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Jin-Song Xu

Abstract

Two aspects of the principal oscillation pattern (POP) analysis are used to study the large-scale modes of the coupled atmosphere–ocean system. First, P0Ps can be considered as the normal modes of the system; one way of studying these normal modes is to estimate them from data. Second, a POP analysis can be viewed as a multivariate spectral analysis and the spectral characteristics of the modes are by-products of the POP analysis. Both aspects are studied using a combined dataset that includes both atmospheric (sea level pressure, 700-mb, and 200-mb zonal winds) and oceanic (sea surface temperature, Pacific sea level, and Pacific subsurface temperature) parameters.

Six joint modes of the coupled atmosphere-system are found in this study. For modes with small eigenvalues the atmosphere plays an important role. The associated oceanic anomalies appear to be generated by the anomalous atmospheric conditions. For the other modes, which have most of their power on much longer time scales, the ocean is more actively involved. Modes 4 and 5 describe decadal time scale variations. Mode 4 is characterized by changes in SST in all three tropical oceans, and in organized convection over the west Pacific. The results allow us to speculate that these tropical features might excite changes in the extratropical tropospheric and oceanic circulations. Mode 5 shows global-scale SST anomalies and large atmospheric anomalies in the Southern Hemispheric circulation. Mode 6 is the only oscillatory normal mode found in the coupled atmosphere-ocean system; it describes the quasi-cyclic behavior of the El Niñio-Southern Oscillation phenomenon.

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Jin-Song Xu

Abstract

Two quasi-periodic oscillations in the tropical atmosphere with similar oscillation period—the stratospheric quasi-biennial and the Southern oscillations—and the relationship between these two oscillations are examined using the Principal Oscillation Pattern (POP) analysis technique.

The POP analysis of the equatorial stratospheric dataset provides a compact description of the QBO. The oscillation features identified by the POP analysis, namely, the spatial structure, the characteristic times of the oscillation, and the asymmetry in downward propagation, are almost identical to those found by earlier studies using more conventional analyses. The simultaneous POP analysis of the equatorial zonal surface wind and sea surface temperature indicates a well-defined cyclic behavior of the SO. In contrast to the very regular QBO, the SO appears to be much more noisy with intermittent quiet phases. A spectral analysis of the complex POP coefficient time series and the SO index reveals a negligible correlation between the two processes. A POP analysis of the combined equatorial dataset of stratospheric wind, zonal surface wind, and SST also indicates no relation between the QBO and the SO. Two independent modes are identified, one of them completely describing the QBO and the other representing the entire SO. No linear relationship is found between the two modes either in space or in time. It is concluded that the SO and the QBO are two independent processes in the tropical atmosphere with similar time scales.

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Delun Xu, Paul A. Hwang, and Jin Wu

Abstract

Breaking of wind-generated waves was studies in a laboratory tank. The critical surface slope and global wave steepness for inception of breaking were evaluated. Besides the frequency of occurrence, two other characteristic quantities, height and duration of breaking, were measured. The frequency of breaking was found to increase rapidly with wind velocity, following a power law U 2.2. The period of breaking remained about 7% of the wave period at all wind velocities. The height of breaking was about 30% of the wave height. Portions of these results compare favorably with other available measurements.

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Mark C. Green, Jin Xu, and Narendra Adhikari

Abstract

Typical diurnal wind patterns and their relationship to transport of atmospheric aerosol in the Columbia River gorge of Oregon and Washington are addressed in this paper. The measurement program included measurements of light scattering by particles (b sp) with nephelometers, and wind speed and direction, temperature, and relative humidity at seven locations in the gorge. Winds are shown to respond to along-gorge pressure gradients, and five common patterns were identified: strong, moderate, and light westerly (west to east), light easterly, and winter easterly. The strong westerly and winter easterly patterns were the most common summer and winter patterns, respectively, and represented strong gap flow. The light westerly and light easterly patterns occurred most frequently in spring and autumn transition periods. Winter easterly had the highest light scattering and indicated sources east of the gorge mainly responsible for haze. During summer, as westerly winds increased diurnally, a pulse of hazy air from the Portland, Oregon, metropolitan area is transported eastward into the gorge, arriving later with distance into the gorge. During light easterly flow impacts to haze from the city of The Dalles, Oregon, are noted as the wind shifts direction diurnally.

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Jin-Song Xu and Hans Von Storch

Abstract

Principal oscillation pattern (POP) analysis is a diagnostic technique for deriving the space-time characteristics of a dataset objectively. A multiyear dataset of monthly mean sea level pressure (SLP) in the area 15°S to 40°S is examined with the POP technique. In the low-frequency band one physically significant pair of patterns is identified, which is clearly associated with the Southern Oscillation (SO).

According to the POP analysis, the 50 may be described as a damped oscillatory sequence of patterns …→P 1P 2-P 1-P 2P 1… having a time scale of two to three years. The first pattern, P1, representative of the “peak” phase of ENSO, exhibits a dipole with anomalies of opposite sign over the central and eastern Pacific and over the Indian Ocean/Australian sector. The second, P2, pattern is dominated by an anomaly in the SPCZ region and describes an intermediate, or “onset” phase.

The time coefficients of the two patterns, P1 and P2, may be interpreted as a bivariate index of the SO. Generalizing the original diagnostic concept, the POP framework is used to predict this index and the traditional univariate SO index.

The POP prediction scheme is tested in a series of hindcast experiments. The scheme turns out to be skillful for a lead time of two to three seasons. In terms of a correlation skill score, the POP model is better than persistence and a conventional ARMA model in hindcasting the traditional SO index.

