Diurnal cycle of cirrus cloud and its associated radiative effects at the SACOL site

Abstract Diurnal cycle of cirrus cloud (DCCci) can affect cloud interactions with both the solar radiation and terrestrial radiation. However, evaluation on how DCCci influences the radiative effects is relatively few, especially in the semi-arid region, which is one of the most sensitive areas in response to global climate change. In this study, we investigate the physical properties and associated radiative effects of DCCci using two-year, high-resolution Ka-band Zenith Radar (KZAR) observations at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) site in Northwest China. We find that cirrus clouds occur more frequently during nighttime than during daytime, with a maximum occurrence frequency of 48% at midnight and a minimum occurrence frequency of 35% at midday, which drastically influences the diurnal variation of cirrus radiative effects. The diurnal variation of cirrus cloud radiative forcing (CRF) is calculated using the Fu-Liou model, involving 96 cirrus profiles per day to represent DCCci accurately. In each season, the diurnal cycle amplitude of CRF is more than 40 W/m2 for shortwave (SW), and less than 16 W/m2 for longwave (LW). During daytime, the net CRF at the top of the atmosphere (TOA) ranges from −16 to 30 W/m2; during nighttime, it varies from 30 to 33 W/m2. Based on the accurate simulation of CRF with DCCci, we then calculate the daily-mean CRF and compare it with the simulated results derived from different averaged cirrus profiles that do not fully represent the diurnal cycle, to evaluate the radiative biases induced by not having accurate DCCci. We find that the absolute bias of net CRF at the TOA can reach 11 W/m2 at the SACOL site when only one daily averaged cirrus property profile is used in the simulation, demonstrating that neglecting DCCci in the model will result in significant bias of cirrus net CRF. This evaluation suggests that DCCci need to be well considered in climate models to reduce the uncertainty of cirrus radiative effects.