3.2.2 Convective event on 13th May, 2018
Development of the convective system on 13th May 2018
(16:00-22:30 IST) is captured in the plan position indicator (PPI)
diagrams of radar reflectivity field at consecutive times during the
event (Figure 8). The convective clouds started developing over the land
around 25 km east of the radar location at 16:00 IST and then gradually
it started moving westward. This movement of the system was due to the
prevailing easterly wind (Figure 7a). The cloud system passed over NCESS
location around 18:00 IST (Figure 8d). As soon as it reached over the
NCESS location extremely heavy rainfall started, which was observed in
the rain rate measured by disdrometer (Figure 9a). The rain rate crossed
100 mm h-1 and sustained in that range for over an
hour. Gradually the rain intensity declined to a range of 0.1 - 1 mm
h-1, which was basically the stratiform precipitation
following the main convective activity. The rain DSD obtained by the
disdrometer shows an abundance of bigger raindrops (diameter
> 3 mm) during this intense convective spell followed by
smaller drops at the later stage of the event. The deep convective cloud
system eventually moved over the Arabian Sea around 30 km westward from
the radar location and meanwhile it turned into a stratiform system
(Figure 8g-8i). The IMD weather report also mentioned about the rainfall
during these hours. This event was associated with rapid development of
deep convective clouds as observed in the evolution of the cloud top
infrared brightness temperature (IRBT) measured from satellite
(INSAT-3DR). A lower brightness temperature signifies a higher cloud top
height. Figure 10a-10e shows the spatial and temporal evolution of the
brightness temperature during this event. Around 18:00 IST much of the
region was having brightness temperature below 200 K revealing the
occurrence of deep clouds over most of the region. Figure 11 (red curve)
shows the temporal evolution of the brightness temperature over the
NCESS location (averaged over a 12x12 km area centered at NCESS). A
rapid decrease in the brightness temperature started at 15:45 IST and
reached a minimum value of 185 K at 17:45 IST, which demonstrates how
fast such a deep system can develop within such a short span of time.
Also, cloud base height measured by ceilometer shows (Figure 12a) the
presence of multilevel clouds. Before 17:00 IST mostly high-level clouds
are detected (CBH ~ 7 km) and then just before the
precipitation starts all three cloud layers are having cloud base below
2.5 km. Such a low cloud base height and high cloud top height (inferred
from low IRBT values) measures the depth of the cloud system. Once the
rain rate reduced it detected multilevel clouds. The CAPE value of 1713
J kg-1 was observed from the nearest radiosonde
measurements in the mooring hour (05:30 IST) which was indicative of
already existing moderate instability in the atmosphere which built up
further and eventually led to strong updraft during evening hours.
The vertical structure of the storm in terms of DWR polarimetric
measurements and associated hydrometeor identification is shown in
Figure 13. Averaged reflectivity between 2.5 and 3.5 km height during
rapid initial development stage of the storm shows active convective
regions (Figure 13a). Then a vertical cross section along the convection
line AB has been considered to analyse the vertical structure of the
storm. Figure 13b shows the vertical cross section of reflectivity at
horizontal polarization (Zh) along the convection line
AB. The x-axis represents the distance from point A towards point B.
Reflectivity values greater than 30 dBZ reaching up to 10 km height
signifies the existence of strong updraft within the convective core
region. This strong updraft can keep the larger hydrometeors (bigger
raindrops, graupels etc.) float aloft for longer period giving them more
time to grow further by the collision-coalescence process for raindrops
and by riming process for ice particles (Schuur et al. 2001). Since,
reflectivity is proportional to the 6th power of the
particle diameter (Bringi & Chandrasekar 2001), these larger particles
produce such strong reflectivity values even at higher altitudes.
Figure 13c shows the vertical cross section of differential reflectivity
(Zdr) along the convection line. Zdrvalue gives a measure of the oblateness of precipitation particles and
hence could be useful in distinguishing between larger raindrops, hail,
and graupel due to differences in shape and orientation. Since raindrops
(diameter> 1 mm) are deformed into oblate spheroid shape
due to aerodynamic forces (Pruppacher & Beard, 1970) with a preferred
orientation of their major axes in the horizontal direction (and
therefore Zh> Zv),
Zdr is positive and increases with raindrop size. This
increase in the value of Zdr with raindrop size is shown
quantitatively in Bringi et al. (2009) in terms of a polynomial fit
between observed Zdr and mean drop diameter measured by
disdrometer. Zdr values greater than 2 dB were observed
which indicate presence of bigger raindrops or melting bigger ice
particles (Anderson et al. 2011) below 4 km height. Bigger raindrops are
also observed in the disdrometer measurements of rain DSD (Figure 9a).
Zdr values are much smaller at higher altitudes (above
0° isotherm ~5 km height) as the ice particles such as
aggregate, graupel, hail, tend to be spherically symmetric or tumble
while falling, causing low values of Zdr. The lower
value of dielectric constant for ice compared to water is another factor
behind lower Zdr for ice particles. Within the strong
convective region at heights above the melting layer, a higher value of
Zdr along with high value of Kdpindicates supercooled liquid drops above freezing level (Hubbert et al.,
1998). The ρhv shows high values (>0.95)
throughout the entire cross section (Figure 13d) and ρhvdepends on several factors such as eccentricity, distribution of canting
angle, irregular shape and mixture of different types of hydrometeors.
Relatively lower values of ρhv at the central region and
at higher altitudes within the cross-section, could be attributed to
mixture of ice particles with rain.
The estimated Kdp (Figure 13e) shows that the spatial
pattern of Kdp is in tandem with that of reflectivity
though there are differences. High values (greater than 5°
km-1) of Kdp below melting level
suggest the presence of intense
convective precipitation with bigger raindrops formed due to the
coalescence process or due to melting graupel. As drop eccentricity
increases with diameter, the differential propagation phase increases
causing higher values of Kdp within regions of intense
convective precipitation. A similar structure of Kdpwithin convective regions is reported by Ryzhkov et al. (2002). Higher
values of Kdp above freezing level suggests prevalence
of supercooled droplets which can help in formation of graupel particles
via the riming process.
Identified hydrometeor types are shown in Figure 13f. Below the melting
level, it is mainly dominated by rain (RN) and above melting level, ice
aggregates (AG) are the dominating hydrometeors. At heights between 4.5
to 8 km, within the convective core regions graupel (HDG) particles are
abundant. Similar findings are obtained in Dolan et al. (2013), in which
HDG was found close to the melting level and LDG at higher heights.
Within such convective cores reaching up to 10 km height, liquid
droplets are pushed to heights much above the freezing level and they
stay there as unstable supercooled droplets. Upon contact with
ice-aggregates they immediately freeze onto the surface forming bigger
ice particles viz. graupel. The strong updraft can sustain these
graupels in air for longer helping them grow even further. The presence
of vertical ice indicates the existence of electric field which forces
these particles to orient vertically and this could be due to the
charging via the collisions between graupels and smaller ice particles,
as confirmed by different laboratory experiments (Takahashi, 1978;
Jayaratne et al., 1983; Saunders et al., 1991).