A surface temperature and moisture intercomparison study of the Weather Research and Forecasting model, in‐situ measurements and satellite observations over the Atacama Desert

Abstract

Good knowledge of the environmental conditions of deserts on Earth is relevant for climate studies. The Atacama Desert is of particular interest as it is considered to be the driest region on Earth. We have performed simulations using the Weather Research and Forecasting (WRF) model over the Atacama Desert for two week‐long periods in the austral winter season coincident with surface temperature and relative humidity in‐situ observations at three sites. We found that the WRF model generally overestimates the daytime surface temperature, with biases of up to 11°C, despite giving a good simulation of the relative humidity. In order to improve the agreement with observed measurements, we conducted sensitivity experiments in which the surface albedo, soil moisture content and five tuneable parameters in the Noah Land Surface Model (namely soil porosity, soil suction, saturated soil hydraulic conductivity, the b parameter used in hydraulic functions and the quartz fraction) are perturbed. We concluded that an accurate simulation is not possible, most likely because the Noah Land Surface Model does not have a groundwater table that may be shallow in desert regions. The WRF‐predicted land surface temperature is also evaluated against that estimated from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. While at night the satellite‐derived and ground‐based measurements are generally in agreement, during the day MODIS estimates are typically lower by as much as 17°C. This is attributed to the large uncertainty in the MODIS‐estimated land surface temperatures in arid and semi‐arid regions. The findings of this work highlight the need for ground‐based observational networks in remote regions such as the Atacama Desert where satellite‐derived and model products may not be very accurate.

On the Analysis of the Performance of WRF and NICAM in a Hyperarid Environment

“…This can be attributed to (i) an underestimation of the surface albedo, as found to be the case in comparison with eddy-covariance measurements at Al Ain in the April event, with the estimated albedo at Al Ain of around 0.314, contrasting with 0.262 for NICAM and 0.216 for WRF; (ii) the underprediction of the observed cloud cover; (iii) deficiencies in the LSMs, may explain the biases in the air temperature. Fonseca et al (2019), and for simulations over the Atacama Desert, found that an increase in the surface albedo of 15% leads to a decrease of the daytime surface temperature by about 0.58-18C, and a change in the air temperature of an even smaller magnitude, not exceeding 0.58C. Given this, factors (ii) and (iii) are likely to play the largest roles in the models' air temperature biases.…”

“…The main goal of the work is to better understand the limitations of the two models in a hyperarid environment, where numerical models are known to underperform and satellite-derived meteorological variables, such as surface temperature, are not very reliable (e.g., Gunwani and Mohan 2017;Ozturk et al 2012;Wehbe et al 2017Wehbe et al , 2018Fonseca et al 2019). This study is particularly relevant as desert regions are predicted to expand in a hypothetical warmer world and are known to be very sensitive to climate change (e.g., Feng et al 2014;Huang et al 2017;Kumar et al 2017;Aldababseh et al 2018).…”

“…Should this underprediction of the albedo be the case elsewhere in the UAE, it can also explain the warmer daytime temperatures predicted by both models. However, and in light of the results of Fonseca et al (2019), the lack of clouds and potential biases in the LSMs are likely to play the largest role in the daytime air temperature biases. As a result of this, the ground surface is warmer during the daytime, leading to an enhanced upward longwave radiation flux, mostly on 13 and 14 April.…”

“…While both models were largely used worldwide, their verification in the context of a hyperarid environment like the one in the UAE has not been thoroughly addressed in the literature. Fonseca et al (2019) ran the WRF Model over the Atacama Desert for two-week long periods in the local winter season and reported very large biases in the daytime surface temperature, which exceeded 118C. What is more, in situ surface temperature measurements were found to be very different from the satellite-derived surface temperatures from the Moderate Resolution Imaging Spectroradiometer (MODIS; Wan and Dozier 1996) payload imaging sensors.…”

Vegetation growth and landscape genetics of Tillandsia lomas at their dry limits in the Atacama Desert show fine‐scale response to environmental parameters

“…In these hyperarid areas, mean annual precipitation (MAP) is <25 mm (Thomas, 2011), and the aridity index, defined as the ratio of MAP to annual potential evapotranspiration, is <0.05 (Middleton & Thomas, 1997). The respective soil systems are among the harshest terrestrial biomes on Earth (Frossard et al., 2015) with surface temperatures ranging from 0 to 50°C (Eckardt et al., 2013; Fonseca et al., 2019). Water is extremely rare, and availability is often highly stochastic (Kumar et al., 2015).…”