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A Numerical Study of the Mesoscale Convective System Observed Okinawa Islands in June 1987

A Numerical Study of the Mesoscale Convective System Observed Okinawa Islands in June 1987. (MONTHLY WEATHER REVIEW. 1991). CHUAN-YONG CHANG AND MASANORI YOSHIZAKI. 2004 年 9 月 3 日　野村真奈美. Introduction.

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A Numerical Study of the Mesoscale Convective System Observed Okinawa Islands in June 1987

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  1. A Numerical Study of the Mesoscale Convective System Observed Okinawa Islands in June 1987 (MONTHLY WEATHER REVIEW 1991) CHUAN-YONG CHANG AND MASANORI YOSHIZAKI 2004年9月3日　野村真奈美

  2. Introduction A mesoscale convective system (MCS) that developed early in the morning of 6 June 1987 over Okinawa Island . Observation analysis →Akaeda et al.(1991)

  3. Leeside convection This study Privailing wind at low levels →south easterly Banta and Schaaf (1987) Akaeda et al.(1991) What is the mechanism?

  4. The leeside convergence zone (Banta（1984）)

  5. The evolution of the system The MCS has two stages of evolution. Stationary Orographical effect? Propagate rapidly eastward

  6. Objective In order to solve the following two questions, using a two-dimensional cloud model, a series of simulations were performed regarding the formation and evolution of the MCS. • Why did the system form in the leeside of the • mountain? Did it form accidentally or was it orographically forced to form in spite of the relatively low mountain height ? • Did the transition from the stationary stage to • propagating stage has a anything to do with • orographically?

  7. Model and environmental fields A two dimensional compressible model developed by Yoshizaki and Ogura (1988). Terrain following coordinates The variables wind velocity (u,v,w), pressure p potential temperature θ mixing water ratios of water substances (water vapor, cloud water, rain) Warm rain process The mountain shape is smooth with a single peak. (along the X1X2 line ) The elevation of the terrain surface zS = HMAX exp [-(x/xH)2] Domain the x direction 360km Vertical direction Grid interval 63m The model top 21km

  8. Model and environmental fields

  9. Model and environmental fields

  10. Results of the CNTL simulation 1. A time-X section ・The system moves right ward very slowly. ・Individual cells move leftward. ・The system has two stages of evolution. Stationary stage Propagating stage leeside upwind New cells form at the leading edge. Speed 4.5 m/s

  11. 2. Vertical structures mountain Cloud water (solid) and rain (dashed) Deviation of potential temperature H H H L

  12. Comparison with the observation result

  13. Sensitivity of the MCS to the mountain height mountain height 0m 400m 800m ・The high-mountain simulation The model MCSs stay near the mountain without propagation. ・The low-mountain simulation The model MCSs have stationary and propagating stages in their evolution.

  14. Vertical structures in the H4 simulation time Stationary stage The mountain ・forced lifting to the MCS in the upwind side ・block the outflow spreading in the leeside. Propagating stage The density current overtakes the orpgraphic effects.

  15. Sensitivity of the MCS to the environmental field S1 S2 The moist region near 780 hPa is crucial for the system to form in the lee side, while the inversion near the surface is not.

  16. Sensitivity of the MCS to the environmental field Examine the moist distribution by using the dry model Hmax= 400m, 200m Features are similar to Hmax=800m. Region A is more humid. Deep convection can develop in the leeside by forcing associated with mountain waves. Hmax = 800m X

  17. Conclusion Using a two-dimensional cloud model, a series of simulations were performed to answer two questions regarding the formation and evolution of the MCSs that occurred in the early morning of 6 June 1987 over Okinawa Island. 1. Why did the system form in the leeside of the mountain? Ascending motion associated with mountain-induced waves made the low-to midlevel air saturated in the leeside and the model convection formed there. ( The model was able to produce deep convection only when the observed sounding was modified to make the lower atmosphere more moist and unstable. )

  18. Conclusion 2. Why did the system undergo two stages of evolution ( the stationary and propagating stage ) ? ・The convective system stayed leeside until the cold pool beneath the cloud base deepened sufficiently and overtook the orographic effects. ( Forced lifting , damming of the cold pool ) ・The cold air overflowed the crest of the mountain and system started propagating. The formation of the cold pool in the leeside of the mountain and orographic effects make the stationary stage longer and remarkable.

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