Tetsuya Kawano

Numerical sensitivity study of rainband precipitation and evolution

Abstract Using a deep, two-dimensional rainband model, the authors examined the effect of different microphysical treatments on rain evolution and precipitation processes in an otherwise fixed environment. Not only rain evolution but cloud cell activity and heat release patterns also varied greatly depending on the microphysical processes used to drive the model. When maritime microphysics were used, in the early stage, models produced intense rainfall peaks at both the front and the rear of the emerging system. Then, between these two first peaks, new rain peaks successively appeared, forming a broad rainband. Heat was released rather uniformly throughout the cloud depth above the freezing level. Strong winds that blew near freezing level were a common feature. Updraft was enhanced during the intermittent development of new cloud cells. In contrast, when continental microphysics were used, in the early stage, models produced heavy rainfall at the rear of the emerging system and only weak rainfall at the ...

Videosonde observation of torrential rain during Baiu season

Abstract Water accumulation processes producing torrential rain over the southern part of Kyushu Island, Japan during Baiu season have been investigated. The cloud systems involved rain cells that became stationary over Kagoshima City and increased in height. Small, new cells subsequently developed upwind or to the side of the stationary cell and merged with it. Heavy rain was recorded as each cell merged. Data from videosonde ascents in the merging cells showed many large graupel in a narrow area of the cloud just above the freezing level. It is suggested that the intense accumulation of water near that level is produced by the rapid growth of graupel through the capture of supercooled drops transported from the merging cell. Cold dome formation is suggested as the reason for the sudden cessation of cell movement observed in many instances of heavy rain.

An estimation of water origins in the vicinity of a tropical cyclone’s center and associated dynamic processes

To clarify the time evolution of water origins in the vicinity of a tropical cyclone (TC)’s center, we have simulated Typhoon Man-yi (July 2007) in our case study, using an isotopic regional spectral model. The model results confirm that the replacement of water origins occurs successively as the TC develops and migrates northward over the western North Pacific. It is confirmed that, in this case, a significant proportion of total precipitable water around the cyclone center comes from external regions rather than the underlying ocean during the mature stage of a TC. Similar features can also be seen in the proportion of each oceanic origin to total condensation. Indian Ocean, South China Sea, and Maritime Continent water vapors begin to increase gradually at the developing stage and reach their peak at the decay stage when the TC approaches southwestern Japan. These remote ocean vapors are transported to the east of the cyclone via the moisture conveyor belt, a zone characterized by distinct low-level moisture flux that stretches from the Indian Ocean to the TC, and are further supplied into the inner region of the TC by inflow within the boundary layer associated with its secondary circulation. Since it takes time to undergo these two dynamic processes, the delayed influence of remote ocean vapors on the TC appears to become evident during the mature stage.

Evidence for the significant role of sea surface temperature distributions over remote tropical oceans in tropical cyclone intensity

The role of remote ocean sea surface temperature (SST) in regulating tropical cyclone (TC) characteristics has been examined by performing numerical experiments with a regional scale model. Model simulations have been carried out to simulate typhoon Man-yi (July 2007), in our case study, under a range of SST conditions over the Indian Ocean and the South China Sea. The intensity and track of the cyclone have been systematically changed in sensitivity simulations of cool and warm SSTs over that region, following its peak phase. Warm oceans can substantially reduce the intensification of western North Pacific cyclones, whereas cool oceans can enhance their strength. This is intimately associated with the enhancement/weakening of the moisture supply through the moisture conveyor belt (MCB) in the lower troposphere, from the Indian Ocean and South China Sea into the vicinity of the cyclone center. When the MCB is interrupted over the South China Sea in warm SST occurrences, the large-scale transport of moisture into the cyclone system is significantly reduced, leading to the weakening of the cyclone intensity and to the eastward shift of its track. This study shows that changes in remote tropical ocean SST can also modulate TCs and thus can help in improving the forecasting of TC intensities and tracks.

