meteorological and synoptic aspects of the formation and evolution of the novorossiysk bora

Meteorological and Synoptic Aspects of the Formation

and Evolution of the Novorossiysk Bora

E. K. Semenov, N. N. Sokolikhina, and E. V. Sokolikhina

Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991 Russia

Received February 13, 2013

Abstract—The genetic and synoptic classifications of the Novorossiysk Bora are created using the dataof daily observations at the Novorossiysk meteorological station and other available synoptic informa-tion. Obtained are the quantitative criteria of these classifications, and on this base worked out are thebasic scenarios of the generation and evolution of this dangerous phenomenon on the Black Sea coast ofRussia. According to the genetic classification, the Bora was divided into four types: frontal, air-mass,monsoon, and gravity. Quantitative criteria worked out for each type can be used for the more accurateforecast of this destructive phenomenon near Novorossiysk. According to the synoptic classification,four classes were distinguished: Azores, North Atlantic, Siberian, and Arctic.

DOI: 10.3103/S1068373913100026

INTRODUCTION

Bora is a “strong and gusty wind directed down the mountain slope and causing the significant tempera-ture drop in winter. It is observed in the areas, where the not high mountain ridge borders upon the sea” [7].Descending to the water surface, this downward flow provokes the storm wind causing significant seawaves. All this is accompanied by the dramatic temperature drop and icing of ships.

Systematic observations of the Novorossiysk Bora started in the late 19th century. At the same time,some scientific papers made attempts to explain this phenomenon. The first paper was by F.F. Vrangel’whose suppositions were based on the data of the study of downward motions in the cloud from VaradaRidge during the Bora [2]. Another researcher of the Novorossiysk Bora was N.A. Korostelev who revealedan important regularity using the data from 1891 to 1900: the Bora is formed, “…when the air pressure insidethe country rises rapidly forming the barometric maximum and, thus, the dramatic pressure drop towardsthe sea is formed, where the favorable conditions are created for the generation of barometric minima” [4].He first made a conclusion that the Bora mainly depends on the large-scale circulation.

Theoretical aspects of the formation and evolution of the Novorossiysk Bora were considered by manyscientists of the Soviet period [1]. The significant contribution to studying the Novorossiysk Bora wasmade by the group of researchers headed by A.M. Gusev [5] which processed the archive data for the periodfrom 1891 to 1954, compiled the first calendar of the Novorossiysk Bora, and proposed its four regimes(types): frontal, air-mass, monsoon, and gravity. This dangerous weather phenomenon is studied nowadaysas well [3, 6, 8].

The main priority in studying the Novorossiysk Bora is its timely forecast. To issue such forecast, it isnecessary to construct correctly different variations of climatic and prognostic models which could predictthis hazardous phenomenon with a high degree of probability several days before its beginning. The tem-perature of the air, of the underlying surface, and of the Black Sea surface, the air pressure, the air humidity,and the direction and speed of the wind should be the key parameters for constructing such forecastingmodels.

The present paper deals with the comprehensive study of the conditions of formation and evolution ofthe Novorossiysk Bora and with the development of genetic and synoptic classifications. At first, quantita-tive criteria of distinguishing the Bora of different regimes were determined using the calendar of theNovorossiysk Bora [5]. Afterwards, on the base of these criteria, the dataset was formed; it contains majormeteorological variables for the cold period of the year (November–April) for two decades: from 1891 to1900 and from 1998 to 2007. These two decades were selected, because, according to the data of the Univer-sity of East Anglia (http://www.uea.ac.uk), the first one characterizes the period of climate cooling and the

ISSN 1068-3739, Russian Meteorology and Hydrology, 2013, Vol. 38, No. 10, pp. 661–668. � Allerton Press, Inc., 2013.

Original Russian Text � E.K. Semenov, N.N. Sokolikhina, E.V. Sokolikhina, 2013, published in Meteorologiya i Gidrologiya, 2013, No. 10, pp. 16–28.

661

second one, the period of climate warming. The analysis of the obtained information enabled to carry outthe genetic and synoptic classifications of the Novorossiysk Bora.

