Development of nowcasting technique and evaluation of convective indices for thunderstorm prediction in Gangetic West Bengal (India) using Doppler Weather Radar and upper air data

Thunderstorm and hailstorm are well known short term severe weather phenomena which sometimes turn in to natural hazard especially in Gangetic West Bengal region of India. Large vertical extent of the cumulonimbus cloud, very high reflectivity, squally wind speed sometimes exceeding 100 km/h and heavy rainfall are the main features of these thunderstorms during pre-monsoon period in this region. A study of 70 thunderstorms has been carried out during the pre-monsoon season (March-May) of the year 2005 around Kolkata (22.5° N, 88.5° E) using Doppler Weather Radar and Upper air data. Standard convective indices like CAPE, CINE, LI, BRN and VGP have been evaluated and analyzed statistically. As no definite thresholds of the convective indices are available for thunderstorm prediction in this region, an attempt has been made to find threshold of these indices for possible occurrences of thunderstorms in Gangetic West Bengal region after the analysis of the thunderstorms during year 2005. The validity of these convective indices has been checked with 34 occurrences of thunderstorms during 2006-2007 recorded by Doppler Weather Radar Kolkata. The study reveals that nowcasting of thunderstorms may be done at least 2-3 hrs in advance witha fair degree of accuracy using Doppler radar products only. However, the lead time of nowcasting may be further improved if the convective indices are also analyzed and used in addition to the DWR data. A simple technique has been suggested by the authors for better prediction of thunderstorms at least three to four hours in advance.

SIMULASI NUMERIK MEKANISME TURBULENSI DEKAT AWAN KONVEKTIF

Intisari Turbulensi yang dialami oleh pesawat komersial rute Jakarta-Medan telah dilaporkan mengalami Clear Air Turbulence (CAT) di atas Sumatera Utara pada tanggal 24 Oktober 2017. Namun berdasarkan data citra satelit Himawari dari Badan Meteorologi, Klimatologi, dan Geofisika (BMKG) Indonesia menyebutkan bahwa di sekitar lokasi turbulensi terdapat awan kumulonimbus. Penelitian ini memanfaatkan model WRF-ARW dengan resolusi spasial dan temporal tinggi untuk mengetahui secara detail proses yang terjadi pada awan konvektif penyebab Near Cloud Turbulence (NCT). Turbulensi tersebut disebabkan oleh bilangan Richardson rendah yang terbentuk di wilayah udara jernih (clear air) yang berjarak 300-700 m di atas puncak awan dan diperkuat dengan adanya Turbulensi Energi Kinetik (TKE) mencapai 4,4 m2 / s2 dan geser angin vertikal (VWS) oleh arus keluar awan konvektif. Abstract Turbulence encountered by commercial aircraft Jakarta-Medan routes has been reported that experienced Clear Air Turbulence (CAT) over North Sumatra on October 24th, 2017. However, based on Himawari satellite imagery data produced by Agency for Meteorology, Climatology, and Geophysics (BMKG), Indonesia stated that there was a cumulonimbus cloud around the turbulence location. This study utilizes WRF-ARW models with a high spatial and temporal resolution to find out in detail the processes that occur in convective clouds causing Near Cloud Turbulence (NCT). The turbulence was caused by a low Richardson number formed in the clear-air area, which has a distance of 300 - 700 m above the cloud top and reinforced by the existence of Turbulence Kinetic Energy (TKE) reaching 4,4 m2/s2 and vertical wind shear (VWS) by deep convection’s outflow.