Assessment of 1D and 3D model simulated radiation flux based on surface measurements and estimation of aerosol forcing and their climatological aspects

Abstract

Ground reaching solar radiation flux was simulated using a 1-dimensional radiative transfer (SBDART) and a 3-dimensional regional climate (RegCM 4.4) model and their seasonality against simultaneous surface measurements carried out using a CNR4 net Radiometer over a sub-Himalayan foothill site of south-east Asia was assessed for the period from March 2013–January 2015. The model simulated incoming fluxes showed a very good correlation with the measured values with correlation coefficient R2 ~0.97. The mean bias errors between these two varied from −40 W m−2 to +7 W m−2 with an overestimation of 2–3% by SBDART and an underestimation of 2–9% by RegCM. Collocated measurements of the optical parameters of aerosols indicated a reduction in atmospheric transmission path by ~20% due to aerosol load in the atmosphere when compared with the aerosol free atmospheric condition. Estimation of aerosol radiative forcing efficiency (ARFE) indicated that the presence of black carbon (BC, 10–15%) led to a surface dimming by −26.14 W m−2 τ−1 and a potential atmospheric forcing of +43.04 W m−2 τ−1. BC alone is responsible for >70% influence with a major role in building up of forcing efficiency of +55.69 W m−2 τ−1 (composite) in the atmosphere. On the other hand, the scattering due to aerosols enhance the outgoing radiation at the top of the atmosphere (ARFETOA ~−12.60 W m−2 ω−1), the absence of which would have resulted in ARFETOA of ~+16.91 W m−2 τ−1 (due to BC alone). As a result, ~3/4 of the radiation absorption in the atmosphere is ascribed to the presence of BC. This translated to an atmospheric heating rate of ~1.0 K day−1, with ~0.3 K day−1 heating over the elevated regions (2–4 km) of the atmosphere, especially during pre-monsoon season. Comparison of the satellite (MODIS) derived and ground based estimates of surface albedo showed seasonal difference in their magnitudes (R2 ~0.98 during retreating monsoon and winter; ~0.65 during pre-monsoon and monsoon), indicating that the reliability of the satellite data for aerosol radiative forcing estimation is more during the retreating and winter seasons.