On the mechanism of eastward-propagation of super cloud clusters (SCCs) over the equatorÂ

1. ICMCS-V_061101 On the mechanism of eastward-propagation of super cloud clusters (SCCs) over the equator  – Impact of precipitation activities on climate of East Asia – Masanori YOSHIZAKI and Tomoe NASUNO (IORGC/JAMSTEC) Topics 1. Simple model: linear, 4-layer model with constant N and no basic wind 2. Extension of simple model using a NICAM output (Diabatic heating: positive-only wave CISK) Thanks to Drs. T. Nasuno and M. Sato for providing NICAM data

2. East day History and motivations ・Hayashi･Sumi (1986) found an eastward- propagating mode around the equator in the aqua-planet numerical experiment. ・Eastward-propagating super cloud clusters （SCCs) were obtained by satellite data, too. (e.g., Nakazawa，Murakami，Takayabu et al.) Many theories to explain the mechanisms of eastward-propagating modes: 1) Atmospheric instability ・Moisture convergence ・Surface evaporation 2) Atmospheric response to independent forcing ・Tropical intraseasonal stationary forcing ・Tropical stochastic forcing ・Lateral forcing Zhang (2003) Eastward propagating Which mechanisms are working? Atmospheric instability, or atmospheric response to independent forcing? Westward propagating Intraseasonal variation >>> MJO (Madden-Julian Oscillation) Nakazawa

3. Yoshizaki (1991a,1991b): a simple model of SＣＣs ・Atmosphere with constant N and no basic wind, ・Equatorial-beta plane system (βE), ・4 layers in the vertical, ・Linear system, ・Heating: positive-only wave-CISK, ・Large second-order horizontal diffusion. 0　　　　　　：ｗB< 0 Q= 　　　　ｗB・f(z)　：ｗB> 0 Model: * Horizontal direction: Grid * Vertical direction: Mode expansion

4. * Vertical mode expansion Height Model top Total Q Each modes of Q ｗB bottom Q/N2 0 1 * Two heating profiles were considered: Only 1st mode Combination of two modes Height η1 = 1.5, η2=0.0 η1=1.5, η2=-1.5 ｗB * Top-heavy heating profile can be expressed as a combination of positive 1st mode and negative 2nd mode

5. η1 = 1.5, η2=0.0 Height η1=1.5, η2=-1.5 ｗB Time Eastward-propagating mode grows faster than westward-propagating mode in βE . Convective mode moves westward in βE . → Changes of vertical heating profiles induce different characteristic features of propagation! along the equator

6. * In this model, it is assumed that diabatic heating is greater than adibatic cooling due to upward motion in some layers. However, is the ‘>’ case right, observationally or numerically?; Disturbance driven by convection for the ‘>’ case, or neutral wave in the stable stratification for the ‘<‘ case. >>> Which mechanisms are working? Atmospheric instability , or atmospheric response to independent forcing? Further study could not be pursued in 1990’s, however, because there was no step to check above-mentioned features. Recently, numerical outputs using a global NH model (NICAM) were available.

7. Snapshot of ‘NICAM’ precipitation - Aqua planet - NICAM: Nonhydrostatic ICosahedral Atmospheric Model = Global cloud-resolving nonhydrostatic model

8. x - t distribution of diabatic heating 40000 km / 30 days ～15.4 m / s 2S – 2N average 7 km resolution SCC

10. (1) Comparison of Q (diabatic heating) and adiabatic cooling due to upward motion

11. (2) Comparison of Q (diabatic heating) and adiabatic cooling due to upward motion D is positive in some layers in the vertical direction. Disturbances driven by convection ( or atmospheric instability )

12. Governing equations Positive only wave-CISK * 54 vertical grid model is used. * Parameter ε: 0 or 1 ε1 : Linear or nonlinear (NL) ε2 : With or without basic eastward wind ε3 : Including or excluding Rayleigh damping (function of z)

