vertical structure of convection during the dry and wet spells of...
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0.0
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1.0
He
igh
t (k
m)
West East West
(a) Dry Period (b) Wet Period
East
72.75 73 73.25 73.5 73.75 74 74.25 74.5 74.75
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15
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24
3
6
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12
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15
-dB
Z E
TH
(k
m)
Longitude (o E) Longitude (o E)
Loca
l T
ime
(H
rs.)
72.75 73 73.25 73.5 73.75 74 74.25 74.5 74.75
12
15
18
21
24
3
6
9
12
Windward Lee Windward Lee
Dry period : associated with strong diurnal cycle and weak
upstream winds
Wet Period: associated with weak diurnal cycle and strong
upstream winds
Inferred that the observed convective features during dry
(wet) periods are associated with the thermally
(mechanically) induced circulations.
0
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0.0 0.2 0.4 0.6 0.8 1.0Normalized Occurrence
0-d
BZ
ET
H (
km
)
-0.6 -0.3 0.0 0.3 0.6 0.9 1.2
0
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He
igh
t (k
m)
Latent Heating (K hr-1)
-7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.02
3
4
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9
r = -0.79
Low Level Static Stability (dTv/dz
low)
30
-dB
Z E
TH
(k
m)
d)
b)
-7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.02
3
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Low Level Static Stability (dTv/dz
low)
r = - 0.44
30
-dB
Z E
TH
(k
m)
c)
0
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10
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18
0.0 0.2 0.4 0.6 0.8 1.0
Normalized Occurrence
0-d
BZ
ET
H (
km
)
-0.6 -0.3 0.0 0.3 0.6 0.9 1.2
0
2
4
6
8
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18
Latent Heating (K hr-1)
He
igh
t (k
m)
a)
Dry Period Wet Period
1 3 5 7 9 11 13 15 17 19 21 23
3
4
5
6
7
8 0-dBZ ETH
15-dBZ ETH
30- dBZ ETH
1 3 5 7 9 11 13 15 17 19 21 23
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7
8
-15 -10 -5 0 5 10 15 20
0
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15
18
mode: 5.53 ms-1
Horizontal Wind Cross-Shore Component (ms-1)
Oc
cu
rre
nc
e (
x 1
02)
-15 -10 -5 0 5 10 15 200
3
6
9
12
15
18
Horizontal Wind Cross-Shore Component (ms-1)
mode: 18.14 ms-1
a) b)
ET
H (
km
)
c) d)
Weak Strong
Medium
Weak Strong
Medium
Wet Period Dry Period
Local Time (hrs.) Local Time (hrs.)
Vertical Structure of Convection during the Dry and Wet spells of Monsoon over the Western Ghats of India
Within ISM domain, some of the highest & most spatially variable rainfall rates
are found along & upstream of the Western Ghats (WG), which acts as anchors
for generating convective centers of monsoon [Xie et al. 2006].
Mesoscale precipitation pattern:
steep west-to-east gradient of
precipitation across WG.
Within season, rainfall over WG fluctuates between dry and wet spells usually
occurring during active & break spells of large-scale rainfall over monsoon zone
NWP models: continuing efforts to improve simulation of organization of clouds
and precipitation in the monsoon ISO scale; but suffers owing to limited
observations of mesoscale convective features under different ISO settings.
Past studies on orographic precipitation & intra-seasonal variability over the WG,
were mainly based on satellite and reanalysis data sets.
In present study, time-continuous X-band radar observations are used to
characterize the vertical structure and evolution of convective cloud fields during the
dry and wet epochs of monsoon & relate small-scale convective features (as obtained
form radar data) to the large-scale atmospheric state. Auxiliary data from satellites,
reanalysis and Lightning Location Network are used.
Background, data, and methodology Lightning, convective tops & area fractions
Work in this study is supported by IITM, Pune We thank DST-Inspire, for fellowship of
Utsav Bhowmik & Thunderstorm Dynamics group at IITM for LLN data.
Echo top heights relationship with LH profiles & low-level stability
Figure 6: Time zonal sections of composite diurnal 15‐dBZ ETH cycle
(latitudinal average over radar domain) during (a) dry & (b) wet periods.
Mean elevation profile of topography above the plots is shown in gray
Utsav Bhowmik, Sachin Deshpande*, Subrata Das, G. Pandithurai and Dev Niyogi
Figure 4: Top Panel: Vertical profiles of 0 dBZ ETH distribution
and TRMM 3G31 LH profiles for a) dry and b) wet period..
Bottom Panel: Scatter plot of 30 dBZ ETH and low level static
stability (1.5-4.5km) for c)dry and d) wet periods.
