Direct observations of the Cloud Radiative Forcing at Earth’s Surface over a coastal station in the south-west Peninsular India

Clouds are the principal modulators of the Radiation Budget of the Earth-Atmosphere system, and their magnitude is quite large over the Tropics. Clouds reflect part of the incoming solar radiation back to space, causing a net reduction in the down-welling shortwave flux reaching the earth's surface. In contrast, they absorb the long-wave radiation emitted by the earth's surface. Characterizing the radiative impact of clouds is of paramount importance in the understanding their role on Climate. Cloud Radiative Forcing (CRF) at TOA have been carried out in the past based on Satellite observations (ERBE, CERES), but directobservations of the CRF at Surface are rather sparse which, have significant role in the surface energetics.

The present study focuses on the estimation of cloud radiative forcing at the earth's surface over a coastal station Thumba (8.5 N; 77ºE) situated in the southwest peninsular India, which is the gateway of Asian Summermonsoon. Instantaneous short-wave cloud radiative forcing (SWCRF) at surface is defined as the down-welling shortwave radiative flux reaching the earth's surface under the prevailing cloudy condition minus the corresponding values under clear sky condition. Similar isthe definition for long-wave cloud radiative forcing (LWCRF). The CRF at the earth's surface has been calculated using direct measurements of the incoming short-wave (SW) fluxes from up looking Kipp & Zonen CMP 22 Pyranometer and CGR4 Pyrgeometer. The instrument measures the hemi-spherically integrated down-welling SW fluxes in the spectrum spans from 0.25 to 4 µm in the time interval of one minute during May 2011- December 2013. Monthly seasonal and inter-annual variation in cloud radiative forcing is also investigated. The seasons are classified as winter (December-January-February), Pre-monsoon (March-April-May),Summer monsoon(June-July-August-September) and Post-monsoon (October-November).

Diurnal variation of SWCRF is maximum during Pre-noon period during the Pre-monsoon and Summer monsoon seasons and noon or afternoon during the Post-monsoon and Winter seasons. Diurnal variation in LWCRF is maximum during night and minimum during day. LWCRF at the earth's surface is significantly smaller than the magnitude SWCRF. Monthly variation in SWCRF during June-September is about -70 Wm-2 and about -30 Wm-2 during January-February and monthly variation in LWCRF is maximum during post-monsoon and pre-monsoon season (20 to 40 Wm-2) (relatively higher cloud base) and less during summer monsoons season (lower cloud base) and least in January due to least cloud occurrence. SWCRF shows significant inter-annual variation during summer monsoon and its magnitude during June 2013 is about two times larger than the value during the previous years.