Thunderstorms over a tropical Indian station, Minicoy: Role of vertical wind shear

In this study, an attempt has been made to bring out the observational aspects of vertical wind shear in thunderstorms over Minicoy. Case studies of thunderstorm events have been examined to find out the effect of vertical wind shear and instability on strength and longevity of thunderstorms. Role of vertical wind shear in thunderstorms and its mechanism has been explored in this study. Results reveal that for prolonged thunderstorms high and low instability along with moderate to high vertical wind shear (moderate: 0.003 S-1 vertical wind shear 0.005 S-1 and high: > 0.005 S-1) play a significant role in longevity and strength of thunderstorms. The mechanism of vertical wind shear in thunderstorms was investigated in a few cases of thunderstorm events where the duration of thunderstorm was covered by the radiosonde/rawin ascent observation taken at Minicoy. Empirical model has been developed to classify thunderstorm type and to determine the strength and longevity of thunderstorms. Model validation has been carried out for selected cases. Model could classify thunderstorm type for most of the cases of thunderstorm events over island and coastal stations

Seasonal variation of lightning activity over the Indian subcontinent

The seasonal variation of lightning flash activity over the Indian subcontinent (0°N–35°N and 60°E–100°E) is studied using the quality checked monthly lightning flash data obtained from lightning imaging sensor on board the Tropical Rainfall Measuring Mission satellite. This paper presents results of spatio-temporal variability of lightning activity over the Indian subcontinent. The study of seasonal total lightning flashes indicates that the lightning flash density values are in qualitative agreement with the convective activity observed over this region. Maximum seasonal total flash counts are observed during the monsoon season. The propagation of the inter-tropical convergence zone over this region is also confirmed. Synoptic conditions responsible for variation of lightning activity are also investigated with the help of an observed dataset. The mean monthly flash counts show a peak in the month of May, which is the month of maximum temperatures over this region. Maximum flash density (40.2 km−2 season−1) is observed during the pre-monsoon season at 25.2°N/91.6°E and the annual maximum flash density of 28.2 km−2 year−1 is observed at 33.2°N/74.6°E. The study of the inter-annual variability of flash counts exhibits bimodal nature with the first maximum in April/May and second maximum in August/Septembe

Dynamical features of incessant heavy rainfall event of June 2013 over Uttarakhand, India

The southward penetration of mid-latitude westerlies and their interaction with monsoon current are harbinger of intense rainfall activity over northern and central India. Such synoptic condition prevailed during June 14–17, 2013, and unleashed relentless rainfall over the state of Uttarakhand. Observational aspects of this event have been explored using surface, satellite and reanalysis data. Precipitation features have been explored using data from Precipitation Radar Onboard TRMM satellite in conjunction with TRMM-TB42, Automatic Weather Station and Automatic Rain Gauge Station data. The ERA interim dataset has been used to explore prevalent synoptic conditions, and Modern-Era Retrospective Analysis for Research and Applications reanalysis fields revealed that prevalent synoptic features led to moisture flux convergence in lower troposphere. Vertically integrated water vapor transport over the India and adjoining oceanic region is evaluated to gain an insight into the dynamical mechanism of rainfall activity over Uttarakhand. Results reveal that interplay between movement of monsoon low along the monsoon trough resulting in strong low-level convergence and constant feeding of moisture from Arabian Sea and Bay of Bengal and strong upper-level divergence owing to southward intrusion of mid-latitude westerly trough resulted in heavy rainfall activity over Uttarakhand

Atmospheric ions and new particle formation events at a tropical location, Pune, India

Characteristics of small, intermediate and large ions and nanometre particles, and the influences of environmental conditions and local topography on the interactions of these ions and particles in the size range of 3.85-47.8 nm diameter are studied during new particle formation (NPF) events observed at Pune, India from 20 March to 23 May 2012. During the NPF events, the formation rate of 5 nm particles ranged from 3.5 to 13.9 cm(-3) s(-1)and growth rates of both positive and negative ions were greater than those of neutral particles. Total ion concentrations in this size range were approximately double on event days than on non-event days. The NPF events were generally preceded by 2-3 h periods of enhanced concentration of ions and aerosols (PECIA) which are associated with katabatic winds from the surrounding hill-slopes at our site. During PECIA, ions 8 nm diameter grew faster and attained mode diameters greater than those of neutral particles. PECIA provided the ions of sub-8 nm diameter and dramatically reduced the condensation sinks and thus contributed to the development of conditions conducive for NPF. During PECIA, total concentrations of the ions and particles become almost equal and the Boltzmann charge distribution breaks down. During NPF events, mode diameters of the ions and particles first steeply decreased accompanied by a steep increase in condensation sink but later gradually increased at identical rates. Observations of charging ratio of 3-4 nm particles being higher than the charge equilibrium value and non-observation of charge equilibrium for >5 nm ions and particles indicate occurrence of ion nucleation and/or advection during the PECIA

Investigation of Uttarakhand (India) disaster-2013 using weather research and forecasting model

A natural disaster in the form of severe flash floods due to extreme precipitation occurred at Kedarnath (Uttarakhand), India, on 16–17 June 2013 and is being considered as one of the worst disasters in India (Das in J Geol Soc India 82:201, 2013). The catastrophe in the form of flash flood and associated debris flow caused major devastation leading to a high death toll of locals and visiting pilgrims. The very early migration of monsoon trough (MT) towards northern India and its interaction with an incoming western disturbance (WD) formed a transient cloud system that led to extreme precipitation. Using WRF model with triple-nested domain for simulation at finer resolutions, this high-intensity precipitating event is analysed. Interaction of the MT with WD over the foothills of the Himalayas usually causes a break period in the Indian monsoon, but the interaction of MT and WD during this storm event showed different characteristics. Such an association of WD with the MT has been termed as pulsatory extension of the monsoon (PEM) towards Himalayas (Pisharoty and Desai in Indian J Meteorol Geophys 7:333–338, 1956; Mooley in Indian J Meteorol Hydrol Geophys 8:253–260, 1957). The interaction of the WD with the MT exactly over the Uttarakhand region forms an occluded discontinuity between the mid to upper-tropospheric WD frontal system (colder) and the lower-troposphere MT (warm and humid). The precursor of this front caused formation of steep temperature gradient over the Indian region that led to the early advance of MT towards Himalayas. Formation of this strong front develops augmented convective instability, which is further enhanced by orographic lifting, leading to the configuration of this large organized storm causing extreme precipitation over a large spatial region