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Yuan Yuan, Song Yang, and Zuqiang Zhang

Abstract

The authors examine different evolution features of the low-level anticyclone over the tropical northwestern Pacific between eastern Pacific (EP) El Niño events and central Pacific (CP) El Niño events. During EP El Niño, the low-level anticyclone shows an eastward movement from the northern Indian Ocean to the east of the Philippines. During CP El Niño, however, the anticyclone is mostly confined to the west of the Philippines. It is weaker, exhibits a shorter lifetime, and lacks eastward movement compared to the Philippine Sea anticyclone (PSAC) during EP El Niño. Investigation into the possible impact of Indian Ocean (IO) sea surface temperature (SST) on the evolution of the low-level anticyclone during EP and CP El Niño indicates that both SST and low-level atmospheric circulation over the IO are related more strongly with EP El Niño than with CP El Niño. The IO SST tends to exert a more prominent influence on PSAC during EP El Niño than during CP El Niño. During the developing summer and autumn of EP El Niño, the anomalous anticyclone over the northern Indian Ocean excited by positive IO dipole may contribute to an early development of the PSAC. During the winter and decaying spring, the anomalous anticyclone to the east of the Philippines instigated by the IO basin-wide warming mode also favors a larger persistence of the PSAC. During CP El Niño, however, IO SST shows a negligible impact on the evolution of the anticyclone.

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Yuan Zhang and George F. Hepner

Abstract

The accurate prediction of plant phenology is of significant importance for more sustainable and effective land management. This research develops a framework of phenological modeling to estimate vegetation abundance [indicated by the normalized difference vegetation index (NDVI)] 7 days into the future in the geographically diverse Upper Colorado River basin (UCRB). This framework uses phenological regions (phenoregions) as the basic units of modeling to account for the spatially variant environment–vegetation relationships. The temporal variation of the relationships is accounted for via the identification of phenological phases. The modeling technique of Multivariate Adaptive Regression Splines (MARS) is employed and tested as an approach to construct enhanced predictive phenological models in each phenoregion using a comprehensive set of environmental drivers and factors. MARS has the ability to deal with a large number of independent variables and to approximate complex relationships. The R 2 values of the models range from 91.62% to 97.22%. The root-mean-square error values of all models are close to their respective standard errors ranging from 0.016 to 0.035, as indicated by the results of cross and field validations. These demonstrate that the modeling framework ensures the accurate prediction of short-term vegetation abundance in regions with various environmental conditions.

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Shushi Zhang, David B. Parsons, and Yuan Wang

Abstract

This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

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Yuan Zhang, Valentin Dymnikov, and John M. Wallace

Abstract

By using the POP (principal oscillation pattern) system matrices as examples, this study introduces to the meteorological community the concept and application of spectral portrait (or pseudospectra) of a nonsymmetric matrix. An analytical formula is provided to estimate the system error of a POP matrix due to the inversion of a covariance matrix. By comparing the spectral portrait and the system error, this study demonstrates a possible tool to test the sensitivity of a POP analysis.

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Yuan Zhang, John M. Wallace, and Naoto Iwasaka

Abstract

Indices of the dominant spatial patterns of wintertime Northern Hemisphere 500-mb height and North Pacific sea surface temperature are strongly correlated with one another on interannual and interdecadal timescales, and both am correlated with indices of the El Niñ-Southern Oscillation. One possible interpretation of these relationships is that the tropical SST anomalies associated with ENSO force the atmospheric circulation over the North Pacific, and these atmospheric anomalies, in turn, give rise to the observed SST anomalies over the extratropical North Pacific.

In this study, linear relationships between ENSO and extratropical variables are examined in two different ways. First, the component of the observed extratropical variability that is linearly dependent upon ENSO is removed. The dominant spatial patterns in the residual variability of 500-mb height and SST anomalies over the North Pacific are shown to be similar to their counterparts in the total fields and remain strongly coupled on both interannual and interdecadal timescales. Second, the 44 winters used in the analysis are divided into strong ENSO and weak ENSO groups in accordance with the absolute magnitude of ENSO SST anomalies during that winter. Consistent with the analysis of the residual fields, the dominant patterns in extratropical 500-mb height and SST over the North Pacific are strongly coupled, even during winters in which tropical Pacific SST anomalies are weak. An alternative analysis, in which a 15-year record of MSU tropical precipitation data is used as a basis for defining the ENSO signal, yields similar results. The linear relation between SST in the western tropical Pacific and extratropical circulation anomalies is also examined and found to be weak.

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John M. Wallace, Yuan Zhang, and Louis Bajuk

Abstract

Monthly Mean time series of 1) surface air temperature anomalies averaged over the Northern Hemisphere based on data from land stations, 2) sea surface temperature anomalies averaged over the Northern Hemisphere oceans, and 3) 1000–500-mb thickness anomalies averaged over the region poleward of 40°N are examined. The data are stratified in terms of warm (May–October) and cold (November–April) seasons. Time series 1) and 3) based on cold-season data exhibit much larger month to month variability and a richer spectrum of interannual variability than those based on warm-season data: the interdecadal to century scale variability stands out more clearly in the warm-season time series. Cold- and warm-season time series for 1) differ substantially, even when heavily smoothed.

