Cirrus cloud radiative effect on surface-level shortwave and longwave irradiances at regional and global scale

Abstract. Data collected at four ground-level sites are analyzed (1) to determine the surface cloud radiative effect (CRE) induced by cirrus clouds at regional scale for shortwave (CRE_SW) and longwave (CRE_LW) fluxes and (2) to derive the sensitivity of surface CRE_SW to the cloud optical thickness (COT) modulated by the solar zenith angle and the atmospheric turbidity (noted CRE_SW*) and the sensitivity of surface CRE_LW to the infrared emissive power of cirrus cloud modulated by the water vapor content (noted CRE_LW*). The average CRE_SW* is −120 W m⁻² COT⁻¹ but it ranges from −80 to −140 m⁻² COT⁻¹ depending on the solar illumination with a residual variability ranges from +40 and −40 W m⁻² COT⁻¹ from pristine to turbid conditions, respectively. The CRE_LW*, that corresponds to the infrared transmissivity of the atmosphere, ranges from 3% to 40% from dry to wet atmospheric conditions, respectively. The subvisible cirrus class (COT<0.03) over mid-latitude sites, that represents 20% of the population, induces a significant increase in surface LW irradiance at the 2–7 W m⁻² level. The semi-transparent cirrus class (0.03<COT<0.3), that represents 45% of the population, will affect the surface SW irradiance by −12 to −25 W m⁻². Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and the Atmospheric Infrared Sounder (AIRS) are used here to estimate the surface radiative effect at global scale. Global CRE estimations show very significant zonal and seasonal variability of each component of the CRE_NET. CRE_NET is 0.4 W m⁻² during winter/autumn for 15–75° N and 1 W m⁻² for 45–75° S whereas it is near −3 W m⁻² for 15° S–15° N. The summer period shows a cirrus cloud global cooling at all the latitudes except for 75–45° S with a quasi null effect and a peak at −3.6 W m⁻² for 15° S–45° N. The global average cumulative CRE is −2.8, 1.7 and −1.1 W m⁻² for CRE_SW, CRE_LW, and CRE_NET, respectively.