Structure and Evolution Characteristics of Atmospheric Intraseasonal Oscillation and its impact on the summer rainfall over eastern China

Transcription

1 Structure and Evolution Characteristics of Atmospheric Intraseasonal Oscillation and its impact on the summer rainfall over eastern China Yanjun QI Chinese Academy of Meteorological Sciences,CMA Acknowledgement: Tim LI, Renhe ZHANG 8 Aug, 2016, Chengdu

2 Outlines ISO features of summer precipitation in eastern China The relationship between ISO and summer mean rainfall Structures and evolution of ISO Impacts of ISO on summer rainfall Effect of Mean flow on ISO

3 EOF analysis of 160-station precipitation within China during May-Aug. in YRB (27-32 N, E)

4 Power spectrum of summer rainfall over YRB (27-32 N, E) days Green dashed line red noise spectrum Red solid line indicates the 95% confidence level.

5 Standard deviations of ISO rainfall and seasonal variation in MJJA rainfall in Intensity of ISO in precip. during MJJA

6 Northward propagation of summer rainfall in 1998 Latitude-time cross section of daily precipitation in 1998 averaged over ( E)

7 Summer rainfall in 1998 Daily precip. for period May- Aug. in 1998 band filtered and daily precip. for period May- Aug. in 1998

8 For 29 years ( ), ISO intensity ~ summer mean precip. over YRB positive correlation R=0.68 Wet years: 1980,1993,1995,1996,1998,1999,2002 Dry years: 1981,1985,1986,1990,2001,2007

9 Wet/dry years phase composite of ISO Rainfall & wind850 Dry Wet C C A C A C C A C C C A A A

10 ISO vorticity & divergence/convergence (contour, x 10-6 /s) Dry Wet

11 Dry Wet/dry years,specific Humidity & Omega Wet

12 Dry Wet omega rainfall shum

13 Omega equation:primarily determined by vertical differential of vorticity advection + temperature advection Positive vorticity advection increase with height upward motion Warm advection upward motion Diagnose the vertical motion of boundary layer in wet or dry years

14 Dry Wet

15 Height-phase cross section of day filtered advection of temperature over YRB during wet and dry summer years. Unit: x10-5 /s Dry Wet In both wet and dry summers, warm advection during peak phase and it increase with the height in the boundary layer below 850 hpa, resulting in upward motions over YRB.

16 Height-phase cross section of day filtered vorticity advection with height over YRB (unit: x10-10 /s 2 /p a ) Dry Wet In wet summers, the positive A # anomalies increase with height in the boundary layer and peak at Phase 3. The Phase 5 is the transition stage from positive to negative vorticity in lower troposphere. The increase with height of positive vorticity advection during Phase 4 and 5 causes the upward motion, although it is weak. In dry summers, the A # decreases with height from Phase 3 and peak during Phase 5 and 6 in the boundary layer. The increases (decreases) with height of A # during peak phase in wet (dry) summers explain why it is ascending motion in wet summers and descending motion in dry summers in the boundary layer.

17 To examine the effect process of vorticity advection on vertical motion in boundary layer, the low-frequency background mean state (including annual mean and semi-annual cycle and cycles with the period longer than 90 days) the intraseasonal component (30-90 days) and the higher frequency eddy component (with a period less than 30 days). u = u + u ( + u (( v = v + v ( + v (( ζ = ζ + ζ ( + ζ ((

18 the mean state vorticity, ISO vorticity perturbation and eddy vorticity disturbance advected by the mean horizontal winds, ISO winds, and eddy winds: A # = V ζ v ζ ( + v ζ ( ( + v ζ (( ( + v ζ ( + v ζ ( + v ζ ( + v ζ ( + v ζ ( + v ζ ( To investigate which term in the decomposed nine components show the largest contribution to the intraseasonal vorticity advection

19 Phase_5 adv. of vorticity at 850hPa dry wet

20 Dry-Wet Difference of vorticity adv. during phase 5 at 850hPa between dry and wet summers. advection of eddy vorticity by the eddy flow v ζ

21 Difference distribution pattern of High-frequency eddy winds and vorticity (shading; unit: x10-5 /s) at 850 hpa at Phase 5 between dry and wet summer years. v ζ

22 Effects of mean flow on ISO perturbation

23 at phase 5, 850hPa, over YRB Mean flow, ISO vorticity/temp/humidity vorticity advection( v ζ ( ) wet dry temperature adv. ( v T ( ) wet dry moisture adv. ( v q ( ) wet dry

24 Summary The summer rainfall in eastern China exhibits remarkable intraseasonal and interannual variations. The interannual relationship between the intraseasonal oscillation (ISO) and summer mean rainfall over eastern China is positive. The composite spatial structure and temporal evolution of ISO associated with the summer rainfall over eastern China are analyzed. The low-level circulation anomalies are quite different in the wet and dry summers. In wet summers, the coupled cyclonic-anticyclonic circulation pattern propagates northwestward from WNP. (wind convergence, upward motion, moisture ) Impact of ISO perturbation on the summer precipitation from its structural and evolutional characteristics descending motion in the PBL restrains the development of persistent precipitation over YRB in dry summers Diagnosis of omega equation, the temperature advection has the same contributions to the ascending motion in peak phase in both wet and dry summers. The ascending and descending motions are primarily attributed to the vertical differential of horizontal vorticity advection. Advection of eddy vorticity by the eddy wind at 850hPa not only causes the ascending motion in the wet summers but also lead to strong descending motion in the PBL in dry summers. Effect of seasonal mean state on the ISO perturbation

25 Thank you!

26 Distribution of ISO intensity in precip. during boreal summer Averaged summertime ISO intensity for period of (unit: mm/day).

27 涡度平流的高度 - 位相剖面图 Dry 年涡度平流在对流层的 peak phase 随高度减小, 引起下沉运动 Wet 年在 peak phase 随高度增加, 产生上升运动 Dry 年,ph4-6, 对流层低层 750hPa 以下, 涡度平流随高度增加而减小, 会引起下沉运动 Wet 年,ph3-5, 对流层低层, 涡度平流随高度增加而增加, 容易导致上升运动的发生

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29 ( v q ( )