumn (Riesco et al., 2013). Jansa et al., (2001) reported that approximately 90% of heavy precipitation episodes in the Internal Basins of Catalonia between 1996 and 2002 were associated with warm and moist flows at low levels generated by extratropical cyclones. Rigo and Llasat (2004) show that 15 during the period 1996-2000, forty-three heavy rainfall events have been detected in Catalonia (Northeastern of Spain) and the most of these events caused floods and serious damage. Ramis et al., (2009) studied heavy precipitation in the western Mediterranean and found that deep convection was the main cause. Several studies have classified the meteorological patterns associated with heavy precipitation. Thus, Romero et al., (1999) found the average geopotential field at the 925 hPa and 500 hPa pressure levels to be associated with heavy precipitation, and Martin-Vide (2002) classified the days with 20 precipitation higher than 200 mm on the Spanish Mediterranean coast. Riesco et al., (2013) reported that severe precipitation episodes in the southern Iberian Peninsula may be classified in three Types according to moisture flux at the 850 hPa pressure level and the lifted index. All of these studies have analysed precipitation occurring in the same area and hence we believe it is necessary to explore other areas of the Iberian Peninsula where heavy precipitation also falls. Fernandez-Montes et al., (2014) reported that a great amount of precipitation in the Iberian Peninsula concentrates in 25 relatively few days, primarily conditioned by the atmospheric circulation and the moisture content, and they investigated the relationship between synoptic circulation Types (CTs) and the frequency of precipitation extremes (> 90th percentile) in spring and autumn at 44 different stations. Merino et al., (2016) conducted an analysis of precipitation extreme events (PEEs) in Spain between 1960 and 2011. Thresholds for determining event severity were defined using 99th percentiles, regions of extreme weather risk were identified and then trends of extreme precipitation index were analysed using the 30 Mann-Kendall test. Álvarez et al., (2017) analyses the competence of Spanish historical precipitation network and explore the physiographic influence of elevation and orientation at a national scale. Nearly 12000 monthly precipitation series 3 recorded from the 19 th century until the hydrological year 2004/2005 are used and comparable statistics and precipitation lapse rates are managed once a completion of gaps is accomplished. González and Bech (2017) present a regional and seasonal study of extreme point rainfall scaling from 10 min to 2 years. The highest point-based rainfall list based on these temporal periods was calculated from the Spanish Meteorological Service (AEMET) precipitation database with more than 11000 rain-gauge stations, with the longest series ranging from 1805 to 2014. Results indicate that daily precipitation 5 extremes are mostly concentrated over the Mediterranean coast while from durations from one month to two years extremes are located in southern and northwest Spain.
The use of information obtained from statistical or case studies of precipitation episodes obtained from post-event investigation is needed to improve the forecasting of precipitation and is the basis of effective operative warning for flooding due to extreme heavy precipitation, such as that based on the European precipitation index (Alfieri and Thielen, 10 2015). In the present study we analysed episodes that featured more intense orographic precipitation and also had high instability index values during years 1958 to 2010, which affected the Central range of Iberian Peninsula but specifically those that occurred in the southern part of the area. This work is supplemented by including a study of meteorological indices that can help in the characterization of intense orographic precipitation. Finally, we analyse in detail two examples of episodes that demonstrate the direct application of the indices described above and that outline the difficulties involved 15 in the study of orographic precipitation.

Study area and data
The study area is focused on the eastern part of the Central range, comprising the Gredos range (SGRED, Figure   1). This mountain range separate valleys of the Duero basin from those of the Tajo basin and have a maximum altitude of 20 2592 m and an average elevation of 1200 m a.s.l. A highly irregular relief characterizes this zone and the barrier is not uniform in height, width or orientation, such that from the point of view of index calculation many simplifications must be made. The eastern part of the Central range comprises the Gredos range, which forms the massif of the Central range and is very wide. In addition, most of the Gredos range, where the south slopes are much more abrupt than the leeward side, configure the Central range separating the southern plateau, with a lower altitude (about 400 m a.s.l) from the northern 25 plateau (800 m a.s.l).
