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A multi-year study of the anomalous propagation conditions along the coast of the Adriatic sea
Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 75–84
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A multi-year study of the anomalous propagation conditions along the coast of the Adriatic sea Mladen Viher a,n, Maja Teliˇsman Prtenjak b, Branko Grisogono b a b
Croatian Air Force HQ, Croatia Andrija Mohorovicˇic´ Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Croatia
a r t i c l e i n f o
abstract
Article history: Received 29 April 2012 Received in revised form 13 January 2013 Accepted 25 January 2013 Available online 24 February 2013
Aerological balloon soundings are an abundant and reliable source of modified refractive index profiles. A time series spanning 15 years, collected from 4 Adriatic aerological stations, shows frequent anomalous propagation (anaprop) phenomena over the coast of the Adriatic Sea: radio ducts, superrefractions and subrefractions. Radio ducts are the rarest phenomena, but they have the most dramatic effects on propagation. Radio ducts generally occur in the lower third of the troposphere. Superrefractions are the most frequent anaprop phenomena, occurring mostly in the lower half of the troposphere. Subrefractions occur frequently in cold and dry air higher in the atmosphere; therefore, the vertical distribution shows a significant number of elevated subrefractions. Rare cases of subrefractions in the stratosphere also occur. All anaprop phenomena show distinct annual cycles in both number and vertical distributions. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Anomalous refraction Anaprop Aerological balloon sounding
1. Introduction As a part of the Mediterranean Sea, the Adriatic Sea is an important corridor for sea and air traffic. Air and surface traffic safety depends on reliable radio-frequency transmissions; radio, radar and navigation aids must operate with no significant impairment. These radio systems, as well as radio and TV diffusion, are strongly influenced by refractive conditions. According to Barrios and Patterson (2002), the refractive environment can be accurately characterised through the use of balloon-borne probes. Therefore, radiosoundings represent a reliable and accessible resource of perennial meteorological data that is necessary for the descriptions of vertical profiles and the annual cycle of anomalous propagation (hereafter referred to as anaprop) phe¨ nomena (e.g., Viher, 1995, 2006; Rueger, 2002). Refractive conditions in the atmosphere are defined by the atmospheric refractive index n. According to Maxwell equations, the refractive index is function of the permittivity e and perme¨ ability m of the atmosphere (e.g., Battan, 1973; Rueger, 2002): pffiffiffiffiffiffi ð1Þ n ¼ em, Both permittivity and permeability are difficult to determine in field conditions; therefore, an empirical and more practical relation (2) is proposed based on measurable quantities: tem¨ perature T, pressure p and relative humidity rh of the air (Rueger, n
Corresponding author. Tel.: þ385 91 571 1965. E-mail addresses: [email protected], [email protected] (M. Viher), [email protected] (M.T. Prtenjak), [email protected] (B. Grisogono). 1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.01.014
2002). Because the refractive index of radio waves has a value of approximately 1.000340 at the Earth’s surface and decreases with height (to a value of 1.0 in a space vacuum), a new quantity, N, was suggested (Skolnik, 1980; Barrios and Patterson, 2002) that represents only the significant digits of the refractive index: N ¼ ðn1Þ 106 ¼
y¼
77:6 p y 3:73 105 þ ; T T2
rh 6:105 expðxÞ 100
x ¼ 25:22
T273:15 T 5:31 ln ; T T0
ð2Þ
ð3Þ
where the value T0 ¼273.15 K. Eqs. (2) and (3) are valid only at the surface. At a certain height, we must take into account the difference between the curvature of the horizontal surface at altitude h and the actual curvature of the radio rays (Fig. 1). In this study, h varies from 0 m to 40 km above ground level (a.g.l.). In all cases where ha0, the centre of the arc 4/3 (Rþh) does not lie at the centre of the Earth (an approximation of the spherical Earth with radius R). Because hZ0, the curvature of the standard refraction is always smaller than the curvature of the level at height h, and according to this definition, an actual value of standard refraction (expressed in N ‘‘units’’) depends on h. Therefore, the refractive index N must be transformed into a new quantity, termed the modified refractive index M (Skolnik, 1980; Barrios and Patterson, 2002): M¼
h h 106 þ ðn1Þ 106 ¼ 106 þ N ¼ 0:157 h þ N; R R
where R represents radius of the Earth ¼6371 km.
ð4Þ
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The ‘‘standard 4/3 refraction’’ is the geometric solution with the assumption of a linear dependence of the refractive index. An alternative exponential refractive profile was proposed (Bean and
Fig. 1. Sketch of the limiting values of the modified refractive index M (Eq. (4)) between four types of refraction: standard refraction, subrefraction, superrefraction and radio ducts. Limiting values of the refraction types are geometrically determined (Skolnik, 1980; Barrios and Patterson, 2002) according to the local horizontal plane at altitude h, represented with a dashed line. Radio electronic equipment is designed to evaluate standard refraction.
Thayer, 1959; International Telecommunications Union, 2003; American National Standards Institute and American Institute of Aeronautics and Astronautics, 2004), but this profile only provides a good approximation for the lowest part of the troposphere. Recently, Viher et al. (2011) and Viher and Prtenjak (2012) investigated multi-year refractive index profiles along the Adriatic coast by means of daily aerological soundings. These studies showed that a widely used, simple exponential model is acceptable only for the lowermost 1000–1500 m, which limits its application only to terrestrial telecommunications. Therefore, Viher and Prtenjak (2012) proposed two models with significantly better characteristics: a polynomial model for both the troposphere and the stratosphere up to 40 km and a much simpler linear model for the stratosphere alone. However, the average characteristics of the refractive conditions were not investigated in that study. An example of the M profile as an output of the application Advanced Refractive Effects Prediction System (AREPS; Barrios and Patterson, 2002) is shown in Fig. 2. This particular event with three strong elevated anaprops occurred over Brindisi on 5 July 1996 at 05 UTC. The reasons of these anaprops could be easily analysed by comparing the Skew-T diagram of this probing in Fig. 2a with the M profile in Fig. 2b (based on Eq. (4)). Inside the lower trapping layer (which produces radio duct) in the range of 204–433 m a.g.l., a positive temperature gradient and, simultaneously, a strong negative gradient of the dew point occurred (sudden drying of the air). In the second, higher trapping layer
Fig. 2. (a) Skew-T diagram of sounding above Brindisi (40.651N, 17.951E) on July 5, 1996 at 05 UTC. (b) Vertical M profile (see Eq. (4)) with a graphical representation of the M profile and the corresponding refractive types, obtained by the Advanced Refractive Effects Prediction System (AREPS; Barrios and Patterson, 2002). source: http://weather.uwyo.edu/upperair/sounding.html
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from 1817–1899 m a.g.l., a similar situation was repeated with a particularly strong negative drop in the dew point (from 2.4 1C to 10.4 1C). Between those two radio ducts, a subrefractive layer, located at 788–1204 m a.g.l., was produced by an isothermal layer along with a positive gradient of the dew point (from 2.8 1C to 10.4 1C). The layer with positive temperature and negative dew
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point gradients at an altitude of approximately 4100 m was not sufficiently strong to produce anomalous phenomena. Up to the present time, few studies have presented detailed distributions of the refractive anomalies that occur above the Mediterranean area. One common anaprop presentation is the difference between the actual surface refractive index and the
Fig. 3. Global distribution of the surface refractive anomalies in August (from International Telecommunications Union (2003)). The source data consisted of a 5-year time series (1955–1959) from 99 aerological stations worldwide.
