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

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.

An All-Weather Land Surface Temperature Product Based on MSG/SEVIRI Observations

“…Daytime and nighttime cases were analyzed separately, averaging half-hourly timeslots between 00:00UTC and 02:30UTC for nighttime and between 12:00UTC and 14:30UTC for daytime. In general, the results in Figure 4 are similar to those found in previous comparison studies between model skin temperature and satellite LST [10,11,88,89]. Namely, they show a tendency for model surface temperatures to have a lower daily amplitude, particularly over arid and semi-arid regions -see the strong negative daytime bias of −4 to −7 K over the Iberian Peninsula in July, of −2 K to −4 K over part/most of Northern Africa in July/January and of −1 K to −5 K in Southern Africa in January.…”

“…In an attempt to increase the use of LST for climate-related studies, Good [8] explored relationships of LST and near-surface (2 m) air temperature over a number of standard weather stations worldwide. LST is also useful for assessing and improving parameters in surface schemes of numerical weather prediction (NWP) and Earth system models to provide more accurate surface and near-surface diagnostics [9][10][11]. It has been recognized that land surface models often struggle to correctly represent clear sky skin temperature (T skin ), particularly regarding its diurnal amplitude over arid and semi-arid regions with an underestimation of daytime and a small overestimation of night-time values [10][11][12][13].…”

“…LST is also useful for assessing and improving parameters in surface schemes of numerical weather prediction (NWP) and Earth system models to provide more accurate surface and near-surface diagnostics [9][10][11]. It has been recognized that land surface models often struggle to correctly represent clear sky skin temperature (T skin ), particularly regarding its diurnal amplitude over arid and semi-arid regions with an underestimation of daytime and a small overestimation of night-time values [10][11][12][13]. Johannsen et al [11] showed that inaccuracies in the representation of vegetation cover could largely explain the biases observed over the Iberian Peninsula.…”

“…It has been recognized that land surface models often struggle to correctly represent clear sky skin temperature (T skin ), particularly regarding its diurnal amplitude over arid and semi-arid regions with an underestimation of daytime and a small overestimation of night-time values [10][11][12][13]. Johannsen et al [11] showed that inaccuracies in the representation of vegetation cover could largely explain the biases observed over the Iberian Peninsula. However, it is unclear if other factors could be contributing to the observed bias.…”

Comparisons of Diurnal Variations of Land Surface Temperatures from Numerical Weather Prediction Analyses, Infrared Satellite Estimates and In Situ Measurements

“…The two reanalyses tended to overestimate LST at night and underestimate it in the summer daytime, which is consistent with previous results [40]. Johannsens [40] evaluated the impacts of vegetation parameters on ECMWF LSTs over the Iberian Penisula, by using GLCC (Global Land Cover Characterization which was also used in the reanalyses) and several other land-cover datasets. They found that the inaccuracies of vegetation cover used in the reanalyses could explain part of the discrepancy between the reanalyses and SEVIRI.…”