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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
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
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
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
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
Propagating signals evident near coastlines
3B42 Annual Mean 1998-2007
3B42 Time of Maximum Accumulation 1998-2007
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
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)
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
1-10 June 1998
10-15 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. 2004 CMORPH
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 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
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
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
1
NESA
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
BMRC C-POL Radar
Animation
15 May 1998
08-20 L
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
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
1 2
NESA
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
BMRC C-POL Radar
Animation
5 June 1998
02-14 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
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
South China coastlineSouth China coastline
11
Afternoon convection over land
7 m s-1
14
MAX
MIN
3rd Intl TRMM Sci Conf 8 February 2008 Las Vegas, NV
15 m s-1
10 m s-1
15-20 May 1998
Dongsha7 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
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.
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
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
€
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)
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
NS 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
15-26 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