Influence of the Vertical Turbulent Exchange Parameterization on the Results of Reanalysis of the Black Sea Hydrophysical Fields

Abstract

Comparative analysis of two simulations of the Black Sea hydrophysical fields' long-term variability in 1993-2012 is performed. Two reanalyses are done using various methods of parametrizing the vertical turbulent exchange in the circulation model. The first variant implies a simple parameterization including the constant coefficient of vertical turbulent viscosity and the coefficient of vertical turbulent diffusion in a form of climatic profiles. The second simulation includes the turbulence model of the Mellor-Yamada type to parametrize the vertical exchange processes. Having been compared, the reanalysis results and the measurement data show that application of the turbulent model for parametrizing the vertical exchange yields a better description of the Black Sea water thermohaline structure. In particular, structure of the summer seasonal thermocline and location of the main halocline (in both of them maximum deviations of the model temperature and salinity from the in-situ measurement data are observed) are reproduced more accurately. Different methods of parametrizing the momentum vertical exchange influence the horizontal circulation vertical structure.The first variant of simulation shows that a significant part of the currents' kinetic energy is concentrated in the thin 10-m layer, and this fact affects the circulation features of shallow water on the northwestern shelf; whereas the simulation based on the turbulent model implies more intensive transfer of the kinetic energy to the lower layers. The carried out investigation shows that the results of the hydrophysical fields' reanalysis depend on accurate description of the thermodynamic processes in the sea upper layer and the vertical turbulent exchange ones, in particular.Acknowledgements: the investigation is carried out within the framework of the state task on the theme No. 0827-2018-0002 "Development of the methods of operational oceanology based on the inter-disciplinary studies of the marine environment formation and evolution processes, and mathematical modeling using the data of remote and direct measurements" (code "Operational oceanology") (numerical modeling with satellite data assimilation) under support of the RSF No.17-77-30001 "New methods and supercomputer technologies of the World Ocean and Arctic basin nowcast and forecast" (analysis of the obtained results).For citation: Sukhikh, L.I. and Dorofeyev, V.L., 2018. Influence of the vertical turbulent exchange parameterization on the results of reanalysis

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“…Note that in this work, for creating the most economical approach to assimilation of observations, we used Pacanowski -Philander formula adapted for the Black Sea. However, in the work of other authors [23] during the reanalysis of the Black Sea fields for 1993-2012 with SST assimilation in the MHI model using Mellor -Yamada turbulence model, the maximum value of temperature RMS deviation for winter turned out to be 0.6 ºС at 50 m depth and for summer 2.8 ºС at 20 m depth. A noticeable difference between the obtained RMS deviation values in our calculation, as noted above, is associated with the special conditions of the summer thermocline formation in 2012.…”

“…The performed study shows that the proposed methodology for assimilation of seawater temperature and salinity pseudo-measurements in the model using Pacanowski -Philander parameterization, supplemented by the satellite SST assimilation, provides an opportunity to satisfactorily describe the hydrological fields in the entire thickness of the Black Sea. In order to assess the efficiency of Pacanowski -Philander parameterization application on climatic scales, it is advisable to compare it with Mellor -Yamada model used in [23], calculated for 1993-2012. At the same time, one should consider the possibility of using seasondependent coefficients in the Pacanowski -Philander parameterization to describe better the interannual variability of the summer thermocline characteristics in the Black Sea.…”

“…However, even with such a solution, it is unlikely that it will be possible to avoid an abnormal increase in RMS deviation in particular years that differ from climatic fields. Another alternative is the use of less economic turbulence models of Mellor -Yamada type [23,24].…”

Modelling of Water Pollution Propagation in the Sevastopol Bay

“…According to the reanalysis data, the maximum value of the annual salinity RMSD is 0.42 ‰ at 75 m horizon. This is by 0.06 ‰ less than the maximum value of the averaged salinity RMSD over the entire reanalysis array for 1993-2012, performed with assimilation of altimetry data and sea-surface temperatures in the MHI model using the equations and turbulence relations from Mellor-Yamada family of models [28]. What is important is that square root of the dispersion of salinity fields measured by the Argo floats in 50-200 m layer significantly exceeds RMSD of the reconstructed fields of salinity pseudo-measurements and calculated in the reanalysis for 2012 (Fig.…”

“…Obviously, the best result can be achieved by assimilating satellite observations of the sea surface temperature. However, it should be noted that even with the assimilation of sea-surface temperature in the MHI model, the maximum value of averaged temperature RMSD for summer, according to the reanalysis from [28], is 3.4°C at  18 m depth, which is exclusively due to the aggravated seasonal thermocline in the Black Sea.…”