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Wook Jang and Hye-Yeong Chun

Abstract

The effects of topography on the evolution of Typhoon Saomai (2006) are investigated by conducting a series of numerical simulations with the Weather Research and Forecasting (WRF) Model using 100%, 75%, 50%, and 25% of terrain heights of the Central Mountain Range (CMR) in Taiwan. Differences in the track and intensity of Typhoon Saomai between the experiments are strongly related to those of Tropical Storm Bopha, which passed Taiwan earlier than the typhoon. In the sensitivity experiments, the higher CMR drifts Bopha more southward, which results in the weakening of Bopha by prohibiting the interaction between the CMR and Bopha, and the flows induced by Bopha force Saomai to propagate along a more southerly track. The higher CMR weakens the easterly flow in the lower troposphere and suppresses the northerly flow in the upper troposphere to the west of Saomai. The resultant weak vertical wind shear keeps warm air near the typhoon center in the upper troposphere, which promotes the intensification of the typhoon. To examine the direct effects of topography on the track and intensity of Saomai, additional simulations involving the removal of Bopha from the initial condition with 100% and 50% of CMR are conducted. The results without Bopha showed that Saomai moves more southward at a slower speed and with greater intensity, due to the stronger northerly wind to the west of Saomai, which was not canceled out by the southerly wind to the east of Bopha, and there is no significant difference in the tracks or intensity with respect to the mountain heights.

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Wook Jang and Hye-Yeong Chun

Abstract

The statistical and dynamical characteristics of binary tropical cyclones (TCs) observed in the western North Pacific (WNP) for 62 years (1951–2012) are investigated by using best track and reanalysis data. A total of 98 binary TCs occurred with an annual average of 1.58. The occurrence frequency of binary TCs shows significant year-to-year variations and there are two peaks in the mid-1960s and early 1990s. Three-fourths (76.3%) of the binary TCs occurred between July and September, which is consistent with the high activity season of TCs in general. A relatively higher track density for binary TCs is present to the east of the maximum track density for total TCs. This result is likely due to the differences in the locations of genesis and environmental steering flow between binary and total TCs. The poleward steering flow, weaker vertical wind shear, and warmer sea surface temperature are pronounced for binary TCs, and these result in a longer lifetime of TCs, which can increase the chances that they would be detected as binary TCs. By applying the clustering analysis technique, six representative trajectories of the binary TCs are obtained. The transitional speed and recurving location are significantly different with respect to the clustered types. The trajectories of each type are strongly related to the temporal variations in the environmental steering flow and the location of the North Pacific high.

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So-Young Kim and Hye-Yeong Chun

Abstract

Stratospheric gravity waves generated by Typhoon Saomai (2006) were simulated using a mesoscale model in a moving frame of reference following the typhoon. Waves with large amplitudes appear near the domain center because of strong convection in the eyewall of the typhoon. Convection bands propagating outward from the storm center also generate waves propagating to the stratosphere. Convective forcing is significant in various propagation directions, with maximum power in slowly moving eastward components due to convection in the eyewall. The forcing exhibits large amplitude at a speed of 8–16 m s−1 in the eastward direction in which spiral bands are mainly developed. Induced gravity waves in the stratosphere are dominant in the eastward, northeastward, and southeastward propagation directions, since westward waves are mostly filtered by the background wind below z = 25 km. While the typhoon moves northwestward for 78 h, the wave characteristics vary through time depending on the evolution of the eyewall and spiral bands. Horizontal wavelengths of waves are longer in the mature and decaying stages than in the developing stage of the typhoon, likely because of a more dominant concentric eyewall in the mature and decaying stages. The spectral peak of the waves is at ∼20 km (∼50 km) horizontal wavelength in the developing (mature) stage, and the wave amplitudes are larger in the developing stages. The dominant contribution to the momentum flux is from waves with horizontal wavelengths longer than 80 km. Positive momentum flux decreases with overall height and the resultant positive drag can cause deceleration of northeasterly background wind. Sensitivity of the model results to horizontal resolution reveals that small-scale waves resolved in the present simulations with 3-km resolution cannot be fully represented with 9- or 27-km resolutions.

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Dan-Bi Lee and Hye-Yeong Chun

Abstract

At 0247 UTC 13 February 2013, a South Korean commercial aircraft encountered moderate-level clear-air turbulence at ~24 000 ft (~7.3 km) over the Yellow Sea (121.25°E, 38.55°N) en route from Incheon, South Korea, to Tianjin, China. Two crew members were severely injured by this event. To investigate the possible mechanisms of this event, a high-resolution numerical simulation using the Weather Research and Forecasting Model was conducted. In the synoptic-scale flow pattern, one of two bifurcated jet streams passed over the Yellow Sea, and strong horizontal and vertical gradients of the wind occurred on the northern edge of the jet stream near the flight route. An upper-level frontal system on the cyclonic shear side of the jet intensified as it moved northward toward a strengthening upper-level trough in northeastern China. The developed jet–frontal system induced strong vertical wind shear and tropopause folding, which extended down to about z = 5 km, near the observed turbulence region. Despite a relatively high stability with an intrusion of stratospheric air with tropopause folding, the strong vertical wind shear led to a small Richardson number in the incident region, which in turn induced the aviation turbulence through the Kelvin–Helmholtz instability. Although small-scale mountain waves were evident during the passage of flight before the incident time, breaking of these waves was not likely the key factor for the observed turbulence, given that the wave amplitudes were weak and that the strong zonal wind on the upstream of the mountain waves prohibited wave saturation and breakdown.

