Diurnal Cycles of the Boundary Layer Structure Simulated by WRF in Beijing

The planetary boundary layer (PBL) is directly affected by the presence of the earth's surface, responding to such forcing as frictional drag, solar heating and evapotranspiration. The PBL over land generally undergoes significant diurnal cycles. The daytime PBL develops when the upward net surface heat flux increases after sunrise. The mass and wind fields in this mixed layer adjust quickly to produce a state of equilibrium until the PBL reaches its maximum depth in the late afternoon. After sunset, the rapid radiative heat losses occur at the ground so that a second temperature inversion starts to grow near the bottom surface.Although many studies focus on the sensitivity of the structure of the PBL simulation to the PBL parameterization, little attention has been paid to the land surface processes' contribution to the development of the PBL, especially the wind field in surface layer. Two planetary boundary layer parameterizations and 3 land surface model (LSM) in WRF model are used to simulate the PBL structures in Beijing area during July 1—4, 2004. The diurnal cycles of surface winds and temperatures and PBL height are analyzed, which shows the temperature and wind fields in the surface layer are sensitive to both the PBL parameterization and the land surface process. The simulations show MYJ scheme gives the more reasonable results than YSU. Noah land surface model considers the soil moisture variation in addition to soil temperature, as the result, the sensitivity of the temperature and wind in surface layer to the PBL scheme becomes strong as the rain appears when use Noah LSM. Noah LSM can produce the heat island phenomenon because it couples a single layer urban canopy model. As a new LSM implement in WRF, RUC cannot describe the variation of the PBL very well. All results of the PBL schemes and land surface models simulated wind speeds in surface layer are much stronger than observations in urban area due to insufficient consideration of the urban building effects. Not only the urban area-averaged potential temperature vertical profiles but also the wind vertical profiles are sensitive to both the PBL schemes and land surface processes.