DATA AND RESEARCH METHODS

The data of ground-based daily observations at Novorossiysk meteorological station for the periodsfrom 1891 to 1900 (three observation times), from 1952 to 1954, and from 1998 to 2007 (eight observationtimes) on the air temperature, the surface pressure, relative humidity, dew-point temperature, the speed anddirection of the wind, amount of cloudiness, and precipitation were used for the study. All data were consid-ered for the cold period of the year (November–April).

To study the Novorossiysk Bora during the period from 1952 to 1954, the calendar of the Bora types wasused [5]. This calendar was compiled using the results of the comparison of horizontal pressure gradientscomputed from the pressure difference at a number of points located in the direction of the pressure gradient(Gelendzhik–Krymskaya, Krymskaya–Slavyanskaya, Slavyanskaya–Akhtarsk, and Akhtarsk–Eisk). As aresult of the comparison between this calendar and all meteorological information, the quantitative criterionwas worked out of distinguishing the Bora of different regimes (Table 1). Such criteria were the pressuretendency, the surface pressure, the air temperature, the wind speed, and precipitation.

Using the criteria and the results of the analysis of meteorological data, the calendar of the Bora typeswas compiled for the first time for two periods, the period of cooling (1891–1990) and the period of warming(1998–2007) of the climate. The frequency of different regimes, the maximum wind speed (Vmax), and thedeviations of temperature observed before the beginning of the phenomenon, from the minimum tempera-ture during the process of the Bora evolution (�T =T

0– Tmin) were also studied for the two selected decades.

To carry out the synoptic classification for the period of 1998–2007, the daily weather charts were used(http://www.wetterzentrale.de; of the surface synoptic analysis, of the surface constant-pressure levels su-perimposed on the level of 500 hPa, the surface temperature) as well as the NCEP/NCAR reanalysis dailydata on the geopotential at the level of 1000 hPa and the daily data of the radio sounding in Rostov-on-Donand Tuapse. As a result, the typical schemes of synoptic processes were singled out: Azores, North Atlantic,Siberian, and Arctic.

To estimate quantitatively the synoptic conditions of the Novorossiysk Bora formation, the charts ofdifferences in the values of geopotential H1000 on the whole selected territory and in Novorossiysk H1000N

were plotted using the reanalysis data for the period from 1998 to 2007 for the territory from 40� to 65� Nand from 25� to 60� E with the grid space of 2.5�:

�H H Hi j1000 1000 1000� �, ,N

where i, j are the coordinates of the grid point.

Such charts were plotted for all days, on which the Bora was observed as well as for two days and for aday before the beginning of the phenomenon development. Therefore, the proposed approach can be used topredict this hazardous phenomenon.

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662 SEMENOV et al.

Table 1. Quantitative criteria of the genetic types of the Bora

CriteriaGenetic type

frontal air-mass monsoon gravity

Pressure tendency,hPa

Pressure, hPaWind speed, m/sTemperature, �C

Precipitation

Pressure drop from –2 to–4 in front of the coldfront (CF)Intensive pressure risefrom 3 to 8 behind the CF995–101520–35–6...–15(dramatic drop)Moderate and heavy

Continuous gradual pres-sure rise from 1 to 6

1015–104015–252...–8(gradual drop)Not observed

Small pressure ten-dencies of both signsfrom 3 to –3

>102025–40–10...–20(very dramatic drop)Light

Small pressure risefrom 0.2 to 0.8

>102010–200...–5(gradual drop)

Not observed

GENETIC CLASSIFICATION

As it has been mentioned above, four regimes of the Bora evolution (frontal, air-mass, monsoon, andgravity) are proposed in [5]. Using the present-day meteorological information, the composite characteris-tics of separated regimes were obtained.

The flowing of the powerful stream over the ridge is typical of the first regime. It should be noted thatthe height of the ridge is small as compared with the vertical extent of the flow. Such regime is formed inthe process of the cold front passage over Novorossiysk or shortly after it and is accompanied by the tem-perature drop on the whole northeastern coast of the Caucasus. Therefore, this regime can be referred to thefrontal type.

