The Synoptic Characteristics, Causes, and Mechanisms of Kahlaa Tornado in Iraq on 14th April 2016

In this study, an analysis of the synoptic characteristics, causes and mechanisms of Kahlaa tornado event was carried out. This tornado occurred on 10:30 UTC (1:30 pm Iraq Local Time) on 14 April 2016 to the north of Kahlaa town in Maysan governorate. We analyzed surface and upper charts, weather conditions, the damage indices, the dynamical features and the instability of the tornado. The analysis showed that there was a low pressure system which was an extension of the Monsoon low in addition to a supercell thunderstorm and a jet stream aloft. The cold trough and high relative vorticity at 500 hPa level, the humid warm wind blowing from the south and the dry cold wind from the north contributed to the initiation of the tornado. According to the damage amount, Kahlaa tornado can be classified as EF2 degree (considerable) on Enhanced Fujita scale. Three indices were calculated to estimate the instability of the tornado. The values of the convective available potential energy (CAPE), K-index, and lifted index were (≥2500 J/kg), (35.3 o C), and (-7), respectively. All these indices confirmed the instability required to form severe thunderstorm essential to tornado formation. Although the forecasting of tornadoes occurrence is difficult, there would be indications that may lead to expect of occurrence. These may include the availability of moisture, heat, and significant wind direction changes with altitude. However, the vital factors were the existence of high instability and a supercell thunderstorm.


Introduction:
Tornadoes are the most destructive winds event created by nature. Sometimes they are strong enough to destroy most things in their path (1). The tornado is a tilting column of air touching the ground and hanging from a position under a deep thunderstorm base. Tornado is becoming visible when the water vapor condenses to droplets inside, or when the tornado uplifts debris and dust from the ground. The funnel shape of the hanging cloud is an indication of the tornado occurrence (2). Most of tornadoes occur in United States with more than 1000 tornados a year. It is very common that "Tornado Alley" in the central Great Plains is the area with higher frequency. However, research has identified evidence of a new spot in "Dixie Alley," which represents an eastward extension of "Tornado Alley" (3). On the other side of the Atlantic Ocean, it was estimated that at least 100 tornados occur each year in Europe. It is responsible for significant damages with impacts on many social activities, human health and economy (4,5). Most tornadoes rotate cyclonically. Tornadoes diameters are ranging from 100 m to about 600 m or more. Most tornadoes continue for minutes with a path of about 7 km but in very rare situations, tornadoes with hundreds of kilometers and duration of some hours have been recorded (6). Observing and forecasting tornadoes is a difficult task because it is very diverse in space and very short in lifetime. In addition, it is of small diameter that cannot be easily observed by satellites (7). The number of tornado occurrences around the world is variable but much related to the surface topography and distribution of land and sea (8). The possibility to generate tornadoes increases as the thunderstorm developed to a supercell cloud, which is a severe circulating cloud. However, not every supercell generates a tornado. In the same time, not every supercell generate tornado (9). Tornado may contain single vortex, but the most powerful tornadoes contain whirls inside, robust and smaller called suction vortices, which circulate around the center of the bigger tornado in anticlockwise direction (multivortex tornadoes) (10). Due to the strong pressure gradient in tornado, the wind speed may reach a maximum of 480 km per hour, which is a strong and destructive speed. In the late 1960's, Theodor Fujita suggested a scale to classify tornadoes owing to their rotational speed called Fujita Scale. The speed of tornadoes is estimated depending solely on the damage that produced by the tornado. In 2007, a new modified version of this scale was suggested, the Enhanced Fujita Scale (EF Scale). The new scale tried to give a wide range of criteria in determining the tornado speeds by using 28 damage indicators (6). There was a great deal of research focused on tornadoes on a global basis.
In Turkey, the country that located north of Iraq, 225 tornadoes were recorded in the last five years. This number is more than halve the total number experienced during two centuries. This increase was attributed to the climate changes impacts (11). Paul (2001) (12) in analyzed 304 tornado cases in France and showed that the regions of high occurrence of tornadoes are the northwestern and the southeastern of France. Most of the tornadoes there occur in spring and summer. Dotzek (2001) (13) implemented a study of tornadoes in Germany by using 517 cases starting from 1870. The study showed that the highest activity of tornadoes was in July and the lowest activity was from November to February mostly at afternoon and before sunset.  (15) presented a statistical climatological model to study the climate of tornadoes occurrence. It was found that the increase of population by half contributed to the increasing of the calls about the tornadoes occurrence by 13%. A survey, which was conducted in china for tropical cyclone tornadoes from 2006 to 2018, counted 64 cases with an average of five per year. The study determined that most tornadoes occur before and within 36 hours after the tropical cyclone passage (16).
The case study in this paper is concerned with the tornado that happened in Kahlaa town, Maysan governorate, South of Iraq at 1:30 pm local Iraq time (10:30 am UTC) on 14 April 2016. There was no recorded case of a previous tornado in Iraq. Tornadoes, in general, may cause huge damage of buildings, fields, electric power towers, cars, and any other entities that come into its path. In our case, Kahlaa tornado causes destruction of some houses in the region of effect. This tornado also dropped down many high power towers and overturned some cars. This study is an attempt to investigate the causes of Kahlaa tornado and the geographical and meteorological conditions that led to such a phenomenon. Instability indices can be useful in determining the potential weather in forming the severe thunderstorms and tornado. One of the parameters describing atmospheric stability is CAPE. CAPE indicates the amount of energy available for convection (17): The K index (KI) (Eq. 1) is particularly useful for identifying convective and heavy rain producing environments. Values exceeding 40 point out the best potential for thunderstorm with heavy rain (18): = ( 850 − 500 ) + 850 − ( 700 − 700 ) 1 Where: 850 is the temperature at 850 hPa, 500 is the temperature at 500 hPa, 850 is the dew point at 850 hPa, 700 is the temperature at 7oo hPa, 700 is the dew point at 700 hPa. Environments not characterized by high values of CAPE or vertical wind shear like the situation in Iraq receive less attention, but they can still support severe weather and tornadoes (19). The inclusion of the dew point depression term reflects the unique emphasis of K on assessing the vertical penetration of low-level moisture, thought to be essential for the formation of air mass storms (20). A value ≥ 30 °C is typically expected for severe thunderstorms (21). Lifted Index (Eq. 2) is presented by Galway (22) to be the temperature difference between the observed 500 hPa temperature and the assumed 500 hPa temperature of a mean parcel lifted from the boundary layer next to the ground.

