Rain cells are the most elementary unit of precipitation system in nature. In this study, fundamental geometric and physical characteristics of rain cells over tropical land and ocean areas are investigated by using 15-yr measurements of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). The rain cells are identified with a minimum bounding rectangle (MBR) method. The results indicate that about 50% of rain cells occur at length of about 20 km and width of 15 km. The proportion of rain cells with length > 200 km and width > 100 km is less than 1%. There is a a log-linear relationship between the mean length and width of rain cells. Usually, for the same horizontal geometric parameters, rain cells tend to be square horizontally and lanky vertically over land, while vertically squatty over ocean. The rainfall intensity of rain cells varies from 0.4 to 10 mm h⁻¹ over land to 0.4–8 mm h⁻¹ over ocean. Statistical results indicate that the occurrence frequency of rain cells decreases as the areal fraction of convective precipitation in rain cells increases, while such frequency remains almost invariant when the areal fraction of stratiform precipitation varies from 10% to 80%. The relationship between physical and geometric parameters of rain cells shows that the mean rain rate of rain cells is more frequently associated with the increase of their area, with the increasing rate over land greater than that over ocean. The results also illustrate that heavy convective rain rate prefers to occur in larger rain cells over land while heavy stratiform rain rate tends to appear in larger rain cells over ocean. For the same size of rain cells, the areal fraction and the contribution of convective precipitation are about 10%–15% higher over land than over ocean.

雨单元是自然界中降水系统中最基本的单元。在这项研究中，通过使用热带雨量测量任务（TRMM）降水雷达（PR）进行的15年测量研究了热带陆地和海洋区域上雨单元的基本几何和物理特征。使用最小边界矩形（MBR）方法识别雨单元。结果表明，大约50％的雨单元出现在大约20 km的长度和15 km的宽度上。长> 200 km，宽> 100 km的雨单元所占比例不到1％。雨室的平均长度和宽度之间存在对数线性关系。通常，对于相同的水平几何参数，雨单元在水平方向上往往呈正方形，在陆地上垂直时呈瘦高，而在海洋上则垂直呈鳞状。^-1在陆地上至0.4–8 mm h ^-1在海洋上空。统计结果表明，雨单元的发生频率随着对流降水的面积分数的增加而降低，而当层状降水的面积分数从10％变化到80％时，这种频率几乎保持不变。雨单元的物理参数与几何参数之间的关系表明，雨单元的平均降雨率与其面积的增加更为相关，陆地上的增长率大于海洋上的增长率。结果还表明，大对流降雨率倾向于出现在比陆地更大的雨单元中，而重层状降雨率倾向于出现在海洋上更大的雨单元中。对于相同大小的雨单元，