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Guanghua Chen, Yukari N. Takayabu, and Chie Yokoyama

Abstract

Using 10-yr high-resolution satellite and reanalysis data, the synoptic-scale dual structure of precipitable water (PW), in which the southern and northern bands straddled at the ITCZ produce zonally propagating meridional dipoles, is observed over the eastern Pacific (EP) during boreal summer and fall. Composites indicate that the PW dipole, concurrent with the dipole-like filtered divergence, has a shift to the west of the anomalously cyclonic circulation. The vertical structure of filtered meridional wind is characterized by a wavenumber-1 baroclinic mode, and the vertical motion has two peaks situated at 850 and 300 hPa, respectively. To the east of the PW dipole, the shallow convection is embedded within the deep convection, forming a multilevel structure of meridional wind on the ITCZ equatorward side. To the west of the PW dipole, the deep convection tends to be suppressed because of the invasion of midlevel dry air advected by northerly flows. The generation and propagation of the dual PW band can be attributed to the divergence and advection terms related to specific humidity and three-dimensional wind. By comparison, the PW anomalies over the western North Pacific, only exhibiting a single band, coincide with the centers of synoptic disturbances with a barotropic vertical structure. Because of the weakening of lower-level divergence, the vertical motion, and the horizontal gradient of PW, the synoptic-scale PW signal is reduced significantly. The typical cases and statistics confirm that the strong meridional dipoles and westward-propagating disturbances are closely associated with the distortion and breakdown of ITCZ over the EP.

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Atsushi Hamada, Yuki Murayama, and Yukari N. Takayabu

Abstract

Characteristics and global distribution of regional extreme rainfall are presented using 12 yr of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) measurements. By considering each rainfall event as a set of contiguous PR rainy pixels, characteristic values for each event are obtained. Regional extreme rainfall events are defined as those in which maximum near-surface rainfall rates are higher than the corresponding 99.9th percentile on a 2.5° × 2.5° horizontal-resolution grid.

The geographical distribution of extreme rainfall rates shows clear regional differences. The size and volumetric rainfall of extreme events also show clear regional differences. Extreme rainfall rates show good correlations with the corresponding rain-top heights and event sizes over oceans but marginal or no correlation over land. The time of maximum occurrence of extreme rainfall events tends to be during 0000–1200 LT over oceans, whereas it has a distinct afternoon peak over land. There are also clear seasonal differences in which the occurrence over land is largely coincident with insolation.

Regional extreme rainfall is classified by extreme rainfall rate (intensity) and the corresponding event size (extensity). Regions of “intense and extensive” extreme rainfall are found mainly over oceans near coastal areas and are likely associated with tropical cyclones and convective systems associated with the establishment of monsoons. Regions of “intense but less extensive” extreme rainfall are distributed widely over land and maritime continents, probably related to afternoon showers and mesoscale convective systems. Regions of “extensive but less intense” extreme rainfall are found almost exclusively over oceans, likely associated with well-organized mesoscale convective systems and extratropical cyclones.

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Satoru Yokoi, Yukari N. Takayabu, and Hiroyuki Murakami

Abstract

This paper performs an attribution analysis of future changes in the frequency of tropical cyclone (TC) passages over the western North Pacific basin projected by seven general circulation models. The models project increases in the passage frequency over the tropical central North Pacific and decreases in regions to the west and northwest, including East Asian countries. The attribution analysis reveals that while changes of the basinwide TC count would decrease the frequency of passages throughout the basin, the gross horizontal contrast in the passage frequency changes is caused by a projected eastward shift of main TC development regions, probably caused by El Niño–like sea surface temperature changes. The change in the frequency of passages is also caused by changes of TC translation vectors and preferable tracks. In particular, the translation vector would rotate clockwise to point in a more easterly direction over oceanic regions south of Japan, decreasing the passage frequency over the Korean peninsula and western Japan while increasing it over eastern Japan. This change in translation direction may be caused by the southward shift of the subtropical jet axis and resultant intensification of westerly steering flows. The El Niño–like change and westerly steering flow change are consistent not only among the seven models but also among a number of other climate models, which suggests the reliability of these results from the viewpoint of intermodel agreement.

