C. D. Whiteman

The relationship between overlying synoptic-scale flows and winds within a valley

Abstract The relationship between winds above and within the Tennessee Valley is investigated climatologically and with an atmospheric numerical model. For the climatological analyses, winds above the valley were determined by interpolation from four surrounding rawinsonde stations, while winds within the valley were measured on four 100-m towers. Tennessee Valley winds are generally weak and bidirectional, oriented along the valleys axis. The valley wind direction depends strongly on the component of the synoptic-scale pressure gradient that is superimposed along the valleys axis at ridge-top level, with winds blowing along the valleys axis from high toward low pressure. This relationship between winds above and within the valley can result in countercurrents similar to those observed in the Rhine Valley. While winds in the Tennessee Valley are driven primarily by this pressure-driven channeling mechanism, downward momentum transport can cause afternoon winds within the valley to approach the wind dir...

Particulate Air Pollution in Mexico City: A Collaborative Research Project

PM10, PM25, precursor gas, and upper-air meteorological measurements were taken in Mexico City, Mexico, from February 23 to March 22, 1997, to understand concentrations and chemical compositions of the citys particulate matter (PM). Average 24-hr PM10 concentrations over the period of study at the core sites in the city were 75 H g/m3. The 24-hr standard of 150 μ g/m3 was exceeded for seven samples taken during the study period; the maximum 24-hr concentration measured was 542 μ g/m3. Nearly half of the PM10 was composed of fugitive dust from roadways, construction, and bare land. About 50% of the PM10 consisted of PM2.5, with higher percentages during the morning hours. Organic and black carbon constituted up to half of the PM2.5. PM concentrations were highest during the early morning and after sunset, when the mixed layers were shallow. Meteorological measurements taken during the field campaign show that on most days air was transported out of the Mexico City basin during the afternoon with little day-to-day carryover.

Cold Pools in the Columbia Basin

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.

Boundary layer evolution and regional‐scale diurnal circulations over the and Mexican plateau

Data collected in a measurement campaign in February and March 1997 showed that the Mexico Basin (also called the Valley of Mexico), located atop the Mexican plateau, fails to develop the strong nocturnal inversions usually associated with basins and does not exhibit diurnally reversing valley wind systems. Data analyses, two-and three-dimensional numerical simulations with the Regional Atmospheric Modeling System (RAMS), and a Lagrangian particle dispersion model are used to interpret these observations and to examine the effects of topography and regional diurnal circulations on boundary layer evolution over the Mexico Basin and its surroundings during fair weather periods in the winter dry season. We show that the boundary layer evolution in and above the basin is driven primarily by regional diurnal circulations that develop between the air above the Mexican Plateau and the generally cooler surrounding coastal areas. A convective boundary layer (CBL) grows explosively over the plateau in the late morning to reach elevations of 2250 m agl (4500 m msl) by noon, and a strong baroclinic zone forms on the edges of the plateau separating the warm CBL air from its cooler surroundings. In early afternoon the rates of heating and CBL growth are slowed as cool air leaks onto the plateau and into the basin through passes and over low-lying plateau edges. The flow onto the plateau is retarded, however, by the strongly rising branch of a plain-plateau circulation at the plateau edges, especially where mountains or steep slopes are present. An unusually rapid and deep cooling of the air above the plateau begins in late afternoon and early evening when the surface energy budget reverses, the CBL decays, and air accelerates onto the plateau through the baroclinic zone. Flow convergence near the basin floor and the associated rising motions over the basin and plateau produce cooling in 3 hours that is equivalent to half the daytime heating. While the air that converges onto the plateau comes from elevations at and above the plateau, it is air that was modified earlier in the day by a cool, moist coastal inflow carried up the plateau slopes by the plain-plateau circulation.

Evaluation of an Inexpensive Temperature Datalogger for Meteorological Applications

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 8C over the range from 258 to 1508C with a resolution of 0.048C or better. Sensor time constants were 122 6 6s . 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.

Deep Valley Radiation and Surface Energy Budget Microclimates. Part I: Radiation

Abstract Solar and longwave radiation data are presented for five sites in Colorados 650 m deep semiarid Brush Creek Valley (39°32′N, 108°24′W) during September 1984. During the sunlit period of a nearly clear day, individual sites received 0.73–0.81 of the theoretical extraterrestrial solar radiation. Incoming solar radiation increased with elevation in the valley. Direct radiation made up 0.86– 0.88 of the downward shortwave flux. On average, 0.12–0.21 of the incoming shortwave radiation was reflected at the individual sites. Strong variations in reflected solar radiation and outgoing longwave radiation occurred from site to site. Because of the large direct beam component, aspect and inclination angles of the valley surfaces had a strong effect on the solar radiation received. Contrasts between a southwest- and northeast-facing sidewall were significant. Shading from surrounding topography produced inter-site differences in both instantaneous and daily total radiation. Inter-site differences in most d...

Minimum Temperatures, Diurnal Temperature Ranges, and Temperature Inversions in Limestone Sinkholes of Different Sizes and Shapes

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.

Turbulent Entrainment Into Natural Gravity-Driven Flows

Observations of entrainment into natural gravity-driven flows on sloping surfaces are described. It is shown that the laboratory-based entrainment law of Ellison & Turner (1959), which is often used for modelling of atmospheric and oceanic flows, underestimates the entrainment rates substantially, arguably due to the fact that the laboratory flows have been conducted at Reynolds numbers

A Sinkhole Field Experiment in the Eastern Alps

(Re \lesssim 10^3)

Morning transition tracer experiments in a deep narrow valley

below what is required for mixing transition (