Fig. 4. Locations of the meteorological stations used in the present study. The rectangles represent aerological stations where actual refractive profiles were computed, and the circles represent the main meteorological stations with surface data only. source of blind map: http://d-maps.com
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Table 1 Main characteristics of the measurement sites, including latitude (j) and longitude (l) and the analysed period with records of altitude, pressure, temperature and moisture. The sites are also shown in Fig. 3. WMO] and station
16044 16144 14430 16320
Udine San Pietro Zadar Brindisi
Geographic location (u, k)
(46.031N, (44.651N, (44.101N, (40.651N,
13.181E) 11.611E) 15.351E) 17.951E)
Period Starting
Ending
1.11.1995 1.1.1994 15.3.2006 1.7.1996
15.6.2009 15.6.2009 1.6.2009 15.6.2009
average surface refractive index (global average is Navg Mavg ¼340). The difference for the actual case is DN¼Nactual Navg. For example, in a global study conducted by the International Telecommunication Union (International Telecommunications Union, 2003), a global distribution of the surface refractive anomalies (DN) was made, indicating three distinct areas of summer surface anaprops: the West Coast of North America, the Mediterranean area and the Persian Gulf area (Fig. 3). However, these results were provided on a global scale, which is not sufficiently precise for their application to the Adriatic Sea area. Furthermore, the recommendation of the ITU (International Telecommunications Union, 2003) focused mostly
Fig. 5. (a) Radio sounding data for Zadar (44.101N, 15.351E). Every circle represents one record of four variables: altitude, air temperature, total air pressure and relative humidity, which enables the computation of M according to Eq. (4). The standard levels are shown as dense records that appear as irregular lines because of the altitude variations in the standard pressure levels (e.g., 1000 hPa, 850 hPa, 500 hPa). Significant levels occur between standard levels, where large gradients in all measurements were recorded. (b) The vertical profile of the air temperature (1C) at Zadar. The reason for the poor coverage of stratospheric altitudes at Zadar in the range of 10 km–30 km was a limitation in the temperature sensors, which record a minimum temperature of 40 1C for many measurements.
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on surface refractive conditions, reflecting their interest in terrestrial microwave communications. Another study focused on the Iberian Peninsula over the coasts of Catalonia (Bech et al., 2002). This study showed that the anaprop conditions at low altitudes significantly affect meteorological radar outputs. Their findings about annual cycles of the number of anaprop occurrences raised a question about the similarity of the refractive conditions between the Catalonian and Adriatic coasts. The authors also used a two-year dataset, which is still not sufficient enough to reveal certain average characteristics, and their analysis covered only the lowermost 1000 m of the atmosphere. Because the Adriatic Sea is strongly influenced by the Mediterranean Sea and because there is no detailed climatological study of the refractive conditions in this area, the main goal of this study is an objective statistical description of the refractive conditions over the Adriatic Sea. Aerological balloon probing data were obtained for a 15-year period from 1994 to 2009, allowing us to explore the radio wave refractivity pattern along the Adriatic coast. Furthermore, these data and their analysis will expand our existing knowledge concerning refractive index profiles of up to 40 km for the Adriatic area, thus supplementing long-term research of the surface refractive index by the International Telecommunications Union (2003). These findings will also aid in the optimal construction of a dynamic model of the atmosphere, which will provide a detailed spatial representation of the phenomena involved and could even provide a shortterm anaprop forecast (Bech et al., 2004).
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The sources of data for air pressure and temperature were regular aerological balloon probings of four stations around the Adriatic Sea: Udine (WMO]16044), San Pietro Capofiume (WMO]16144), Zadar (WMO]14430) and Brindisi (WMO]16320). While Brindisi (10 m above sea level, a.s.l.) is a coastal station, the other stations, Zadar (80 m a.s.l.), San Pietro Capofiume (38 m a.s.l.) and Udine (94 m a.s.l.), are located inland at distances of 10 km–40 km away from the coast. The data series covered a period of up to 15 years (Table 1) with a temporal resolution of 12 h (Zadar and San Pietro) or 6 h (Udine and Brindisi). The data were collected on the NOAA site ‘‘ESRL Radiosonde Database Access’’ (http://www.esrl.noaa.gov/ raobs/). This dataset provides records of all required variables: altitude, pressure, temperature and moisture. The radio probe is also strongly influenced by wind, especially at higher altitudes, so standard probe positioning includes radio-theodolite (for wind measurements) or uses a more sophisticated NAVAID, such as GPS, Loran-C or Alpha. The accuracy of the probe’s position depends on the actual positioning system used. According to the manufacturer’s data (http://www.vaisala.com), we assume an absolute error of less than 10 m for altitude measurements. Due to a change in the measurement methodology at the Zadar station beginning on March 15, 2006, we restricted the data to those collected after that date (Fig. 5a and Table 1). Zadar’s probes were also unique due to the limitations in their thermal sensors, which were set to a minimum temperature of 40 1C. For this reason, many stratospheric data points were lost in the altitude range of 10–30 km (Fig. 5b).
2.2. Climatological characteristics of the Adriatic region 2. Material and methods 2.1. Measurements The Adriatic Sea is a stretched basin in a NW–SE direction, located in the northern-central Mediterranean between the Apennine Peninsula and the Balkan Peninsula (Fig. 4 and Table 1).
Fig. 6. Annual cycle of (a) the mean monthly surface 2-m air temperature and (b) the surface relative humidity recorded at the Adriatic meteorological stations shown in Fig. 3 (Penzar et al., 2001).
Through the year, atmospheric conditions vary due to the influence of moving weather formations of alternating low and high air pressure systems. During the winter, early spring and late autumn seasons, stationary anticyclonic systems exchange with fast-moving cyclonic systems with typical winds: the northwesterly bora wind (more in e.g., Grisogono and Beluˇsic´, 2009) and the strong southerly sirocco wind. In summer, cyclonic activity is mostly localised northward, allowing sunny and dry weather and the development of local circulations (sea/land breeze and/or
Fig. 7. Tropospheric distributions of the refractive types computed by all tropospheric data, including surface measurements.
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slope winds) above the mostly mountainous coast (e.g., Prtenjak and Grisogono, 2007). The mean annual air temperature gradually rises southward, ranging from approximately 13 1C in the northern region to 16 1C in the middle Adriatic and up to 17–18 1C in the southern region (e.g., Penzar et al., 2001). The mean annual temperature field has two particularities: (i) two warmer regions above the middle and southern Adriatic over the deepest sea area and (ii) the eastern Adriatic coast is warmer than the western coast due to sea currents. Along the eastern Adriatic, the warmest month is July, and the coldest months are January and February (Fig. 6a). Above the Adriatic, air temperatures and the frequent bora wind significantly affect the average annual values of relative humidity, which are decreased southward and eastward (Fig. 6b). Annually, the relative air humidity course has an irregular shape, usually experiencing two maxima and two minima. The primary maximum occurs at the end of autumn or during the winter season, and the secondary maximum occurs in May. The notably pronounced primary minimum is in July and, occasionally, a secondary minimum occurs in February.