Materials and Methods:
The tornado event was studied on many aspects: The weather conditions, which might contribute in tornado occurring. Also, its form and destruction, its synoptic, dynamical and thermodynamic properties. small with less than 5 km × 5 km area. The inhabitants of this town may reach about 85000 people. The town center surrounded by a flat area, mostly agricultural lands ( Fig. 1). Houses there do not exceed 10 m in height.
To consider the case study of tornado, the routinely surface weather data was based on the datasets from the Iraqi Meteorological and Seismology Organization. The other data was acquired from the European Center of Moderate Weather Forecasting (ECMWF).   Fortunately, the tornado did not cause casualties though it damaged some high power towers, houses, and cars. There was another funnel vortex hanging out of the cloud but did not reach the ground (Figs. 5-7). To study the synoptic analysis of the weather, charts of temperature, surface air pressure, dew point were drawn for the times before and around the event. The dataset used to visualize the charts was extracted from the European Centre for Medium-Range Weather Forecasts (ECMWF) dataset. Figures. 8 and 9 show the surface air temperature on 13   To study the dynamical and instability situation, one should track the behavior of the geopotential heights for different levels in addition to the instability. Hence, the charts of pressure levels (850 hPa, 700 hPa, 500 hPa) were drawn. More emphasis was placed on 850 hPa level by taking the geopotential height, temperature, wind speed and relative humidity into account, Figs. 14-19. Also, the charts the relative vorticity (at 500 hPa), jet stream (  Spring in Iraq is a season with a significant quantity of rain. The humidity and heat impose convection process and large cumuliform clouds. Looking at the observations of the day of tornado (April 14, 2016) in addition to the day before and after for Amarah city shows occasional thunderstorms. The meteorological station of Amarah city is the nearest station to Kahlaa city. Most of the clouds during these days were stratocumulus and altostratus with significant amounts of cumulonimbus (Cb).
The plots of wind speed, air temperature, and air pressure, Figs 2-4 do not show a specific trend at the time of tornado event, namely 1:30 pm local time (10:30 am UTC) on April 14.
Unfortunately, Kahlaa city does not have meteorological station and hence the data of Amarah station, which located at tens of kilometers apart, did not reveal the exact meteorological situation at Kahlaa.

