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    Map showing region of Peninsular India.

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    Spatial variation of lightning (FC per year) over Peninsular India.

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    Annual variation of lightning FC over Peninsular India based on monthly averages computed from LIS 1998–2007 data.

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    Seasonal scenario of spatial variation of lightning FC over the Indian Peninsula: (a) premonsoon, (b) monsoon, (c) postmonsoon, and (d) winter.

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    Relationship between lightning FC and SST during 1998–2007: FC vs month (a) Arabian Sea and (c) Bay of Bengal; FC vs SST (b) Arabian Sea and (d) Bay of Bengal.

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Relationship between Lightning Activity over Peninsular India and Sea Surface Temperature

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  • 1 Indian Institute of Tropical Meteorology, Pune, India
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Abstract

This paper presents a study of spatiotemporal variation of lightning activity over Peninsular India (8°–22°N, 72°–88°E) by using monthly satellite-based lightning flash grid (1° × 1°) data for a period of 10 yr (1998–2007). The data are examined in terms of spatial, annual, and seasonal distribution of the lightning activity. It is found that lightning activity is higher over south Peninsular India and eastern India. On a seasonal time scale, the lightning activity shows two maxima—first in the month of May and then in the month of September. The lightning activity in the monsoon period is noticed to be considerable because of the occurrence of the low-level jet and increase in the monsoon break period. During the postmonsoon, the activity is mainly due to the presence of the convective nature of the disturbed weather during the northeast monsoon season over most parts of the east coast of south Peninsular India. The relationship between lightning activity over Peninsular India and sea surface temperature in the bordering seas (Arabian Sea and Bay of Bengal) is also examined. The results disclose a significant link between them.

Corresponding author address: Kaushar Ali, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune 411 008, India. Email: kaushar@tropmet.res.in

Abstract

This paper presents a study of spatiotemporal variation of lightning activity over Peninsular India (8°–22°N, 72°–88°E) by using monthly satellite-based lightning flash grid (1° × 1°) data for a period of 10 yr (1998–2007). The data are examined in terms of spatial, annual, and seasonal distribution of the lightning activity. It is found that lightning activity is higher over south Peninsular India and eastern India. On a seasonal time scale, the lightning activity shows two maxima—first in the month of May and then in the month of September. The lightning activity in the monsoon period is noticed to be considerable because of the occurrence of the low-level jet and increase in the monsoon break period. During the postmonsoon, the activity is mainly due to the presence of the convective nature of the disturbed weather during the northeast monsoon season over most parts of the east coast of south Peninsular India. The relationship between lightning activity over Peninsular India and sea surface temperature in the bordering seas (Arabian Sea and Bay of Bengal) is also examined. The results disclose a significant link between them.

Corresponding author address: Kaushar Ali, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune 411 008, India. Email: kaushar@tropmet.res.in

1. Introduction

Lightning discharges in thunderstorms are an indication of the intensity of atmospheric convection. Atmospheric convection occurs under unstable atmospheric conditions, either due to the heating of the boundary layer by solar radiation during the day, or by the mixing of air masses of different densities. High lightning frequencies are therefore related to the regions of greatest instability in the earth’s atmosphere. These regions of instability do not occur randomly around the planet, but have an organized pattern related to the climate of the earth, which is driven by the differential heating of the earth’s surface by the sun. If we change the climate we will change the regions of convection, their intensity, and hence will likely change the lightning patterns around the globe (Price 2009).

There have been numerous studies of the differences in lightning characteristics occurring over continental and oceanic areas. Though it is clear from global studies that most lightning activity takes place over land (Turman and Edgar 1982; Orville and Henderson 1986; Christian et al. 1999a), a considerable amount of lightning activity takes place over the oceans and along coastal areas. Lightning is a manifestation of cumulonimbus cloud dynamics and microphysics and is, therefore, physically related to convective development. It can be used to estimate convective rainfall (e.g., Petersen and Rutledge 1998), to forecast precipitation in convective systems (Blanchard and Lopez 1985), and to have further insight into the synoptic systems (Reap 1994) or low-level wind flows (Lopez and Holle 1987). Meteorologists have long considered the surface properties of oceans and seas [in particular the sea surface temperature (SST)] as a prime candidate for use in the analysis of the influence of air–sea interactions on atmospheric variability in different geographical areas. In particular, SST is strongly related to instability. For instance, the convergence flow in the planetary boundary layer is driven by the SST gradient in tropical zones, and also, nonneutral stratification may be partially determined by the distribution of SST (Numaguti 1995).

