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C. David Whiteman and Xindi Bian

A short review of solar semidiurnal atmospheric tides is presented. Semidiurnal atmospheric tides have been documented in the troposphere primarily through analyses of long time series of surface pressure measurements, although the winds produced by these tides have, by now, been well documented in the middle and upper atmosphere. Recent research using UHF and very-high-frequency radar wind profilers has now identified tidal wind perturbations in tropospheric data. This review focuses on the tidal wind characteristics and the distinctive signature of this wind system in radar profiler data analyses.

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Warren E. Heilman and Xindi Bian

Abstract

Recent research suggests that high levels of ambient near-surface atmospheric turbulence are often associated with rapid and sometimes erratic wildland fire spread that may eventually lead to large burn areas. Previous research has also examined the feasibility of using near-surface atmospheric turbulent kinetic energy (TKEs) alone or in combination with the Haines index (HI) as an additional indicator of anomalous atmospheric conditions conducive to erratic or extreme fire behavior. However, the application of TKEs-based indices for operational fire-weather predictions in the United States on a regional or national basis first requires a climatic assessment of the spatial and temporal patterns of the indices that can then be used for testing their operational effectiveness. This study provides an initial examination of some of the spatial and temporal variability patterns across the United States of TKEs and the product of HI and TKEs (HITKEs) using data from the North American Regional Reanalysis dataset covering the 1979–2008 period. The analyses suggest that there are regional differences in the behavior of these indices and that regionally dependent threshold values for TKEs and HITKEs may be needed for their potential use as operational indicators of anomalous atmospheric turbulence conditions conducive to erratic fire behavior. The analyses also indicate that broad areas within the northeastern, southeastern, and southwestern regions of the United States have experienced statistically significant positive trends in TKEs and HITKEs values over the 1979–2008 period, with the most substantial increases in values occurring over the 1994–2008 period.

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L. Ruby Leung, Yun Qian, and Xindi Bian

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The regional climate of the western United States shows clear footprints of interaction between atmospheric circulation and orography. The unique features of this diverse climate regime challenges climate modeling. This paper provides detailed analyses of observations and regional climate simulations to improve our understanding and modeling of the climate of this region. The primary data used in this study are the 1/8° gridded temperature and precipitation based on station observations and the NCEP–NCAR global reanalyses. These data were used to evaluate a 20-yr regional climate simulation performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5) driven by large-scale conditions of the NCEP–NCAR reanalyses. Regional climate features examined include seasonal mean and extreme precipitation; distribution of precipitation rates; and precipitation intensity, frequency, and seasonality. The relationships between precipitation and surface temperature are also analyzed as a means to evaluate how well regional climate simulations can be used to simulate surface hydrology, and relationships between precipitation and elevation are analyzed as diagnostics of the impacts of surface topography and spatial resolution. The latter was performed at five east–west transects that cut across various topographic features in the western United States.

These analyses suggest that the regional simulation realistically captures many regional climate features. The simulated seasonal mean and extreme precipitation are comparable to observations. The regional simulation produces precipitation over a wide range of precipitation rates comparable to observations. Obvious biases in the simulation include the oversimulation of precipitation in the basins and intermountain West during the cold season, and the undersimulation in the Southwest in the warm season. There is a tendency of reduced precipitation frequency rather than intensity in the simulation during the summer in the Northwest and Southwest, which leads to the insufficient summer mean precipitation in those areas. Because of the general warm biases in the simulation, there is also a tendency for more precipitation events to be associated with warmer temperatures, which can affect the simulation of snowpack and runoff.

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C. David Whiteman, Shiyuan Zhong, and Xindi Bian

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Wintertime temperature profiles in the Grand Canyon exhibit a neutral to isothermal stratification during both daytime and nighttime, with only rare instances of actual temperature inversions. The canyon warms during daytime and cools during nighttime more or less uniformly through the canyon’s entire depth. This weak stability and temperature structure evolution differ from other Rocky Mountain valleys, which develop strong nocturnal inversions and exhibit convective and stable boundary layers that grow upward from the valley floor. Mechanisms that may be responsible for the different behavior of the Grand Canyon are discussed, including the possibility that the canyon atmosphere is frequently mixed to near-neutral stratification when cold air drains into the top of the canyon from the nearby snow-covered Kaibab Plateau. Another feature of canyon temperature profiles is the sharp inversions that often form near the canyon rims. These are generally produced when warm air is advected over the canyon in advance of passing synoptic-scale ridges.

