Search Results

You are looking at 1 - 10 of 38 items for

  • Author or Editor: Shiyuan Zhong x
  • Refine by Access: All Content x
Clear All Modify Search
Marwan Katurji and Shiyuan Zhong

Abstract

A high-resolution numerical investigation of a cold-air pooling process (under quiescent conditions) is carried out that systematically highlights the relations between the characteristics of the cold-air pools (e.g., slope winds, vertical temperature and wind structure, and cooling rate) and the characteristics of the topography (e.g., basin size and slope angle) under different ambient stabilities. The Advanced Regional Prediction System model is used to simulate 40 different scenarios at 100-m (10 m) horizontal (vertical) resolution. Results are within the range of similar observed phenomena. The main physical process governing the cooling process near the basin floor (<200 m in height) was found to be longwave radiative flux divergence, whereas vertical advection of temperature dominated the cooling process for the upper-basin areas. The maximum downslope wind speed is linearly correlated with both basin size and slope angle, with stronger wind corresponding to larger basin and lower slope angle. As the basin size increases, the influence of slope angle on maximum downslope wind decreases and the maximum is located farther down the slope. These relationships do not appear to be sensitive to stability, but weaker stability produces more cooling in the basin atmosphere by allowing stronger rising motion and adiabatic cooling. Insight gained from this study helps to improve the understanding of the cold-air pooling process within the investigated settings.

Full access
Shiyuan Zhong and Jerome Fast

Abstract

This study presents what is, to the authors' knowledge, the first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MMS), the Regional Atmospheric Modeling System (RAMS), and the NCEP Meso-Eta, at horizontal resolution finer than 1 km. Simulations were carried out for both weak and strong synoptic forcing cases during the Vertical Transport and Mixing (VTMX) field campaign conducted in the Salt Lake valley in October of 2000. Both upper-air and surface observations at high spatial and temporal resolution were used to evaluate the simulations with a focus on boundary layer structures and thermally driven circulations that developed in the valley. Despite differences in the coordinate systems, numerical algorithms, and physical parameterizations used by the three models, the types of forecast errors were surprisingly similar. The common errors in predicted valley temperature structure include a cold bias extending from the surface to the top of the valley atmosphere, lower than observed mixed-layer depths when the observed mixed layers were relatively high, and much weaker nocturnal inversion strengths over the valley floor. Relatively large wind forecast errors existed at times in the midvalley atmosphere even in the case of strong synoptic winds. The development of valley, slope, and canyon flows and their diurnal reversals under weak synoptic forcing were captured better by RAMS and MM5 than by Meso-Eta. Meso-Eta consistently underpredicted the strengths of these terrain-induced circulations and the associated convergence and divergence over the valley floor. As operational mesoscale modeling moves toward subkilometer resolution in the near future, more detailed forecasts of the circulation patterns and boundary layer structure can be produced for local-scale applications. However, this study shows that relatively large forecast errors can still exist even with sufficiently fine spatial resolution, indicating that the future for accurate local forecasting still lies in improved model parameterization of longwave radiation and turbulent mixing.

Full access
Lejiang Yu and Shiyuan Zhong

Abstract

Strong wind events (SWEs) over Antarctica and its surrounding oceans are investigated using gridded surface wind data from the ERA-Interim for the 1979–2017 period. Throughout the year, SWEs are more prevalent over the coastal region of East Antarctica where mean surface wind speeds are also higher. The occurrences of SWEs appear to be accompanied by positive anomalies in surface temperature and negative (positive) anomalies in mean sea level pressure related to cyclone (anticyclone) activity over the Ronne and Ross Ice Shelves and coastal regions (the inland areas of East Antarctica). The interannual variability of the SWE occurrences appears to be related to the southern annular mode (SAM) and, to a lesser degree, ENSO. The trends of SWE in the recent four decades exhibit considerable regional variations that are consistent with the trends in seasonal mean wind speed and surface air temperature, and can be largely explained by the variations in the sea level pressure trends across the region.

