2021 EGU (Session AS1.5 -Tropical meteorology and Tropical ...

The Synoptically-Influenced Extreme Precipitation Systems

over Asian-Australian Monsoon Regionobserved by TRMM Precipitation Radar

Wei-Ting ChenDepartment of Atmospheric Sciences,

National Taiwan University, Taipei, Taiwan

Collaborators: Hong-Wen Jian, Peng-Jen Chen, Chien-Ming Wu (NTU) & Kristen Rasmussen (CSU, USA)

2021 EGU (Session AS1.5 - Tropical meteorology and Tropical Cyclones)

Jian, H.-W., W.-T. Chen*, C.-M. Wu, and K. L. Rasmussen (2021), The synoptically-influenced extreme precipitation systems over Asian-Australian monsoon region from TRMM PR Measurements, J. Meteo. Soc. Japan, 99 (2), 269-285, doi:10.2151/jmsj.2021-013

Extreme Rainfall in the Asian-Australian Monsoon Region

• Extreme rainfall events have caused severe loss of life and properties over the Asian-Australian Monsoon (AAM) region

• Previous satellite (TRMM) studies have provided statistics of the spatial-temporal distribution, size, intensity, and internal structures of extreme systems [e.g., Hamada et al., 2014; 2015; Rasmussen et al., 2015]• Coastal areas are hotspots of extensive type extreme systems• Occurrence is strongly tied to the seasonal monsoon circulation

• Understanding of the various “ingredients” producing the extreme systems still requires further research, especially over the lower latitudes• Mid-latitude – dominant synoptic control of extreme case [e.g., Yokoyama et

al., 2020, JMSJ]• Tropics -- synoptic condition, topographical effects, coastal land-sea breeze

circulation, meso-scale organization processes…

“Ingredients” for Extreme System can be Multi-scale -- Example 1: W Coast Philippines with remote vortex (boreal summer)

• Synoptic: Onshore westerly enhanced by remote TY

• Topography and probably land-sea breeze effects

• High CWV, strong low-level shear à favors convection organization

• Extreme rain rate occurs in the boundary of convective and stratiform rainfall

[Jian et al., 2021, JMSJ]

Goal: Identify the different synoptic states

associated with extreme rainfall systems over the AAM region,

then examine the linkage between extreme and environment

Convection: System-based approach +

Synoptic: Minimalist approach (get the most distinct feature first)

Step 1: Rainy Systems from TRMM PR

Rainy system database over the TRMM PR area (37.5N-37.5S) during 1998-2014

Step 2: Define Tropical Extreme Systems that are both Large and Intense

• Rank the maximum rainfall within system and the horizontal size of all systems

• Top 0.1 % intense (>60.7 mm/hr) and top 0.1 % extensive (>271 km)

à N=3457 extreme systems (0.015% of all systems) during 1998-2014

à 27.8% (N=916) of these extreme systems are over the AAM region (black box; 90-160E, 12S-27N)

[Jian et al., 2021, JMSJ]

Step 3: Classify into Four Major Types of Synoptic Flow Patterns

Vortex type:Convection system embedded in TY/TD

• Subjectively classify the 916 extreme systems into four synoptic types, based on 850 hPa circulation patterns, rainfall distribution, and distance to coastlines < 2.5o

Coastal type:System close to coastlines with monsoonal background wind

Remote VortexCoastal:System close to coastlines with remote vortex enhancing onshore wind

(vortex=closed cyclonic circulation of TY, TD, monsoon lows, tropical waves…)

None of the above:Open ocean convective clusters or systems associated with mid-latfront

Percentage of Synoptic Types of Extreme Systems

N= 402 268 66 153(total N =916)Lightning probability 41% 54% 56% 35%

Intensity, Size, and Lightning of Extreme Types

• Similar median values of max rain rate among four types

• Vortex type has highest variability

• Similar horizontal scales for V, VC and C types (median ~ 300 km)

• N type has larger size

Seasonal distribution: Boreal Winter (blue) vs. Summer (red)

Vortex type (44%)

Vortex-impacted Coastal type (7%)

Costal type (29%)

None of the above type (20%)

[Jian et al., 2021, JMSJ]• Vortex type: mainly over the off-equatorial areas in boreal summer. • C and VC types: significant seasonal variations in occurrence locations

• Similar internal structure across all four types:• Broad stratiform and wide convective core. • Max. rainfall locates mostly in convective area, near convective-stratiform boundary in

the system

Internal Structures and Environment

• Environment for C and VC types : high CWV and existence of low-level shear

[Jian et al., 2021, JMSJ]

Conclusion

• AAM contains 28% of the extremely large and intense systems (>271.2 km & >60.7 mm/hr) over the Tropics• Four types of the synoptic patterns associated with

extreme systems in AAM: V (44%), C (29%), VC (7%), N (20%)• V type can be the most intense; N type can be the largest;

C and VC more frequently accompanied by lightning flash, with seasonally distinct locations; • Similar internal structure: either WCC or B; max. rainfall

near the convective-stratiform boundary

Thanks for your attention!Contact: [email protected]