Comparison of Cloud Structures of Storms Producing Lightning at Different Distance Based on Five Years Measurements of a Doppler Polarimetric Vertical Cloud Profiler
2. Data and Methods
2.1. Milešovka Meteorological Observatory
2.2. Cloud Radar METEK
- Power in co-channel (pow) and cross-channel (pow-cx) of reflected signal [dBm]
- Phase in co-channel (pha) and cross-channel (pha-cx) of reflected signal [deg]
- Equivalent radar reflectivity factor (Ze [dBZ])
- Doppler radial velocity (V [m/s]) oriented towards the radar
- Linear Depolarization Ratio (LDR) [dB]
- Co-polar correlation coefficient (RHO)
- Doppler spectrum width (RMS [m/s])
- Differential phase (Phi [deg] in range −180–180°)
2.3. Lightning Data
2.4. Preparation of Radar Data and Their Processing
- We are aware that the determination of the localization of lightning is inaccurate with an error that may be around 1 km in the case of CC discharges, which significantly predominate (about 85%) over CG discharges that are usually located more accurately. We divide data in the way that ND and FD are clearly separated.
- Since ND dataset is usually smaller than FD, we need a reasonable number of ND to get sufficiently robust results. Therefore, the radius around the radar selecting ND must not be too small. Conversely, the use of too long distance will cause the data to no longer be characteristic.
- When selecting data, it is also necessary to avoid cases when there is no storm/cloud above the radar. In the data, we had cases when a discharge was recorded at a distance of only several hundred meters from the radar but, above the radar, there was a low cloud with its top below 2 km. Based on tests, we applied a condition that we consider only those data with Ze > 10 dBZ at the height of 5 km or higher above the radar.
- During thunderstorms, lightning often occurs almost simultaneously at different distances from the radar. In order not to include data in both ND and FD datasets simultaneously, we first determine ND, and then we select FD so that it differs from ND by at least 5 min.
- We are aware that in the case of FD, we do not know exactly where we are in terms of the storm center. The measured data may be at both the front and the back side of the storm or at the lateral sides of the storm. We neglected this information in the study.
3. Results and Discussion
3.1. Comparison of Vertical Profiles of Radar Quantities for ND and FD Close to the Discharge Time
3.2. Comparison of Vertical Profiles for ND and FD before and after Discharges
- The structure of the storms differs in all compared characteristics depending on the discharge distance. The difference is not only in the time of the discharge but also when comparing the tendencies of the variables just before and after the registered discharge. Although the differences are visually obvious, their statistical significance is usually around 0.6 due to the large variability of storms.
- The main differences in variables between ND and FD are seen at altitudes from 4 to 10 km. The interpretation of some differences is not straightforward because the measured quantities are influenced by various factors.
- Comparing the vertical profiles for ND, the data clearly give higher Ze and LDR in the middle and upper troposphere, indicating the presence of hail and raindrops. Higher Doppler radial velocity in absolute value is consistent with this. The increase in RMS values indirectly confirms that the storm is in its active phase and contains a mixture of various hydrometeors. This is also consistent with lower RHO values as compared to that of FD.
- Phi values differ substantially between ND and FD in some parts of the vertical profile. However, these data have a completely different character and properties than the other variables. There is a large number of zero values in the measured data, and the values vary widely from one vertical level to another. Therefore, the applied method of their evaluation is not suitable as it frequently leads to null values in the profile. The possibility of using Phi measured by a vertical cloud radar for cloud structure analysis needs to be addressed in further research.
- When comparing pow, the differences between ND and FD are similar to those in the case of Ze. This is not surprising. Significant differences in pha at altitudes above 10 km are difficult to explain. This may be due to the small amount of ND and of data at that altitude. Significant differences in pha-cx at altitudes around 6 km are also difficult to explain as it is not seen in pha.
- The time evolution of the quantities is clear in the order of tens of s. The time evolution of ND differs from that of FD. The time evolution is clearly more pronounced for ND.
- In ND, Ze increases close to the time of the discharge, and V increases in absolute value in the middle and upper troposphere, i.e., where large hydrometeors such as hail and water droplets increase in concentration. In FD, it is the opposite. For ND, the vertical profiles of Ze, V, and LDR show that after the discharge, the convective storm continues to grow for at least some time. The discharges do not represent the end of storm development.
- An opposite trend between ND and FD is observed in the case of LDR. Higher pre-discharge LDR values for ND at altitudes around 6 km, together with rather lower Ze values, can be attributed to the presence of the electric field and its influence on the organization of ice crystals.
- In general, there is a noticeable difference in the trends between ND and FD. Near discharges clearly indicate the active part of the storm, which evolves quickly in time. The time evolution of FD is small compared to ND.
- In our previous work , we expressed the possibility of using a similarly performed analysis of radar data for the classification of a convective storm in terms of the occurrence of lightning in the close vicinity of the radar position. This study does not explicitly address this application, but the results also showed that some of the observed characteristics derived from the radar data differ for lightning storms that occur in the immediate vicinity of the radar versus storms where the lightning is farther from the radar. However, due to the small frequency of the occurrence of lightning near the radar, the obtained results are still burdened with considerable uncertainty even if we use five years of measurements.
- We cannot directly compare the obtained results with the results of other works, as we did not find any similar studies which would use a similar technique of storm analysis using radar data. This study presents a relatively new view on the internal structure of storms and on their development in connection with the occurrence of lightning. This adds new information to the knowledge of thunderstorms.
Data Availability Statement
Conflicts of Interest
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|Variable||Altitude [km]||ND Max. Altitude [km]||FD Max. Altitude [km]||ND: N|
(R = 1 km)
(R = 7.5–10 km)
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Sokol, Z.; Popová, J.; Skripniková, K.; Torcasio, R.C.; Federico, S.; Fišer, O. Comparison of Cloud Structures of Storms Producing Lightning at Different Distance Based on Five Years Measurements of a Doppler Polarimetric Vertical Cloud Profiler. Remote Sens. 2023, 15, 2874. https://doi.org/10.3390/rs15112874
Sokol Z, Popová J, Skripniková K, Torcasio RC, Federico S, Fišer O. Comparison of Cloud Structures of Storms Producing Lightning at Different Distance Based on Five Years Measurements of a Doppler Polarimetric Vertical Cloud Profiler. Remote Sensing. 2023; 15(11):2874. https://doi.org/10.3390/rs15112874Chicago/Turabian Style
Sokol, Zbyněk, Jana Popová, Kateřina Skripniková, Rosa Claudia Torcasio, Stefano Federico, and Ondřej Fišer. 2023. "Comparison of Cloud Structures of Storms Producing Lightning at Different Distance Based on Five Years Measurements of a Doppler Polarimetric Vertical Cloud Profiler" Remote Sensing 15, no. 11: 2874. https://doi.org/10.3390/rs15112874