On the Anomalies between Clear-Sky Measured and Modeled Shortwave Radiation during the Ganges Valley Aerosol Experiment

Description

Radiative transfer models have been used extensively for simulating radiation fluxes and radiative forcing due to trace constituents; however, the evaluations of such calculations are sparse, particularly over the tropical Indian region. Solar shortwave radiation at the surface is strongly influenced by the presence of clouds and aerosols, as well as the distribution of atmospheric water vapour. Over mountainous regions such as the foothills of the Himalayas, accurate computations of radiative fluxes are challenging due to the inadequacy of models to produce accurate atmospheric temperature and humidity profiles and lack of extensive aerosols measurements. In this work, we utilize the extensive dataset of aerosol properties, meteorology, and radiation measurements, collected as a part of the Ganges Valley Aerosol Experiment (GVAX), to investigate the anomalies between measured and modeled downward shortwave (0.3-3 µm) radiation at the surface. We focus on the effect of poor representation of atmospheric temperature and humidity profiles by repeating the computations with a “Tropical Standard Atmosphere”, with hourly retrievals of meteorological profiles from a Microwave Radiometer Profiler (MWRP), with average radiosonde climatology and with meteorology simulated from a regional model (WRF). Results show that when a tropical standard atmosphere is used, the total shortwave radiation at the surface is significantly (by 45.1±26.8 Wm-2) underestimated. The anomalies also show strong diurnal variation with afternoon minima. Inclusion of hourly profiles improves the model performance significantly and reduces the average anomalies to 1.3±16.6 Wm-2. Aerosol Radiative Forcing (ARF) was also estimated to be -6.4±6.8 at the Top of the Atmosphere (TOA) and -31.8±14.6 Wm-2 at the surface. The ARF at TOA is significantly impacted (~5 Wm-2), particularly in the high ARF (-20 Wm-2) conditions. Use of meteorological profiles from WRF model reproduces the mean shortwave radiation reasonably well; however the variability of the results is larger than that of the MWRP driven results. A “GVAX climatological atmosphere” recommended for future use over this region is provided in the appendix. With this atmosphere, the average anomaly between the model and measured radiation is -4.9 ± 25.1 Wm-2.