Wintertime winds in the main canyon are not classical diurnal along-valley wind systems. Rather, they are driven along the canyon axis by the horizontal synoptic-scale pressure gradient that is superimposed along the canyon’s axis by passing synoptic-scale weather disturbances. They may thus bring winds into the canyon from either end at any time of day.

The implications of the observed canyon boundary layer structure for air pollution dispersion are discussed.

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Steven J. Ghan, Xindi Bian, and Lisa Corsetti

Abstract

The low-level jet frequently observed in the Great Plains of the United States forms preferentially at night and apparently influences the timing of thunderstorms in the region. The authors have found that both the European Centre for Medium-Range Weather Forecasts general circulation model and the National Center for Atmospheric Research Community Climate Model simulate the low-level jet rather well, although the spatial distribution of the jet frequency simulated by the two GCMs differs considerably. Sensitivity experiments have demonstrated that the simulated low-level jet is surprisingly robust, with similar simulations at much coarser horizontal and vertical resolutions. However, both GCMs fail to simulate the observed relationship between clouds and the low-level jet. The pronounced nocturnal maximum in thunderstorm frequency associated with the low-level jet is not simulated well by either GCM, with only weak evidence of a nocturnal maximum in the Great Plains.

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C. David Whiteman, Xindi Bian, and Shiyuan Zhong

Abstract

The Colorado Plateau, surrounded by a ring of mountains, has the meteorological characteristics of a basin. Deep, persistent potential temperature inversions form in this basin in winter. The formation, maintenance, and dissipation of these inversions are investigated using two to four times daily radiosonde data from the winter and early spring of 1989–90. In winter, inversion evolution is forced primarily by synoptic-scale events. The buildup takes place over one or more days as warm air advection occurs above the basin with the approach of high pressure ridges. The breakup, which occurs with cold air advection above the basin as troughs approach, can occur over periods less than 12 h. Many approaching troughs modulate inversion strength and depth but are too weak to destroy the persistent inversion. Later in the winter and spring, the radiation-induced nocturnal inversion is destroyed nearly every day by the daytime growth of convective boundary layers from the basin floor and sidewalls.

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C. David Whiteman, Xindi Bian, and Shiyuan Zhong

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A climatology of the Great Plains low-level jet (LLJ) is developed from 2 yr of research rawinsonde data obtained up to eight times per day at a site in north-central Oklahoma. These data have better height and time resolution than earlier studies, and show that jets are stronger than previously reported and that the heights of maximum wind speed are closer to the ground. LLJs are present in 47% of the warm season soundings and 45% of the cold season soundings. More than 50% of the LLJs have wind maxima below 500 m above ground level (AGL). Because the 404-MHz radar profiler network in the central United States has its first data points at 500 m AGL, it is likely to miss some LLJ events and will have inadequate vertical resolution of LLJ wind structure.

Previous studies have identified LLJs on the basis of a wind speed profile criterion. This criterion fails to separate the classical southerly LLJs from the less frequent northerly jets, which differ in both structure and evolution. Classical southerly jets are more frequent; they occur year round, with the highest frequency in the summer and at night. Southerly LLJ wind speed maxima are most frequently found at 300–600 m AGL, and peak speeds, typically between 15 and 21 m s−1, are attained at 0200 CST. The height of the wind speed maximum varies little during nighttime—a period when surface-based inversions grow in depth but generally remain below the jet. Winds at the nose of the southerly jets exhibit a distinct diurnal clockwise turning in wind direction and an oscillation in speed.

Northerly jets occur year round. They are generally associated with cold air outbreaks and are found in the cold air behind southward-moving cold fronts. In winter, their frequency of occurrence rivals that of the southerly jets. Their occurrence, however, is less dependent on time of day, with a weak daytime maximum. They are more variable in the heights of their wind speed maxima, are associated more frequently with elevated frontal inversions, and do not exhibit a clockwise turning with time. The heights of the jet speed maxima are found to increase with distance behind the surface cold front.

