5:15 PM - 7:15 PM
[AAS03-P18] Contribution of Moist Absolute Unstable Layer (MAUL) Region to the Evolution of a Precipitation System
Keywords:Moist Absolute Unstable Layer (MAUL), heavy rainfall, back-building , compensating downdraft
Occurrences of heavy rainfall events should be caused by intrusion of abundant water vapor not only in the lower troposphere but also in the middle one. Several recent studies recently pointed out that the formation of a moist absolutely unstable layer (MAUL) in the vicinity of heavy rainfall regions contributes to the events. The authors have investigated the heavy rainfall event that occurred in Kyushu on July 4, 2020. They demonstrated that a thick MAUL region had formed upstream of the heavy rainfall one. Furthermore, the heavy rainfall and MAUL regions were directly connected. On the other hand, the MAUL regions were separated from the heavy rainfall one due to compensating downdrafts associated with convective clouds in certain regions. This suggests that investigation between the MAUL and heavy rainfall regions should be required by detailed spatiotemporal analysis at the convective-cell scale. This study aims to clarify the contribution of the MAUL region to the evolution of the precipitation system.
The CReSS-3DVAR reanalysis data developed in National Research Institute for Earth Science and Disaster Resilience (NIED) were used in this study. The reanalysis data had approximately 1.5-km horizontal grid resolution around Kyushu area. The JMA-LFM GPV data were applied as initial and boundary conditions and surface wind at AMeDAS stations and Doppler velocity obtained by XRAIN were assimilated by incremental analysis updates (IAU) method at every 10 min. The output interval of the data was 10 minutes.
The heavy rainfall region was a quasi-stationary convective band (QSCB) extending in the east-west direction. The QSCB should be a back-building type. The MAUL region was formed over a wide area to the upstream (southwest) side of the QSCB. Convective cells developed at the western edge of the QSCB, where they received an abundant supply of water vapor and cloud water from the MAUL region in the lower and middle troposphere. They moved eastward by westerly wind at the middle troposphere, transitioned into their mature stage, and formed a compensating downdraft region to the south of the QSCB. As a result, the development of convective cells on the southern side of the QSCB should not occur, although the MAUL is distributed to the south of the compensating downdraft region. The intrusion of water vapor from the MAUL region should be inhibited due to compensating downdrafts. The phenomena might be caused due to the small angle between the orientation of the QSCB and the direction of water vapor intrusion, that is, lower-level wind direction.
The CReSS-3DVAR reanalysis data developed in National Research Institute for Earth Science and Disaster Resilience (NIED) were used in this study. The reanalysis data had approximately 1.5-km horizontal grid resolution around Kyushu area. The JMA-LFM GPV data were applied as initial and boundary conditions and surface wind at AMeDAS stations and Doppler velocity obtained by XRAIN were assimilated by incremental analysis updates (IAU) method at every 10 min. The output interval of the data was 10 minutes.
The heavy rainfall region was a quasi-stationary convective band (QSCB) extending in the east-west direction. The QSCB should be a back-building type. The MAUL region was formed over a wide area to the upstream (southwest) side of the QSCB. Convective cells developed at the western edge of the QSCB, where they received an abundant supply of water vapor and cloud water from the MAUL region in the lower and middle troposphere. They moved eastward by westerly wind at the middle troposphere, transitioned into their mature stage, and formed a compensating downdraft region to the south of the QSCB. As a result, the development of convective cells on the southern side of the QSCB should not occur, although the MAUL is distributed to the south of the compensating downdraft region. The intrusion of water vapor from the MAUL region should be inhibited due to compensating downdrafts. The phenomena might be caused due to the small angle between the orientation of the QSCB and the direction of water vapor intrusion, that is, lower-level wind direction.