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

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

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

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

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

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

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

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

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

3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV South China coastline GMS Brightness Temperatures °E (South China Sea) 1 May – 30 June °N MAY 1JUNE 1 1 NESA

3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV BMRC C-POL Radar Animation 15 May L

3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV South China coastline GMS Brightness Temperatures °E (South China Sea) 1 May – 30 June °N MAY 1JUNE NESA

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

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

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

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

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

South China coastline 11 Afternoon convection over land 7 m s MAX MIN

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

3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV Possible Explanations for System Motion 1. 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 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 u 0 = 0. Possible Explanations for System Motion

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

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

3. Gravity wave dynamics, I Gravity waves generated by elevated heat source over coastal mountains Mapes et al. (2003) ANDES Convection over Panama Bight Possible Explanations for System Motion But elevation of mountain range in southern China is low

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

3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV 5. Discrete propagation Possible Explanations for System Motion 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  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  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  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  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 END

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

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

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

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