The frontal type of the Bora is characterized by high values of the pressure tendency (to –4 hPa), lowpressure (not higher than 1015 hPa), dramatic temperature rise, and precipitation. The wind speed can reach20–35 m/s (Table 1). The frontal Bora is never localized near Novorossiysk only. It always occupies thesignificant part of the coast and can last up to 10 days. In the case of the rapid passage of the front, it lasts fortwo–three days but afterwards it can be transformed into the air-mass Bora.

At the second regime, the ridge is overflowed by the stream with the strength which is a bit larger thanthe height of the obstacle. In this case, there is no dramatic strengthening of the wind over the ridge, becausetemperature inversions are weakly pronounced or absent, wave processes are almost not registered, and thedifferences in the temperature regime between the Kuban Lowland and the coast are small. The continuousgradual pressure rise and the gradual temperature drop take place, the wind speed does not exceed 25 m/s,and no precipitation is observed (Table 1). This regime is referred to the air-mass type. Thus, the air-massBora does not reach the catastrophic force and continues from two to five days. It often precedes or, as it hasbeen already mentioned above, completes the frontal Bora.

The third regime is formed, as a rule, in transition seasons or in winter against a background of north-eastern storms over the Black and Azov seas. Then the catastrophic Bora of the monsoon type is developeddue to significant temperature contrasts between the cooled continent and the warm sea in the area ofAnapa–Gelendzhik. From time to time, the situation is aggravated as a result of the development of anticy-clones over the European part of Russia that come from the polar latitudes and bring the air masses of lowtemperature. If the position of anticyclones over the European part of Russia is stationary during severaldays, the temperature in the area of the anticyclone drops due to the cooling and due to the continuing in-flow of cold air masses from the northern regions. The horizontal temperature gradient in the area of thenorthern coast of the Black Sea increases, the dramatic temperature drop (–10…–20�C) takes place, and thewind speed can reach 40 m/s (Table 1). The duration of the monsoon Bora can amount from three to elevendays, i.e., such type of the Bora is the most durable.

The fourth regime is never of catastrophic nature. The wind speed in the area of Novorossiysk is nothigher than 20 m/s. Such Bora is mainly observed at night and in the morning during the cold period of theyear. It is formed if the night-time cooling forms the significant air temperature contrast to the northeast ofthe mountains and along the Caucasian coast of the Black Sea. When the layer of the cooled surface air be-comes strong enough, it starts to flow over the mountains and then flow down the mountain slopes. Underthe impact of gravity forces, the speed of the descending flow grows and reaches the storm values near themountain foot. This regime is referred to the gravity type. In the case of such Bora, low gradients of temper-ature and pressure are observed everywhere except the area over the narrow coastal zone. The duration ofthe gravity Bora is several hours.

After that, using the worked-out quantitative criteria (Table 1) and the available meteorological data, thecalendar of the Novorossiysk Bora according to the genetic type was compiled for two decades under study,and the frequency of each type was computed (Table 2).

It is clear from the data of Table 2 that the number of the Bora cases during the period of cooling(1891–1900) is larger by 20% than during the period of warming; the air temperature drop in the case ofcooling was more intensive than in the case of warming (1998–2007); the wind speed in the case of coolingwas maximal. Besides, the frontal type of the Bora was observed more often (73%) in the period of cooling,whereas the number of cases with the air-mass Bora increased during the warm period (up to 42%).Thisoccurred due to the fact that in November the number of the Bora cases increased (up to 20% of the totalnumber of cases), and Novembers were also abnormally warm against a background of global warming.So, November became a kind of the continuation of the warm period of the year that caused formation ofthe large number of cases of the air-mass Bora.

Besides, the year distribution during the first cold period is rather even, it amounts to about 12–13 casesof the Novorossiysk Bora per year from November to April. During the second period, the maximum num-


METEOROLOGICAL AND SYNOPTIC ASPECTS OF THE FORMATION AND EVOLUTION 663

ber of Bora cases was observed in 2002–2003 (20 cases); at the beginning of the period (1998–1999), thisnumber was equal to only six during the cold period. Considering the monthly frequency of the Bora,maximum values for both decades under consideration are observed in November–January (60% of thetotal number of cases).

