Sea-Breeze Front Observations with Water Vapor Lidar and Doppler Lidar at Tokyo Bay—Case Study of Local Heavy Rainfall on 19 August 2017

The frequency of occurrence of localized heavy rainfall that can cause extensive damages, has been increasing in Japan for the last forty years (Japan Meteorological Agency, 2018). Previous studies on the local heavy rainfalls occurred in Tokyo metropolitan area reported that the initiation of deep convection and subsequent heavy rainfall often occurred when the southerly sea breeze front (SBF) from Tokyo Bay collides with the east-northerly SBF from Kashima-nada over the Kanto Plain (e.g., Fujibe et al., 2002; Kawabata et al., 2007; Saito et al., 2018). However, the detailed process on the initiation of deep convection has not been well understood because partly of lack of the observational data on the water vapor and wind fields and their evolution. To improve our understanding of this process, the vertical distributions of water vapor, aerosols and clouds, and horizontal wind were measured using a mobile Raman lidar (RL) and a Doppler lidar (DL) during the passage of an SBF on 19 August 2017 at Tokyo Bay when local heavy rainfall occurred inland. The mobile RL is an automated and compact system developed by the Meteorological Research Institute (MRI) (Sakai et al., 2019), while the DL is operated by the Japan Meteorological Agency (JMA) for aviation weather services. Furthermore, a high-resolution simulation using the JMA nonhydrostatic model (JMA-NHM) (Saito et al., 2006) was conducted to study the evolution of the SBF and its influence on the initiation of convection.

The passage of the SBF was identified by the DL-derived horizontal winds showing the change of wind direction toward the land below an altitude of 0.2 km above mean sea level (MSL) at 10 local time (LT) over the measurement sites. The wind speed increased from 2 m/s to 7 m/s from 10 LT to14 LT in the altitude region. The cloud base height that was estimated from the elastic backscatter signal of the RL was below 0.3 km MSL before the passage of frontal head and increased over 1.0 km MSL after that. The RL-derived water vapor mixing ratio decreased by 1-2 g/kg below 0.5 km MSL after the passage of the SBF.

The result of the simulation using the JMA-NHM showed that the simulated vertical distributions of water vapor and horizontal wind in the SBF were consistent with those obtained from the lidar measurements. The time evolution of the simulated SBF showed that it moved inland at a mean speed of 2 m/s and collided with the gust front at 15 LT, where warm and moist air ahead of the SBF ascended and formed cloud and precipitation. The location and timing of the precipitation were consistent with the radar observation. This result suggests that the SBF partly contributed the heavy rainfall in Tokyo on the studied case.

In summary, the measurements of water vapor and wind profiles with RL and DL are useful to validate the model and improve our understanding of the mechanism of initiation of deep convection that can cause heavy rainfalls.

References

Fujibe, F., K. Sakagami, K. Chubachi, and K. Yamashita, 2002: Surface wind patterns in Tokyo in the preceding afternoon short-time heavy rainfall of midsummer days. Tenki, 49, 395–405 (in Japanese).

Japan Meteorological Agency, 2018: Climate Change Monitoring Report 2017, p40, https://www.jma.go.jp/jma/en/NMHS/ccmr/ccmr2017_low.pdf.

Kawabata, T., H. Seko, K. Saito, T. Kuroda, K. Tamiya, T. Tsuyuki, Y. Honda, and Y. Wakazuki, 2007: An assimilation and forecast experiment of the Nerima heavy rainfall with a cloud-resolving nonhydrostatic 4-dimensional variational data assimilation system. J. Meteor. Soc. Japan, 85, 255–276, http://doi.org/10.2151/jmsj.85.255.

Saito, K., T. Fujita, Y. Yamada, J. Ishida, Y. Kumagai, K. Aranami, S. Ohmori, R. Nagasawa, S. Kumagai, C. Muroi, T. Kato, H. Eito, and Y. Yamazaki, 2006: The operational JMA nonhydrostatic mesoscale model. Mon. Wea. Rev., 134, 1266–1298, https://doi.org/10.1175/MWR3120.1.

Saito, K., M. Kunii, and K. Araki, 2018: Cloud Resolving Simulation of a Local Heavy Rainfall Event on 26 August 2011 Observed in TOMACS. J. Meteor. Soc. Japan, 96A., 175–199, http://doi:10.2151/jmsj.2018-027.

Sakai, T., Nagai, T., Izumi, T., Yoshida, S., and Shoji, Y., 2019: Automated compact mobile Raman lidar for water vapor measurement: instrument description and validation by comparison with radiosonde, GNSS, and high-resolution objective analysis. Atmos. Meas. Tech., 12, 313–326, https://doi.org/10.5194/amt-12-313-2019.