Damage Class:
According to its damages (See Figs 5-7), Kahlaa tornado can be described as considerable on Enhanced Fujita scale (EF2). It is with a wind speed of 100 knots, the one that uproots large trees and causes destruction of weak structures (7).

The Synoptic Analysis:
Charts of air temperature, Figs 8,9 showed an increase at the day of tornado and revealed that the hottest span on Kahlaa city was on 14 April, the day of tornado. Charts of mean sea level pressure, Figs 10, 11 show a low-pressure system (Monsoon) extended from the south to the north of Iraq. Humidity was high in the region as it is clear in charts of dew point temperature, Figs 12, 13. The thunderstorm was not frontal but of air mass type.

The Dynamical and Instability Analysis:
The dynamical situation can be analyzed in this manner: at the surface, there was a warm humid air from the south east caused by the effect of monsoon. At the level of 850 hPa, there was a wedge of warm humid wind veered slightly with a significant wind speed blowing from the southeast, This successive turning of wind with height caused a spinning effect, which contributed to the tornado formation. The chart of vertical wind speed shows a significant value of the ascending wind at the surface, greater than 6 m/s, which was much greater than the usual speed that is about 1 m/s (See Fig  22). This mechanism summed by the high instability in the region supported the chances to build up the huge cell of thunderstorm. Study of the Convective Available Potential Energy (CAPE) is of importance in investigating tornado occurrence. The deep convection is caused by the strong instability of air and it is vital in thunderstorm and tornado formation. Figs 23,24 show the high values of CAPE on the day of tornado occurrence in the region of Kahlaa. CAPE index suggested that the layer was mostly unstable and developed a deep convection with a value of more than 2500 J/kg. This value is higher than those of the tornadoes that occurred in other places such as in Czech Republic (Average from 500 J/kg to 900 J/kg) (23) Also, the K-index value in the region was calculated by using eq.1 to be 35.3 o C. This value suggests numerous activity of thunderstorms. In a study in Greece for a total number of 34 o C significant tornadoes, showed that the mean value of K-index is 31.5 and the maximum value is 40 o C (24). Hence, Kahlaa tornado corresponds to an average value of significant tornadoes.
The lifted index was calculated by using eq.2 for the time of the tornado, it was -7 which is a negative value and refers to very instable condition.
The above synoptic regime and the atmospheric environment: Convective storm, the low level moisture, the vertical wind shear (the change of wind speed and direction with altitude), CAPE and lifted index was taken into account in many studies as effective factors in the process of tornado genesis (6 ,25) In the end, it can be said that despite the difficulty of predicting the location, time, intensity and horizontal path of a tornado, there would be indications that may lead to an increase in the expectation of occurrence. This may include the availability of moisture and heat with significant wind direction changes with altitude. The vital factors are the existence of high instability and a supercell thunderstorm. These factors were also confirmed by many research works (26)(27)(28)

Conclusions:
The study of the tornado event in Kahlaa shows that the tornado is formed in very instable conditions. It is initiated within the effect of the low pressure system (Monsoon) with sufficient heat and humidity. The turning of wind with height played an important role in offering the spinning effect of the tornado. It was found that the tornado was of EF2 degree, which is called "considerable" on Enhanced Fujita Scale. Many factors affect the instability of the air in the region of the tornado. The high relative vorticity, jet stream, and surface vertical wind. This instability is confirmed by the instability indices such as CAPE index, K-index, and LI index with values of 2500 J/kg, 35.3 o C, and -7, respectively. These values suggest that the layer is mostly unstable and developed a deep convection and a numerous activity of thunderstorm.
Despite the difficulty in predicting the location, time, intensity and horizontal path of a tornado, there would be indications that may lead to an increase in the expectation of occurrence. This may include the availability of moisture and heat with significant wind direction changes with altitude. The vital factors are the existence of high instability and a supercell thunderstorm.