2. Data and Methodology
We have identified eleven convective events over the southern peninsular
India during the pre-monsoon period (i.e., Mar to May) of 2018 using
C-band radar reflectivity field. For these events convective-stratiform
separation has been done to obtain statistics on radar reflectivity over
convective and stratiform regions. Two prominent convective events on
13th May and 25th May, 2018 have
been selected as representative cases for further analysis. Besides the
DWR data, we have used rain drop size distribution (DSD) data from
disdrometer, cloud base height (CBH; m) data from ceilometer, brightness
temperature data from INSAT satellite, ERA5 reanalysis data and also
radiosonde measurements. Disdrometer and ceilometer were installed over
the rooftop of the National Centre for Earth Science Studies (NCESS;
8.5228N, 76.9097E). Locations of the DWR and NCESS along with the
topography of the surrounding area are shown in Figure 1. A brief
description of the instruments and data is summarized in Table 1.
Optical disdrometer (model: OTT Parsivel, manufactured by OTT Hydromet,
Germany) is a laser-based system that detects all types of precipitation
at the surface (Löffler-Mang and Joss, 2000; Friedrich et al., 2013). It
measures rain DSD and fall velocity distribution in 32 size and velocity
classes as well as it provides rain rates (R; mm h-1)
and radar reflectivity (dBZ). The size of measurable liquid
precipitation particles ranges from 0.2 to 8 mm and it varies from 0.2
to 25 mm for solid precipitation particles. It can measure the particles
fall velocity from 0.2 to 20 ms-1. The temporal
resolution of this data is 1 minute. The disdrometer used in this study
was installed over rooftop of NCESS.
Ceilometer (model: CHM15k-Nimbus manufactured by Lufft Mess-und
Regeltechnik GmbH) is a ground-based remote sensing device that uses
standard lidar method to determine the cloud base height (CBH) from the
altitude profile of backscattered signals. It can provide cloud
thickness where the cloud layers do not totally attenuate the laser
beam. But the signals get attenuated in a rainy situation depending on
the number concentration and size of raindrops and hence signal to noise
ratio of the ceilometer decreases with increasing rain rate (Clothiaux
et al., 2000). Technical details of CHM15k can be obtained from the
previous studies by Heese et al. (2010) and Sumesh et al. (2019). The
CHM15k is operated with a vertical resolution of 15 m and the CBH is
measured with a temporal resolution of 15 s.
Brightness temperature data (Infrared Brightness Temperature, IRBT) has
been used as a proxy for the cloud top height. This data is obtained
from INSAT-3DR which is a multi-purpose geosynchronous spacecraft and
provides data with spatial resolution of 4x4 km and temporal resolution
of 30 minutes, of mesoscale phenomena in the visible and infrared (IR)
spectral bands (0.55-12.5 μm) over the Indian region. This data is
freely available through the https://www.mosdac.gov.in/ server.
The synoptic circulations over the study region were analysed using the
geopotential (m2 s-2), u-wind (m
s-1) and v-wind (m s-1) variables
from ERA5 reanalysis hourly data having spatial resolution of
0.25°x0.25°. Radiosonde measurements from India Meteorological
Department (IMD), Thiruvananthapuram have been utilized to analyse the
Convective available potential energy (CAPE; J kg-1),
vertical profiles of temperature (K), mixing ratio (g
kg-1), wind speed (m s-1) and wind
direction (deg).