3. Results and discussion Because the main task was to determine the statistical characteristics of the anomalous refractive conditions above the Adriatic area, particularly the altitude range where anomalous propagations frequently occur, we estimated four different refractive conditions: standard refraction, subrefraction, superrefraction and radio ducts based on aerological station data. Fig. 7 shows the relative proportions of the anaprop phenomena for all four stations in the troposphere. These four stations present good series of tropospheric data readings, making direct comparisons possible. A recent study (Viher and Prtenjak, 2012) also showed that anomalous propagation conditions in the stratosphere are rare (generally subrefractions) and that the vertical gradient of the refractive index is linear with a gradient of 152 km 1 up to an altitude of 40 km, meaning that stratospheric data could be excluded. With regard to each type of anomalous refractive condition, superrefractions are the most frequent anaprop formations followed by subrefractions and radio ducts. According to the results in Fig. 7, superrefractions are phenomena which occur more often along the western Adriatic coast. Meteorological conditions that are favourable for radio ducts are similar to those that are favourable for superrefractions, but extreme gradients of temperature and humidity are required for radio duct formation.
Therefore, radio ducts are less common than superrefractions, and their frequency decreases northward. Fig. 8 shows the differences that are caused by daily variations in the refractive index. Because the San Pietro and Zadar cites have only two probing terms per day, only the corresponding noon and nocturnal terms were compared for all four stations. Terms are timed to cover rare cases of probe launching outside of the regular schedule (00 UTC and 12 UTC) but no more than 3 h earlier or later. A relatively high percentage of daily anaprop over Zadar and a relatively low percentage over Udine are observed, which could be due to differences in the station locations with respect to the coastline and typical local meteorological pattern. San Pietro and Brindisi have similar percentages for nocturnal events. The overall anaprop occurrence is similar to that found over the Catalonian coast (Bech et al., 2002) and the Alpine and sub-Alpine region (Viher, 2006), which quantitatively confirms the ITU prediction for the Adriatic region as a region with significant number of anaprop phenomena (Fig. 3). On a daily scale, nocturnal anaprop occurrences are more frequent than daytime occurrences for all stations. However, the difference in the number of anaprop occurrences on a daily scale is smaller if the location is (1) closer to the sea (e.g., Brindisi) and (2) more strongly influenced by mountainous terrain (e.g., Udine). Above Zadar and San Pietro, the daytime anaprop frequency decrease is approximately 30–50% for the overall anaprop cases. In Udine, the decrease is 25%, while in Brindisi, a decrease of only 10% is found. Nevertheless, the possible influence of daily thermally generated circulations, stratification and convection in the lower half of the troposphere on the anaprop frequency or its daytime decrease cannot be determined satisfactorily by soundings. Further research based on 3D meteorological models at a high resolution could offer additional insight into anaprop characteristics over the Adriatic. In Fig. 9, the vertical distributions of all anaprop conditions for all four Adriatic aerological stations soundings are shown. The percentage of anomalous phenomena was computed by dividing the height into steps of 100 m for each station. These figures revealed that anaprop conditions are unequally distributed with altitude, with the largest number of occurrences being observed near the surface due to their direct dependence on intense fluxes of heat and moisture. As mentioned previously, for anaprop phenomena in general, the largest proportions of radio ducts are recorded (i) near the surface, especially when the inversion layer is humid, and (ii) at approximately 1500 m due to the sudden temperature change across the top of the boundary layer. Significant numbers of elevated radio ducts are also observed in the
Fig. 8. Frequencies of the anomalous tropospheric phenomena that were detected during two standard probing terms, 00 UTC and 12 UTC for all four aerological stations along the Adriatic coast.
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Fig. 9. Vertical distributions of the anomalous propagations over four Adriatic aerological stations; (a) radio ducts, (b) superrefractions and (c) subrefractions. Percentages of anomalous propagations are computed for each station separately by dividing the altitude into 100-m steps.
lower third of the troposphere, above the boundary layer. This finding largely coincides with the findings of Haack and Burk (2001) along the coast of California. These researchers found that the average observed trapping layer thickness of ducts was approximately 100 m within the lowermost 500 m. In this study, the large number of ducts in the lowermost 400 m is presumably associated with the sea/land breeze formation, as was also
demonstrated in e.g., Atkinson and Zhu (2006). These researchers used two models, a meteorological MM5 numerical model and an AREPS propagation model, to produce radar coverage over a coastal area in the Persian Gulf for one chosen case. According to their results, the sea breeze trapped radio waves in the lowermost few hundred metres during a sea breeze penetration of more than 100 km inland.
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Fig. 10. Annual cycle of the monthly percentage of each anaprop formation, computed for each station separately and compared on the same graph: (a) radio ducts, (b) superrefractions and (c) subrefractions. The maximum of each anaprop phenomena during the summer months corresponds to the maximum surface air temperature (Fig. 5a) and minimum relative humidity (Fig. 5b) as recorded at the selected Adriatic stations.
