Effects of aerosol on the local heat budget of the lower atmosphere

TLDR: In this paper, the authors derived an equation to show how the net effect of aerosol on the lower atmosphere depends on the reflection coefficient of the surface and on the aerodynamic and surface resistances to vapour transfer.

Abstract: SUMMARY The convergence of heat below subsidence inversions was estimated from radiosonde ascents on fine summer days. Sensible heat input from the ground was estimated from measurements of heat fluxes and net radiation over a wheat field. Absorption and back scattering of solar radiation by aerosol throughout the atmosphere were calculated from measurements of solar radiation at the ground. Radiation absorbed by aerosol in the thermal boundary layer heated the lower atmosphere at an average rate of 3.3degC day-’ (60Wrn-’), about twice as fast as estimated for gaseous constituents. An equation is derived to show how the net effect of aerosol on the lower atmosphere depends on the reflection coefficient of the surface and on the aerodynamic and surface resistances to vapour transfer. Over the wheat, the net effect was almost zero when the crop was transpiring fast, but when the crop was mature, aerosol caused net cooling. I. INTRODUCTION As part of a wider attempt to identify possible causes of climatic change, attention has recently been directed to the absorption and scattering of solar radiation by aerosol, processes which modify the heat balance of the atmosphere and the radiation budget at the earth’s surface. Calculations by Mitchell (1971), and by others, demonstrate that aerosol may be responsible either for cooling or for heating the lower atmosphere depending on factors such as the height, thickness and optical properties of the layer and the relative inputs of sensible and latent heat from the ground. The main sources of uncertainty in predicting thermal effects of aerosol in the atmosphere are an incomplete knowledge of (i) optical properties and distribution of aerosol, and (ii) the relationship between net radiation and sensible heat flux at the earth’s surface. We have made direct measurements, during summer anticyclonic conditions in Britain, of heat storage in the lower atmosphere, coupled with measurements of surface radiation and heat fluxes. In the first instance, the measurements were analysed to show the importance of radiant energy absorbed by aerosol in determining the energy budget of the lower atmosphere. The analysis was then extended to estimate the influence of atmospheric aerosol on the input of sensible heat to the atmosphere from the ground. It appears that aerosol may be responsible for a net heating or cooling of the lower atmosphere at a rate which depends on surface reflectivity and on the ratio of aerodynamic and surface resistances to vapour transfer. 2. THEORY During anticyclonic weather in Britain, inversions formed by subsidence are common at heights between 1 and 2km. Below the main inversion, the atmosphere is heated by radiation and by an input of sensible heat from the ground. Heat may also be entrained from the warmer air above the inversion as discussed by Cattle and Weston (1975). For simplicity, we shall consider the heat budget of a ‘thermal boundary layer’ defined as that layer of the atmosphere above which there was no measurable change of temperature due 95