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Huiyan Xu
,
Guoqing Zhai
, and
Xiaofan Li

Abstract

In this study, the WRF Model is used to simulate the torrential rainfall of Typhoon Fitow (2013) over coastal areas of east China during its landfall. Data from the innermost model domain are used to trace trajectories of particles in three major 24-h accumulated rainfall centers using the Lagrangian flexible particle dispersion model (FLEXPART). Surface rainfall budgets and cloud microphysical budgets as well as precipitation efficiency are analyzed along the particles’ trajectories. The rainfall centers with high precipitation efficiency are associated with water vapor convergence, condensation, accretion of cloud water by raindrops, and raindrop loss/convergence. The raindrop loss/convergence over rainfall centers is supported by the raindrop gain/divergence over the areas adjacent to rainfall centers. Precipitation efficiency is mainly determined by hydrometeor loss/convergence. Hydrometeor loss/convergence corresponds to the hydrometeor flux convergence, which may be related to the increased vertical advection of hydrometeors in response to the upward motions and upward decrease of hydrometeors, whereas hydrometeor gain/divergence corresponds to the reduction in hydrometeor flux convergence, which may be associated with the decreased horizontal advection of hydrometeors in response to the zonal decrease in hydrometeors and easterly winds and the meridional increase in hydrometeors and southerly winds. The water vapor convergence and associated condensation do not show consistent relationships with orographic lifting all the time.

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Richard L. Pfeffer
,
Robin Kung
, and
Guoqing Li

Abstract

The amplitude and phase of a topographically forced wave in a baroclinic flow are studied both experimentally and theoretically. The experiments were conducted in a thermally driven fluid in a rotating annulus with two-wave bottom topography. Analysis of velocity data at a single level in seven different experiments at the same imposed temperature contrast and successively larger rotation rates (Ω) reveals that the forced wave is displaced upstream from the topography by an amount which increases with increasing Ω. The wave amplitude increases as we progress from low to moderate Ω, beyond Which it becomes smaller.

Linear equivalent barotropic and baroclinic theory (the latter incorporating vertical density stratification) give an upstream phase displacement which increases with increasing Ω, in qualitative agreement with the experimental data. The phase lag in the theory is controlled by the “β-effect” (produced by the slope of the free surface) and by Ekman layer dissipation (measured by the ratio of the square root of the Ekman number to the Rossby number). The theoretical phase displacement increases with Ω more slowly at low Ω, and more rapidly at high Ω, than the experimentally determined displacement. The wave amplitude derived from the linear theory is too large and increases monotonically with Ω, peaking at resonance, which is found outside the range of rotation rates imposed in the experiments. The discrepancies between the theoretically and experimentally determined phase are attributed to variations in the vertical shear of the basic state velocity with Ω, which the present measurements were not designed to observe. The required variations are consistent with those observed in a related series of experiments without bottom topography. The discrepancies in the amplitude determinations are attributed to nonlinear wave-wave interactions that are not taken into account in the theory.

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Richard J. Greatbatch
,
Guoqing Li
, and
Sheng Zhang

Abstract

This paper investigates the hindcasting of interdecadal climate events using an ocean circulation model driven by different combinations of time-varying surface flux, sea surface temperature (SST), and sea surface salinity (SSS) data. Data are generated from a control run, against which the subsequent model experiments are compared. The most robust results are obtained using flux boundary conditions on both surface temperature and salinity. For these boundary conditions, model results am relatively insensitive to noise in the surface data and take about 20 years to overcome the imposition of an incorrect initial condition. Model results are much more sensitive to noisy inputs when run using SST and SSS data. To obtain meaningful results, SST data alone are not sufficient; SSS data are also required. This is related to the well-known instability of ocean climate models upon a switch to mixed boundary conditions. Time-varying SSS data cannot be replaced by climatology; using a best-fit TS relation, to calculate anomalies in SSS from those in SST is also found to give disappointing results. The difficulty of trying to correct for inaccuracies in surface heat flux using SST data, while at the same time using a flux boundary condition on surface salinity, is demonstrated.

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Guo-Qing Li
,
Robin Kung
, and
Richard L. Pfeffer

Abstract

A series of laboratory experiments was performed in a thermally-driven rotating annulus of fluid with and without two-wave bottom topography. Velocity measurements were made by illuminating a thin layer of fluid at mid-depth and photographing successive positions of tracer particles suspended in the fluid. Streamfunctions were determined from the calculated vorticity. Comparison of the rotational velocity field with the measured velocity field revealed negligible differences, indicating that the flow was horizontally quasi-nondivergent.

