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