| Ice clouds are mainly composed of non-spherical ice crystal particles,which are complex and variable in time and space,and their formation mechanisms vary significantly in different regions.Meanwhile,because of their unique physical and radiative properties,ice clouds play a special and important role in the regional radiative energy balance.However,there are currently some problems with the accurate characterization of ice cloud physical properties in climate models,which results in important implications for the correct calculation of ice cloud radiative effects and the accurate assessment of their role in climate feedbacks.Thus,this paper analyzed the macro-and micro-physical properties of ice cloud at different time scales using six-year Ka-band Zenith Radar(KZAR)observations with high spatial and temporal resolution from January 2014 to December 2019 at the Semi-Arid Climate and Environment Observatory of Lanzhou University(SACOL)site in Northwest China,and simulated the radiative effects of ice cloud combining with the radiative transfer model.Meanwhile,based on the accurate separation of single-layer water cloud,single-layer ice cloud and double-layer cloud and consideration of their daily variation,we computed the radiative forcing of three different cloud types and their contribution to total cloud radiative forcing(CRF)at the SACOL site.Additionally,we analyzed the radiative effects of upper tropospheric cirrus clouds at the global low-and mid-latitude regions using the ISCCP cloud properties observations combining with the radiative transfer model.By using ground-based cloud radar,satellite observation data and radiative transfer model,this paper mainly focuses on the physical properties and radiative effects of ice clouds.The major research contents and conclusions are summarized as follows:(1)This paper investigated the diurnal variation characteristics of physical properties and radiative effects of ice cloud,and the radiative bias induced by neglecting the diurnal variation of ice cloud using the ground-based cloud radar observations at the SACOL site.Results show that ice clouds occur more frequently during nighttime than during daytime at the SACOL,with a minimum occurrence frequency of 36%at midday and a maximum occurrence frequency of 44%at midnight.Its physical thickness,ice water path and particle effective diameter are also greater at night.Thus,we accurately simulated the diurnal variation of radiative effects of ice clouds by inputting 96 ice cloud profiles per day into the Fu-Liou model.During daytime,due to combined effect of shortwave(SW)cooling and longwave(LW)warming effects,the daily variation of net CRF at the top of the atmosphere(TOA)ranges from-14 to 28W/m~2;during nighttime,only affected by the LW warming effect,it varies from 27 to29 W/m~2.Also,to evaluate the radiative bias induced by neglecting the daily variation of ice clouds,we calculated the CRF of ice cloud under the condition that the daily variation of cloud properties cannot be reasonably characterized,such as one daily averaged cloud profile,two day and night averaged cloud profiles,and compared them with the results under the condition that the daily variation of ice clouds is accurately characterized.Results find that the absolute bias of net CRF at the TOA can reach 11W/m~2 at the SACOL site when only one daily averaged cloud vertical profile is used in the model,suggesting that neglecting the diurnal variation of ice cloud properties in the model will result in significant bias of the CRF.(2)Based on the accurate consideration of diurnal variation characteristics of ice clouds,this paper further analyzed the seasonal and inter-annual variation characteristics of physical properties and radiative effects of ice cloud by using the ground-based cloud radar observations.Results show that ice cloud occurs more frequently during cold seasons than during warm seasons at SACOL,and its occurrence frequency reach the maximum of 54.1%in March,and the minimum of 23.6%in July.Also,the geometric thickness of ice cloud are greater during cold seasons than those during warm seasons,and its ice water path and particle effective diameter are larger in spring and autumn.Due to the influence of incoming solar radiation,and the occurrence frequency and physical properties of ice cloud,the net heating effects at the TOA induced by ice cloud are stronger during cold seasons than those during warm seasons,with the monthly mean values varying from 8 to 15 W/m~2.During the study period,ice cloud induces net heating effects of 11 W/m~2 on the Earth-atmosphere system.For the atmospheric heating rate,the SW cloud raditive heating rate(CHR)changes from negative to positive with increasing height,and the LW CHR changes from positive to negative.Additionally,during warm seasons,the SW heating effects are stronger inside the cloud layer;during cold seasons,the LW cooling effects are stronger above the cloud layer.The variation of net radiative heating rate with height and time is similar to that of LW,indicating the long-wave radiation dominates.(3)Based on the accurate separation of single-layer water cloud,single-layer ice cloud and double-layer cloud and consideration of their daily variation,this paper investigated the radiative forcing of three different cloud types and their contribution to total CRF at the SACOL site by using the ground-based cloud radar data combining with the Fu-Liou radiative transfer model.Results show that ice cloud is most abundant at SACOL,with the occurrence frequencies of single-layer ice cloud,single-layer water cloud,and double-layer cloud being 29.1%,3.3%,and 8.3%,respectively.Single-layer ice cloud occurs more frequently in cold seasons,while single-layer water cloud and double-layer cloud occur more frequently in warm seasons.The three different cloud types all contribute significantly to TOA SW CRF with the annual percentage contribution of single-layer ice cloud,double-layer cloud,and single-layer water cloud being 43%,40%,and 17%,respectively.For LW,single-layer ice cloud dominates the warming effects,providing 70%contribution to the LW CRF at the TOA.Combining the SW and LW radiative effects,single-layer water cloud and double-layer cloud cause a net radiative cooling effect of 8.5 W/m~2 on the Earth-atmosphere system,and their annual percentage contributions are 26%and 27%,respectively.However,single-layer ice cloud induces a net radiative warming effect of 7.4 W/m~2with the annual percentage contributions being 47%.(4)Based on the study of physical properties and radiative effects of ice clouds at SACOL site,and to conduct related studies on a larger scale,this paper calculated the radiative forcing of upper tropospheric cirrus clouds at global low-and mid-latitude regions by using the ISCCP cloud properties observations during January 2012 to December 2016 combining with the SBDART radiative transfer model.The effect of daily variation of cirrus clouds on radiative forcing on a large scale suggests that the SW cooling and LW warming effects induced by cirrus clouds are overestimated by0?25 W/m~2 over the continental regions and most oceanic regions if the diurnal variation of cirrus physical properties is not considered in model.The largest absolute bias of net CRF induced by neglecting the diurnal cycle of cirrus cloud occurs over the tropical deep convection regions,which exceeding 4 W/m~2.Therefore,on the basis of considering the daily variation of cirrus clouds,we accurately simulated the radiative forcing of upper tropospheric cirrus cloud in the global low-and mid-latitude regions.The results show significant differences in spatial distribution.Over the tropical deep convection regions,cirrus CRF exceeds 5 W/m~2,inducing a net radiative heating effect.In the mid-latitude oceanic regions,cirrus CRF is negative value with less than 2 W/m~2in magnitude,inducing a net cooling effect on the Earth-atmosphere system.In addition,during the study period,the SW and LW CRF are-3.3 and 5.2 W/m~2 over the global low-and mid-latitude regions,respectively,and thus cirrus clouds induce a net warming effect of 1.9 W/m~2 on the Earth-atmosphere system. |
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