Despite the above, there has been occasional work done across the globe on the possible relationship between lightning and SSTs (Laing et al. 2008). On the other hand, there has not been any research work related to the relationship between SSTs of the Arabian Sea/Bay of Bengal and the lightning activity over the Indian peninsular region. Reeve and Toumi (1999) suggested that SST is poorly correlated with lightning activity over the sea. However, it is over land where lightning mainly occurs and it is sensitive to deep convection (Mackerras et al. 1998). Reason and Mulenga (1999) have found that SST is significantly linked to rainfall over land, and have suggested that the increase in rainfall arises from an enhancement of moisture advection driven by the SST. Lightning activity is mostly affected by the moisture content (Petersen et al. 1996; Rivas Soriano et al. 2001; de Pablo and Soriano 2002), and this suggests that it might be possible to find a relationship between SSTs and lightning over land. So, the present work aims at studying the spatiotemporal variation of lightning activity over Peninsular India and investigating the relationship between lightning activity over Peninsular India and sea surface temperature in the bordering seas (Arabian Sea and Bay of Bengal) to understand the climatology of lightning activity over Peninsular India.

2. Data and source description

The Lightning Imaging Sensor (LIS) is a satelliteborne instrument used to detect the lightning flashes occurring in the tropics (Christian et al. 1999b; Bond et al. 2002). It is a National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) 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 (4–7 km) over a large region (600 km × 600 km) of the earth’s surface. LIS was launched on 28 November 1997 into low earth orbit to circle Earth at an altitude of 350 km. 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 little regional bias (Boccippio et al. 2000). However, according to Boccippio et al. (2002), flash detection efficiency is predicted to be 93% ± 4% at night and 73% ± 11% during the day. LIS records both cloud-to-ground and intracloud lightning and shows them as a total flash count.

The present study involves LIS flash count data for a period of 10 yr (1998–2007) over the selected domain of Peninsular India (8°–22°N, 72°–88°E) as shown in Fig. 1. The data are available online (http://thunder.msfc.nasa.gov) in 1° × 1° grids for individual months. Data for monthly lightning flash count occurring over the region and during the period under study have been collected from the above data source and then have been compiled according to seasons encountered by the Indian region. For this purpose a year has been divided, as per India Meteorological Department categorization, into four seasons, namely, premonsoon (March–May), monsoon (June–September), postmonsoon (October–November), and winter (December–February). Monthly mean sea surface temperature (Arabian Sea: 8°–21°N, 72°–78°E; Bay of Bengal: 8°–21°N, 78°–88°E) data for the same period are extracted from the Climatic Data Center of the National Oceanic and Atmospheric Administration (NOAA).

3. Results and discussion

a. Spatial variation of lightning activity over Peninsular India

Figure 2 shows the lightning flash count (FC) for a 10-yr period (1998–2007) over Peninsular India. It is seen from the figure that the southern Indian Peninsula and eastern India record maximum lightning flash counts (∼2200–2400 FC yr−1). The area intercepting such high lightning activity is very small in the southern peninsula, whereas it is very large in the eastern part of India. A minimum lightning flash count is observed over a vast region extending from the mid- to northwest peninsula. The lightning flash count over this region ranges from 400 to 1000 FC yr−1. The observed scenario of the spatial variation of lightning flash count embedded in the thunderstorms is supported by the description and explanation presented by Wang (2006). The description states that

