1 Introduction

The local circulations are important meteorological phenomena which affect and impact most the human activities concerning, the local meteorology, the air quality, the pollution transport and dispersion and the sustainability of ecosystems. In the Mediterranean coastal regions, where significant lack of horizontal homogeneity of the surface is present, these implications can even be intensified. Climatological studies (Ferretti et al. 2003; Mastrantonio et al. 2006, 2008) based on long-term conventional meteorological observations confirmed that in the coastal regions of the Italian peninsula, the sea/land breeze circulation is usually predominant for most part of the year with a pronounced diurnal cycle (Mastrantonio et al. 1994; Leuzzi and Monti 1997).

The interaction between the local and synoptic circulation can determine a variability in the low-level mesoscale circulation and in some cases, the resulting flow can mask some component of the local circulation. Savijarvi and Liya (2001) revealed that the local circulations associated to the urban “heat island” affect the local climate and drive the local transport of the pollutants. Accurate forecast of the local circulation could help to prevent critical situations of pollutants concentration in the highly populated urban areas. Numerical investigations of Caballero and Lavagnini (2002), Monti and Leuzzi (2005) and Gariazzo et al. (2007) have provided other information on the low-level atmospheric circulation and on its influence on pollutant dispersion, in the Latium region (Italy).

The characterization of the local circulation in the area of the Estate of Castelporziano is important to evaluate the effect of the re-circulation of pollutants in the city of Rome. Allegrini et al. (1999) have highlighted the influence of the local meteorology on the pollutants concentration inside the Estate of Castelporziano and recorded atmospheric compounds of urban origin. In fact, particularly in nighttime with favorable synoptic conditions, the air masses flowing from the Tiber valley and crossing the urban area of Rome, reach the Estate of Castelporziano being loaded with atmospheric pollutants collected during the transit over the city. In the low Tiber valley, the local circulations are also affected by the interaction with the synoptic circulation. The accurate experimental characterization of the low-level circulation is also fundamental in evaluating the quality of numerical models in reproducing the local meteorological systems. Recently, Petenko et al. (2011) have examined the nocturnal and diurnal components of the local circulation in relation with the large-scale atmospheric flows. This paper presents some results of the local atmospheric circulation observed during summer and winter of the year 2007 at the Estate of Castelporziano, by sodar measurements, to show the presence of different patterns of the diurnal behavior of the low-level wind field.

2 Site and instrumentation

The experimental site is located in the Estate of Castelporziano (“Tenuta di Castelporziano”, hereafter TCP), at the southern boundary of the city of Rome and 8 km northeast the Tyrrhenian coastline (41°44′N, 12°24′E, 30 m agl). The map of the area including the location of the measurement site is given in Fig. 1. The area of TCP is a forest that covers 59 km2 bounded by the ‘urbanized’ area in the northerly and westerly directions, while eastward the landscape rises in altitude towards the Albani hills that reach elevations of 500–1,000 m at a distance of about 15 km. In the south-southwest direction, the Estate opens with 3 km of coast to the Tyrrhenian Sea.

Fig. 1
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Map of the area around the measurements site at the estate of Castelporziano. Map of the low Tiber valley. The dot indicates the sodar site in the Estate of Castelporziano (TCP)

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The data used in this study have been collected by the triaxial monostatic Doppler sodar system, developed by the Institute of Atmospheric Sciences and Climate of the National Research Council (Italy). The data processing algorithms and electronics have been described by Mastrantonio and Fiocco (1982) and Elisei et al. (1986). Contini et al. (2004) proposed a procedure to improve the accuracy in determining the vertical component of the wind velocity measured by sodar. The sodar installed in the Estate of Castelporziano is configured in the following way. The three antennas simultaneously emit into the atmosphere acoustic bursts with a duration of 0.1 s each at different frequencies, 1,750, 2,000 and 2,250 Hz, one for each channel-antenna at a repetition rate of 6 s. One antenna is vertically pointing while the other two, tilted of 20° from the vertical, are oriented along the geographical axis N–S and E–W. The height range of measurements is from 39 to 1,000 m with a step of about 27 m. The backscattered signals received by the each antenna/channel are sampled after appropriate filtering, and a fast Fourier transform is performed. The sequence of the instantaneous profiles of echo intensity is used to depict the time evolution of the thermal structure of the atmospheric boundary layer. An example of the echo intensity facsimile reproduction is shown in Fig. 2. In the same time, the radial wind velocity is calculated from the obtained Doppler spectra to reconstruct the vertical profile of wind field.

Fig. 2
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Example of the thermal structure of atmospheric the boundary layer depicted by the sodar at the estate of Castelporziano on July 2007

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The sodar at Castelporziano runs continuously since September 2004. The vertical profiles of the wind speed and direction calculated on 10-min averaging base are collected and stored in the “Mediterranean Ecosystems Observatory” data repository with other atmospheric parameters used to monitor the ecosystem variability in the estate.

