Wednesday, 30 August 2023: 11:00 AM
Great Lakes BC (Hyatt Regency Minneapolis)
An isolated thunderstorm is selected in this paper to study the signatures of vertical alignment ice particles on the top of the thundercloud before the first in-cloud (IC) lightning flash by an X-band Dual Polarized Phased Array Weather Radar (DP-PAWR). Polarimetric parameters of DP-PAWR and lightning data by LIDEN system are used to analyze signatures of vertical ice particles orientation related to strong electric fields in thunderstorms. The observation shows the negative KDP appears on the top of the thundercloud and decreases before the first in-cloud (IC) lightning flash, then increases with lightning flash discharging.
In this paper, an isolated thunderstorm is selected by using composite ZH which ZH is larger than 40 dBZ at a -10 °C isothermal layer. By introducing composite KDP which is defined as the minimum KDP value in the vertical column of all elevation scans at each horizontal 2D grid point above the assigned altitude, the negative KDP signatures associated with vertically oriented ice particles in the strong electric fields on the top of the cloud are analyzed. At 7 minutes before the first IC lightning flash, the average composite KDP at -10 ℃ layer starts to decrease from about 0.3 to -0.2 °/km in 3.5 minutes. The averaged composite KDP at -20 ℃ layer starts to decrease 6 minutes before the first IC lightning flash and decreased from about 0.5 to -0.65 °/km in 5 minutes, and the averaged composite KDP at -25 ℃ layer decreases from about 0 to -0.7 °/km in 1 minute which is 2.5 minutes before the first IC lightning flash while the average composite KDP at -30 ℃ layer decreases from -0.4 to -1 °/km in 2 minutes at 2.5 minutes before the first IC lightning flash. Similar results are obtained by using the average KDP on the top of the thundercloud, such as the average KDP at -25 ℃ layer decreases from 0.1 °/km to -0.6 °/km in 1 minute at 2.5 minutes before the first IC lightning flash, while the average KDP at -30 ℃ layer decreases from -0.7 °/km to -1.2 °/km in 30 seconds at 1 minute before the first IC lightning flash.
The result shows that averaged composite KDP decreases from a positive value to a negative value or from a negative value to a more negative value at a different layer from -10 ℃ isothermal level to the top of the thundercloud before the first IC lightning flash, and with the time approaching the first IC lightning flash, the composite KDP decrease to a more negative value at higher altitude, the descending characteristics of composite KDP at higher levels are closer to the time of the first lightning occurrence. These signatures probably relate to the early developing stage of the thunderstorm, the volume of cloud at higher altitude increase with the updraft, and stronger electric fields are gradually forming at high altitudes with charged ice particles moving upward, and higher concentration of ice particles on the top of cloud are vertically aligned due to stronger electric fields.
In this paper, it is also observed that the change of electric field strength above and below the -30 ℃ layer in the isolated thunderstorm may be inferred from the difference between the -30 ℃ layer and other layers warm than it, and the initial conclusion is consistent with the distribution of electric field at different altitude indicated by average KDP on the top of the cloud. It probably indicates that the change of electric field strength and the altitude of electric fields on the top of the cloud could be reflected by the changing of composite KDP at the corresponding altitude.
With high spatial and temporal resolution, DP-PAWRs have the ability not only to detect the rapid evolution of microphysical structures in thunderstorms, but also to observe the initial electrification of thunderstorms, which will facilitate forecasting lightning flash initiation by using the polarization parameters of DP-PAWRs.
In this paper, an isolated thunderstorm is selected by using composite ZH which ZH is larger than 40 dBZ at a -10 °C isothermal layer. By introducing composite KDP which is defined as the minimum KDP value in the vertical column of all elevation scans at each horizontal 2D grid point above the assigned altitude, the negative KDP signatures associated with vertically oriented ice particles in the strong electric fields on the top of the cloud are analyzed. At 7 minutes before the first IC lightning flash, the average composite KDP at -10 ℃ layer starts to decrease from about 0.3 to -0.2 °/km in 3.5 minutes. The averaged composite KDP at -20 ℃ layer starts to decrease 6 minutes before the first IC lightning flash and decreased from about 0.5 to -0.65 °/km in 5 minutes, and the averaged composite KDP at -25 ℃ layer decreases from about 0 to -0.7 °/km in 1 minute which is 2.5 minutes before the first IC lightning flash while the average composite KDP at -30 ℃ layer decreases from -0.4 to -1 °/km in 2 minutes at 2.5 minutes before the first IC lightning flash. Similar results are obtained by using the average KDP on the top of the thundercloud, such as the average KDP at -25 ℃ layer decreases from 0.1 °/km to -0.6 °/km in 1 minute at 2.5 minutes before the first IC lightning flash, while the average KDP at -30 ℃ layer decreases from -0.7 °/km to -1.2 °/km in 30 seconds at 1 minute before the first IC lightning flash.
The result shows that averaged composite KDP decreases from a positive value to a negative value or from a negative value to a more negative value at a different layer from -10 ℃ isothermal level to the top of the thundercloud before the first IC lightning flash, and with the time approaching the first IC lightning flash, the composite KDP decrease to a more negative value at higher altitude, the descending characteristics of composite KDP at higher levels are closer to the time of the first lightning occurrence. These signatures probably relate to the early developing stage of the thunderstorm, the volume of cloud at higher altitude increase with the updraft, and stronger electric fields are gradually forming at high altitudes with charged ice particles moving upward, and higher concentration of ice particles on the top of cloud are vertically aligned due to stronger electric fields.
In this paper, it is also observed that the change of electric field strength above and below the -30 ℃ layer in the isolated thunderstorm may be inferred from the difference between the -30 ℃ layer and other layers warm than it, and the initial conclusion is consistent with the distribution of electric field at different altitude indicated by average KDP on the top of the cloud. It probably indicates that the change of electric field strength and the altitude of electric fields on the top of the cloud could be reflected by the changing of composite KDP at the corresponding altitude.
With high spatial and temporal resolution, DP-PAWRs have the ability not only to detect the rapid evolution of microphysical structures in thunderstorms, but also to observe the initial electrification of thunderstorms, which will facilitate forecasting lightning flash initiation by using the polarization parameters of DP-PAWRs.
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