“thunderstorms/lightning are mostly isolated, but sometimes widespread, occur over the west coast of Sri Lanka, the south of Peninsular India, eastern India, central India and Nepal in April-May, if the upper level features such as the penetration of deep westerly trough up to 15°N over India are favorable. In-phase positioning of the waves in the mid-tropospheric tropical easterlies and subtropical westerlies over the Peninsular India are conspicuous in producing occasional widespread Peninsular thunderstorms in April-May as low-level moisture is provided from the neighbouring seas. The peaks of thunderstorm activity over the south Peninsular of India occur between mid-April to the end of April about 4–6 weeks prior to the onset of the monsoon. The trigger for release of convective latent instability is provided by the day time heating, orographic features and sea breeze penetration.”

b. Annual variation of lightning activity over Peninsular India

Figure 3 shows the annual variation of lightning flash count over Peninsular India on the basis of one month’s total FC values averaged over a period of 10 yr from 1998 to 2007. The figure exhibits two maxima during a year—one occurring in the month of May (2397) and the other in the month of September (1491). The minimum flash count is observed during December–February and ranges from 20 to 102. Afterward, the flash count rises steeply up to the month of May and then decreases until July. It remains nearly constant during July and August (555 and 552, respectively). Again, the flash count rises until September and then decreases steeply until December.

During March–May, air over Peninsular India is very warm and mostly dry. Low-level wind flow is northeasterly or variable. Low-level anticyclones over the Arabian Sea as well as over the Bay of Bengal generally occur. Often, easterly waves move across the peninsula or the anticyclone over the Bay strengthens and southeasterly flow occurs over the peninsula (Puranik and Karekar 2006). Moisture from the Arabian Sea moves into the peninsula, causing an elevated discontinuity in the humidity distribution, aligned southwest–northeast. In the second kind of situation, there is an incursion of upper-tropospheric dry air over the peninsula. The atmosphere becomes very unstable along the boundary of the incoming dry air. In both of these cases sudden thunderstorms occur at any time of the day. The third type of wind discontinuity occurs where two low-level airstreams from the Arabian Sea and the Bay of Bengal meet east of the Western Ghats (8°–21°N, 73°–75°E, as shown in Fig. 1), causing evening thunderstorms. The convective latent instability in this case is provided by the daytime heating, orographic features, and sea-breeze penetration (Puranik and Karekar 2006). During premonsoon season, the maximum lightning activity occurs at places lying in the neighborhood of the Western Ghats (India Meteorological Department 1933) and northeastern region of the Indian Peninsula as seen in Fig. 4a. It may be noted that, during the premonsoon period, the peninsular land areas receive large moisture content from the Bay of Bengal up to a height of about 2 km. These moist winds blow into the peninsula only below latitude around 15°N. Differential heating over the peninsular land region and over the surrounding sea/oceanic region sets up a large current of air from the Indian Ocean, south of the equator, to Peninsular India during this period. This further provides conditions conducive to the development of thunderstorms and hence more lightning activity over Peninsular India is recorded during the premonsoon season (Das 1984).

There is considerable lightning activity over the peninsula even during the monsoon period (Fig. 4b). The maximum value of lightning flash count over the peninsula is ∼900. This is mainly due to the influence of a cross-equatorial low-level jet (LLJ) that, during the active phase of the monsoon, passes through this region with its core centered around 900 hPa.

The low-level jet near the top of the atmospheric boundary layer (900 hPa) is the main artery through which moisture is fed into the flow over southern parts of South Asia and is then carried across the Bay of Bengal to adjoining Southeast Asia, and even into East Asia. The LLJ passes across different SST regimes over the Arabian Sea before reaching the west coast of India. Airflow in the upper troposphere is reversed with an even more extensive clockwise gyre located over Tibet (the south Asian high) with divergent wind flow becoming easterly-northeasterly toward the equator. It attains maximum speed exceeding 40–50 m s−1 along 15°–20°N over south Peninsular India (Koteswaram 1958; Krishnamurti and Ramanathan 1982). However, the wind flow through Peninsular India in the lower troposphere is dominated by LLJ (Joseph and Sijikumar 2004). The occurrence over the peninsular region of India of upper-tropospheric cold and dry air mass from the northern region and lower-tropospheric warm and moist air mass from the oceanic region generates favorable conditions for the development of thunderstorms and lightning.

