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Ma, Ding

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Ma

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Ding

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Ma, Ding

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2014 - 20174

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Author's Publications: search.filters.isAuthorOfPublication.c091da21-1768-4e61-8731-5118993e2139

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Now showing 1 - 4 of 4
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    Publication
    Investigations of Three Dominant Patterns of Large-Scale Circulation Variability in the Atmosphere
    (2016-09-12) Ma, Ding; Kuang, Zhiming; Farrell, Brian F.; Tziperman, Eli; Huybers, Peter
    This work covers the investigations of three dominant patterns of large-scale circulation variability in the atmosphere, namely the South Asian summer monsoon, the Madden-Julian Oscillation (MJO) and the annular mode. The role of the orography on South Asian monsoon is comprehensively examined using numerical simulations. The results show that the Tibetan Plateau creates a strong monsoon mainly by insulating the thermal maximum south of the Himalayas from extratropical air, as the monsoon strength is not sensitive to the orography and surface heating perturbations north of the Himalayas. It is also found that free-tropospheric moisture anomaly may lead to deviations from the theory of strict convective quasi-equilibrium, implying complexities that need to be included in existing theories of monsoon strength. The MJO is the dominant mode of intraseasonal variability in the tropics. A mechanism- denial study is conducted to examine the importance of different physical processes to the MJO using numerical modeling. The results show that the essential MJO dynamics are internal to the equatorial Indian and Pacific Oceans. The wind-evaporation feedback tend to slow down propagation of the MJO and the radiative-convective feedback is important to MJO amplitude. In an observational study, the vertical distributions of radiative heating anomalies associated with the MJO is constructed using radiative heating profiles from CloudSat. Bottom-heavy radiative heating is seen in actively convecting regions of the MJO, which is argued to strengthen the MJO. The annular mode corresponds to the meridional shift of the eddy-driven jet. Using a linear response function (LRF), the present study confirms unequivocally a positive eddy feedback in the annular mode dynamics in an idealized GCM. The strength of the eddy-jet feedback is accurately quantified, and the result from the LRF is used to evaluate three statistical methods. Because of the quasi-oscillatory nature of eddies, the mean-state- independent eddy forcing reduces the accuracy of two statistical methods proposed by previous studies. The results from a new method proposed here converge to the value produced by the LRF as the mean-state-independent eddies are mostly filtered out. The statistical methods are then applied to the reanalysis data.
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    Publication
    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 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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    Publication
    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.