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Ayako Seiki and Yukari N. Takayabu

Abstract

The mechanism of synoptic-scale eddy development in the generation of westerly wind bursts (WWBs) over the western–central Pacific, and their relationship with the El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO), were examined. In the WWB occurrences, barotropic structures of equatorial eddy westerlies with cyclonic disturbances were found from the surface to the upper troposphere. The dominant contributions to substantial eddy kinetic energy (EKE) were the barotropic energy conversion (KmKe) in the lower and middle tropospheres and the conversion from eddy available potential energy (PeKe) in the upper troposphere. Low-frequency environmental westerlies centered near the equator preceded strong zonal convergence and meridional shear, resulting in the substantial KmKe. The activation of synoptic convection also contributed to an increase in EKE through PeKe. These energies were redistributed to the lower-equatorial troposphere through energy flux convergence (GKe). These results showed that environmental fields contribute to the EKE increase near the equator and are important factors in WWB occurrences. Next, eddy growth was compared under different phases of MJO and ENSO. The MJO westerly phases of strong MJO events were classified into two groups, in terms of ENSO phases. Higher EKE values were found over the equatorial central Pacific in the WWB–ENSO correlated (pre–El Niño) periods. The energetics during these periods comported with those of the WWB generations. In the uncorrelated periods, the enhancement of eddy disturbances occurred far from the equator near the Philippines, where the activities of the easterly wave disturbances are well known. It is noteworthy that the enhanced region of the disturbances in the pre–El Niño periods coincided with the vicinity of large-scale MJO convection. It is suggested that coincidence corresponds with an enhancement of the internal disturbances embedded in the MJO, which is found only when the environmental conditions are favorable in association with ENSO.

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Chie Yokoyama and Yukari N. Takayabu

Abstract

Synoptic-scale westward-propagating disturbances over the eastern Pacific (EP) are analyzed in boreal autumn, utilizing spectral analysis, composite analysis, and energy budget analysis. The results are compared with those over the western Pacific (WP).

Spectral peaks of total precipitable water (TPW) and vertical velocity at 850 hPa (ω850), and outgoing longwave radiation (OLR) are detected at periods of ~3–7 days over the EP. Meanwhile over the WP, a spectral peak of OLR is pronounced, but peaks of TPW and ω850 are not detected. Composite analysis reveals that disturbances that have a coupled structure, with a vortex at its center near ~9°N and a mixed Rossby–gravity (MRG) wave–type disturbance, frequently exist over the EP. At the same time, the disturbances have a double-deck structure associated with divergence both in the upper and in the middle to lower troposphere. These disturbances are associated with both deep convection and congestus, which generate kinetic energy of the disturbance in the upper and in the lower troposphere, respectively.

Examining diabatic heating in relation to the coupled disturbances, deep heating with the peak at the height of ~7.5 km is greatest in the northeastern part of the vortex. The coupled MRG wave–type disturbance provides a relatively deep cross-equatorial southerly flow into the northeastern part of the vortex. It is suggested that deep rain is maintained with the existence of deep convergence produced by the coupled disturbances over the EP, where a very shallow convergence field exists on average.

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Chie Yokoyama and Yukari N. Takayabu

Abstract

Three-dimensional rain characteristics of tropical cyclones (TCs) are statistically quantified, using Tropical Rainfall Measuring Mission (TRMM) data from December 1997 to December 2003. Tropical cyclones are classified into four maximum intensity classes (<34, 34–64, 64–128, and ≥128 kt) and three stages (developing, mature, and decaying). First, rain characteristics of TCs are compared with those of the equatorial (10°N–10°S) mean. A notable finding here is that the average stratiform rain ratio (SRR), which is the contribution from stratiform rain in the total rainfall, of TCs is 52%, while it is 44% for the equatorial oceanic mean and 46% for the Madden–Julian oscillation in its mature phase. Stronger rain is observed in TCs both for convective and stratiform rain. Second, radial rain characteristics of TCs suggest that the region 0–60 km can be classified as “the inner core,” and 60–500 km as “the rainband.” The inner core is characterized with small SRR, very high rain-top height, and a large flash rate, indicating the vigor of convective activity. In contrast, the rainband is characterized with large SRR and relatively large rain yield per flash, indicating a large rainfall amount with a moderate convective activity. An important implication of this study is that TCs are listed in the high end of tropical oceanic organized rain systems, in terms of the organization levels of rain. Last, we use the above composite results to calculate the rainfall contribution of TCs to total annual rainfall between 35°N and 35°S as 3.3% ± 0.1%.

