Numerical simulation of a quasi-tropical cyclone over the Black Sea

The paper describes results of numerical experiments on the simulation of a mesoscale quasi-tropical cyclone, a rare event for the Black Sea, with the MM5 regional atmospheric circulation model. General characteristics of the cyclone and its evolution and physical formation mechanisms are discussed. The balances of the momentum components have been estimated, and sensitivity experiments have been performed. It is shown that, according to its main physical properties and energy supply mechanisms, the cyclone can be related to quasi-tropical cyclones.     

一次黄海爆发性气旋的观测分析和数值模拟研究

本文利用再分析资料和WRFV3.9模式（Weather Research and Forecasting Model）对2020年7月22－24日发生在黄海海域的一次爆发性气旋进行了研究,并对其演变过程和发展机制进行了详细分析。该气旋22日12UTC在山东南部生成,入海后开始爆发性发展,最大加深率达到1.2 Bergeron,23日在黄海中部气压降至最低990 hPa左右,24日在韩国登陆。高空强辐散、低层的暖舌结构、水汽输送和下垫面热通量的变化增强了大气斜压性,使其迅速发展。使用WRF模式对气旋进行模拟,涡度的诊断分析表明,大气低层强斜压性主要通过涡度方程的散度项对气旋的发展起作用,对流项在涡度发展旺盛的时刻也有一定影响。海温的敏感性试验表明,海温变化对气旋移动路径和中心气压影响明显。     

On numerical modelling of currents over the north-west shelf of the Black Sea

Water circulation in the north-west part of the Black Sea during the summer period is calculated by means of a complete non-linear thermohydrodynamic model. We have also studied the contribution of wind forcing, the thermohaline regime, the coastline configuration, and the bottom topography to the generation and transformation of a flow field. It has been shown that the thermohaline regime and wind forcing account for 30 and 70% of the total circulation, respectively. Variation in the nature of circulation at weak winds is due to thermohaline factors. Vertical motion plays a crucial role in theT, S- andH ₂ S-field formation.Translated by V. Puchkin.     

A numerical model of online forecasting Black Sea currents

A numerical three-dimensional nonlinear model of the hydrophysical fields of the Black Sea is presented. The properties of model discrete equations are described. The results of test experiments on the choice of model finite-difference approximations and parameters (as applied to the online forecasting of currents) are given. The results of prognostic calculations of the hydrophysical fields of the Black Sea are given for the period of March 31, 2005, to September 26, 2006. These results show that this numerical model with consideration for real atmospheric forcing can yield a satisfactory forecast of the parameters of the upper layers of the sea for 18 months of model time.     

Influence of vertical motions on maintaining the nitrate balance in the Black Sea based on numerical simulation

The upwelling of deep water associated with the influence of cyclonic wind curl and the difference in the buoyancy of the inflows in the lower and upper water layers is observed in the central part of the Black Sea. The resulting vertical water motions contribute to the transport of ammonium to the upper boundary of the anaerobic zone. In the suboxic zone, ammonium is converted to nitrate via nitrite as a result of the nitrification, and thus it can supply the nitrocline in the water basin. Within the framework of this paper we discuss the effectiveness of this mechanism on the basis of the numerical simulation. The calculations were performed using a one-dimensional physical-biogeochemical model for the upper 600-m sea water layer, which takes into account seasonal variations in atmospheric parameters and vertical motions. The model describes the biological and redox processes in the suboxic zone. We have estimated the contribution of different constituents into the balance of nitrogen compounds in the euphotic water layer. It is shown that ammonium nitrogen coming from the deep water due to vertical water motion plays a significant role in maintaining the balance of nitrates in the central part of the Black Sea.     