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Chie Yokoyama, Yukari N. Takayabu, and Sachie Kanada

Abstract

Contrasts in precipitation characteristics across the baiu front are examined with Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data near Japan during June–July (1998–2011). The vertical structure of atmospheric stratification differs between the tropics and midlatitudes. On an average, the baiu front is found around the latitude that roughly divides the midlatitude atmosphere from the tropical atmosphere. Precipitation characteristics are compared between the southern and northern sides of the reference latitude of the baiu front, which is detected with equivalent potential temperature at 1000 hPa of 345 K in terms of the boundary between the tropics and midlatitudes.

The results show that there are obvious differences in precipitation characteristics between the southern and northern sides. In the south, convective rainfall ratios (CRRs) are 40%–60%, which are larger than those in the north (20%–40%). Greater rainfall intensity and taller/deeper precipitation are also observed in the south. Moreover, the characteristics of precipitation features (PFs), which are contiguous areas of nonzero rainfall, differ between the southern and northern sides. In the north, wide stratiform precipitation systems with CRRs of 0%–40% and heights of 8–11 km are dominant. In the south, organized precipitation systems with heights of 12–14 km and CRRs of 30%–50% and those with very large heights (14–17 km) and CRRs of 50%–80% are dominant in addition to wide stratiform precipitation systems. These results suggest that the mechanisms to bring rainfall are different between the southern and northern regions of the baiu front.

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Chie Yokoyama, Yukari N. Takayabu, and Takeshi Horinouchi

Abstract

A quasi-stationary front, called the baiu front, often appears during the early-summer rainy season in East Asia (baiu in Japan). The present study examines how precipitation characteristics during the baiu season are determined by the large-scale environment, using satellite observation three-dimensional precipitation data. Emphasis is placed on the effect of subtropical jet (STJ) and lower-tropospheric convective instability (LCI).

A rainband appears together with a deep moisture convergence to the south of the STJ. Two types of mesoscale rainfall events (REs; contiguous rainfall areas), which are grouped by the stratiform precipitation ratio (SPR; stratiform precipitation over total precipitation), are identified: moderately stratiform REs (SPR of 0%–80%) representing tropical organized precipitation systems and highly stratiform REs (SPR of 80%–100%) representing midlatitude precipitation systems associated with extratropical cyclones. As the STJ becomes strong, rainfall from both types of mesoscale precipitation systems increases, with a distinct eastward extension of a midtropospheric moist region. In contrast, small systems appear regardless of the STJ, with high dependency on the LCI.

The results indicate that the STJ plays a role in moistening the midtroposphere owing to ascent associated with secondary circulation to the south of the STJ, producing environments favorable for organized precipitation systems in the southern part of the rainband. The horizontal moisture flux convergence may also contribute to precipitation just along the STJ. On the other hand, the LCI plays a role in generating shallow convection. In high-LCI conditions, deep convection can occur without the aid of mesoscale organization.

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Nagio Hirota, Yukari N. Takayabu, and Atsushi Hamada

Abstract

Reproducibility of summer precipitation over northern Eurasia in climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is evaluated in comparison with several observational and reanalysis datasets. All CMIP5 models under- and overestimate precipitation over western and eastern Eurasia, respectively, and the reproducibility measured using the Taylor skill score is largely determined by the severity of these west–east precipitation biases. The following are the two possible causes for the precipitation biases: very little cloud cover and very strong local evaporation–precipitation coupling. The models underestimate cloud cover over Eurasia, allowing too much sunshine and leading to a warm bias at the surface. The associated cyclonic circulation biases in the lower troposphere weaken the modeled moisture transport from the Atlantic to western Eurasia and enhance the northward moisture flux along the eastern coast. Once the dry west and wet east biases appear in the models, they become amplified because of stronger evaporation–precipitation coupling. The CMIP5 models reproduce precipitation events well over a time scale of several days, including the associated low pressure systems and local convection. However, the modeled precipitation events are relatively weaker over western Eurasia and stronger over eastern Eurasia compared to the observations, and these are consistent with the biases found in the seasonal average fields.

