The behaviour of a surface temperature/vegetation index (TVX) matrix derived from 10-day composite AVHRR images over monsoon Asia

摘要

Satellite remote sensing of surface-related variables is essential for understanding regional or global land-surface processes. This study focuses on a Temperature/Vegetation Index (TVX) matrix that combines surface temperature (Ts) and a normalized difference vegetation index (NDVI). The results of our study show the behaviour of the TVX matrix on continental scales. The study domain includes eastern Eurasia and monsoon Asia-regions with great variability in land-surface conditions. The data used included a 10-day composite Advanced Very High Resolution Radiometer (AVHRR) dataset compiled by the US Geological Survey (USGS). The relaxation in the determination algorithm for TVX slope (an upper envelope line in a TVX matrix box) was conducted to obtain both the negative and positive slope. The TVX slope can be derived from previous studies as the monsoon advanced and retreated over the tropics. However, over the Tibetan Plateau, a time series of the TVX slope showed an opposite sign compared to those in previous studies (represented by a positive TVX slope). Scatter plotting of the TVX matrix pixel sets was conducted for the evaluation of a variety of TVX matrix pixels. The TVX slope error sometimes occurred over arid regions because of a few green pixels corresponding to oases or irrigated areas. On the Tibetan Plateau, 'two' TVX slopes, both negative and positive, were found in the scatterplot. The reason for the two TVX slopes is the energy consumption in the soil, particularly, the phase change from frozen to liquid water. However, further study will be required to understand the mechanisms on the Plateau.