Riming Electrification in Hokuriku Winter Clouds and Comparison with Laboratory Observations

AbstractRiming electrification is the main charge separation mechanism of thunderstorms, occurring mainly during graupel particle–ice crystal collisions. Laboratory experiments have found that charge separation polarity and magnitude depend critically on cloud water content and temperature. Several groups have mapped this dependence, but there are substantial differences between their results. These conflicting laboratory-derived riming electrification topographies can be tested by comparing them to field observations. Here, direct and simultaneous sonde-based measurement of both precipitation particle type and charge (videosonde) and cloud water content [hydrometeor videosonde (HYVIS)] in lightning-active Hokuriku winter clouds at Kashiwazaki, Niigata Prefecture, Japan, are reported. With decreasing height, summed graupel charge transitioned from negative to positive at a mean temperature of −11°C, and the mean peak cloud water content in the positive graupel domain was 0.4 g m−3. Thus, in cloud regions ...

A positive feedback process between tropical cyclone intensity and the moisture conveyor belt assessed with Lagrangian diagnostics

Using a cloud-resolving regional model and Lagrangian diagnostics, we assess a positive feedback process between tropical cyclone (TC) intensity and the moisture conveyor belt (MCB), which connects a TC and the Indian Ocean (IO), the South China Sea (SCS), and the Philippine Sea (PS) vapors, from a macroscopic view. We performed sensitivity experiments that modified the observed sea surface temperature (SST) field over the IO and the SCS to regulate the MCB behavior, and we examined the remote response of a prototypical TC. The results show that the connection between MCB formation and TC development is very robust, which was also observed in another TCs case. The MCB plays a vital role in transporting lots of moist air parcels toward the TC from the IO, SCS, and PS regions. The transported parcels, which further gained the underlying ocean vapor along the MCB, are easily trapped in the inner core by radial inflow in the atmospheric boundary layer and, subsequently, release latent heat around the eyewall, resulting in the TCs intensifying. This acts to further penetrate the moist parcels of remote ocean origin into the inner core through the enhanced and expanded inflow. An additional experiment suggested that the MCB is not formed unless the westward propagation of equatorial waves induced by TC heating overlaps with the background monsoon westerlies. These findings support the reliability and validity of TC-MCB feedback.

Dynamical Modulation of Wintertime Synoptic-Scale Cyclone Activity over the Japan Sea due to Changbai Mountain in the Korean Peninsula

The dynamical impact of the Changbai Mountain Range in the Korean Peninsula on the extratropical cyclone activity over the Japan Sea in early winter is examined using the Weather Research Forecasting model. We have conducted two independent long-term integrations over 15 winter months (December only) from 2000 to 2014 with and without modified topography. The results show that the Changbai Mountain Range plays a vital role in increasing cyclone track frequency, low-level poleward eddy heat flux, and the local deepening rate over the Japan Sea through enhancement of the lower-tropospheric baroclinic zone (LTBZ). This mountain range gives rise to activation of the synoptic-scale cyclone activity over that region. From our case study on three typical cyclones, it is found that mesoscale structures in the vicinity of a cyclone’s center are dynamically modulated when it passes through the LTBZ and that cyclogenesis is triggered around that zone. A vorticity budget analysis shows that the stretching term relevant to enhanced low-level convergence plays a dominant role in intensifying cyclonic vorticities. We confirmed that the composite features of the three typical cases are consistent with the statistical ones of the dynamical modulation of the Changbai Mountain on synoptic-scale cyclone activity.

A Possible Mechanism of Tornadogenesis Associated with the Interaction between a Supercell and an Outflow Boundary without Horizontal Shear

AbstractUsing the insert–restart method developed in this study, tornadogenesis processes associated with the interaction between a supercell and an outflow boundary were investigated. A highly idealized outflow boundary promoted surface vortex intensification in a preconditioned supercell having a strong surface vortex. In particular, a tornado-like vortex was observed in an experiment with a moderate outflow boundary. This intensification was associated with the enhancement of the near-surface horizontal convergence by the outflow boundary. The optimal coldness of the outflow boundary was explained by the balance between the enhancement of the near-surface horizontal convergence and the buoyancy reduction by the outflow boundary. Sensitivity experiments demonstrated that, despite a less favorable environment, a storm with a boundary interaction might reach a similar maximum near-surface vorticity as a storm without a boundary in a more favorable environment.