It is hypothesized that the richer spectrum of variability in the cold-season time series is dynamically induced. To test this hypothesis, departures of cold-season data for 1) and 3) from their respective smoothed warm- season time series are regressed upon the hemispheric 1000–500-mb thickness field. Anomalously warm cold- season months are shown to be characterized by circulation regimes conducive to positive thickness anomalies over the high-latitude continents. A simple index of this distinctive “cold ocean–warm land” (COWL) pattern accounts for 65% of the variance of 1) during the cold-season months and for most of the discrepancies between its 5-yr running mean cold- and warm-season time series. The anomalous warmth of the winters of the 1980s is consistent with the strong positive bias of the COWL pattern index during the cold seasons of that decade.

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Yi Zhang, Rucong Yu, Jian Li, Weihua Yuan, and Minghua Zhang
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Yi Zhang, Rucong Yu, Jian Li, Weihua Yuan, and Minghua Zhang

Abstract

Given the large discrepancies that exist in climate models for shortwave cloud forcing over eastern China (EC), the dynamic (vertical motion and horizontal circulation) and thermodynamic (stability) relations of stratus clouds and the associated cloud radiative forcing in the cold season are examined. Unlike the stratus clouds over the southeastern Pacific Ocean (as a representative of marine boundary stratus), where thermodynamic forcing plays a primary role, the stratus clouds over EC are affected by both dynamic and thermodynamic factors. The Tibetan Plateau (TP)-forced low-level large-scale lifting and high stability over EC favor the accumulation of abundant saturated moist air, which contributes to the formation of stratus clouds. The TP slows down the westerly overflow through a frictional effect, resulting in midlevel divergence, and forces the low-level surrounding flows, resulting in convergence. Both midlevel divergence and low-level convergence sustain a rising motion and vertical water vapor transport over EC. The surface cold air is advected from the Siberian high by the surrounding northerly flow, causing low-level cooling. The cooling effect is enhanced by the blocking of the YunGui Plateau. The southwesterly wind carrying warm, moist air from the east Bay of Bengal is uplifted by the HengDuan Mountains via topographical forcing; the midtropospheric westerly flow further advects the warm air downstream of the TP, moistening and warming the middle troposphere on the lee side of the TP. The low-level cooling and midlevel warming together increase the stability. The favorable dynamic and thermodynamic large-scale environment allows for the formation of stratus clouds over EC during the cold season.

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Yuan Zhang, Joel R. Norris, and John M. Wallace

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This paper attempts to resolve a long-standing paradox concerning the season-to-season memory of sea surface temperature anomalies (SSTAs) over the North Pacific. Summertime SSTAs are confined to a shallow mixed layer that is obliterated by wind-driven mixing in late autumn or early winter storms. The mixing exposes waters that were last in contact with the surface during the previous spring. Hence, SSTAs at fixed locations exhibit little memory from summer to the next winter. Yet despite this apparent lack of memory, a well-defined pattern of summertime and autumn SSTAs exhibits significant correlations with the sea level pressure field over the North Pacific a season later.

It is shown that the dominant mode of SSTA variability over the North Pacific, as inferred from empirical orthogonal function (EOF) analysis, exhibits a rather similar spatial structure year-round, with highest amplitude during summer. By means of singular value decomposition analysis, it is shown that this pattern is much more persistent from one season to the next (and particularly from summer to the next winter) than SSTAs at fixed grid points. It is substantially more persistent from one summer to the next than from one winter to the next, reflecting the relatively greater prominence of the interdecadal variability in the summertime SSTAs.

The minor differences in the structure of the winter and summer EOFs can be attributed to the coupling with the atmospheric Pacific North American pattern during winter, which induces SSTAs off the west coast of North America opposite in polarity to those in the central and western Pacific.

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Tao Luo, Renmin Yuan, Zhien Wang, and Damao Zhang

Abstract

In this study, collocated satellite and buoy observations as well as satellite observations over an extended region during 2006–10 were used to quantify the humidity effects on marine boundary layer (MBL) aerosols. Although the near-surface aerosol size increases with increasing near-surface relative humidity (RH), the influence of RH decreases with increasing height and is mainly limited to the lower well-mixed layer. In addition, the size changes of MBL aerosols with RH are different for low and high surface wind () conditions as revealed by observations and Mie scattering calculations, which may be related to different dominant processes (i.e., the hygroscopic growth process during low wind and the evaporation process during sea salt production during high wind). These different hygroscopic processes under the different conditions, together with the MBL processes, control the behaviors of the MBL aerosol optical depth () with RH. In particular, under high conditions, the MBL stratifications effects can overwhelm the humidity effects, resulting in a weak relationship of MBL on RH. Under low conditions, the stronger hygroscopic growth can overwhelm the MBL stratification effects and enhance the MBL with increasing RH. These results are important to evaluate and to improve MBL aerosols simulations in climate models.

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