Temporal Characteristics and Synoptic Conditions of Extreme Bora Formation in Novorossiysk

V. V. Efimov✉, O. I. Komarovskaya, T. M. Bayankina

Marine Hydrophysical Institute, Russian Academy of Sciences, Sevastopol, Russian Federation

✉ e-mail: vefim38@mail.ru

Abstract

Purpose. Bora in Novorossiysk is the strong, cold, gusting northeast wind directed to the sea on a ridge slope. As the dangerous phenomenon of weather it happens in winter at the wind speed more than 30 m/s, therefore, bora cases with maximum speed and minimum subzero temperature are of practical and all-physical interest.

Methods and Results. According to long-term daily data from the archives at the station Novorossiysk and observations from the Russian Hydrometeorological Institute (https://rp5.ru/) for the period 1901–2017 statistics on the number of days with bora for cold and warm periods and the number of bora cases with different duration of the day and their repeatability by month for the period 1901‒2017 are obtained. Modern regional numerical models developed in the Marine Hydrophysical Institute permit to take into account the role of orography in generation of the Novorossiysk bora, to define its localization and to study evolution of thermodynamic fields of a strong bora during its development and attenuation. The estimation of the repeatability of extreme wind speeds at true bora by the method of approximation of the cumulative function of the series by the Weibull distribution is given. In the analysis of sinoptic features of bora formation, the data of RegCM regional reanalysis for the period 1979–2013, as well as data of ERA Interim reanalysis at model levels were used. Roses of winds for points at windward and leeward slopes are constructed.

Conclusions. It was found that the average annual number of days with bora for the study time series is 30. The greatest duration of cases with bora falls on the winter months and can be up to 7 days. From the arrays of reanalysis data, the fields of pressure and wind speed corresponding to a strong bora are selected and classified by the cluster analysis method. Composites of pressure and wind fields are constructed. It is shown that the condition for the development of a strong bora is the formation of two pressure centers – the northern cyclonic and southern anticyclonic. Vertical profiles of temperature, wind speed and buoyancy frequency for 2 main clusters are presented. According to the measurements on the Novorossiysk HMS, the time characteristics of the bora were estimated. It is shown that the main features necessary for the formation of a true bora is an intense low – atmospheric air flow with velocities of about 10-15 m/c, accompanied by temperature inversion at an altitude of about 1 km and the formation of the underlying layer of stably stratified air.

Keywords

bora in Novorossiysk, repeatability, cluster analysis, Froude number, buoyancy frequency

Acknowledgements

The investigation is carried out within the framework of the state task on the theme No. 0827-2018-0001 “Fundamental studies of the interaction processes in the ocean-atmosphere system conditioning the regional spatial-temporal variability of natural environment and climate” (code “Ocean and atmosphere interaction”).

Original russian text

Original Russian Text © V. V. Efimov, O. I. Komarovskaya, T. M. Bayankina, 2019, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 35, Iss. 5, pp. 409–422 (2019)

For citation

Efimov, V.V., Komarovskaya, O.I. and Bayankina, T.M., 2019. Temporal Characteristics and Synoptic Conditions of Extreme Bora Formation in Novorossiysk. Physical Oceanography, 26(5), pp. 361-373. doi:10.22449/1573-160X-2019-5-361-373

DOI

10.22449/1573-160X-2019-5-361-373

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