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).