Impact of Atmospheric and Oceanic Conditions on the Frequency and Genesis Location of Tropical Cyclones over the Western North Pacific in 2004 and 2010

This study examines the impact of atmospheric and oceanic conditions during May–August of 2004 and 2010 on the frequency and genesis location of tropical cyclones over the western North Pacific. Using the WRF model, four numerical experiments were carried out based on different atmospheric conditions and SST forcing. The numerical experiments indicated that changes in atmospheric and oceanic conditions greatly affect tropical cyclone activity, and the roles of atmospheric conditions are slightly greater than oceanic conditions. Specifically, the total number of tropical cyclones was found to be mostly affected by atmospheric conditions, while the distribution of tropical cyclone genesis locations was mainly related to oceanic conditions, especially the distribution of SST. In 2010, a warmer SST occurred west of 140°E, with a colder SST east of 140°E. On the one hand, the easterly flow was enhanced through the effect of the increase in the zonal SST gradient.The strengthened easterly flow led to an anomalous boundary layer divergence over the region to the east of 140°E, which suppressed the formation of tropical cyclones over this region. On the other hand, the colder SST over the region to the east of 140°E led to a colder low-level air temperature, which resulted in decreased CAPE and static instability energy. The decrease in thermodynamic energy restricted the generation of tropical cyclones over the same region.

Energy Budgets on the Interactions between the Mean and Eddy Flows during a Persistent Heavy Rainfall Event over the Yangtze River Valley in Summer 2010

In this study,a persistent heavy rainfall event（PHRE） that lasted for around 9 days（from 0000 UTC 17 to0000 UTC 26 June 2010） and caused accumulated precipitation above 600 mm over the Yangtze River valley,was reasonably reproduced by the advanced research WRF model.Based on the simulation,a set of energy budget equations that divided the real meteorological field into the mean and eddy flows were calculated so as to understand the interactions between the precipitation-related eddy flows and their background circulations（BCs）.The results indicated that the precipitation-related eddy flows interacted with their BCs intensely during the PHRE.At different layers,the energy cycles showed distinct characteristics.In the upper troposphere,downscaled energy cascade processes appeared,which favored the maintenance of upper-level eddy flows;whereas,a baroclinic energy conversion,which reduced the upper-level jet,also occurred.In the middle troposphere,significant upscaled energy cascade processes,which reflect the eddy flows＇ reactionary effects on their BCs,appeared.These effects cannot be ignored with respect to the BCs＇ evolution,and the reactionary effects were stronger in the dynamical field than in the thermodynamical field.In the lower troposphere,a long-lived quasi-stationary lower-level shear line was the direct trigger for the PHRE.The corresponding eddy flows were sustained mainly through the baroclinic energy conversion associated with convection activities.Alongside this,the downscaled energy cascade processes of kinetic energy,which reflect the direct influences of BCs on the precipitation-related eddy flows,were also favorable.A downscaled energy cascade of exergy also appeared in the lower troposphere,which favored the precipitation-related eddy flow indirectly via the baroclinic energy conversion.

The Multiscale Factors Favorable for a Persistent Heavy Rain Event over Hainan Island in October 2010

A case study is presented of the multiscale characteristics that produced the record-breaking persistent heavy rainfall event（PHRE） over Hainan Island,northern South China Sea（SCS）,in autumn 2010.The study documents several key weather systems,from planetary scale to mesoscale,that contributed to the extreme rainfall during this event.The main findings of this study are as follows.First,the convectively active phase of the MJO was favorable for the establishment of a cyclonic circulation and the northward expansion of the Intertropical Convergence Zone（ITCZ）.The active disturbances in the northward ITCZ helped direct abundant moisture from adjacent oceans towards Hainan Island continuously throughout the event,where it interacted with cold air from the midlatitudes and caused heavy rain.Second,the 8-daylong PHRE can be divided into three processes according to different synoptic systems：peripheral cloud clusters of a tropical depression-type disturbance over the central SCS in process 1;interactions between the abnormally far north ITCZ and the invading cold air in process 2;and the newly formed tropical depression near Hainan Island in process 3.In the relatively stable synoptic background of each process,meso-α and meso-β-scale cloud clusters repeatedly traveled along the same path to Hainan Island.Finally,based on these analyses,a conceptual model is proposed for this type of PHRE in autumn over the northern SCS,which demonstrates the influences of multiscale systems.