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Kuang, Zhiming

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Kuang

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Zhiming

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Kuang, Zhiming
Author's Publications: search.filters.isAuthorOfPublication.00054155-0a1a-4fde-b4a5-35181e56738b

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Now showing 1 - 10 of 24
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    Excitation of Intraseasonal Variability in the Equatorial Atmosphere by Yanai Wave Groups via WISHE-Induced Convection
    (American Meteorological Society, 2011) Solodoch, Aviv; Boos, William; Kuang, Zhiming; Tziperman, Eli
    A mechanism is presented, based on multiscale interactions via nonlinear wind-induced surface heat exchange (WISHE), that produces eastward-propagating, intraseasonal convective anomalies in the tropical atmosphere. Simulations of convectively coupled disturbances are presented in two intermediate-complexity atmospheric models. One is a shallow water model with a simple WISHE-motivated heating term. The other model is also based on a first baroclinic mode but has an additional prognostic equation for humidity and a simple representation of moist convection based on a quasi-equilibrium approximation. In spite of many differences between the models, they robustly produce a coherent signal in westerly winds and convection that travels eastward at \(4–10 m s^{−1}\). It is shown here that this slow signal is a forced response to an eastward-propagating Yanai (mixed Rossby–gravity) wave group. The response takes the form of a forced Kelvin wave that is driven nonlinearly, via WISHE, by meridional wind anomalies of the Yanai wave group and that travels considerably more slowly than the free convectively coupled Kelvin waves in these models. The Yanai waves are destabilized in the models used here by WISHE in the presence of mean easterlies, but more generally they could also be excited by stratiform instability in the absence of mean easterlies so that the mechanism described here could also operate without mean easterlies. Similarities to and differences from the Madden–Julian oscillation (MJO) and convectively coupled tropical waves are discussed.
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    Enhanced MJO-like Variability at High SST
    (American Meteorological Society, 2013) Arnold, Nathan P.; Kuang, Zhiming; Tziperman, Eli
    The authors report a significant increase in Madden–Julian oscillation (MJO)–like variability in a superparameterized version of the NCAR Community Atmosphere Model run with high sea surface temperatures (SSTs). A series of aquaplanet simulations exhibit a tripling of intraseasonal outgoing longwave radiation variance as equatorial SST is increased from 26° to 35°C. The simulated intraseasonal variability also transitions from an episodic phenomenon to one with a semiregular period of 25 days. Moist static energy (MSE) budgets of composite MJO events are used to diagnose the physical processes responsible for the relationship with SST. This analysis points to an increasingly positive contribution from vertical advection, associated in part with a steepening of the mean vertical MSE profile in the lower troposphere. The change in MSE profile is a natural consequence of increasing SST while maintaining a moist adiabat with a fixed profile of relative humidity. This work has implications for tropical variability in past warm climates as well as anthropogenic global warming scenarios.
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    Role of surface heat fluxes underneath cold pools
    (John Wiley and Sons Inc., 2016) Gentine, Pierre; Garelli, Alix; Park, Seung‐Bu; Nie, Ji; Torri, Giuseppe; Kuang, Zhiming
    Abstract The role of surface heat fluxes underneath cold pools is investigated using cloud‐resolving simulations with either interactive or horizontally homogenous surface heat fluxes over an ocean and a simplified land surface. Over the ocean, there are limited changes in the distribution of the cold pool temperature, humidity, and gust front velocity, yet interactive heat fluxes induce more cold pools, which are smaller, and convection is then less organized. Correspondingly, the updraft mass flux and lateral entrainment are modified. Over the land surface, the heat fluxes underneath cold pools drastically impact the cold pool characteristics with more numerous and smaller pools, which are warmer and more humid and accompanied by smaller gust front velocities. The interactive fluxes also modify the updraft mass flux and reduce convective organization. These results emphasize the importance of interactive surface fluxes instead of prescribed flux boundary conditions, as well as the formulation of surface heat fluxes, when studying convection.
