Observations And Simulations Of Low-level Jets And Urban Boundary Layer Characteristics In Beijing

In this paper,the basic characteristics of low-level jets and urban heat island were summarized using the ball intensive sounding data,automatic meteorological station data and WRF model simulation results in Beijing from August 28-September 2,2016,and the effects of low-level jets on the temperature and humidity environment in the urban boundary layer were discussed.In addition,the three-dimensional variational assimilation technique of GRAPES was used to add sounding data for simulation,the structural characteristics of the boundary layer in the context of low-level jets were analyzed,and the effects of single assimilation and cyclic assimilation on various meteorological elements in the boundary layer at different times were summarized.The main conclusions are as follows:Low-level jets are frequent during the transition period from night to early morning.The jets in urban areas are higher in height,weaker in intensity and shorter in duration.Heat islands dissipate after sunrise,form at sunset,and persist at night with high intensity;the vertical development is from the ground upward,with the strongest intensity near the ground and gradually weakening upward.The impacts of low-level jets on heat islands and dry islands in the urban boundary layer are manifested as follows:(1)4)is less than 0.25 at both urban and suburban sites,the momentum at high altitude is transferred downward,and the increased wind speed near-surface transports heat downstream from urban areas to suburban areas,causing the canopy heat islands to drift downwind and expand their area by about 1.00×10~3 km~2.(2)Wind shear is the main mechanism for the enhancement of turbulent kinetic energy in the urban boundary layer.The jets enhance the vertical mixing over the urban area,and the turbulent kinetic energy near the ground reaches 0.52 m~2/s~2,and heat mixing still exists at higher heights,resulting in a vertical temperature decrease rate of only 0.3?/100m near the ground in the urban area,which is less than half of that in the absence of jets(0.7?/100m),causing the vertical development of the heat island to rise to 200 m.(3)Warm advection below 200 m can cause the temperature rise by 8.5?/h at the downstream of the urban area.The upper-air temperature advection transport makes the three-dimensional structure of the boundary layer heat island tilt in the downwind direction,and finally behaves like a plume wake.(4)The canopy urban heat island has a good correspondence with the dry island in both shape and extent size,and the effect of jets on the heat island also acts on the dry island.The effects of data assimilation on the boundary layer simulation are as follows:(1)the addition of intensive sounding data at the time of jet in single assimilation improves the jet simulation significantly,the root mean square error of wind speed is reduced by 1.1 m/s compared with the control experiment,the correlation coefficient is improved by 0.19,and the deviation is reduced by 43%;the error of the jet intensity is reduced by 2.0 m/s,the height is consistent with the observation,and the location and range are consistent with the observation.The height,and the location and range are consistent with the observation.(2)The results of cyclic assimilation are better than those of single assimilation,and the effect of cyclic assimilation every 3-h is slightly better than that of assimilation every 6-h.The wind field of every 3-h cyclic assimilation has obvious location revision and improvement of the simulation of the jet intensity,which simulates the existence of urban and suburban inverse temperature and high temperature near-surface in the northern suburbs,reflecting the role of the jet in transporting heat from the urban area to the downstream suburbs.(3)The forecasting effect of cyclic assimilation is better than that of single assimilation,and the forecast accuracy is best in the first 3 h,and the error increases thereafter.The correlation coefficient of wind velocity after3 h of the forecast is still greater than 0.9,and the root-mean-square error is between 1.0 and1.1 m/s with a deviation of less than 10%,which is better than the assimilation effect of other schemes.