Study of the Lee waves formation over Zagros Mountain and its influences on Clear Air Turbulence (CAT)

Clear Air Turbulence (CAT) refers to a micro scale turbulence which normally happens in upper troposphere and lower stratosphere when there is neither cloud in the sky nor any significant convective activities. The turbulence intensity is from low to sever and due to invisibility; it can cause irreparable damage to passengers in flights.
Many factors are effective in formation of clear air turbulence, including: wind shear, the waves, the tropopause, jet streams, the high level fronts, perturbations and breaking of the gravity waves caused by obstacles (such as Lee waves) or gravity waves caused by convection.
As stated, perturbations of micro-scale to meso-scale Lee waves are among the main factors in development of CAT. According to many studies which were done in mountainous areas throughout the world, mountain waves may form when any of these specific meteorological conditions happens: 1-The wind blows to the peak of the mountain in direction of 30° to perpendicular line; 2- The wind speed for the high mountains and hills is more than 30 and 15 knots, respectively; and 3- The stability around the peak is much greater than other levels of atmosphere. These conditions are used as some techniques for prediction of clear air turbulence associated with the mountain waves.
To the best of our knowledge, few studies on occurrence of clear air turbulence in Iran have been done. Hence, according to the necessity of knowing much more about this phenomenon in Iran and considering the influences of mountain waves on flights over country, we have conducted this study for Zagros Mountains. The position of Zagros Mountains is a north-west to south-east over west of Iran. Prevailing winds in this area are from the west and south-west. Therefore, the formation of mountain waves and the chance of occurrence Clear Air Turbulence is favourable in this area. In this paper, the Dena peak as the highest peak in Zagros Mountains is considered as the study area.
According to the mentioned meteorological conditions of the Lee wave formation, the days with Lee waves over Zagros Mountain is estimated for a period of 3 years from 2010 to 2012 using the actual maps and SKEW-T diagrams. The formation of Lee waves in the studied days is double checked by considering the Scorer Parameter and dimensionless Froude number. Furthermore, in order to indicate the range of wave formation, horizontal divergence is calculated and plotted using the WRF model output. Finally, the gradient Richardson number is calculated as an index for the CAT occurrence. Based on these results, the vertical momentum flux of Lee wave in the ridge axis is obtained typically in the range of 0.1-7.3 . In addition, the turbulence caused by the presence of the mountain waves is well indicated by Richardson number. The probability of the mountain wave formation in the Dena peak region during the 2010-2012 is higher in winter. We have also shown that the probability of turbulence occurrence with “moderate to severe” intensity in both 00 and 12UTC can happen in 550, 600, 650, 700hPa levels. Furthermore, turbulence with the same intensity occurred mostly at 12UTC in 650hPa level which is equal to just above the height of the Dena peak.