Sensitivity of Nocturnal Boundary Layer to Tropospheric Aerosol Radiative Forcing Under Clear Sky Conditions

1 2 Since the middle of the last century, global surface air temperature exhibits an increasing trend, 3 with nocturnal temperatures increasing at a much higher rate. Proposed causative mechanisms 4 include the radiative impact of atmospheric aerosols on the nocturnal boundary layer (NBL) 5 where the temperature response is amplified due to shallow depth and its sensitivity to potential 6 destabilization. A one-dimensional version of the Regional Atmospheric Modeling System 7 (RAMS) is used to examine the sensitivity of the nocturnal boundary layer temperature to 8 surface longwave radiative forcing (SLWRF) from urban aerosol loading and doubled 9 atmospheric carbon dioxide concentrations, for typical mid-latitude nocturnal boundary layer 10 case days from the CASES-99 field experiment. The analysis is further extended to urban sites 11 in Pune and New Delhi, India. For the cases studies, locally the nocturnal SLWRF from urban 12 atmospheric aerosols (2.7 – 30 W m-2) is comparable or exceeds that caused by doubled 13 atmospheric carbon dioxide (3 W m-2), with the surface temperature response ranging from a 14 compensation for daytime cooling to an increase in the nocturnal minimum temperature. The 15 sensitivity of the NBL to radiative forcing is approximately four times higher compared to the 16 daytime boundary layer. Nighttime warming or cooling may occur depending on the nature of 17 diurnal variations in aerosol optical depth (AOD). Soil moisture also modulates the magnitude 18 of SLWRF, decreasing from 3 W m-2 to 1 W m-2 when soil saturation increases from 37% to 19 70%. These results show the importance of aerosols on the radiative balance of the climate 20 system. 21