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Yukari N. Takayabu, George N. Kiladis, and Victor Magaña

Abstract

Insights by Professor Michio Yanai on tropical waves, which have been vital ingredients for progress in tropical meteorology over the last half-century, are recollected. This study revisits various aspects of research on tropical waves over the last five decades to examine, in Yanai’s words, “the nature of ‘A-scale’ tropical wave disturbances and the interaction of the waves and the ‘B-scale’ phenomena (cloud clusters),” the fundamental problem posed by Yanai at the design phase of the GARP Atlantic Tropical Experiment (GATE) in 1971. The various contributions of Michio Yanai to the current understanding of the dynamics of the tropical atmosphere are briefly reviewed to show how his work has led to several current theories in this field.

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Yukari N. Takayabu, K-M. Lau, and C-H. Sui

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Detailed structure of the quasi-2-day oscillation observed in the active phase of the Madden–Julian oscillations during the intensive observation period of Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE IOP) was described. A variety of observational platforms is used including high-resolution GMS infrared histogram, rain-rate estimate from TOGA and MIT radar measurements, upper-air soundings, and boundary layer profiler winds from the Integrated Sounding System and surface data from the IMET buoy.

The quasi-2-day mode had a westward propagation speed of 12°–15° day −1, a horizontal wavelength of 25°–30° longitude. A coupling with the westward-propagating n = 1 inertio–gravity waves was hypothesized from the space–time power spectral distribution of the cloud field. The wind disturbance structure was consistent with the hypothesis. The vertical wave structure had an eastward phase tilt with height below 175 hPa and vice versa above, indicating the wave energy emanating from the upper troposphere.

Four stages in the life cycle of the oscillating cloud–circulation system were identified:. 1) the shallow convection stage with a duration time of 12 h, 2) the initial tower stage (9 h), 3) the mature stage (12 h), and 4) the decaying stage (15 h). Surface and boundary layer observations also showed substantial variation associated with the different stages in the life cycle. Results suggest that the timescale of quasi-2-day oscillation is determined by the time required by the lower-tropospheric moisture field to recover from the drying caused by deep convection.

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Nagio Hirota, Yukari N. Takayabu, Masaya Kato, and Sho Arakane

Abstract

Precipitation in excess of 100 mm h−1 in Hiroshima, Japan, on 19 August 2014, caused a flash flood that resulted in 75 deaths and destroyed 330 houses. This study examined the meteorological background of this fatal flood. During this event, considerable filamentary transport of water vapor from the Indochina Peninsula to the Japanese islands occurred, forming a so-called atmospheric river (AR). This AR had a deep structure with an amount of free tropospheric moisture comparable with that of the boundary layer. Furthermore, a cutoff low (COL), detached from the subtropical jet over the central Pacific, moved northwestward to the Japanese islands. Instability associated with the cold core of the COL and dynamical ascent induced in front of it, interacted with the free tropospheric moisture of the AR, which caused the considerable precipitation in Hiroshima. Moreover, the mountains of the Japanese islands played a role in localizing the precipitation in Hiroshima. These roles were separately evaluated on the basis of sensitivity experiments with a cloud-resolving model.

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Ayako Seiki, Yukari N. Takayabu, Takuya Hasegawa, and Kunio Yoneyama

Abstract

The lack of westerly wind bursts (WWBs) when atmospheric intraseasonal variability (ISV) events occur from boreal spring to autumn is investigated by comparing two types of El Niño years with unmaterialized El Niño (UEN) years. Although high ocean heat content buildup and several ISV events propagating eastward are observed in all three types of years, few WWBs accompany these in the UEN years. The eddy kinetic energy budget analysis based on ISV shows that mean westerly winds in the lower troposphere facilitate the development of eddy disturbances, including WWBs, through convergence and meridional shear of zonal winds. In the UEN years, these westerly winds are retracted westward and do not reach the equatorial central Pacific mainly as a result of interannual components. In addition, positive sea surface temperature anomalies in the western Pacific, which are conducive to active convection, spread widely in a meridional direction centered on 15°N. Both westward-retracted mean westerlies and off-equatorial warming enhance off-equatorial eddies, which result in a reduction in equatorial eddies such as WWBs. The characteristics of the UEN years are significantly different from those observed during the eastern Pacific El Niño (EP-EN) years, which are characterized by anomalous cooling (warming) and suppressed (enhanced) convective eddies in the off-equatorial (equatorial) western Pacific. The central Pacific El Niño years show mixed features during both EP-EN and UEN years. Different background states not only in the equatorial region but also in the off-equatorial region can be a reason for the lack of WWBs in the UEN years.

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