Thus, against a background of the total climate cooling, the distribution of cases of the NovorossiyskBora during the cold period of the year is even, but this trend is not observed against a background of warm-ing, and the total number of Bora cases decreases.

SYNOPTIC CLASSIFICATION

The following typical schemes of synoptic processes which can result in the generation of the Novoros-siysk Bora were chosen for the synoptic classification: Azores, Siberian, North Atlantic, and Arctic.

The Azores class of the Bora development. Anticyclones (or their spurs) are stretched from west toeast, and the cyclonic activity moves to the north of Europe (Fig. 1a). These are relatively warm (for winter)air masses; however, if their stationary position over the European part of Russia lasts long, they are cooled,and cold masses flow in from the north. Due to this fact, besides the high horizontal pressure gradients, thetemperature gradients may arise and lead to the development of the Bora.

The data of the radio sounding obtained at Rostov-on-Don and Tuapse (Fig. 2a) enabling to observe thevariations of thermodynamic conditions of the air mass were used for the fuller and deeper analysis. InRostov-on-Don which was influenced by the Azores High on February 7, 2006, the stable stratification andthe whole “bunch” of subsidence inversions were observed. On the contrary, the unstable stratification istypical of Tuapse due to the flowing of the air mass over the ridge (Fig. 2b). This is explained by the fact thatthe dry cold anticyclonic air flows over the warmer and more humid air that intensifies the air massturbulization.

The North Atlantic class of the Bora development. Anticyclones that form the Bora of North Atlanticclass and the Bora of the Azores class come from the Atlantic Ocean. Their difference consists in the factthat North Atlantic anticyclones are the completing link in the cyclonic series of Atlantic cyclones (Fig. 1b).

The Siberian class of the Bora development. The Siberian anticyclone is stationary in winter. Unlikethe spur of the Azores anticyclone, the spur of the Siberian anticyclone stretches towards Novorossiyskfrom the eastern part of Russia (Fig. 1c) and it is stronger and colder. Due to its stationary nature, it canresult in catastrophic effects of the Novorossiysk Bora, for example, as on February 1–13, 2001.

In Figs. 2c and 2d, an example is given of synchronous radio sounding on January 7, 2006. In Rostov-on-Don which was influenced by the cold Siberian anticyclone, the subsidence inversion was observed aswell as in the case of the Azores anticyclone. In Tuapse, unstable stratification was formed.


664 SEMENOV et al.

Table 2. Frequency of the genetic types of the Bora during the periods of cooling and warming of the climate

Type of the Bora Number of cases Frequency, % �T T T� �0 min , �C Vmax, m/s

1891–1900

FrontalAir-massMonsoonGravity

7530124

632683

10.48.9

16.16.1

23.822.333.318.8

1998–2007

FrontalAir-massMonsoonGravity

35431211

35421211

8.08.8

16.54.7

20.918.931.212.3

Note: Number of cases of the Bora development is 121 for the period from 1891 to 1900 and 101 for the period from1998 to 2007. �T = 10.4�C andVmax = 24.6 m/s for 1891–1900; �T = 9.5�C andVmax = 20.8 m/s for 1998–2007.

The Arctic class of the Bora development. During the period from 1998 to 2007, the Arctic anti-cyclones had two main trajectories:

—the very intense inflow of the cold air from Greenland took place over Scandinavia and resulted in theformation of the Arctic anticyclone and in its further movement to the European part of Russia;

—the already formed Arctic anticyclone moved rapidly from the Kara Sea to the European part ofRussia and weakened quickly (Fig. 1d).

Let us consider the frequency of different classes of the Bora in different years and months from 1998 to2007. The monthly course for any month of the cold period of the year is rather smooth, no clear regularitieswere revealed. However, the authors succeeded to distinguish three periods. From 1998 to 2001, the Azores



Fig. 1. Typical cases of synoptic processes resulting in the formation of the Novorossiysk Bora. (a) Azores class; (b) North

Atlantic class; (c) Siberian class; (d) Arctic class.

