Abstract

Cyclone Sidr, one of the most devastating tropical cyclones that resulted in several thousand deaths and substantial damages, developed in the north Indian Ocean and made landfall over the Bangladesh coast on 15 November 2007. Observation and simulation results show that Sidr was embedded in a nonuniform environment and contained an intense outer rainband to the east of its center and a significant frontal band to the northwest. A detailed study of the outer rainband is performed by numerical simulation.

The eastern band was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone center. This band was composed of convective cells that developed southeast of the center within a synoptic-scale convergence zone and propagated along the band toward the northeast quadrant. The speed of the downwind-propagating cells was greater than that of the cyclone, which resulted in a convective cluster northeast of the center. Only the downwind portion of the band consisted of convection with stratiform rain, whereas the upwind and middle portions contained active convective cells without stratiform rain.

The band was located between the synoptic-scale flows of a weakly sheared, gradient-balanced westerly and a strongly sheared, nongradient-balanced prevailing southerly caused by the complex terrain of the Bay of Bengal’s southeast region. Low-level convergence along the band was dominated by cross-band flow from both sides of the band and was confined below 3 km. As the cyclone moved northward, the convergence zone resulted in the extension of band length up to ~800 km. The southerly at the eastern side of the center gradually accelerated and was directed toward the center by a strong pressure gradient force. The flow accumulated a substantial amount of water vapor from the sea in addition to the increased moisture in the lower troposphere, resulting in further intensification of the convective cells.

Abstract

Dimension characteristics in precipitation properties of cumulonimbus clouds are basic parameters in understanding the vertical transport of water vapor in the atmosphere. In this study, the dimension characteristics and precipitation efficiency of cumulonimbus clouds observed in the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) Huaihe River Basin Experiment (HUBEX) are studied using data from X-band Doppler radars and upper-air soundings. The maximum echo area (EA_max) of the cumulonimbus clouds ranged from 0.5 to 470 km², and the maximum echo top (ET_max) ranged from 2 to 19 km. The total number of cells (TNC) within the cumulonimbus clouds over their lifetime was from 1 to 25.

The ET_max, TNC, area time integral (ATI), and total rainfall amount (R _tot) strongly correlate with the EA_max of the cumulonimbus clouds. The cell-averaged ATI (ATI_cell = ATI/TNC), maximum rainfall intensity (RI_max), and cell-averaged rainfall amount (R _cell = R _tot/TNC) increase when the EA_max is smaller than 100 km². On the other hand, they are almost constant when the EA_max is larger than 100 km². The rain productivity of small clouds (<100 km² in EA_max) increases not only by the increase of the TNC but also by the intensification of cells, while that of large cumulonimbus clouds (>100 km² in EA_max) increases by the increase of the TNC rather than by the intensification of cells.

In the present study, precipitation efficiency (ε_p ) is defined as the ratio of the total rainfall amount (R _tot) to the total water vapor amount ingested into the cloud through the cloud base (V _tot). The ε_p was calculated for six clouds whose vertical velocity data at the cloud-base level were deduced by dual-Doppler analysis throughout their lifetime. The ε_p ranged from 0.03% to 9.31% and exhibited a strong positive correlation with the EA_max. This indicates that more than 90% of the water vapor that enters the clouds through the cloud base is consumed to moisten the atmosphere and less than 10% is converted to precipitation and returned to the ground. The cumulonimbus clouds in the region far south from the mei-yu front over the eastern Asian continent efficiently transport water vertically and humidify the upper troposphere.

