3.2.2 Convective event on 13th May, 2018
Development of the convective system on 13th May 2018 (16:00-22:30 IST) is captured in the plan position indicator (PPI) diagrams of radar reflectivity field at consecutive times during the event (Figure 8). The convective clouds started developing over the land around 25 km east of the radar location at 16:00 IST and then gradually it started moving westward. This movement of the system was due to the prevailing easterly wind (Figure 7a). The cloud system passed over NCESS location around 18:00 IST (Figure 8d). As soon as it reached over the NCESS location extremely heavy rainfall started, which was observed in the rain rate measured by disdrometer (Figure 9a). The rain rate crossed 100 mm h-1 and sustained in that range for over an hour. Gradually the rain intensity declined to a range of 0.1 - 1 mm h-1, which was basically the stratiform precipitation following the main convective activity. The rain DSD obtained by the disdrometer shows an abundance of bigger raindrops (diameter > 3 mm) during this intense convective spell followed by smaller drops at the later stage of the event. The deep convective cloud system eventually moved over the Arabian Sea around 30 km westward from the radar location and meanwhile it turned into a stratiform system (Figure 8g-8i). The IMD weather report also mentioned about the rainfall during these hours. This event was associated with rapid development of deep convective clouds as observed in the evolution of the cloud top infrared brightness temperature (IRBT) measured from satellite (INSAT-3DR). A lower brightness temperature signifies a higher cloud top height. Figure 10a-10e shows the spatial and temporal evolution of the brightness temperature during this event. Around 18:00 IST much of the region was having brightness temperature below 200 K revealing the occurrence of deep clouds over most of the region. Figure 11 (red curve) shows the temporal evolution of the brightness temperature over the NCESS location (averaged over a 12x12 km area centered at NCESS). A rapid decrease in the brightness temperature started at 15:45 IST and reached a minimum value of 185 K at 17:45 IST, which demonstrates how fast such a deep system can develop within such a short span of time. Also, cloud base height measured by ceilometer shows (Figure 12a) the presence of multilevel clouds. Before 17:00 IST mostly high-level clouds are detected (CBH ~ 7 km) and then just before the precipitation starts all three cloud layers are having cloud base below 2.5 km. Such a low cloud base height and high cloud top height (inferred from low IRBT values) measures the depth of the cloud system. Once the rain rate reduced it detected multilevel clouds. The CAPE value of 1713 J kg-1 was observed from the nearest radiosonde measurements in the mooring hour (05:30 IST) which was indicative of already existing moderate instability in the atmosphere which built up further and eventually led to strong updraft during evening hours.
The vertical structure of the storm in terms of DWR polarimetric measurements and associated hydrometeor identification is shown in Figure 13. Averaged reflectivity between 2.5 and 3.5 km height during rapid initial development stage of the storm shows active convective regions (Figure 13a). Then a vertical cross section along the convection line AB has been considered to analyse the vertical structure of the storm. Figure 13b shows the vertical cross section of reflectivity at horizontal polarization (Zh) along the convection line AB. The x-axis represents the distance from point A towards point B. Reflectivity values greater than 30 dBZ reaching up to 10 km height signifies the existence of strong updraft within the convective core region. This strong updraft can keep the larger hydrometeors (bigger raindrops, graupels etc.) float aloft for longer period giving them more time to grow further by the collision-coalescence process for raindrops and by riming process for ice particles (Schuur et al. 2001). Since, reflectivity is proportional to the 6th power of the particle diameter (Bringi & Chandrasekar 2001), these larger particles produce such strong reflectivity values even at higher altitudes.
Figure 13c shows the vertical cross section of differential reflectivity (Zdr) along the convection line. Zdrvalue gives a measure of the oblateness of precipitation particles and hence could be useful in distinguishing between larger raindrops, hail, and graupel due to differences in shape and orientation. Since raindrops (diameter> 1 mm) are deformed into oblate spheroid shape due to aerodynamic forces (Pruppacher & Beard, 1970) with a preferred orientation of their major axes in the horizontal direction (and therefore Zh> Zv), Zdr is positive and increases with raindrop size. This increase in the value of Zdr with raindrop size is shown quantitatively in Bringi et al. (2009) in terms of a polynomial fit between observed Zdr and mean drop diameter measured by disdrometer. Zdr values greater than 2 dB were observed which indicate presence of bigger raindrops or melting bigger ice particles (Anderson et al. 2011) below 4 km height. Bigger raindrops are also observed in the disdrometer measurements of rain DSD (Figure 9a).
Zdr values are much smaller at higher altitudes (above 0° isotherm ~5 km height) as the ice particles such as aggregate, graupel, hail, tend to be spherically symmetric or tumble while falling, causing low values of Zdr. The lower value of dielectric constant for ice compared to water is another factor behind lower Zdr for ice particles. Within the strong convective region at heights above the melting layer, a higher value of Zdr along with high value of Kdpindicates supercooled liquid drops above freezing level (Hubbert et al., 1998). The ρhv shows high values (>0.95) throughout the entire cross section (Figure 13d) and ρhvdepends on several factors such as eccentricity, distribution of canting angle, irregular shape and mixture of different types of hydrometeors. Relatively lower values of ρhv at the central region and at higher altitudes within the cross-section, could be attributed to mixture of ice particles with rain.
The estimated Kdp (Figure 13e) shows that the spatial pattern of Kdp is in tandem with that of reflectivity though there are differences. High values (greater than 5° km-1) of Kdp below melting level suggest the presence of intense convective precipitation with bigger raindrops formed due to the coalescence process or due to melting graupel. As drop eccentricity increases with diameter, the differential propagation phase increases causing higher values of Kdp within regions of intense convective precipitation. A similar structure of Kdpwithin convective regions is reported by Ryzhkov et al. (2002). Higher values of Kdp above freezing level suggests prevalence of supercooled droplets which can help in formation of graupel particles via the riming process.
Identified hydrometeor types are shown in Figure 13f. Below the melting level, it is mainly dominated by rain (RN) and above melting level, ice aggregates (AG) are the dominating hydrometeors. At heights between 4.5 to 8 km, within the convective core regions graupel (HDG) particles are abundant. Similar findings are obtained in Dolan et al. (2013), in which HDG was found close to the melting level and LDG at higher heights. Within such convective cores reaching up to 10 km height, liquid droplets are pushed to heights much above the freezing level and they stay there as unstable supercooled droplets. Upon contact with ice-aggregates they immediately freeze onto the surface forming bigger ice particles viz. graupel. The strong updraft can sustain these graupels in air for longer helping them grow even further. The presence of vertical ice indicates the existence of electric field which forces these particles to orient vertically and this could be due to the charging via the collisions between graupels and smaller ice particles, as confirmed by different laboratory experiments (Takahashi, 1978; Jayaratne et al., 1983; Saunders et al., 1991).