The Cooling Efficiency Factor Index (CEFI): A New Satellite-Based Indicator for Research and Operational Monitoring of Land Surface Processes and Beyond

The cooling efficiency of the land surface, i.e. its ability to dissipate absorbed radiation and moderate temperature rise, is manifested by its apparent heat capacity, a property of the land surface that varies throughout the day in response to the intensities of the sensible and latent heat fluxes. Under clear sky conditions, the daytime increase in apparent heat capacity can be reliably estimated from geostationary satellite data and is used to define the Cooling Efficiency Factor Index (CEFI), which uniquely characterizes the temperature response to radiation at a given location on a given day. At longer time scales, the temporal variability of CEFI is modulated by several factors associated with changes in land surface state, including land cover, soil moisture availability, as well as the structure and dynamics of the atmospheric boundary layer. These relationships can be exploited to derive proxies for variables and processes that are otherwise difficult to observe, especially in real time. Here, we recall the definition of CEFI and we also present several applications. As already demonstrated, the CEFI index can serve as an indicator of vegetation drought stress, the condition when plants close their stomata due to soil water limitation and excessive atmospheric moisture demand, and vegetation productivity. In addition, the CEFI can act as a proxy for surface wind stress over arid regions with sparse vegetation. Consequently, CEFI can be involved in the detection of flash drought and to estimate fire risk in natural ecosystems, crop production losses in agricultural areas, and dust formation in desert regions. The CEFI can also be applied to quantify the cooling efficiency of urban areas. Finally we introduce the estimation of the apparent heat capacity over the sea surface, with potential implications for estimating wind over the sea and mixed layer depth from a purely observed perspective. Given its broad range of applications, our next step is to compute CEFI using multiple geostationary satellites to extend its spatial coverage as well as to further demonstrate its applicability range.