Fig. 2. Synchronous data of radio sounding in (a, c) Rostov-on-Don and (b, d) Tuapse in the case of (a, b) Azores and (c, d) Si-

berian classes of the Bora formation.

class prevailed, which favors the development of the frontal and air-mass Bora. In 2002–2003, the cases ofthe Bora of Siberian and Arctic classes prevailed. During this period, the maximum number of the Boracases during the whole decade was also observed. It should be noted that the cold period of 2002–2003 wasthe coldest (the mean temperature drop in the Northern Hemisphere was observed during the whole decadeunder study). In other years, the total number of the Bora cases was rather small and no dominating classeswere revealed.

To estimate the synoptic conditions of the formation and forecasting of the Novorossiysk Bora duringthe period from 1998 to 2007, the charts were plotted of the difference in the values of the geopotentialH1000 between the whole selected territory and Novorossiysk (�H1000) for the territory from 40� to 65� N andfrom 15� to 60� E. These charts were plotted for two days and for one day before the beginning and on thefirst day of the Bora development.

Two days before the development of the Bora of the Azores class against a background of the low-gra-dient field of high pressure an anticyclone which afterwards caused the Bora had not been seen over Novo-rossiysk yet (Figs. 3a, 3c, and 3e). One day before the beginning of the Bora development, it could be seenthat its center was located over Germany. At that time, the maximum positive values of �H1000 (+140 gpm)were observed over Novorossiysk. At the beginning of the Bora evolution, the center of the anticyclonemoved towards Belarus and Ukraine. In the area of Novorossiysk, �H1000 was equal to +100 gpm.


666 SEMENOV et al.

Fig. 3. Distribution of the difference in the values of geopotential �H1000 (gpm) between the whole selected territory and

Novorossiysk for (a, c, e) the Azores and (b, d, f) North Atlantic classes for (a, b) two days and (c, d) for one day before the

beginning, and (e, f) on the first day of the Bora development.

The North Atlantic anticyclones are notable for their rather rapid movement and are often not observedon the chart at two days before the Bora development (Figs. 3b, 3d, and 3f) either. Moreover, the values of�H1000 at that time were negative over Western Europe and close to zero over Novorossiysk. The center ofthe anticyclone was already situated in the area of Denmark and Germany a day before the beginning of theBora development and over Poland and Ukraine on the day of the beginning of the Bora development. Thevalues of �H1000 in Novorossiysk increased up to +130 gpm (Fig. 3c).

Another situation is typical of the stationary Siberian anticyclone which can be located over the sameterritory and can be observed long enough within the limits of the chart. However, the increase in theabsolute values of �H1000 in the area of Novorossiysk was observed two days before the beginning of theBora development, and these values were negative (–120 gpm) (Figs. 4a, 4c, and 4e). A day before thebeginning of the Bora development, they reached maximal absolute values (–200 gpm).

Unlike the Siberian anticyclone, the Arctic anticyclone is mobile and nonstationary. As clear fromFigs. 4b, 4d, and 4f, two days before the beginning of the Bora development, the chart shows the saddlewith the center of the Arctic anticyclone over the Kara Sea and the ridge directed towards Karelia and withthe center of the Azores anticyclone over the Caspian Sea and the ridge over Novorossiysk. Unlike in the caseof the Azores class of the Bora development, it is easier to issue the timely forecast in this situation becausethe Arctic anticyclone is much stronger than that Azores one. A day before the beginning of the Boradevelopment over Novorossiysk, the increase was observed in absolute but negative values of �H1000

(–100 gpm). This is associated with the fact that the center of the Arctic anticyclone moved to the south, to



Fig. 4. The same as in Fig. 3 for (a, c, e) the Siberian and (b, d, f) Arctic classes of the Novorossiysk Bora development.

Arkhangelsk oblast, and that of the Azores anticyclone started weakening. The Bora begins to develop,when the Arctic anticyclone moves to the center of the European part of Russia.