Full access
Shiyuan Zhong and C. David Whiteman

Abstract

The characteristics of well-developed downslope winds observed by tethered balloon soundings at multiple locations over a low-angle slope in the Salt Lake Valley are studied using the Regional Atmospheric Modeling System (RAMS). The model successfully simulated the key properties of the observed wind and temperature structure and evolution and provided insight into the forcing mechanisms. The results show that, although the slope angle is only 1.6°, the buoyancy force associated with the local temperature perturbation caused by nocturnal cooling of the slope surface is capable of producing the unusually strong and deep downslope winds observed by the tethersondes. The hypothesis that the flow is produced locally by the temperature deficit is further confirmed by analysis of the momentum budget that indicates a very small contribution from advection to the downslope mass flux. The analysis also reveals the importance of the along-slope pressure gradient force, which has been neglected by some previous investigators. On an isolated slope, the pressure gradient force, which develops as the downslope-flow layer deepens with downslope distance, is important mostly in the upper part of the downslope wind layer where it counterbalances the buoyancy force. On a slope in a valley, the pressure gradient force interacts with the valley inversion to produce intermittency in the downslope jet and may also significantly slow the flow as the inversion strengthens during the night. The simulations for two different observational nights indicate that the maximum downslope wind speed is sensitive to ambient stability, with near-neutral ambient stability yielding a stronger downslope jet than does a more stable ambient atmosphere. Sensitivity studies suggest that an increase in down-valley winds leads to a decrease in the maximum downslope wind speed and an increase in the thickness of the downslope wind layer. An increase in slope roughness, on the other hand, increases the height of the downslope jet but has little effect on other properties. The downslope wind is stronger over a gentle 1.6° slope than over a much steeper slope of 11°, mainly because of the combination of the stronger buoyancy and weaker pressure gradient over the gentle slope.

Full access
Shiyuan Zhong and J. C. Doran

Abstract

A numerical modeling study was conducted to examine the response of the atmospheric boundary layer to inhomogeneous surface fluxes. The study was used to extend the results obtained from a field experiment carried out in spring 1992 in north-central Oregon over a region characterized by warm, dry sagebrush and grassland steppe and cooler, irrigated farmland. Characteristic scales of the two prominent land-use types were on the order of 10 km or more. A series of numerical experiments were carried out to analyze boundary-layer behavior on three days selected for detailed study, to perform a set of sensitivity tests to identify the principal mechanisms responsible for secondary circulations in the region, and, with selected two-dimensional simulations, to verify the role of advection in maintaining well-mixed layers over cool surfaces. Although contrasts in land use produce measurable secondary circulations over the study area, terrain effects and ambient winds can mask much of the response to the differential heating over the warm and cool regions. The depth of the mixed layer is poorly correlated with the local underlying surface heat fluxes but is governed instead by a combination of local fluxes, horizontal advection, convergence or divergence, and shear production of turbulence. An analysis of mesoscale heat fluxes, that is, the fluxes associated with secondary circulations that would not be resolved by a coarse resolution model, shows that the contribution, arising from land-use differences, to the domain-averaged atmospheric heating rate is small for this area. The authors suggest that modeling studies based on idealized terrain and land-use configurations may tend to overestimate the effect of mesoscale fluxes on the temperature structure predicted by coarse-resolution models applied to real world conditions. Even so, secondary circulations may be significant for other boundary-layer properties, such as mixed-layer depth and cloud formation.

Full access
Shiyuan Zhong and Eugene S. Takle

Abstract

A three-dimensional mesoscale numerical model has been used to examine the effects of large-scale background winds on the characteristics of the sea–land-breeze circulations over an area with an irregular coastline and complex surface-heating patterns at Kennedy Space Center/Cape Canaveral in Florida. A series of numerical experiments was performed in which the large-scale winds were varied in both speed and direction. The surface heating was based on measured surface-temperature variation from the Kennedy Space Center Atmospheric Boundary Layer Experiment (KABLE) during the spring season when the land–sea temperature gradient reaches its maximum. The results from the simulations compared reasonably well with data available from KABLE.

The results show that an onshore large-scale flow produces weaker sea-breeze perturbations compared to those generated by an offshore flow. However, the coastal rivers and lagoons create intense surface convergence with strong vertical motion on the seaward side of the river by the merging of the onshore flow with the offshore river breezes, and such strong vertical motion can last for several hours. The disturbances caused by the inland water bodies are significant in the sea-breeze phase but are very minor in the land-breeze phase. An onshore synoptic wind causes an earlier onset of the sea breeze, but delays the onset of the land breeze, and a strong onshore flow of more than 5 m s−1 does not allow the land breeze to develop at all. The maximum offshore wind speed and vertical motion at night are less sensitive to the magnitude of surface cooling than to the large-scale flow and daytime surface heating, which together determine the initial flow at the beginning of the land–breeze phase. The results also show that the magnitude, the sense of rotation, and the diurnal variation of the dominant forces governing the wind-vector rotation change as the orientation of the synoptic wind direction changes. The rate of rotation in the sea-breeze phase is dominated mainly by the balance between the mesoscale pressure gradient and friction; at night, the Coriolis effect also contributes significantly to the balance of forces in the land-breeze phase.

