2.1. DWR data and quality control
C-band polarimetric Doppler Weather Radar (DWR), installed at VSSC, Thiruvananthapuram (8.5374N, 76.8657E, 27 m above mean sea level) operates at a frequency of 5.625 GHz and have a peak transmitting power of 250 kW. The radar performs a volumetric scan of the surrounding atmosphere within a radius of 240 km at 11 elevation angles (0.5°, 1°, 2°, 3°, 4°, 7°, 9°, 12°, 15°, 18° and 21°) with an azimuthal and radial resolutions of 1° and 150 m respectively. One full volume scan takes around 15 minutes. The radar provides base products such as reflectivity at horizontal polarization (Zh), differential reflectivity (Zdr), differential propagation phase (Φdp), cross-correlation(ρhv), radial velocity (Vr) and Spectral width(σ). Zdris the difference between reflectivities (in decibel) at horizontal and vertical polarization, Φdp is the phase difference between the horizontally and vertically polarized pulses. More information about these variables can be found in Doviak and Zrnic (1993) and also in Bringi and Chandrasekar (2001). A comprehensive detail about the radar is given in Mishra et al. (2020). The validation of the radar data with other instruments showed that the DWR reflectivity agrees quite well with GPM satellite measurements and also the radar retrieved precipitation have a good correlation (0.89) with ground based in-situ measurements (Kumar et al., 2020).
Received signal by radar is often contaminated by signals reflected from non-meteorological objects such as hills, birds etc., anomalous propagation and also attenuation of the electromagnetic wave by different types of hydrometeors (Ryzhkov & Zrnic, 1998; Friedrich et al., 2006; Unal, 2009; Lakshmanan et al., 2014). Even though the radar signal processor takes into account many factors to give reasonably accurate base products from the return signal, still the data needs certain quality control measures. The use of simple thresholds for different variables can be quite useful in removing unwanted echoes (Ryzhkov & Zrnic, 1998; Lakshmanan et al., 2014). The following quality control measures are considered for this study- (i) pixels with Zh > 70 dBZ or ρhv<0.7 are ignored, (ii) topography data from Shuttle Radar Topography Mission (SRTM) (Farr et al., 2007) to remove ground clutter from hills present towards 40 km east of the radar (Figure 1) using the method proposed by Friedrich et al., (2006). Figure 2 shows the radar reflectivity during an event on 13thMay, 2018 before quality control (Figure 2a) and after quality control (Figure 2b). The clutter due to hills is present on the reflectivity field before quality control, which is removed nicely after applying the above-mentioned quality control measures. Other variables (Zdr, Φdp and ρhv) were processed similarly.
Table 1 . Overview of the instruments and the data used in this study