Characteristics of Summer Precipitation

around the Western Ghats and Myanmar

West Coast

Dibas Shrestha

Nepal Academy of Science and Technology (NAST)

2/2/2015

Study Area and Summer Rainfall

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WG: Western Ghats and MWC: Myanmar West Coast

Mean summer rainfall 1998-2010

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Data

Near-surface: Lowest point in the clutter-free ranges

Average rain rate (mm/h) = Rain frequency × Rain conditioned rain rate (mm/h)

Rain frequency = Number of rain samples / Number of samples

Conditional rain rate = Near-surface rain rate / Number of rain sample

Data

source

Period Horizontal

resolution

Data characteristics

TRMM/PR

(V6)

1998- 2010

0.05° × 0.05°

2A25 - Instantaneous near-surface

rainfall and rain-type

2A23 - Storm-top height (Height of

the precipitation column)

JRA-25 1979-2008 1.125° × 1.125° Temperature, wind, specific

humidity

GTOPO30 0.05° × 0.05 ° Elevation

Central Himalayan Region (Shrestha et al. 2012)

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Clear double band of rainfall

maxima over CHR.

Intense rainfall over

low-altitude areas.

High frequency of rainfall

over higher altitude areas.

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Horizontal distribution of rainfall

(Xie. et al., 2005) Rain rate (mm/d)

Cond. rain rate (m/h) Frequency (%)

(Contour: 500 m)

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Typical examples of cross-shore distribution

MWC WG Elev. (m)

WG

Topography (m)

Freq (%)

Cond rain rate (mm/h)

Rain rate (mm/d)

Rai

n r

ate

(mm

/d),

Fre

q. (%

)

Co

nd

. R

ain

rat

e (m

m/h

)

MWC MWC

WG: Major rain peak appears near the top of the mountains corresponds to high rain frequency.

MWC: Peak rainfall coincides with high conditional rain rate. Broader peak of rain frequency

extended from mountain top to the hundreds of kilometer off the coastline.

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Storm-top height and vertical profile

• Downward increasing rain rate—oceanic type

• Deeper convections over MWC regions

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Vertical structure of rainfall

2/2/2015 Page 8 Lon (Deg.)

Hei

gh

t (m

)

WG MWC

WG: The rain-top height is higher over

the top of the mountains and coastal

ocean.

MWC: The rain-top height is higher

and homogenous.

Coastline Coastline

WG MWC

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Atmospheric stability WG MWC

Shaded: Equivalent potential temperature (K)

Vector: Horizontal wind (m/s)

Contour: Specific humidity (gm/kg)

• Remarkable difference in low-level θe.

• High humidity caused higher θe

in the MWC regions.

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Schematic of precipitation mechanisms

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Thank you

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General features

Rai

nfa

ll (

mm

/d)

Elevation (m) 0

0

Wet case Dry case

Ele

vat

ion

Wet case Dry case

Low-level moisture condition is a key factor.

• Rainfall, topography and atmospheric instability

Central Himalayas

Wet case Dry case

Summer monsoon Pre-monsoon

Active

periods (wet)

Break periods

(dry)

Coastal regions

Myanmar west coast (wet) Western Ghats (dry)

Andes regions

Central Andes (wet) Southern Andes (dry)

Summary

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• Central Himalayan region

More intense rainfall during pre-monsoon season, particularly over higher

altitudes.

Double rain peaks: First, along the Sub-Himalayas (~500-700 m) attributed to

intense rainfall; second, along the steep slope of Lesser Himalayas (2,000-2,200)

to frequent rainfall.

• Coastal regions

More humid condition favors broader rainfall maxima over Myanmar west

coast.

• Andes regions

Maximum rainfall over central and southern Andean foothills is attributed to

rainfall frequency and intensity, respectively.

• The characteristics of the precipitation along the continental mountains are distinct

from those of coastal mountains.

• Steep mountains favor more-frequent nonextreme convections, which contribute a

significant amount of rainfall over slopes in regions with large rain totals.

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Thank you very much for

your kind attention!

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Presentation outline

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1. Introduction

2. Objective

3. Study area and subregions of study

4. Data and methodology

5. Results

I. South Asia

(a) Central Himalayan region

(b) Coastal regions (Western Ghats: WG and Myanmar

west coasts: MWC)

II. Andes regions

6. Summary

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Introduction (Cont…) • Dairaku et al. (2004), Engman and Hershfield (1969), Sokol and Bližňák, (2009)

Larger quantities of rainfall recorded at higher altitudes are attributed to

greater duration and higher frequency of rainfall.

• Bookhagen and Burbank (2006), and Bookhagen and Strecker (2008)

Topography, relief and TRMM derived rainfall relationships

Large-scale relationships.