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    Effects of Orography and Surface Heat Fluxes on the South Asian Summer Monsoon
    (American Meteorological Society, 2014) Ma, Ding; Boos, William; Kuang, Zhiming
    A high-resolution (40 km horizontal) global model is used to examine controls on the South Asian summer monsoon by orography and surface heat fluxes. In a series of integrations with altered topography and reduced surface heat fluxes, monsoon strength, as indicated by a vertical wind shear index, is highly correlated with the amplitude of the maximum boundary layer equivalent potential temperature (θeb) over South Asia. Removal of the Tibetan Plateau while preserving the Himalayas and adjacent mountain ranges has little effect on monsoon strength, and monsoon strength decreases approximately linearly as the height of the Himalayas is reduced. In terms of surface heat flux changes, monsoon strength is most sensitive to those in the location of the θeb maximum just south of the Himalayas. These results are consistent with the recent idea that topography creates a strong monsoon by insulating the thermal maximum from dry extratropical air. However, monsoon strength is found to be more sensitive to variations in the θeb maximum when topography is altered than when surface heat fluxes are reduced, and it is suggested that free-tropospheric humidity changes lead to deviations from strict convective quasi equilibrium and cause this difference. When topography is reduced, dry extratropical air intrudes into the troposphere over the θeb maximum and is entrained by local deep convection, requiring a higher θeb to achieve convective equilibrium with a given upper-tropospheric temperature and associated balanced monsoon flow. These results illustrate potential complexities that need to be included in simple theories for monsoon strength built on strict convective quasi equilibrium.
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    A moisture budget perspective of the amount effect
    (Wiley-Blackwell, 2014) Moore, Mary; Kuang, Zhiming; Blossey, P. N.
    A stable water isotopologue-enabled cloud-resolving model was used to investigate the cause of the amount effect on the seasonal (or longer) time scales. When the total water (vapor and condensed phase) budget of the precipitating column of air is considered, our results indicate that as convection becomes stronger and the precipitation rate increases, the δD of precipitation (δDp) depends on the isotopic composition of the converged vapor more than that of surface evaporation. Tests with disabled fractionation from rain evaporation demonstrate that this mechanism does not account for the amount effect as has been previously suggested. If the isotopic content of converged vapor is made uniform with height with a value characteristic of surface evaporation, the amount effect largely disappears, further supporting the dominance of converged vapor in changes to the δDp signal with increasing precipitation. δDp values were compared to the water budget term math formula, where P is precipitation and E is evaporation. Results from this comparison support the overall conclusion that moisture convergence is central in determining the value of δDp and the strength of the amount effect in steady state.
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    The Linear Response Function of an Idealized Atmosphere. Part I: Construction Using Green’s Functions and Applications
    (American Meteorological Society, 2016) Hassanzadeh, Pedram; Kuang, Zhiming
    A linear response function (LRF) determines the mean response of a nonlinear climate system to weak imposed forcings, and an eddy flux matrix (EFM) determines the eddy momentum and heat flux responses to mean-flow changes. Neither LRF nor EFM can be calculated from first principles owing to the lack of a complete theory for turbulent eddies. Here the LRF and EFM for an idealized dry atmosphere are computed by applying numerous localized weak forcings, one at a time, to a GCM with Held–Suarez physics and calculating the mean responses. The LRF and EFM for zonally averaged responses are then constructed using these forcings and responses through matrix inversion. Tests demonstrate that LRF and EFM are fairly accurate. Spectral analysis of the LRF shows that the most excitable dynamical mode, the neutral vector, strongly resembles the model’s annular mode. The framework described here can be employed to compute the LRF and EFM for zonally asymmetric responses and more complex GCMs. The potential applications of the LRF and EFM constructed here are (i) forcing a specified mean flow for hypothesis testing, (ii) isolating/quantifying the eddy feedbacks in complex eddy–mean flow interaction problems, and (iii) evaluating/improving more generally applicable methods currently used to construct LRFs or diagnose eddy feedbacks in comprehensive GCMs or observations. As an example for (iii), in Part II, the LRF is also computed using the fluctuation–dissipation theorem (FDT), and the previously calculated LRF is exploited to investigate why FDT performs poorly in some cases. It is shown that dimension reduction using leading EOFs, which is commonly used to construct LRFs from the FDT, can significantly degrade the accuracy owing to the nonnormality of the operator.
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    The Linear Response Function of an Idealized Atmosphere. Part II: Implications for the Practical Use of the Fluctuation–Dissipation Theorem and the Role of Operator’s Nonnormality
    (American Meteorological Society, 2016) Hassanzadeh, Pedram; Kuang, Zhiming
    A linear response function (LRF) relates the mean response of a nonlinear system to weak external forcings and vice versa. Even for simple models of the general circulation, such as the dry dynamical core, the LRF cannot be calculated from first principles owing to the lack of a complete theory for eddy–mean flow feedbacks. According to the fluctuation–dissipation theorem (FDT), the LRF can be calculated using only the covariance and lag-covariance matrices of the unforced system. However, efforts in calculating the LRFs for GCMs using FDT have produced mixed results, and the reason(s) behind the poor performance of the FDT remain(s) unclear. In Part I of this study, the LRF of an idealized GCM, the dry dynamical core with Held–Suarez physics, is accurately calculated using Green’s functions. In this paper (Part II), the LRF of the same model is computed using FDT, which is found to perform poorly for some of the test cases. The accurate LRF of Part I is used with a linear stochastic equation to show that dimension reduction by projecting the data onto the leading EOFs, which is commonly used for FDT, can alone be a significant source of error. Simplified equations and examples of 2 × 2 matrices are then used to demonstrate that this error arises because of the nonnormality of the operator. These results suggest that errors caused by dimension reduction are a major, if not the main, contributor to the poor performance of the LRF calculated using FDT and that further investigations of dimension-reduction strategies with a focus on nonnormality are needed.
