the diurnal cycle of convection over the northern south china sea

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

The Diurnal Cycle of Convection over the Northern South China Sea

The Diurnal Cycle of Convection over the Northern South China Sea

Richard H. JohnsonPaul E. Ciesielski

Andrew J. Newman

Richard H. JohnsonPaul E. Ciesielski

Andrew J. Newman

Colorado State UniversityColorado State University


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


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

3B42

1998-2007


Propagating signals evident near coastlines

3B42 Annual Mean 1998-2007

3B42 Time of Maximum Accumulation 1998-2007


Propagating signals evident near coastlines3B42 Time of Maximum Accumulation 1998-2007

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)


1-10 June 1998

10-15 m s-1

●

●

●

China

Dongsha Is.Shiyan 3

SCSMEX


Southeast Asia: JJA 2003

Kousky et al. 2004 CMORPH


BMRC C-POL RADAR

South China Sea Monsoon Experiment (SCSMEX) – May-June

1998


20°N

10°

EQ


South China

coastline

GMS Brightness Temperatures 110-120°E

(South China Sea) 1 May – 30 June 1998 35°N

30

25

20

15

10

5MAY 1 JUNE 1

NESA

Monsoon onset over northern

SCS;diurnal signal

Convection shifts to central SCS; diurnal signal still present


South China

coastline



30

25

20

15

10

5MAY 1 JUNE 1

1

NESA


BMRC C-POL Radar

Animation

15 May 1998

08-20 L


South China

coastline



30

25

20

15

10

5MAY 1 JUNE 1

1 2

NESA


BMRC C-POL Radar

Animation

5 June 1998

02-14 L


RAINFALL MINIMUM


Low-level flow across and along a sharp SST gradient:

• Weak sensible and latent heat fluxes

• Low-level convergence (reduced mixing)

1998

LLJ


SST-Wind Coupling: Convergence (Wallace et al. 1989)


South China coastlineSouth China coastline

11

Afternoon convection over land

7 m s-1

14

MAX

MIN


15 m s-1

10 m s-1

15-20 May 1998

Dongsha7 m s

-1


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


2. Gravity current (cold pool) dynamics

02/1)/( bughkV vv +Δ= θθ

vθΔ 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.



SOUTHWARD-MOVING SQUALL LINES ENCOUNTER WARMER WATER; STRONGER COLD POOL?


3. Gravity wave dynamics, I

Gravity waves generated by elevated heat source over

coastal mountains

Mapes et al. (2003)

ANDES

Convection over Panama Bight


But elevation of mountain range

in southern China is low


4. Gravity wave dynamics, II

€

c igw =N

m≈13 m s−1

for stratiform cooling region, found to be instrumental in producing surface high/low pressure couplet (Haertel and Johnson 2000)


COOLING


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)


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


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


END


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


Why does convection form over cool water and in an environment of relatively low

CAPE?

CAPE decrease

SST cooling

RAINY PERIOD


Evening

NS South China Sea

LLJ

Morning

Sea breeze

S N

LLJ

South China Sea

Land breeze

COASTAL CONVECTION


Stratiform areal fraction increases with distance

from coast

15-26 May 1998

Based on BMRC radar data


~ 11 m s-1

Convection over land

Convection over ocean

the diurnal cycle of convection over the northern south china sea

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