A three-dimensional model study on ocean dynamic response to traveling cyclone over the Huanghai Sea

On the basis of numerical simulation of the mean circulation and relevant thermal-salinity fields in June with a three-dimensional ocean model (ECOM-si), the model outputs are used as first guess of initial fields for numerical integration of the model equations and the numerical results are applied to investigating the dynamical responses of the Huanghai Sea and the East China Sea (HECS) in the course of a weak land-to-sea cyclone‘s passage over the Huanghai Sea on 15-16 June 1999. Predominance of the dynamic impact of cyclone over the thermal one in June in the HECS is justified using observations and model simulations.The cyclone and its surrounding weather system, i.e,, subtropical high ridge to its south could influence current and thermal fields in the Bohai Sea, the Huanghai Sea and the northern East China Sea even though the intensity of cyclone was rather weak. The response of oceanic currents to the wind stresses driven by the cyclone and its southern subtropical high were strongly characterized by the wind drift with its extent of equivalent scale of cyclone in the horizontal and of Ekman layer in the vertical. The sea response at a given site was closely related to the transient local wind speed and direction,especially was sensitive to the local wind direction,which is demonstrated at three points locating at the southern and western Huanghai Sea and the northern East China Sea. So the sea responses at locations differed considerably from one another. Current responded to the wind stress in a simple way:directly to the wind-driven current and subsequent gradient current and slope current, etc., whereas sea temperature responded to the wind stress in two ways: directly to the cyclone-induced cooling and indirectly to water movements both in the horizontal and the vertical by the cyclone‘ s wind stress. So the sea temperature variation under the influence of cyclone was more complicate than the current. The HECS in response to the cyclone and its ambient weather system was likely to be a fast process and such a response could last at least for more than 1d. Current increased with the duration of wind stress exerted on the surface and decreased with the increasing depth. Affected by the cyclone, the maximum sea surface temperature decreased by almost 1.6℃ during the 24h cyclone.     

Statistics of mesoscale cyclonic Eddies over the Black Sea

On the basis of the results obtained by using the PRECIS regional climate model with high space resolution (25 km), we study the mesoscale specific features of the atmospheric circulation in the Black-Sea region for a period of 30 yr. To separate and trace the subsynoptic eddies, we use a method based on the Okubo–Weiss criterion. Several types of cyclonic eddies are selected and described: winter Caucasian coastal eddies, summer Caucasian separated eddies, cyclonic eddies of the open sea, and seldom quasitropical cyclones. For various types of eddies, we present the statistics of their lifetime and intensity and the histograms of diurnal and seasonal cycles.     

A numerical experiment on modelling synoptic eddies in the Black Sea during the summer season

An energetically-balanced model has been used to perform an eddy-resolving experiment on the calculation of the baroclinic circulation in the Black Sea. As the initial and boundary conditions, historical data on the density and tangential wind stress for the summer season have been used. Following the integration of a set of model equations, an array of anticyclonic eddies formed on days 50–60 on the margins of the Black Sea rim current. Off the Caucasus, Crimean, and Bulgarian coast, anticyclonic eddies are shown to be quasi-stationary. The north-western Black Sea and Anatolia coastal zone are areas of Black Sea rim current instability, where synoptic eddies — both cyclonic and anticyclonic — are generated.Translated by Vladimir A. Puchkin.     

A numerical experiment on computations of the Black Sea density fields and current velocities during summer

A near-bottom pressure equation consistent with the original set of difference equations is derived in the framework of a numerical energetically-balanced model. The model is used to simulate numerically three-dimensional density and current velocity fields and in the Black Sea for a specific season. The climatic values of temperature, salinity, and tangential wind stress during summer are applied as the original conditions. Analysis of the hydrophysical fields at the final moment of integrating is performed. The difference between new data and data derived earlier is demonstrated.Translated by V. Puchkin.     

Seasonal fields of tangenital wind stress over the Black Sea

Tangential wind stress fields with a 40×60 spatial step are calculated from the data on atmospheric pressure distribution over the Black Sea over a decade. Their space-time variability is studied. It is shown that maximum values of the tangential wind stress in all seasons are located in the areas southwest of the Crimea and south of the Taman Peninsula. The tangenital wind stress retrieved from wind measurements is compared with the one determined from the atmospheric pressure data.Translated by V. Puchkin.     

