Observed instantaneous cirrus radiative effect on surface‐level shortwave and longwave irradiances

Data collected at the SIRTA Observatory, 20 km south of Paris, are analyzed to determine the instantaneous surface cloud radiative effect (CRE) induced by cirrus clouds. CRE is here defined as the difference between overcast‐sky and clear‐sky surface radiative fluxes obtained by ground‐based measurement of broadband fluxes and clear‐sky parametric models, respectively. Clear‐sky periods detected by a double threshold based on lidar and radiative fluxes analysis show a root mean square error for clear‐sky models smaller than 6.5 W m −2 for shortwave flux and 4 W m −2 for longwave flux. Over 100 h in 2003–2006 characterized by homogeneous overcast cirrus clouds are analyzed. Fifty percent of this cirrus population is subvisible and semitransparent, that is, with optical thickness less than 0.3. The mean surface shortwave cirrus cloud radiative effect (CRE SW ) is found near −50 W m −2 . We establish the relationship between CRE SW and cirrus optical thickness (COT) to be about −90 W m −2 per unit of COT. This SW sensitivity ranges from −80 W m −2 COT −1 to −100 W m −2 COT −1 for turbid to pristine atmospheres, respectively. We also establish the relationship between surface longwave cloud radiative effect (CRE LW ) and the irradiance emitted by the cirrus cloud derived from cloud infrared emissivity and cloud temperature. The average surface CRE LW is about +5 W m −2 . CRE LW is found to be about 10% of the cloud irradiance. This LW effect ranges from 5 to 15% of the cirrus irradiance depending on atmospheric humidity for the wet and dry atmosphere, respectively.