Maintenance and Sudden Change of a Strong Elevated Ducting Event Associated with High Pressure and Marine Low-Level Jet

摘要

Capture of a strong elevated ducting event,especially its maintenance and sudden change,is of great value to airborne radar to achieve its beyond-the-line-of-sight detection.However,the knowledge is not easily accessible over the open ocean and hence very rare.During the Air–Sea Interaction Survey(ASIS)over the western North Pacific(WNP)in May 2016,a strong elevated ducting event with a long-life period and sudden change in its evolution was observed.Measurements from the ASIS,images from the Himawari-8 satellite,reanalysis data from the ECMWF,and Weather Research and Forecasting(WRF)model,were used to analyze the maintenance and sudden change of this strong ducting event,together with the model performance on simulating it.The results showed that the maintenance of strong elevated ducts,with their tops ranging from 750 to 1050 m and average strength of approximately 38 M units,was caused by a strong dry air mass capping over the wet marine atmospheric boundary layer(MABL),together with the subsidence inversion associated with high pressure.The WRF model performs well in simulating them.However,a sudden increase in duct height with a slight decrease of strength was recorded by the subsequent GPS radiosonde,which was finally contributed to the mechanical turbulent inversion and hydrolapse associated with the marine low-level jet(MLLJ).The height of the maximum horizontal wind speed(Umh)of the MLLJ corresponds well with the bottom of the trapping layer.However,these jet-relevant ducts are generally weak and it is difficult to accurately simulate them by using the mesoscale numerical model,since the wind-shear produced eddies are too small to be properly parameterized.