Characteristics of severe thunderstorms over Bangladesh and neighborhood: Observations & WRF- ARW model simulations Senaka Basnayake Asian Disaster Preparedness Centre Thailand WRF Lecture Series, 20 – 24 September BCCR - Norway

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Severe thunderstorms or Nor’westers

Favorable Synoptic conditions for Nor’westers Presence of an active elongated trough of low over the Gangetic plains of north India across north Bangladesh on the mean seal level pressure chart Presence of southerly low level jet with a pole ward meridional component in the lower tropospheric wind flow High magnitude of moisture in the humidity field intruding from the Bay of Bengal into Bangladesh and thereafter north India Presence of sub-tropical jet stream with strong vertical wind shear in the low to middle troposphere levels over Bangladesh and neighborhood Intrusion of high convective available potential energy (CAPE) from the Bay of Bengal into Bangladesh and presence of a low convective inhibition energy (CINE) regime. A pocket of strong warm temperature advection at 850 hpa and cold temperature advection at 300 hpa

S. NoDateNo of events /dayReported StationReported Wind Speed km/h Wind Direction (’ly) Reported Time (UTC) aRajshahi56NW bRangpur41NE a44SW0949 1b52W0952 1c44W0955 2Rangpur63N a46NW1756 2bM.M.O Dhaka65S SW a52NW b57NW1450 1c74NW1508 1d56NW e44NW1524 2a56NW bCox’sbazar46N N W Sylhet58NW W a51E1025 1bHatiya51SE1402 2Rangpur83NW aKutubdia93S1414 3bCox’sbazar83SWS N Sylhet70W Rangpur67E Sylhet67NW NW pre-monsoon

2009 pre-monsoon season

(29) NW SE (2) 8) W S (3) NE (9 N (10) E (3) (4) SW 41 – 60 km/h ~ 41, 61 – 90 km/h ~ 27, 91 – 120 km/h ~ pre-monsoon season

CAPECINE LI SI Threshold Value > > ( NEGATIVE) Week instability CAPE < 1000 J/kg Moderate instability 1000 J/kg < CAPE < 2500 J/kg Strong instability CAPE > 2500 J/kg Very Stable atmospheric condition LI ≥ 6.0 Stable condition - unlikely to form thunderstorms 1.0 < LI < 6.0 Slightly Unstable - possible thunderstorms with mechanical lifting -2.0 < LI < 0.0 Unstable - likely to form thunderstorms with strong mechanical lifting -6.0< LI < -2.0 Very Unstable - likely to form severe thunderstorms with strong mechanical lifting LI < -6.0 Possible showers or thunderstorms SI ≤ +3.0 Possible severe convective activitySI ≤ -3.0

TTISWEAT BRN Indicator of occurrence of thunderstorm TTI ≥ 40 Indicator of occurrence of thunderstorm with tornado intensity TTI ≥ 47 Indicator for potential strong convectionSWEAT = +250 Indicator for potential severe thunderstormsSWEAT = +300 ndicator for potential tornados SWEAT = +400 Thunderstorms unlikely due to too much wind shear BRN < 10 Possibility of Supercells10 < BRN < 45 Storms with multicells than supercellsBRN > 45

(d) (e) (f)

Weather Research and Forecasting (WRF) Developed by the Mesoscale and Microscale Meteorology Division of National Centre for Atmospheric Research (NCAR) of USA National Centre for Environment Prediction (NCEP) real time analysis (FNL) data as initial and lateral boundary conditions (LBCs) at six hourly intervals Domain parameters & Model physics; Horizontal Resolution : 9 km No. of grid point along X : 153 No. of grid point along Y : 139 No. of Vertical sigma levels : 27 Iterative Time step : 50 seconds Integration scheme : 3 rd order Runga-Kutta mp_physics: WSM 3 Cumulus parameterization : Kain-Fritch scheme Sf_sfclay_physics : Monin-Obukhov with Carslon-Bolan viscous sub-layer option Sf_surface_physics : Noah LSM ra_lw_physics : RRTM ra_sw_physics : Dudhia Boundary layer parameterization : Yonsei University scheme

Date Model Simulated Time (UTC) Observed Place Observed Wind Speed (km/h) Observed Time (UTC) Squall speed Nor’westers (41-60 km/h) 31 /03/ Chittagong /05/ Dhaka Cox's Bazar/Ctg /05/ Rangpur /05/ Dhaka /05/ Dhaka /05/ Dhaka /05/ Saidpur Light Nor'wester (61-90 km/h) 29/03/ Dhaka Dhaka /05/ Sylhet /05/ Dhaka /05/ Chittagong /05/ Chittagong Moderate Nor'wester ( km/h) 17/04/ Kutubdia /05/ Barrackpore, India931240

S. No. ParameterSquall speedLightModerate Obs.ModelObs.ModelObs.Model 1 Cloud Top Altitude (hPa) Precipitation rate at surface (mm/h) Direction of movementNW ly NNW ly W ly WNW ly NW ly 4 Speed of movement (km/h) Maximum wind speed at surface (m/s) Length of squall line (km) Altitude of Core ppt n (hPa)N/A830N/A800N/A750 8 Intensity of Core ppt n (mg/kg)N/A195N/A740N/A26 9 Updrafts speed (maximum) (cm/s)N/A120N/A100N/A Downdrafts Speed (maximum) (cm/s)N/A60N/A120N/A60 11 Liquid water Content (mg/kg)N/A221N/A301N/A365 Composite Characteristics

Concluding Remarks…. Convective indices at Patna, Ranchi, Guwahati and Agartala stations are good indicators for identifying possible thunderstorm days over Bangladesh as the systems initiate or conceive over West Bengal, Assam and Meghalaya regions. Some of the factors which are essential for triggering of Nor’westers such as low level moist southerly jet from the head Bay of Bengal, mid-trophospheric cold dry westerly trough, etc. are well simulated by the model. WRF-ARW model is able to simulate some salient features of thunderstorms with some temporal and spatial shifting.

Acknowledgement SAARC Meteorological Research Centre (SMRC) Bangladesh Meteorological Department (BMD) Dr. Someshwar Das and research team at SMRC

Tusen Takk

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