Spatio-temporal Distribution of Lightning Phenomenon in Iran Using TRMM Lightning Image Sensor (LIS) Data

Lightning is a sudden electrostatic discharge during an electrical storm between electrically charged regions of a cloud (called intra-cloud lightning), between two clouds (CC lightning), or between a cloud and the ground (CG lightning).This phenomenon is one of the most important featureswhich are associated with extreme storms and seize the life of about 2,000 people in the world each year.The occurrence of lightning is related to the cloud microphysics in the mixed-phase layer becauselightning is frequent in convective clouds that contain many large hydrometeors in the mixed-phase layer.Also, air on the windward side of a mountain is forced to rise; anditoften leads tothecloudand lightning.In fact, lightning activities are highly variable on many spatial and temporal scales, and to some extent depend on the local convective regime.Lightning activities are not registered in the synoptic stations, but the thunder day statistics determined by human observers and compiled by the World Meteorological Organization (WMO) are one of the best sources of proxy information concerning lightning activity worldwide.Lightning day might include more than one hundred events; therefore, it cannot be a good representative for the lightning activitywhilethese stationsdo notshow offa good distribution.Accordingly, using remote sensing technology can be accurately measured lightning activity. Several space borne instruments have measured the global distribution of lightning one of which is Lightning Image Sensor (LIS).In this paper, diurnal, spatial, and temporaldistribution of the lightning phenomenon in Iran werestudied using LIS data.
Study Area: Iran, with an area of 1,648,195 km2 is located in the southern part of the temperate zone of the northern hemisphere.It is situated between 25° and 47°northern latitudes, and 44° to 63° eastern longitudes.This area is generallymountainous and semi-arid.The lowest area is 28 m lower than sea level located on northern Iranand its highest peak is Damavand with an altitude of 5,671 m/ASL.Existence of this diversity in roughness of the ground causes different climatic characteristics in various parts of the country.
Material and Two satellite-based lightning sensors have been successfully used by NASA since April1995. The sensors can detect the total lightning activities (cloud-to-ground flash and intra-cloud flash) on a global scale. One of these sensors is Lightning Image Sensor (LIS) which was launched on November 28, 1997 aboard the Tropical Rainfall Measuring Mission (TRMM). The LIS sensor detects lightning with storm-scale resolution of 3∼6km over a region of 600km×600kmof the earth’s surface. The LIS circles the earth with a velocity of 7km·s−1, and observes a point on theearth or a cloud for about 90s. This short sampling time during the satellite overpass limits the data usage for forecast and requires several years to compute high resolution climatology. Nowadays, LIS has collected lightning measurements for over 16 years making possible the compilation of total lightning climatology maps in high resolution such as 0.250 and 0.100 of horizontal resolution. In this paper, diurnal, spatial and temporal distribution of the lightning phenomenon in Iran werestudied using LIS data. Some geoprocessing functions in ArcGIS were applied tocalculate statistical values and to identify the locations of statistically significant lightning clusters. For generalizing geographic locations of lightning occurrence to an entire area a Kernel Density Interpolation estimator was introduced. Basically, a Kernel density tool calculates the density of point features such as lightning occurrence locations in a radius searcharound all similar features. Conceptually, a smooth, curved surface is fitted over each lightning flash pointincidentin Kernel density procedure regarding all observations. The surface value is highest at the location of the occurrence point and diminishes with increasing distance from the point, reaching zero at the search radius distance from the point. In practice, the density rate at each output raster cell is calculated by adding the values of all the Kernel surfaces where they overlay the raster cell centre, based on a quadratic Kernel function.
The result showed that diurnal cycle of lightning display a local maximum in flash rate in early afternoon (between 12 and 15 local time) and local minimum in flash rate in early morning to late morning (between 01 and 11 local time). Monthly variation of lightning indicated that maximum frequency of lightning occurs in April whereas the minimum happensin January and September. Annual distribution of lightning data indicated that the maximum frequency of lightning coincides with mountain areas. A majority of the lightning activities over the mountain region occurs primarily in southern slopes ofthe mountains. More specifically, this maximum occurs over the south and southeast facing slopes of the mountainous areaslikeZagros, Alborz, Binalud, Barez, etc. Western and south-western slopes of the Zagros Mountains have the highest rate of annual lightning in Iran.Central regions of Iran have the lowest frequency of lightningwhich are generally flat and arid.
The result of Kernel density function showed that distribution of lightning in January, November and December are alike and maximum density of lightning occurs in southwest of Iran (between Khozestan and Lorestan provinces). The maximum density of lightning in February, March and October are also in southwest of Iran but the lightning occurred in a wider area. The peakfrequency oflightningactivityoccurs inApril and Mayanditsspreadismuch more thanother months. In these months, west, southwest and northeast of Iran have maximum frequencies of lightning. In June, July, August and September, the distribution of lightning activities are different from other months and the maximum density of lightning are in southern Kerman, Sistan and Baluchestan and some areas of Hormozgan province.
Although lightning activity occurs in all regions, it appears that some areas havemore favorable conditions for the occurrence of this phenomenon. This study investigated diurnal, spatial and temporal distribution of lightning activity with 16 years (1998–2013) of LIS.The results provide valuable information on the distribution of lightning activity in Iran, sinceno study had been carried outbefore the distribution of this phenomenon in Iran.Results of diurnal cycle indicated that there was a marked daily distribution of lightning frequency during the afternoons peaking between 3PM and 5PM hours. These results nearly match the pervious findings; in such studies it was shown that all maximums in lightning were observed during the afternoons between 3pm and 7pm (EST). The increase in storms during this period is primarily due to the proliferation in energy provided by the sun during the warmer spring and summer months. The monthly distribution of lightning showed a distinct tendency indeed for all lightning to occur during March to May.The increase instorms during this period is primarily due to the increase in energy provided by the sun during the warmer spring. The result of lightning distribution analysis indicated that a majority of the lightning activity over the mountain region occurs primarily over the southern slopes ofthe mountains. Western and south-western slopes of the Zagros Mountains have the highest rate of annual lightning in Iran. Maximum frequency of lightning in January, February, March, October, November and December are also in this region but in warm season (June, July, August and September), south and southeast of Iran have maximum frequency of lightning activity.