Feasibility study of cloud base height remote sensing by ground-based sky thermal infrared brightness temperature measurements

Infrared remote sensing of clouds is one of the main subjects in the field of atmospheric remote sensing from satellites. As its characteristics of intense infrared radiation and absorption, the heights of cloud top can generally be derived from the brightness temperature obtained from satellite infrared cloud images, and then a series of study about cloud morphologic, microphysics and dynamics can be conducted. On the same principle, the heights of cloud base can also be determined by sky infrared brightness temperature observations on the ground. Some preliminary studies were made in the past, but have not been extended. The reason is that the range of zenith angle is very wide for ground-based remote sensing of cloud base heights. From near horizontal ground up to zenith, at different zenith angle, the background infrared brightness temperature of clear sky atmosphere is very different, it is the function of the atmospheric temperature and humidity profiles as well as the zenith angle. Hence the question must be considered is how to obtain the correct information about the heights of cloud base from sky background brightness temperatures in large variational range. In addition, the contributions of aerosols in near ground atmosphere to sky brightness temperatures observed on the ground cannot be neglected. In order to correctly derive cloud base heights from sky brightness temperatures, the effect of aerosol layer must be studied seriously. Using radiative transfer equation to study the sensitivities of thermal infrared brightness temperature values observed on the ground to different aerosols and different cloud base heights is an important basic work. In this paper systematic study on the variations of sky infrared brightness temperatures caused by different kinds of clouds under the different kinds and the different intensities(represented by different ground visibilities) of aerosols using MODTRAN 4.0 model is presented. In this study the feasibility of cloud base height remote sensing by ground-based sky infrared brightness temperature observation and the quantitative effects of aerosol layer on it can be found. From the calculated results by model the following can be concluded: 1. For low and middle clouds, the sky thermal infrared brightness temperature observed on the ground is very sensitive to the variations of cloud base height, therefore as the "clear" sky brightness temperature is known it can be used to retrieve and remote sensing of cloud base height. 2. The influence of aerosols (visibilities and aerosol types) above the ground on the sky background brightness temperatures is considerable, it must be corrected. Because the aerosol's influence on brightness temperatures with different zenith angles is fairly clear, it can be corrected very well.