| << | Interaction of Aerosols, Clouds and Radiation/Transboundary Transport of Air Pollution | >> |
Abstract
In this study, we analyze two datasets: ground-based and satellite measurements. The firsts corresponds to solar and infrared irradiance measurements, cloud and aerosol Lidar backscattering profiles, microwave radiometer brightness temperatures, radiosonde profiles, and sun-photometer extinctions monitored at four observatories located in the midlatitudes, the Tropics and the Arctic. This dataset permits to estimate the Cirrus cloud Radiative Forcing on surface-level shortwave (CRFSW) and longwave (CRFLW) irradiances. The sensitivity of CRFSW to Cloud Optical Thickness (noted CRFSW*) is established and ranges from 100 W m-2 to 200 W m-2 per unit of cloud optical thickness. The important variability of aerosols and water vapor content obtained in studying the 4 observatories allows us to quantify the combined influence of aerosol optical thickness and integrated water vapor on CRFSW* : 10 to 20 % CRFSW* range for turbid and pristine atmosphere. Moreover, the sensitivity of the CRFLW to both cloud emissivity and cloud temperature (noted CRFLW*) is established and the influence of integrated water vapor on CRFLW* quantified. The long-term dataset allows quantifying the impact of subvisible cirrus cloud on the anomaly of direct and diffuse solar irradiances at the surface over ARM SGP site since 10 years.
Satellite measurements are used next as input parameters to the cirrus cloud radiative forcing parameterizations to calculate CRFSW and CRFLW at global scale. CALIOP provide aerosol and cirrus cloud properties and AIRS the integrated water vapor. Meridian distribution are shown and discussed. They reveal a positive cirrus cloud net radiative effect (CRFSW + CRFLW) from 30°N poleward during boreal winter and from 45°S during austral winter. The cumulative cirrus cloud net radiative effect reaches +1.5 W m-2 for these two winter cases and -8 W m-2 near the equator.