Association of Rainfall and Stability Index with Lightning Parameter over the Indo-Gangetic Plains

The Lightning Imaging Sensor (LIS) based satellite lightning grid data for 10 year period (1998-2007) were used to study the association of rainfall and stability index with lightning parameter over Indo-Gangetic plain (IGP) region. The spatial variation of flash rate density (FRD) is found to be (40 fl·km·yr) higher over northern region of IGP as compare to that of eastern IGP region. The annual variation of FRD exhibits bimodal distributions, while the precipitation rate shows unimodal distributions. The results show that the FRD peaked 2 months (premonsoon) in advance to the monsoon months where rainfall peak occurred due to environmental lapse rates more than 7.0oC/km during pre-monsoon which is evident from the temperature profile for correlation coefficient between temperature (700 mb) and FRD with coefficient of 0.70, p ≤ 0.0001 during pre-monsoon. The annual variation of lifted index show negative value over March to September due to intense insolation, convective available potential energy (CAPE) and also availability of moisture. The convective cloud transform into thundercloud with the development of mixed-phase (cloud water + ice) which subsequently produce the lightning. During monsoon, seasonal thermal heating diminishes and even on revival after break monsoon period, K-index is found to be less as the orography does not allow the highly moist air of low temperature to reach to large height above freezing level. They can be referred as maritime clouds of intermediate height with moderate updraft and hence minimum lightning activity during the monsoon season. Lifted index are proved to be indicators of thunderstorm conditions. This is because that rising air parcel is much warmer than its surroundings and can accelerate rapidly and create severe thunderstorms.


Introduction
Lightning and precipitation have received a lot of attention in the scientific literature, not only because it is an impressive atmospheric phenomenon but also because it is associated with severe storms that may cause important damages to agriculture, electric power networks, property and life [1] [2] [3] [4] [5]. Stability indices are a measure of the atmospheric static stability. Their values are used to quickly assess the potential of the atmosphere to produce convection and consequently severe weather. LI indicates the likelihood of severe thunderstorms. Lifted index (LI) is a measure of upper level instability and takes into account elevated convection which tends to occur when upper level disturbances move across unstable equilibrium environment aloft. Negative LI indicates the possibility of convection. Values less than −9˚C reflect extreme instability. A value of LI between 0˚C and −2˚C indicates a small chance of having a severe thunderstorm. Air mass thunderstorms can occur even when the LI is slightly positive [6] [7] [8]. K Index has been proved useful in indicating the probability of air mass thunderstorms. As the K Index increases, so does the probability of having an air mass thunderstorm. K Index takes into account three components which are important for convective instability: temperature lapse rate, lower tropospheric humidity and the vertical extent of the moist layer. Values of K Index lower than 15˚C [9] indicate very small likelihood of thunderstorms, while K Index values higher than 35˚C indicate high probability for the development of numerous and/ or severe thunderstorms.
Large body of literature is available for thunderstorm and lightning research [10]- [16]. Also, over the Indian region, attempts have been made to understand relation of thunderstorms to wet bulb temperature [17] [18] [19] [20], lightningrainfall relationship [21] and also, thunderstorm climatology [22]. The analysis of satellite data in this study focuses on the association of rainfall and stability index with lightning parameter over the hot spot region of Indo-Gangetic Plains.