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Yuh-Lang Lin and Hye-Yeong Chun

Abstract

The response of a two-dimensional, stably stratified shear flow to diabatic cooling, which represents the evaporative cooling of falling precipitation in the subcloud layer, is examined using both a linear analytical theory and a nonlinear numerical model. The ambient wind is allowed to reverse its direction at a certain height and the cooling is specified from the surface to a height below the wind reversal level.

From a scale analysis of the governing equations a nonlinearity factor of the thermally induced finite-amplitude wave, gQ0l(cpT0U02N), is found. From a scale analysis of the linear system, it is shown that the wind shear can modify the condition in which the upstream propagation of the density current is opposed by the ambient wind. When the shear and the basic wind are of opposite sign, small basic wind is enough to prevent the upstream propagation of the density current. This is because part of the cooling is used to compensate the positive vorticity associated with the positive wind shear. Therefore, the effective cooling rate, or the speed of the density current, becomes smaller than that in the uniform wind case.

In the nonlinear numerical simulations, it is found that the response of the atmosphere to a steady cooling in a shear flow may be categorized as either a stationary cold pool or a density current, depending upon the strength of the effective cooling. For a strong shear flow, the cold pool is stationary with respect to the upstream flow because most of the cooling is used to compensate the positive vorticity associated with the positive wind shear. In this case, the response is similar to the linear steady-state case. For a weak shear flow, the cold pool is able to propagate upstream because the effective cooling, which increases with time, is strong enough to push the outflow against the basic wind. From the comparison of linear and nonlinear numerical model simulations, it is found that the nonlinearity appears to reduce the wave disturbance below the critical height and above the cooling top, while it tends to strengthen the density current or cold pool near the surface.

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Hye-Yeong Chun and Jong-Jin Baik

Abstract

Gravity wave momentum flux induced by thermal forcing representing latent heating due to cumulus convection is investigated analytically from a viewpoint of a subgrid-scale drag for the large-scale flow, and a possible way to parameterize the momentum flux in large-scale models is proposed. For the formulations of the momentum flux and its vertical derivative, two-dimensional, steady-state, linear perturbations induced by thermal forcing in a uniform basic-state wind are considered. The calculated momentum flux is zero below the forcing bottom, varies with height in the forcing region, and remains constant above the forcing top with the forcing top value. The sign of the momentum flux at the forcing top depends on the basic-state wind according to the wave energy–momentum flux relationship. Inside the forcing region, there exists a vertical convergence or divergence of the momentum flux that can influence the zonal mean flow tendency. The maximum magnitude of the zonal mean flow tendency contributed by the wave momentum flux in the forcing region is as large as 24 m s−1 d−1.

A parameterization scheme of subgrid-scale convection-induced gravity wave momentum flux for use in large-scale models is proposed. Even though the momentum flux in the cloud region can be parameterized based on the analytical formulation, it is not practically applied in large-scale models because subgrid-scale diabatic forcing considered in this study comes from cumulus parameterization that is activated only in a conditionally unstable atmosphere. Thus, the convection-induced momentum flux is parameterized from the cloud-top height. The momentum flux at the cloud-top height is parameterized based on the analytical formulation, while above it two methods can be used following mountain drag parameterization. One method is to specify a linearly decreasing vertical profile with height and the other is to apply the wave saturation theory in terms of the Richardson number criterion. The formulations of the minimum Richardson number and saturation momentum flux are surprisingly analogous to those in mountain drag parameterization except that the nonlinearity factor of thermally induced waves is used instead of the Froude number. Gravity wave drag by convection can have a relatively strong impact on the large-scale flow in midlatitude summertime when the surface wind and stability are weak and in the tropical area where deep cumulus convection persistently exists.

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Jung-Hoon Kim and Hye-Yeong Chun

