Role of Aerosol-Induced Radiative Cooling on the Evening Transitions Observed in the Atmospheric Boundary Layer

Vertical temperature profile close to the ground controls many micrometeorological processes. These include development of inversion layer, occurrence of fog, pollution dispersion and vertical transport of heat and moisture.  We here present results from an extensive field study, conducted at the observation site next to the north runway at the Kempegowda International Airport, Bengaluru, India (13.208°N, 77.704°E). At the site, we have deployed a HATPRO microwave radiometer, a Windcube Lidar, a set of 4-component radiative flux sensors, a weather station, a 2m mast carrying humidity and temperature sensors for monitoring temperature and humidity, along with a soil temperature profiler and two soil heat flux sensors. With this arrangement at the site, we continuously monitor the vertical profile of temperature, relative and absolute humidity from surface to 10 km height.

Evening transition of the atmospheric boundary layer (ABL) observed during and after the sunset (under calm and clear sky conditions) indicates the development of Lifted Temperature Minimum (LTM) type vertical temperature profiles at the site. Boundary layer cooling observed after the sunset extends more than 200 m from the surface. Cooling is strong near the ground. This leads to formation of penetrative-convection layer close to the ground. Above this unstable convective layer, a stable inversion layer develops that extends to several hundred meters in height.

We will present results from the numerical simulation of the ABL, by initializing from the radiometer observed vertical profile of temperature before sunset. Numerical simulations are based on a high-resolution, one-dimensional radiation model, coupled with ground temperature with and without aerosols' presence. LTM height, intensity, and its evolution with time observed from the field experiments and simulations have been compared and analyzed regarding radiation budget, aerosol property, number density, and soil emissivity. Results presented here indicate that the observed temperature profile in the field experiments matches closely with the simulations only when the presence of aerosols is considered in the numerical simulations. A high concentration of the aerosol in the surface layer, close to the ground enhances the radiative cooling and leads to the formation of the LTM profile.