Influences of the Three Gorges Dam in China on Precipitation over Surrounding Regions

Impacts of the Three Gorges Dam（TGD） in China on the regional pattern and annual amount of precipitation around the Three Gorges Reservoir（TGR） are examined by comparing observations before and after the operation of TGD（1984–2003 and 2004–13）. Empirical orthogonal function（EOF） analysis of the annual precipitation anomalies clearly indicates that the land-use change associated with the construction of TGD has not significantly changed the precipitation pattern. To investigate the impacts of TGD on the rainfall amount, we compare the relative variations of atmospheric variables related to precipitation formation in three spatial bands： over TGR, near TGR, and far from TGR. It is found that the differences in annual rainfall over TGD between the two periods before and after the operation of TGD are small, suggesting a weak impact of TGD on the rainfall amount. The TGD water level increased from 66 m before June 2003 to 175 m after 2010, and this may have slightly reduced precipitation on the local scale.

Impact of 10-60-Day Low-Frequency Steering Flows on Straight Northward-Moving Typhoon Tracks over the Western North Pacific

This study investigates the impact of low-frequency(intraseasonal and interannual) steering flows on straight northward-moving(defined as a meridional displacement two times greater than the zonal displacement) typhoons over the western North Pacific using observational data. The year-to-year change in the northward-moving tracks is affected by the interannual change in the location and intensity of the subtropical high. A strengthened northward steering flow east of 120°E and a weakened easterly steering flow south of the subtropical high favor more frequent straight northward tracks. Examining each of the individual northward-moving typhoons shows that they interact with three types of intraseasonal(10-60-day) background flows during their northward journey. The first type is the monsoon gyre pattern, in which the northward-moving typhoon is embedded in a closed cyclonic monsoon gyre circulation. The second type is the wave train pattern, where a cyclonic(anticyclonic) vorticity circulation is located to the west(east) of the northward-moving typhoon center. The third type is the mid-latitude trough pattern, in which the northward-moving typhoon center is located in the maximum vorticity region of the trough.

Impacts of Steering Flows with Different Timescales on the Track of Typhoon Sanba(2012)

Typhoon Sanba(2012),the strongest tropical cyclone(TC)of the year worldwide,moved northward almost along130°longitude during its lifetime and passed through different background flows from low to high latitudes.The steering flows with different timescales for Sanba are retrieved by using the NCEP reanalysis data with the total wind field separated into:a mean state,an interannual component,an intraseasonal component,and a synoptic component.Our analysis indicates that the intraseasonal timescale wave train(WT)with east–west oriented circulations made the largest contribution to the movement of Sanba.The effects of the environmental steering with different timescales on Sanba’s movement are investigated with numerical simulations using the Weather Research and Forecasting(WRF)model.In the control simulation,total fields from the NCEP reanalysis are used as initial and boundary conditions,and the northward motion of Sanba is well captured.In sensitivity experiments,each of the intraseasonal and interannual components is removed one at a time.The steering vectors associated with these timescales can explain their influences on the movement of Sanba in the experiments.Vorticity budget analyses indicate that the horizontal vorticity advection made the largest contribution to the movement of the storm.

Factors controlling northward and north-eastward moving tropical cyclones near the coast of East Asia

The impacts of multi-time-scale flows on northward and north-eastward moving tropical cyclones(TCs)near the east coast of China in August and September are investigated using reanalysis data from 1982 to 2012.TCs of interest are under the influence of the subtropical high-pressure system in the western North Pacific(WNP).In August when the subtropical highpressure system is strong and close to the coast line,most TCs in the region move northward,while more TCs move north-eastward in September when the subtropical highpressure system retreats to the east.To investigate the influence from different time-scales,the environmental flow is divided intofour components,the synoptic flow,the intraseasonal flow,the interannual flow and the climatological background field.Analysis of steering flows between 25°N and 30°N indicates that the meridional steering vectors from all time-scales point to the north,dominated by the intraseasonal component.The deciding factor on whether a TC moves to the north or north-east between 25°N and 30°N is the zonal steering vector.For the northward moving group,the sum of the zonal steering from all time-scales is very small.On the other hand,the north-east moving group has a net eastward zonal component mainly contributed by the climatological mean flow.Several individual cases that stood out from the majority of the group are analyzed.For those cases,the intraseasonal flow plays an important role in affecting the movement of the TCs mainly through the wave train,in which a cyclonic circulation is located to the north-west(north)and an anticyclonic circulation to the south-east(east)of TCs.The analysis of the steering vectors indicates the importance of all components with different timescales to the movement of TCs.