Vertical proportion profiles in refractions
profiles of superrefractions (Fig. 9b) show a greater of elevated phenomena compared to the radio duct Fig. 9a. A significant number of elevated superis in the lower half of the troposphere. Favourable
meteorological conditions for subrefractions occur in cold, dry air when the temperature profile shows a sudden temperature rise. Both temperature and humidity drop with height; therefore, a relatively large proportion of elevated subrefractions are detected (Fig. 9c). However, in the stratosphere, the anaprop phenomena are almost exclusively subrefractions due to the following reasons: (i) dry and cold stratospheric air, (ii) temperature increase due to stratospheric dynamics and (iii) a stratospheric vertical gradient of the modified refractive index about 152 km 1 that represents a value that is close to the limiting value for subrefractions (Viher and Prtenjak, 2012). With regard to the annual variations in the overall anaprop phenomena, distinct annual cycles can be noted in both the number of phenomena and the proportion of elevated anaprop (Fig. 10). The percentages of all anaprop phenomena for all four Adriatic stations show minima during November to March, increases in the spring season, maxima during July–August and a subsequent decrease during the fall season. Generally, annual cycles in the percentages of anomalous phenomena (Fig. 10) exhibit good overlap for radio ducts and subrefractions. The annual cycle of superrefractions for Zadar shows a discrepancy due to the relatively low data quality. The Brindisi station has a month-long lag in the maxima of all anaprop phenomena relative to Zadar and San Pietro. The June–August maximum in the Adriatic anaprop percentage corresponds to a maximum of the near-surface air temperature T as recorded in numerous Adriatic stations (Fig. 6a) and a minimum in the surface relative humidity rh (Fig. 6b). Because the refractive index (Eq. (2)) is mostly dependent on rh and T and is approximately 104 times less dependent on p (shown by the partial derivation in Eq. (2) over p, T and rh), the minimum of rh during the summer months leaves changes in T as the main reason for the anomalous phenomena. Keeping in mind that the vertical distributions shown in Fig. 9 are not stationary, the monthly vertical distributions were computed for each station. Due to the irregular nature of the vertical distributions, the median and 95% percentage were chosen as representative parameters of the distributions. The results are shown in Fig. 11. The curve marked with ‘‘M’’ shows a median altitude for each month, and the curve marked ‘‘95%’’ is the altitude above the surface, where 95% of the particular phenomena were recorded. Annual changes in the radio duct average median tend to rise during the cold season (Fig. 11a), confirming the dominance of surface radio ducts, shown in Fig. 9a. The monthly average altitude of the boundary below which 95% of radio ducts occur (Fig. 11a) experience minima during March (in the range of 1717– 2325 m) for all stations, except Udine, which has a minimum in January (2494 m). The monthly average maximal altitude of the 95% occurrences for the radio ducts were recorded in July for Zadar and Udine (3867 m and 4197 m, respectively), while San Pietro has a maximum in September (3285 m), and Brindisi has an extended 95% boundary of radio ducts from August to September (3497–3489 m). Superrefractions are the most frequent anaprop, and they have a larger proportion of elevated phenomena (Fig. 11b) compared to radio ducts (Fig. 11a). Therefore, it is expected that both the median and 95% occurrences occur at higher altitudes compared to radio ducts. Annual changes in the average median for superrefractions have minimal altitudes during February, and in the case of Brindisi, this minimum extends from January to March. Fig. 11b shows the annual change in the vertical distribution of superrefractions. The minimal monthly average altitudes are in the range of 2731 m (San Pietro) to 3270 m (Brindisi). In addition, the mean altitudes of the 95% occurrences are higher for superrefractions than for radio ducts, indicating more elevated superrefractions than elevated radio ducts. For example, the mean
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Fig. 11. Annual variations in the vertical profiles for anomalous propagation phenomena, represented by the average monthly median (M) and average monthly altitude of the 95% probability of the phenomenon: (a) radio ducts, (b) superrefractions and (c) subrefractions.
heights of the 95% occurrences range from 4537 m (Brindisi) to 5339 m (Zadar). Maximal values were recorded in August over Zadar and Udine, while San Pietro recorded a wider maximum range from July to August, and Brindisi recorded the widest maximum, extending from June to September. Subrefractions are the anaprop phenomena that have the greatest proportion of elevated occurrences (Fig. 11c). It is expected that both the mean monthly averages of the median and 95% occurrences have the greatest values compared to the other two anaprop phenomena. Minimal average monthly median altitudes of the subrefractions were recorded in the range of 94 m (Zadar) to 1148 m (Brindisi). This minimal value of the median altitude was
recorded during June, while other three stations recorded minimal values during February and March (Fig. 11c). The annual cycle of the mean monthly altitude of the 95% occurrences is less welldefined for subrefractions than for superrefractions (Fig. 11c and b). Both minimal and maximal values show that subrefractions reach the greatest heights among all anaprop events. For example, the minimal values of the 95% occurrences range from 4670 m (San Pietro) to 5362 m (Brindisi). These minimal values were recorded from December to March. The maximal altitudes of the 95% occurrences for the subrefractions are in the range from 6356 m (Zadar)–6832 m (Udine). These maximal values were recorded during the summer months of July to September.
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4. Conclusions
Acknowledgements
In this study, the statistical characteristics of the refractive conditions along the Adriatic coast were determined for three different anomalous refractive conditions: subrefractions, superrefractions and radio ducts using aerological sounding data. The data were provided by four stations (one Croatian site and three Italian sites) along the Adriatic coast for a 15-year period (1994–2009). The data analysis showed that aerological balloon soundings represent a useful tool for calculating modified refractive index profiles. The main advantages of aerological soundings are significant level reports, which are often connected to anaprop phenomena. Thus, every anaprop phenomenon can be detected in a particular vertical profile. Because of the high numbers anaprop phenomena that are detected around the Adriatic Sea, we conclude that these phenomena cannot be ignored. Our results show that superrefractions are the most frequent anaprop phenomena followed by subrefractions and radio ducts. The greatest number of anaprop phenomena was recorded from June to September with the exception of the Zadar data. This finding could be explained by the relative humidity minimum in the air over the Adriatic Sea during this period. In relatively dry air, sudden changes in the temperature profile become the dominant driving factor for anaprop phenomena. According to the vertical distributions of the anaprop phenomena, a majority of cases were recorded near the surface or at low altitudes. With respect to elevated phenomena, subrefractions are the most frequent phenomena and reach up to 6800 m a.g.l. Superrefractions usually appear in the lower half of the troposphere, while elevated radio ducts are generally recorded in the lower third of the troposphere. These radio ducts are most likely caused in this level by the sea/land breeze flow, raising a question regarding the effect of the sea/land breeze on the radar coverage across the Adriatic coast. The results of this study could be a good starting point for the construction of a numerical model of the anaprop phenomena occurring over the Adriatic Sea. A long time series of vertical profiles is a prerequisite for better numerical model optimisation in both spatial and temporal domains. Such numerical models will provide insight into three-dimensional spatial distributions of anaprop phenomena, particularly due to the high vertical resolution. Moreover, with proper time resolution, the models could describe short-term phenomena that only exist for several hours and are not detected with the time resolution of typical balloon probings.
