Comprehensive study of energy and water exchange process over the Tibetan Plateau: A review and perspective

Containing elevated topography, the Tibetan Plateau (TP) has significant thermodynamic effects for regional environment and climate change, where understanding energy and water exchange process (EWEP) is an important prerequisite. However, estimation of the exact spatiotemporal variability of the land-atmosphere energy and water exchange over heterogeneous landscape of the TP remains a big challenge for scientific community. Focused on the above scientific question, a series of atmospheric scientific experiments and research programs have been conducted since the 1960s, quantitatively evaluating both the spatial distribution and the multi-timescale variation of EWEP via observation, remote sensing, and numerical simulation. Based on the three main approaches, the major advances on EWEP over the past 25 years are systematically summarized in this work. Observations reveal distinct characteristics of the energy balance components and micrometeorological parameters. The roughness length for momentum is generally one order of magnitude higher than that for heat, and a distinct diurnal cycle of the excess resistance for heat transfer (kB^-1) is captured. These progresses via observations further contributed to the improvement of remote sensing parameterization and numerical simulation of EWEP, e.g., the daily sensible heat flux can be overestimated by approximately 50% using a fixed , while this overestimation can be mitigated with the observation-captured diurnal variation in  taken into consideration. Moreover, multisource (multispectral, thermal, and microwave) satellite data have been successfully used to retrieve key land–atmosphere properties, which offers a feasible way to monitor EWEP at different spatiotemporal scales: A decreasing trend of sensible heat flux and an increasing trend of latent heat flux over the TP from 2001 to 2012 were reported. Hourly data of land surface heat fluxes over the entire TP were first obtained, with root mean square errors of 76.6 W m⁻² (net radiation flux), 60.3 W m⁻² (sensible heat flux), 71.0 W m⁻² (latent heat flux) and 37.5 W m⁻² (soil heat flux), superior to the previous flux products. The total annual evaporation is approximately 51.7 ± 2.1 km³ year^-1 for high-elevation lakes with ice sublimation component accounting for around 10-25%. In addition, different numerical models have been evaluated and improved to study EWEP over heterogeneous land surfaces. The simulation accuracy of land surface temperature and surface energy balance in arid and semiarid areas was improved via an improved heat roughness parameterization scheme in Noah. The sensible heat flux was also effectively improved in the CoLM model by adopting an independent method to determine aerodynamic roughness length. All these results advanced the understanding of different aspects of EWEP over the TP by using in situ measurements, multisource satellite data and numerical modeling. Future studies are recommended to focus on the optimization of the current three-dimensional comprehensive observation system, the development of advanced parameterization schemes and the investigation of EWEP on weather and climate changes over the TP and surrounding regions.