Influences Of Boundary Layer Vertical Mixing And Land Surface Parameterizations On Numerical Simulations Of Landfalling Hurricanes

Landfalling hurricanes are the significant weather-related threat both onshore and inland.They cause great loss due to their strong winds,heavy rainfall,soil erosion,and flooding.Accurate prediction of landfalling hurricanes and their further evolution over land is a great challenge and a critical scientific issue for numerical weather prediction(NWP).Understanding the evolution of landfalling hurricanes is essential to improving their forecasts.Hurricanes usually undergo a weakening process during their landfall.With a research version of the Hurricane Weather Research and Forecasting(HWRF)model,this study examines the effects of boundary layer vertical mixing on the evolution of three landfalling hurricanes(Dennis,Katrina,and Rita in 2005),all of which underwent a normal and continuous decay process after landfall.It is found that the simulation of landfalling hurricanes is very sensitive to vertical mixing in the PBL.Specifically,increasing vertical mixing in the hurricane boundary layer over land can well portray the hurricane decay process after landfall with great improvement in the simulation of storm track,intensity,synoptic flow,rainfall structure,etc.However,decreasing vertical mixing in the hurricane boundary layer tends to overestimate storm intensity over land.Further diagnoses indicate that,increased vertical mixing in the hurricane boundary layer can result in a decrease in the vertical wind shear and an increase in the vertical gradient of virtual potential temperature,which destroys the turbulent kinetic energy in the hurricane boundary layer and inhibits hurricane development over land.In other words,increasing vertical mixing in the hurricane boundary layer stabilizes the hurricane boundary layer and limits its maintenance.Further investigation modified the parameterization of vertical eddy diffusivity in the PBL for both momentum(Km)and thermal(Kh).Results show that,the impacts of the vertical eddy diffusivity parameterization are mainly on hurricane inner-core structures.Specifically,the modification leads to stronger vertical mixing in the hurricane boundary layer over land,and changes in the hurricane boundary layer are closely related to the estimation of the Km parameter in HWRF.Consequently,simulated landfalling hurricanes attenuate effectively with the modified Km while they mostly inherit their characteristics over ocean and decay inefficiently with the original Km.This sensitivity study of the parameterization of vertical eddy diffusivity highlights the great importance and uncertainty of the parameterization of Km in the HWRF PBL scheme in the simulation of landfalling hurricanes.Overall,with efficient vertical mixing in the hurricane boundary layer,simulated storms are weaker and tend to have a drier and colder environment,larger eyewall slope,larger storm sizes,weaker inflow/outflow,and weaker updrafts and downdrafts.They also have a weaker warm core,a weaker spiral wind structure,a shallower boundary layer and weaker,looser,and less organized convective rainbands that evolve far from the storm center.Energy supplied through surface enthalpy fluxes is essential to hurricane development and maintenance.A sensitivity study of the effects of land surface parameterizations on the evolution of post-landfall Tropical Storm Bill(2015),which decays slowly with a much longer lifespan over land compared to most of the normal landfall cases,further illuminates the importance of land surface parameterization on the simulation of storm evolution over land.Comparison of results between the SLAB and the NOAH land surface schemes show that,NOAH produces better simulation results than SLAB in terms of storm track,intensity and rainfall structures during the storm's interaction with mid-latitude westerlies over the Great Plains.Specifically,the simulated storm with NOAH has higher cyclonic vorticity than those with SLAB,leading to a weaker eastern steering wind and the evident improvement of the storm track simulation.Further comparison shows that the response of boundary layer vertical mixing to different land surface parameterizations has noticeable discrepancy in both the storm environment and the storm itself.Compared to the SLAB scheme,vertical mixing in the storm boundary layer is stronger in NOAH scheme,leading to weaker storm over land.Moreover,the diural effects in the storm environment are more evident with NOAH scheme than with SLAB scheme.In particular,in the storm environment,a warmer land surface with stronger surface heating is significant during the daytime,leading to significant upward surface sensible and latent heat fluxes;meanwhile,this warmer land surface also produces an unstable,deep boundary layer with stronger vertical mixing.Therefore,the energy supplement to the storm environment from the land surface is significant during the daytime,which could be important to the maintenance of storm evolution over land.During the nighttime,however,the land surface becomes colder,resulting in the stabilization of the boundary layer with a decrease in its thickness,destroyed turbulence,weaker vertical mixing,and weaker surface enthalpy fluxes.Therefore,storm maintenance over land is inhibited.In other words,the energy supplement with higher from the storm environment during the daytime is important to the maintenance of the storm over land.This higher air is stronger in NOAH than in SLAB because of the associated stronger heating effects from the land surface.Simulations highlight the importance of land surface parameterizations in the simulation of landfalling hurricanes.A sensitivity study of the effects of initial soil moisture on the simulation of Tropical Storm Bill(2015)over land is also conducted.Results indicate that increasing initial soil moisture makes the storm weaker during its landfall since it tends to cool the land surface and decrease surface enthalpy fluxes over both the storm and its environment.Meanwhile,it also increases the vertical mixing in the storm boundary layer.Overall,simulation results highlight that initial soil moisture could be a key factor that influences the simulation of landfalling hurricanes in numerical models.