Aerosol Radiative Forcing over­­­­­­ North-East India: Synergy of Model simulation and ground based observations

Abstract:

The diurnal evolution of shortwave solar radiance at the surface has been investigated from Kipp and Zonen CNR4 net radiometer measurements in a humid sub-tropical location Dibrugarh in the North Eastern region of India. Data for a total of 345 clear days within a span of two years during March 2013- January 2015 are analyzed which are further utilized to validate the Santa Barbara Discrete Ordinate Radiative Transfer (SBDART) simulated flux. The diurnal evolution of solar radiation maximizes in its amplitude in monsoon months (JJAS) and is minimum during the winter months (DJF) prescribed by the Northern Hemisphere routine. The net shortwave radiation increases from the minimum value of ~100 Wm^-2 at the beginning of the year and attains maximum ~300 Wm-2 during monsoon. Both the measured and model simulated diurnal and seasonal solar flux exhibit similar behaviour at the surface with good correlation with R²~ 0.98-0.99. The present study also focuses on the validation of the surface albedo and the albedo retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) measurements by the CNR4 net Radiometer measurements, which again shows a good agreement. This validation is essential for the reliability of satellite retrieved surface reflectance that are being utilised in the radiative transfer models. In order to study the influence of the aerosols upon the incoming solar irradiances the aerosol radiative forcing (ARF) and aerosol radiative forcing efficiency (ARFE) is estimated. The ARFE_surface during the Winter is the highest (-75.02 ± 8.03 W m^-2 τ^-1) and minimum during Retreating Monsoon (ON) (-58.40 ±25.03 W m^-2 τ^-1). For both the modeled and the field based estimation, the aerosol radiative forcing obtained during the study period ranged from -39 ±6 Wm^-2 to -10 ±4 Wm^-2 at the surface and 10±3 Wm^-2 to 28±7 Wm^-2 at the atmosphere and -7±4 Wm^-2 to -10 ±3 Wm^-2 at the TOA. The measured and the model ARF values differ by 5 - 8 % in winter and premonsoon and almost ~6% in monsoon. The average atmospheric heating rate is maximum in pre-monsoon for both the estimations. The observation of ARF is further compared with the ICTP’s RegCM4 model in order to acquire the model utility in the location where measurements are not feasible.