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David C. Fritts and Ling Wang

Abstract

Part I of this paper employs four direct numerical simulations (DNSs) to examine the dynamics and energetics of idealized gravity wave–fine structure (GW–FS) interactions. That study and this companion paper were motivated by the ubiquity of multiscale GW–FS superpositions throughout the atmosphere. These DNSs exhibit combinations of wave–wave interactions and local instabilities that depart significantly from those accompanying idealized GWs or mean flows alone, surprising dependence of the flow evolution on the details of the FS, and an interesting additional pathway to instability and turbulence due to GW–FS superpositions. This paper examines the mechanical and thermal energy dissipation rates occurring in two of these DNSs. Findings include 1) dissipation that tends to be much more localized and variable than that due to GW instability in the absence of FS, 2) dissipation statistics indicative of multiple turbulence sources, 3) strong influences of FS shears on instability occurrence and turbulence intensities and statistics, and 4) significant differences between mechanical and thermal dissipation rate fields having potentially important implications for measurements of these flows.

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Zheng Ling, Yuqing Wang, and Guihua Wang

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The South China Sea (SCS) is affected by two intraseasonal components in summer: the Madden–Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO). In the present study, the impacts of the MJO and QBWO on tropical cyclones (TCs) locally formed in the SCS (local TCs) in summer are investigated. The results show that both the MJO and QBWO can affect the genesis frequency, location, and motion of the local TCs. More TCs form in the convectively active phases of the MJO and QBWO in the northern SCS. With the northward propagation of the MJO and QBWO convective signals, the major TC genesis location also shifts northward. Since the western Pacific subtropical high shifts eastward (westward) when convection associated with the MJO and QBWO in the northern SCS is enhanced (suppressed), the steering flow in the major TC genesis region is favorable for the eastward (westward) movement of TCs. Results from the composite analysis of the steering flow indicate that both the MJO and QBWO play an important role in controlling the motion of the eastward-moving TCs.

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Ling Ling Liu, Wei Wang, and Rui Xin Huang

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Wind stress and tidal dissipation are the most important sources of mechanical energy for maintaining the oceanic general circulation. The contribution of mechanical energy due to tropical cyclones can be a vitally important factor in regulating the oceanic general circulation and its variability. However, previous estimates of wind stress energy input were based on low-resolution wind stress data in which strong nonlinear events, such as tropical cyclones, were smoothed out.

Using a hurricane–ocean coupled model constructed from an axisymmetric hurricane model and a three-layer ocean model, the rate of energy input to the world’s oceans induced by tropical cyclones over the period from 1984 to 2003 was estimated. The energy input is estimated as follows: 1.62 TW to the surface waves and 0.10 TW to the surface currents (including 0.03 TW to the near-inertial motions). The rate of gravitational potential energy increase due to tropical cyclones is 0.05 TW. Both the energy input from tropical cyclones and the increase of gravitational potential energy of the ocean show strong interannual and decadal variability with an increasing rate of 16% over the past 20 years. The annual mean diapycnal upwelling induced by tropical cyclones over the past 20 years is estimated as 39 Sv (Sv ≡ 106 m3 s−1). Owing to tropical cyclones, diapycnal mixing in the upper ocean (below the mixed layer) is greatly enhanced. Within the regimes of strong activity of tropical cyclones, the increase of diapycnal diffusivity is on the order of (1 − 6) × 10−4 m2 s−1. The tropical cyclone–related energy input and diapycnal mixing may play an important role in climate variability, ecology, fishery, and environments.

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Ling-Yun Chiao and Yu-Huai Wang

Abstract

To account for the uneven sampling of current measurements collected by acoustic Doppler current profilers (ADCPs), a robust, three-dimensional interpolation scheme based on the multiresolution representation of the regional mean and tidal current fields is proposed. Instead of reconstructing the tidal field by getting bin-averaged time series that rely on heavy sampling or invoking radial basis function expansions, such as using the biharmonic splines with subjectively selected knots, the resolving capability of the proposed scheme relies fundamentally on the scale hierarchy of the resolvable local information constrained by the data. It is demonstrated that the proposed scheme flexibly incorporates the merits of the two conventional techniques. It enforces the resolution of model information while accommodating the local sampling density. Since it is based on a knots network defined by regular grids, attempts at experimentally and subjectively constructing the proper number and locations of controlling nodes are avoided. Constructing multiresolution representation of the current fields in terms of the three-dimensional wavelet basis is implemented by the computationally effective discrete wavelet transform of coefficients of the interpolation equations. Applications of the proposed multiresolution scheme on artificial as well as field datasets of the ADCP measurements demonstrate that it is a promising approach.

