Effect And Mechanism Of Direct Radiative Effects Of Inhomogeneous Sea-salt Aerosols On The Precipitation Of Landfalling Tropical Cyclone

The problem of precipitation produced by landfalling tropical cyclones has long been paid close attention at home and abroad.Tropical cyclones spend most of their life over ocean,and the major aerosols over the ocean come from sea salt.In current numerical models,SSAs are normally treated as homogeneous spherical liquid droplets at different relative humidity levels to characterize their radiation properties.However,studies show that sea salt particles are present an inhomogeneous state of water surrounding the core solute.during the deliquesce and crystallization processes.The physical process associated with the inhomogeneity of sea salt particles has never been considered in research and numerical forecast of TCs,whether and how the inhomogeneity of sea salt aerosols influences precipitation of TCs remains unclear,especially over the high sea-salt-production oceans.Based on the observational data,the influence and mechanism of the inhomogeneous sea salt aerosols on the precipitation of the landing typhoon Fitow(No.23 strong typhoon in 2013)are investigated through thermodynamic analysis and numerical experiments.The main conclusions are summarized as follows.Inhomogeneous sea salt aerosols influence the direct radiation effect through local cooling.The results of using the two-layer coated sphere model to represent the inhomogeneous sea salt aerosols show that the inhomogeneity of sea salt aerosols have little effect on the extinction coefficient and single albedo,but significantly reduces the asymmetry factor,particularly in the short wavelength range.According to the definition of the asymmetry factor,a decrease in the asymmetry factor means an increase in the backscattering of particles.Therefore,the inhomogeneity of sea salt aerosols will influences the direct radiation effect by affecting the optical properties such as reducing the asymmetry factor.When a tropical cyclone moves to the coast and is about to make landfall,it is usually intruded by environmental dry air from the continent on the northwest.Studies show that sea salt aerosols will partially crystallize in a dry environment and thus show a inhomogenity,so the homogeneous sea salt aerosols model commonly used in most numerical models is no longer applicable.In this paper,the inhomogeneous state of sea salt is described by a double sphere model,which is coupled to the Weather Research and Forecasting(WRF)model.Results show that the direct radiative effects of sea salt aerosols hardly affect the track and intensity of Fitow,but influence the rainfall of the typhoon.The increased backscattering(decreased asymmetry factor)associated with the inhomogeneity of sea salt aerosols leads to a cooling effect in the atmosphere,which increases the stability of the lower troposphere,weakens the vertical velocity and finally suppresses the rainfall of Fitow.When the environment changes from wet to dry,the sea salt aerosols have the optical property of continuing to maintain the wet state,so that the change of its state lags the change of the relative humidity,which is called the hysteresis effect of sea salt aerosols.The study shows that due to the different locations of sea-salt aerosols with different humidity,after further considering the hysteresis effect of inhomogeneous sea salt aerosols,the vertical profiles of the atmospheric stability of landfall typhoons has changed,thus affecting the precipitation of landfall typhoons.When only the inhomogeneous sea salt aerosols is considered,the cooling effect of sea salt aerosols is located in the lower layer,resulting the stabilized layer is mainly located in the lower troposphere(850 to 700 h Pa),which weakens updraft and the decrease of precipitation;after further considering the hysteresis effect of the inhomogeneous sea salt aerosols,the cooling effect is located in the middle layer,causing stabilized layer moves upper to the mid-to-upper troposphere(650 to 420 h Pa),which triggers significant downdraft and associated divergence in the mid-to-upper and mid troposphere,respectively.The convection is confined in the lower troposphere,the precipitation is further decreased with the most realistic result with the observation.In summary,the sea salt aerosols have an impact on the precipitation of landfalling Typhoon Fitow through the direct radiative effects.After considering the direct radiative effects of sea salt aerosols,the atmospheric stability increased and produce downward motion,which eventually reduce the precipitation of the typhoon.The inhomogeneity of sea salt aerosols lead to a cooling effect in the atmosphere,which increases the stability of the lower troposphere,weakens the vertical velocity and finally suppresses the rainfall of Fitow.After further considering the inhomogeneity of SSAs,the atmospheric stability further increased and causes up to 10% rainfall decrease compared to the control simulation.On this basis,after considering the hysteresis effect of inhomogeneous sea salt aerosols,the vertical structure of the atmospheric stability has changed,and the stabilized layer moves upper to the mid-to-upper troposphere,which lead to the Fitow convection confined.the precipitation is further decreased with the most realistic result with the observation.Given the current overestimation of typhoon precipitation forecasts and simulations relative to observations,our results provide a new clue for improving the physical processes associated with tropical cyclone precipitation in numerical models.