Showing papers by "Shih-Yu Wang published in 2010"

Coherence between the Great Salt Lake Level and the Pacific Quasi-Decadal Oscillation

Abstract: The lake level elevation of the Great Salt Lake (GSL), a large closed basin lake in the arid western United States, is characterized by a pronounced quasi-decadal oscillation (QDO). The variation of the GSL elevation is very coherent with the QDO of sea surface temperature anomalies in the tropical central Pacific (also known as the Pacific QDO). However, such coherence denies any direct association between the precipitation in the GSL watershed and the Pacific QDO because, in a given frequency, the precipitation variation always leads the GSL elevation variation. Therefore, the precipitation variation is phase shifted from the Pacific QDO. This study investigates the physical mechanism forming the coherence between the GSL elevation and the Pacific QDO. Pronounced and coherent quasi-decadal signals in precipitation, streamflow, water vapor flux, and drought conditions are found throughout the Great Basin. Recurrent atmospheric circulation patterns develop over the Gulf of Alaska during the warm-to-cool and cool-to-warm transition phases of the Pacific QDO. These circulation patterns modulate the water vapor flux associated with synoptic transient activities over the western United States and, in turn, lead to the QDO in the hydrological cycle of the Great Basin. As the GSL integrates the hydrological responses in the Great Basin, the hydrological QDO is then transferred to the GSL elevation. Because the GSL elevation consistently lags the precipitation by a quarter-phase (about 3 yr in the quasidecadal time scale), these processes take an average of 6 yr for the GSL elevation to eventually respond to the Pacific QDO. This creates a half-phase delay of the GSL elevation from the Pacific QDO, thereby forming the inverse, yet coherent, relationship between them. Tree-ring reconstructed precipitation records confirm that the quasi-decadal signal in precipitation is a prominent feature in this region.

A planetary-scale land - sea breeze circulation in East Asia and the western North Pacific

Abstract: The diurnal wind variation over the East Asian continent is commonly considered to be a combination of a land-sea breeze near the coast and a mountain –valley breeze along the slopes of the Tibetan Plateau. The local land –sea breeze along the coastline typically spans <100 km into the ocean. However, a detailed examination of the global reanalysis data suggests that this local land –sea breeze circulation apparently couples with the global-scale diurnal atmospheric pressure tide to produce a planetary-scale land –sea breeze with a spatial scale of ∼1000 km over the western North Pacific. Computations of the momentum budget and equivalent potential temperatures indicate that the atmospheric diurnal tidal wave contributes the most to this circulation feature. A diagnosis of the water vapour budget further suggests that the convergence of water vapour flux, which is related to the convergence of low-level wind induced by the seasonal change of diurnal tidal wave, leads to different times of occurrence of maximum diurnal rainfall over East Asia between summer and winter. Copyright c

Atmospheric Scale Interaction on Wintertime Intermountain West Low-Level Inversions

Abstract: Persistent winter inversions result in poor air quality in the Intermountain West of the United States. Although the onset of an inversion is relatively easy to predict, the duration and the subsequent breakup of a persistent inversion event remains a forecasting challenge. For this reason and for this region, historic soundings were analyzed for Salt Lake City, Utah, with reanalysis and station data to investigate how persistent inversion events are modulated by synoptic and intraseasonal variabilities. The results point to a close linkage between persistent inversions and the dominant intraseasonal (30 day) mode that characterizes the winter circulation regime over the Pacific Northwest. Meteorological variables and pollution (e.g., particulate matter of #2.5-mm diameter, PM2.5) revealed coherent variations with this intraseasonal mode. The intraseasonal mode also modulates the characteristics of the synoptic (6 day) variability and further influences the duration of persistent inversions in the Intermountain West. The interaction between modes suggests that a complete forecast of persistent inversions is more involved and technically beyond numerical weather prediction models intended for the medium range (;10 day). Therefore, to predict persistent inversions, the results point to the adoption of standard medium-range forecasts with a longerterm climate diagnostic approach.

Circumglobal teleconnection and early summer rainfall in the US Intermountain West

Abstract: Physical processes responsible for the abnormally wet condition in the Central Intermountain West (CIW) of the United States in June 2009 were investigated. It was illustrated that persistent rainy conditions over the CIW during June 2009 were associated with a pronounced circumglobal teleconnection pattern, which is characterized as a short Rossby wave train along the jet stream waveguide with a wave number 5 structure. The ascending motion and moisture flux convergence over the CIW associated with the cyclonic action center over the US West Coast in the teleconnection wave train could be essential for the persistent local rainfall during June 2009. Further analysis suggested that the June 2009 circulation pattern is consistent with a prevailing mode of the summer circumglobal teleconnection pattern. The findings in this study provide information for improved understanding of the early summer rainfall regime in the CIW.

CFS prediction of winter persistent inversions in the Intermountain Region

Abstract: A recent study by Gillies and others of persistent inversion events in the Intermountain West of the United States found a substantive linkage between the intraseasonal oscillation (ISO) and the development of persistent inversion events. Given that NCEP’s Climate Forecast System (CFS) has demonstrated skill in the prediction of the ISO as far out as 1 month, it was decided to examine the CFS forecast’s capability in the prediction of such winter persistent inversions. After initial analysis, a simple regression scheme is proposed that is coupled to the CFS output of geopotential height as a way to predict the occurrence of persistent inversion events for Salt Lake City, Utah. Analysis of the CFS hindcasts through the period 1981‐2008 indicates that the regression coupled with the CFS can predict persistent inversion events with lead times of up to 4 weeks. The adoption of this coupled regression‐CFS prediction may improve the forecasting of persistent inversion events in the Intermountain West, which is currently restricted to the more limited time span (;10 days) of medium-range weather forecast models.