Data and Source Description
The Lightning Imaging Sensor (LIS) is a satellite-borne instrument used to detect the lightning flashes occurring in the tropics [23] [24]. It is a National Aeronautics and Space Administration's (NASA) Earth Observing System (EOS) instrument on the Tropical Rainfall Measuring Mission (TRMM) Satellite. The LIS is designed with a higher sensitivity and spatial accuracy than the Optical Transient Detector (OTD). It detects lightning flash with storm-scale (spatial) resolution (4 to 7 km) over a large region (600 × 600 km) of the Earth's surface. LIS was launched on 28 November 1997 into low Earth orbit to circle the Earth at an altitude of 350 km. The TRMM satellite was boosted from an average altitude of about 350 km before August 2001 to about 400 km after August 2001. The higher altitude (at an orbital inclination of 35˚) gives a correspondingly larger field of view for each of the sensor pixels and swath and longer sampling duration that increases the total flash counts [25]. Lightning flash illuminates a cloud top area significantly larger than the field of view of an LIS pixel. Also, the LIS detection threshold settings were not changed after boost. Thus, there is no change in detection efficiency. The inclination of the orbit is 35˚, which allows the LIS to observe the lightning activity in the tropical regions of the globe. It detects the lightning activity with detection efficiency of 90% with negligible regional bias [26] for flash with detection efficiency 93% ± 4% and 73% ± 11% during night and day respectively. LIS records both cloud-to-ground and intra-cloud lightning in terms of total flash count.
The present study involves LIS flash rate density data for a period of 10 years    Figure   2 may be due to formation of thunderstorms due to retreating ITCZ and synoptic scale systems [31]. When ITCZ approaches the equator from its northernmost convection associated with the large-scale circulation dominates over the landmass and orography distribution which produces a bimodal type of variation as seen in Figure 3. Figure 3 and Figure 4 shows annual variation of average FRD and precipitation rate (mm/day). From these figures, it is seen that during the annual course, the curve for lightning activity show bimodality and rainfall activity exhibit a peak.

The FRD shows a consistent increase from February that attains a maximum in
May and thereafter it starts decreasing consistently till the end of August and again increases to a second lower peak in September. Pre-monsoon monthly (March-May) convective instability is shown in Figure 5(a) and correlation of air temperature (at 700 mb) with FRD is shown in Figure 5(b). FRD peaked in pre-monsoon before the rainfall peak (monsoon) as seen in Figure 3. This is attributed to environmental lapse rates (>7.0˚C/km) during pre-monsoon and al- so, evident from the altitudinal air temperature variation analysis ( Figure 5(a)) and a high correlation (R = 0.70, p ≤ 0.0001) of air temperature (at 700 mb) with FRD during pre-monsoon ( Figure 5(b)).
The bimodality of lightning activity is related to the variation in air temperature in the tropics. Variations in the incoming shortwave radiation from the Sun are attributed [32] to be ultimately responsible for variations in global meteorological parameters on the seasonal timescale. The rainfall activity also shows a similar increase where steep increase is in April and reaches its maximum in July, and then it shows decrease and reaches to its minimum in November. This observed peak of rainfall activity is the significant climatological feature pattern of the monsoon rainfall and it is the difference between the ascending and descending phase (onset and withdrawal phase) of the monsoon [33]. The onset of the monsoon in early June is followed by the progressive increase in rainfall over the entire country. This phase attains its maximum in the month of July [17]. A careful examination of this figure suggests that there is a lag of nearly 2 months in the occurrence of peaks in lightning and rainfall activity. The plausible explanation for this lag is given below: Prior to the onset phase of the monsoon, the Indian land mass region experiences maximum convective instability caused by insolation [18] [34]. Hence  It is seen from Figure 6 that the negative LI is seen over March to September.  Figure 7). The intense conditional instability over the region develops mixed phase clouds (water + ice) which subsequently produce the lightning activity.

Conclusions
The paper presents the association of rainfall and stability index with lightning activity over Indo-Gangatic plain (IGP) region. The results indicate the following concluding remarks.
1) The spatial variation of flash rate density (FRD) and AOD is found to be higher by 40 fl·km −2 ·yr −1 over the northern region of IGP to that of eastern IGP region.
2) The annual variation of FRD exhibits bimodal distributions and the precipitation rate shows only one peak in its distributions. The time-lag of 2 months in the peak occurrence of FRD and rainfall rate may be attributed to the environmental lapse rates of more than 7.0˚C/km (thunderstorm condition) during premonsoon as is evident from the temperature profiles. Also, high correlation (R = 0.70, p =< 0.0001) of air temperature (at 700 mb) and FRD during pre-monsoon support this hypothesis.
3) The annual variation in LI distribution shows negative value from March-September and also, K-index during pre-monsoon and withdrawal phase of monsoon is due to intense insolation, availability of moisture and on account of large CAPE. The intense conditional instability over the region develops mixed phase clouds (water + ice) which subsequently produce the lightning activity.
4) The thermal heating diminishes on account of monsoonal rainfall and revival of monsoon rain even after break period, the LI is not strong enough to lift the moisture above freezing level. These warm clouds