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Satoru Yokoi

Abstract

This study conducts a multireanalysis comparison of variability in column water vapor (CWV) represented in three reanalysis products [Japanese 55-year Reanalysis Project (JRA-55), JRA-25, and ECMWF Interim Re-Analysis (ERA-Interim)] associated with the Madden–Julian oscillation (MJO) in boreal winter, with emphasis on CWV tendency simulated by forecast models and analysis increment calculated by data assimilation systems. Analyses of these variables show that, while the JRA-55 forecast model is able to simulate eastward propagation of the CWV anomaly, this model tends to weaken its amplitude. The multireanalysis comparison of the analysis increment further reveals that this weakening bias is related to excessively weak cloud radiative feedback represented by the model. This bias in the feedback strength makes anomalous moisture supply by the vertical advection term in the CWV budget equation too insensitive to precipitation anomaly, resulting in reduction of the amplitude of CWV anomaly. ERA-Interim has a nearly opposite feature: the forecast model represents excessively strong feedback. These results imply the necessity of accurate representation of the cloud radiative feedback strength for a short-term MJO forecast and may be evidence to support the argument that this feedback is essential for the existence of MJO. Furthermore, this study demonstrates that the multireanalysis comparison of the analysis increment will provide useful information for examining model biases and potentially for estimating parameters that are difficult to estimate from observational data, such as gross moist stability.

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Satoru Yokoi and Takehiko Satomura

Abstract

Mechanisms of the northward movement of submonthly scale vortices over the Bay of Bengal during the boreal summer (May–September) are studied with the use of a vorticity budget analysis applied to the ECMWF 40-yr Re-Analysis (ERA-40) data. To quantitatively evaluate the contribution from each term that constitutes the vorticity anomaly equation to the movement of the vortices, a vector measure, termed the forcing vector (FV), is used in the present study. Because the axis of the submonthly scale relative vorticity anomaly does not tilt meridionally below the 200-hPa level, the mechanisms of the northward movement of a composite submonthly scale vortex integrated from the surface to the 100-hPa level [the barotropic component (BTC)] are studied. The barotropic vortex moves northwestward, with northward speeds of 0.9° day−1. The meridional component of the FV (MFV), which represents the contribution to the meridional component of the movement, reveals that the primary and secondary terms that contribute to the northward movement are the advection of the vortex by the environmental meridional wind, and the tilting effect of the environmental horizontal vorticity vector by the vertical pressure velocity anomaly associated with the vortex, respectively. The former term works mainly in the lower troposphere, while the latter operates in the middle and upper troposphere. The first baroclinic component (FBCC) of the vortex in the troposphere also moves northwestward with almost the same northward speed as the BTC. Mechanisms of the northward movement of the FBCC are also clarified in the present study through examination of the MFV. The primary contributing term is the same as that of the BTC, while the tilting term hinders the northward movement of the FBCC. For the FBCC, the secondary contributing term is the advection of the planetary vorticity by the meridional wind anomaly associated with the horizontal convergence and divergence anomalies in the lower and upper troposphere, respectively. The present study also discusses the phase relation between the BTC and the FBCC from the viewpoint of their northward movement in an environment of easterly vertical shear.

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Satoru Yokoi and Takehiko Satomura

Abstract

This study reveals remarkable differences in the geographical distribution of variance between two types of intraseasonal variations in daily-mean radar reflectivity data in the western part of the Indochina Peninsula. In this region, the Downa Range lies parallel to the coast and separates the inland region from the coastal region. The 30–60-day variation of reflectivity factor dominates most of the coastal region, while its variance in the inland region is less than that of background red noise with the same frequency band. Horizontal gradients in the variance are largest over the range, implying that the mountain range plays a significant role in the geographical contrast. Correlation analysis with reanalysis data shows that the variation only in the coastal region is associated with a synoptic-scale zonal wind anomaly with the same time scale, suggesting the importance of an orographic rainfall process that brings a large amount of precipitation only to the windward side of the Downa Range. In contrast, while the 10–20-day variation of reflectivity factor has larger variance in the inland region than in the coastal region, the variation in both of the regions is correlated with synoptic-scale cyclonic circulation anomaly. A possible reason for the differences between the two types is also discussed in terms of the relationship between synoptic-scale wind anomaly field and the orientation of the Downa Range.

