1. Cheng-Zhong Zhang and Hiroshi Uyeda Hydroshperic Atmospheric Research Center, Nagoya University 1 November 2006 in Boulder, Colorado Possible Mechanism for Prevalence of Midlevel Convections to the South of the Meiyu Front What are the clouds to south of the Meiyu/Baiu front?

2. Cheng-Zhong Zhang and Hiroshi Uyeda Hydroshperic Atmospheric Research Center, Nagoya University 1 November 2006 in Boulder, Colorado Possible Mechanism for Prevalence of Convection of Medium Depth to the South of the Meiyu Front In order to understand the characteristics of precipitation systems to south of the Meiyu/Baiu front, a concept of Convection of Medium Depth (CMD) is introduced. Midlevel Convections Convection of Medium Depth (CMD) We tried to explain the prevalence of the CMD to south of the Meiyu front by using Hefei Doppler radar data and cloud resolving numerical model (CReSS).

3. Target Area

4. 3 years data (Jun/Jul, 2001-2003) HeFei Doppler radar 150km 60km 0 ℃ Convection of Medium Depth Statistical Analyses of Vertical Structure on Precipitation Systems Zhang et al. 2006 (JMSJ)

5. Classification of precipitation type: + Used Bright Band Fraction (BBF) Method for 3km x 3km column. + CMD is extracted from convective portion. Classification of meso-beta scale convective systems: + Decide the location of the Meiyu front by using Doppler velocity gradient. + Consider the relative location and the motion speed of the precipitation system Analyses of 3D Doppler radar data

6. Criteria: 1) The altitude of reflectivity core is lower than 4km. 2) The height of 15 dBZ echo-top is equal to or less than 8 km. Convection of Medium Depth (CMD) CMDDC CONVECTION OF MEDIUM DEPTH

7. Resonfeld (1995) & Cheng et al. (2001) BBF is defined as the fraction of grid point with intensive reflectivity in 2 km thickness air layer including 0 °C level to that in the whole air column. B BF = a/N a-- the number of point with reflectivity larger than threshold within 2 km thickness air layer including melting level. The threshold is determined firstly by place the reflectivity in the whole column to descend order from Zmax to Zmin, then set the Nth intense reflectivity within whole air column as threshold. N-- the number of grid point within 2 km thickness air layer around the melting level. BBF method BBF<0.4 convective 0.4<BBF<0.6 transition BBF>0.6 stratiform

8. 2150UTC 09JUL2003 Distribution of different properties of precipitation H =2 km B'B' B A A'A' B'B' B AA'A' CMD

9. Classification of precipitation type: + Used Bright Band Fraction (BBF) Method for 3km x 3km column. + CMD is extracted from convective portion. Classification of meso-beta scale convective systems: + Decide the location of the Meiyu front by using Doppler velocity gradient. + Consider the relative location and the motion speed of the meso-beta scale convective system Analyses of 3D Doppler radar data

10. Schematic illustration of CMD and DC over the Meiyu front Zhang et al. 2006 (JMSJ) ～ 2 km

11. SNF TYPE (1 case) SAF TYPE (6 cases) FAF TYPE (4 cases) SSF TYPE (12 cases) Ratio of estimated precipitation amount for four types STRATIFORM TRANSITION CMD CONVECTIVE Z=300 R 1.37 for convective (CMD) Z=205 R 1.48 for stratiform and transition (Fujiwara 1965)

12. 300  300 grid points, with the spacing of 5 and 1 km (outer and inner domains) 70 levels vertically, with stretching from 100 m to 400 m with increasing to 20 km. Initialized at 00 UTC 22 July 2002 by using RANAL data. (for 12 hours) The simulation for the inner domain started 3 hours after that for the outer domain. CReSS 5 km 1 km

13. Radar reflectivity at 3 km at 0628 and 0700 UTC 22 July 2002. The solid line indicates the Meiyu front, and five cells (CMD) are respectively labeled as C1, C2, C3, C4, and C5. The radar site is indicated by the + symbol Radar Echo

14. Model-simulated mixing ratio of rainwater (g kg -1 ) and wind (vector) at 3 km at 0730 and 0800 UTC. The convective cells CMD1  5 (CMD2 dissipated at 0730 UTC) are convection of medium depth. DPC1 indicates deep convection. The thick solid line represents the Meiyu front. S1 and S2 at 08 UTC represent the areas immediately ahead and south of the front, respectively. Simulation

15. East-west cross section of CMD3 along the lines AA, BB and CC in the previous figure. The black contour line indicates divergence, the red contour line indicates convergence. CMD3

16. (a) Time variation of the maximum mixing ratio of precipitation hydrometeors and corresponding vertical velocity for CMD3. The solid and short-dashed lines represent the vertical velocity; the color area is representative of the mixing ratio of rainwater; and the red dashed line represents the mixing ratio of snow and graupel. (b)Time variation of simulated hourly rainfall intensity within 2 km  2 km rectangle region under the peak. CMD3 2kmx2km Column

17. Same as in the previous figure, but for deep convection DPC1. DPC1 2kmx2km Column

18. (a) Distribution of LNB and (b) vertical cross section for relative humidity along AA' in (a). The solid line in (a) and the arrow in (b) indicate the position of the Meiyu front. The blank areas in (a) indicate atmosphere is absolutely stable. MEIYU FRONT RELATIVE HUMIDITYLEVEL OF NEUTRAL BUOYANCY S N Low LNB

19. Summary In order to understand the characteristics of precipitation systems to south of the Meiyu/Baiu front, a concept of Convection of Medium Depth (CMD) is introduced. Data analyzed is Hefei Doppler radar for June and July from 2001 to 2003 over the downstream region of Yangtze River. We found that CMD (echo top height ranges from 3 km to 8 km during its duration) predominates in the convective systems categorized as slow-moving (< 3 m/s) and south-of-front (SSF) types. We employed cloud-resolving numerical model to replicate the evolution of CMD. The structure of CMD from the simulation well agreed with the observation through the life. It is concluded that because of existence of abundant moist air to south of the Meiyu front, convection is easily formed in low atmosphere. The moderate updraft within the convective cell couldn’t uplift large amount of precipitation particles penetrating the melting level, resulting in formation of CMD to south of the Meiyu front. Further studies on CMD would be necessary to understand the formation mechanism of heavy rainfalls along the Meiyu/Baiu front.