A study on radiative properties of Indian summer monsoon clouds

Abstract

Possible causes behind the unusual cooling by summer monsoon clouds over India are investigated. Results suggest that the causes behind the cooling over the Bay of Bengal, India (BBI) and Arabian Sea (AS) within the Indian monsoon region are different. Over the BBI, clouds are tall. A unique upper tropospheric easterly jet stream exists over India during the summer monsoon season, which horizontally spreads the vertically growing deep convective clouds and thereby increases the cloud cover. Hence, more incoming solar radiation is reflected back to space, which leads to cooling. A radiative transfer study employing the Santa Barbara DISORT Atmospheric Radiative Transfer model supports this view. Over the Arabian Sea, clouds are shallow, and hence the upper tropospheric jet cannot affect them. Due to their proximity to the ground, Arabian Sea clouds exert less warming effect, but they exert a considerable cooling effect, which arises because of the high reflectivity of the clouds. Over the Equatorial Indian Ocean (EIO), where the monsoon clouds originate and propagate towards the monsoon trough region, both cooling and warming effects are nearly canceled out. The upper tropospheric jet is located hundreds of kilometers north of the EIO, and hence it does not disturb the deep convective clouds of the EIO. Therefore, they behave similarly to other deep convective clouds in the tropical belt.

Appendix

SBDART is a combination of a sophisticated, discrete ordinate radiative transfer module with a low-resolution atmospheric transmission model and is designed for analysis of a wide variety of RT problems encountered in satellite remote sensing. In SBDART, the radiative transfer equations are numerically integrated with a discrete ordinate radiative transfer (DISORT) module (Stamnes et al. 1988). This method uses a numerically stable algorithm to solve the equations of plane-parallel radiative transfer in a vertically inhomogeneous atmosphere (Ricchiazzi et al. 1998). In SBDART, the intensity of scattered and thermally emitted radiation can be computed at different directions and heights. Computations can be performed in up to 65 atmospheric layers and 40 radiation streams. The radiative processes included in the model are Rayleigh scattering, gaseous absorptions, cloud and aerosol scattering and absorptions. The simulation was done by keeping the cloud cover range as observed in various cloud data over the study region. The computation of radiative transfer in a cloudy atmosphere required knowledge of different scattering parameters, which are computed in SBDART using a Mie-scattering code for spherical cloud droplets with statistical distribution of the drop radius. For radiative transfer through cloud, the parameters considered are the altitude of the cloud layer, its optical thickness and droplet size. In SBDART, altitudes of cloud layers (km) can be specified as separate cloud layers or as a range of altitudes that will be filled by cloud.