Research On Tropical Cyclone Boundary Layer Processes And Parameterizatio

The boundary layer process is an important part of tropical cyclone（TC）,in order to understand the boundary layer process and parameterization of TCs,using the Weather Research and Forecasting（WRF）model,with a series of numerical experiments,this study examined the sensitivity of Typhoon Mangkhut（2018）simulation during the intensification period to different surface layer air-sea flux parameterization schemes（isftcflx option from Revised MM5 surface layer scheme）,different planetary boundary layer（PBL）parameterization schemes（including YSU,MYJ,QNSE,MYNN2,MYNN3,ACM2,and BouLac PBL scheme）,and modified turbulent diffusivity K_m（from YSU PBL scheme）.The main results as follows:（1）The results from the effects of different surface layer air-sea flux parameterization on the TC simulation show that 3 schemes basically reproduce TC track and intensity of observation,and the simulated exchange coefficient of three schemes is consistent with theoretical results.Using the same upper limit of C_d as OPTO and OPT2,OPT 1 has much larger C_k than the other two options,which leads to larger latent heat（and sensible heat）flux and produces stronger inflow（within boundary layer）and updrafts（around eyewall）,and thus stronger TC intensity.Meanwhile,the results that larger C_k/C_d corresponds with stronger TC in the mature stage are consistent with Emanuel’s potential intensity theory.The fact that C_k in OPT1 is evidently larger than the C_k from previous studies leads to produce a better TC intensity simulation.（2）The results from the effects of different PBL parameterization on the TC simulation show that,compared with observation,all the numerical simulations generally reproduced TC track and intensity,and YSU,QNSE,and BouLac generally captured the rapid intensification processes and its simulated TC intensity is close to the observation.During the intensification period,these 3 PBL schemes produced stronger tangential wind,radial inflow in the boundary layer,and updraft around the eye wall than other PBL schemes,which is consistent with the results of TC intensity.QNSE PBL scheme with QNSE surface layer scheme simulated the largest C_k/C_d ratio,leading to its stronger TC intensity based on potential intensity theory.And among the simulation using the same surface layer scheme,YSU and BouLac simulated weaker PBL turbulent diffusivity with smaller K_m and shallower boundary layer mixing height,which resulted in their stronger TC intensity.Generally,PBL and surface layer parameterization both affect simulated TC intensity.QNSE with the largest C_k/C_d,and YSU and BouLac with weaker turbulent diffusivity,have stronger warm core structures,potential vorticity within the inner core,radial inflow at low-level and larger updraft along the eyewall,which leads to their stronger TC intensity.（3）The results from the effects of boundary layer turbulent diffusivity on the TC simulation show that,a smaller K_m tends to produce a smaller grid-point minimum central sea level pressure（MSLP）,but a larger area-averaged MSLP and smaller area-averaged maximum 10 m wind speed（VMAX）,consistent with a weaker radial inflow averaged over the area of 500 km in radius.The surface latent heat flux shows a nearly linear relationship to K_m throughout the simulation.Overall,a larger K_m tends to produce a greater diabatic heating rate,although the relationship between K_m and diabatic heating is not as clean as that between K_m and latent heat flux due to the spontaneous asymmetric convective eddy features.Since a larger K_m tends to produce a larger 10 m tangential wind,the size of a TC defined by the radius where 10 m tangential wind speed becomes 10 m s^-1,shows a nearly linear relationship to K_m with a larger K_m corresponding to a larger R10.Generally,we should use more reasonable air-sea flux parameterization based on observation to improve TC intensity simulation.