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Xin Zhang

Abstract

A multitaper 2D spectral estimation method is developed for increasing the degree of freedom of the estimation. The core of this method is 2D Slepian eigen windows that are optimum in the sense of minimizing the spectral leakage. Wavenumber spectra of very short wind wave slopes are calculated by this method. The advantages of the multitaper technique are shown by obtaining smooth wavenumber spectra from a limited amount of image data. The data used for the spectral estimation were measured in the laboratory with a water surface gradient detector developed by the authors. Important features of the spatial distribution of short-wave energy are newly revealed. The widening of angular spreading of energy density spectra is not monotonic with increasing wave-number. There is a local plat region of minimum angular spreading in the spectral band of parasitic capillary waves that suggests that there is another upstream of energy cascade in spite of the energetic gravity-wave spectral peak. The input energy of parasitic waves from energetic long gravity waves with a narrow angular distribution is dominant over energy cascade down from spectrally close short waves with a broad angular distribution. The omnidirectional energy spectra also show features related to the change of energy spreading. There is a local wave energy maximum of parasitic waves and a local wave energy minimum of gravity–capillary waves.

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Meng Zhang, Fuqing Zhang, Xiang-Yu Huang, and Xin Zhang

Abstract

This study compares the performance of an ensemble Kalman filter (EnKF) with both the three-dimensional and four-dimensional variational data assimilation (3DVar and 4DVar) methods of the Weather Research and Forecasting (WRF) model over the contiguous United States in a warm-season month (June) of 2003. The data assimilated every 6 h include conventional sounding and surface observations as well as data from wind profilers, ships and aircraft, and the cloud-tracked winds from satellites. The performances of these methods are evaluated through verifying the 12- to 72-h forecasts initialized twice daily from the analysis of each method against the standard sounding observations. It is found that 4DVar has consistently smaller error than that of 3DVar for winds and temperature at all forecast lead times except at 60 and 72 h when their forecast errors become comparable in amplitude, while the two schemes have similar performance in moisture at all lead times. The forecast error of the EnKF is comparable to that of the 4DVar at 12–36-h lead times, both of which are substantially smaller than that of the 3DVar, despite the fact that 3DVar fits the sounding observations much more closely at the analysis time. The advantage of the EnKF becomes even more evident at 48–72-h lead times; the 72-h forecast error of the EnKF is comparable in magnitude to the 48-h error of 3DVar/4DVar.

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Xin Zhang, Xiang-Yu Huang, and Ning Pan

Abstract

The authors propose a new technique for parallelizations of tangent linear and adjoint codes, which were applied in the redevelopment for the Weather Research and Forecasting (WRF) model with its Advanced Research WRF dynamic core using the automatic differentiation engine. The tangent linear and adjoint codes of the WRF model (WRFPLUS) now have the following improvements: A complete check interface ensures that developers write accurate tangent linear and adjoint codes with ease and efficiency. A new technique based on the nature of duality that existed among message passing interface communication routines was adopted to parallelize the WRFPLUS model. The registry in the WRF model was extended to automatically generate the tangent linear and adjoint codes of the required communication operations. This approach dramatically speeds up the software development cycle of the parallel tangent linear and adjoint codes and leads to improved parallel efficiency. Module interfaces were constructed for coupling tangent linear and adjoint codes of the WRF model with applications such as four-dimensional variational data assimilation, forecast sensitivity to observation, and others.

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Rongwang Zhang, Xin Wang, and Chunzai Wang

Abstract

Simulations of the global oceanic latent heat flux (LHF) in the CMIP5 multimodel ensemble (MME) were evaluated in comparison with 11 LHF products. The results show that the mean state of LHF in the MME coincides well with that in the observations, except for a slight overestimation in the tropical regions. The reproduction of the seasonal cycle of LHF in the MME is in good agreement with that in the observations. However, biases are relatively obvious in the coastal regions. A prominent upward trend in global-mean LHF is confirmed with all of the LHF products during the period of 1979–2005. Despite the consistent increase of LHF in CMIP5 models, the rates of increase are much weaker than those in the observations, with an average of approximately one-ninth that in the observations. The findings show that the rate of increase of near-surface specific humidity q a in MME is nearly 6 times that in the observations, while the rate of increase of the near-surface wind speed U is less than one-half that in the observations. The faster increase of q a and the slower increase of U could both suppress evaporation, and thus latent heat released by the ocean, which may be one of the reasons that the upward trend of LHF in the MME is nearly one order of magnitude lower than that in the observations.

