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J. C. Doran and S. Zhong

Abstract

Current approaches to parameterizations of sub-grid-scale variability in surface sensible heat fluxes in general circulation models normally neglect the associated variability in mixed-layer depths. Observations and a numerical mesoscale model are used to show that the magnitude of such variability can be significant. Over a domain of (41 km)2, the standard deviation of simulated mixed-layer depths was found to be 21%–24% of the mean noontime values on three days, and the mean depths were not simply related to the mean sensible heat fluxes. Results obtained with two-dimensional simulations over idealized distributions of warm, dry and cool, or moist surfaces show that as the characteristic sizes of individual patches increase, the distributions of mixed-layer depths tend to assume a bimodal nature. under these conditions, the mean mixed-layer depth may have little physical relevance. Finally, the use of domain-averaged values of wind and temperature to compute surface fluxes is shown to be another potential source of error in flux parameterizations.

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J. C. Doran and S. Zhong

Abstract

A southeasterly flow in the form of a low-level jet that enters the Mexico City basin through a mountain gap in the southeast corner of the basin developed consistently in the afternoons or early evenings during a four-week 1997 winter field campaign. Peak wind speeds often exceeded 10 m s−1. Although these winds have not been studied previously, the observations suggest that they are a regular feature of the basin wind system, at least during the winter months. The jets were found more frequently during the early part of the experiment, when conditions in the basin were generally warmer, drier, and less cloudy, than in the later part when conditions were cooler, more humid, and cloudier. The winds usually were stronger during the early period, also. Temperature measurements from radiosondes launched inside and outside the basin showed a dependence of the gap wind strength on the temperature differences between the two regions. A three-dimensional numerical model was used to simulate the characteristics of the gap flows to provide information on the mechanisms responsible for their development. The maximum speed of the jet usually is reached several hours after the occurrence of the maximum temperature gradient between the basin and the region to the south. An analysis of the momentum balance shows that the gap wind is initiated by a north–south pressure gradient across the gap in the lower boundary layer arising from temperature differences between the warmer basin and the cooler exterior air. Penetration of the gap wind into the basin is caused primarily by horizontal advection. The gap wind plays an important role in the formation of a convergence zone, which can have an important effect on surface air pollutant distributions in the basin.

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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.

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J. C. Doran and S. Zhong

Abstract

An analysis of regional drainage flows in the Pacific Northwest is presented using results from a network of surface observations and a series of simulations carried out with a nested mesoscale model. The flows, which occur regularly in southeastern Washington during the late spring and summer months, are marked by an increase in wind speed and a shift to northwesterly wind directions early in the evening. The phenomenon occurs when a deep mixed layer forms cast of the Cascade Range, drawing cooler air from the west over the mountain crest. Anabatic and katabatic forcing, terrain channeling, and turning by the Coriolis force combine to produce the characteristic flow patterns.

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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.

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S. Zhong and J. C. Doran

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Measurements from the Southern Great Plains Cloud and Radiation Testbed site, which is situated in Oklahoma and Kansas and extends over an area approximately 300 km × 350 km in extent, are combined with results from a three-dimensional mesoscale model to study the sensitivity of boundary layer properties to spatially varying surface sensible and latent heat fluxes. Four cloud parameterization schemes are used to estimate the fractional cloudiness expected over the site on three case study days with settled weather conditions during the summer of 1994. Comparisons between observations and model predictions show good qualitative agreement. Although local responses to varying surface fluxes can be found, the replacement of the spatially varying surface conditions with uniform ones makes little difference in the simulated cloud cover or the vertical profiles of potential temperature and water vapor mixing ratio when these are averaged over the full site. Spatial variations in the ambient meteorology were found to be more important than variations in surface fluxes in determining cloud amount and areas of preferred cloud formation. This conclusion is supported by additional simulations in which both the ambient meteorology and surface conditions are averaged over scales ranging from 6.25 km to 300 km. The results call into question the importance of mesoscale fluxes (i.e., fluxes arising from secondary circulations induced by heating contrasts over different surfaces) in coarse-resolution models such as general circulation models, at least for settled weather conditions similar to those considered in this study.

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Xin-Zhong Liang, Min Xu, Kenneth E. Kunkel, Georg A. Grell, and John S. Kain

Abstract

The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)-based regional climate model (CMM5) simulations of U.S.–Mexico summer precipitation are quite sensitive to the choice of Grell or Kain–Fritsch convective parameterization. An ensemble based on these two parameterizations provides superior performance because distinct regions exist where each scheme complementarily captures certain observed signals. For the interannual anomaly, the ensemble provides the most significant improvement over the Rockies, Great Plains, and North American monsoon region. For the climate mean, the ensemble has the greatest impact on skill over the southeast United States and North American monsoon region, where CMM5 biases associated with the individual schemes are of opposite sign. Results are very sensitive to the specific methods used to generate the ensemble. While equal weighting of individual solutions provides a more skillful result overall, considerable further improvement is achieved when the weighting of individual solutions is optimized as a function of location.

