The Planetary Boundary Layer Height Variability over the Arabian Peninsula

We investigate the Planetary Boundary Layer (PBL) height and its variability at different time scales in the Arabian Peninsula (AP) as simulated by a high resolution assimilative Weather Research and Forecasting (WRF) regional model. The model is configured at 5-km resolution and 59 vertical layers, 30 of which covering the surface to the lower troposphere to well resolve the boundary layer characteristics. The ERA-Interim reanalysis fields are used as initial and boundary conditions and the model is initialized at 1200 UTC on a daily basis and integrated for 36-hours using boundary conditions updated every six hours. All available observations in the region are assimilated on 6-hourly 3DVAR cycles, after which the first 12-hour are discarded as spin-up and the hourly outputs from +12 till +36 hour simulations are stored to generate a 38 years regional climate reanalysis for the AP between 1980 and 2016.

The simulated PBL height was first validated against the Radiosonde and MERRA gridded data. The mean spatial and temporal variability of model PBL height is in good agreement with the observations at different time scales, suggesting 20-25% errors in the shallow boundary layers and 5-10% in the deeper boundary layers. The model simulated deeper PBL heights than the observations, due higher surface temperatures simulated by the PBL scheme in the daytime. This resulted in stronger sensible-heat fluxes to the atmosphere, with the associated deeper turbulent mixing causing an increase in the depths of the turbulent transfer which enhances the PBL heights. Overall, the AP reanalysis well described the regional variability of PBL height compared to the data.

Analysis of the AP reanalysis suggests that the mean PBL height climatology exhibits different daytime and nighttime seasonal patterns; daytime heights are larger in summer than winter, but nighttime heights are larger in winter. The annual climatological PBL heights are typically around 2.0 km during daytime and around 0.5 km at night. The hourly PBL heights outlines the diurnal cycle of the AP PBL height, with a maximum in the afternoon, and seasonally (larger in summer) and regionally (higher over the desert regions than the mountain and coastal regions) varying magnitudes.