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Takeshi Horinouchi

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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Takeshi Horinouchi

Abstract

Satellite-derived brightness temperature has been used to estimate tropical precipitation. Ricciardulli and Garcia applied it to quantify forcing of atmospheric waves that are excited by tropical cumulus convection and propagate into the middle atmosphere. Because of the broad coverage of the satellite data, this method provides exclusively dense information on wave forcing and is especially valuable for middle-atmosphere modeling. However, the validity of the method has not been investigated, which is done in this study using radar-derived precipitation during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) field experiment. The method is shown to overestimate the variance of precipitation so that the wave forcing derived with it is too strong. The overestimation is most severe at coarse resolution, reaching nearly an order of magnitude at a grid scale of 2°, which is comparable to typical resolutions of current global climate models. Although the comparison was made using data from a limited region in the western Pacific, it is suggested that the method overestimates wave forcing globally. The probability distribution of the mean radar precipitation on the meso-β scale fits well to the gamma distribution, while that for the satellite-derived precipitation does not. The latter shows a high probability of extreme grid mean precipitation, and this contributes to the overestimation. The frequency spectra of radar and satellite precipitation showed some similarity in shape, but differences are evident at subdiurnal frequencies. In addition to the satellite method, the Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI) is also investigated. GPI shows a similar, but better, performance to estimate wave forcing.

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Takeshi Horinouchi

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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Udai Shimada and Takeshi Horinouchi

Abstract

Strong vertical wind shear produces asymmetries in the eyewall structure of a tropical cyclone (TC) and is generally a hostile environment for TC intensification. Typhoon Noul (2015), however, reintensified and formed a closed eyewall despite 200–850-hPa vertical shear in excess of 11 m s−1. Noul’s reintensification and eyewall formation in strong shear were examined by using Doppler radar and surface observations. The evolution of the azimuthal-mean structure showed that the tangential wind at 2-km altitude increased from 30 to 45 m s−1 in only 5 h. During the first half of the reintensification, the azimuthal-mean inflow penetrated into the ~40-km radius, well inside the radius of maximum wind (RMW), at least below 4-km altitude, and reflectivity inside the RMW increased. As for the asymmetric evolution, vigorous convection, dominated by an azimuthal wavenumber-1 asymmetry, occurred in the downshear-left quadrant when shear started to increase and then moved upshear. A mesovortex formed inside the convective asymmetry on the upshear side. The direction of vortex tilt between the 1- and 5-km altitudes rotated cyclonically from the downshear-left to the upshear-right quadrant as the vortex was vertically aligned. In conjunction with the alignment, the amplitude of the wavenumber-1 convective asymmetry decreased and a closed eyewall formed. These features are consistent with the theory that a vortex can be vertically aligned through upshear precession. The analysis results suggest that the vortex tilt, vigorous convection, and subsequent intensification were triggered by the increase in shear in a convectively favorable environment.

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Takeshi Horinouchi and Shigeo Yoden

Abstract

The interaction between convectively excited waves and the mean zonal wind in the equatorial lower stratosphere is investigated with a simplified general circulation model (GCM). The model has T42 truncation, and the vertical resolution is about 700 m in the stratosphere. Although it is an “aquaplanet” model with uniform sea surface temperature, cumulus convection in low latitudes has realistic hierarchical structures with reasonable space–time spectral distributions. The model produced an oscillation having quite similar features to the equatorial quasi-biennial oscillation (QBO), although the period is 400 days.

Waves in the equatorial lower stratosphere of the model are excited mainly by the cumulus convection in low latitudes. The energy of these waves is a little larger than that observed in the real atmosphere. The dominant waves are gravity waves having an equivalent depth of about 200 m and those of 40–100 m. About half of the transport and deposition of zonal momentum contributing to the oscillation is accounted for by the gravest symmetric gravity modes: eastward momentum by Kelvin waves and westward momentum by n = 1 gravity waves. The momentum deposition is done over a wide range of zonal wavenumber (2–30), while about half of it is done over a period of 1–3 days. The deposition has rather continuous phase speed distributions and a considerable portion of it is provided by waves having critical levels. Since gravity waves with small intrinsic phase speeds have small vertical wavelengths, vertical grid spacings of 700 m or less appear to be required in the lower stratosphere for GCMs in order to simulate the QBO.

