High-resolution dual polarized micropulse lidar (MPL) observations have been used to investigate the diurnal evolution of atmospheric boundary layer (ABL) during winter (2008–2011) over Thiruvananthapuram (8.5°N, 77°E), a tropical coastal station located at southwest Peninsular India, adjoining the Arabian Sea. The lidar observations are compared with the boundary layer characteristics derived from concurrent balloon-borne radiosonde observations. This study shows that the mixed layer height over this coastal station generally increases from <300 m in the morning to ∼1500 m by the afternoon. Growth rate of the mixed layer height is rapid (∼350 m/hr) during 09–11 IST and slows down with time to <150 m/hr during 11–14 IST and <90 m/hr during 14–16 IST. Thermal internal boundary layer during the afternoon, caused by sea breeze circulation, extends up to ∼500 m altitude and is characterized by highly spherical aerosols, while a distinctly non-spherical aerosol layer appear above this altitude, in the return flow arising from the landmass.
High-resolution observations of downwelling spectral irradiance in the wavelength band of 350-2500 nm carried out using Spectroradiometer at Thiruvananthapuram (8.5°N, 77°E), a coastal station located in the southwest Indian Peninsula adjoining the Arabian Sea, have been used to investigate the instantaneous spectral aerosol direct radiative forcing efficiency (ISADRFE) and its variation with solar zenith angle. This study shows that, during the pre-monsoon season (March-May), magnitude of the ISADRFE maximizes at the wavelength of ~460 nm around noon, with mean values ranging between -270 to -330 Wm<sup>-2</sup>μm<sup>-1</sup>τ500 <sup>-1</sup> at 15°<SZA<50°. The wavelength at which the ISADRFE peaks as well as the spectral width of the peak increases with solar zenith angle (SZA), especially at SZA>70°. About 47% of the to the total (broad band) instantaneous shortwave aerosol radiative forcing is contributed by the photosynthetically active radiation at solar zenith angle of 20°, while the corresponding percentage contribution decreases to ~40% at SZA of 75°. At solar zenith angle of 20°, ~8% of IADRFE is contributed by the wavelength bands above 1.5 μm, while at SZA of 75°, the corresponding contribution is ~17%.