In this article, the climatological state and the seasonal variability of the Arabian heat low (AHL) and the intertropical discontinuity (ITD) are investigated over the Arabian Peninsula using the 1979-2019 ERA-5 reanalysis data.The AHL is a summertime feature, mostly at 15 -35 N and 40 -60 E, exhibiting a clear strengthening over the last four decades in line with the observed increase in surface temperature. However, no clear shift in its position is detected. The AHL, driven by both thermodynamic and dynamic forcing, is broader and stronger during daytime and exhibits considerable variability on day-to-day timescales, likely due to the convection associated with the Asian summer monsoon. The ITD is the boundary between the hot and dry desert air and the cooler and more moist air from the Arabian Sea. It lies along the Arabian Peninsula's southern coastline in the cold season but reaches up to 28 N between 50 and 60 E in the summer months. While in the former it has a rather small diurnal variability; in the latter it shows daily fluctuations of up to 10 in latitude. The presence of the Sarawat Mountains over southwestern Saudi Arabia precludes a northward migration of the ITD in this area. The ITD exhibited a weak northward migration in the 41-year period, likely due to the increased sea surface temperatures in the Arabian Sea. On inter-annual timescales, the El Niño-Southern Oscillation, the Indian Ocean Dipole, and solar-geomagnetic effects play an important role in the AHL's and ITD's variability.

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A thorough evaluation of the Weather Research and Forecasting (WRF) model is conducted over the United Arab Emirates, for the period September 2017 - August 2018. Two simulations are performed: one with the default model settings (control run), and another one (experiment) with an improved representation of soil texture and land use land cover (LULC). The model predictions are evaluated against observations at 35 weather stations, radiosonde profiles at the coastal Abu Dhabi airport, and surface fluxes from eddy-covariance measurements at the inland city of Al Ain. It is found that WRF’s cold temperature bias, also present in the forcing data and seen almost exclusively at night, is reduced when the surface and soil properties are updated, by as much as 3.5 K. This arises from the expansion of the urban areas, and the replacement of loamy regions with sand, which has a higher thermal inertia. However, the model continues to overestimate the strength of the near-surface wind at all stations and seasons, typically by 0.5 to 1.5 m s-1. It is concluded that the albedo of barren/sparsely vegetated regions in WRF (0.380) is higher than that inferred from eddy-covariance observations (0.340), which can also explain the referred cold bias. At the Abu Dhabi site, even though soil texture and LULC are not changed, there is a small but positive effect on the predicted vertical profiles of temperature, humidity, and horizontal wind speed, mostly between 950 and 750 hPa, possibly because of differences in vertical mixing.