Abstract
The East Asian summer climate displays a marked change after the late 1990s. This is principally due to a weakening of the Pacific–Japan (PJ) teleconnection pattern that was a dominant ...driver of precipitation variability over East Asia. Nevertheless, western Japan has frequently experienced heavy rainfall events over the past several years. Atmospheric reanalysis and observational datasets are used to investigate summer precipitation variability over East Asia in a view of interdecadal changes from 1979 to 2020. East Asian summer precipitation has increased the most in Japan, especially over and around the Southwest Islands in southern Japan, where cumulus convection has been more dominant for the characteristic of precipitation variability including mei-yu–baiu rainfall than frontal structure since the mid-2000s. Atmospheric analysis in vertical structures and convective instability indicates that moist instability is neutralized by cumulus convection over the southern East China Sea and off the Pacific coast of Japan, where recent warming in sea surface temperature (SST) along the Kuroshio exceeds the SST threshold for convection. This decadal shift in precipitation variability has a close relationship with the second precipitation mode over East Asia, which has taken the place of the PJ pattern as a leading driver of precipitation variability over western Japan in the past decade. The enhanced cumulus convection is enabled to act as a forcing mechanism for the Rossby wave train from East Asia toward North America along the westerly jet or for the eastward extension of the Silk Road pattern, which possibly favors a heatwave in the Pacific Northwest areas of North America.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Estimation of the radius of maximum wind (RMW) of tropical cyclone (TC) is helpful for the disaster prevention and mitigation. If RMWs are estimated from infrared (IR) imagery taken by ...geostationary meteorological satellites, their estimation is available densely in time, regardless of the ocean basin. Kossin et al. showed that when TCs have clear eyes, the eye radii estimated from IR images have a high correlation with the RMW estimated from aircraft reconnaissance. The regression of the former onto that latter was shown to have a mean absolute error (MAE) of 4.7 km. We revisit the IR-based RMW estimation by using C-band synthetic aperture radar (SAR) sea surface wind estimates. The criteria for selecting clear-eye cases are simplified. The MAE of the Kossin et al. method is found to be smaller than previously suggested: 3.1 km when the proposed relation is used and 2.7 km when the regression is revised with the SAR-measured RMWs. We further propose an improvement of the IR-based method to estimate the eye radii. The resultant MAE is shown to be 1.7 km, which indicates that the IR-based RMW estimation is more accurate than has been suggested. A strong correlation between eyewall slope and eye size is confirmed. We also investigated cloud features in the eye that may be closely related to RMW and wind structure around RMW. Potential applications of highly accurate RMW estimation are discussed.
Significance Statement
The radius of maximum wind (RMW) of tropical cyclone (TC) is an important factor for TC intensity estimation and disaster prevention. A previous study suggested that the RMWs of TCs with clear eyes can be estimated from geostationary satellite images at a mean absolute error (MAE) of 4.7 km. Here we improved the method, reducing the MAE by more than one-half. Since the method does not require aircraft or satellite in low Earth orbit, it helps TC monitoring at high frequency. The method can also improve initialization of models used to predict TC hazards and further our physical understanding and the climatology of the wind structures near the centers of TCs.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This study provides an overall understanding of the summertime synoptic variability of precipitation and moisture transport at mid-latitude from the eastern coastal region of China to the ...northwestern Pacific. Using satellite precipitation and reanalysis data, a clear relationship is found between upper tropospheric disturbances (Rossby waves), surface precipitation, and lower tropospheric humidity through July and August. The upper tropospheric disturbances are characterized by the undulation of the 1.5 PVU contours of potential vorticity (PV) on the 350 K isentropic surfaces. Case studies suggest that a precipitation band of several hundred kilometers wide and a thousand to several thousand kilometers long is formed very frequently on the equatorward and low-PV side of the northernmost 1.5 PVU contours, which meander together around 40°N. Lower tropospheric specific humidity is also enhanced there, and it falls sharply to the north of these contours. The synoptic situations associated with it include, but are not limited to, a common situation in which moist convection is enhanced ahead of upper-level troughs. These results are confirmed by a composite analysis over the 12 summers from 2001. A novel method of analyzing the forcing of the quasi-geostrophic potential enstrophy, in which boundary contributions are incorporated, reveals that upper tropospheric disturbances in the area are propagated predominantly from the west along the Asian jet, and that they exert a significant forcing onto near-surface levels, while the upward forcing from near-surface levels to upper tropospheric disturbances is weak. A Q-vector analysis shows that the upwelling associated with the precipitation bands is forced predominantly by confluence. This process is frontogenetic, and surface fronts are often formed therein. The upwelling is enhanced by latent heating. The latitudinal extent of humid air masses is affected not only by this circulation but by low-level flows induced by upper-level disturbances in a cooperative manner.
Observations of the winds in tropical cyclones are still limited. We propose a new method for deriving the tangential winds in tropical cyclones, which employs a spectral analysis of high‐frequency ...cloud imaging by latest‐generation geostationary meteorological satellites such as Himawari‐8. The method was applied to the visible images of boundary layer clouds in the eye of Typhoon Lan (2017) over an 8.5‐hr period. The low‐level tangential winds over the central two thirds of the eye in radius were close to a rigid body rotation and increased with time. On its outside was a region with striating clouds rotating at much higher angular velocities, which may have been supergradient. Asymmetric motions were visualized as the deviation from the inner rotation, and the vorticity of some mesovortices were quantified. These asymmetric motions are suggested to transport angular momentum to accelerate the inner rotation.
