Prediction of the clear air turbulence over western Iran (Tehran–Ahwaz and Tehran–Ardebil) using the WRF model simulations

The clear air turbulence (CAT) is one of the atmospheric phenomena that can endanger the flights, and may creat restrictions in the flight surveillance. CAT can be defined as all turbulences in the free atmosphere (about >10000m AGL) of interest in aerospace operations that is not in or adjacent to visible convective activity. As Iran is located in the gate of southwest Asia and Eastern Europe, investigation of the occurrence of the CAT in Iran is essential. To this end, the CAT indices such as Richardson number, vertical wind shear and Dutton's index are calculated via post processing of outputs of the WRF model. Using the Global Forecast System (GFS) data as input to the Weather Research and Forecasting (WRF) model, prediction of the CAT is provided for a 24-hour lead time. The GFS data with 0.5 degree horizontal resolution are used as the initial and boundary conditions for running the WRF model, while reports of pilots and aviation maps are used to evaluate the model performance. The WRF model was run with a three nested domains with horizontal resolutions of 18, 6 and 2 km, respectively. The CAT is diagnosed for two flight routes over Iran area: Tehran to Ardabil on 9 March 2018 and Ahwaz to Tehran on 24 April 2018. Both of routes are embedded in the Alborz and Zagros mountains. Results indicate that the CAT can be better predicted using the Dutton when the YSU planetary boundary layer scheme, the KF Cumulus schemes and Lin and Kessler microphysics schemes are used in WRF model setup. Prediction of the CAT based on wind shear index is better achieved when the MYJ planetary boundary layer scheme, the KF Cumulus scheme and the WSM3 are employed in WRF model. Based on the Richardson index, the CAT is better predicted using the MYJ and YSU planetary boundary layer schemes, the KF cumulus scheme and the WSM3 and Lin microphysics schemes. Based on the results of the evaluations, for the horizontal resolution of 18 km, the best indices for the weak to moderate CAT are Richardson number index, vertical wind shear index and Dutton's index for the Tehran to Ardabil flight route, and Richardson number index, Dutton's index and vertical wind shear index for the Ahwaz to Tehran flight route. The CAT in these routes is accompanied with the upper-tropospheric (200 to 300 hPa) jet streams and troughs and ridges.