A detailed analysis was made of one experiment with and one without topography at the same point in dimensionless parameter space. The results indicate that the effect of topography is 1) to modulate the synoptic-scale waves in both space and time, 2) to suppress the odd modes and 3) to force a “planetary” scale mode which oscillates about a climatological mean position (with high pressure centers located in this experiment approximately 22° upstream of the mountain ridges). Synoptic wavenumbers 4 and 6 have a common frequency of wave passage, which is the same as the frequency of oscillation of the planetary scale wavenumber 2. The wave amplitudes, as well as the zonal mean velocity profile, also vacillate with time at this same frequency. The combined energy of the synoptic-scale waves is concentrated about 45° downstream from the mountain ridges.

A hierarchy of experiments is recommended for the future in which sloping upper and/or lower boundaries are used to simulate the β effect at other points in parameter space and additional Fourier components are added to the bottom topography with relative amplitudes and phases equal to those found on the earth.

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Richard L. Pfeffer
,
Jon Ahlquist
,
Robin Kung
,
Yehui Chang
, and
Guoqing Li

Abstract

Complex principal component analysis is applied to data from three laboratory experiments of flow over two-wave sinusoidal bottom topography in a thermally driven, rotating annulus of fluid. The experiments are conducted at the same imposed temperature contrast (Δ T) and at three different rotation rates (Ω). In each case, the intensity of the wave activity is maximum downstream of the two topographic ridges. The analysis, however, reveals a fundamental difference in the behavior of the waves at lower rotation rates than at the highest rotation rate. At the lower Ω's, the baroclinic waves travel over the topographic ridges with diminished intensity and amplify on the other side of each ridge, with the result that the flows downstream of the two ridges are coherent. At the largest Ω, at which the Rossby number, Ro, is very small and the friction parameter, r = E½/Ro (where E is proportional to the Ekman number), is rather large, the waves downstream of each ridge are decoupled from those downstream of the other ridge, such that there is no coherence between them. It is thought that this behavior might be related to the small Rossby radius of deformation and large effective Ekman layer dissipation associated with baroclinic waves at large rotation rates.

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Jicai Zhang
,
Guoqing Li
,
Jiacheng Yi
,
Yanqiu Gao
, and
Anzhou Cao

Abstract

Temporal vertical eddy viscosity coefficient (VEVC) in an Ekman layer model is estimated using an adjoint method. Twin experiments are carried out to investigate the influences of several factors on inversion results, and the conclusions of twin experiments are 1) the adjoint method is a capable method to estimate different kinds of temporal distributions of VEVCs; 2) the gradient descent algorithm is better than CONMIN and L-BFGS for the present problem, although the posterior two algorithms perform better on convergence efficiency; 3) inversion results are sensitive to initial guesses; 4) the model is applicable to different wind conditions; 5) the inversion result with thick boundary layer depth (BLD) is slightly better than thin BLD; 6) inversion results are more sensitive to observations in upper layers than those in lower layers; 7) inversion results are still acceptable when data noise exists, indicating the method can sustain noise to a certain degree; 8) a regularization method is proved to be useful to improve the results for present problem; and 9) the present method can tolerate the existence of balance errors due to the imperfection of governing equations. The methodology is further validated in practical experiments where Ekman currents are derived from Bermuda Testbed Mooring data and assimilated. Modeled Ekman currents coincide well with observed ones, especially for upper layers. The results demonstrate that the assumptions of depth dependence and time dependence are equally important for VEVCs. The feasibility of the typical Ekman model, the imperfection of Ekman balance equations, and the deficiencies of the present method are discussed. This method provides a potential way to realize the time variations of VEVCs in ocean models.

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Yong Liu
,
Huopo Chen
,
Guoqing Zhang
,
Jianqi Sun
,
Hua Li
, and
Huijun Wang

Abstract

The lake area in the Inner Mongolian Plateau (IMP) has experienced a rapid reduction in recent decades. Previous studies have highlighted the important role of intensive human activities in IMP lake shrinkage. However, this study found that climate change–induced summer precipitation variations can exert great influences on the IMP lake area variations. The results suggest that the decadal shift in the IMP summer precipitation may be the predominant contributor to lake shrinkage. Further analysis reveals that the Atlantic multidecadal oscillation (AMO) and Arctic sea ice concentration (SIC) play important roles in the IMP summer precipitation variations. The AMO seems to provide beneficial large-scale circulation fields for the decadal variations in the IMP summer precipitation, and the Arctic SIC decline is favorable for weakening the IMP summer precipitation intensity after the late 1990s. Evidence indicates that the vorticity advection related to the Arctic SIC decline can result in the generation of Rossby wave resources in the midlatitudes. Then, the strengthened wave resources become favorable for enhancing the stationary wave propagation across Eurasia and inducing cyclonic circulation over the Mongolia–Baikal regions, which might bring more rainfall northward and weaken the IMP summer precipitation intensity. Consequently, due to the decreased rainfall and gradual warming after the late 1990s, the lake area in the IMP has experienced a downward trend in recent years.

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