# Study of the Algorithm for Wind Shear Detection with Lidar Based on Shear Intensity Factor
## 1 Introduction  
Wind shear is defined as the change of the wind speed vector between two points in space along a certain direction. When the wind shear occurs below 1600 ft, it means that when the low-level wind shear occurs [1, 2], it can easily cause flight accidents [3, 4] due to changes in the dynamic performance of aircraft and an insufficient response time for the pilot. Therefore, low-level wind shear could seriously affect the safety of aircraft takeoff and landing. According to data from the National Transportation Safety Commission of the United States, low-level wind shear has been responsible for more than 80% of fatal air accidents in the United States since 1975 [5]. Therefore, low-level wind shear is also known as the \"aircraft killer\" in the aviation industry [6]. Currently, the number of airports with a Low-Level Wind Shear Alert System (LLWAS) in China is very low [7]. With the increasing number of airports and more frequent aviation activities in China, studies on low-level wind shear monitoring and early warning have important practical implications and application prospects. The technology of wind shear monitoring and early warning mainly involves two aspects: the accurate acquisition of wind field information and the detection algorithm for wind shear based on the obtained wind field data. To acquire wind field data, many devices have been developed and lidar has numerous advantages over traditional wind detection equipment. The data from lidar have a higher space-time resolution and can also be used for wind field detection in the clear sky. Lidar can also realize the high space-time resolution and high-precision detection for the slide of aircraft path, making up for the shortcomings of traditional wind measurement equipment. At this time, lidar has been widely used in flight meteorological support at large international airports abroad [8]. Therefore, this paper studies the improvement of the alerting algorithm for low-level windshear with lidar. At present, many alerting algorithms for low-level wind shear with lidar have been proposed to alert wind shear according to the energy change of aircraft in the wind field, including the F-factor algorithm [9] and the modified F-factor algorithm [10]. In addition, an algorithm to alert wind shear by calculating the eddy current dissipation rate has been put forward [11]. The least-square fitting algorithm [12] and 8-neighborhood algorithm [13] can also be used for wind shear monitoring. Hong Kong Observatory's [PERSON] and [PERSON] et al. proposed the slide path scanning (SP) method based on lidar and designed the ramp detection algorithm for wind shear [12; 14]. To find the maximum shear intensity and location, the algorithm extends and contracts the detection (ramp) length according to the changing wind speed trend. The algorithm has been successfully applied to the Hong Kong International Airport, with the hit rate for wind shear being 76%. However, this algorithm only uses the changing trend of wind speed to adjust the ramp length when determining the maximum shear position, but the wind shear intensity is affected by the changing trend of the wind speed and the ramp length itself. Therefore, adjusting the ramp length based on the changing wind speed trend cannot detect the value and position of maximum shear alone. [PERSON] and [PERSON] et al. improved the ramps detection algorithm [15] and F-factor algorithm [10], respectively. The conventional single ramp detection was improved to a double ramps' detection algorithm. The new algorithm not only calculates the shear value along the ramp, but also calculates the difference between the shear values on two adjacent single ramps. The final result is combined with the detection results of single and double ramps. The improved algorithm can effectively improve the accuracy of wind shear detection. However, ramp length extension and contraction strategy have not been improved.