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Satoru Yokoi and Jun Matsumoto

Abstract

This paper reveals synoptic-scale atmospheric conditions over the South China Sea (SCS) that cause heavy rainfall in central Vietnam through case study and composite analyses. The heavy rainfall event discussed in this study occurred on 2–3 November 1999. Precipitation in Hue city (central Vietnam) was more than 1800 mm for these 2 days. Two atmospheric disturbances played key roles in this heavy rainfall. First, a cold surge (CS) northerly wind anomaly in the lower troposphere, originating in northern China near 40°N, propagated southward to reach the northern SCS and then lingered there for a couple of days, resulting in stronger-than-usual northeasterly winds continuously blowing into the Indochina Peninsula against the Annam Range. Second, a southerly wind anomaly over the central SCS, associated with a tropical depression–type disturbance (TDD) in southern Vietnam, seemed to prevent the CS from propagating farther southward. Over the northern SCS, the southerly wind anomaly formed a strong low-level convergence in conjunction with the CS northeasterly wind anomaly, and supplied warm and humid tropical air. These conditions induced by the CS and TDD are favorable for the occurrence of the heavy orographic rainfall in central Vietnam. The TDD can be regarded as a result of a Rossby wave response to a large-scale convective anomaly over the Maritime Continent associated with equatorial intraseasonal variability.

Using a 24-yr (1979–2002) reanalysis and surface precipitation datasets, the authors confirm that the coexistence of the CS and TDD is important for the occurrence of heavy precipitation in central Vietnam. In addition, it is observed that CSs without a TDD do not lead to much precipitation.

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Satoru Yokoi and Takehiko Satomura

Abstract

In the present study, intraseasonal variations of precipitation over Thailand [Thailand daily rainfall (TDR)] during 1998 and the accompanying variations of the tropospheric circulation are analyzed, mainly with the use of rain gauge data and the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis version 1.5.

Wavelet analysis reveals that the amplitudes of the intraseasonal variations of the 1998 TDR exhibit two peaks. One peak is categorized as a 30–60-day variation that appeared in the early half of the rainy season, while the other peak is categorized as a 10–20-day variation that appears in the later half of the rainy season.

The variation of atmospheric circulation associated with the latter peak of the TDR is analyzed. When the 10–20-day variation of the TDR is in the most active phase, a negative streamfunction anomaly at the 850-hPa level exists over the Bay of Bengal, and a positive anomaly is found in the Southern Hemisphere to the south of the negative anomaly. Both anomalies are associated with cyclonic vortices and are accompanied by negative height anomalies at the same level. Such horizontal structures resemble the horizontal structure of an equatorial Rossby wave of the gravest meridional mode (n1ERW). The cyclonic disturbances move westward with speeds of 6–6.4 m s−1.

In the vertical, the disturbances have a first-baroclinic structure in the troposphere, with a node at the 300–400-hPa levels. In addition, the amplitude of the streamfunction anomaly in the lower troposphere is smaller than that in the upper troposphere over the western Pacific but larger than that in the upper troposphere over the Bay of Bengal.

The mechanism for the longitudinal difference in the vertical structure and comparison of the analyzed disturbance with n1ERW-type disturbances over the Pacific are also discussed.

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Satoru Yokoi and Yukari N. Takayabu

Abstract

Variability in tropical cyclone (TC) activity is a matter of direct concern for affected populations. On interannual and longer time scales, variability in TC passage frequency can be associated with total TC frequency over the concerned ocean basin [basinwide frequency (BF)], the spatial distribution of TC genesis in the basin [genesis distribution (GD)], and the preferable track (PT) that can be considered as a function of genesis locations. To facilitate investigation of mechanisms responsible for the variability, the authors propose an approach of decomposing anomalies in the passage frequency into contributions of variability in BF, GD, and PT, which is named the Integration of Statistics on TC Activity by Genesis Location (ISTAGL) analysis. Application of this approach to TC best track data in the western North Pacific (WNP) basin reveals that overall distribution of the passage frequency trends over the 1961–2010 period is mainly due to the PT trends. On decadal time scales, passage frequency variability in midlatitudes is primarily due to PT variability, while the BF and GD also play roles in the subtropics. The authors further discuss decadal variability over the East China Sea in detail. The authors demonstrate that northward shift of the PT for TCs generated around the Philippines Sea and westward shift for TCs generated in the eastern part of the WNP contribute the variability with almost equal degree. The relationships between these PT shifts and anomalies in environmental circulation fields are also discussed.