Quasi-decadal spectral peaks of tropical western Pacific SSTs as a precursor for tropical cyclone threat

Abstract: [1] A recent study identified significant spectral peaks in sea surface temperatures (SSTs) at quasi-decadal frequency bands, which were unique to the tropical western Pacific (TWP). Using the multitaper method for spectral and coherence analysis, this study finds that the TWP SSTs at these quasi-decadal time scales are coherent with western Pacific tropical cyclone threat [as measured by the power dissipation index (PDI)], but that the PDI lags TWP SSTs by about two years. Thus, the quasi-decadal peaks in TWP SSTs may be a precursor for enhanced tropical cyclone threat two years later. Composite analyses are shown to illustrate how areal tropical cyclone frequencies and relevant atmospheric fields evolve following the quasi-decadal peaks in TWP SSTs. The fields during peak PDI years strongly resemble those observed during El Nino events.

Sudden surface warming/drying events caused by typhoon passages across Taiwan

Abstract: Typhoon passages across Taiwan can generate sudden surface warming in downslope regions. Special characteristics and mechanisms for 54 such warming events that were identified during the 1961–2007 period are examined. Preferred warming regions were identified in northwest Taiwan, where warming is generated by downslope flow from east or northeast winds in westward-moving typhoons, and in southeast Taiwan, where it is generated by downslope flow from west or northwest winds in northwestward-moving typhoons. In addition to the orographic effect, warmings occurred exclusively within nonprecipitation zones of typhoons. Most northwest (southeast) warmings occur during the day (night) with an average lifetime of 4 (5) h, which roughly corresponds to the average time a nonprecipitation zone remains over a station. During the period examined, three typhoons generated warming events in both northwest and southeast Taiwan, and only Typhoon Haitang (2005) generated warmings with comparable magnitudes (∼12-K...

NAM Model Forecasts of Warm-Season Quasi-Stationary Frontal Environments in the Central United States

Abstract: Using a composite procedure, North American Mesoscale Model (NAM) forecast and observed environments associated with zonally oriented, quasi-stationary surface fronts for 64 cases during July–August 2006–08 were examined for a large region encompassing the central United States. NAM adequately simulated the general synoptic features associated with the frontal environments (e.g., patterns in the low-level wind fields) as well as the positions of the fronts. However, kinematic fields important to frontogenesis such as horizontal deformation and convergence were overpredicted. Surface-based convective available potential energy (CAPE) and precipitable water were also overpredicted, which was likely related to the overprediction of the kinematic fields through convergence of water vapor flux. In addition, a spurious coherence between forecast deformation and precipitation was found using spatial correlation coefficients. Composite precipitation forecasts featured a broad area of rainfall stretched p...

The Pacific QDO as a natural predictor for the Great Salt Lake elevation

Abstract: The lake elevation of the Great Salt Lake (GSL), a large closed basin in the arid western United States, is characterized by a pronounced quasi-decadal oscillation (QDO) (Lall and Mann 1995). Wang et al. (2009a) found that the variation of the GSL elevation is very coherent with the QDO of sea surface temperature anomalies in the tropical central Pacific, known as the Pacific QDO (White and Liu 2008a, b). The Pacific QDO can be depicted by the SST anomalies in the NINO4 region (Allan 2000), denoted as ΔSST (NINO4). Fig. 1 illustrates the coherence between the GSL elevation and ΔSST (NINO4). However, such a coherence denies any direct association between the precipitation over the GSL watershed and the Pacific QDO because, in a given frequency, the precipitation variations always lead the GSL elevation variations. In other words, a direct link between the Pacific QDO and the precipitation source of the GSL is absent. What causes the GSL elevation to vary so coherently with ΔSST (NINO4) is therefore an intriguing question.

NOTES AND CORRESPONDENCE NAM Model Forecasts of Warm-Season Quasi-Stationary Frontal Environments in the Central United States

Abstract: Using a composite procedure, North American Mesoscale Model (NAM) forecast and observed environments associated with zonally oriented, quasi-stationary surface fronts for 64 cases during July‐August 2006‐08 were examined for a large region encompassing the central United States. NAM adequately simulated the general synoptic features associated with the frontal environments (e.g., patterns in the low-level wind fields) as well as the positions of the fronts. However, kinematic fields important to frontogenesis such as horizontal deformation and convergence were overpredicted. Surface-based convective available potential energy (CAPE) and precipitable water were also overpredicted, which was likely related to the overprediction of the kinematic fields through convergence of water vapor flux. In addition, a spurious coherence between forecast deformation and precipitation was found using spatial correlation coefficients. Composite precipitation forecasts featured a broad area of rainfall stretched parallel to the composite front, whereas the composite observed precipitation covered a smaller area and had a WNW‐ESE orientation relative to the front, consistent with mesoscale convective systems (MCSs) propagating at a slight right angle relative to the thermal gradient. Thus, deficiencies in the NAM precipitation forecasts may at least partially result from the inability to depict MCSs properly. It was observed that errors in the precipitation forecasts appeared to lag those of the kinematic fields, and so it seems likely that deficiencies in the precipitation forecasts are related to the overprediction of the kinematic fields such as deformation. However, no attempts were made to establish whether the overpredicted kinematic fields actually contributed to the errors in the precipitation forecasts or whether the overpredicted kinematic fields were simply an artifact of the precipitation errors. Regardless of the relationship between such errors, recognition of typical warm-season environments associated with these errors should be useful to operational forecasters.