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[MIS13-04] Conditions for Increased Lightning Activity Preceding Downbursts in Multi-cell Convective Clouds
Keywords:Lightning, downburst, graupel, numerical simulation
Lightning Jump (LJ[1]), a predictive indicator defined as the increased frequency of lightning preceding downbursts, has been reported in observational studies. However, the mechanism of LJ based on the characteristics of graupel, which is required for both downbursts and lightning, remains insufficiently understood. To elucidate the condition of the LJ mechanism, we investigated the formation of the environments of riming electrification of graupel and the riming growth of graupel. This study conducted idealized experiments focusing on multi-cell convective clouds, using the meteorological model SCALE[2][3] implementing bulk lightning model[4] explicitly calculating lightning processes.
The idealized experiment was conducted as CTL experiments using the sounding data of a downburst event with lightning observed that occurred in Misato, Saitama Prefecture, on September 8, 1994, following the approach of Guo et al. (1999)[6]. We set a warm bubble with temperatures max to 4 K above the model domain average which was located southwest of the model domain, and simulations are conducted over 90 minutes. Additionally, the sensitivity experiment was conducted 1.2x the x-direction wind speed to intensify vertical wind shear.
As simulated results of the CTL experiment, LJ was characterized by an increase in the flash origin density as a lightning flash rate (33.3 min) in the multi-cell convective clouds formed by the initial cold pool, followed by a downburst 14.7 minutes later (48.0 min). In contrast, in the sensitivity experiment, despite the formation of multi-cell convective clouds by a similar cold pool, a downburst occurred (33.0 min) before the peak of lightning (36.6 min). Thus, these results indicated the presence of LJ in the CTL experiment and the absence of LJ in the sensitivity experiment.
The factors affecting the occurrence of LJ, whether it happens or not, were investigated based on the tilt of convection originating from the balance between the cold pool and vertical wind shear[7]. In the CTL experiment, firstly, a convective cell that was initially generated tilted convection which created an environment suitable for riming electrification. Next, the following convective cell grew upright which created an environment suitable for riming growth of graupel. These sequences suggest the mechanism of LJ in multi-cell convective clouds. In contrast, the sensitivity experiment showed that an initially generated convection cell tilted less than in the CTL experiment, failing to create an environment suitable for riming electrification. As a result, a downburst occurred without LJ.
Reference
1. Williams, E. R., and Coauthors, 1999: The behavior of total lightning activity in severe Florida thunderstorms. Atmos. Res., 51, 245–264. doi:10.1016/S0169-8095(99)00011-3
2. Nishizawa, S., Yashiro, H., Sato, Y., Miyamoto, Y. & Tomita, H. 2015: Influence of grid aspect ratio on planetary boundary layer turbulence in large-eddy simulations. Geosci. Model Dev. 8, 3393–3419 (2015).
3. Sato, Y. S. Nishizawa, H. Yashiro, Y. Miyamoto, Y. Kajikawa, & H. Tomita. Impacts of cloud microphysics on trade wind cumulus: which cloud microphysics processes contribute to the diversity in a large eddy simulation? Prog. Earth Planet. Sci. 2, (2015).
4. Sato, Y., Y. Miyamoto, and H. Tomita, 2019: Large dependency of charge distribution in a tropical cyclone inner core upon aerosol number concentration. Prog. Earth Planet. Sci., 6, 62, doi:10.1186/s40645-019-0309-7.
5. Takayama, H., H. Niino, S. Watanabe, and J. Sugaya, 1997: Downbursts in the northwestern part of Saitama Prefecture on 8 September 1994. J. Meteor. Soc. Japan, 75, 885–905, doi:10.2151/jmsj1965.75.4_885.
6. Guo, X. L., Niino, H., & Kimura, R., 1999. Numerical Modeling on a Hazardous Microburst-Producing Hailstorm. Toward Digital Earth Proceedings of the International Symposium on Digital Earth (Vol. 1, pp. 383– 398). Science Press.