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Xin Lin, Sara Q. Zhang, and Arthur Y. Hou

Abstract

Global microwave rainfall retrievals from a five-satellite constellation, including the Tropical Rainfall Measuring Mission Microwave Imager, Special Sensor Microwave Imager from the Defense Meteorological Satellite Program F13, F14, and F15, and the Advanced Microwave Scanning Radiometer from the Earth Observing System Aqua, are assimilated into the NASA Goddard Earth Observing System (GEOS) Data Assimilation System using a 1D variational continuous assimilation (VCA) algorithm. The physical and dynamical impact of rainfall assimilation on GEOS analyses is examined at various temporal and spatial scales. This study demonstrates that the 1D VCA algorithm, which was originally developed and evaluated for rainfall assimilations over tropical oceans, can effectively assimilate satellite microwave rainfall retrievals and improve GEOS analyses over both the Tropics and the extratropics where the atmospheric processes are dominated by different large-scale dynamics and moist physics, and also over land, where rainfall estimates from passive microwave radiometers are believed to be less accurate. Results show that rainfall assimilation renders the GEOS analysis physically and dynamically more consistent with the observed precipitation at the monthly mean and 6-h time scales. Over regions where the model precipitation tends to misbehave in distinctly different rainy regimes, the 1D VCA algorithm, by compensating for errors in the model’s moist time tendency in a 6-h analysis window, is able to bring the rainfall analysis closer to the observed. The radiation and cloud fields also tend to be in better agreement with independent satellite observations in the rainfall–assimilation run especially over regions where rainfall analyses indicate large improvements. Assimilation experiments with and without rainfall data for a midlatitude frontal system clearly indicate that the GEOS analysis is improved through changes in the thermodynamic and dynamic fields that respond to the rainfall assimilation. The synoptic structures of temperature, moisture, winds, divergence, and vertical motion, as well as vorticity, are more realistically captured across the front.

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Jubao Zhang, Raymond W. Schmitt, and Rui Xin Huang

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Scaling analysis of the oceanic thermohaline circulation has been done under two types of surface boundary conditions: (i) Under “relaxation” conditions (sea surface temperature and salinity are relaxed to prescribed values), there is a two-thirds power law dependence of the meridional overturning (and the poleward heat transport) on the diapycnal diffusivity. For any given external forcing, there is only one equilibrium state for the thermohaline circulation. (ii) Under “mixed” boundary conditions (temperature is relaxed to prescribed values and a virtual salt flux condition is used for salinity), multiple equilibria become possible. For a given thermal forcing, the existence of multiple equilibria depends on the relative contributions of diapycnal diffusivity and the hydrologic forcing: for each diapycnal diffusivity K, there is a threshold freshwater flux E c = CK 2/3 (C is a constant) below which three modes are possible with one stable thermal mode, one unstable thermal mode, and a stable haline mode and above which only one stable haline mode can exist.

Numerical experiments are also implemented to test the above scaling arguments. Consistent results have been obtained under the two types of boundary conditions. The relationship derived here focuses attention on the need to better understand both the diapycnal mixing in the ocean and the strength of the hydrologic forcing at its surface.

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Jubao Zhang, Raymond W. Schmitt, and Rui Xin Huang

Abstract

The effect of double-diffusive mixing on the general circulation is explored using the GFDL MOM2 model. The motivation for this comes from the known sensitivity of the thermohaline circulation to the vertical diffusivity and the earlier work of Gargett and Holloway, who studied the effects of a simple nonunity ratio between heat and salt diffusivities in a GCM. In this work, a more realistic, yet conservative, parameterization of the double-diffusive mixing is applied, with the intensity depending on the local density ratio R ρ = αT z/βS z. A background diffusivity is used to represent non-double-diffusive turbulent mixing in the stably stratified environment. The numerical model is forced by relaxation boundary conditions on both temperature and salinity at the sea surface. Three control experiments have been carried out: one with the double-diffusive parameterization (DDP) determined by the local density ratio, one with constant but different diffusivities for heat and salt as previously considered by Gargett and Holloway (GHD), and the other with the same constant diapycnal eddy diffusivity for both heat and salt (CDD). The meridional overturning in run DDP is 22% less than in run CDD, and the maximum poleward heat transport is about 8% less. In comparison, the overturning rate and poleward heat transport in run GHD display reductions that are about half as large. The interior temperature and salinity in run DDP and GHD are higher than in run CDD, with the change in run DDP more than twice that in run GHD. In addition, in DDP and GHD, the density ratio distribution becomes closer to unity than in run CDD, with the change in run DDP being larger than in GHD. Interestingly, the double diffusion is stronger in the western boundary current region than the interior, implying a close relation between vertical shear and the intensity of double diffusion. These results indicate a greater sensitivity of the thermohaline circulation to double diffusion than had previously been suspected due to the tendency of the double-diffusive mixing to generate self-reinforcing flows. This effect appears to be more significant when the double-diffusive mixing is applied only when the stratification is favorable rather than uniformly applied. In addition, parameter sensitivity experiments suggest that double diffusion could have stronger effects on the meridional overturning and poleward heat transport than modeled here since the parameterizations chosen are rather conservative.