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Wei-Chyung Wang, Joseph P. Pinto, and Yuk Ling Yung

Abstract

Using a one-dimensional radiative-convective model, we perform a sensitivity study of the effect of ozone depletion in the stratosphere on the surface temperature. There could be a cooling of the surface temperature by ∼0.2 K due to chlorofluoromethane-induced ozone depletion at steady state (assuming 1973 release rates). This cooling reduces significantly the greenhouse effect due to the presence of chlorofluoromethanes. Carbon tetrafluoride has a strong ν3 band at 7.8 μm, and the atmospheric greenhouse effect is shown to be 0.07 and 0.12 K (ppbv)−1 with and without taking into account overlap with CH4 and N2O bands. At concentration higher than l ppbv, absorption by the ν3 band starts to saturate and the greenhouse effect becomes less efficient.

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Ling Wang, Marvin A. Geller, and M. Joan Alexander

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Five years (1998–2002) of U.S. high vertical resolution radiosonde data are analyzed to derive important gravity wave parameters, such as intrinsic frequencies, vertical and horizontal wavelengths, and vertical propagation directions in the lower stratosphere and troposphere.

Intrinsic frequencies ω̂ increase with increasing latitude, with larger values in the troposphere. In the lower stratosphere, ω̂ is higher in winter than in summer, especially at mid- and high latitudes. Intrinsic frequencies divided by the Coriolis parameter f are ∼4 in the troposphere, and ∼2.4–3 in the lower stratosphere. The lower-stratospheric ω̂/f generally decreases weakly with increasing latitude. The latitudinal distributions of the lower-stratospheric ω̂/f are explained largely by the propagation effects. The seasonal variations of ω̂ in the lower stratosphere are found to be related to the variations of the background wind speeds.

Dominant vertical wavelengths decrease with increasing latitude in the lower stratosphere, and maximize at midlatitudes (35°–40°N) in the troposphere. They are generally longer in winter than in summer. The variations of the dominant vertical wavelengths are found to be associated with the similar variations in gravity wave energies. Dominant horizontal wavelengths decrease with increasing latitude, with larger values in the lower stratosphere.

Approximately 50% of the tropospheric gravity waves show upward energy propagation, whereas there is about 75% upward energy propagation in the lower stratosphere. The lower-stratospheric fraction of upward energy propagation is generally smaller in winter than in summer, especially at mid- and high latitudes. The seasonal variation of upward fraction is likely an artifice due to the analysis method, although a small part of it may be interpreted by the variations in background wind speeds.

Results suggest that propagation effects are much more important than source variations for explaining the large-scale time-average properties of waves observed by radiosondes.

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Ling Wang, Marvin A. Geller, and David C. Fritts

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Thorpe analysis has been used to study turbulence in the atmosphere and ocean. It is clear that Thorpe analysis applied to individual soundings cannot be expected to give quantitatively reliable measurements of turbulence parameters because of the instantaneous nature of the measurement. A critical aspect of this analysis is the assumption of the linear relationship C = L O/L T between the Thorpe scale L T, derived from the sounding measurements, and the Ozmidov scale L O. It is the determination of L O that enables determination of the dissipation rate of turbulence kinetic energy ε. Single atmospheric and oceanic soundings cannot indicate either the source of turbulence or the stage of its evolution; different values of C are expected for different turbulence sources and stages of the turbulence evolution and thus cannot be expected to yield quantitatively reliable turbulence parameters from individual profiles. The variation of C with the stage of turbulence evolution is illustrated for direct numerical simulation (DNS) results for gravity wave breaking. Results from a DNS model of multiscale initiation and evolution of turbulence with a Reynolds number Re (which is defined using the vertical wavelength of the primary gravity wave and background buoyancy period as length and time scales, respectively) of 100 000 are sampled as in sounding of the atmosphere and ocean, and various averaging of the sounding results indicates a convergence to a well-defined value of C, indicating that applying Thorpe analysis to atmospheric or oceanic soundings and averaging over a number of profiles gives more reliable turbulence determinations. The same averaging study is also carried out when the DNS-modeled turbulence is dominated by turbulence growing from the initial instabilities, when the turbulence is fully developed, when the modeled turbulence is decaying, and when the turbulence is in a still-later decaying stage. These individual cases converge to well defined values of C, but these values of C show a large variation resulting from the different stages of turbulence evolution. This study gives guidance as to the accuracy of Thorpe analysis of turbulence as a function of the number of profiles being averaged. It also suggests that the values of C in different environments likely depend on the dominant turbulence initiation mechanisms and on the Reynolds number of the environment.