This work has been supported by the Ministry of Science, Education and Sport (BORA Project no. 119-1193086-1311). Reviewers’ comments and constructive criticism which led to the paper improvements are highly appreciated. References American National Standards Institute and American Institute of Aeronautics and Astronautics, 2004. Guide to Reference and Standard Atmosphere Models, ANSI/AIAA G-003B-2004, pp. 90, ISBN:1-56347-744-0. Atkinson, B.W., Zhu, M., 2006. Coastal effects on radar propagation in atmospheric ducting conditions. Meteorological Applications 13, 53–62. Barrios, A.E., Patterson, W.L., 2002. Advanced propagation model v.1.3.1 computer software configuration item (CSCI) documents, SPAWAR Systems Center, San Diego, Technical Document 3145. Battan, L.J., 1973. Radar observation of the atmosphere. University of Chicago Press, Chicago. Bean, B.R., Thayer, G.D., 1959. Central Radio Propagation Laboratory (CRPL) Exponential Reference Atmosphere—National Bureau of Standards Monograph 4, US Department of Commerce, p. 67. Bech, J., Toda, J., Codina, B., Lorente, J., Bebbington, D., 2004. Using mesoscale NWP model data to identify radar anomalous propagation events, Proceedings of ERAD, pp. 310–314. Bech, J., Codina, B., Lorente, J., Bebbington, D., 2002. Monthly and daily variations of radar anomalous propagation conditions: how ‘‘normal’’ is normal propagation? Proceedings of ERAD, Copernicus GmbH, pp. 310–314. Grisogono, B., Beluˇsic´, D., 2009. A review of recent advances in understanding the meso- and microscale properties of the severe bora wind. Tellus 61A, 1–16. Haack, T., Burk, S.D., 2001. Summertime marine refractivity conditions along coastal California. Journal of Applied Meteorology 40, 673–687. International Telecommunications Union (ITU), 2003. Recommendation ITU-R P.453-9 the radio refractive index; its formula and refractivity data, ITU. Penzar, B., Penzar, I., Orlic´, M., 2001. Weather and Climate of the Eastern Adriatic, (in Croatian). Nakladna kuca Dr. Feletar, Zagreb, pp. 258. Prtenjak, M.T., Grisogono, B., 2007. Sea/land breezes climatological characteristics along the northeastern Adriatic coast. Theoretical and Applied Climatology 90, 201–215, http://dx.doi.org/10.1007/s00704-006-0286-9. ¨ Rueger, J.M., 2002. Refractive Index Formulae for Electronic Distance Measurement with Radio and Millimetre Waves, Unisurv Report S-68, School of Surveying and Spatial Information Systems, University of New South Wales, UNSW SYDNEY NSW 2052, pp. 1–52. Skolnik, M.I., 1980. Introduction to Radar Systems, 2nd Edition McGraw-Hill 441–469. Viher, M., 1995. A practical method of modified refraction index computing for the microwave radiation using atmospheric probing measurements. Croatian Meteorological Journal 30, 63–70. Viher, M., 2006. A study of the modified refraction indices over the Alpine and subAlpine region. Meteorologische Zeitschrift 15, 625–630. Viher, M., Kos, T., Markezic, I., 2011. A Study of the Maximal Height for the Exponential Model of the Refractivity Indeks Vertical Profile in the Republic of Croatia. In: Proceedings of the 53rd International Symposium Electronics in Marine (ELMAR) 2011, pp. 187–190. Viher, M., Prtenjak, M.T., 2012. A multi-year study of radio-wave refractivity profiles above the Adriatic Sea up to an altitude of 40 km. Meteorologische Zeitschrift 21, 365–370.
Contents lists available at SciVerse ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
A multi-year study of the anomalous propagation conditions along the coast of the Adriatic sea Mladen Viher a,n, Maja Teliˇsman Prtenjak b, Branko Grisogono b a b
Croatian Air Force HQ, Croatia Andrija Mohorovicˇic´ Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Croatia
a r t i c l e i n f o
abstract
Article history: Received 29 April 2012 Received in revised form 13 January 2013 Accepted 25 January 2013 Available online 24 February 2013
Aerological balloon soundings are an abundant and reliable source of modified refractive index profiles. A time series spanning 15 years, collected from 4 Adriatic aerological stations, shows frequent anomalous propagation (anaprop) phenomena over the coast of the Adriatic Sea: radio ducts, superrefractions and subrefractions. Radio ducts are the rarest phenomena, but they have the most dramatic effects on propagation. Radio ducts generally occur in the lower third of the troposphere. Superrefractions are the most frequent anaprop phenomena, occurring mostly in the lower half of the troposphere. Subrefractions occur frequently in cold and dry air higher in the atmosphere; therefore, the vertical distribution shows a significant number of elevated subrefractions. Rare cases of subrefractions in the stratosphere also occur. All anaprop phenomena show distinct annual cycles in both number and vertical distributions. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Anomalous refraction Anaprop Aerological balloon sounding
1. Introduction As a part of the Mediterranean Sea, the Adriatic Sea is an important corridor for sea and air traffic. Air and surface traffic safety depends on reliable radio-frequency transmissions; radio, radar and navigation aids must operate with no significant impairment. These radio systems, as well as radio and TV diffusion, are strongly influenced by refractive conditions. According to Barrios and Patterson (2002), the refractive environment can be accurately characterised through the use of balloon-borne probes. Therefore, radiosoundings represent a reliable and accessible resource of perennial meteorological data that is necessary for the descriptions of vertical profiles and the annual cycle of anomalous propagation (hereafter referred to as anaprop) phe¨ nomena (e.g., Viher, 1995, 2006; Rueger, 2002). Refractive conditions in the atmosphere are defined by the atmospheric refractive index n. According to Maxwell equations, the refractive index is function of the permittivity e and perme¨ ability m of the atmosphere (e.g., Battan, 1973; Rueger, 2002): pffiffiffiffiffiffi ð1Þ n ¼ em, Both permittivity and permeability are difficult to determine in field conditions; therefore, an empirical and more practical relation (2) is proposed based on measurable quantities: tem¨ perature T, pressure p and relative humidity rh of the air (Rueger, n
Corresponding author. Tel.: þ385 91 571 1965. E-mail addresses: [email protected], [email protected] (M. Viher), [email protected] (M.T. Prtenjak), [email protected] (B. Grisogono). 1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.01.014
2002). Because the refractive index of radio waves has a value of approximately 1.000340 at the Earth’s surface and decreases with height (to a value of 1.0 in a space vacuum), a new quantity, N, was suggested (Skolnik, 1980; Barrios and Patterson, 2002) that represents only the significant digits of the refractive index: N ¼ ðn1Þ 106 ¼
y¼
77:6 p y 3:73 105 þ ; T T2
rh 6:105 expðxÞ 100
x ¼ 25:22
T273:15 T 5:31 ln ; T T0
ð2Þ
ð3Þ
where the value T0 ¼273.15 K. Eqs. (2) and (3) are valid only at the surface. At a certain height, we must take into account the difference between the curvature of the horizontal surface at altitude h and the actual curvature of the radio rays (Fig. 1). In this study, h varies from 0 m to 40 km above ground level (a.g.l.). In all cases where ha0, the centre of the arc 4/3 (Rþh) does not lie at the centre of the Earth (an approximation of the spherical Earth with radius R). Because hZ0, the curvature of the standard refraction is always smaller than the curvature of the level at height h, and according to this definition, an actual value of standard refraction (expressed in N ‘‘units’’) depends on h. Therefore, the refractive index N must be transformed into a new quantity, termed the modified refractive index M (Skolnik, 1980; Barrios and Patterson, 2002): M¼
h h 106 þ ðn1Þ 106 ¼ 106 þ N ¼ 0:157 h þ N; R R
where R represents radius of the Earth ¼6371 km.
ð4Þ
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The ‘‘standard 4/3 refraction’’ is the geometric solution with the assumption of a linear dependence of the refractive index. An alternative exponential refractive profile was proposed (Bean and
Fig. 1. Sketch of the limiting values of the modified refractive index M (Eq. (4)) between four types of refraction: standard refraction, subrefraction, superrefraction and radio ducts. Limiting values of the refraction types are geometrically determined (Skolnik, 1980; Barrios and Patterson, 2002) according to the local horizontal plane at altitude h, represented with a dashed line. Radio electronic equipment is designed to evaluate standard refraction.