The comparison between the synoptic and genetic classifications for the period from 1998 to 2007 dem-onstrated that the frontal type of the Bora passed against a background of synoptic processes of the Azoresclass in 38% of cases, of the North Atlantic class, in 31% of cases, and of the Siberian class, in 28% ofcases; the air-mass type passed against a background of synoptic processes of Azores and Siberian classesin 35 and 39% of cases, respectively; the monsoon type developed in the case of synoptic processes of Arc-tic class in 67% of cases and the gravity type of the Bora in the case of Azores and North Atlantic classes, in46 and 35% of cases, respectively.

CONCLUSIONS

The statistical processing and systematization of the whole available information enabled to obtain thequantitative criteria for the genetic classification of the Novorossiysk Bora and to compile using this infor-mation the calendar of genetic types of the Bora for two periods of time, namely, for the cold period from1891 to 1900 and for the warm period from 1998 to 2007.

It was revealed that the Bora in the area of Novorossiysk during the period of cooling was observedmuch more frequently as compared with the period of warming, the wind speed was considerably higher,and the temperature drop was more dramatic. It is referred to the greater degree to the frontal Bora and tothe smaller degree, to other genetic types.

The analysis of atmospheric circulation conditions during the periods of the Novorossiysk Bora evolu-tion demonstrated that the development of this phenomenon can be observed under different types of thelarge-scale circulation with participation of main types of air masses: the tropical air of Mediterranean ori-gin (the Azores class); the Atlantic moderate air (the North Atlantic class); the continental moderate air (theSiberian class); the marine Arctic air of the Norwegian and Barents seas (the Arctic class).

Typical schemes of synoptic processes and prognostic charts of differences in H1000 between the areas ofEastern Europe and Novorossiysk for two days and for one day before the beginning and on the first day ofthe Bora development were obtained for each class. It was revealed that the most difficult conditions of theforecast are associated with the coming of mobile Arctic anticyclones to the European part of Russia andthe simplest ones, with the stationary Siberian anticyclones.

The analysis of the radio sounding data demonstrated that the strongly pronounced stable stratification inthe middle troposphere prevailed in all types of air masses before their flowing over the Caucasian Ridge.After the flowing over the ridge, the stratification in the area of Tuapse over the relatively warm sea becameunstable.

The comparison between the synoptic and genetic classification demonstrated that the frontal type of theBora passed against a background of synoptic processes of the Azores class in 38% of cases, of the NorthAtlantic class, in 31% of cases, and of the Siberian class, in 28% of cases; the air-mass type passed against abackground of synoptic processes of Azores and Siberian classes in 35 and 39% of cases, respectively; themonsoon type developed in the case of synoptic processes of the Arctic class in 67% of cases and the grav-ity type of the Bora in the case of Azores and North Atlantic classes, in 46 and 35% of cases, respectively.

REFERENCES

1. E. A. Burman, Local Winds (Gidrometeoizdat, Leningrad, 1969) [in Russian].

2. F. F. Vrangel’, The Novorossiysk Bora and Its Theory (St. Petersburg, 1875) [in Russian].

3. A. Yu. Ivanov, “The Novorossiysk Bora: a View from Space,” Issledovanie Zemli iz Kosmosa, No. 2 (2008) [inRussian].

4. N. A. Korostelev, “The Novorossiysk Bora,” Trans. Imperial Academy of Sciences, No. 2, 15 (1904) [in Russian].

5. The Novorossiysk Bora, Ed. by A. M. Gusev (Moscow Hydrophysical Inst., Moscow, 1959), Vol. 14 [in Russian].

6. S. Ya. Seregin, E. A. Yaili, S. N. Tsai, and I. A. Potekhina, Climate and Nature Management of KrasnodarBlack-Sea Area (RGGMU, St. Petersburg, 2001) [in Russian].

7. S. P. Khromov and L. I. Mamontova, Meteorological Dictionary (Gidrometeoizdat, Leningrad, 1963) [in Russian].

8. W. Alpers, A. Yu. Ivanov, and K.-F. Dagestad, “Observation of Local Wind Fields and Cyclonic AtmosphericEddies over the Eastern Black Sea Using Envisat Synthetic Aperture Radar Images,” Issledovanie Zemli iz Kos-mosa, No. 5 (2010) [in Russian].


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meteorological and synoptic aspects of the formation and evolution of the novorossiysk bora

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