13. X - time section of vertical motions at the height of 3.7 km along the equator Blue ： upward motion Red : downward motion Full model: ε1=ε2=ε3 = 1 η=60 16 days ~ 29 m / s Time (day) Horizontal section Vertical section Yoshizaki (1991a,1991b) * Linear * No zonal wind * Constant N * 4 layers in the vertical * Mode expansion in the vertical * Combination of 1st and 2nd modes Present calculation * Nonlinear * Zonal wind * Variable N * 54 layers in the vertical * Grid in the vertical * Diabatic heating simulated by NICAM X (10,000 km)

14. Vertical pattern of simulated SCCｓ　 Rayleigh damping is working well. θ Full model : Basic wind u + N + positive-only wave-CISK + Rayleigh damping Heating Z X

15. Conclusions 1) Diabatic heating is larger than adiabatic cooling due to upward motion in some vertical layers: SCCs appeared in NICAM is disturbances driven by convection. Then, SCCs are excited due to atmospheric instability. 2) The simple model is extended using the NICAM output. 3) When positive-only wave-CISK is applied as diabatic heating, eastward-propagating disturbances appear as a dominant mode. 4) MJO (or SCC) is responsible for the formation of tropical cyclones affecting East Asia. Thus, this study is important. Further studies 1) This vertical grid model should extend to a vertical mode model, to confirm results obtained by a simple vertical mode. 2) Rayleigh damping is important to eliminate the reflection of vertically propagating gravity waves. The differences between inclusion/exclusion of Rayleigh damping should be studied. 3) Multi-scale horizontal feature is not simulated due to selection rule of convection.

19. Horizontal pattern of simulated SCCｓ

20. Two heating profiles were considered: Case of one vertical mode Case of two vertical modes Height η1=1.5, η2=-1.5 η1 = 1.5, η2=0.0 ｗB * Top-heavy heating profile can be expressed as a combination of positive 1st mode and negative 2nd mode Time → Changes of vertical profiles of heating induce different characteristic features of propagation! along the equator

21. Why does the difference of heating profiles produce different features? Case of one vertical mode (η1>1) η1 = 1.5, η2=0.0 Similarly to an usual convection, disturbances with no propagation are excited Only convective mode excited Convective mode grows without propagation in no βE. X along the equator Convective mode moves westward in βE .

22. Why does the difference of heating profiles produce different features? Case of two vertical modes (η1>1, η2<0) Convective and oscillation modes excited simultaneously Oscillation mode can be separated into EP and WP modes. EP mode grows faster than WP mode in βE .

23. Horizontal diffusion = 0 Small horizontal diffusion Large horizontal diffusion Growth rate Growth rate Growth rate Small Small Small Large Large Large Horizontal wavenumber Horizontal wavenumber Horizontal wavenumber Selection rule ofconvection In the linear atmosphere system, there are two independent modes; (1) neutral wave modes and (2) exponentially growing modes. (1) Gravity wave, Kelvin wave, Rossby wave and so on: When forced, a selection rule does not work: All waves stimulated by forcing are evenly excited and appear following a dispersion relation. (2) Baroclinic waves, Benard convection, shear instability and so on: Modes with maximum growth rate grow fastest and a selection rule works. In this model, a positive-only wave-CISK works like usual convection and disturbances with maximum growth rate are infinitesimally small without horizontal diffusion (and viscosity). >>>> A large horizontal diffusion is included to get modes with horizontal scales of 1000 km. >>>> No multi-scale horizontal structure!

24. X - time section of vertical motions at the height of 3.7 km along the equator Blue ： upward motion Red : downward motion Full model: ε1=ε2=ε3 = 1 η=60 Time (day) X (10,000 km)

25. Governing equations Parameter ε: 0 or 1 *ε1 : Linear or nonlinear (NL) *ε2 : With or without basic eastward wind *ε3 : Including or excluding Rayleigh damping (function of z)

26. Case of one vertical mode Case of two vertical modes Height η1 = 1.5, η2=0.0 η1=1.5, η2=-1.5 ｗB Time Eastward-propagating mode grows faster than westward-propagating mode in βE . Convective mode moves westward in βE . → Changes of vertical profiles of heating induce different characteristic features of propagation! along the equator