Impact of Roughness Length on WRF Simulated Land‐Atmosphere Interactions Over a Hyper‐Arid Region

Abstract

The aerodynamic roughness length is a crucial parameter that controls surface variables including the horizontal wind, surface temperature, and heat fluxes. Despite its importance, in the Weather Research and Forecasting (WRF) model, this parameter is typically assigned a predefined value, mostly based on the dominant land-use type. In this work, the roughness length is first estimated from eddy-covariance measurements at Al Ain in the United Arab Emirates (UAE), a hyper-arid region, and then ingested into WRF. The estimated roughness length is in the range 1.3-2.2 mm, one order smaller than the default value used in WRF. In line with previous studies, and from WRF model simulations during the warm and cold seasons, it is concluded that, when the roughness length is decreased by an order of magnitude, the horizontal wind speed increases by up to 1 m s −1 , the surface temperature rises by up to 2.5°C, and the sensible heat flux decreases by as much as 10 W m −2 . In comparison with in situ station and eddy covariance data, and when forced with the updated roughness length, WRF gives more accurate 2-m air temperature and sensible heat flux predictions. For prevailing wind speeds >6 m s −1 , the model underestimates the strength of the near-surface wind, a tendency that can be partially corrected, typically by 1-3 m s −1 , when the updated roughness length is considered. For low wind speeds (<4 m s −1 ), however, WRF generally overestimates the strength of the wind.

Characteristics of Atmospheric Aerosols Over the UAE Inferred From CALIPSO and Sun Photometer Aerosol Optical Depth

“…They also found that the AOD values were higher in the summer months when the near‐surface wind is stronger. This is in line with other published work (Naizghi & Ouarda, 2016; Nelli et al., 2020a, 2020b).…”

Characteristics of atmospheric aerosols over the UAE inferred from CALIPSO and Sun Photometer Aerosol Optical Depth

“…They also found that the AOD values were higher in the summer months when the near-surface wind is stronger. This is in line with other published work (Naizghi & Ouarda, 2016;Nelli et al, 2020aNelli et al, , 2020b.…”

“…Such dust layers are more likely at inland sites where the boundary layer is deeper (Filioglou et al, 2020;Reid et al, 2008), than at coastal sites like Masdar where boundary layer depths are generally below 2 km (Reid et al, 2008;Temimi et al, 2020). The deeper boundary layers inland are caused not only by the higher surface temperatures, but also the increased surface roughness, which leads to a slowdown of the near-surface wind and consequently low-level wind convergence and vertical ascent (Nelli et al, 2020b). Other explanations for these mismatches, as highlighted in (Omar et al, 2013), include differences in the instrument view angle, inhomogeneities in the target region and column, CALIPSO retrieval errors, and detection limits.…”

On the analysis of a summertime convective event in a hyperarid environment

“…As at Hatta, WRF has a tendency to overestimate the strength of the near-surface wind, but F I G U R E 11 As Figure 10 but for 2 m temperature (shading; • C) and 10 m horizontal wind (vectors; m⋅s −1 ) the biases at this station are even more pronounced. The stronger winds in the model have been noted by Fonseca et al (2020), Nelli et al (2020b), Schwitalla et al (2020), and Temimi et al (2020b), and may be explained by a poor representation of its subgrid-scale fluctuations, likely more significant over complex terrain as is the case at the Al Hajar mountains, and/or of the surface drag parametrization. As at Hatta, WRF does not capture the observed cloud cover, is colder at night and takes longer to warm up in the morning, in particular in the GFS runs.…”

“…(2020b) and Nelli et al . (2020b), where high‐resolution simulations in arid/hyperarid regions were also conducted. In the vertical, 50 levels are considered, more closely spaced in the PBL, with the first vertical level at about 25 m above ground and the model top at 30 hPa.…”