Evaluation of the Non-divergence level height over Iran

In this study, the variation of daily and monthly non-divergent level in January and July in 1976-2015 long-term periods is investigated in Iran. The non-divergent level is assumed to be the lowest level between 700 and 300 hPa, in which the horizontal divergence is zero. To calculate the horizontal divergence at the standard pressure levels, the zonal and meridional wind components are extracted from the NCEP / NCAR archive. In Iran, nine points were selected in different regions, and the vertical profiles of divergence and Omega are analyzed. The daily survey of vertical profile of divergent in case study shows that, because low pressure (highs) is accompanied with convergence (divergence) in the lower levels and divergence (convergence) at high levels, at least one non-divergent level exists in the areas affected by these systems. The height of non-divergent level depends on the depth of the underlying convergence, so the deeper the underlying convergence or divergence, the height of the non-divergence level will be more elevated. Actually the non-divergent level of the heat lows )highs(
are seen in lower levels. The mean monthly height of non-divergent level in the statistical period indicate that it is lower in July in the northwest, west, southwest, center, south, east and south east of Iran. This condition is created by the dominance of the heat low and its shallow convergence in July. In the north and northeastern of Iran, the non-divergent level is lower in January than in July. These areas are less affected by heat lows in July. The study of the trend of changes in the mean monthly height of non-divergent level in the long-term period showed that there is an upward trend in western and north-eastern of Iran and downward trend in southern and central of Iran in July. This condition is indicating an increase convergence depth of the heat low-pressure in southern and central (western and north-eastern) of Iran.