Fig. 1: Observation of cloud vertical structure via scanning by the T2 lidar.

The cloud was observed around 20:35 LT after sunset on July 5, 2022 at the Brookhaven National Laboratory Center for Multiscale Applied Sensing observational site. a The colored region shows the distribution of observed photons within a 12.75-m time-gated window (height of the insets) per second. The time-gated window starts below cloud base and shifts upwards by 1.5 m every second. Four inset plots show the distribution of observed photons at a 15-cm resolution within selected time-gated windows, namely below cloud base (CB), at CB, right above CB, and farther above CB. b Fraction F(i) of observed photons within each sublayer i observed for the four selected time-gated windows in (a). The 12.75-m time-gated window is divided into 85 sublayers, each of thickness 15 cm. The thick lines show 20-point moving averages; the blue, red, and yellow lines are approximately linear, whose slopes are explained by the minimal model of first-arriving photons (see main text). c The profile of observed photon rate within the first 1.5 m of each time-gated window. The coloring of the dots in a–c corresponds to the four selected time-gated windows: blue for the haze region, purple marking the beginning of the haze-cloud transition, and red and yellow are both within the cloud. The relative backscatter (χ) of the yellow point is 11%, defined as the ratio of the observed photon rate at that level to its peak value in the profile.