From the climatological data network, we selected the 18 episodes with the heaviest precipitation accumulated in 24 hours (> 100 mm in at least one observatory inside the study area) in the western part of the Central range during the years 1958 to 2010 (Table 1). Additionally, it must be noted that episode 19 was included as a case study despite having a maximum precipitation daily slightly less than 100 mm, because is used us example of the importance of the time duration 30 of the rain episode. The data regarding precipitation were obtained from the climatological database of the Spanish Meteorological Service [Agencia Estatal de Meteorologia (AEMET)]. They are world-class observatories equipped with 4 staff, automatic stations and meteorological collaborators and they are subjected to various quality controls. Table 2 shows the accumulated precipitation and the precipitation in 24 hours corresponding to the observatory that reported the maximum value in the study area. Data for the other meteorological fields were obtained from the ERA-40 database of the European Centre for Medium Range Weather Forecasting (ECMWF) for the 1958-2002period (Uppala et al., 2005 and directly from the ECMWF model for the 2003-2010 interval (http:// apps.ecmwf.int/datasets/data/era40). 5 The following parameters were calculated for each precipitation episode: mean and maximum precipitation; meridional moisture flux at the 850 hPa pressure level (obtained by multiplying the 850 hPa meridional component of wind velocity and the specific humidity, Vq); the dry and moist Froude numbers (calculated as F = V(Nh) -1 , with V the meridional wind, N the dry or moist Brunt-Vaisala frequency, and h the obstacle height); Convective Available Potential Energy (CAPE) and the Total Totals and K instability indices. It is worth mentioning here that we used the meridional 10 wind component instead of the perpendicular component. The Central range is clearly west-east oriented, so we believe that the wind meridional component is an appropriate estimate of the flow in the mountains and therefore our calculated values can be considered estimates of the Froude number. In the present study, the above-mentioned parameters were calculated (Kriging with ArcGIS© software) at 40º 16´ 31`` ºN, 5º 09` 32`` ºW (belonging to the Gredos range area, Figure   1b), where southerly winds must surpass a height difference of ~ 1,500 m, and the Central range reaches its maximum 15 altitude (2592 m a.s.l.), and the maximum slope (Δh/Δx) is ~ 0.09 with a mean value in the southern hills of 0.05. It should be mentioned that precipitation is very difficult to interpolate in mountainous areas, so that we considered that the impact of the interpolation method on the area-averaged precipitation is small and used the kriging interpolation method because is the default method which comes built-in in the ArcGIS© software.

Results and discussion
The values of the meteorological parameters considered in this study for each precipitation event are shown in Table 1. Figure 2 shows the composite fields at 12 UTC of sea level pressure, wind and specific humidity at the 850 hPa pressure level and geopotential at the 500 hPa pressure level. These maps were built using data from the 18 episodes of severe rainfall indicated in Table 1. It is worth mentioning that the synoptic situation of each individual event is similar to 25 that of the average. Figure 2a shows a low-pressure system at the sea level located on the north-western Iberian Peninsula with south-westerly winds over the Central range ( Figure 2b). This is a moist flow and thus, increased the humidity over the study area, especially on the south slopes, as shown in Figure 2c. The figure also shows a tongue of moist air at low levels over the south-western Iberian Peninsula, which reached values greater than 6 g kg -1 over the study area. Moreover, a high wind speed was observed at the 850 hPa pressure level (Figure 2b), where a low-level jet stream, with a wind speed 30 greater than 20 m s -1 , was apparent. A north-south oriented mid-level trough located to the west of the Iberian Peninsula (see Figure 2d) and moderate values of the instability indices (Table 1) were other common characteristics associated with 5 the heavy precipitation events studied in this work. The average values in the mean and maximum meridional moisture fluxes were 110 and 147 mg(s kg) -1 respectively, and in most cases the individual values were higher than 100. As mentioned, Lin et al., (2001) defined the U(Δh/Δx)q index (U is the meridional component of wind velocity, Δh/Δx is the mountain slope, and q is the mixing ratio) and reported that heavy orographic precipitation in the Alps was associated with values higher than 4.7 ms -1 at low levels. When this index was calculated for the Central range using the data in Table 1, 5 the mean value was 5.5. ms -1 . Thus, this index also indicates the risk of heavy orographic precipitation for the Central range. Regarding the dry and wet Froude numbers, these ranged between 0.7 and 1.9, the maximum averaged values being 1.2 and 1.3 respectively, corresponding to moderate values of this index. The shape of the mountain can be approached by using a slope of 1.6 km/35 km ≈ 0.05 of the windward mountain barrier (which is the mean value in the southern hill). Chu and Lin (2000) and Chen and Lin (2005) established four types of flow regimes as a function of F w and CAPE: 10 • Type I (F w small): a convective system of up-stream propagation.
