## 2 Materials and Methods
### ISEA4H Subdivision Model of the Temporal and Spatial Grid of the Earth System's Layers
#### 2.2.5 The ISEA4H-ESSG Layered Block Subdivision Model of the Earth-System Spatiotemporal Grid  
The subdivision of the three-dimensional space of the Earth's layers is carried out in two directions: the layer surface and the layer radius. The single spherical surface obtained from the hexagonal grid subdivision based on the regular icosahedron can serve as the subdivision surface for the radial subdivision of the Earth's space. After dividing the three-dimensional space of the Earth's layers into several reference layer bodies, the spatial subdivision will be carried out around the hierarchical subdivision of the near-Earth layer surface. The three-dimensional space of the Earth's layers is divided into a series of spatial units by using the subdivision surface, which can be called subdivision block elements. The geometric structure of each subdivision block element can be described as: V[S, E, P], where V represents the layer body, S represents the surface of the layer body, E represents the edges of the layer body, and P represents the faces of the layer body. Any subdivision block is composed of 12 vertices, 18 edges, and 8 faces. As shown in Figure 8, it is the division of each layer of the Earth. The layer body that this study focuses on is the near-Earth space layer, that is, the layer within the height from the Earth's surface to the low-orbit satellites, which can be regarded as a spherical  
Figure 7: The global subdivision result at the third level. shell with a certain thickness. The division of the reference layer bodies is based on the layered structure of the Earth system and the characteristics of human activities in each layer. Through the reference layer bodies, the division range of the layered spatial grid can be defined, and the hierarchical subdivision of the layered spatial grid can be made more in line with the needs of modeling and analysis in a specific layered space. According to the specific modeling needs, setting the reference layer body is actually an expansion of the division of the reference layer bodies. The reference layer body can be regarded as a finer layered structure. For example, reference layer bodies are set up for the stratosphere and troposphere of the atmosphere layer. In this way, the spatial distribution of the grid units in the layered space can be closer to the requirements of the specific modeling space. For the spherical shell between the reference layer body and the target layer body, a radial subdivision model is established. Influenced by the regular subdivision mechanism of SDOG (Degenerate Quadtree Grid), a radial regular subdivision model is established. For the Earth system, the subdivision is carried out by using the independent subdivision methods of the layer radius and the layer surface. The independent subdivision of the layer radius means that the layer radius is subdivided relatively until it meets specific requirements, and the grid of the layer surface remains unchanged during this process. The independent subdivision of the layer surface means that the layer surface of each layer's spherical shell is subdivided until it meets specific requirements, and the division of the layer radius remains unchanged during this process. Different subdivided-unit spherical shells on the layer radius can obtain the scale closest to the specific requirements according to the rules of spherical subdivision, and the grids on each spherical surface and the grids on the edges of each layer body radius can be infinitely subdivided, respectively. In the scientific research of the Earth system, data with uneven spatial sampling are often used, such as NCEP FNL, which includes parameters such as vertical velocity and temperature. The resolution of these data in the radial direction of the Earth is 0.1-0.5 km, and the resolution on the spherical surface is 110 km-110 km (close to \(1^{\circ}\times 1^{\circ}\)). For this kind of data  
Figure 8: The subdivision model of the layer radius.