Operationalisation of Thunderstorm Nowcasting Services over NE Region using DWR data
North Eastern Space Applications Centre (NESAC) has been providing thunderstorm nowcasting (forecasting up to 4 hours) services for North Eastern Region (NER) of India since 2015 under the North Eastern Regional node for Disaster Risk Reduction (NER-DRR) initiatives. This was done using the data from satellite imager and sounder onboard INSAT-3D / INSAT-3DR, automatic weather station data, and by analysing numerical weather forecast data. However, it was difficult to detect, track and forecast using this data alone as most of the thunderstorms being localised event, extending only over a few tens of km and having lifetime of less than one hour. The availability of DWR data has opened a new window for precise identification of thunderstorm weather systems, track them and forecast the probable areas which may get affected, albeit with lesser lead time.
The first S-band dual polarimetric Doppler Weather Radar (DWR) was installed at Cherrapunjee, Meghalaya which was dedicated to nation by Shri Narendra Modi, Hon’ble Prime Minister of India on May 27, 2016. NESAC is operating the DWR continuously since then, and the data is made available in near real time for public through the MOSDAC (Meteorological and Oceanographic data archival centre) and IMD websites. The DWR is calibrated at regular intervals and the data and products are being validated. It has unobstructed coverage for the entire state of Meghalaya, Tripura, Southern Assam, and part of Mizoram and Manipur. For the western and central Assam region, the DWR has coverage beyond 3 degree elevation only. The DWR also sees large part of India’s neighboring country, Bangladesh. The radar completes one volume scan in 11 minutes, comprising of 360 degree azimuth scan for 10 elevation angles ranging from 0.5 to 21 degrees. It also allows sector scan (in both azimuth and elevation) for high temporal observation of any event. The DWR covers a distance of 250 km (up to 500 km only for Z) with spatial resolution of 300 m.
Thunderstorm is a pre-monsoon season (April-May) phenomenon over the NER of India. The data collected by the DWR during 2016 was used to understand the thunderstorm and storm signatures and calibrate the nowcasting model. During 2017 the nowcasting service was made operational. Severe thunderstorm nowcasting services for Southern Assam, Meghalaya, and Tripura were done primarily using the DWR data and for the rest of the NER, the earlier methodology was used. In addition to the Z (radar reflectivity), S (spectral width) and V (velocity) data collected by the DWR, extensive use of the polarimetric data like ZDR (differential reflectivity) and ρHV (Correlation coefficient) were also made to differentiate thunderstorm clouds from non-thunderstorm clouds.
Almost all the thunderstorms affecting Meghalaya, southern Assam, and Tripura could be identified using the data from Cherrapunjee DWR during the pre-monsoon season of 2017. The spectral width (S) data was used to characterise the internal turbulence within a cloud system that helped in quantifying the storm potential (S>5) of a particular cloud. Hail potential could also be identified using ZDR (ranging from 0.5 to - 0.5) and ρHV (~0.9) data. The radar reflectivity, Z and its vertical profile (Max Z plots) was the backbone of thunderstorm identification and identification of individual storm cells in large cloud bands that appear in satellite imageries. All clouds having Z value more than 40 dBZ could become a thunder cloud and were tracked in real time. Z of 50-56 dBZ was categorised as severe thunderstorm and Z of more than 56 dBZ was categorised as very severe thunderstorm with hail. The reflectivity and velocity of clouds were studied to track every individual storm and forecast the likely affected areas within certain time intervals, based on sustenance potential of such storms, estimated by analysing the local atmospheric condition.
The use of the Cherrapunjee DWR data has improved the thunderstorm nowcasting accuracy over Meghalaya, Southern Assam, and Tripura states. Altogether 48 severe and very severe thunderstorms were forecasted in these three states during April 1 to June 15, 2017 period. The accuracy of nowcasting was more than 90% with lead time varying from 30 minutes to more than 2 hours. The nowcasting services were disseminated through NER-DRR website and also through direct communication to the concerned at state level.
The DWR, Cherrapunjee coverage for elevation angle of 3 degree (left). Calibration of the DWR using metal sphere attached to hydrogen gas filled balloon & Pisharoty sonde (right)
A squall line formed over Meghalaya, Southern Assam, and Manipur which is clearly identified using DWR data (left). INSAT-3D TIR1 channel image of the same time is also shown (right). The DWR data helped in identifying the individual storm cloud cells in a large cloud band as seen in satellite image
Max V (left) and Max S (right) data from DWR, Cherrapunjee. Max V is used to estimate the velocity at which a weather system is moving and Max S gives an idea about the internal turbulence within cloud system