Estimation of Long-Term Surface Downward Longwave Radiation over the Global Land from 2000 to 2018

Chunjie Feng,Xiaotong Zhang,Yu Wei,Xianhong Xie,Weiyu Zhang,Ning Hou,Shuyue Yang,Bo Jiang,Jiawen Xu

doi:10.3390/rs13091848

Abstract

It is of great importance for climate change studies to construct a worldwide, long-term surface downward longwave radiation (Ld, 4–100 μm) dataset. Although a number of global Ld datasets are available, their low accuracies and coarse spatial resolutions limit their applications. This study generated a daily Ld dataset with a 5-km spatial resolution over the global land surface from 2000 to 2018 using atmospheric parameters, which include 2-m air temperature (Ta), relative humidity (RH) at 1000 hPa, total column water vapor (TCWV), surface downward shortwave radiation (Sd), and elevation, based on the gradient boosting regression tree (GBRT) method. The generated Ld dataset was evaluated using ground measurements collected from AmeriFlux, AsiaFlux, baseline surface radiation network (BSRN), surface radiation budget network (SURFRAD), and FLUXNET networks. The validation results showed that the root mean square error (RMSE), mean bias error (MBE), and correlation coefficient (R) values of the generated daily Ld dataset were 17.78 W m−2, 0.99 W m−2, and 0.96 (p < 0.01). Comparisons with other global land surface radiation products indicated that the generated Ld dataset performed better than the clouds and earth’s radiant energy system synoptic (CERES-SYN) edition 4.1 dataset and ERA5 reanalysis product at the selected sites. In addition, the analysis of the spatiotemporal characteristics for the generated Ld dataset showed an increasing trend of 1.8 W m−2 per decade (p < 0.01) from 2003 to 2018, which was closely related to Ta and water vapor pressure. In general, the generated Ld dataset has a higher spatial resolution and accuracy, which can contribute to perfect the existing radiation products.

Highlights

The surface downward longwave radiation (Ld, 4–100 μm) is an indispensable component needed to study the Earth’s surface radiation budget and energy balance [1]
After resampling to a 5-km resolution, the ERA5 Ta, ERA5 relative humidity (RH), ERA5 total column water vapor (TCWV), global land surface satellite (GLASS) shortwave radiation (Sd), and GMTED2010DEM elevation datasets were extracted according to the latitude, longitude, and time corresponding to the ground stations; Training the gradient boosting regression tree (GBRT) model
Ld estimates for the training and test datasets against the ground measurements collected at the AmeriFlux, AsiaFlux, baseline surface radiation network (BSRN), FLUXNET, and surface radiation budget network (SURFRAD) networks from 2000 to 2018

Summary

Introduction

The surface downward longwave radiation (Ld , 4–100 μm) is an indispensable component needed to study the Earth’s surface radiation budget and energy balance [1]. Ld is not always treated as a conventional observation as other common meteorological parameters are, such as air temperature (Ta), relative humidity (RH), etc. Its observation stations are sparsely distributed and even entirely absent in certain areas due to a high cost, a difficult calibration process, and a required quality control step [2,3,4,5]. Establishing a more accurate long-term global Ld dataset is useful for improving the knowledge of the surface radiation balance but is helpful for perfecting the existing Ld products

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Methods

Results

Discussion

Conclusion