Impact of surface heat flux differences on mixed-layer height: ground-based remote sensing measurements at Yatir forest, Israel

The growth of the convective boundary layer, or mixed layer (ML) is driven by heat transfer from the ground, but it is not clear to what extent the ML responds to spatial differences in surface forcing. The Yatir forest in Israel is excellently suited for studying the impact of surface heterogeneities on the ML height under cloud-free conditions. In the summer at noon time, the sensible heat flux is approximately 300 W m^-2 larger above the forest than above the surrounding semi-arid desert, due to the low albedo of the forest and the increased turbulence above the canopy. In order to verify whether such large spatial differences in surface sensible heat flux on the scale of 10 km affect the height of the ML, ground-based remote sensing measurements were performed in the desert and the forest from 21 August to 10 September 2013, using a ceilometer and a Doppler wind lidar. Under the assumption that the top of the ML follows the topography, we found that on half of the days during the measurement campaign, the ML above the forest is significantly deeper than above the desert, whereas on the other days, no significant differences between forest and desert were detected. On 5 of the 16 measurement days, the rise of the ML above the forest is accompanied by a positive mean vertical wind velocity, which is strongest in the middle of the convective boundary layer. This suggests that a secondary circulation develops above the forest. During the afternoon hours, we found a strong decrease in ML height above the forest as well as above the desert. That could be attributed to the advection of a marine boundary layer from the Mediterranean Sea during the early afternoon hours, which is associated with a cool and humid sea breeze at the surface.