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Chie Yokoyama and Yukari N. Takayabu

Abstract

Differences in the characteristics of rain systems in the eastern Pacific (EP) intertropical convergence zone (ITCZ) and the western Pacific (WP) warm pool are quantitatively examined in relation to the large-scale environment. This study mainly uses precipitation feature (PF) data observed by the precipitation radar (PR) on board the Tropical Rainfall Measuring Mission (TRMM). The PFs are classified into four types according to their areas and maximum heights. Rain from tall unorganized systems and very tall organized systems tends to be dominant in high-SST regions such as the WP. On the other hand, the EP has more rain from congestus and organized systems with moderate heights than the WP. It is shown that shallow rain from congestus and moderately deep rain from organized systems are highly correlated with shallow (1000–925 hPa) convergence fields with coefficients of 0.75 and 0.66, respectively. These relationships between characteristics of rain systems and the large-scale environment are robust through all seasons.

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Ayako Seiki and Yukari N. Takayabu

Abstract

Statistical features of the relationship among westerly wind bursts (WWBs), the El Niño–Southern Oscillation (ENSO), and intraseasonal variations (ISVs) were examined using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis data (ERA-40) for the period of January 1979–August 2002. WWBs were detected over the Indian Ocean and the Pacific Ocean, but not over the Atlantic Ocean. WWB frequencies for each region were lag correlated with a sea surface temperature anomaly over the Niño-3 region. WWBs tended to occur in sequence, from the western to eastern Pacific, leading the El Niño peak by 9 months to 1 month, respectively, and after around 11 months, over the Indian Ocean. These results suggest that WWB occurrences are not random, but interactive with ENSO. Composite analysis revealed that most WWBs were associated with slowdowns of eastward-propagating convective regions like the Madden–Julian oscillation (MJO), with the intensified Rossby wave response. However, seasonal and interannual variations in MJO amplitude were not correlated with WWB frequency, while a strong MJO event tended to bear WWBs. It is suggested that the strong MJO amplitude promotes favorable conditions, but it is not the only factor influencing WWB frequency. An environment common to WWB generation in all regions was the existence of background westerlies around the WWB center near the equator. It is inferred that ENSO prepares a favorable environment for the structural transformation of an MJO, that is, the intensified Rossby wave response, that results in WWB generations. The role of the background wind fields on WWB generations will be discussed in a companion paper from the perspective of energetics.

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Hiroki Tsuji and Yukari N. Takayabu

Abstract

A significant enhancement of precipitation can result from the interplay between two independent, large-scale phenomena: an atmospheric river (AR) and a cutoff low. An AR is a long, narrow region with a deep moist layer. A cutoff low is an upper-level cyclonic eddy isolated from the meandering upper-level westerly jet. Herein, we construct composites of cutoff lows both close to an AR (AR-close category) and distant from an AR (AR-distant category) over a 14-yr period across the western North Pacific region. A comparison between the two categories shows an enhanced precipitation area to the northwest of the cutoff low and to the south of the AR axis in the AR-close category. The horizontal formation among the AR, cutoff low, and enhanced precipitation area in the composite coincides with that in a disastrous flood event that occurred in Hiroshima, Japan, in 2014. The deep moist layer associated with the AR, and the destabilization and isentropic up-gliding effect associated with the cutoff low are also observed in both the composite and the Hiroshima cases. We further evaluate the distribution of quasigeostrophic forcing (Q vector) for vertical motion. This shows that warm air advection associated with the AR overcomes the descending forcing inherent in the northwest of the cutoff low and makes the instability and up-gliding effect in that region more effective. These results indicate that the interplay between ARs and cutoff lows is a common mechanism in the enhancement of precipitation and the Hiroshima case is an extreme precipitation event caused by this interplay.