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Satoru Yokoi, Takehiko Satomura, and Jun Matsumoto

Abstract

With the use of daily rain gauge data observed at 210 stations in the Indochina Peninsula (ICP) for the 26 yr from 1978 to 2003, this paper describes climatological characteristics of 2 types of intraseasonal variations (ISVs): the 30–60-day variation (30–60DV) and the 10–20-day variation (10–20DV). The authors find that these characteristics are quite different from place to place in the ICP.

During the rainy season, variance of the 30–60DV is generally larger in coastal regions than over inland regions and it has two local maxima: one found in the coastal region of Myanmar (CMY) and the other in the southern Laos and central Vietnam region (SLCV). Wavelet analysis reveals that the 30–60DV in the CMY is active throughout the rainy season (May–October) and exhibits the maximum activity in May–June. In addition, its typical time scale shifts from 40 days in the early half of the rainy season to 50 days in the latter half. Cross-correlation analysis reveals that its signal propagates northward. On the other hand, the 30–60DV in the SLCV is active only during July–October, and its signal propagates northwestward.

The largest variance of the 10–20DV is found in the coastal regions of northern and central Vietnam (CNCV), while the variance in other coastal regions is generally smaller than that in inland regions. In contrast to the 30–60DV, the 10–20DV activity varies significantly over the course of the rainy season. The 10–20DV in the inland regions is active in May and September and inactive in July, while that in the CNCV is active during August–November. The 10–20DV exhibits high spatial coherence over most of the ICP, and its signal propagates west-northwestward.

Relationship of the ISV in the ICP with synoptic-scale ISV structures is also discussed.

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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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Kazuaki Yasunaga, Satoru Yokoi, Kuniaki Inoue, and Brian E. Mapes

Abstract

The budget of column-integrated moist static energy (MSE) is examined in wavenumber–frequency transforms of longitude–time sections over the tropical belt. Cross-spectra with satellite-derived precipitation (TRMM-3B42) are used to emphasize precipitation-coherent signals in reanalysis [ERA-Interim (ERAI)] estimates of each term in the budget equation. Results reveal different budget balances in convectively coupled equatorial waves (CCEWs) as well as in the Madden–Julian oscillation (MJO) and tropical depression (TD)-type disturbances. The real component (expressing amplification or damping of amplitude) for horizontal advection is modest for most wave types but substantially damps the MJO. Its imaginary component is hugely positive (it acts to advance phase) in TD-type disturbances and is positive for MJO and equatorial Rossby (ERn1) wave disturbances (almost negligible for the other CCEWs). The real component of vertical advection is negatively correlated (damping effect) with precipitation with a magnitude of approximately 10% of total latent heat release for all disturbances except for TD-type disturbance. This effect is overestimated by a factor of 2 or more if advection is computed using the time–zonal mean MSE, suggesting that nonlinear correlations between ascent and humidity would be positive (amplification effect). ERAI-estimated radiative heating has a positive real part, reinforcing precipitation-correlated MSE excursions. The magnitude is up to 14% of latent heating for the MJO and much less for other waves. ERAI-estimated surface flux has a small effect but acts to amplify MJO and ERn1 waves. The imaginary component of budget residuals is large and systematically positive, suggesting that the reanalysis model’s physical MSE sources would not act to propagate the precipitation-associated MSE anomalies properly.

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Satoru Yokoi, Shuichi Mori, Fadli Syamsudin, Urip Haryoko, and Biao Geng

Abstract

The diurnal cycle over tropical coastal waters is characterized by offshore migration of precipitation area during nighttime. This study analyzes in situ observational data collected during the YMC-Sumatra 2017 field campaign around the western coast of Sumatra Island, Indonesia, to examine the offshore migration phenomenon during 5–31 December 2017, when the Research Vessel Mirai was deployed about 90 km off the coast to perform observation. The offshore migration is observed in only less than a half of the 27 days. A comparison of radiosonde data at the vessel between days with and without the offshore migration reveals that vertical wind shear in the lower troposphere is a key environmental condition. In late afternoon of the days with the offshore migration, offshore (northeasterly) wind shear with height with considerable magnitude is observed, which is due to weaker daily mean southwesterly wind in the lower free troposphere, stronger southwesterly wind in the boundary layer, and sea breeze. As this condition is considered favorable for regeneration of convective cells to the offshore side of old ones, these results support an idea that the regeneration process is critical for the offshore migration. The Madden–Julian oscillation and cold surges play some roles in the weakening of the free-tropospheric wind. The migration speed is estimated at 2–3 m s−1, which is lower than that observed in another field campaign conducted in 2015 (Pre-YMC 2015). This difference is partly due to the difference in the environmental wind in the lower to midtroposphere.

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