It has been found in the present work that lightning activity is more frequent during the break period than during the active period of the monsoon season. This is in agreement with the result obtained by Manohar et al. (1999) and Manohar and Kesarkar (2005). Joseph and Simon (2005) have shown that mean monsoon flow through India, from the surface to 1.5-km altitude, had a significant decreasing trend between latitudes 10° and 20°N and significant increasing trend between 2.5° and 7.5°N during the period from 1950 to 2003. As a consequence, the duration of break monsoon spells during the monsoon as defined by an index of wind flow through Peninsular India increased by about 30% during this period. It is mentioned here that a break monsoon situation is considered when the monsoon trough hugs the Himalayan rim; the low-level easterlies disappear entirely along the Indo-Gangetic Plains and are replaced by the west-northwest flow along the periphery of the Himalayas. According to Rao (1976), break periods are the periods during the monsoon season when the monsoon trough is shifted northward and located close to the foothills of the Himalayas, leading to a striking decrease in rainfall over most parts of India but increase along the Himalayas, parts of northeast India, and the southern peninsula. The revival of the monsoon after a break situation is due to the synoptic-scale oscillatory character of the monsoon trough. The revival of monsoon activity after a prolonged break is distinctly linked with propagating intraseasonal oscillation (ISO) of the monsoon trough (Sikka and Gadgil 1980; Wang and Rui 1990) and higher lightning activity (Manohar et al. 1999). Thus, the occurrence of considerable lightning activity over the peninsula during this season may also be due to the large duration of the break situation.

Lightning activity is greater over southern Peninsular India during the postmonsoon season (Fig. 4c) and decreases northward. Also, it is greater over the west and east coasts of the peninsula and decreases over inland region. This is associated with the convective nature of the disturbed weather occurring mainly over most parts of the east coast of south Peninsular India during the northeast monsoon season. The above observed result and its explanation are supported by the publications of the India Meteorological Department (1979) and Manohar et al. (1999). They have shown that the frequency of cyclonic storms developing over the Bay of Bengal peaks during September, the end of the monsoon month, and during October, the beginning of the postmonsoon month of the year. These storms strike the east coast of south India and cause copious rain and lightning activity over the region. However, the maximum occurrence of lightning flashes over the southwest peninsula may be due to weather disturbances over the Arabian Sea.

The winter season records a minimum number of lightning flash counts over the peninsula as compared to those recorded in the other three seasons (Fig. 4d). The maximum FC is ∼100, found around the geographical location of 20.5°N, 79°E. Another less pronounced maximum value is recorded over the Arabian Sea near the southern tip of the peninsula. It may be noted that lightning activity during winter is mainly associated with tropical cyclones that occasionally occur over the southern Bay of Bengal and the southern Arabian Sea regions during this season. Furthermore, the cyclones are more frequent in the Bay of Bengal than in the Arabian Sea (Wang 2006).

c. Lightning activity over Peninsular India vis-à-vis SST of neighboring seas

Thunderstorms and associated lightning are important weather phenomena in understanding many issues related to weather. At the same time, it may be noted that the influence of subtle changes of SST on the atmosphere has long been recognized (Barrows et al. 2007; Kandalgaonkar et al. 2002). So, an attempt has been made in this work to investigate the sensitivity of the SSTs of the Bay of Bengal and of the Arabian Sea to the occurrence of lightning activity over Peninsular India.

Figures 5a and 5c are plots of the annual cycle of lightning flash counts over Peninsular India based on one month’s total value of FCs averaged over the 10-yr period from 1998 to 2007. Figure 5 also includes plots of monthly mean SST of the Arabian Sea and the Bay of Bengal for the same period. The plot shows semiannual oscillation in the SSTs of both seas with the amplitude of the first maximum (May) higher than the amplitude of the second maximum (October). Similar semiannual oscillation is observed in the lightning flash count also. There is nearly concurrence in the annual variation of lightning activity and of SSTs. The concurrency between the above two parameters is further supported by a significant correlation coefficient between them. It is found that the correlation coefficient between lightning flash count and SST is 0.847 (significant at 0.05%) for the Arabian Sea and 0.922 (significant at better than 0.01%) for the Bay of Bengal.

