Structures Of Conveyor Belts And Development Mechanisms In The Extratropical Cyclone

The high resolution simulations of the idealized moist baroclinic wave and northwest Pacific extratropical cyclone are performed to study the structures and characteristics of conveyor belts,and to analyze the development of northwest Pacific cyclone from the PV perspective.As the results show that,the conveyor belts within the extratropical cyclones present more fine scale structures in the high resolution simulations.With the rapid development of idealized frontal cyclone,the warm conveyor belt generally splits into"forward-sloping ascent"(fWCB)and "rearward-sloping ascent"(rWCB)branches as the previous described.They ascend abruptly,transport great moisture and low PV into the upper levels,accounting for major parts of precipitation.The cold conveyor belt can be classified into four types.The "wrapping around forward sloping"(cfCCB)and "wrapping around rearward sloping"(crCCB)airstreams,that originate far from the warm front,travel at lower levels around the cyclone center and turn separately anticlockwise and clockwise as moving southwest of the cyclone,resulting in positive PV anomaly in the lower troposphere.The "forward-sloping ascent"(fCCB)and"rearward-sloping ascent"(rCCB)patterns,which are similar to the warm conveyor belt,start from the cold sector around the warm front and ascend respectively anticyclonically and cyclonically relative to the low center.What's more,the"rearward-sloping ascent" branch was not identified before.The dry intrusion airstreams transport drier air with large potential vorticity into the middle and lower levels.The new types of "slight descent and forward"(sfDI)and "slight descent and rearward"(srDI)airstreams were also analysed apart from the "direct descent"(ddDI)and "descent and then ascent"(drDI)flows.The high precision numerical simulation of the extratropical cyclone occurring in the northwest Pacific from Feb.12th to Feb.16th in 2015 reproduces successfully the development process.The structures and physical characteristics of conveyor belts are analysed using simulated results and the same selection criteria as the idealised cyclone.The study shows that part kinds of conveyor belts exist and play roles in the real case.The WCB could be also divided into "forward-sloping ascent" and"rearward-sloping ascent" branches.However,the CCB can only be classified intotwo patterns of "rearward-sloping ascent" and "wrapping around forward sloping"and the DI has only the "direct descent" type.The cyclone system presents multiple mesoscale low pressure centers.The development is greatly affected by the center born in the Sea of Japan and east of Japan Island,and is closely related to the process of upper PV anomaly intrusion downward and lower PV anomaly transport upward.The mechanisms vary among different centers,and the upper PV anomaly transports downward significantly around the center in the Sea of Japan while the upward lower PV anomaly is vital to the center east of Japan.The intensity of the cyclone increases as the upper and lower PV anomaly getting close.When the upper and lower anomaly are connected and their axes are nearly in a straight line,the cyclone weakens and even decays.The analysis of PV budget shows that,the advection effect is the main influence factor of the PV change and the diabatic process affects greatly the lower-level PV change in the rainfall regions,while the influence of friction is the weakest.The vertical advection effect also shows the transport of positive PV anomalies in the upper and lower level.