Introduction: Boundary layer processes play an important role in controlling the daily mean air temperature through exchange of turbulent fluxes of water and energy between the ground surface and the atmosphere. Therefore, it is important to explore the role of boundary layer parameterization in simulating the daily mean air temperature particularly on a regional scale.

Methods: To accomplish this task, two simulations were conducted to assess the sensitivity of the mean air Temperature (TMP) of Egypt to boundary layer schemes (Holtslag; HOLT and University of Washington; UW) within the framework of the Regional Climate Model (RegCM4). The RegCM4 was downscaled by the ERA-Interim reanalysis of 1.5 degrees with 25 km horizontal grid spacing over the period 1997-2017. Additionally, ERA5 reanalysis product was used as the observational dataset for the variables: the Total Cloud Cover (CLT), surface short and long wave radiation fluxes (RSDS and RLDS), Skin Temperature (TS), surface Wind Speed (WS), Sensible Heat Flux (HFSS) and TMP.

Results: There is no difference between HOLT and UW regarding the CLT and RSDS; while the UW reduces the bias of the RLDS and WS relative to HOLT particularly in the summer and autumn seasons in comparison with ERA5. Furthermore, HOLT and UW schemes showed a considerable impact on the simulated TS. For instance, UW reduces the TS bias relative to HOLT in all seasons but in the spring and winter seasons; UW induces a negative bias more than HOLT. Concerning the HFSS, UW shows a higher bias than HOLT mainly in the summer and autumn seasons. Because of the noted changes of the simulated HFSS and TS, the UW succeeds in reducing the warm bias of the TMP (noted in the HOLT) in all seasons in comparison with the ERA5. With respect to in-situ observations, the HOLT/UW performance varies with the location as well as the month.

Conclusion: The UW scheme can be recommended for future studies concerning the future climate change of Egypt concerning the regional scale. However for a point scale, caution is needed in interpreting the results with respect to the in-situ observations except for Asswan where HOLT can be recommended.