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

Abstract

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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Kazuaki Yasunaga, Kunio Yoneyama, Qoosaku Moteki, Mikiko Fujita, Yukari N. Takayabu, Junko Suzuki, Tomoki Ushiyama, and Brian Mapes

Abstract

A field observational campaign [i.e., the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO)] was conducted over the central equatorial Indian Ocean in October–December 2006. During MISMO, large-scale organized convection associated with a weak Madden–Julian oscillation (MJO) broke out, and some other notable variations were observed.

Water vapor and precipitation data show a prominent 3–4-day-period cycle associated with meridional wind υ variations. Filtered υ anomalies at midlevels in reanalysis data [i.e., the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS)] show westward phase velocities, and the structure is consistent with mixed Rossby–gravity waves. Estimated equivalent depths are a few tens of meters, typical of convectively coupled waves. In the more rainy part of MISMO (16–26 November), the 3–4-day waves were coherent through the lower and midtroposphere, while in the less active early November period midlevel υ fluctuations appear less connected to those at the surface.

SST diurnal variations were enhanced in light-wind and clear conditions. These coincided with westerly anomalies in prominent 6–8-day zonal wind variations with a deep nearly barotropic structure through the troposphere. Westward propagation and structure of time-filtered winds suggest n = 1 equatorial Rossby waves, but with estimated equivalent depth greater than is common for convectively coupled waves, although sheared background flow complicates the estimation somewhat.

An ensemble reanalysis [i.e., the AGCM for the Earth Simulator (AFES) Local Ensemble Transform Kalman Filter (LETKF) Experimental Reanalysis (ALERA)] shows enhanced spread among the ensemble members in the zonal confluence phase of these deep Rossby waves, suggesting that assimilating them excites rapidly growing differences among ensemble members.

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Xianan Jiang, Duane E. Waliser, William S. Olson, Wei-Kuo Tao, Tristan S. L’Ecuyer, King-Fai Li, Yuk L. Yung, Shoichi Shige, Stephen Lang, and Yukari N. Takayabu

Abstract

Capitalizing on recently released reanalysis datasets and diabatic heating estimates based on Tropical Rainfall Measuring Mission (TRMM), the authors have conducted a composite analysis of vertical anomalous heating structures associated with the Madden–Julian oscillation (MJO). Because diabatic heating lies at the heart of prevailing MJO theories, the intention of this effort is to provide new insights into the fundamental physics of the MJO. However, some discrepancies in the composite vertical MJO heating profiles are noted among the datasets, particularly between three reanalyses and three TRMM estimates. A westward tilting with altitude in the vertical heating structure of the MJO is clearly evident during its eastward propagation based on three reanalysis datasets, which is particularly pronounced when the MJO migrates from the equatorial eastern Indian Ocean (EEIO) to the western Pacific (WP). In contrast, this vertical tilt in heating structure is not readily seen in the three TRMM products. Moreover, a transition from a shallow to deep heating structure associated with the MJO is clearly evident in a pressure–time plot over both the EEIO and WP in three reanalysis datasets. Although this vertical heating structure transition is detectable over the WP in two TRMM products, it is weakly defined in another dataset over the WP and in all three TRMM datasets over the EEIO.

The vertical structures of radiative heating QR associated with the MJO are also analyzed based on TRMM and two reanalysis datasets. A westward vertical tilt in QR is apparent in all these datasets: that is, the low-level QR is largely in phase of convection, whereas QR in the upper troposphere lags the maximum convection. The results also suggest a potentially important role of radiative heating for the MJO, particularly over the Indian Ocean. Caveats in heating estimates based on both the reanalysis datasets and TRMM are briefly discussed.

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