Theoretical examination of non-summer daytime boundary layer height variability over coastal-mountain-valley topography: the case of central Israel

The center of Israel presents relatively simple vertical topographical cross-section: The eastern Mediterranean coast on the west, Judea and Samaria mountain in the center and the deep steep Jordan valley on the east.To the best of our knowledge, the variability of the boundary layer height (BLH) over Israel during the non-summer period was not investigated. This work presents climatological examination of the daytime BLH variability during non-summer months according to WRF simulations with 3 km horizontal resolution during 5 years. The monthly average BLH above the mountain peak and its easterly slope is found to be lower than that over the coastal area (1-2.5 .vs. 2-3 km) during the winter months, November-February, while during the rest of the year (except March) the situation is reversed.In order to track the mechanism responsible for the reduction of the BLH over the mountain peak area during winter, an examination of the BLH variability during 10-13 UTC was performed. Accordingly, events with relatively weak pressure gradients and weak (< 5 m/s) easterly flow were found to be responsible for minimal BLH (< 350 m). The synoptic pressure during these events is characterized by ridge from the south in the middle troposphere (500 hPa) and central Red Sea Trough or high to the east or to the north of Israel next to the surface. Fohn winds which locally reduce the humidity and enhance the temperature due to subsidence flow over the eastern slope of the valley and the western slope of the mountain. In such cases, the mild synoptic pressure gradients are responsible for the maintenance of the local cooling over the mountain peak. The advection of hot air to the mountain peak is limited and therefore relatively stable thermal stability is obtained over the mountain peak and its eastern slope.