Thayer, 1959; International Telecommunications Union, 2003; American National Standards Institute and American Institute of Aeronautics and Astronautics, 2004), but this profile only provides a good approximation for the lowest part of the troposphere. Recently, Viher et al. (2011) and Viher and Prtenjak (2012) investigated multi-year refractive index profiles along the Adriatic coast by means of daily aerological soundings. These studies showed that a widely used, simple exponential model is acceptable only for the lowermost 1000–1500 m, which limits its application only to terrestrial telecommunications. Therefore, Viher and Prtenjak (2012) proposed two models with significantly better characteristics: a polynomial model for both the troposphere and the stratosphere up to 40 km and a much simpler linear model for the stratosphere alone. However, the average characteristics of the refractive conditions were not investigated in that study. An example of the M profile as an output of the application Advanced Refractive Effects Prediction System (AREPS; Barrios and Patterson, 2002) is shown in Fig. 2. This particular event with three strong elevated anaprops occurred over Brindisi on 5 July 1996 at 05 UTC. The reasons of these anaprops could be easily analysed by comparing the Skew-T diagram of this probing in Fig. 2a with the M profile in Fig. 2b (based on Eq. (4)). Inside the lower trapping layer (which produces radio duct) in the range of 204–433 m a.g.l., a positive temperature gradient and, simultaneously, a strong negative gradient of the dew point occurred (sudden drying of the air). In the second, higher trapping layer
Fig. 2. (a) Skew-T diagram of sounding above Brindisi (40.651N, 17.951E) on July 5, 1996 at 05 UTC. (b) Vertical M profile (see Eq. (4)) with a graphical representation of the M profile and the corresponding refractive types, obtained by the Advanced Refractive Effects Prediction System (AREPS; Barrios and Patterson, 2002). source: http://weather.uwyo.edu/upperair/sounding.html
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from 1817–1899 m a.g.l., a similar situation was repeated with a particularly strong negative drop in the dew point (from 2.4 1C to 10.4 1C). Between those two radio ducts, a subrefractive layer, located at 788–1204 m a.g.l., was produced by an isothermal layer along with a positive gradient of the dew point (from 2.8 1C to 10.4 1C). The layer with positive temperature and negative dew
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point gradients at an altitude of approximately 4100 m was not sufficiently strong to produce anomalous phenomena. Up to the present time, few studies have presented detailed distributions of the refractive anomalies that occur above the Mediterranean area. One common anaprop presentation is the difference between the actual surface refractive index and the
Fig. 3. Global distribution of the surface refractive anomalies in August (from International Telecommunications Union (2003)). The source data consisted of a 5-year time series (1955–1959) from 99 aerological stations worldwide.
Fig. 4. Locations of the meteorological stations used in the present study. The rectangles represent aerological stations where actual refractive profiles were computed, and the circles represent the main meteorological stations with surface data only. source of blind map: http://d-maps.com
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Table 1 Main characteristics of the measurement sites, including latitude (j) and longitude (l) and the analysed period with records of altitude, pressure, temperature and moisture. The sites are also shown in Fig. 3. WMO] and station
16044 16144 14430 16320
Udine San Pietro Zadar Brindisi
Geographic location (u, k)
(46.031N, (44.651N, (44.101N, (40.651N,
13.181E) 11.611E) 15.351E) 17.951E)
Period Starting
Ending
1.11.1995 1.1.1994 15.3.2006 1.7.1996
15.6.2009 15.6.2009 1.6.2009 15.6.2009
average surface refractive index (global average is Navg Mavg ¼340). The difference for the actual case is DN¼Nactual Navg. For example, in a global study conducted by the International Telecommunication Union (International Telecommunications Union, 2003), a global distribution of the surface refractive anomalies (DN) was made, indicating three distinct areas of summer surface anaprops: the West Coast of North America, the Mediterranean area and the Persian Gulf area (Fig. 3). However, these results were provided on a global scale, which is not sufficiently precise for their application to the Adriatic Sea area. Furthermore, the recommendation of the ITU (International Telecommunications Union, 2003) focused mostly
Fig. 5. (a) Radio sounding data for Zadar (44.101N, 15.351E). Every circle represents one record of four variables: altitude, air temperature, total air pressure and relative humidity, which enables the computation of M according to Eq. (4). The standard levels are shown as dense records that appear as irregular lines because of the altitude variations in the standard pressure levels (e.g., 1000 hPa, 850 hPa, 500 hPa). Significant levels occur between standard levels, where large gradients in all measurements were recorded. (b) The vertical profile of the air temperature (1C) at Zadar. The reason for the poor coverage of stratospheric altitudes at Zadar in the range of 10 km–30 km was a limitation in the temperature sensors, which record a minimum temperature of 40 1C for many measurements.
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on surface refractive conditions, reflecting their interest in terrestrial microwave communications. Another study focused on the Iberian Peninsula over the coasts of Catalonia (Bech et al., 2002). This study showed that the anaprop conditions at low altitudes significantly affect meteorological radar outputs. Their findings about annual cycles of the number of anaprop occurrences raised a question about the similarity of the refractive conditions between the Catalonian and Adriatic coasts. The authors also used a two-year dataset, which is still not sufficient enough to reveal certain average characteristics, and their analysis covered only the lowermost 1000 m of the atmosphere. Because the Adriatic Sea is strongly influenced by the Mediterranean Sea and because there is no detailed climatological study of the refractive conditions in this area, the main goal of this study is an objective statistical description of the refractive conditions over the Adriatic Sea. Aerological balloon probing data were obtained for a 15-year period from 1994 to 2009, allowing us to explore the radio wave refractivity pattern along the Adriatic coast. Furthermore, these data and their analysis will expand our existing knowledge concerning refractive index profiles of up to 40 km for the Adriatic area, thus supplementing long-term research of the surface refractive index by the International Telecommunications Union (2003). These findings will also aid in the optimal construction of a dynamic model of the atmosphere, which will provide a detailed spatial representation of the phenomena involved and could even provide a shortterm anaprop forecast (Bech et al., 2004).
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The sources of data for air pressure and temperature were regular aerological balloon probings of four stations around the Adriatic Sea: Udine (WMO]16044), San Pietro Capofiume (WMO]16144), Zadar (WMO]14430) and Brindisi (WMO]16320). While Brindisi (10 m above sea level, a.s.l.) is a coastal station, the other stations, Zadar (80 m a.s.l.), San Pietro Capofiume (38 m a.s.l.) and Udine (94 m a.s.l.), are located inland at distances of 10 km–40 km away from the coast. The data series covered a period of up to 15 years (Table 1) with a temporal resolution of 12 h (Zadar and San Pietro) or 6 h (Udine and Brindisi). The data were collected on the NOAA site ‘‘ESRL Radiosonde Database Access’’ (http://www.esrl.noaa.gov/ raobs/). This dataset provides records of all required variables: altitude, pressure, temperature and moisture. The radio probe is also strongly influenced by wind, especially at higher altitudes, so standard probe positioning includes radio-theodolite (for wind measurements) or uses a more sophisticated NAVAID, such as GPS, Loran-C or Alpha. The accuracy of the probe’s position depends on the actual positioning system used. According to the manufacturer’s data (http://www.vaisala.com), we assume an absolute error of less than 10 m for altitude measurements. Due to a change in the measurement methodology at the Zadar station beginning on March 15, 2006, we restricted the data to those collected after that date (Fig. 5a and Table 1). Zadar’s probes were also unique due to the limitations in their thermal sensors, which were set to a minimum temperature of 40 1C. For this reason, many stratospheric data points were lost in the altitude range of 10–30 km (Fig. 5b).