Lindzen and Nigam (1987) have linked SST gradients to increased convection. They state that differences in SSTs lead to different pressures over the water because of differences in air density. Higher SSTs are linked to lower air densities and surface pressures, whereas lower SSTs are linked to higher air densities and surface pressures. Such a situation leads to pressure gradient over the sea surface causing wind to flow from a region of high pressure to a region of low pressure. This results in low-level convergence and convection over the warmer sea surface region. Finally, there may be development of thunderclouds–lightning over the region of higher SSTs. The theory explains the association of thunderstorm development to warm SST over the marine areas. As the coastal continental region remains warmer than the adjoining sea surface, pressure gradient force is directed toward the continental areas. On the other hand, the air mass over inland locations of the Indian Peninsula lacks the moisture that is available in the air mass over the sea and the coastal areas. This is the reason that peninsular inland locations record less lightning activity than coastal areas during all four seasons (Fig. 4).

4. Conclusions

This paper presents a 10-yr climatology of lightning activity over Peninsular India. The results of the study suggest the following:

Spatial variation of lightning activity is seen at a maximum over south Peninsular India and southeast India. In-phase positioning of the waves in the midtropospheric tropical easterlies and subtropical westerlies over Peninsular India is conspicuous in producing occasional widespread peninsular thunderstorms in April–May as low-level moisture is provided from the neighboring seas. The trigger for the release of convective latent instability is provided by the daytime heating, orographic features, and sea-breeze penetration.

Differential heating over the peninsular land region and over the surrounding sea/oceanic region provides conditions conducive to the development of thunderstorms/lightning over the region during the premonsoon season.

The occurrence, over the peninsular region of India, of upper-tropospheric cold and dry air mass from the northern region and lower-tropospheric warm and moist air mass from the oceanic region generates favorable conditions for the development of thunderstorms and lightning during the monsoon season. Also, an increase in the duration of the break spell during the monsoon may be considered as probable cause for the increased lightning activity.

Cyclonic storms that developed over the Bay of Bengal and weather disturbances over the Arabian Sea during the postmonsoon season are understood to be the driving factors for the occurrence of lightning activity over the coastal and adjoining continental region of the peninsula.

Lightning activity during winter is also mainly associated with tropical cyclones that occasionally occur over the southern Bay of Bengal and the southern Arabian Sea regions during this season.

Sea surface temperatures of the Arabian Sea and Bay of Bengal play a crucial role in the development of thunderstorms and lightning over the adjoining peninsular region of India.

Acknowledgments

The authors are thankful to Professor B. N. Goswami, Director of IITM, Pune, for his kind support and valuable guidance to this work. Thanks are also given to the head of the Physical Meteorology and Aerology Division for his encouragement to carry out this research work.

REFERENCES

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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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Fig. 1.
Fig. 1.

Map showing region of Peninsular India.

Citation: Journal of Applied Meteorology and Climatology 49, 4; 10.1175/2009JAMC2199.1

Fig. 2.
Fig. 2.

Spatial variation of lightning (FC per year) over Peninsular India.

Citation: Journal of Applied Meteorology and Climatology 49, 4; 10.1175/2009JAMC2199.1

Fig. 3.
Fig. 3.

Annual variation of lightning FC over Peninsular India based on monthly averages computed from LIS 1998–2007 data.

Citation: Journal of Applied Meteorology and Climatology 49, 4; 10.1175/2009JAMC2199.1

Fig. 4.
Fig. 4.

Seasonal scenario of spatial variation of lightning FC over the Indian Peninsula: (a) premonsoon, (b) monsoon, (c) postmonsoon, and (d) winter.

Citation: Journal of Applied Meteorology and Climatology 49, 4; 10.1175/2009JAMC2199.1

Fig. 5.
Fig. 5.

Relationship between lightning FC and SST during 1998–2007: FC vs month (a) Arabian Sea and (c) Bay of Bengal; FC vs SST (b) Arabian Sea and (d) Bay of Bengal.

Citation: Journal of Applied Meteorology and Climatology 49, 4; 10.1175/2009JAMC2199.1

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