عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The purpose of this paper was to study the dynamical and thermodynamical structures of the Siberian high pressure (SH) and some of the effective parameters in its development. The data used were from the National Centers for EnvironmentalPrediction–NationalCenter for Atmospheric Research (NCEP–NCAR) reanalysis winter time data for a 60-year period (1948-2008). To identify the most significant feature points using the Siberian High Index (SHI) the 25 strongest cases were selected from the period of the study. The range of the fields investigated included the mean sea level pressure, the lower- and upper-tropospheric geopotential heights, wind, temperature, and the potential vorticity (PV), as well as the pressure field on the tropopause surface (PV = 2 PVU) and the wave activity vector.
The results showed that in the sea level pressure field, the Siberian high pressure has been strengthening around its climatological position at the developing stage until the peak time. After that the high pressure has started to extend and its central cell has been divided into two distinct cells with one moving southeastward into the Far East and consequently cold surge over there while the ridge of the other cell extends westerly toward Europe and the North East of Iran.
The composite maps of the anomalies suggest that the vertical structures of the SH are different in the downstream and upstream portions of the surface high. A noticeable feature was that the downstream portion of the SH exhibited a thermal structure, while its upstream portion showed a dynamical structure. In addition, although the SH was generally recognized as an anticyclonic circulation in the lower troposphere, the vertical structure of the wind anomalies indicated that there were cyclonic and anticyclonic circulations in the upper troposphere, respectively, in the downstream and upstream parts of the central area (40-65°N, 80-120°E) of the SH.
At the amplification stage of the SH, the appearance of negative pressure anomalies over the Mediterranean Sea implies that this stage can enhance favorable conditions for cyclogenesis over the Mediterranean Sea. This indicates that the SH could have some impacts on the meteorological fields outside its source area.
The other finding was that the SH may have profound effects on the meteorological fields in the middle- and upper-troposphere. Examples include the occurrence of a tropopause folding in the downstream side of the SH and the formation of a blocking ridge, as a part of a quasi-stationary external Rossby wave train, in the upstream side when the surface high is amplified.
The calculation of the horizontal component of the wave activity flux for the stationary Rossby wave revealed that the Rossby wave originated from the Euro-Atlantic sector and the blocking ridge was a component of this approaching wave. Also, during the development of the blocking ridge, the wave activity flux diverges from the negative height anomalies located at the upstream of the ridge and converges into the amplifying blocking ridge.
By evaluating each term of the horizontal temperature advection based on a composite field for the 850 hPa level, it was found that the advection of the basic state temperature by wind anomalies had an important role in developing a surface cold high throughout the amplification stage of the Siberian high pressure.
Finally, through a qualitative analysis, it was seen that the coupling between the negative PV anomalies at the surface due to the low-level cold anomalies and upper-level positive PV anomalies due to the tropopause folding lead to the amplification of the SH as well as the blocking ridge. The main conclusion was that the SH was not simply a local thermal system along with the restricted effects in the low-level troposphere.
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