# Review of Land Surface Albedo: Variance Characteristics, Climate Effect and Management Strategy
## 2 Literature Analysis of Surface Albedo
### Definition Distinction  
Figure 1 shows major albedo variables associated with the surface, top of atmosphere (TOA), atmosphere, and cloud layers, where cloud albedo makes a major contribution to Earth's albedo [4]. Moreover, albedo in other fields is also described. Numerous studies have been carried out on surface albedo over the decades, and it has been defined differently between early [1] and recent research stages [21]. In the field of remote sensing, a recent rigorous definition is provided by [4]: \"Surface shortwave broadband albedo represents the surface hemispheric reflectivity integrated over the solar spectrum (0.2-5 \(\upmu\)m)\". Surface albedo is an important variable in surface energy balance, which can be expressed by Equation (1) [4]. The surface all-wave net radiation \(R_{n}\) represents the balance between incoming radiation from the atmosphere and outgoing radiation from the Earth's surfaces, which is a sum of shortwave net radiation (\(R_{n}{}^{s}\)) and longwave net radiation (\(R_{n}{}^{l}\)). \(\alpha_{sw}\) and \(F_{d}{}^{s}\) refer to surface shortwave broadband albedo and shortwave downward irradiance, respectively. In longwave downward (\(F_{d}{}^{l}\)) and upwelling radiation variables (\(F_{u}{}^{l}\)), \(\epsilon\), \(T_{s}\), and \(\sigma\) refer to surface longwave broadband emissivity, surface skin temperature, and Stefan-Boltzmann's constant, respectively. \[R_{n}=R_{n}^{s}+R_{n}^{l}=(1-\alpha_{sw})F_{d}^{s}+F_{d}^{l}-F_{u}^{l}=(1- \alpha_{sw})F_{d}^{s}+\epsilon F_{d}^{l}-\sigma\epsilon T_{s}^{4} \tag{1}\]  
Figure 1: The scenario for several essential albedo variables. Currently, satellite data provide a unique way to retrieve \(\alpha_{sw}\) at large scales, which relies on satellite-acquired basic quantity of reflectance that characterizes (geometrically) the reflecting properties of a specific surface. Considering the natural surface is intermediate between rough Lambert with isotropic diffuse reflection and smooth surface with specular reflection, the intrinsic reflectance anisotropy (i.e., BRDF) of a natural surface is useful primarily as an underlying concept of reflectance [33]. BRDF is defined as the ratio of increments from reflected radiance to that from incident solar irradiance over a tiny surface element at viewing and solar geometries, respectively. Solar irradiance involved in BRDF refers to the incident radiant flux onto a surface element \(dA\), and reflected radiance represents the radiant flux over the projected area of \(dA\) at a \"conical\" solid angle of reflection. Consequently, directional reflectance varies with incident and reflected geometries (i.e., solar zenith angle (SZA, \(\theta_{i}\)), view zenith angle (VZA, \(\theta_{v}\)), and relative azimuth angle (RAA, \(\varphi\))) and spectrum wavelength \(\lambda\). Before the retrieval, satellite-observed TOA reflectances at limited sun-viewing geometries are always transferred into surface reflectances by eliminating atmospheric effects [34], and then the entire BRDF change can be reconstructed by models [11]. To derive the albedo from reflectance measurements, the directional hemispherical albedo (i.e., black-sky albedo (BSA)) and bihemispherical albedo (i.e., white-sky albedo (WSA)) are computed using Equations (2) and (3) [35], which are further combined to measure blue-sky albedo weighted by a percentage of diffuse skylight S as shown in Equation (4) as a function of aerosol optical depth (AOD) [36; 37]. Finally, Equation (5) shows the integration of blue-sky albedos from bands \(\lambda_{1}\) to \(\lambda_{2}\) to obtain shortwave broadband albedo at band \(\Lambda\). In practice, narrowband-to-broadband conversion coefficients are always utilized to simplify this process based on albedos at several typical narrow bands (e.g., blue, green, red, near-infrared (NIR)), and coefficients for multiple sensors onboard polar orbit and geostationary satellites have been proposed and widely used [12; 38; 39; 40].