Mixed layer in the Sea of Japan: numerical simulation and long-term data analysis

Seasonal variation and topography of the mixed layer in the Sea of Japan are studied by comparison of results from long-term observation data analysis and from numerical simulation with the MHI oceanic model (Shapiro. 1998. Marine Hydrophysical Journal, 6:26～40). The data are retrieved from Oceanographic Atlas of the Bering Sea, Okhotsk Sea, and Japan/East Sea (Rostov, Rostov, Dmitrieva, et al. 2003. Pacific Oceanography, 1 (1):70-72). The simulated and long-term patterns are compared. An impact of surface buoyancy flux, wind, and convergence/divergence of surface currents upon the mixed layer in the Sea of Japan is analyzed.     

Assimilation of altimetry data in the numerical model of the circulation in the Black Sea: The model numerical experiments

The numerical algorithm of the Kalman optimum filtration generalized for the case of three-dimensional fields and a multicomponent vector of the ocean state, with level measurements discrete over space and time being available, is given. The results of model numerical experiments on the assimilation of data on the Black Sea level are given. An attempt to estimate the effect of the time interval of data input on the results of field reconstruction was made.Translated by Mikhail M. Trufanov.     

渤海海冰季节演变的数值模拟

MM5数值产品作为大气强迫,利用冰-海洋耦合模式模拟2003～2004冬季渤海海冰演变过程。与海冰遥感、海洋站监测资料等实测资料相比,初冰日和终冰日和观测数据比较接近,模拟海冰各个发展阶段主要分布特征和MODIS遥感图像相似,但模拟的海冰厚度与辽东湾93平台、202平台的冰厚观测数据相比存在较大出入,说明该模型还有待改善。总体上看,冰-海洋耦合模式基本具备模拟渤海海冰季节演变过程的能力。     

Comparing satellite and meteorological data on wind velocity over the Black Sea

Wind-velocity data obtained from in situ measurements at the Golitsyno-4 marine stationary platform have been compared with QuikSCAT scatterometer data; NCEP, MERRA, and ERA-Interim global reanalyses and MM5 regional atmospheric reanalysis. In order to adjust wind velocity measured at a height of 37 m above the sea surface to a standard height of 10 m with stratification taken into account, the Monin–Obukhov theory and regional atmospheric reanalysis data are used. Data obtained with the QuikSCAT scatterometer most adequately describe the real variability of wind over the Black Sea. Errors in reanalysis data are not high either: the regression coefficient varies from 0.98 to 1.06, the rms deviation of the velocity amplitude varies from 1.90 to 2.24 m/s, and the rms deviation of the direction angle varies from 26° to 36°. Errors in determining the velocity and direction of wind depend on its amplitude: under weak winds (<3 m/s), the velocity of wind is overestimated and errors significantly increase in determining its direction; under strong winds (>12 m/s), its velocity is underestimated. The influence of these errors on both spatial and temporal estimates of the characteristics of wind over the Black Sea is briefly considered.     

Upwelling in the Black Sea by the results of numerical experiments and satellite data

The paper analyses results of the numerical simulation of upwelling events in the north-western part of the Black Sea, mostly near the South Crimea. The calculations were performed using a numerical model based on primitive hydrodynamics equations. Emphasis is laid on the case when a salinity front simulating the Black Sea rim current is prescribed in the initial conditions. The interaction of the Black Sea rim current's stream with the coastline and bottom topography leads to the development of an upwelling near the Crimea's coast, even in the absence of wind forcing. The paper discusses the structure of the three-dimensional circulation of waters in the shelf area of the NW Black Sea. Numerical modelling results are matched up with the satellite data obtained by the HRPT receiving station. Translated by Vladimir A. Puchkin.     