2.2. Climatological characteristics of the Adriatic region 2. Material and methods 2.1. Measurements The Adriatic Sea is a stretched basin in a NW–SE direction, located in the northern-central Mediterranean between the Apennine Peninsula and the Balkan Peninsula (Fig. 4 and Table 1).
Fig. 6. Annual cycle of (a) the mean monthly surface 2-m air temperature and (b) the surface relative humidity recorded at the Adriatic meteorological stations shown in Fig. 3 (Penzar et al., 2001).
Through the year, atmospheric conditions vary due to the influence of moving weather formations of alternating low and high air pressure systems. During the winter, early spring and late autumn seasons, stationary anticyclonic systems exchange with fast-moving cyclonic systems with typical winds: the northwesterly bora wind (more in e.g., Grisogono and Beluˇsic´, 2009) and the strong southerly sirocco wind. In summer, cyclonic activity is mostly localised northward, allowing sunny and dry weather and the development of local circulations (sea/land breeze and/or
Fig. 7. Tropospheric distributions of the refractive types computed by all tropospheric data, including surface measurements.
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slope winds) above the mostly mountainous coast (e.g., Prtenjak and Grisogono, 2007). The mean annual air temperature gradually rises southward, ranging from approximately 13 1C in the northern region to 16 1C in the middle Adriatic and up to 17–18 1C in the southern region (e.g., Penzar et al., 2001). The mean annual temperature field has two particularities: (i) two warmer regions above the middle and southern Adriatic over the deepest sea area and (ii) the eastern Adriatic coast is warmer than the western coast due to sea currents. Along the eastern Adriatic, the warmest month is July, and the coldest months are January and February (Fig. 6a). Above the Adriatic, air temperatures and the frequent bora wind significantly affect the average annual values of relative humidity, which are decreased southward and eastward (Fig. 6b). Annually, the relative air humidity course has an irregular shape, usually experiencing two maxima and two minima. The primary maximum occurs at the end of autumn or during the winter season, and the secondary maximum occurs in May. The notably pronounced primary minimum is in July and, occasionally, a secondary minimum occurs in February.
3. Results and discussion Because the main task was to determine the statistical characteristics of the anomalous refractive conditions above the Adriatic area, particularly the altitude range where anomalous propagations frequently occur, we estimated four different refractive conditions: standard refraction, subrefraction, superrefraction and radio ducts based on aerological station data. Fig. 7 shows the relative proportions of the anaprop phenomena for all four stations in the troposphere. These four stations present good series of tropospheric data readings, making direct comparisons possible. A recent study (Viher and Prtenjak, 2012) also showed that anomalous propagation conditions in the stratosphere are rare (generally subrefractions) and that the vertical gradient of the refractive index is linear with a gradient of 152 km 1 up to an altitude of 40 km, meaning that stratospheric data could be excluded. With regard to each type of anomalous refractive condition, superrefractions are the most frequent anaprop formations followed by subrefractions and radio ducts. According to the results in Fig. 7, superrefractions are phenomena which occur more often along the western Adriatic coast. Meteorological conditions that are favourable for radio ducts are similar to those that are favourable for superrefractions, but extreme gradients of temperature and humidity are required for radio duct formation.
Therefore, radio ducts are less common than superrefractions, and their frequency decreases northward. Fig. 8 shows the differences that are caused by daily variations in the refractive index. Because the San Pietro and Zadar cites have only two probing terms per day, only the corresponding noon and nocturnal terms were compared for all four stations. Terms are timed to cover rare cases of probe launching outside of the regular schedule (00 UTC and 12 UTC) but no more than 3 h earlier or later. A relatively high percentage of daily anaprop over Zadar and a relatively low percentage over Udine are observed, which could be due to differences in the station locations with respect to the coastline and typical local meteorological pattern. San Pietro and Brindisi have similar percentages for nocturnal events. The overall anaprop occurrence is similar to that found over the Catalonian coast (Bech et al., 2002) and the Alpine and sub-Alpine region (Viher, 2006), which quantitatively confirms the ITU prediction for the Adriatic region as a region with significant number of anaprop phenomena (Fig. 3). On a daily scale, nocturnal anaprop occurrences are more frequent than daytime occurrences for all stations. However, the difference in the number of anaprop occurrences on a daily scale is smaller if the location is (1) closer to the sea (e.g., Brindisi) and (2) more strongly influenced by mountainous terrain (e.g., Udine). Above Zadar and San Pietro, the daytime anaprop frequency decrease is approximately 30–50% for the overall anaprop cases. In Udine, the decrease is 25%, while in Brindisi, a decrease of only 10% is found. Nevertheless, the possible influence of daily thermally generated circulations, stratification and convection in the lower half of the troposphere on the anaprop frequency or its daytime decrease cannot be determined satisfactorily by soundings. Further research based on 3D meteorological models at a high resolution could offer additional insight into anaprop characteristics over the Adriatic. In Fig. 9, the vertical distributions of all anaprop conditions for all four Adriatic aerological stations soundings are shown. The percentage of anomalous phenomena was computed by dividing the height into steps of 100 m for each station. These figures revealed that anaprop conditions are unequally distributed with altitude, with the largest number of occurrences being observed near the surface due to their direct dependence on intense fluxes of heat and moisture. As mentioned previously, for anaprop phenomena in general, the largest proportions of radio ducts are recorded (i) near the surface, especially when the inversion layer is humid, and (ii) at approximately 1500 m due to the sudden temperature change across the top of the boundary layer. Significant numbers of elevated radio ducts are also observed in the
Fig. 8. Frequencies of the anomalous tropospheric phenomena that were detected during two standard probing terms, 00 UTC and 12 UTC for all four aerological stations along the Adriatic coast.
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Fig. 9. Vertical distributions of the anomalous propagations over four Adriatic aerological stations; (a) radio ducts, (b) superrefractions and (c) subrefractions. Percentages of anomalous propagations are computed for each station separately by dividing the altitude into 100-m steps.
lower third of the troposphere, above the boundary layer. This finding largely coincides with the findings of Haack and Burk (2001) along the coast of California. These researchers found that the average observed trapping layer thickness of ducts was approximately 100 m within the lowermost 500 m. In this study, the large number of ducts in the lowermost 400 m is presumably associated with the sea/land breeze formation, as was also
demonstrated in e.g., Atkinson and Zhu (2006). These researchers used two models, a meteorological MM5 numerical model and an AREPS propagation model, to produce radar coverage over a coastal area in the Persian Gulf for one chosen case. According to their results, the sea breeze trapped radio waves in the lowermost few hundred metres during a sea breeze penetration of more than 100 km inland.
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Fig. 10. Annual cycle of the monthly percentage of each anaprop formation, computed for each station separately and compared on the same graph: (a) radio ducts, (b) superrefractions and (c) subrefractions. The maximum of each anaprop phenomena during the summer months corresponds to the maximum surface air temperature (Fig. 5a) and minimum relative humidity (Fig. 5b) as recorded at the selected Adriatic stations.