Plain Language Summary
Observations of the winds in tropical cyclones (TCs) are still limited. The latest‐generation geostationary meteorological satellites such as Himawari‐8 capture TCs' cloud motions that were previously unobservable. We propose a method for deriving the tangential winds in TCs using a spectral analysis of high‐frequency cloud imaging by these satellites. The proposed method was applied to the visible images of lower clouds in the eye of Typhoon Lan (2017) over an 8.5‐hr period of daytime observations. The low‐level tangential winds over the central two thirds of the eye in radius were close to a rigid body rotation (rotational motion which has no deforming component), and its speed was increased with time. On its outside was a region with striating clouds rotating at much higher angular velocities. Asymmetric motions were visualized as the deviation from the inner rigid body rotation, and the vorticity of some mesovortices (small scale rotational features found in the TCs) were quantified. These asymmetric motions are suggested to transport angular momentum to accelerate the inner rotation. This study proves the usefulness of geostationary satellites to diagnose and study wind structures of TCs.
Key Points
The latest‐generation geostationary meteorological satellites capture tropical cyclones' cloud motions that were previously unobservable
A new method for estimating the tangential winds of tropical cyclones by using space‐time Fourier analysis of satellite images is proposed
The method was applied to Typhoon Lan (2017), and residual asymmetric motions were derived, providing a diagnosis on its intensification
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The western Pacific subtropical high (WPSH) has a significant effect on droughts, heat waves, and tropical cyclone tracks over East Asia and the northwest Pacific. The WPSH has intensified during the ...past three decades, but its causes are not yet well understood. Here we show that the Pacific Decadal Oscillation (PDO) is responsible for the long-term changes in the WPSH through the meridional shift of the subtropical jet, based on comprehensive data analysis and model results. El Niño-Southern Oscillation (ENSO) is the leading forcing of WPSH variability over interannual timescales, whereas the PDO accounts for its low-frequency variability, resulting in it being independent of ENSO with regard to WPSH variability. The PDO in summer can be interpreted as a coupling with the WPSH. Our results provide useful information for projecting long-term changes in the WPSH.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Until recently, the lower to middle cloud region of Venus had been supposed to be dynamically quiet, accommodating nearly steady superrotating westward flow. However, observations of the regions by ...Akatsuki, the latest Venus orbiter operating since 2015, have revealed a variety of cloud features indicative of vortices and waves. Here we report another, and arguably the most conspicuous, example. Akatsuki's near‐infrared imager IR2 captured gigantic vortices rotating cyclonically on 25 August 2016. By using winds estimated by cloud tracking, the feature is shown to be quantitatively consistent with barotropic instability. The size of the vortices (∼1,000 km) and their spacing (∼2,500 km) are more than several times greater than the vortex‐like features reported previously from the observations of Venus, and they are also greater than the largest barotropic instability observed in the Earth's troposphere.
Plain Language Summary
Hydrodynamical instabilities play important roles in the general circulation of the Earth and planetary atmospheres. Barotropic instability is a kind of instability that arises from horizontal differences in predominantly parallel horizontal flows. We report herewith the first concrete evidence of its occurrence in the atmosphere of Venus. Before this study, reports are limited to vortex‐like cloud features whose appearance is consistent with this instability, but no analyses of flows have been conducted before. The cloud feature like a vortex street reported in this study has a spatial scale far greater than any of previously reported ones, and our study shows that it is dynamically consistent with barotropic instability.
Key Points
An event of barotropic instability, whose scale is greater than 1,000 km, was found by the Akatsuki Venus orbiter
The discovery reinforces the recent view that the lower to middle cloud regions are dynamically active, promoting further studies
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Abstract
Aquaplanet simulations for a given sea surface temperature (SST) are conducted to elucidate possible roles of transient variability in the Hadley circulation and the intertropical ...convergence zone (ITCZ). Their roles are best illustrated with globally uniform SSTs. For such SSTs, an ITCZ and a Hadley circulation that are nearly equatorially symmetric emerge spontaneously. Their strength varies over a wide range from being faint to climatologically significant depending on a tunable parameter of the model’s cumulus parameterization. In some cases asymmetric Hadley circulations formed along with long-lived tropical cyclones.
The tunable parameter affects the transient variability of tropical precipitation. In the runs in which well-defined near-symmetric ITCZs formed, tropical precipitation exhibited clear signatures of convectively coupled equatorial waves. The waves can explain the concentration of precipitation to the equatorial region, which induces the Hadley circulation. Also, the meridional structures of simulated ITCZs are consistent with the distribution of convergence/divergence associated with dominant equatorial wave modes.
Even when the pole–equator temperature gradient is introduced, the dependence of the strength of the circulation to transient disturbances remains. Therefore, transient variability may have a broader impact on tropical climate and its numerical modeling than has been thought.
The reason that a wide variety of circulation is possible when the SST gradient is weak is because the distribution of latent heating can be interactively adjusted while a circulation is formed. Angular momentum budget does not provide an effective thermodynamic constraint, since baroclinic instability redistributes the angular momentum.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The relationship between the interannual variations of the activity of convectively coupled equatorial waves and seasonal mean precipitation in the tropical western to central Pacific Ocean ...is investigated. It is found that the convectively coupled mixed Rossby–gravity (MRG) waves are highly and negatively correlated with the seasonal precipitation near the equator in boreal summer. It is suggested that the MRG waves, which have convection centers off the equator, suppress the equatorial precipitation. The relation is insignificant in the other seasons, when the interannual variation of sea surface temperature near the equator is greater than in boreal summer. Also, a similar relation is not found in the eastern Pacific in any season.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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