3 Diurnal wind pattern from sodar data

The main features of the wind diurnal variation analyzed for the year 2007 present a clear day-to-night alternation of the flow characterizing the low-level atmospheric circulation. To show some aspects of the local circulation observed during the year 2007, the period January–February and the period June–August have been selected as representatives of the winter and summer seasons, respectively. In Figs. 3 and 4, the diurnal behavior of the wind data for both selected periods are summarized. Three levels are presented to show also the variability with the height of the diurnal behavior within the layer the most interested by the local circulation. The color scale intensity is proportional to the occurrence of each speed/direction value at a specific time of the day for the months considered. The marked line represents the mean value for the wind speed (left panel) and the maximum occurrence for the wind direction distribution (right panel) distributions. In winter (Fig. 3) the diurnal cycle shows two periods in which the wind speed reaches the maximum intensity: one in the night hours between 0200 and 0600 CET and the other in the afternoon to evening between 1500 and 2400 CET. This is intensification of the wind speed could be due to the stability condition of the atmosphere, but also due to the contribution of the synoptic circulation forcing that can enhance some components of the low-level circulation in this particular orographic area (Petenko et al. 2011). The behavior of the wind direction plotted in Fig. 3 shows the presence of two nocturnal clusters concentrated around the sectors 0°–90° and 120° in correspondence the increase of wind speed during the night. The northeastern cluster (0°–90°) has high occurrence values at lower range (39 m) decreasing with the height. On the contrary, the cluster centered at 120° maintains high values of occurrence with height showing in winter predominant southeastern wind directions for most part of the day. The persistency of these directions can last until 1200 CET when the circulation tends to rotate to southern direction, and in fair weather conditions, a weak sea breeze can also be observed. But, while this rotation is clear above 100 m, at the lowest level the wind direction, during the central part of the day has a prevailing from the northwestern sector (315°–360°). The southeastern component of the wind in winter seems to be statistically dominant all day long and the diurnal component of the local circulation cannot be very well appreciated. A deeper analysis should be provided to identify the forcing factors that determine such a behavior in the wind field.

Fig. 3
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Diurnal behavior of the wind speed (left panel) and direction (right panel) during winter months January and February 2007

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Fig. 4
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Diurnal behavior of wind speed (left) and direction (right) from summer months July to August 2007

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In summer the situation is quite different and the well marked diurnal cycle can be highlighted. Two wind speed maxima can be observed as shown in Fig. 4: one during the night, around 0600 CET, and another, as expected during the warmest part of the day around 1200–1500 CET. The end of nocturnal circulation ends around 0900 CET with a drop down of the wind speed corresponding to the rapid change of the wind direction occurring when the sea breeze starts. In this case the prevailing wind direction is from south-southwest sectors. During the sea breeze events, that can last until 1800 CET, an increase of the wind speed is observed. Even in summer, during the night hours the wind directions concentrate in around northeast and southeast sectors (Fig. 4), but contrary to what happens in winter the occurrence of winds from northeastern sector are prevailing. Moreover, as well as the sea breeze starts and increases between 0900 and 1200 CET, the southeastern flow occurrence weakens leaving to the sea breeze forcing the wind from southeast to south direction. In the early evening around 1800 CET, as the solar heating decreases, the main diurnal cluster splits in two parts: one concentrated in the southeast sector, the other tends to concentrate in the northwest sector that seems to link up with the northern nocturnal component of the circulation.

These features represent the signature of the different wind regimes in the diurnal cycle of the local circulation which take place alternatively depending on some external factors. Petenko et al. (2011) have described the evolution of the diurnal pattern in the evening. In some cases the diurnal wind direction from southwest links up with the nocturnal regime by a veering back to south east. In other cases, the wind direction continues to rotate clockwise to southwest to match the nocturnal pattern. Similarly to that observed at Pratica di Mare (Petenko et al. 2011), probably external forcing determines the alternation of the two regimes at the Estate of Castelporziano in nighttime as well as in the day and evening wind pattern.

4 Distribution of nocturnal wind speed and direction occurrence

As described above the origin of nocturnal component of the local circulation seems to drive the wind pattern for the rest of the day. To describe in detail the characteristics of the wind field the statistical distribution during the night hours in winter and summer 2007 are shown in the histograms of Figs. 5 and 6, respectively. The histograms show the distribution of the occurrence in percent of wind speed (left panels) and the wind direction (right panels) at four levels representing the average obtained over several layers. The value indicated in the legend corresponds to the height of central point of the layers considered. In winter (Fig. 5) during the nights hour (0000–0006 CET) the maximum occurrence, of the wind speed is centered around 4–6 m/s speed almost at all heights. Wind speed greater than 7 m/s, prevail at higher levels.