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On 2 April 2007, nine cases of moderate-or-greater-level clear-air turbulence (CAT) were observed from pilot reports over South Korea during the 6.5 h from 0200 to 0830 UTC. Those CAT events occurred in three different regions of South Korea: the west coast, Jeju Island, and the eastern mountain areas. The characteristics and possible mechanisms of the CAT events in the different regions are investigated using the Weather Research and Forecasting model. The simulation consists of six nested domains focused on the Korean Peninsula, with the finest horizontal grid spacing of 0.37 km. The simulated wind and temperature fields in a 30-km coarse domain are in good agreement with those of the Regional Data Assimilation and Prediction System (RDAPS) analysis data of the Korean Meteorological Administration and observed soundings of operational radiosondes over South Korea. In synoptic features, an upper-level front associated with strong meridional temperature gradients is intensified, and the jet stream passing through the central part of the Korean Peninsula exceeds 70 m s−1. Location and timing of the observed CAT events are reproduced in the finest domains of the simulated results in three different regions. Generation mechanisms of the CAT events revealed in the model results are somewhat different in the three regions. In the west coast area, the tropopause is deeply folded down to about z = 4 km because of the strengthening of an upper-level front, and the maximized vertical wind shear below the jet core produces localized turbulence. In the Jeju Island area, localized mixing and turbulence are generated on the anticyclonic shear side of the enhanced jet, where inertial instability and ageostrophic flow are intensified in the lee side of the convective system. In the eastern mountain area, large-amplitude gravity waves induced by complex terrain propagate vertically and subsequently break down over the lee side of topography, causing localized turbulence. For most of the CAT processes considered, except for the mountain-wave breaking, standard NWP resolutions of tens of kilometers are adequate to capture the CAT events.

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Hye-Yeong Chun and Jong-Jin Baik

Abstract

The weakly nonlinear response of a two-dimensional stably stratified atmosphere to prescribed diabatic heating in a uniform flow is investigated analytically using perturbation expansion in a small value of the nonlinearity factor for the thermally induced waves. The diabatic heating is assumed to have only a zeroth-order term specified to be vertically homogeneous between the surface and a certain height and bell shaped in the horizontal. The first-order (weakly nonlinear) solutions are obtained using the FFT algorithm after solutions in wavenumber space are obtained analytically. The forcing (F) to the first-order perturbation equation induced by the Jacobian of the zeroth-order (linear) solutions always represents cooling in the lower layer regardless of specified forcing type (cooling or heating). The vertical structure of F is related to the nondimensional heating depth (d) or the inverse Froude number. The first-order solutions are valid for relatively small values (<3) of d. The main nonlinear effect is to produce a strong convergence region near the surface associated with the zeroth-order perturbations regardless of the value of d. This convergence is responsible for producing upward motion in the center of the forcing region that extends upstream. As a result, the zeroth-order downward motion becomes weaker according to a degree of nonlinearity. The relative magnitude of the zeroth-order downward motion and the first-order upward motion upstream of the forcing can be determined by d. The source of the first-order wave energy is found to come mainly from the horizontal advection of the zeroth-order total wave energy by the first-order perturbation horizontal wind.

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Hyun-Joo Choi and Hye-Yeong Chun

Abstract

The excessively strong polar jet and cold pole in the Southern Hemisphere winter stratosphere are systematic biases in most global climate models and are related to underestimated wave drag in the winter extratropical stratosphere—namely, missing gravity wave drag (GWD). Cumulus convection is strong in the winter extratropics in association with storm-track regions; thus, convective GWD could be one of the missing GWDs in models that do not adopt source-based nonorographic GWD parameterizations. In this study, the authors use the Whole Atmosphere Community Climate Model (WACCM) and show that the zonal-mean wind and temperature biases in the Southern Hemisphere winter stratosphere of the model are significantly alleviated by including convective GWD (GWDC) parameterizations. The reduction in the wind biases is due to enhanced wave drag in the winter extratropical stratosphere, which is caused directly by the additional GWDC and indirectly by the increased existing nonorographic GWD and resolved wave drag in response to the GWDC. The cold temperature biases are alleviated by increased downwelling in the winter polar stratosphere, which stems from an increased poleward motion due to enhanced wave drag in the winter extratropical stratosphere. A comparison between two simulations separately using the ray-based and columnar GWDC parameterizations shows that the polar night jet with a ray-based GWDC parameterization is much more realistic than that with a columnar GWDC parameterization.

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Hye-Yeong Chun and Jong-Jin Baik

Abstract

An updated parameterization of gravity wave drag forced by subgrid-scale cumulus convection (GWDC) in large-scale models is proposed. For an analytical formulation of the cloud-top wave stress, two-dimensional, steady-state, linear perturbations induced by diabatic heating are found in a two-layer structure with a piecewise constant shear with a critical level in the lower layer, a uniform flow in the upper layer, and piecewise constant buoyancy frequencies in each layer. The dynamical frame considered is relative to the diabatic forcing and the gravity waves obtained are stationary relative to the diabatic forcing, not necessarily stationary relative to the ground. The cloud-top wave momentum flux is proportional to the square of the magnitude of the convective heating, inversely proportional to the basic-state wind speed, and related to the buoyancy frequencies in each layer. The effect of wind shear in the convective region on the cloud-top momentum flux is negligible, while a difference in the stability between the two layers affects the momentum flux significantly. The cloud-top momentum flux increases as the stability in the convective region decreases and the stability above it increases. A global distribution of the 200-mb wave stress calculated using climatological data reveals that the wave stress in the present study is larger than that in a uniform wind and stability case. This is mainly due to the stability difference between the convective region and the region above it. A methodology of parameterizing GWDC in large-scale models using the wave saturation hypothesis is presented.

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