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Jerome D. Fast, Shiyuan Zhong, and C. David Whiteman

Abstract

A mesoscale model is used to simulate the nocturnal evolution of the wind and temperature fields within a small, elliptical basin located in western Colorado that has a drainage area of about 84 km2. The numerical results are compared to observed profiles of wind and potential temperature. The thermal forcing of the basin wind system and the sources of air that support the local circulations are determined. Individual terms of the basin atmospheric heat budget are also calculated from the model results.

The model is able to reproduce key features of the observed potential temperature profiles over the basin floor and winds exiting the basin through the narrow canyon that drains the basin. Complex circulations are produced within the basin atmosphere as a result of the convergence of drainage flows from the basin sidewalls. The strength of the sidewall drainage flow varies around the basin and is a function of the source area above the basin, the local topography, and the ambient winds. Flows on the basin floor are affected primarly by the drainage winds from the northern part of the basin. The near-surface sidewall drainage flows converge within the southern portion of the basin, producing a counterclockwise eddy during most of the evening. Evaluation of the individual terms of the atmospheric heat budget show that the forcing due to advection and turbulent diffusion is significantly larger above the sidewalls than over the basin floor; therefore, measurements made over the basin floor would not be representative of the basin as a whole. The cooling in the center of the basin results from the local radiative flux divergence and the advection of cold air from the sidewalls, and the cooling above the basin sidewalls is due primarily to turbulent sensible heat flux divergence. A high rate of atmospheric cooling occurs within the basin throughout the evening, although the strongest cooling occurs in the early evening hours. Sensitivity tests show that the thermal structure, circulations, and rate of cooling can be significantly affected by ambient wind direction and, to a lesser extent, vegetation coverage.

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Craig B. Clements, C. David Whiteman, and John D. Horel

Abstract

The evolution of potential temperature and wind structure during the buildup of nocturnal cold-air pools was investigated during clear, dry, September nights in Utah's Peter Sinks basin, a 1-km-diameter limestone sinkhole that holds the Utah minimum temperature record of −56°C. The evolution of cold-pool characteristics depended on the strength of prevailing flows above the basin. On an undisturbed day, a 30°C diurnal temperature range and a strong nocturnal potential temperature inversion (22 K in 100 m) were observed in the basin. Initially, downslope flows formed on the basin sidewalls. As a very strong potential temperature jump (17 K) developed at the top of the cold pool, however, the winds died within the basin and over the sidewalls. A persistent turbulent sublayer formed below the jump. Turbulent sensible heat flux on the basin floor became negligible shortly after sunset while the basin atmosphere continued to cool. Temperatures over the slopes, except for a 1–2-m-deep layer, became warmer than over the basin center at the same altitude. Cooling rates for the entire basin near sunset were comparable to the 90 W m−2 rate of loss of net longwave radiation at the basin floor, but these rates decreased to only a few watts per square meter by sunrise. This paper compares the observed cold-pool buildup in basins with inversion buildup in valleys.

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C. D. Whiteman, J. M. Hubbe, and W. J. Shaw

Abstract

Recent advances in electronics miniaturization have allowed the commercial development of sensor/datalogger combinations that are sufficiently inexpensive and appear to be sufficiently accurate to deploy in measurement arrays to resolve local atmospheric structure over periods of weeks to months. As part of an extended wintertime field experiment in the Columbia Basin of south-central Washington, laboratory and field tests were performed on one such set of battery-powered temperature dataloggers (HOBO H8 Pro from Onset Computer, Bourne, Massachusetts). Five loggers were selected for laboratory calibration. These were accurate to within 0.26°C over the range from −5° to +50°C with a resolution of 0.04°C or better. Sensor time constants were 122 ± 6 s. Sampling intervals can be varied over a wide range, with onboard data storage of more than 21 000 data points. Field experiences with a set of 15 dataloggers are also described. The loggers appear to be suitable for a variety of meteorological applications.

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R. A. Ferrare, S. H. Melfi, D. N. Whiteman, K. D. Evans, F. J. Schmidlin, and D. O'C. Starr

Abstract

This paper examines the calibration characteristics of the NASA/GSFC Raman water vapor lidar during three field experiments that occurred between 1991 and 1993. The lidar water vapor profiles are calibrated using relative humidity profiles measured by AIR and Vaisala radiosondes. The lidar calibration computed using the AIR radiosonde, which uses a carbon hygristor to measure relative humidity, was 3%–5% higher than that computed using the Vaisala radiosonde, which uses a thin film capacitive element. These systematic differences were obtained for relative humidities above 30% and so cannot be explained by the known poor low relative humidity measurements associated with the carbon hygristor. The lidar calibration coefficient was found to vary by less than 1% over this period when determined using the Vaisala humidity data and by less than 5% when using the AIR humidity data. The differences between the lidar relative humidity profiles and those measured by these radiosondes are also examined. These lidar–radiosonde comparisons are used in combination with a numerical model of the lidar system to assess the altitude range of the GSFC lidar. The model results as well as the radiosonde comparisons indicate that for a lidar located at sea level measuring a typical midlatitude water vapor profile, the absolute error in relative humidity for a 10-min, 75-m resolution profile is less than 10% for altitudes below 8.5 km. Model results show that this maximum altitude can be extended to 10 km by increasing the averaging time and/or reducing the range resolution.