Vertical proportion profiles in refractions
profiles of superrefractions (Fig. 9b) show a greater of elevated phenomena compared to the radio duct Fig. 9a. A significant number of elevated superis in the lower half of the troposphere. Favourable
meteorological conditions for subrefractions occur in cold, dry air when the temperature profile shows a sudden temperature rise. Both temperature and humidity drop with height; therefore, a relatively large proportion of elevated subrefractions are detected (Fig. 9c). However, in the stratosphere, the anaprop phenomena are almost exclusively subrefractions due to the following reasons: (i) dry and cold stratospheric air, (ii) temperature increase due to stratospheric dynamics and (iii) a stratospheric vertical gradient of the modified refractive index about 152 km 1 that represents a value that is close to the limiting value for subrefractions (Viher and Prtenjak, 2012). With regard to the annual variations in the overall anaprop phenomena, distinct annual cycles can be noted in both the number of phenomena and the proportion of elevated anaprop (Fig. 10). The percentages of all anaprop phenomena for all four Adriatic stations show minima during November to March, increases in the spring season, maxima during July–August and a subsequent decrease during the fall season. Generally, annual cycles in the percentages of anomalous phenomena (Fig. 10) exhibit good overlap for radio ducts and subrefractions. The annual cycle of superrefractions for Zadar shows a discrepancy due to the relatively low data quality. The Brindisi station has a month-long lag in the maxima of all anaprop phenomena relative to Zadar and San Pietro. The June–August maximum in the Adriatic anaprop percentage corresponds to a maximum of the near-surface air temperature T as recorded in numerous Adriatic stations (Fig. 6a) and a minimum in the surface relative humidity rh (Fig. 6b). Because the refractive index (Eq. (2)) is mostly dependent on rh and T and is approximately 104 times less dependent on p (shown by the partial derivation in Eq. (2) over p, T and rh), the minimum of rh during the summer months leaves changes in T as the main reason for the anomalous phenomena. Keeping in mind that the vertical distributions shown in Fig. 9 are not stationary, the monthly vertical distributions were computed for each station. Due to the irregular nature of the vertical distributions, the median and 95% percentage were chosen as representative parameters of the distributions. The results are shown in Fig. 11. The curve marked with ‘‘M’’ shows a median altitude for each month, and the curve marked ‘‘95%’’ is the altitude above the surface, where 95% of the particular phenomena were recorded. Annual changes in the radio duct average median tend to rise during the cold season (Fig. 11a), confirming the dominance of surface radio ducts, shown in Fig. 9a. The monthly average altitude of the boundary below which 95% of radio ducts occur (Fig. 11a) experience minima during March (in the range of 1717– 2325 m) for all stations, except Udine, which has a minimum in January (2494 m). The monthly average maximal altitude of the 95% occurrences for the radio ducts were recorded in July for Zadar and Udine (3867 m and 4197 m, respectively), while San Pietro has a maximum in September (3285 m), and Brindisi has an extended 95% boundary of radio ducts from August to September (3497–3489 m). Superrefractions are the most frequent anaprop, and they have a larger proportion of elevated phenomena (Fig. 11b) compared to radio ducts (Fig. 11a). Therefore, it is expected that both the median and 95% occurrences occur at higher altitudes compared to radio ducts. Annual changes in the average median for superrefractions have minimal altitudes during February, and in the case of Brindisi, this minimum extends from January to March. Fig. 11b shows the annual change in the vertical distribution of superrefractions. The minimal monthly average altitudes are in the range of 2731 m (San Pietro) to 3270 m (Brindisi). In addition, the mean altitudes of the 95% occurrences are higher for superrefractions than for radio ducts, indicating more elevated superrefractions than elevated radio ducts. For example, the mean
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Fig. 11. Annual variations in the vertical profiles for anomalous propagation phenomena, represented by the average monthly median (M) and average monthly altitude of the 95% probability of the phenomenon: (a) radio ducts, (b) superrefractions and (c) subrefractions.
heights of the 95% occurrences range from 4537 m (Brindisi) to 5339 m (Zadar). Maximal values were recorded in August over Zadar and Udine, while San Pietro recorded a wider maximum range from July to August, and Brindisi recorded the widest maximum, extending from June to September. Subrefractions are the anaprop phenomena that have the greatest proportion of elevated occurrences (Fig. 11c). It is expected that both the mean monthly averages of the median and 95% occurrences have the greatest values compared to the other two anaprop phenomena. Minimal average monthly median altitudes of the subrefractions were recorded in the range of 94 m (Zadar) to 1148 m (Brindisi). This minimal value of the median altitude was
recorded during June, while other three stations recorded minimal values during February and March (Fig. 11c). The annual cycle of the mean monthly altitude of the 95% occurrences is less welldefined for subrefractions than for superrefractions (Fig. 11c and b). Both minimal and maximal values show that subrefractions reach the greatest heights among all anaprop events. For example, the minimal values of the 95% occurrences range from 4670 m (San Pietro) to 5362 m (Brindisi). These minimal values were recorded from December to March. The maximal altitudes of the 95% occurrences for the subrefractions are in the range from 6356 m (Zadar)–6832 m (Udine). These maximal values were recorded during the summer months of July to September.
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4. Conclusions
Acknowledgements
In this study, the statistical characteristics of the refractive conditions along the Adriatic coast were determined for three different anomalous refractive conditions: subrefractions, superrefractions and radio ducts using aerological sounding data. The data were provided by four stations (one Croatian site and three Italian sites) along the Adriatic coast for a 15-year period (1994–2009). The data analysis showed that aerological balloon soundings represent a useful tool for calculating modified refractive index profiles. The main advantages of aerological soundings are significant level reports, which are often connected to anaprop phenomena. Thus, every anaprop phenomenon can be detected in a particular vertical profile. Because of the high numbers anaprop phenomena that are detected around the Adriatic Sea, we conclude that these phenomena cannot be ignored. Our results show that superrefractions are the most frequent anaprop phenomena followed by subrefractions and radio ducts. The greatest number of anaprop phenomena was recorded from June to September with the exception of the Zadar data. This finding could be explained by the relative humidity minimum in the air over the Adriatic Sea during this period. In relatively dry air, sudden changes in the temperature profile become the dominant driving factor for anaprop phenomena. According to the vertical distributions of the anaprop phenomena, a majority of cases were recorded near the surface or at low altitudes. With respect to elevated phenomena, subrefractions are the most frequent phenomena and reach up to 6800 m a.g.l. Superrefractions usually appear in the lower half of the troposphere, while elevated radio ducts are generally recorded in the lower third of the troposphere. These radio ducts are most likely caused in this level by the sea/land breeze flow, raising a question regarding the effect of the sea/land breeze on the radar coverage across the Adriatic coast. The results of this study could be a good starting point for the construction of a numerical model of the anaprop phenomena occurring over the Adriatic Sea. A long time series of vertical profiles is a prerequisite for better numerical model optimisation in both spatial and temporal domains. Such numerical models will provide insight into three-dimensional spatial distributions of anaprop phenomena, particularly due to the high vertical resolution. Moreover, with proper time resolution, the models could describe short-term phenomena that only exist for several hours and are not detected with the time resolution of typical balloon probings.
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