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Shiyuan Zhong, C. David Whiteman, and Xindi Bian

Abstract

The diurnal evolution of the three-dimensional summer-season mean wind and temperature structure in California's Sacramento and San Joaquin Valleys (collectively called the Central Valley) is investigated using data from 22 radar wind profiler/radio acoustic sounding systems (RASS) operated as part of the Central California Ozone Study in 2000. The profiler network revealed, for the first time, that the persistent summer-season flow pattern documented by surface observations extends 800–1000 m above the surface. At most locations, up-valley winds persist during both day and night except at the upper ends of the valleys and close to the valley sidewalls where diurnal wind reversals occur. Wind speeds exhibit pronounced diurnal oscillations, with amplitudes decreasing with height. A low-level wind maximum occurs in the lowest 300 m, with a sharp decrease in speed above the maximum. Especially well defined nocturnal low-level jets occur at sites in the southern San Joaquin Valley, where maximum speeds of 10 m s−1 or more occur 1–2 h before midnight at heights near 300 m. The afternoon mixed layer, generally deeper than 1000 m, increases in depth with up-valley distance in both valleys. At night, temperature inversions develop in the lowest several hundred meters with near-isothermal layers above. Mean temperatures in the lowest 500 m of the valleys are always warmer than at the same altitude over the coast, and temperature increases from the lower to upper valleys. The diurnal oscillations of the coast– valley and along-valley temperature and pressure difference reach a maximum in late afternoon and a minimum in early morning. These oscillations are in phase with the diurnal variation of westerly onshore flows. The along-valley wind maxima, however, occur 1–2 h before midnight, whereas the along-valley pressure gradient maxima are usually found just before sunset.

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Shiyuan Zhong, Jerome D. Fast, and Xindi Bian

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A detailed case study of one complete episode of a typical summertime Great Plains low-level jet (LLJ) using data collected by the NOAA wind profiler demonstration network is presented. The high temporal and spatial resolution of the data from the profiler network permits a much more detailed picture of the Great Plains LLJ than is possible from previous studies of this phenomenon. A three-dimensional mesoscale numerical model is also used to simulate the episode and to provide information on the physical mechanisms responsible for the initiation, evolution, maintenance, and decay of the LLJ. The position and width of the jet core, as well as the diurnal variation of wind speed and direction inside the jet core are well predicted by the model. The analysis and modeling suggest that the diurnal oscillation of horizontal pressure gradient over sloping terrain is secondary to the inertial oscillation mechanism resulting from the release of frictional constraint in the evening and throughout the night in driving this example of the summertime Great Plains LLJ. The meridional variation of the Coriolis parameter as air moves northward appears to enhance the strength of the jet. A larger amplitude of the diurnal oscillation of the jet speed is found to be associated with drier soil, while rising motion downstream of the jet core is stronger for wetter soil. This enhanced vertical motion appears to be associated with latent heat release due to precipitation. A horizontal variation of soil moisture content also appears to be important in reproducing the observed convergence and precipitation patterns in this case.

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L. Ruby Leung, Yun Qian, Xindi Bian, and Allen Hunt

Abstract

The hydroclimate of the western United States is influenced by strong interannual variability of atmospheric circulation, much of which is associated with the El Niño–Southern Oscillation (ENSO). Precipitation anomalies during ENSO often show opposite and spatially coherent dry and wet patterns in the Northwest and California or vice versa. The role of orography in establishing mesoscale ENSO anomalies in the western United States is examined based on observed precipitation and temperature data at 1/8° spatial resolution and a regional climate simulation at 40-km spatial resolution. Results show that during El Niño or La Niña winters, strong precipitation anomalies are found in northern California, along the southern California coast, and in the northwest mountains such as the Olympic Mountains, the Cascades, and the northern Rockies. These spatial features, which are strongly affected by topography, are surprisingly well reproduced by the regional climate simulation.

A spatial feature investigated further is the positive–negative–positive precipitation anomaly found during El Niño years in the Olympic Mountains, and on the west side and east side of the Cascades in both observations and regional simulation. Observed streamflows of river basins located in those areas are found to be consistent with the precipitation anomalies. The spatial distribution of the precipitation anomalies is investigated by relating flow direction and moisture to the orientation of mountains and orographic precipitation. On the west side of the north–south-oriented Cascade Range, the increase in atmospheric moisture is not enough to compensate for the loss of orographic precipitation associated with a change in flow direction toward the southwest during El Niño years. In California, both the increase in atmospheric moisture and shift in wind direction toward the southwest enhance precipitation along the Sierra, which is oriented northwest to southeast. The spatial signature of the interactions between large-scale circulation and topography may provide useful information for seasonal predictions or climate change detection.

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