7. Rotunno, R., Klemp, J. B., & Weisman, M. L. (1988). A Theory for Strong, Long-Lived Squall Lines. Journal of the Atmospheric Sciences, 45, 463–485. https://doi.org/10.1175/1520-0469(1988)045<0463:ATFSLL>2.0.CO;2
The idealized experiment was conducted as CTL experiments using the sounding data of a downburst event with lightning observed that occurred in Misato, Saitama Prefecture, on September 8, 1994, following the approach of Guo et al. (1999)[6]. We set a warm bubble with temperatures max to 4 K above the model domain average which was located southwest of the model domain, and simulations are conducted over 90 minutes. Additionally, the sensitivity experiment was conducted 1.2x the x-direction wind speed to intensify vertical wind shear.
As simulated results of the CTL experiment, LJ was characterized by an increase in the flash origin density as a lightning flash rate (33.3 min) in the multi-cell convective clouds formed by the initial cold pool, followed by a downburst 14.7 minutes later (48.0 min). In contrast, in the sensitivity experiment, despite the formation of multi-cell convective clouds by a similar cold pool, a downburst occurred (33.0 min) before the peak of lightning (36.6 min). Thus, these results indicated the presence of LJ in the CTL experiment and the absence of LJ in the sensitivity experiment.
The factors affecting the occurrence of LJ, whether it happens or not, were investigated based on the tilt of convection originating from the balance between the cold pool and vertical wind shear[7]. In the CTL experiment, firstly, a convective cell that was initially generated tilted convection which created an environment suitable for riming electrification. Next, the following convective cell grew upright which created an environment suitable for riming growth of graupel. These sequences suggest the mechanism of LJ in multi-cell convective clouds. In contrast, the sensitivity experiment showed that an initially generated convection cell tilted less than in the CTL experiment, failing to create an environment suitable for riming electrification. As a result, a downburst occurred without LJ.
Reference
1. Williams, E. R., and Coauthors, 1999: The behavior of total lightning activity in severe Florida thunderstorms. Atmos. Res., 51, 245–264. doi:10.1016/S0169-8095(99)00011-3
2. Nishizawa, S., Yashiro, H., Sato, Y., Miyamoto, Y. & Tomita, H. 2015: Influence of grid aspect ratio on planetary boundary layer turbulence in large-eddy simulations. Geosci. Model Dev. 8, 3393–3419 (2015).
3. Sato, Y. S. Nishizawa, H. Yashiro, Y. Miyamoto, Y. Kajikawa, & H. Tomita. Impacts of cloud microphysics on trade wind cumulus: which cloud microphysics processes contribute to the diversity in a large eddy simulation? Prog. Earth Planet. Sci. 2, (2015).
4. Sato, Y., Y. Miyamoto, and H. Tomita, 2019: Large dependency of charge distribution in a tropical cyclone inner core upon aerosol number concentration. Prog. Earth Planet. Sci., 6, 62, doi:10.1186/s40645-019-0309-7.
5. Takayama, H., H. Niino, S. Watanabe, and J. Sugaya, 1997: Downbursts in the northwestern part of Saitama Prefecture on 8 September 1994. J. Meteor. Soc. Japan, 75, 885–905, doi:10.2151/jmsj1965.75.4_885.
6. Guo, X. L., Niino, H., & Kimura, R., 1999. Numerical Modeling on a Hazardous Microburst-Producing Hailstorm. Toward Digital Earth Proceedings of the International Symposium on Digital Earth (Vol. 1, pp. 383– 398). Science Press.
7. Rotunno, R., Klemp, J. B., & Weisman, M. L. (1988). A Theory for Strong, Long-Lived Squall Lines. Journal of the Atmospheric Sciences, 45, 463–485. https://doi.org/10.1175/1520-0469(1988)045<0463:ATFSLL>2.0.CO;2