Assessment of Convective Storm and Its Overshooting Tops in West of IranUsing SEVIRI Image

Document Type : Research Article

Author

• Somayeh Rafati

Sayyed Jamaleddin Asadabadi University

10.22067/geo.v6i1.58187

Abstract

1. Introduction
Overshooting tops (OTs) are the product of deep convective storm updraft cores of sufficient strength to rise above the storms general equilibrium level in the tropopause region and penetrate into the lower stratosphere. Thunderstorms with OTs frequently produce hazardous weather such as heavy rainfall, damaging winds, large hail, and tornadoes.OTs are most identified in visible channel imagery as having a lumpy texture. However, they are only identifiable during the day and also most easily observed during early morning or late afternoon hours when the sun angle is low and the shadows on the cloud anvil cast by the overshooting tops are well pronounced. One of the most commonly used methods for detecting OTs is based upon the BTDs between the 6.2 μm and 10.8 μm (WV–IRW) channels. This BTD is positive above deep convective clouds and is related to the presence of water vapor above the cloud tops. Warmer temperatures in the lower stratospheric water vapor result in greater observed WV–IRW BTD values. The BTD of the ozone channel (9.7 μm) and the IRW channel also show a positive signature for cloud tops above 11 km. The signal in the BTD between the ozone and IRW channels is even more significant than the signal in the BTD between theWV and IRW channel near the tropopause.Thus, this BTD could be a better indicator of deep convective activity. The BTD between the carbon dioxide (13.4 μm) and the IRW channel is a good indicator of the height of the opaque clouds. The reason is that with higher cloud tops, the absorption effect of CO2 becomes smaller, producing a BTD of the CO2 and IRW channel that is close to 0 or positive, in the case of very deep convective clouds.

2. Material and Methods
In this study the convective system and its related overshooting tops that occurred in 27 march 2007 were studied using High Rate SEVIRI Level 1.5 Image Data from Second Generation Meteosat satellite. The data is transmitted as High Rate transmissions in 12 spectral channels. Level 1.5 image data corresponds to the geolocated and radiometrically pre-processed image data, with a spatial resolution (pixel size) of 3 x 3 km at nadir and a temporal resolution of 15 minutes. Preprocesses such as calibration, geo-referencing and brightness temperature calculation weredone using Envi software. Then, RGB color composites were used for monitoring convection as follow: Red: Cloud depth and amount of cloud water and ice, provided by the visible reflectance at 0.6 μm. Green: Cloud particle size and phase, approximated by the 1.6 μm solar reflectance component. Blue: Temperature, provided by the 10.8 μm channel. Overshooting tops weredetected using visible reflectance at 0.6 μm. Three OT detection methods, WV–IRW, CO2–IRW, O3-IRW brightness temperature difference (BTD), which use combinations of SEVIRI channels in the form of brightness temperature differences, have been tested and compared with OT detection in visible images. Then, the most appropriate BTD threshold was determined in any methods and areas identified as OT was compared in different methods. Finally, CAPE, low level jet, air flow pattern, relative and specific humidity Maps and graphs were presented and interpreted to understanding OTs occurrence conditions. To do this, all data was collected from ECMWF with resolution of 0.125˚*0.125˚ latitude/longitude.

3. Results and Discussion
Results showed that most of theOTs (but not all of them) wereformed in the region where the IR brightness temperature was lower than 215K. Using lower brightness temperature difference threshold (+1 or +2 for CO2–IRW and WV–IRW and +10 K for O3–IRW) led to exaggeration of OT number and extension in all three methods however they could identify all OTs. While using higher threshold (4 K for CO2–IRW and WV–IRW and 13 K for O3–IRW) led to the method failed to identify many of OTs. Thus, it was tried to determine the optimized thresholds that succeed to identify OTs as much as possible and have the least false detection. Finally, the thresholds weredetermined as follow: 3.1 for CO2-IRW, 3.5 for WV-IRW and 12 for O3-IRW. By these thresholds some OTs have not been detected and some pixels have been detected as OT falsely in all three methods, which was due to low spatial resolution. CO2-IRW BTD represent the best results because of fewer missed and false detections. So, while SEVIRI image and these methods have not enough to detectOTs precisely, they are very useful to assess temporal and spatial as well as their occurrence conditions.
Assessment of OTs occurrence conditions in 27 march 2007 revealed that there was a low pressure in west of Iran surrounded by some high pressureat low levels of atmosphere in occurrence day. High pressure in north of the Caspian Sea, north of the Mediterranean Sea and northeast of Africa was advected moisture air mass of the Caspian Sea, the Mediterranean Sea and the Red Sea to the low pressure in west of Iran. Also, there was a weak high pressure in the Arabian Sea which was advected warm and moisture air mass from the Arabian Sea, the Persian Gulf and the Red Sea to the low pressure in west of Iran. Low level jet in the study area has accelerated advection of warm and moisture air mass to this low pressure. Thus, air flow pattern resulted in moisture convergence from all resources in the region (the Persian Gulf, the Arabian Sea, the Red Sea,the Caspian Sea and the Mediterranean Sea) which has major role in the Mesoscale convective system (MCS) and OTs occurrence. So that special and relative humidity had reached 8-12 gr/kg and 100% respectively. Also, the special and relative humidity was high to tropopause and had reached 0.2 gr/kg and 90-100% in 200 hPa respectively. The maximum of the convective available potential energy (CAPE) was about 1200 Jkg-1 to 1500 Jkg-1 at the time of system formation Maximum of the lifted index was observed in the Red Sea convergence zone with value about -3 to -6,which induced to deep convection in this day.

4. Conclusion
In this study the convective system and its related overshooting tops occurred in 27 March 2007 was studied using High Rate SEVIRI Level 1.5 Image Data. Preprocesses such as calibration, geo-referencing and brightness temperature calculation weredone using Envi software. Then, RGB color composites were used for monitoring convection.Three OT detection methods, WV–IRW, CO2–IRW, O3-IRW brightness temperature difference (BTD) have been tested and compared with OT detection in visible images. Then, the most appropriate BTD threshold was determined in any methods and areas identified as OT was compared in different methods. CO2-IRW BTD represent the best results because of fewer missed and false detections. While SEVIRI image and these methods have not enough to detect OTs precisely, they are very useful to assess temporal and spatial as well as their occurrence conditions.
Low level jet in the study area has advected warm and moisture air mass in occurrence and previous day. Air flow pattern resulted in moisture convergence from all resources in the region (the Persian Gulf, the Arab Sea, the Red Sea, and the Mediterranean Sea) which has major role in the Mesoscale convective system (MCS) and OTs occurrence.The maximum of the convective available potential energy (CAPE) was about 1200 Jkg-1 to 1500 Jkg-1 at the time of system formation.Maximum of the lifted index was observed in the Red Sea convergence zone with value about -3 to -6. It revealed that low atmosphere was also instable in study daywhich induced to deep convection in this day.

Keywords

• Convective systems
• Overshooting tops
• Brightness Temperature
• SEVIRI image

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