# Presentation: 205921

## 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

-  Much of world’s heaviest rainfall in the tropics occurs within ITCZ/SPCZ, and also along coastlines  diurnal cycle is important

## 1998-2007

*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

*Propagating signals evident near coastlines*

*3B42 Annual Mean 1998-2007*

*3B42 Time of Maximum Accumulation 1998-2007*

## Propagating signals evident near coastlines

*Propagating signals evident near coastlines*

*3B42 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)*

## China

*1-10 June 1998*

- 10-15 m s-1

- ●

- ●

- ●

*China*

- Dongsha Is.

- Shiyan 3

*SCSMEX *

## Southeast Asia: JJA 2003

*Kousky et al. 2004*

*CMORPH*

## South China Sea Monsoon Experiment (SCSMEX) –  May-June 1998

*BMRC       C-POL RADAR*

**South China Sea Monsoon Experiment (SCSMEX) –  May-June 1998**

## 20°N

**20****°N**

**10****°**

**EQ**

## GMS Brightness Temperatures     110-120°E  (South China Sea)                                       1 May – 30 June 1998 

**South China coastline**

**GMS Brightness Temperatures**     110-120°E  (South China Sea)                                       1 May – 30 June 1998

- 35°N

- 30

- 25

- 20

- 15

- 10

- 5

**MAY 1**

**JUNE 1**

***NESA***

**Monsoon onset over northern SCS;diurnal signal**

**Convection shifts to central SCS; diurnal signal still present**

## GMS Brightness Temperatures     110-120°E  (South China Sea)                                       1 May – 30 June 1998 

**South China coastline**

**GMS Brightness Temperatures**     110-120°E  (South China Sea)                                       1 May – 30 June 1998

- 35°N

- 30

- 25

- 20

- 15

- 10

- 5

**MAY 1**

**JUNE 1**

- 1

***NESA***

## BMRC       C-POL Radar Animation

**BMRC       C-POL Radar Animation**
**15 May**** ****1998**
**08-20 L**

## GMS Brightness Temperatures     110-120°E  (South China Sea)                                       1 May – 30 June 1998 

**South China coastline**

**GMS Brightness Temperatures**     110-120°E  (South China Sea)                                       1 May – 30 June 1998

- 35°N

- 30

- 25

- 20

- 15

- 10

- 5

**MAY 1**

**JUNE 1**

- 1

- 2

***NESA***

## BMRC       C-POL Radar Animation

**BMRC       C-POL Radar Animation**
**5 June**** ****1998**
**02-14 L**

## RAINFALL MINIMUM

*RAINFALL MINIMUM*

## 1998

*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)

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

## Slide 18

## South China coastline

*South China coastline*

*11*

*Afternoon convection over land*

*7 m s**-1*

*14*

*MAX*

*MIN*

## 15 m s-1

**15 m s****-1**

**10 m s****-1**

- 15-20 May 1998

*Dongsha*

**7 m s****-1**

## 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

##          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.

-          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

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

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

## Slide 24

## Possible Explanations for System Motion

- 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

## Possible Explanations for System Motion

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

*COOLING*

- 

## 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)

##   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

-   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

-   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

*END*

##   JMA fluxes over NESA adjusted based on intercomparison with NCAR values at Shiyan 3

*  **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*

## N

**Evening**

**N**

**S**

***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 

- Stratiform areal fraction increases with distance from coast

- 15-26 May 1998

*Based on BMRC radar data*

## ~ 11 m s-1

*~ 11 m s**-1*

*Convection over land*

*Convection over ocean*