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    A mechanism-denial study on the Madden-Julian Oscillation with reduced interference from mean state changes
    (Wiley-Blackwell, 2016) Ma, Ding; Kuang, Zhiming
    Mechanism-denial experiments using Superparameterized Community Atmosphere Model are conducted to investigate the importance of extratropical and circumnavigating waves, wind-evaporation feedback, and radiative-convective feedback to the Madden-Julian Oscillation (MJO). A common issue with mechanism-denial studies is the interference from mean state changes when processes are turned off in the model. Here time-invariant forcing and nudging on effective timescales longer than the intraseasonal timescale are implemented to maintain the mean state. The MJO activity remains largely unchanged with suppressed extratropical and circumnavigating waves when the mean state is maintained to be close to that of the control run, suggesting that excitation of MJO by extratropical and circumnavigating waves is not necessary for the existence of MJO in this model. It is also shown that the wind-evaporation feedback slows down eastward propagation of the MJO, and the radiative-convective feedback amplifies the MJO.
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    Quantifying the Eddy–Jet Feedback Strength of the Annular Mode in an Idealized GCM and Reanalysis Data
    (American Meteorological Society, 2017) Ma, Ding; Hassanzadeh, Pedram; Kuang, Zhiming
    A linear response function (LRF) that relates the temporal tendency of zonal-mean temperature and zonal wind to their anomalies and external forcing is used to accurately quantify the strength of the eddy–jet feedback associated with the annular mode in an idealized GCM. Following a simple feedback model, the results confirm the presence of a positive eddy–jet feedback in the annular mode dynamics, with a feedback strength of 0.137 day−1 in the idealized GCM. Statistical methods proposed by earlier studies to quantify the feedback strength are evaluated against results from the LRF. It is argued that the mean-state-independent eddy forcing reduces the accuracy of these statistical methods because of the quasi-oscillatory nature of the eddy forcing. Assuming the mean-state-independent eddy forcing is sufficiently weak at the low-frequency limit, a new method is proposed to approximate the feedback strength as the regression coefficient of low-pass-filtered eddy forcing onto the low-pass-filtered annular mode index. When time scales longer than 200 days are used for the low-pass filtering, the new method produces accurate results in the idealized GCM compared to the value calculated from the LRF. The estimated feedback strength in the southern annular mode converges to 0.121 day−1 in reanalysis data using the new method. This work also highlights the significant contribution of medium-scale waves, which have periods less than 2 days, to the annular mode dynamics. Such waves are filtered out if eddy forcing is calculated from daily mean data. The present study provides a framework to quantify the eddy–jet feedback strength in GCMs and reanalysis data.
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    Microphysical controls on the isotopic composition of wintertime orographic precipitation
    (Wiley-Blackwell, 2016) Moore, M.; Blossey, P. N.; Muhlbauer, A.; Kuang, Zhiming
    The sensitivity of mixed-phase orographic clouds, precipitation, and their isotopic content to changes in dynamics, thermodynamics, and microphysics is explored in idealized two-dimensional flow over a mountain barrier. These simulations use the Weather Research and Forecasting (WRF) model with stable water isotopologues (HDO and math formulaO), which have been integrated into the Thompson microphysics scheme within WRF as part of the present project. In order to understand how the isotopic composition of precipitation ( math formula) is fixed, the mountain height, temperature, and the prescribed cloud droplet number concentration (CDNC) have been varied in a series of simulations. For the given range of values explored in this work, changes in mountain height and temperature induce stronger responses in domain-averaged math formula than do changes in CDNC by a factor of approximately 10. The strongest response to changing CDNC leads to local variations of math formula of about 3‰, though those occur in regions of weak precipitation (<0.1 mm h−1). Changes in math formula can be understood through the microphysical pathways by which precipitable hydrometeors are formed and by the isotopic signature associated with each pathway. The decrease in math formula with increasing mountain height is not just a function of decreasing temperature but also reflects the changing contributions and distinct isotopic signatures of riming of cloud liquid and vapor deposition onto snow, the leading sources of precipitation in these simulations. The changes in math formula with mountain height, temperature, and CDNC are governed in part by the microphysical pathways through which precipitating hydrometeors are formed and grow.