Cold Bias of ERA5 Summertime Daily Maximum Land Surface Temperature over Iberian Peninsula

Johannsen Johannsen,Dutra Dutra,Trigo Trigo,Nogueira Nogueira,Martins Martins,Ermida Ermida

doi:10.3390/rs11212570

Abstract

Land surface temperature (LST) is a key variable in surface-atmosphere energy and water exchanges. The main goals of this study are to (i) evaluate the LST of the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim and ERA5 reanalyses over Iberian Peninsula using the Satellite Application Facility on Land Surface Analysis (LSA-SAF) product and to (ii) understand the main drivers of the LST errors in the reanalysis. Simulations with the ECMWF land-surface model in offline mode (uncoupled) were carried out over the Iberian Peninsula and compared with the reanalysis data. Several sensitivity simulations were performed in a confined domain centered in Southern Portugal to investigate potential sources of the LST errors. The Copernicus Global Land Service (CGLS) fraction of green vegetation cover (FCover) and the European Space Agency’s Climate Change Initiative (ESA-CCI) Land Cover dataset were explored. We found a general underestimation of daytime LST and slightly overestimation at night-time. The results indicate that there is still room for improvement in the simulation of LST in ECMWF products. Still, ERA5 presents an overall higher quality product in relation to ERA-Interim. Our analysis suggested a relation between the large daytime cold bias and vegetation cover differences between (ERA5 and CGLS FCocver) with a correlation of −0.45. The replacement of the low and high vegetation cover by those of ESA-CCI provided an overall reduction of the large Tmax biases during summer. The increased vertical resolution of the soil at the surface, has a positive impact, but much smaller when compared with the vegetation changes. The sensitivity of the vegetation density parameter, that currently depends on the vegetation type, provided further proof for a needed revision of the vegetation in the model, as there is a reasonable correlation between this parameter and the Tmax mean errors when using the ESA-CCI vegetation cover (while the same correlation cannot be reproduced with the original model vegetation). Our results support the hypothesis that vegetation cover is one of the main drivers of the LST summertime cold bias in ERA5 over Iberian Peninsula.

Highlights

Land Surface Temperature (LST) is a key variable for the surface-atmosphere energy and water exchanges and it was recently integrated as an Essential Climate Variable (ECV) into the Global Climate Observing System (GCOS) [1]
These results suggest that the large systematic underestimation of RdeamyottiemSeensL. 2S0T19c, a11n, 2b5e70partially attributed to an overestimation of total vegetation cover in ERA51.1Tohf 2is1 overestimation of vegetation will be reflected in a higher coupling between the skin layer and the atmosphere via turbulent exchanges
We found a general underestimation of daytime LST and slightly overestimation at night-time

Summary

Introduction

Land Surface Temperature (LST) is a key variable for the surface-atmosphere energy and water exchanges and it was recently integrated as an Essential Climate Variable (ECV) into the Global Climate Observing System (GCOS) [1]. The use of satellite LST has been steadily increasing during the last decades, from the evaluation and improvement of land surface models [5,6,7,8] to filling gaps in 2-meters air temperature (T2m), in areas where station coverage is poor [9]. The latter has great potential to improve the quality of T2m observation datasets [10]. This is relevant due to LST’s globally available datasets, while T2m is only measured in in-situ stations

Objectives

Methods

Findings

Discussion

Conclusion