Wataru Yanase

Dependence of Polar Low Development on Baroclinicity and Physical Processes: An Idealized High-Resolution Numerical Experiment

Abstract Polar low dynamics in an idealized atmosphere in which baroclinicity, stratification, and average temperature are varied in the typically observed range is investigated using a 5-km-resolution nonhydrostatic model. The baroclinicity is found to be the most important factor that strongly controls the polar low dynamics. When the baroclinicity is weak, a small, nearly axisymmetric vortex develops through a cooperative interaction between the vortex flow and cumulus convection. The surface friction promotes the vortex dynamics by transporting the sensible heat and moisture into the vortex center. The vortex development has a strong sensitivity to the initial perturbation. As the baroclinicity is increased, most of the characteristics of polar low dynamics change smoothly without showing any significant regime shift. The vortex for an intermediate baroclinicity, however, moves northward, which is a unique behavior. This is caused by vortex stretching on the northern side of the vortex where intense c...

A polar low over the Japan Sea on 21 January 1997. Part II: A numerical study

Abstract A remarkable isolated polar low observed over the Japan Sea on 21 January 1997 is studied by numerical simulations using a 20-km-mesh regional primitive equation model [Regional Spectral Model (RSM) of the Japan Meteorological Agency] and a 5-km-mesh nonhydrostatic mesoscale model [Meteorological Research Institute Nonhydrostatic Model (MRI-NHM)] and by sensitivity experiments using the MRI-NHM. The 24-h integration of RSM starting from 0000 UTC 21 January reproduces quite well the polar lows movement and development, including the initial wrapping of the west part of an east–west-oriented vorticity zone. The vorticity budget analysis indicates that the stretching term plays a dominant role in the polar low development. The simulation by MRI-NHM reproduces surprisingly well the wrapping of the western part of the east–west-oriented vortcity zone associated with a cloud band, the spiral-shaped cloud pattern, and the “eye” structure near the vortex center. The wind speed and resulting sea surface ...

Seasonal and Intraseasonal Modulation of Tropical Cyclogenesis Environment over the Bay of Bengal during the Extended Summer Monsoon

AbstractThe environmental field of tropical cyclogenesis over the Bay of Bengal is analyzed for the extended summer monsoon season (approximately May–November) using best-track and reanalysis data. Genesis potential index (GPI) is used to assess four possible environmental factors responsible for tropical cyclogenesis: lower-tropospheric absolute vorticity, vertical shear, potential intensity, and midtropospheric relative humidity. The climatological cyclogenesis is active within high GPI in the premonsoon (~May) and postmonsoon seasons (approximately October–November), which is attributed to weak vertical shear. The genesis of intense tropical cyclone is suppressed within the low GPI in the mature monsoon (approximately June–September), which is due to the strong vertical shear. In addition to the climatological seasonal transition, the authors’ composite analysis based on tropical cyclogenesis identified a high GPI signal moving northward with a periodicity of approximately 30–40 days, which is associat...

Multiscale Interactions in the Life Cycle of a Tropical Cyclone Simulated in a Global Cloud-System-Resolving Model. Part II: System-Scale and Mesoscale Processes*

Abstract The life cycle of Tropical Storm Isobel was simulated reasonably well in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model. The evolution of the large-scale circulation and the storm-scale structure change was discussed in Part I. Both the mesoscale and system-scale processes in the life cycle of the simulated Isobel are documented in this paper. In the preconditioned favorable environment over the Java Sea, mesoscale convective vortices (model MCVs) developed in the mesoscale convective systems (MCSs) and convective towers with cyclonic potential vorticity (PV) anomalies throughout the troposphere [model vortical hot towers (VHTs)] appeared in the model MCVs. Multiple model VHTs strengthened cyclonic PV in the interior of the model MCV and led to the formation of an upright monolithic PV core at the center of the concentric MCV (primary vortex enhancement). As the monolithic PV core with a warm core developed near the circulation center, the intensif...

Multiscale Interactions in the Life Cycle of a Tropical Cyclone Simulated in a Global Cloud-System-Resolving Model. Part I: Large-Scale and Storm-Scale Evolutions*

Abstract The Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model, successfully simulated the life cycle of Tropical Storm Isobel that formed over the Timor Sea in the austral summer of 2006. The multiscale interactions in the life cycle of the simulated storm were analyzed in this study. The large-scale aspects that affected Isobel’s life cycle are documented in this paper and the corresponding mesoscale processes are documented in a companion paper. The life cycle of Isobel was largely controlled by a Madden–Julian oscillation (MJO) event and the associated westerly wind burst (WWB). The MJO was found to have both positive and negative effects on the tropical cyclone intensity depending on the location of the storm relative to the WWB center associated with the MJO. The large-scale low-level convergence and high convective available potential energy (CAPE) downwind of the WWB center provided a favorable region to the cyclogenesis and intensification, whereas the st...