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Jianping Duan, Qi-bin Zhang, and Li-Xin Lv

Abstract

The recent increase in the frequency of winter cold extremes has received particular attention in light of the climate's warming. Knowledge about changes in the frequency of winter cold extremes requires long-term climate data over large spatial scale. In this study, a temperature-sensitive tree-ring network consisting of 31 sampling sites collected from seven provinces in subtropical China was used to investigate the characteristics of cold-season temperature extremes during the past two centuries. The results show that the percentage of trees in a year that experienced an abnormal decrease in radial growth relative to the previous year can serve as an indicator of interannual change in January–March temperature in subtropical China. The frequency of extreme interannual decreases in cold-season temperature has increased since the 1930s. The change in cold-season temperature was significantly correlated with the intensity of the Siberian high, yet the correlation was much weaker in the period preceding the 1930s. The findings provide evidence of a frequency change in the occurrence of interannual cold-season temperature extremes in the past two centuries for subtropical China. Particularly, the pattern in the variation of cold-season temperature suggests a change in climate systems around the 1930s.

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Yi-Leng Chen, Xin An Chen, and Yu-Xia Zhang

Abstract

During 31 May–2 June 1987 a low pressure center developed around 28°N, 102°E in the lee of the Tibetan Plateau during the passage of a midlatitude trough. It moved northeastward and intensified over the Yangtze River valley and produced widespread rainfall over southeastern China. A strong southwest flow developed in the lower troposphere over southern China as the cyclone deepened. At 0000 UTC 1 June, a well-defined low-level jet (LLJ) was found between the 850- and 700-hPa levels with wind speed exceeding 15 m s−1 at the 850-hPa level and 20 m s−1 at the 700-hPa level. The LLJ transported warm, moist air from the subtropical ocean, increased the moist static energy, and provided a favorable environment for the development of widespread precipitation.

It is shown that the development of the LLJ is closely related to the developing lee cyclone to the east of the Tibetan Plateau. Our analysis reveals that the diagnosed secondary circulation associated with the jet-front system differs from those predicted by previous theoretical studies. The transverse secondary circulation across the front is characterized by a thermally direct circulation across the baroclinic zone with warm air rising within the southwest monsoon flow and cold air sinking in the postfrontal northeasterlies. A very weak thermally indirect circulation was observed to the south.

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Xidong Wang, Chunzai Wang, Liping Zhang, and Xin Wang

Abstract

This study investigates the variation of tropical cyclone (TC) rapid intensification (RI) in the western North Pacific (WNP) and its relationship with large-scale climate variability. RI events have exhibited strikingly multidecadal variability. During the warm (cold) phase of the Pacific decadal oscillation (PDO), the annual RI number is generally lower (higher) and the average location of RI occurrence tends to shift southeastward (northwestward). The multidecadal variations of RI are associated with the variations of large-scale ocean and atmosphere variables such as sea surface temperature (SST), tropical cyclone heat potential (TCHP), relative humidity (RHUM), and vertical wind shear (VWS). It is shown that their variations on multidecadal time scales depend on the evolution of the PDO phase. The easterly trade wind is strengthened during the cold PDO phase at low levels, which tends to make equatorial warm water spread northward into the main RI region rsulting from meridional ocean advection associated with Ekman transport. Simultaneously, an anticyclonic wind anomaly is formed in the subtropical gyre of the WNP. This therefore may deepen the depth of the 26°C isotherm and directly increase TCHP over the main RI region. These thermodynamic effects associated with the cold PDO phase greatly support RI occurrence. The reverse is true during the warm PDO phase. The results also indicate that the VWS variability in the low wind shear zone along the monsoon trough may not be critical for the multidecadal modulation of RI events.

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