Abstract: Positive cloud-to-ground (CG) natural lightning typically exhibits higher energy release than negative CG lightning, with greater peak current, longer duration, and larger charge transfer. However, positive CG flashes only account for 10%-20% of total lightning occurrences, and their detection remains challenging due to limited radiation monitoring capabilities. High-resolution optical observation systems are essential for investigating positive CG lightning development. 25 downward positive CG flashes during 2010-2023 recorded at Tall-object Lightning Observatory in Guangzhou (TOLOG) are analyzed using synchronized optical and electromagnetic field measurements. Pickup points are determined through integration of two-station optical triangulation with data from Guangdong-Hong Kong-Macao Lightning Location System (GHMLLS). Based on geometric inclination characteristics, positive leaders are classified into 2 distinct types: Approximately vertical and predominantly horizontal. Statistical analysis of their two-dimensional development speeds shows that leader speed is strongly correlated with channel morphology. For approximately vertical positive leader channels, the speed increases significantly over time during two periods: From initial optical detection to -1 ms relative to return stroke onset, and from -1 ms to 0 ms. Velocity ranges are 1.15×10⁴-1.73×10⁶ m·s^-1 and 1.73×10⁵-6.90×10⁶ m·s^-1, respectively. Notably, the mean speed in the later period (1.43×10⁶ m·s^-1) is approximately 7 times higher than that in the earlier period (2.17×10⁵ m·s^-1). For predominantly horizontal positive leader channels, the velocities oscillate around their mean values within ranges of 4.83×10⁴-1.61×10⁶ m·s^-1 and 7.31×10⁴-3.48×10⁶ m·s^-1 for these periods, respectively. The mean speed in the later period (1.05×10⁶ m·s^-1) is only 3 times than that in the earlier period (3.26×10⁵ m·s^-1).
In addition, the speed of approximately vertical positive leader channels increases significantly with decreasing altitude, reaching a mean of 1.67×10⁶ m·s^-1 below 500 m, about 7 times higher than the mean of 2.25×10⁵ m·s^-1 at 1000-1500 m altitude. For predominantly horizontal positive leader channels, the speed also varies with height but less dramatically: The average speed below 500 m (6.31×10⁵ m·s^-1) is only 1.3 times as fast as the speed at 1000-1500 m section (4.68×10⁵ m·s^-1).