Diurnal cycle of convection and its control by upstream winds
Figure 5: Top panel: Diurnal cycle of 0, 15 and 30 dBZ during a)
dry & b) wet period. Bottom panel: Histogram of cross shore
horizontal wind at 850 hPa during c) dry & d) wet days. Spell identification & large scale features
Figure 2 : Identification of dry and wet spells
of Monsoon 2014 (JJAS) using gridded IMD
rainfall (0.25ox 0.25o) over radar domain.
A period is marked as wet (dry) if the
standardized rainfall anomaly is above
(below) 0.5 (-0.5) for 3 consecutive days
Figure 8: Diurnal variations of (a) total
lightning, (b) fractional area with 30‐dBZ
echo‐top heights (ETHs) >5 km, and (c)
convective area fraction with their standard
errors for dry period (red line) and for wet
period (blue line).
Convection relative to precipitation maxima
Figure 7: Scatter plot of total lightning as a function of 30‐dBZ
echo‐top height (ETH) for (a) dry and (b) wet periods. Radar volumes
with zero flashes have been excluded in this analysis.
Dry period : Strong diurnal amplitude; two broad maxima due
to deep convection of lee
Wet Period: Small diurnal amplitude on both windward &
leeward sides. Shallow ETHs over upslope
Dry period: Penetration of 30-dBZ ETH above the 0°C
isotherm : indicator of the existence of large particles in mixed
phase region of the cloud, supported by updrafts well
correlated with increased lightning occurrence
Wet period: Seldom electrical activity, which may be related
to insufficient updraft speeds within convective core to
produce lightning.
Summary : pictorial flowchart
Figure 1 : TRMM monsoon precipitation
climatology [Shige et al., 2017]
Jun 1 Jun 30 Jul 31 Aug 31 Sep 30
-1
0
1
2
3
4
5
6
Day (June-September, 2014)
Sta
nd
ard
ize
d
rain
fall
an
om
aly
500-mb Gpt Hgt
OLR
Rain Rate
Precipitable Water
500-mb Gpt Hgt
OLR
Precipitable Water
Rain Rate
Figure 3: Anomalies of
geopotential height at 500hPa
(a & e); OLR (b & f); Precip.
water (c & g) and rain rate (d
& h) during dry & wet spells
of monsoon.
Dry composites
+ve GPH : anticyclonic
circulation: fewer
precipitation systems.
+ve OLR & -ve precip
water
weak rain belt along coast
Wet composites
Negative GPH anomaly:
cyclonic circulation;
anomalous south-west flow;
many precipitation systems
Anomalies of -ve OLR &
positive precipitable water
Rainfall maxima along
WG, extending over Arabian
Sea
Dry: Suppressed heating and sharp decline in 0-dBZ
ETH occurrence
Wet: Enhanced 0-dBZ ETH occurrence :cooling below 2
km & increased heating aloft with maxima at 6 km.
Similar variations in ETH and LH profiles implying that
heating profiles has a distinct signature for different
distribution of cloud tops
ETH of precipitating convection decreases with
increasing low level stability. Low-level static stability
inhibits cloud growth beyond freezing level, promote
cloud detrainment; important factor in determining top
heights of precipitation.
Positive feedback between low-level heating and low-level
static stability maintains shallow nature of convection
over Western Ghats.
Upstream wind at 850 hPa: 16.75o N-19.25o N, 69o E-75o E
over the Arabian Sea [Shige et. al 2017]
Three wind regimes defined: weak (𝑈 ≤ 𝑈 ̅ − 𝜎), medium (𝑈 ̅ − 𝜎 < 𝑈 < 𝑈 ̅ + 𝜎), strong (𝑈 ≥ 𝑈 ̅ + 𝜎)
Reduced dimension analysis Total lightning & 30-dBZ ETH
Dry: An afternoon peak in lightning
production is observed accompanied by
increased penetration of 30-dBZ ETH
> 0°C isotherm, increased convective
area fractions, favouring strong
electrification within convection.
Wet: Overall lightning activity is
reduced by almost 50%; with subdued
Convective area fraction
Figure 9: Daily evolution of a)Rainfall
b)SST c)0 dBZ ETH Occurrence d) IR TB e)
0-dBZ echo coverage f) Relative Humidity &
g) evolution of cumulus, congestrus and
Deep clouds from 0 dBZ ETH
Three episodes of high rainfall (P1,P2,
P3), 3-5 day periodicity
Cloud top heights increase steadily at
about same rate for 1 week prior to
rainfall event.
Mid-tropospheric moisture rapidly
increased over a period of 2 to 3 days
ahead of the increase in cloud top
heights and their areal coverage near
the high precipitation events.
Atmosphere is always moist below 3 km
Maximum moisture variations found
above 3 km
Midtroposphere moistening & drying
occurs during peak occurrence of
congestus & shallow convection modes.
Acknowledgements
Journal of Geophysical Research, 124, February 2019
P1 P2 P3