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C. D. Whiteman, S. Zhong, W. J. Shaw, J. M. Hubbe, X. Bian, and J. Mittelstadt

Abstract

Persistent midwinter cold air pools produce multiday periods of cold, dreary weather in basins and valleys. Persistent stable stratification leads to the buildup of pollutants and moisture in the pool. Because the pool sometimes has temperatures below freezing while the air above is warmer, freezing precipitation often occurs, with consequent effects on transportation and safety. Forecasting the buildup and breakdown of these cold pools is difficult because the interacting physical mechanisms leading to their formation, maintenance, and destruction have received little study.

In this paper, persistent wintertime cold pools in the Columbia River basin of eastern Washington are studied. First a succinct meteorological definition of a cold pool is provided and then a 10-yr database is used to develop a cold pool climatology. This is followed by a detailed examination of two cold pool episodes that were accompanied by fog and stratus using remote and in situ temperature and wind sounding data. The two episodes illustrate many of the physical mechanisms that affect cold pool evolution. In one case, the cold pool was formed by warm air advection above the basin and was destroyed by downslope winds that descended into the southern edge of the basin and progressively displaced the cold air in the basin. In the second case, the cold pool began with a basin temperature inversion on a clear night and strengthened when warm air was advected above the basin by a westerly flow that descended from the Cascade Mountains. The cold pool was nearly destroyed one afternoon by cold air advection aloft and by the growth of a convective boundary layer (CBL) following the partial breakup of the basin stratus. The cold pool restrengthened, however, with nighttime cooling and was destroyed the next afternoon by a growing CBL.

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Kenneth E. Kunkel, Karen Andsager, Xin-Zhong Liang, Raymond W. Arritt, Eugene S. Takle, William J. Gutowski Jr., and Zaitao Pan

Abstract

A regional climate model simulation of the period of 1979–88 over the contiguous United States, driven by lateral boundary conditions from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis, was analyzed to assess the ability of the model to simulate heavy precipitation events and seasonal precipitation anomalies. Heavy events were defined by precipitation totals that exceed the threshold value for a specified return period and duration. The model magnitudes of the thresholds for 1-day heavy precipitation events were in good agreement with observed thresholds for much of the central United States. Model thresholds were greater than observed for the eastern and intermountain western portions of the region and were smaller than observed for the lower Mississippi River basin. For 7-day events, model thresholds were in good agreement with observed thresholds for the eastern United States and Great Plains, were less than observed for the most of the Mississippi River valley, and were greater than observed for the intermountain western region. The interannual variability in frequency of heavy events in the model simulation exhibited similar behavior to that of the observed variability in the South, Southwest, West, and North-Central study regions. The agreement was poorer for the Midwest and Northeast, although the magnitude of variability was similar for both model and observations. There was good agreement between the model and observational data in the seasonal distribution of extreme events for the West and North-Central study regions; in the Southwest, Midwest, and Northeast, there were general similarities but some differences in the details of the distributions. The most notable differences occurred for the southern Gulf Coast region, for which the model produced a summer peak that is not present in the observational data. There was not a very high correlation in the timing of individual heavy events between the model and observations, reflecting differences between model and observations in the speed and path of many of the synoptic-scale events triggering the precipitation.

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Peyman Zawar-Reza, Marwan Katurji, Iman Soltanzadeh, Tanja Dallafior, Shiyuan Zhong, Daniel Steinhoff, Bryan Storey, and S. Craig Cary

Abstract

Measuring routine vertical profiles of atmospheric temperature is critical in understanding stability and the dynamics of the boundary layer. Routine monitoring in remote areas such as the McMurdo Dry Valleys (MDV) of Antarctica is logistically difficult and expensive. Pseudovertical profiles that were derived from a network of inexpensive ground temperature sensors planted on valley sidewalls (up to 330 m above valley floor), together with data from a weather station and a numerical weather prediction model, provided a long-term climatological description of the evolution of the winter boundary layer over the MDV. In winter, persistent valley cold pools (VCPs) were common, lasting up to 2 weeks. The VCPs were eroded by warm-air advection from aloft associated with strong winds, increasing the temperature of the valley by as much as 25 K. Pseudovertical datasets as described here can be used for model validation.

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