Airmass Structure And Cause Of Precipitation In The Comma Head Of A Jiang-Huai Cyclone In Winter

An extreme rainfall-snowfall event that occurred in Shandong Province on 12-13 February 2016 contributed to the warm front of Jiang-Huai cyclone.The precipitation at forty-two stations in Shandong province broke the historic record.In this paper,the mesoscale evolution characteristics,the development mechanism and the air flow structure in the cloud region of the cyclone comma are analyzed by means of multiple observations,WRF model and HYSPLIT trajectory model.Main results show that:(1)The precipitation of this heavy rainfall-snowfall event shows a banded distribution from the northeast to the southwest.It took place in the context of the combination of the middle and south troughs and the development of the Jiang-Huai cyclone moving northward.The lower vortex and the warm shear in the south are favorable for the development of water vapor convergence and vertical motion.The combination of high level jet and low level jet is beneficial to the low-level convergence movement in this region.The low level southwest jet and the southeast airflow provide the water vapor transport condition for precipitation.(2)The comma head of Jiang-Huai cyclone developed from four banded echoes.The radar echo rotated cyclonically and elongated,and formed many strong mesoscale precipitation bands.The horizontal wind field shows that the low-level shear convergence line promotes the development and organization of strong echo,and the direction-changed convergence line corresponds to echo zone to the northeast-southwest direction.The wind profile shows that the cold cushion of the northeast wind is shallow when it is raining.The northeast wind cold cushion is much deeper than the rainfall stage during snowfall.The thinning of the southeast wind layer on the northeast wind cold cushion is a sign of the phase transformation of precipitation.(3)The initial and development stages of banded convection are accompanied by inertial instability and conditional symmetric instability.The low layer is a conditionally symmetric unstable region.With the strengthening of the low-level jet at the rear of the warm front,the airflow condenses along the warm front above the elevation condensation height,and the latent heat releasing promotes the conditional symmetric instability.The inertial instability of the middle and upper layers causes the air parcel to get the inertial acceleration along the isentropic surface,thus enhancing the slant convection.The frontogenesis forcing provides the dynamic condition of symmetrical unstable energy release.(4)The comma head consisted of three vertically stacked air masses:from bottom to top,a boundary layer air mass from the East China Sea,the Bohai Sea or inland,a warm-moist air mass of south China Sea origin,and a drier air mass originating over western Asia and Eastern Europe.During snowfall stage,from bottom to top,the comma head consisted of four vertically stacked air masses that originated from the Siberia,the East China Sea,the South Sea and the Bay of Bengal.These corresponding to the cold conveyor belt,the warm conveyor belt and the dry conveyor belt respectively.The middle layer dry conveyer belt is superimposed on the warm conveyor belt,which makes the moist potential vorticity decrease.The intersection of low-level warm conveyor belt and cold conveyor belt makes the local increasing of frontogenesis.The strong banded precipitation occurs in the northwest quadrant of the negative moist potential vorticity and the south of the frontogenesis axis.In the vertical direction,the strong precipitation is located in the strong ascending branch of the thermodynamic circulation along the edge of the front center.It is also a conditional symmetric unstable region with negative moist potential vorticity on the warm side of frontal zone,with a warm trowal at the back.