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Qin Xu, Chong-Jian Qiu, and Jin-Xiang Yu

Abstract

The simple adjoint method of Qiu and Xu is extended and used to retrieve the low-altitude horizontal wind field from single-Doppler radial wind data measured during the Phoenix II field experiment. Since the extended method uses only the radial momentum equation on a low-altitude horizontal plane with a weak nondivergence constraint for the horizontal winds, the pressure gradient and vertical advection are treated as an unknown residual forcing. The test results show that (i) the method can retrieve the low-altitude time-mean (or running mean) horizontal winds (averaged over a period of several sequential radar scans) from single-Doppler radial wind data; (ii) retrieving the time-mean part of the unknown residual forcing term improves the wind retrieval; (iii) the detailed data treatments and proper settings of the weights considered in Xu et at. remain useful for improving the retrieval.

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Qin Xu, Li Wei, Yi Jin, Qingyun Zhao, and Jie Cao

Abstract

This paper proposes a new method to properly define and accurately determine the vortex center of a model-predicted tropical cyclone (TC) from a dynamic perspective. Ideally, a dynamically determined TC vortex center should maximize the gradient wind balance or, equivalently, minimize the gradient wind imbalance measured by an energy norm over the TC vortex. In practice, however, such an energy norm cannot be used to easily and unambiguously determine the TC vortex center. An alternative yet practical approach is developed to dynamically and unambiguously define the TC vortex center. In this approach, the TC vortex core of near-solid-body rotation is modeled by a simple parametric vortex constrained by the gradient wind balance. Therefore, the modeled vortex can fit simultaneously the perturbation pressure and streamfunction of the TC vortex part (extracted from the model-predicted fields) over the TC vortex core area (within the radius of maximum tangential wind), while the misfit is measured by a properly defined cost function. Minimizing this cost function yields the desired dynamic optimality condition that can uniquely define the TC vortex center. Using this dynamic optimality condition, a new method is developed in the form of iterative least squares fit to accurately determine the TC vortex center. The new method is shown to be efficient and effective for finding the TC vortex center that accurately satisfies the dynamic optimality condition.

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Qingyun Zhao, Qin Xu, Yi Jin, Justin McLay, and Carolyn Reynolds

Abstract

The time-expanded sampling (TES) method, designed to improve the effectiveness and efficiency of ensemble-based data assimilation and subsequent forecast with reduced ensemble size, is tested with conventional and satellite data for operational applications constrained by computational resources. The test uses the recently developed ensemble Kalman filter (EnKF) at the Naval Research Laboratory (NRL) for mesoscale data assimilation with the U.S. Navy’s mesoscale numerical weather prediction model. Experiments are performed for a period of 6 days with a continuous update cycle of 12 h. Results from the experiments show remarkable improvements in both the ensemble analyses and forecasts with TES compared to those without. The improvements in the EnKF analyses by TES are very similar across the model’s three nested grids of 45-, 15-, and 5-km grid spacing, respectively. This study demonstrates the usefulness of the TES method for ensemble-based data assimilation when the ensemble size cannot be sufficiently large because of operational constraints in situations where a time-critical environment assessment is needed or the computational resources are limited.

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Qi Xu, Zhaoyong Guan, Dachao Jin, and Dingzhu Hu

Abstract

Using the NCEP–NCAR reanalysis and Global Precipitation Climatology Project monthly rainfall, we have investigated the regional features of interannual variations of rainfall in the Maritime Continent (MC) and their related anomalous atmospheric circulation patterns during boreal summer by employing the rotated empirical orthogonal function (REOF) analysis. Our results demonstrate that the rainfall variabilities in the MC are of very striking regional characteristics. The MC is divided into four independent subregions on the basis of the leading REOF modes; these subregions are located in central-eastern Indonesia (subregion I), the oceanic area to the west of Indonesia (subregion II+V), the part of the warm pool in the equatorial western Pacific Ocean (subregion III), and Guam (subregion IV+VI).The anomalous precipitation in different subregions exhibits different variation periodicities, which are associated with different circulation patterns as a result of atmospheric response to different sea surface temperature anomaly (SSTA) patterns in the tropical Indo-Pacific sector. It is found that rainfall anomalies in subregion I are induced by the Pacific ENSO, whereas those in subregion II+V are dominated by a triple SSTA pattern with positive correlations in the MC and negative correlation centers in the tropical Pacific and tropical Indian Ocean. Rainfall anomalies in subregion III mainly resulted from an SSTA pattern with negative correlations in the eastern MC and positive correlations in the western equatorial Pacific east of the MC. A horseshoe SSTA pattern in the central Pacific is found to affect the precipitation anomalies in subregion IV+VI. All of the results of this study are helpful for us to better understand both the climate variations in the MC and monsoon variations in East Asia.

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Jin-Song Xu, Hans von Storch, and Harryvan Loon

Abstract

Monthly mean sea level pressure (SLP) data from four low-resolution spectral GCMs–ECMWF T21, CCC, NCAR CCM and GFDL R15–are compared with observations for the Southern Hemisphere.

Characteristics of the observed Southern Hemisphere January and July mean mass distribution are:

(i) high pressure areas in the subtropics;

(ii) a steep meridional gradient at midlatitudes;

(iii) a circumpolar trough in the Antarctic;

(iv) a zonal asymmetry dominated by zonal wave 1, which has an almost complete phase reversal near 40°S;

(v) a double westerly wind maximum during the colder part of the year.

The CCC model reproduces some of these features. The ECMWF model, the NCAR CCM, and the GFDL models fail with respect to (ii) and (iii). All GCMs underestimate the intensity of the stationary eddies. None of the models considered reproduces the double westerly wind maximum.

Another marked feature of the Southern Hemisphere circulation is the semiannual wave that dominates the annual curve of SLP at mid- and polar latitudes. Regardless of the various models’ degree of success in reproducing the mean circulation, all fail in simulating the general features of the semiannual wave.

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