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Takeshi Horinouchi and Ayumu Hayashi

Abstract

It has been revealed that in summertime, precipitation is enhanced to the south of the upper-level tropopausal potential vorticity contours, which are accompanied by instantaneous jets, over the eastern coastal region of China to the northwestern Pacific. It is frequently exhibited as precipitation bands ranging in size from over a thousand to several thousands of kilometers long. In this study, an analysis was conducted to quantify the relationship depending on the phase of upper-level disturbances. With composite analysis, it is shown that the enhancement along the contours occurs at all phases; it occurs not only to the east but also to the west of the upper-level troughs, although it is weaker. The midtropospheric distributions of upwelling and the Q-vector convergence are collocated with the precipitation enhancement, suggesting the importance of dynamical induction by geostrophic flow at all phases. The effects of upper-level disturbances and low-level jets (LLJs) with a southerly component are investigated by using an idealized nondimensional quasigeostrophic model supporting latent heating. While upper-level waves induce upwelling and downwelling to the east and west, respectively, of the upper-level troughs, LLJs tend to offset the downwelling, enabling precipitation to the west too. Both in the observational composite and the idealized model with LLJ, confluence and diffluence contribute to the Q-vector convergence to induce upwelling along the subtropical jet irrespective of upper-level disturbance phases. This induction is explained as a general feature of a veered jet where geopotential isolines rotate clockwise with height without requiring wind variation along the jet.

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Takeshi Imamura, Takeshi Horinouchi, and Timothy J. Dunkerton

Abstract

A modified, equatorial Kelvin wave solution is obtained in the presence of the zonal-mean meridional circulation. The modified Kelvin wave solution, which is obtained via a perturbation expansion of the linearized, primitive equations on an equatorial β plane, possesses a nonzero meridional wind component. This meridional wind component is absent when the background flow is at rest. The combination of the meridional and zonal winds induces a meridional flux of zonal momentum in the upstream direction of the background north–south flow. This flux is divergent in latitude and produces a nonzero wave-induced force even though the waves are linear, steady, and conservative. It is shown that, although such a force violates the traditional nonacceleration theorem in which the mean meridional circulation is negligible at leading order, the result is in accord with a more general nonacceleration theorem obtained from the exact generalized Lagrangian-mean theory in which the mean meridional circulation is nonzero. The meridional circulation, in effect, attempts to advect wave pseudomomentum off the equator, resulting in a nonzero acceleration in the Eulerian reference frame. The meridional flux of momentum for any equatorially trapped mode is derived from the generalized Lagrangian-mean theory. Those modes with eastward (westward) intrinsic phase velocity transport eastward (westward) zonal momentum in the upstream direction of the background meridional flow in the neighborhood of the equator. It is also shown that the vertical flux of zonal momentum is not constant with altitude in a steady vertical flow since diabatic heating/cooling is needed to sustain the vertical wind. Implications of the results for the terrestrial and Venusian atmospheres are discussed.

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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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Hye-Yeong Chun, In-Sun Song, and Takeshi Horinouchi

Abstract

The momentum flux of convectively forced internal gravity waves is calculated using explicitly resolved model-simulated gravity wave data. The momentum flux in a control simulation with nonlinearity and cloud microphysical processes is compared with that in quasi-linear dry simulations with either diabatic forcing or nonlinear forcing. It is found that the momentum flux induced by either of these two sources is significantly different from each other and also from the momentum flux in the control simulation. This is because the spectral distribution and magnitude of each wave source are significantly different and the cancellation of the momentum flux by cross-correlation terms between the two sources cannot be included in the momentum flux by a single source. This suggests that a parameterization of convectively forced gravity waves must take into account nonlinear forcing as well as diabatic forcing in order to qualitatively and quantitatively represent the reference-level momentum flux spectrum.

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Steven C. Sherwood, Takeshi Horinouchi, and Heidi A. Zeleznik

Abstract

Observed temperature trends and interannual variations near the tropical tropopause suggest that temperatures up to the cold point are controlled by the troposphere, but some models indicate otherwise. Here, previous investigations of thermal anomalies and heating profiles associated with tropical convective outbreak are extended, by examining behavior near the tropopause. Observations show that active convective systems are locally associated with warm anomalies in the upper troposphere but cold anomalies in the lower troposphere and near the tropopause. Time-dependent solutions of Laplace's equations demonstrate that the cold anomaly below 100 hPa can be, at least partly, accounted for by adiabatic lofting associated with a transient heating pulse at lower levels. However, detailed examination of the cold-point tropopause in the data reveals that it moves against the lofting, downward toward higher pressure and colder potential temperatures, in response to convection. These variations qualitatively agree with longitudinal and ENSO-related variations in tropopause height and temperature reported in the literature, though seen here on hourly timescales. From this, local mesoscale diabatic cooling of several degrees kelvin per day close to the tropopause during active convection is inferred. This exceeds the likely contribution from cloud-top radiative cooling, suggesting a role for convective turbulence in refrigerating the tropopause.

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