Effect of cloud reflection on direct aerosol radiative forcing: a modelling study based on lidar observations

In the present study, we have utilized light detection and ranging (lidar) and sun photometer observations to derive vertical distribution and optical depth of aerosols over Manora Peak (29.54 degrees N, 79.45 degrees E, 1956 m above mean sea level). The vertical profiles of aerosols revealed the presence of aerosol layer at 0.75 km above the surface and contribute about 68% to the total aerosol optical depth (AOD) at 0.50 mu m wavelength during the observational period. Our observational site being a mountainous terrain site, we observe cloud formation taking place over the surface most of the time. In this context, it is essential to quantify the effect of cloud reflection on aerosol radiative forcing. In order to address the above issue, we have approached an integral way that combining model with observations. Optical Properties of Aerosols and Clouds (OPAC) model is used to derive aerosol and cloud optical properties. The estimated average single scattering albedo is about 0.85 for an AOD of 0.13 at 0.50 mu m wavelength. The assumed effective radius, number density of cloud drop and vertical extent of cloud are 7.33 mu m, 170 cm(-3) and 100 m, respectively, which gives the visible optical depth of 4.0 at 0.50 mu m wavelength. For mean conditions of aerosol optical properties and 25% of cloud cover, the diurnal mean surface aerosol radiative forcing is about -10.0 W m(-2) (for aerosol above and below the cloud case). The top of the atmospheric aerosol radiative forcing is in the range of +6.5 W m(-2) to +0.5 W m(-2), and atmospheric forcing (AF) ranges from +16.5 W m(-2) to +10.5 W m(-2) when the aerosol layer is above and below the cloud. The cloud layer enhances and reduces the AF as compared to clear sky by 4.5 Wm(-2) and 1.5 Wm(-2) when aerosol layer is mostly above the cloud and below the cloud, respectively. Hence, the present study emphasizes the importance of the knowledge of cloud properties along with the aerosol vertical profiles in cloudy atmosphere.