A case study on low level vertical wind shear in Mehrabad Airport

Every two second Radio sound data of Mehrabad airport upper air station in Tehran for January and July 2010 is used in this research to investigate low level vertical wind shear (LLVWS) in the station. Results show in January non-convective LLVWS were caused by temperature inversion and front transitions. Vertical shear occurred in different layers and its weak vertical shear frequency is significant in this month. The highest shear values were observed on 4th and 23rd January at 0000UTC. Survey of the LLVWS on different days of the month show the phenomenon mainly caused by inversion and is greater in the layer 10 to 71m above the ground with different intensities. Frequency of moderate intensities in July is more than January. In this month 35% of cases had LLVWS, within those cases 22% are weak, 11% moderate and 2% are intense.
Introduction. Vertical wind shear is a complicated problem in aviation meteorology and tropical cyclone and has been on the center of attention of researchers. Based on the recommendation of WMO since 1960s; it was considered more drastically and publication No. 230 of WMO published in 1969 which is mainly devoted to results of investigations of world researchers. Among the researchers in the publication which investigated vertical wind shear in boundary layer are Kosano, Robert and Pettitt (1969). Later there have been many specialized studies on the subject which we can name Sinclier (1991), Arkell (2000). Sinklier et.al in a paper investigated wind shear in low level flights to reveal the phenomenon and for use in warnings. Arkell investigated types of wind shear and its forecast procedures from aeronautical meteorology view. The purpose of this study is to determine different layers LLVWS in the Mehrabad airport in July and January as a representative to warm and cold months to ease landing and take off operations.
Materials and methods. The Mehrabad airport station is located at the geographical position of 51.31° E and 35.69° N and height of 1191m above mean sea level. It is an aviation met station with twice daily soundings. To investigate wind shear, every 2 second wind data of the station at 0000 and 1200UTC is used. LLVWS is defined when the vertical wind speed difference is at least 5mps/100ft and is dominant from surface to 2000ft aloft[18],[11]. The surface wind speed is measured at 10m above the ground, so the height 0f 2030 is divided to 20 sub-layers of 100ft. Then, through routine meteorology procedures, data related to the base and top of the shallow layers is obtained. Typically the data is presented in Table 3. We computed vertical wind shear using the equation (1). The phenomenon is considered significant when.
Results and discussion.The data Survey shows non convective vertical wind shear in January results from frontal movements and inversions. During the month, the phenomena occurred in different layers above the station with higher frequencies of weak LLVWS cases. The maxima of the phenomena were at 0000UTC on 4th and 23rd of January. The data Survey in July show inversions are the main reason for vertical wind shear and it occurred with different intensities and more frequent within the 10 to71 m layer above the ground. The survey indicate within July 35% of cases were LLVWS, 63% of them of weak intensity, 31% moderate and 6% severe.
Conclusion. January and July 2010 wind profile data in a 2030ft layer divided to 100ft sub-layers show that atmospheric fronts in January and inversions in July are the main reason for of LLVWSin the Mehrabad meteorology station. The maximum cold season wind shear occurs at 0000UTC and warm season at 1200UTC. Since in warm seasons the dominant heat low on Iran diminishes at 0000UTC and gets its maxima in 1800UTC, thus intensified afternoon easterly winds dominates Iranian plateau and transfers desert warm air toward Mehrabad airport. This mechanism causes low level convection in Mehrabad in mid-day.