Abstract:
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° N, 79.45° 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 μm
wavelength during the observational period. Our observational site being a mountai-
nous 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 μm
wavelength. The assumed effective radius, number density of cloud drop and vertical
extent of cloud are 7.33 µm, 170 cm⁻³ and 100 m, respectively, which gives the visible
optical depth of 4.0 at 0.50 μm 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 empha-
sizes the importance of the knowledge of cloud properties along with the aerosol
vertical profiles in cloudy atmosphere.