Abstract

Intensity change of tropical cyclones (TCs) as they make landfall is closely linked to sustained periods of high surface winds and heavy precipitation. Few studies have investigated the intensity change of intense TCs that make landfall in middle latitudes such as Japan because few intense typhoons make landfall in middle latitudes. In this study, a numerical simulation of intense Typhoon Nancy (1961) was used to understand the intensity change that occurred when Nancy made landfall in Japan. A spectral nudging technique was introduced to reduce track errors in the simulation. During landfall, the simulated storm exhibited the salient asymmetric structure and rapid eyewall contraction. A tangential wind budget indicated that the maximum wind speed decreased concurrent with an increase in surface friction and advection associated with low-level asymmetric flows. Detailed evolution of the storm’s warm core was analyzed with a potential temperature budget. In the upper part of the warm core centered at a 12-km height, cooling due to ventilation by asymmetric flows and longwave radiation overcame heating due to condensation and shortwave radiation during the contraction of eyewall clouds. In the lower part of the warm core, adiabatic cooling more than offset warm-air intrusions associated with asymmetric flows and condensational heating. The condensation was supplied by an abundance of moisture due to evaporation from the ocean in the well-developed typhoon based on a moisture budget. Sensitivity experiments revealed that environmental baroclinicity in the midlatitudes, orography, and radiative processes made minor contributions to the weakening. The weakening was instead controlled by inner-core dynamics and interactions with land surfaces.

Abstract

The Loess Plateau of China consists of dissected flat tablelands with steep gullies. To evaluate the effect of topography on local circulation and cumulus generation over the Loess Plateau, numerical simulations of atmospheric boundary layer (ABL) development were conducted using a cloud-resolving nonhydrostatic model. Two types of numerical simulation were carried out under two sets of bottom boundary conditions: real terrain and flat terrain. The differences in ABL development and cumulus generation between the flat- and real-terrain conditions are described and the local circulation structures induced by ABL development are illustrated. More cumulus clouds were generated over the real terrain than over the flat terrain. In the real-terrain case, large amounts of cumulus cloud were generated on the windward slopes and on the edge of the tableland, with updrafts caused by thermal generation and a local circulation developing with horizontal and vertical scales of several kilometers. Cumulus clouds clearly developed at the top of the ABL because the water vapor is nonhomogeneously lifted by the local circulation on windward slopes and on edge of the tableland. Thus, the topography of the Loess Plateau plays an important role in cumulus generation.

Abstract

Typhoons with long-lived concentric eyewalls (CEs) are more intense than those with short-lived CEs. It is important for more accurate prediction of typhoon intensity to understand the maintenance mechanism of the long-lived CEs. To study the mechanism of the long-term maintenance of CEs, a numerical experiment of Typhoon Bolaven (2012) is performed using a nonhydrostatic model with full physics. Two aspects of the maintenance of simulated CEs are investigated: the maintenance of the inner eyewall and the contraction of the outer eyewall. To examine the maintenance of the inner eyewall, the equivalent potential temperature budget and air parcel trajectories of the simulated inner eyewall are calculated. The results show that the entropy supply to the inner eyewall is sufficient to maintain the inner eyewall after secondary eyewall formation (SEF). During the early period after SEF, entropy is supplied by an axisymmetric inflow, and later it is supplied by nonaxisymmetric flows of the outer eyewall. To examine the contraction of the outer eyewall, the potential vorticity (PV) budget of the outer eyewall is diagnosed. The result reveals that the negative contribution to the contraction of the outer PV peak (i.e., the outer eyewall) in the early period is the negative PV generation due to axisymmetric advection and diabatic heating just inside of the outer PV peak. In the later period, the negative PV generation due to nonaxisymmetric structure is important for the prevention of contraction. The present study reveals that the structure of the outer eyewall plays important roles in the maintenance of long-lived CEs.

Abstract

The characteristics and formation mechanism of an anticyclonic vortex street of meso-β scale, which appeared near a cold front around the Ryukyu Islands, Japan, on 11 April 1995, were examined by an analysis of observational data, a numerical simulation, and a linear stability theory.

The vortex street was generated near the midtroposphere on the north edge of a cloud band along the cold front. The diameter of individual vortices was 70 km, and the interval between vortices was 100–200 km. The phase speed of the vortices was nearly equal to the wind speed at the 400-hPa level.

The cloud band was accompanied by a weak wind region, in which the along-front wind was about 15 m s⁻¹ weaker than that in the environment. A region of large anticyclonic horizontal shear (∼10⁻⁴ s⁻¹) existed on the north side of this weak wind region.

