367395 Understanding of Convection Genesis by Urban Meteorological Model Based on Large Eddy Simulation

Wednesday, 15 January 2020
Hall B1 (Boston Convention and Exhibition Center)
Kosei Yamaguchi, Kyoto Univ., Uji, Kyoto, Japan; and T. Tsuchihashi, D. Konishi, and E. Nakakita

A single or a couple of isolated cumulonimbus clouds sometimes causes localized torrential rainfall, and it even causes flash flood disasters in a small river basin in Japan frequently. It is important to detect the cloud at its early life stage, which we call as a “baby-cell” stage. Nakakita et al. (2017) found that a detection of high vertical vorticity observed by Doppler weather radar in a cumulonimbus cloud at its early stage is useful to predict whether the cloud will develop or not. Ministry of Land Infrastructure Transform and Tourism (MLIT) in Japan employs this method to identify severe storm in real time operation. However, weather radar cannot detect “convection genesis”, that is, air motion of thermal without precipitation particles, because weather radar is sensitive to precipitation particles. Both observational and numerical approaches to understand the convection genesis are much required.
In this study, cumulous cloud simulation over urban area based on large-eddy simulation (LES) with 60 meters resolution that is developed by Yamaguchi et al. (2015) was carried out to clarify the convection genesis. In addition, we evaluated our model analysis by our campaign observation that involves boundary layer radar, Doppler lidar, Ka-band radar, and radio-sonde. Target area is set to Kobe City which is one of the biggest urban cities in Japan, because the convection genesis is concerned to be much affected by urban heat effect. And, our LES model can calculate through urban canopy layer to above boundary layer on a scale possible to resolve buildings explicitly. It can describe process of formation of rain drops by cloud microphysical model, and it also can describe anthropogenic heat.
First, vortex tubes in the simulation were analyzed. The backward facing step flow and vertical wind shear came up behind buildings. They made horizontal vortex tubes near surface. Then, anthropogenic heat, sensible heat from surface, and the convergence of horizontal wind made upward flow. The updraft tilted the horizontal vortex to vertical, which changed into a positive/negative pair of vertical vortex tubes. The vertical vortex tubes had a several hundred meters width. The occurrence area of the updrafts corresponded to the tall buildings or factory that produced turbulence and much heat. The strong updraft thermals can be seen in the following cases, i) a few thermals were merged into one big thermal, ii) a series of thermals (the upper thermal broke the stable layer so that the lower thermal could penetrate above the boundary layer), iii) much moisture not only near surface but also in upper boundary layer. Finally, the strong updrafts made cumulous clouds, and raised the top level of the boundary layer gradually. We will show some animations of the vortex tubes with an interval of three seconds in my talk (See the attached figure that shows high vorticity index).
Second, we evaluated our model results by our campaign observation of the multi sensors. The radio-sonde observation at two locations was carried out. The first location was upstream of Kobe City, and the vertical profile of the observation was given as the initial condition as an inflow of the simulation. The second location was the center of Kobe City for the validation. The top height of boundary layer was well simulated compared with hourly radio-sonde observation. The boundary layer radar observation for measuring upward current was also carried out at the center of Kobe City. The spatial scale and the time scale of the thermal updrafts were also well matched to the simulation, but the observation had smaller perturbations. The Doppler lidar observation for measuring the horizontal wind velocity without precipitation particles was also carried out. The area where the horizontal vortex tubes were tilted up was well matched to the simulation.
In summary, it was found that the vertical vortex tubes occurred by updrafts from the urban surface through urban effects was a key signature for identifying convection genesis.
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