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Stephan F. J. de Wekker, Shiyuan Zhong, Jerome D. Fast, and C. David Whiteman

Abstract

Numerical experiments have been carried out with a two-dimensional nonhydrostatic mesoscale model to investigate the diurnal temperature range in a basin and the thermally driven plain-to-basin winds. Under clear-sky conditions, the diurnal temperature range in a basin is larger than over the surrounding plains due to a combination of larger turbulent sensible heat fluxes over the sidewalls and a volume effect in which energy fluxes are distributed through the smaller basin atmosphere. Around sunset, a thermally driven plain-to-basin flow develops, transporting air from the plains into the basin. Characteristics of this plain-to-basin wind are described for idealized basins bounded by sinusoidal mountains and the circumstances under which such winds might or might not occur are considered. In contrast with a previous numerical study, it is found that the height of the mixed layer over the plains relative to the mountain height is not a critical factor governing the occurrence or nonoccurrence of a plain-to-basin wind. The critical factor is the horizontal temperature gradient above mountain height created by a larger daytime heating rate over the basin topography than over the plains. Subsidence and turbulent heat flux divergence play important roles in this heating above mountain height.

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C. D. Whiteman, T. Haiden, B. Pospichal, S. Eisenbach, and R. Steinacker

Abstract

Air temperature data from five enclosed limestone sinkholes of various sizes and shapes on the Hetzkogel Plateau near Lunz, Austria (1300 m MSL), have been analyzed to determine the effect of sinkhole geometry on temperature minima, diurnal temperature ranges, temperature inversion strengths, and vertical temperature gradients. Data were analyzed for a non-snow-covered October night and for a snow-covered December night when the temperature fell as low as −28.5°C. A surprising finding is that temperatures were similar in two sinkholes with very different drainage areas and depths. A three-layer model was used to show that the sky-view factor is the most important topographic parameter controlling cooling for basins in this size range in near-calm, clear-sky conditions and that the cooling slows when net longwave radiation at the floor of the sinkhole is nearly balanced by the ground heat flux.

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Steven E. Koch, Paul B. Dorian, R. Ferrare, S. H. Melfi, William C. Skillman, and D. Whiteman

Abstract

Detailed moisture observations from a ground-based Raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented. A synthesis of the lidar data with conventional meteorological data, in conjunction with theoretical calculations and comparison to laboratory studies, leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore. The disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site. Bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current. Analysis suggests that the bore was generated by the collapse of the gravity current into a stable, nocturnal inversion layer, and subsequently propagated along this wave guide at nearly twice the speed of the gravity current.

The Raman lidar provided detailed measurements of the vertical structure of the bore and its parent generation mechanism. A mean bore depth of 1.9 km is revealed by the lidar, whereas a depth of 2.2 km is predicted from hydraulic theory. Observed and calculated bore speeds were also found to agree reasonably well with one another (∼ ±20%). Comparison of these observations with those of internal bores generated by thunderstorms in other studies reveals that this bore was exceedingly strong, being responsible for nearly tripling the height of a surface-based inversion that had existed ahead of the bore and dramatically increasing the depth of the moist layer due to strong vertical mixing. Subsequent appearance of the relatively shallow gravity current underneath this mixed region resulted in the occurrence of an elevated mixed layer, as confirmed with the special radiosonde measurements.

A synthesis of the lidar and radiosonde observations indicates that bore-induced parcel displacements attenuated rapidly at the same height as the level of strongest wave trapping predicted from the theory of Crook. This trapping mechanism, which is due to the existence of a low-level jet, results in a long-lived bore, and seems to he a common phenomenon in the environment of thunderstorm-generated bores and solitary waves. Despite the weakening of a capping inversion by this strong and persistent bore, analysis indicates that the 30-min averaged lifting of 0.7 m s−1 was confined to a too shallow layer near the surface to trigger deep convection, and could only produce scattered low clouds as deduced from the lidar measurements.

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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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C. D. Whiteman, T. Haiden, B. Pospichal, S. Eisenbach, and R. Steinacker
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B. B. Demoz, D. O’C. Starr, K. D. Evans, A. R. Lare, D. N. Whiteman, G. Schwemmer, R. A. Ferrare, J. E. M. Goldsmith, and S. E. Bisson

Abstract

Detailed observations of the interactions of a cold front and a dryline over the central United States that led to dramatic undulations in the boundary layer, including an undular bore, are investigated using high-resolution water vapor mixing ratio profiles measured by Raman lidars. The lidar-derived water vapor mixing ratio profiles revealed the complex interaction between a dryline and a cold-frontal system. An elevated, well-mixed, and deep midtropospheric layer, as well as a sharp transition (between 5- and 6-km altitude) to a drier region aloft, was observed. The moisture oscillations due to the undular bore and the mixing of the prefrontal air mass with the cold air at the frontal surface are all well depicted. The enhanced precipitable water vapor and roll clouds, the undulations associated with the bore, the strong vertical circulation and mixing that led to the increase in the depth of the low-level moist layer, and the subsequent lifting of this moist layer by the cold-frontal surface, as well as the feeder flow behind the cold front, are clearly indicated.

A synthesis of the Raman lidar–measured water vapor mixing ratio profiles, satellite, radiometer, tower, and Oklahoma Mesonet data indicated that the undular bore was triggered by the approaching cold front and propagated south-southeastward. The observed and calculated bore speeds were in reasonable agreement. Wave-ducting analysis showed that favorable wave-trapping mechanisms existed; a low-level stable layer capped by an inversion, a well-mixed midtropospheric layer, and wind curvature from a low-level jet were found.

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