A Numerical Study on the Atmospheric Circulation over the Midlatitude North Pacific during the Last Glacial Maximum

Abstract The dynamics of the atmospheric circulation change over the midlatitude North Pacific under the boundary conditions during the last glacial maximum (LGM) have been studied by atmospheric general circulation models (GCMs) with different ocean feedbacks. Three boundary conditions in the LGM were different from those of the present day (PD): ice sheet with elevated topography and high albedo, atmospheric CO2 concentration, and insolation. The ocean component was treated as follows: a full-circulation ocean with dynamical and thermal ocean feedback [coupled general circulation model (CGCM)]; a slab ocean only with thermal feedback used to calculate the surface heat balance [slab ocean GCM (SGCM)]; and no ocean feedback by fixing sea surface temperature (SST) with pure atmospheric dynamics (AGCM). Both CGCM and SGCM simulated a weakened Pacific high pressure system in boreal summer during the LGM compared to the PD and an intensified Aleutian low pressure system in winter. Both in summer and winter, t...

Climatology of Polar Lows over the Sea of Japan Using the JRA-55 Reanalysis

AbstractPolar lows are intense meso-α-scale cyclones that develop over the oceans poleward of the main baroclinic zone. A number of previous studies have reported polar low formation over the Sea of Japan within the East Asian winter monsoon. To understand the climatology of polar lows over the Sea of Japan, a tracking algorithm for polar lows is applied to the recent JRA-55 reanalysis. The polar low tracking is applied to 36 cold seasons (October–March) from October 1979 to March 2015. The polar lows over the Sea of Japan reach their maximum intensity on the southeastern side of the midline between the Japanese islands and the Asian continent. Consistent with previous case studies, composite analysis demonstrates that the polar low development is associated with the enhanced northerly flow on the western side of a synoptic-scale extratropical cyclone, with the cold trough in the midtroposphere and with increased heat fluxes from the sea surface. Furthermore, the present climatological study has revealed ...

Parameter Spaces of Environmental Fields Responsible for Cyclone Development from Tropics to Extratropics

AbstractObjective cyclone tracking applied to a 30-yr reanalysis dataset shows that cyclone development in the summer and autumn seasons is active in the tropics and extratropics and inactive in the subtropics. To understand this geographically bimodal distribution of cyclone development associated with tropical and extratropical cyclones quantitatively, the direct relationship between cyclone types and their environments are assessed by using a parameter space of environmental variables [environmental parameter space (EPS)]. The number of cyclones is analyzed in terms of two different factors: the environmental conditions favorable for cyclone development and the area size that satisfies the favorable condition. The EPS analysis is mainly conducted for two representative environmental parameters that are commonly used for cyclone analysis: potential intensity for tropical cyclones and baroclinicity for extratropical cyclones. The geographically bimodal distribution is attributed to the high sensitivity o...

Idealized Numerical Experiments on Cyclone Development in the Tropical, Subtropical, and Extratropical Environments

AbstractThe development of cyclones, particularly in the Southern Hemisphere summer, is active in the tropics and extratropics but is inactive in the subtropics. To elucidate the influence of environmental fields on the cyclone development in the tropics, subtropics, and extratropics, idealized numerical experiments are conducted using a nonhydrostatic channel model. The experiments examine the development of a weak initial vortex within a zonally uniform environmental field that consists of five factors: the Coriolis parameter, zonal wind, potential temperature, relative humidity, and surface temperature difference between the ocean and atmosphere. The idealized experiments successfully reproduce the significant cyclone development in the tropical and extratropical environment as well as no cyclone development in the subtropical environment. This result confirms the dominant role of the environmental field in controlling the cyclone development. To clarify which environmental factor is responsible for th...

Climatology of Polar Mesocyclones over the Sea of Japan Using a New Objective Tracking Method

AbstractPolar mesocyclones (PMCs) are mesoscale cyclonic vortices that develop poleward of the main polar front. This article reports on a new algorithm for the objective tracking of PMCs, including meso-β-scale vortices, which will facilitate the study of their climatology. The algorithm is based mainly on the vorticity field and consists of three parts: the identification of vortices, the connection of vortices at consecutive time steps, and discrimination between PMCs and synoptic-scale disturbances. The objective tracking method was applied to Mesoscale Analysis (MA) data provided by the Japan Meteorological Agency, which has a horizontal resolution of 5 km. The detected tracks of PMCs were confirmed by subjective analysis of the MA data and satellite images. The method used here to discriminate between PMCs and synoptic-scale disturbances differs from that used in previous studies, which is based on the difference between the sea surface temperature and the temperature at 500 hPa, but gives a consist...