Cirrus and PBL Height Climatology and the Potential Effects of Cirrus on PBL Height in Metro Manila using Ground-based High Spectral Resolution Lidar

We investigate the radiative impacts of tropical cirrus on the planetary boundary layer height (PBLH) over a polluted urban site at Manila Observatory (14.64°N, 121.08°E), Philippines, throughout January to December 2019. A unique ensemble of cloud, aerosol, and radiation measurements from a ground-based High Spectral Resolution Lidar (HSRL) and a broadband pyranometer provided a long-term dataset to examine how cirrus radiative forcing modulates PBLH. An HSRL-based cirrus cloud detection algorithm retrieved cirrus optical depth (τ), while a temporal variance in aerosol-based algorithm for retrieving PBLH produced a 1-year record of cirrus τ and PBLH. Pyranometer measurements show that cirrus τ directly modulates downwelling solar radiation (DSR): clear-sky or thin cirrus maximize DSR, while thicker cirrus reduce DSR. Cirrus τ is consistently negatively correlated with PBLH across seasonal and daily timescales. In the dry season (December-May), thinner cirrus (τmean = 0.20) were observed with higher PBLHmax, (1500 m); in the wet season (June‑November), thicker cirrus (τmean = 0.30) with lower PBLHmax (1300 m). This unique suite of ground-based measurements strongly supports the mechanism whereby thin cirrus leads to enhanced DSR, surface warming, sensible heat flux (SHF) and PBLH. MERRA-2 reanalysis corroborates these findings, showing a robust positive correlation between HSRL-detected PBLH and MERRA-2 SHF. Comparison of cirrus τ and aerosol optical depth highlight the dominant role of cirrus (up to 60%) in modulating DSR relative to aerosols. These results demonstrate that cirrus significantly influence PBL evolution, with implications for air quality and the representation of boundary layer processes in models.