• Type II (F w moderate): a long-lasting convective flow over the mountain.
• Type III (F w large): a convective system over the mountain that propagates similarly to the flow.
• Type IV (F w small and low CAPE): orographic stratiform system, which probably propagates downstream.
Therefore, in principle, the greatest windward precipitation would occur with moderate or small Froude number. 15 With the values obtained in this study, in nearly all cases a Type III flow would be obtained which implies the presence of a convective system propagating similarly to the flow. However, it should be taken into account that with low CAPE and moderate wind intensity the flow Type would be Type IV instead of Type III (Chen and Lin, 2005) giving rise to stratiform precipitation. In any case, both convective and stratiform precipitation would give large amounts of total precipitation (including possible flooding), if the flow persists for many hours. 20 Table 2 shows the precipitation data of the Gredos range (SGRED) (Figure 1b). González and Bech (2017) studied extreme rainfall in Spain. They found that extreme rainfall may be expressed as a potential law, and their reported law gives values around 270 mm in 24 hours for the inland Spain, and 260 mm in 24 hours for the province of Avila.
These numbers are in agreement with the values shown in Table 2, specially taking into account that González and Bech (2017) only considered extreme rainfall in their study . Colle, (2004) reported that with moderate winds (~ 20 m s -1 ) and a 25 freezing level over 750 hPa, the drop in the freezing level increases as mountain barrier height and width increase.
Moreover, mountain waves when the wind is strong, the static stability is low, and the mountain barrier is narrow, may favour intense vertical movement. The orographic and precipitation profile across the SGRED is depicted in Figure 3. This profile was calculated following the line AB indicated in Figure 1b. Figure 3 shows that precipitation over the SGRED increases sharply windward and that maximum values are reached on the upper third of the windward slope. Precipitation 30 decreases sharply on the leeward slope, indicating an "orographic shadow" associated with the SGRED. Maximum precipitation is now located around the summit.

a.-Case study:23-25 November 2006
During this time frame (episode nº16), the precipitation observed at several stations along the Central range was more than 300 mm. Figure 4a shows the infrared channel image from the Meteosat satellite and Figure 4b shows the surface pressure pattern for November 24 at 00 UTC. A low-pressure system was affecting the North Atlantic area, with a 5 minimum sea level pressure of ~ 970 hPa located west of Ireland. A south-westerly moist air flow and several frontal systems were affecting the western Iberian Peninsula, moving from west to east. Figure 4c shows a deep north-southoriented trough at the 300 hPa pressure level located west of the Iberian Peninsula, with a south-westerly low level jet of 60 m/s over the Central Iberian Peninsula. Chen and Lin (2001) reported that the presence of a jet at low levels exacerbates the windward precipitation, in agreement with the observed in this case, where a south-westerly low-level jet stream of 30 10 ms -1 was also observed crossing the Central range at the 850 hPa pressure level on 25 November at 00 UTC (Figure 4d).. This meteorological situation generated a high-moisture flux (see Table 1) and had a value of 7.2 ms -1 for the U(Δh/Δx)q index, clearly higher than the threshold value reported by Lin et al., (2001) for the Alps (4.7 ms -1 ). The cloud-to-ground lightning flashes observed on 24 November are shown in Figure 5a. The discharges were mainly located windward of the Central range indicating that flow was unstable. This is confirmed by the Madrid sounding (40º 27´ ºN, 3º 43 ºW) at 12 15 UTC (Figure 5b) and the values of the instability indices (Table 1) The temporal distribution on 24 November is shown in Figure 6a, and indicates that large amounts of precipitation occurred regularly during that day. The pattern of the spatial distribution of average precipitation   (Figure 6b) 20 shows the relevance of orographic precipitation in this area of the Iberian Peninsula. Figure 7 shows the orographic profile and the precipitation along a south-north-oriented line crossing the SGRED. Precipitation decreased leeward of the broadest part of the Central range, and returned in large amounts windward of the Cantabrian range, located near the northern Spanish coastline (see Figure 1a). This marked "orographic shadow" is consistent with the shape (resembling a C) of the spatial pattern of precipitation shown in Figure 6b. When the 25 line was displaced to the west, the "orographic shadow" was not seen.