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David C. Fritts, Ling Wang, and Joseph A. Werne

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Four idealized direct numerical simulations are performed to examine the dynamics arising from the superposition of a monochromatic gravity wave (GW) and sinusoidal linear and rotary fine structure in the velocity field. These simulations are motivated by the ubiquity of such multiscale superpositions throughout the atmosphere. Three simulations explore the effects of linear fine structure alignment along, orthogonal to, and at 45° to the plane of GW propagation. These reveal that fine structure alignment with the GW enables strong wave–wave interactions, strong deformations of the initial flow components, and rapid transitions to local instabilities and turbulence. Increasing departures of fine structure alignment from the GW yield increasingly less efficient wave–wave interactions and weaker or absent local instabilities. The simulation having rotary fine structure velocities yields wave–wave interactions that agree closely with the aligned linear fine structure case. Differences in the aligned GW fields are only seen following the onset of local instabilities, which are delayed by about 1–2 buoyancy periods for rotary fine structure compared to aligned, linear fine structure. In all cases, local instabilities and turbulence primarily accompany strong superposed shears or fluid “intrusions” within the rising, and least statically stable, phase of the GW. For rotary fine structure, local instabilities having preferred streamwise or spanwise orientations often arise independently, depending on the character of the larger-scale flow. Wave–wave interactions play the greatest role in reducing the initial GW amplitude whereas fine structure shears and intrusions are the major source of instability and turbulence energies.

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Guihua Wang, Zheng Ling, Renguang Wu, and Changlin Chen

Abstract

The present study investigates the impact of the Madden–Julian oscillation (MJO) on the South China Sea (SCS) in summer with three types of models: a theoretical Sverdrup model, a 1.5-layer reduced gravity model, and a regional ocean model [Regional Ocean Modeling System (ROMS)]. Results show that the ocean circulation in the SCS has an intraseasonal oscillation responding to the MJO. During its westerly phase, the MJO produces positive (negative) wind stress curl over the northern (southern) SCS and thus induces an enhanced cyclonic (anticyclonic) circulation in the northern (southern) SCS. This not only cools sea surface temperature (SST) but also decreases (increases) subsurface temperature in the northern (southern) SCS. During its easterly phase, the MJO basically produces a reversed but weaker influence on SCS ocean circulation and temperature. Thus, the MJO can have an imprint on the summer climatology of SCS circulation and temperature. The authors' analysis further indicates that the MJO's dynamic effect associated with wind is generally more important than its thermodynamic effect in modulating the regional ocean circulation and temperature. The present study suggests that the MJO is important for summer ocean circulation and temperature in the SCS.

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Marvin A. Geller, Peter T. Love, and Ling Wang

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The 1-s-resolution U.S. radiosonde data are analyzed for unstable layers, where the potential temperature decreases with increasing altitude, in the troposphere and lower stratosphere (LS). Care is taken to exclude spurious unstable layers arising from noise in the soundings and also to allow for the destabilizing influence of water vapor in saturated layers. Riverton, Wyoming, and Greensboro, North Carolina, in the extratropics, are analyzed in detail, where it is found that the annual and diurnal variations are largest, and the interannual variations are smallest in the LS. More unstable layer occurrences in the LS at Riverton are found at 0000 UTC, while at Greensboro, more unstable layer occurrences in the LS are at 1200 UTC, consistent with a geographical pattern where greater unstable layer occurrences in the LS are at 0000 UTC in the western United States, while greater unstable layer occurrences are at 1200 UTC in the eastern United States. The picture at Koror, Palau, in the tropics is different in that the diurnal and interannual variations in unstable layer occurrences in the LS are largest, with much smaller annual variations. At Koror, more frequent unstable layer occurrences in the LS occur at 0000 UTC. Also, a “notch” in the frequencies of occurrence of thin unstable layers at about 12 km is observed at Koror, with large frequencies of occurrence of thick layers at that altitude. Histograms are produced for the two midlatitude stations and one tropical station analyzed. The log–log slopes for troposphere histograms are in reasonable agreement with earlier results, but the LS histograms show a steeper log–log slope, consistent with more thin unstable layers and fewer thick unstable layers there. Some radiosonde stations are excluded from this analysis since a marked change in unstable layer occurrences was identified when a change in radiosonde instrumentation occurred.

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