4. Summary
The present study is focused on the structure of pre-monsoon convective
systems over a tropical coastal region in southern peninsular India.
Observations from several instruments such as Doppler weather radar
(DWR), disdrometer, ceilometer, INSAT-3DR satellite data, radiosonde
measurements are used in the study. Using the quality controlled DWR
data, 11 convective events have been identified by inspecting the
reflectivity field from DWR. Out of which, the convective events
occurred on 13th May and 25th May
2018 has been analysed in detail to understand the development of
mesoscale cloud systems. Convective-stratiform separation has been done
for all the events. Following are the major conclusions of the study.
- Convective-stratiform separation clearly demarcates the distinct
difference in reflectivity profiles over convective and stratiform
regions. A peak in the mean reflectivity profile near 3 km height is
registered for convective regions. Stratiform regions are
characterized by a peak reflectivity near melting layer signifying the
bright band and almost constant reflectivity profile between 1km and 4
km levels. The distribution of the reflectivity values at a height of
3 km shows bell shaped nature and there is an overlap between
distributions for the convective and stratiform precipitations. It
also shows that using a single threshold for reflectivity or rain rate
may not be useful for convective-stratiform separation.
- The reflectivity PPI captured the spatial and temporal evolution of
the convective cases on 13th and
25th May 2018 from the initiation to the dissipative
stages of both the events. The development of the system on
13th May was much more rapid with cloud tops
reaching much higher altitudes as clearly seen in the brightness
temperature observed from satellite. Disdrometer measurements of rain
rate and DSD during the two events show that the event on
13th May, was associated with high rain rate
(>100 mm h-1) having bigger raindrops
(diameter > 3mm) during the first hour of the event. Even
though the spatial extent of the system on 25th May,
was larger, much lower rain rate (<10 mm
h-1) with relatively smaller (diameter < 3
mm) raindrops was observed.
- Vertical structures inside the storms during rapid development stage
have been obtained by taking
vertical cross sections of
reflectivity through major convective regions. The reflectivity values
show convective cores reaching 10 km height on 13thMay and about 7 km height on 25th May. High values
of Zdr at lower levels were observed on
13th May, due to the oblate spheroid shape of the
bigger raindrops. The structure of Kdp field is quite
similar to that of reflectivity in both the cases. High values of
Kdp reveals the presence of intense rainfall on
13th May, as Kdp is mainly dominated
by bigger raindrops.
- Fuzzy-logic based hydrometeor identification (HID) has been done along
the vertical cross sections over prominent convective regions. HID
analysis shows presence of graupel at middle levels within the
convective core regions revealing presence of strong updrafts. Ice
aggregates and rain are the dominant hydrometeors above and below
melting level respectively. Presence of vertical ice signifies the
presence of electric field inside the storm. Such electric field may
be generated due to non-inductive charging via collision between
graupel and smaller ice crystals.
It would be worth studying the observed lightning activity (if any)
during these events as presence of vertical ice indicates toward
development of electric field. If major lightning activity occurred
during these events, then it would support the collision charging
mechanism as graupels are identified within the convective core regions.