b.-Case study: 27-28 February 2010
The time duration of the rain episode is an important factor. Large amounts of precipitation are not usually associated with episodes of short duration and this was the case of episode nº 19. On 27 February 2010, the extratropical cyclone 30 "Xinthya" crossed the northwestern part of the Iberian Peninsula. This was an event of explosive cyclogenesis, with hurricane-strength winds. It caused heavy rain and local floods in the west of Spain (Hickey, 2011). Figure 8a shows the surface pressure pattern at 12 UTC. The fast-moving deep low of 976 hPa was located close to the northern coastline of Portugal, resulting in strong southerly winds impinging on the Central range. The same flow pattern was seen at medium and high levels (not shown). Doppler radar (Figure 8b) shows a south-westerly low-level jet stream over the area studied. Table 3 shows the sudden increase in the instability indices, Froude number and meridional moisture flux during the morning and central hours of 27 February, together with the low values before and after the event. This was due to the short duration of the flow perpendicular to the mountain range (Figure 9a). Therefore, the expected amount of rain cannot 5 have been large. This is confirmed by the data shown in Figure 9b, which shows the precipitation pattern. There is a broad area where precipitation measured between 30 and 40 mm and the maximum values were around 60 mm.

Summary and conclusions
We have analysed episodes that featured intense orographic precipitation and also had high values in the instability indices that affected the Central range of the Iberian Peninsula during the years 1958 to 2010. The synoptic 10 characteristics associated with the cases analysed were a strong south-westerly tropospheric flow, with jet streams at low and high levels, and high moisture contents at low levels. This flow was the result of low pressure systems over the North Atlantic area, with associated fronts affecting the area studied. In general, the CAPE values and the Total Totals (TT) and K instability indices indicate static instability, in agreement with the lightning flashes observed, especially windward.
The moisture flux associated with the cases of heavy orographic precipitation considered here was > 100 mg (s 15 kg) -1 , and both the dry and moist Froude numbers were >1 on the average (1.3), although in two events the value was 0.8.
These numbers can be considered to be characteristic for heavy precipitation in the Central range when they are associated with long-lasting intervals. The broadest and highest part of this mountain range (Gredos range) generates an "orographic shadow" leeward. Precipitation from frontal systems is increased sharply by orographic vertical movement in the Central range, where updrafts associated with the warm conveyor belt are increased. The precipitation pattern in these cases 20 resembles a "C", with heavy precipitation south of the mountain range. In several cases, the total amount of precipitation was moderate. This seems to have been caused by the short time interval where airflow perpendicular to the orographic barrier persisted.
Author Contribution: M. Mora and F. de Pablo designed the ideas and concepts of the work and they prepared the 25 manuscript. J. Riesco and L. Rivas carried out the analysis of the data and evaluated the results. J.M. Sánchez developed the entire part graphic and checked the language. All the authors conducted a final review of the manuscript.