## 1 Introduction  
Low-level wind shear refers to a sudden, drastic change in wind speed or wind direction in the atmosphere, occurring within 600 m above ground level [1]. Encountering wind shear will cause an instantaneous change in the aircraft airspeed, which will lead to a change in the aircraft lift and deviation from the intended flight path, thereby threatening the flight safety of civil aircraft [2]. In recent years, airport wind shear accidents have often been reported, especially in take-off and landing processes [3, 4, 5]. With the rapid growth in global aviation transportation, it is urgent to improve the ability of detecting, identifying, alerting, and forecasting windshear to ensure the safety, regularity, and efficiency of flight operations [6]. The Terminal Doppler Weather Radar (TDWR) network [7] and the Low-Level Windshear Alert System (LLWAS) system consist of several anemometers installed in the vicinity of runways that have previously been applied in low-level wind shear monitoring for airports [8]. Moreover, balloon soundings, wind profilers [9], ground-based weather radar, lidar, and MET satellites are also adopted in windshear detection [1]. Among these instruments, the coherent Doppler wind lidar (CDWL) is a promising wind detection instrument that provides flexible scanning with high temporal and spatial resolutions. It can cooperate with radar to provide full weather condition detection. The CDWL has been widely used in different areas, such as the detection of the atmosphere boundary layer [10; 11; 12], gravity waves [13], turbulence [14; 15], windshear [16; 17; 2], gust [18], and aircraft vortex [19]. Detection and identification of low-level wind shear using CDWL has become a popular topic of research in aviation [3; 5; 20; 21; 22; 23]. The Doppler lidar system installed at the Hong Kong International Airport (HKIA) has been applied for wind shear alerting since 2002. Comprehensive studies on wind shear events observed by the lidar have been conducted at HKIA, including the design of the glide path scanning and the development of an automatic wind shear detection algorithm [4; 14; 24; 25; 26; 27; 28; 29]. Besides HKIA, lidar systems for wind shear detection have been tested at several international airports, such as Narita International Airport [30], Bratislava Airport [20], Lanzhou Zhongchuan International Airport [5], Beijing Capital International Airport [3], and others [21; 22]. The headwind profile of a single runway can be synthesized from the radial velocity in line with the glide path [28], while it is difficult to be applied to a mega airport (an airport with multiple runways) due to the requirement of multiple lidars and their limited locations. Deploying multiple lidars has double costs: it increases the difficulty and efforts of instrument maintenance and data assembly. Retrieval of a horizontal 2D wind field is a challenge with single lidar [31], and it is difficult to apply to wind shear alerting due to the lack of quantitative, accurate, and efficient methods for evaluating wind shear intensity. In this paper, a new approach using a single CDWL is presented for windshear alerting for multiple runways, through quantitatively evaluating the distribution of shear ramps based on orthogonal wind components. The CDWL system achieves monitor four runways in a detection range of 13 km, providing services for aviation safety at Beijing Daxing International Airport (BDIA). The approach requires enough detection range to cover multiple runways. The degree of air cleanliness, low clouds, low atmospheric visibility, gross air pollution, and extreme weather conditions will lead to a short lidar detection range. In these cases, multiple lidars are usually needed to cover the whole area of a mega-airport to implement this approach. This paper is organized as follows: the wind retrieval and windshear alerting method are introduced in Section 2. The results of windshear, microburst, and vortex structures with alerting information in the field experiment are described in Section 3.