Sensing of rainy clouds using an IR lidar
Authors: Yellapragada Bhavani Kumar 
Published: February 2023

LIDAR is an optical profiler that generally works during clear sky periods.&#160; Lidars operation during disturbed weather conditions is rare [1]. An Infrared lidar sensor was developed indigenously at NARL site to profile clouds and rain during thunderstorm periods. The lidar operates in slant mode through a window and make measurements continuously during cloudburst periods. The developed lidar employs a Nd:YAG laser that operates at its fundamental spectrum in the Infrared band and works in pulsed mode. A spare optical tube assembly (OTA) is employed in the experimental work for collecting the backscattered infrared photons. A high degree of alignment made between laser and OTA units to collect light photons from far ranges. An adjustable conical pin-hole system is employed in the present work, which permitted lidar to function in daylight period. A silicon avalanche photodiode (APD) is used in the demonstration work for optical sensing and signal conditioning. Thin-film interference (IF) filter doublet and a peltier cooled APD supported the lidar measurements at room temperature conditions. An Ethernet interfaced single channel transient recorder unit employed in the receiver measurements, which digitizes signal at 40 MHz rate. The experimental data were recorded at one-second sampling with 7.5 m range resolution. The pump laser uses an in-line optical attenuator that switches at 20 pulses per second. The laser radar probes the atmosphere at a slant angle through a window of the lidar room. The lidar first measurement during thunder clouds and rain is shown in Figure 1. Figure 1 contains two panels. The first panel illustrates the range time intensity map generated using the lidar data that collected between 1200 and 1300 Hrs LT on November 2018 at NARL site. The data plotted in Figure 1 correspond to 7.5 m range resolution at one second time sampling. The other panel of Figure 1 indicates the height profile of lidar range corrected signal (RCS) that obtained at 1203 Hrs LT, which has been shown indicate the lidar signal strength during cloud conditions.&#160; One can notice from Figure 1 the downward movement of thunderstorm cloud deck with time, which further leads fall of rain over land.&#160; One can see rainfall as varying streaks of intensity with range. Different color bands shown in Figure 1 indicate the variations in the intensity of lidar RCS. The red color band indicates the peak value that represents the thunderstorm cloud base. The yellow-orange represents heavy rain events, whereas the shades of green and blue color indicate light rain. The lidar signal overlap occurs at a range of around 100 m, which is 50 m above ground level.Figure 1. Infrared lidar measurements of cloud and rain during thunderstorm period over NARL site through a window.&#160;References&#160;[1]. R. Vishnu, Y. Bhavani Kumar, T. Narayana Rao, Anish Kumar M. Nair, A. Jayaraman , &#8220;Development of lidar sensor for cloud-based measurements during convective conditions,&#8221;, Proc. SPIE. 9876, Remote Sensing of the Atmosphere, Clouds, and Precipitation VI&#160;