Full access
Shiyuan Zhong and Eugene S. Takle

Abstract

The diurnal evolution of the three-dimensional structure of a mesoscale circulation system frequently occurring in the area of Kennedy Span Center-Cape Canaveral has been studied using the data from the Kennedy Space Center Atmospheric Boundary Layer Experiment (KABLE). The case was chosen from the spring intensive data-collection period when the greatest daytime temperature difference between land and water (sea and inland rivers) occurs and the local circulations are most intense. The daytime flow structure was determined primarily by the mesoscale pressure-gradient form created by the temperature contrast between land and water. A strong sea-breeze circulation, the dominant feature of the daytime flow field, was modified by a local inland river breeze known as the Indian River breeze, in that divergent flow over the river enhanced the sea-breeze convergence on the seaward side and generated additional convergence on the landward side of the river. The rivers near the coastline also modified the initial flow field by enhancing convergence in the surrounding areas and speeding up the movement of the sea-breeze front. The nighttime flow structure was dominated by a large-scale land breeze that was relatively uniform over the area and became quasi-stationary after midnight. The nonuniformity of the wind-vector rotation rate suggests that mesoscale forcing significantly modifies the Coriolis-induced oscillation. No clear convergence patterns associated with the rivers were observed at night. Detailed characteristics over a diurnal cycle of the sea-land breeze and of the river breeze onset time, strength, depth, propagation speed and both landward and seaward extension, are documented in this study. Some boundary-layer characteristics needed for predicting diffusion of pollutants released from coastal launch pads, including atmospheric stability, depth of the thermal internal boundary layer, and turbulent mixing are also discussed.

Full access
Larry K. Berg and Shiyuan Zhong

Abstract

The sensitivity of high-resolution mesoscale simulations to boundary layer turbulence parameterizations is investigated using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) and observations from two field campaigns. Three widely used turbulence parameterizations were selected for evaluation, two of which [Blackadar (BK) and Medium Range Forecast (MRF) schemes] are simple first-order nonlocal schemes and one [Gayno–Seaman (GS) scheme] of which is a more complex 1.5-order local scheme that solves a prognostic equation for turbulence kinetic energy (TKE). The two datasets are the summer 1996 Boundary Layer Experiment (BLX96) in the southern Great Plains and the autumn 2000 Vertical Transport and Mixing (VTMX) field campaign in the Salt Lake Valley in Utah. Comparisons are made between observed and simulated mean variables and turbulence statistics. Despite the differences in their complexity, all three schemes show similar skill predicting near-surface and boundary layer mean temperature, humidity, and winds at both locations. The BK and MRF schemes produced daytime boundary layers that are more mixed than those produced by the GS scheme. The mixed-layer depths are generally overestimated by the MRF scheme, underestimated by the GS scheme, and well estimated by the BK scheme. All of the schemes predicted surface latent heat fluxes that agreed reasonably well with the observed values, but they substantially overestimated surface sensible heat fluxes because of a significant overprediction of net radiation. In addition, each parameterization overestimated the sensible and latent heat flux aloft. The results suggest that there is little gain in the overall accuracy of forecasts with increasing complexity of turbulence parameterizations.

Full access
C. David Whiteman and Shiyuan Zhong

Abstract

Thermally driven downslope flows were investigated on a low-angle (1.6°) slope on the west side of the floor of Utah’s Salt Lake Valley below the Oquirrh Mountains using data from a line of four tethered balloons running down the topographic gradient and separated by about 1 km. The study focused on the evolution of the temperature and wind structure within and above the slope flow layer and its variation with downslope distance. In a typical situation, on clear, undisturbed October nights a 25-m-deep temperature deficit of 7°C and a 100–150-m-deep downslope flow with a jet maximum speed of 5–6 m s−1 at 10–15 m AGL developed over the slope during the first 2 h following sunset. The jet maximum speed and the downslope volume flux increased with downslope distance. The downslope flows weakened in the late evening as the stronger down-valley flows expanded to take up more of the valley atmosphere and as ambient stability increased in the lower valley with the buildup of a nocturnal temperature inversion. Downslope flows over this low-angle slope were deeper and stronger than has been reported previously by other investigators, who generally investigated steeper slopes and, in many cases, slopes on the sidewalls of isolated mountains where the downslope flows are not subject to the influence of nighttime buildup of ambient stability within valleys.

Full access
C. David Whiteman, Shiyuan Zhong, and Xindi Bian

Abstract

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.

Full access