Fig. 5
figure 5

Statistical distribution of the wind speed (left) and wind direction (right) from 0000 to 0006 CET in winter (January and February 2007) at four levels within 80 and 400 m

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Fig. 6
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Statistical distribution of the wind speed (left) and wind direction (right) from 0000 to 0006 CET in winter for summer months July and August 2007

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The wind direction distribution indicate two maxima in the lowest layer at 80 m centered around 45° and around 100°. At higher levels (above 200 m) the nocturnal wind direction spans between 120° and 180°, and this can easily be associated to presence of flows driven by the large-scale circulation.

In summer (Fig. 6), the nocturnal wind field shows a different behavior than in winter. Low wind speeds are mainly concentrated in the lower layer, with occurrences greater than those observed in winter. Moderate-to-high wind speeds in the same time interval (0000–0006 CET) prevails only at higher levels, with little influence occurrence on the layer below. The distribution of wind direction concentrates at lower heights mainly around the sector 0°–90° for 15 % of cases, and at a higher layer (404 m) a peak of occurrence is well marked around 120° to indicate again the superimposition of large-scale flow to the local circulation components.

5 Wind direction variability with height

The diurnal wind regime at the low level can be modified by the influence of synoptic circulation present at higher level. To understand the consequences of such influence it is important to characterize the variability of the wind direction with the height. The wind profiles up to 500 m obtained by sodar are used to estimate the depth of layer involved in the nocturnal circulation and identify the main components of the atmospheric circulation. In Fig. 7, the cumulative distribution wind profiles in winter months January and February 2007 is presented for the nocturnal component of the local circulation. The color scale indicates the occurrence, while the marked line indicates the mean value of the wind speed distribution. The nocturnal interval between 0000 and 0006 CET is plotted to evaluate as the wind direction measured at low level varies with the height. As previously described, the nocturnal wind field shows two main peaks of direction within the sector 45°–120° at lower levels. As the height increases the wind directions tend to rotate. The wind from the sector at 45° rotates toward northwest, while the wind from southeast toward south. The southeast wind direction distribution is more frequent and can extend up to 400 m. A cluster of wind direction from southwest appears above 100 m in the southwest direction, apparently without any correlation with the lower layer wind direction.

Fig. 7
figure 7

Vertical distribution of the nocturnal wind speed (above) and wind direction (below) during the winter months of January and February 2007. The prevailing nighttime wind at h ≈ 40 m is from the sectors northeast and southeast but at the higher level the direction varies splitting the clusters definitely to north or to south

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In Fig. 8 the plots are presented for the summer months July and August 2007. The wind direction distribution is vertically confined within two main clusters at 45 and at 120° and no rotation occurs with height. The occurrence of the directions in the clusters at 45° diminishes around 400 m, while the occurrence of the cluster at 120° increases above 400 m and this confirms the presence of the large-scale circulation influence at higher level. The secondary cluster that appears around the northwest sector, even if is not predominant is present within all levels with a slight increase of occurrence between 200 and 300 m.

Fig. 8
figure 8

Vertical distribution of the nocturnal wind speed (above) and wind direction (below) during the summer months of July and August 2007. The prevailing nighttime wind at h ≈ 40 m is from the sectors northeast and southeast. A weak cluster of occurrence is present in the sector of northwest. The wind direction at higher level for each cluster to be consistent with the direction at lower levels, but an increase of the occurrence of southeasterly wind appear at 300–400 m

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6 Summary and conclusions

This study aimed to analyze part of the wind data collected at the Estate of Castelporziano by a Doppler sodar to determine some key features of the behavior of the local circulation. The results of a statistical analysis of the low-level wind field for summer and winter months on 2007 have been shown. The pronounced diurnal cycle, that has been observed for the most part of the year, has been highlighted for the period considered in the present work and confirm that the low-level local circulation is one of the most important phenomena which influences the atmospheric processes at the coastal zone of Italy.

In winter time, the persistence for the whole day of the southeasterly winds can be associated to presence of the large-scale flow that superimposes to the local circulation. This lasts all day long even though the occurrence of northerly components can be observed in the morning in the late evening and in the night. In summer, the diurnal cycle of wind is more evident with a clear separation between the diurnal and nocturnal components of the local circulation. The nocturnal component seems to be less influenced by synoptic circulation. The statistical distribution of wind direction presents great occurrence at low levels within the northeast and around 100° in winter.

Moreover, it has been highlighted that the nocturnal components of the wind direction at low-level changes with the height, and this variability can be associated to the effects induced by the large-scale flow and the local circulation at higher levels. Such effect need to be better investigated in this particular site, with the help of numerical models and by integration of other information on the wind pattern at a higher level, for example by means of radio sounding profiles. This would increase the comprehension of the mechanism that generate the variability from daytime to nocturnal conditions of the low-level circulation.