To examine a detailed structure and formation mechanism of the weak wind region, a numerical simulation was performed. Regions of active convection and convective cloud bands, associated with a convergence line of the surface wind, were reproduced at the position where they were observed. The weak wind region accompanied by the cloud band was also reproduced. A budget analysis of the along-front momentum equation shows that the weak wind region was produced by vertical advection of horizontal momentum in the convective cloud band, which is resolved by the 15-km grid interval of the numerical model.

The stability of the simulated along-front wind and temperature fields near the weak wind region was examined by a linear theory. It is found that there exist a number of growing modes, the disturbance kinetic energy of which is supplied by the kinetic energy of the basic flow (i.e., a barotropic instability). The fastest growing mode has a maximum amplitude at 450 hPa and is confined to the region of 450 hPa ± 100 hPa. The wavelength and phase velocity of the fastest growing mode agree well with those of the observed vortex street.

Abstract

Following an earlier diagnostic study, the present paper performs numerical simulations of the rare wintertime supercell storms during 19–20 December 2002 in a subtropical environment near Taiwan. Using Japan Meteorology Agency (JMA) 20-km analyses and horizontal grid spacing of 1.5 and 0.5 km, the Cloud-Resolving Storm Simulator (CReSS) of Nagoya University successfully reproduced the three major storms at the correct time and location, but the southern storm decayed too early over the Taiwan Strait. The two experiments produce similar overall results, suggesting that the 1.5-km grid spacing is sufficient even for storm dynamics. Model results are further used to examine the storm structure, kinematics, splitting process, and the variation in the mesoscale environment. Over the Taiwan Strait, the strong surface northeasterly flow enhanced low-level vertical shear and helped the storms evolve into isolated supercells. Consistent with previous studies, the vorticity budget analysis indicates that midlevel updraft rotation arose mainly from the tilting effect, and was reinforced by vertical stretching at the supercell stage. As the ultimate source of vorticity generation, the horizontal vorticity (vertical shear) was altered by the baroclinic (solenoidal) effect around the warm-core updraft, as well as the tilting of vertical vorticity onto, and rotation of vortex tubes in the x–y plane, forming a counterclockwise pattern that pointed generally northward (westward) at the right (left) flanks of the updraft. In both runs, model storms travel about 15°–20° to the left of the actual storms, and they are found to be quite sensitive to the detailed low-level thermodynamic structure of the postfrontal atmosphere and the intensity of the storms themselves, in particular whether or not the existing instability can be released by forced uplift at the gust front. In this regard, the finer 0.5-km grid did produce stronger storms that maintained longer across the strait. The disagreement in propagation direction between the model and real storms is partially attributed to the differences in environment, while the remaining part is most likely due to differences not reflected in gridded analyses. Since the conditions (in both the model and real atmosphere) over the Taiwan Strait are not uniform and depend on many detailed factors, it is anticipated that a successful simulation that agrees with the observation in all aspects over data-sparse regions like this one will remain a challenging task in the foreseeable future.

Abstract

In the fall of 1994, the Beaufort and Arctic Storms Experiment (BASE) was held to collect information on the structure and evolution of mesoscale weather systems over the southern Beaufort Sea and the Mackenzie River delta of the western Canadian Arctic. As part of the experiment, X-band Doppler radar observations were carried out at Tuktoyaktuk, a village on the shore of the Beaufort Sea. In this paper, the precipitation features, structure, and moisture transport associated with two distinctly different weather systems that were observed during BASE are described with a variety of datasets. Climatologies of storm activity in the area suggest these two types of different weather systems, the so-called Pacific origin and storm track disturbances, are the most frequently observed in this region during the fall months.

