statistical characteristics of convective clouds over the ...moes/rac-2018/rac... · deep...
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-
m s -1
(e) June (f) July
(g) August (h) September
g kg-1
Statistical Characteristics of Convective Clouds over the Western Ghats
Derived from X-band Radar Observations
Orographic precipitation over Western Ghats (~6000 mm annual): Despite one of
the largest rainfall areas of the SW monsoon, observations on convection lifecycle
are lacking.
Few ground-based studies: [e.g. Konwar et al (2014); Deshpande et al (2015), Das
et al (2017), Kalapureddy et al (2017)]
But most studies confined to long-term satellite data: [e.g. Romatschke and
Houze, 2011, S. Kumar et al., 2014; Shiege et al 2016; Kumar and Bhat 2016]
Satellite lack time-continuous aspects of convection: formation, growth, duration,
movement.
Number of questions remain unanswered:
Where does convection initiate in the Western Ghats?
What is the average size of convective clouds?
What are their propagation aspects, average lifetime, vertical structure and
diurnal cycle?
Continuous X-band radar observations (pertaining to small-scale convective state) at
Mandhardev would provide the best test bed to study these questions.
TITAN cell-tracking algorithm is used to identify, track convective storm (Dixon
& Wiener 1993). “Storm (or cell)” is a 3D contiguous region in space with reflectivity > 35 dBZ at 2 km altitude above the surface, and volume exceeds 15
km3. A storm must last for a minimum of two successive radar scans (>24 min).
Background, Data, and Methodology Frequency Distribution: Cell Properties
Acknowledgements
The work in this study is supported by Indian Institute of Tropical Meteorology, Pune,
MoES, India. We thank DST-Inspire, for fellowship of Utsav Bhowmik.
Spatial Distribution of Convective Cell Properties
Cell Kinematics Reduced Dimension Analysis
Diurnal Variation of Cell Properties
Figure 5: Frequency Distribution of a)speed b) storm direction c)wind direction
at 850 hPa d) wind rose of storm and wind directions d) Storm orientation e)
storms with modal orientation of 90 degree.
Utsav Bhowmik, Sachin Deshpande, Subrata Das, and G. Pandithurai Radar and Satellite Meteorology Group
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125m
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1400Mumbai
Pune
No
rth
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th d
ista
nce
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m R
ad
ar
(km
)
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km
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East-West Distance from Radar (km)
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%
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Topography
35 dBZ Top Heights
Storm Frequency
Volume > 40 dBZ (%)
Figure 3: a)Topography around IITM X band radar
b) Storm Occurrence Frequency c) Storm Top Heights
d) Percentage volume of Storms with reflectivity > 40 dBZ
a) b)
c) d)
IITM’s X-band (precipitation) and Ka-band(cloud) Doppler radars are deployed at Mandhardev (1.3Km
AMSL) on the windward side of the Western Ghats
Increased storm frequency over the Windward slopes of
mountains compared to coastal and Lee sides highlights
orographic influence on storm activity
Shallow storms (7km) dominates the Lee side. Systems over lee
side deeper.
Intense storms with higher volumes of cells exceeding
40 dBZ on Leeward sides, less intense storms with
lesser cell volumes exceeding 40 dBZ on windward side
Effect of Underlying surface on Convective Cell Onset
23:30-2:30
Local Time
2:30-5:30 5:30-8:30
8:30-11:30 11:30-14:30
14:30-17:30
17:30-20:30
20:30-23:30
Height [m]
(a)
(d)
(g)
(b) (c)
(e) (f)
(h)N-S
dis
tan
ce f
rom
rad
ar
(km
)
E-W distance from radar (km)
Figure 4: Temporal variation of Convective Cell Onset w.r.t.
Underlying topography during JJAS 2014.
•In order to identify the occurrence of cumulus convection and the processes that trigger or suppress it, the Convective
cell onset is studied.
•Convective cell onset is defined as the first time occurrence of convective cell with reflectivity of 35 dBZ. Therefore, it
can also be called as single newly-formed storm.
•An eastward progression of convective activity from upstream the barrier through windward slopes of mountains
over to the lee side is observed.
• Cell onset times depend on the combination of local time and the underlying surface.
1 10 40 70 95 99.5 99.999
10
100
0 30 60 90 120 150 180 210 2400
5
10
15
20
25
30
35
40
1 3 5 7 9 11 13 15 170
3
6
9
12
15
18
21
0.01 1 10 40 70 95 99.5 99.9991
10
10 30 50 70 90 110130150170190
0
5
10
15
20
25
30
35
40
2030 50 70 90 98 99.5 99.9910
100
Du
rati
on
(m
in)
Are
a (
km
2)
Top
Hei
gh
t (k
m)
Fre
qu
ency
(%
)
Duration (min)
Fre
qu
ency
(%
)
Area (km2)
Fre
qu
ency
(%
)
Top Height (km)
Accumulated Frequency (%) Accumulated Frequency (%) Accumulated Frequency (%)
Mean Duration = 46 min Mean Area= 27 km2 Mean Top Ht. = 5.5 km
-100 -50 0 50 100 125East-West Distance from radar (km)
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-100 -50 0 50 100 125East-West Distance from radar (km)
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0
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East-West Distance from radar (km)
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N
0
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0.5
0.6
0.7
0.8
0.9
1
(b) June (c) July
(d) August (e) September
Cell Occurrences & Large Scale features
Figure 1 : Month wise variation in Storm Occurrence (a-d) and anomalies of surface
Specific Humidity. and Wind (e-h).
