Micrometeorological measurements in an arid environment: Diurnal characteristics and surface energy balance closure

The goal of this study is to assess the surface energy balance closure (SEBC) in an arid environment through a comprehensive analysis of surface radiation, heat fluxes, and other micrometeorological measurements. These were taken from a 2.3-m eddy covariance (EC) flux tower at Al Ain (24 degrees 16'26.5535 '' N, 55 degrees 37'03.2196 '' E; 243 m a.m.s.l.) during the periods April - October 2017 and February 2018 - January 2019. The SEBC is investigated on seasonal and diurnal time-scales using the Energy Balance Ratio (EBR) and residual flux (Delta E) values. An analyses of the SEBC reveals that during daytime a three-way balance between the sensible heat, ground heat fluxes and the net surface radiative flux prevails, while at night the last two essentially balance each other. These fluxes are roughly 25% larger in the summer compared to the winter season. The surface downward shortwave radiation flux and latent heat flux peak in spring, despite the latter having rather small magnitudes (< 20 W m(-2)) throughout the year. The EBR during daytime is in the range 0.60 to 0.75. The mean Delta E during daytime (nighttime) is 30 W m(-2) (-14 W m(-2)) respectively. Further analysis of the SEBC with respect to the prevailing wind direction revealed that the maximum closure is observed for north-northwesterly winds and the minimum for southerly winds. This may be explained by the surface heterogeneity and the presence of a building to the south of the site. The SEBC is better for daytime and for periods when the friction velocity is higher.