The characteristic feature of a Pacific origin weather system is a pronounced layering of the air masses. In the upper layer, the air mass is of Pacific origin and is associated with a deep low in the Gulf of Alaska. As a result it is moist and is capable of producing precipitation. In contrast, the lower layer is initially of continental origin and is associated with a secondary lee cyclogenesis event in the Mackenzie River basin. As the secondary disturbance moves to the east, there is a shift in the wind direction that advects air from the Beaufort Sea into the lower layer. This results in a moistening of the lower layer that allows precipitation from the upper layer that had previously evaporated in the lower layer to be enhanced and reach the surface. The detailed structure of this type of storm is strongly affected by the topography of the region and the presence of open water in the southern Beaufort Sea.

The storm track weather system is markedly different and is associated with the passage of a mesoscale low over the southern Beaufort Sea. In this sort of system, there is a well-defined frontal structure of a type previously identified in the midlatitudes. Two different precipitation regimes are identified that are associated with the passage of the warm and cold front. In this sort of system, the sources of moisture are the Bering Sea and the open water in the southern Beaufort Sea.

Abstract

During the morning hours on 23 May 2002, a convective line associated with a mei-yu front brought heavy rainfall along the coast of central Taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy (CAPE) of over 3000 m² s⁻². Doppler radar observations indicated that deep convection was organized into a linear shape with a northeast–southwest orientation along the front about 70 km offshore from Taiwan over the northern Taiwan Strait. The system then moved toward Taiwan at a slow speed of about 4 m s⁻¹. In the present study, the effects of Taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at a 2-km horizontal grid size. Experiments with different terrain heights of Taiwan, including full terrain (FTRN), half terrain (HTRN), and no terrain (NTRN), were performed. The control run using full-terrain and cold rain explicit microphysics realistically reproduced the evolution of the convective line and the associated weather with many fine details.

Two low-level convergence zones were found to be crucial in the development of this convective line and the subsequent rainfall distribution over Taiwan. The first was along the mei-yu front and forced mainly by the front, but was terrain enhanced off the northwestern coast of Taiwan due to the blocking of air on the windward side of the Central Mountain Range (CMR). After formation, convective cells along this zone propagated southeastward and produced rainfall over the northwestern coast. As the front moved closer to Taiwan, a second arc-shaped convergence zone with a nearly north–south orientation along about 120°E formed ahead of the front between the prevailing flow and near-surface offshore flow induced by the blocking. This second zone was terrain induced, and convection initiated near its northern end was found to be responsible for the rainfall maximum observed near the coast of central Taiwan. Its intensity and position were highly sensitive to terrain height. In the HTRN run where the terrain was reduced by half, a weaker zone closer to the CMR (by about 50 km) was produced, and the rain fell mostly over the windward slope of the terrain instead of over the coastal plain. When the terrain was removed in the NTRN run, no such zone with the correct orientation formed. It was also found that the frontal movement near northern Taiwan was slightly delayed with the presence of terrain, and this affected the timing and distribution of local rainfall during the later stages of this event.

Abstract

Before landfall in the Philippines, Supertyphoon Haiyan in 2013 experienced a period of rapid intensification (RI) with the maximum wind speed increasing by 31 m s⁻¹ in 24 h. A 2-km horizontal resolution full-physics cloud-resolving model is used to study the RI of Haiyan. To aid in the diagnosis of RI, we employ a dynamic efficiency factor (DEF), which is based on axisymmetric gradient wind vortex theory. The DEF measures the efficacy of convective heating at any point in generating kinetic energy. The DEF is large in the presence of strong baroclinicity near the eyewall. The simulated tropical cyclone (TC) becomes more symmetric and resembles the observations of Haiyan during the RI period. Our results highlight the nonlinear feedback process in which deep convection becomes collocated with the large DEF area near the eyewall. With the intensity increase, the eyewall convection results in significant subsidence warming at the edge of the eye rather than at the center of the eye. The overall eye warming enhances the baroclinicity near the eyewall and the system DEF in the eye-core region is increased by 33% before and after the RI onset. The vortex intensity increase during the RI period also leads to large unbalanced radial inflow in the boundary layer. The large radial inflow causes the deep convection to occur in the region of large inertial stability inside the radius of maximum wind. The deep convection further efficiently intensifies the TC. Our simulation emphasizes the importance of model resolution in simulating the internal nonlinear processes of RI.