Convective Depth and Intensity
2 3 4 5 6 7 8 9 102
4
6
8
10
12
14
16
2 3 4 5 6 7 8 9 102
4
6
8
10
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16
2 3 4 5 6 7 8 9 102
4
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16
2 3 4 5 6 7 8 9 102
4
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10
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100
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
c) August d) September
a) June b) July
0-d
BZ
to
p h
eig
ht
(km
) 0
-dB
Z t
op
he
igh
t (k
m)
35-dBZ top height (km) 35-dBZ top height (km)
%
35-dBZ top height (km) 35-dBZ top height (km)
0 40 80 120 160 200 240 280 320 3600
1
2
3
4
5
6
7
8
F
req
ue
ncy
(%
)
Direction storm moving to (degree)
(b)
0 40 80 120 160 200 240 280 320 3600
5
10
15
20
Fre
qu
en
cy (
%)
850 hPa wind direction (degree)
(c)
Storm Orientation (deg)
Fre
qu
en
cy (
%)
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0
25
50
75
100
125
Storms with modal orientation of 90 degree
East-West Distance from Radar (km)
No
rth
-So
uth
Dis
tan
ce f
rom
Ra
da
r (k
m)
(f)
0 2 4 6 8 10 12 14 16 180
5
10
15
20
Storm Speed (m s-1)
Fre
qu
en
cy (
%)
(a)
0
45
90
135
180
225
270
315 850 hPa Wind Direction Direction storm moving to
(d) 90o
(e)
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18
00
06
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1250
2
4
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18
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00
06
12
West-East distance from Radar (km) South-North distance from Radar (km)
m s-1
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200
400
600
800
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200
400
600
800
West East South North
He
igh
t (m
) Lo
cal
Tim
e (
Ho
urs
)
(a) (b)
(c)
Cross coast Along coast
(d)
Research Advisory Committee Meeting, 22 January 2018, IITM, Pune
Fig 7: Frequency
distribution of 35-
dBZ Top Heights,
Area and Storm
Duration
Convection has
shallow depth,
sub-MCS nature,
short lifetime
Storm properties
obey Lognormality
0
100
200
300
400
1 3 5 7 9 11 13 15 17 19 21 23
1 3 5 7 9 11 13 15 17 19 21 2330
35
40
45
50
55
1 3 5 7 9 11 13 15 17 19 21 23
4.8
5.0
5.2
5.4
5.6
5.8
6.0
43
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49
501 3 5 7 9 11 13 15 17 19 21 23
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.01 3 5 7 9 11 13 15 17 19 21 23
16
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30
1 3 5 7 9 11 13 15 17 19 21 2320
25
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Nu
mb
er
VIL
(k
g m
-2)
Ma
x.
Re
fle
ctiv
ity
(d
BZ
)
Pre
cip
. F
lux
(m
3 s
-1)
Are
a (
km
2)
Vo
lum
e (
km
3)
35
-dB
Z T
op
He
igh
t (k
m)
Du
rati
on
(m
in)
Local time (hrs.)
(b) (a) (c)
(e) (e)
(d)
(g) (h)
Local time (hrs.)
Local time (hrs.) Local time (hrs.)
a) c) e)
b) d) f)
Figure 8: Diurnal evolution of
storm Properties
Classification of Convective Cell Types
Local time (hrs.)
Local time (hrs.)
Storms exhibit small nocturnal
and strong afternoon maxima
Afternoon peak due to
convective system over land &
morning peak to those over
ocean
Convective area peak delayed
by several hours to that of
precipitation.
Spatial distribution of storms influenced by anomalies of surface specific humidity
and wind fields
Figure 6: Topography along a) Cross coast and (b) along
coast dimensions and Diurnal Hovmoller along c)cross coast
and d)along coast dimensions.
Slow Storms: 3-5 ms-1; Storm move east-west; steered by large scale
wind at 850 hPa; modal orientation of 90o parallel to Eat-West ridges,
influence of ridge topography in aligning storms parallel to them.
Figure 9:
a. Spatial distribution
of Cell Types
b. Diurnal evolution
of Cell Types
Congestus dominates. Cumulus and Congestus clusters along windward mountains.
Deep convection on lee side. Lead-Lag relation of Congestus and deep > transition
from shallow to deep (heating and moistening by Congestus important).
Figure 2 : Relative
frequency distribution of
convective cells as a
function of 0 dBZ and 35
dBZ top heights for each
months of monsoon 2014.
Slopping pattern in cross coast direction : Systematic
progression. Horizontal pattern in along coast
direction : slow movement, unorganized convection
0 dBZ Top : Depth of
Storm
35 dBZ Top : Intensity
of Storm
For wide range of 0dBZ Tops, height attained by 35 dBZ is maximum 7km. June
corresponds to the period with deep storms having intense internal structures.
Summary Future Scope
• First–time view of time-continuous aspects of convection, with respect to complex topography.
• Increased frequency of cell initiation along upslope compared to coastal & lee side highlights orographic
response to southwesterlies with superimposed
diurnal cycle.
• Convection has shallow depth, sub-MCS nature, short lifetime, slow movement, east-west alignment.
Cell properties follow log-normal dist. Storms steered by largescale flow at 850 hPa to move in east-
west direction and orient along mountain ridges.
• Radar observed small-scale convective features are
useful in validating CRMs.
• Vertical structure of convection during dry and
wet spells will be studied.
• Combined Radar and Lighting data shall help
linking convective storm
intensity with microphysics
& lightning production.
1 3 5 7 9 11 13 15 17 19 21 230
100
200
300
400
500
Cumulus [< 4 km]
Congestus [4-9 km]
Deep convection [> 9 km]
Local Time (Hrs)
Nu
mb
er
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0
25
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125
Congestus
Cumulus
Deep
convection
No
rth
-So
uth
Dis
tan
ce f
rom
Ra
da
r (k
m)
East-West Distance from Radar (km)