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大气水物质由水汽和大气水凝物(云水)组成,二者在大气水循环过程中密不可分.降水基本来自大气水凝物,而不直接来源于水汽,但前人对水凝物的研究较少.评估一定地区、时段中最大可能降水量时,需要定量评估该地区、时段中所有出现过的大气水凝物总量.其中没有形成地面降水、仍留在空中有可能被开发的水凝物,本文定义为云水资源(CWR).基于大气水分收支平衡方程,本文定义了与大气水凝物、水汽及大气总水物质相关的16个物理量,包括状态量、平流量和源汇量,它们称为云水资源的组成量;在此基础上,提出了包括降水效率(PE)和更新期(RT)在内的云水资源相关12个特征量的定义.基于卫星观测、大气再分析资料、降水产品和云分辨模式,建立了两种云水资源的定量评估方法(细节见文章2),并实现2017年4月和8月华北区域云水资源及其特征量的评估计算.结果表明,云水资源和降水(Ps)是同一个量级,水汽通过凝结过程转化形成的水凝物量(Cvh)是地面降水的主要来源,它与地面降水有显著的正相关,对大区域的云水资源贡献最高;水凝物的状态项和平流项比水汽的相应量低2个量级,但大气水凝物的降水效率高(为百分之几十,水汽为百分之几)、更新期短(仅数小时,水汽约数天);对于逐日的云水资源评估,状态量不可忽略,当评估时段为月或更长时间尺度时,源汇量(即云物理过程量)的贡献最高;平流项的贡献随着评估区域的扩展而减小.“,”The water in the air is composed of water vapor and hydrometeors, which are inseparable in the global atmosphere. Precipitation basically comes from hydrometeors instead of directly from water vapor, but hydrometeors are rarely focused on in previous studies. When assessing the maximum potential precipitation, it is necessary to quantify the total amount of hydrometeors present in the air within an area for a certain period of time. Those hydrometeors that have not participated in precipitation formation in the surface, suspending in the atmosphere to be exploited, are defined as the cloud water resource (CWR). Based on the water budget equations, we defined 16 terms (including 12 independent ones) respectively related to the hydrometeors, water vapor, and total water substance in the atmosphere, and 12 characteristic variables related to precipitation and CWR such as precipitation efficiency (PE) and renewal time (RT). Correspondingly, the CWR contributors are grouped into state terms, advection terms, and source/sink terms. Two methods are developed to quantify the CWR (details of which are presented in the companion paper) with satellite observations, atmospheric reanalysis data, precipitation products, and cloud resolving models. The CWR and related variables over North China in April and August 2017 are thus derived. The results show that CWR has the same order of magnitude as surface precipitation (Ps). The hydrometers converted from water vapor (Cvh) during the condensation process is the primary source of precipitation. It is highly correlated with Ps and contributes the most to the CWR over a large region. The state variables and advection terms of hydrometeors are two orders of magnitude lower than the corresponding terms of water vapor. The atmospheric hydrometeors can lead to higher PE than water vapor (several tens of percent versus a few percent), with a shorter RT (only a few hours versus several days). For daily CWR, the state terms are important, but for monthly and longer-time mean CWR, the source/sink terms (i.e., cloud microphysical processes) contribute the largest; meanwhile, the advection terms contribute less for larger study areas.