A numerical simulation of the distribution of water temperature and salinity in the Seto Inland Sea

Constant flows, as well as oscillatory tidal flow, play an important role in the long-term dispersion of water in the Seto Inland Sea. Two kinds of numerical model (1-line and 2-line models) of the Seto Inland Sea have been developed to determine the role of density-induced currents, one type of the constant flow, in water dispersion in the Inland Sea. The seasonal variations of temperature, salinity and density fields are simulated and the density-induced current field is predicted at the same time. It is found that the most appropriate value of the longitudinal eddy diffusion coefficient,K _x, is 5×10⁶–7×10⁶ cm²sec^–1. The value of the overall mean dispersion coefficient is of the order of 10⁷cm²sec^–1 (Hayami and Unoki, 1970). Consequently, it is suggested that 50–70% of the total dispersion in the Seto Inland Sea can be attributed to currents other than density-induced currents,i.e., tidal currents, tide-induced currents and wind-driven currents.In winter, both density and velocity fields, calculated using the 1-line model, satisfy the conditions for the existence of a coastal front in Kii Channel and in the eastern Iyo-nada.     

Statistical characteristics and the variability of currents over the western Black Sea shelf

Statistical characteristics and the parameters of the temporal variability of currents are analysed using data on current measurements conducted by five moored autonomous buoy stations over Bulgaria's shelf. Estimates of the specific contribution to the variability of synoptic (60%) and mesoscale (40%) oscillations are derived. The qualitative spectrum of the currents is defined, constituted by synoptic oscillations, short-period synoptic oscillations, inertial oscillations, and internal waves. Estimates of periods and oscillation amplitudes are also derived. The kinetic energy of the currents is calculated and the specific contribution of the energy of different-scale oscillations and their interrelations are determined.Translated by V. Puchkin.     

Numerical modeling of quasi-two-dimensional vortices in the atmosphere over the Black Sea

We consider quasi-two-dimensional rapidly dissipating mesoscale atmospheric vortices generated over the Black Sea near the Crimean and Caucasian coasts. Based on the results of numerical modeling for a characteristic example of the Crimean eddy, we determine its structure and parameters and estimate the rate of decay of kinetic energy and enstrophy. In addition to the large-scale secondary circulation in the vortex, we also consider a small-scale secondary circulation induced by Raleigh-Bénard convection.     

Observations on the Rim Current structure,CIW formation and transport in the western Black Sea

CTD and ADCP measurements together with a sequence of satellite images indicate pronounced current meandering and eddy activity in the western Black Sea during April 1993. The Rim Current is identified as a well-defined meandering jet stream confined over the steepest topographic slope and associated cyclonic–anticyclonic eddy pairs located on both its sides. It has a form of highly energetic and unstable flow system, which, as it propagates cyclonically along the periphery of the basin, is modified in character. It possesses a two-layer vertical structure with uniform upper layer speed in excess of 50 cm/s (maximum value ∼100 cm/s), followed by a relatively sharp change across the pycnocline (between 100 and 200 m) and the uniform sub-pycnocline currents of 20 cm/s (maximum value ∼40 cm/s) observed up to the depth of ∼350 dbar, being the approximate limit of ADCP measurements. The cross-stream velocity structure exhibits a narrow core region (∼30 km), flanked by a narrow zone of anticyclonic shear on its coastal side and a broader region of cyclonic shear on its offshore side. The northwestern shelf circulation is generally decoupled from the influence of the basinwide circulation and is characterized by much weaker currents, less than 10 cm/s. The southward coastal flow associated with the Danube and Dinepr Rivers is weak during the measurement period and is restricted to a very narrow coastal zone.The data suggest the presence of temperature-induced overturning prior to the measurements, and subsequent formation of the Cold Intermediate Water mass (CIW) within the Northwestern Shelf (NWS) and interior of the western basin. The newly formed shelf CIW is transported in part along the shelf by the coastal current system, and in part it flows downslope across the shelf and intrudes into the Rim Current convergence zone. A major part of the cold water mass, however, seems to be trapped within the northwestern shelf. The CIW mass, injected into the Rim Current zone from the shelf and the interior region, is then circulated around the basin.