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The Diurnal Cycle of Convection over the Northern South China Sea

The Diurnal Cycle of Convection over the Northern South China Sea. Richard H. Johnson Paul E. Ciesielski Andrew J. Newman. Colorado State University. TRMM 3B42 Rainfall 1998-2007.

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The Diurnal Cycle of Convection over the Northern South China Sea

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  1. The Diurnal Cycle of Convection over the Northern South China Sea Richard H. Johnson Paul E. Ciesielski Andrew J. Newman Colorado State University 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  2. TRMM 3B42 Rainfall 1998-2007 • Much of world’s heaviest rainfall in the tropics occurs within ITCZ/SPCZ, and also along coastlines  diurnal cycle is important 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  3. 1998-2007 EVENING (12-23 LT) MINUS MORNING (00-11 LT) RAIN 3B42 Nocturnal max generally over oceans …but, afternoon/evening max over interior ocean basins and other offshore areas NORMALIZED EVENING (12-23 LT) MINUS MORNING (00-11 LT) RAIN 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  4. 3B42 Annual Mean 1998-2007 3B42 Time of Maximum Accumulation 1998-2007 Propagating signals evident near coastlines 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  5. Indian Ocean: squall lines (Yang and Slingo 2001; Webster et al. 2002) Panama Bight: gravity waves (Mapes et al. 2003) Papua New Guinea: gravity waves (Liberti et al. 2001; Zhou and Wang 2006) Borneo: land breeze (Houze et al. 1981) GATE region: squall line propagation from West Africa 3B42 Time of Maximum Accumulation 1998-2007 Propagating signals evident near coastlines 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  6. China ● Dongsha Is. ● Shiyan 3 10-15 m s-1 SCSMEX ● 1-10 June 1998 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  7. Southeast Asia: JJA 2003 Kousky et al. 2004 CMORPH 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  8. South China Sea Monsoon Experiment (SCSMEX) – May-June 1998 BMRC C-POL RADAR 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  9. 20°N 10° EQ 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  10. GMS Brightness Temperatures 110-120°E (South China Sea) 1 May – 30 June 1998 35°N 30 Monsoon onset over northern SCS;diurnal signal 25 South China coastline 20 NESA Convection shifts to central SCS; diurnal signal still present 15 10 5 MAY 1 JUNE 1 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  11. GMS Brightness Temperatures 110-120°E (South China Sea) 1 May – 30 June 1998 35°N 30 25 1 South China coastline 20 NESA 15 10 5 MAY 1 JUNE 1 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  12. BMRC C-POL Radar Animation 15 May1998 08-20 L 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  13. GMS Brightness Temperatures 110-120°E (South China Sea) 1 May – 30 June 1998 35°N 30 2 25 1 South China coastline 20 NESA 15 10 5 MAY 1 JUNE 1 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  14. BMRC C-POL Radar Animation 5 June1998 02-14 L 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  15. RAINFALL MINIMUM 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  16. 1998 • Low-level flow across and along a sharp SST gradient: • Weak sensible and latent heat fluxes • Low-level convergence (reduced mixing) LLJ 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  17. SST-Wind Coupling: Convergence (Wallace et al. 1989) 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  18. 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  19. Afternoon convection over land South China coastline MAX 7 m s-1 MIN 11 14 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  20. 15-20 May 1998 10 m s-1 15 m s-1 7 m s-1 Dongsha 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  21. Possible Explanations for System Motion • Advection System motion is nearly at right angles to the low- to mid-tropospheric winds Upper-tropospheric (200 hPa) winds are in the direction of propagation, but at speeds 4-8 m s-1 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  22. Possible Explanations for System Motion 2. Gravity current (cold pool) dynamics for SCSMEX cold pools  3 K (at Dongsha Island), which gives V  7 m s-1 using k  1 and assuming h = 500 m and u0 = 0. 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  23. SOUTHWARD-MOVING SQUALL LINES ENCOUNTER WARMER WATER; STRONGER COLD POOL? 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  24. 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  25. Possible Explanations for System Motion 3. Gravity wave dynamics, I Mapes et al. (2003) Gravity waves generated by elevated heat source over coastal mountains ANDES But elevation of mountain range in southern China is low Convection over Panama Bight 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  26. Possible Explanations for System Motion 4. Gravity wave dynamics, II for stratiform cooling region, found to be instrumental in producing surface high/low pressure couplet (Haertel and Johnson 2000)  COOLING 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  27. Possible Explanations for System Motion 5. Discrete propagation New cells forming ahead of the convective line and its associated gust front via gravity wave dynamics/ducting (Houze 1977; Zipser 1977; Crook and Moncrieff 1988; Carbone et al. 1990; Shige and Satomura 2001; Fovell et al. 2006) 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  28. The South China Sea region is dominated by diurnal-cycle effects associated with the nearby land masses; during the summer monsoon there is a regular diurnal cycle of southward propagation from China • Convection typically forms in the coastal environment just offshore in the early morning and moves seaward in late morning/early afternoon as it dissipates • Propagation still evident when convection shifts to central SCS; point of origin south of coastline so coastal effects do not always play a role 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  29. Local minimum in precipitation south of the China coastline may be a consequence of cool coastal water, small sensible and latent heat fluxes, weak instability • Ten years of TRMM PR data show propagation signal consistent with results for SCSMEX year • Propagation mechanisms still uncertain: gravity current dynamics, gravity wave dynamics, discrete propagation 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  30. END 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  31. JMA fluxes over NESA adjusted based on intercomparison with NCAR values at Shiyan 3 • Large differences between model and observed fluxes • Small fluxes due to warm air flowing over cool water 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  32. Why does convection form over cool water and in an environment of relatively low CAPE? RAINY PERIOD SST cooling CAPE decrease 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  33. COASTAL CONVECTION LLJ Sea breeze South China Sea N S Evening LLJ Land breeze N S South China Sea Morning 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  34. Based on BMRC radar data 15-26 May 1998 Stratiform areal fraction increases with distance from coast 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

  35. Convection over ocean Convection over land ~ 11 m s-1 3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV

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