Bookhagen and Burbank (2006)

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TRMM PR uncertainty

Major sources of error (Anders et al., 2006)

• Retrieval error: Remote sensing methods

• Sampling error: Low earth orbit satellite with narrow swath (250 km)

Sampling error is small enough to resolve statistically significant

precipitation patterns in regions with large precipitation totals (Nesbitt and

Anders, 2009).

Shrestha et al. (2012)

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Variation of rainfall with elevation

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• Primary peak (~500 m): Rain conditioned

rain rate is dominant.

• Secondary peak (~2,100 m): Frequency

is dominant

• Linear decrease of rain-conditioned rain rate

Area averaged analysis:- Rainfall characteristics are averaged for each 200 m

altitude interval up to 5,000 m MSL.

0

2

4

6

8

10

12

14

16

18

0 1000 2000 3000 4000 5000Rai

nfa

ll (m

m/d

ay),

Co

nd

. rai

n

rate

(m

m/h

r) &

Fre

q. (

%)

Elevation (m)

JJA

Cond. rain rate

Rainfall

Frequency

0

1000

2000

3000

4000

5000

6000

7000

0 10 20 30 40

Height (m)

Topography

St. ht. (above MSL)

St. ht. (AGL)

S-N pixel

Storm height JJA

Storm height is approximately 6 km, with

clear peak near the top of the slope and a

small peak at the bottom.

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Vertical structure of rain (84.5o E)

MAM: Deep system with intense rain over

low-altitude areas

JJA: Rather uniform rain-top height

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Synoptic conditions

Strong instability during rainy days

in MAM.

Indicator of atmospheric instability:

Difference between equivalent

potential temperature (θe925- θe600

hPa).

Color: Equivalent potential

temperature (K)

Vector: Horizontal wind (m/s)

Contours: Specific humidity (gm/kg)

Gray shading: Elevation above 800 m

MAM: Case study during May in 2004.

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Schematic of precipitation mechanisms in the

central Himalayas

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0

1

2

3

4

5

6

7

0 1000 2000 3000 4000 5000

Elevation (m)

Rai

n r

ate

(mm

/hr)

(a)

Stratiform rain

0

5

10

15

20

0 1000 2000 3000 4000 5000

Elevation (m)

No

. of

even

ts

(b)

• Intense convective rainfall

over the foothills.

• High frequency of stratiform

rainfall over higher elevation.

Convective rain Convective – Stratiform rain rate (mm/h)

Stratiform – Convective rain events

Rain type

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Atmospheric stability

Shaded: Equivalent potential

temperature (K)

Vector: Horizontal wind (m/s)

Contours: Specific humidity

(gm/kg)

MAM: Strong instability

JJA: Higher stability, abundant

supply of water vapor in the

lower atmosphere.

Active: Remarkably similar with

JJA

Break: Wind patterns is just

opposite than that during active

period.

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Break (dry) period Active (wet) period

Vertical structure

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Synoptic conditions in MAM

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Atmospheric stability WG MWC

Shaded: Equivalent potential temperature (K) Vector: Horizontal wind (m/s) Contour: Specific humidity (gm/kg)

• Remarkable difference in low-level θe.

• High humidity caused higher θe

in the MWC regions.

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Storm-type distribution

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Storm-type distribution

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Difference between convective and stratiform rain charactereistics

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Percentage of convective and stratiform rainfall

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Variation of rainfall with elevation

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Vertical profile of rain intensity

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Vertical profile of specific humidity

Central Himalayas

Western Ghats

Myanmar West Coasts

Central Andes

Altitude (

hPa)

Specific humidity (g/kg)

Southern Andes

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0

5

10

15

20

25

0 1000 2000 3000 4000 5000

Rain rate (mm/day)

Cond. Rain rate (mm/hr)

Rain frequency (%)

Sikkim-Bhutan Himalaya

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200 1400 1600

Meghalaya Plateau

0

5

10

15

20

25

30

35

0 200 400 600 800 1000 1200 1400

Western Ghats

0

2

4

6

8

10

12

0 1000 2000 3000 4000 5000

Western Himalayas

0

2

4

6

8

10

12

14

16

18

0 1000 2000 3000 4000 5000

Rai

nfa

ll (m

m/d

ay),

Co

nd

. rai

n

rate

(m

m/h

r) &

Fre

q. (

%)

Elevation (m)

JJA

Cond. rain rate

Rainfall

Frequency

0

2

4

6

8

10

12

14

16

18

20

0 1000 2000 3000 4000 5000

Eastern Himalayas

Rainfall-elevation relationship

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Break Active

600 hPa

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MAM JJA

600 hPa