Surface-based Inversion Research Articles

Structure of an Internal Bore and Dissipating Gravity Current as Revealed by Raman Lidar

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.

The Effect of Building Shadows on the Vertical Temperature Structure of the Lower Atmosphere in Downtown Denver

Abstract Denver's Continuous Air Monitoring Program (CAMP) site, typically recording the highest carbon monoxide levels in the metropolitan area; lies within a large region of downtown Denver shadowed by tall buildings. Two studies conducted during the winters of 1987/88 and 1988/89 indicated several possible scenarios leading to the high-pollution episodes often reported at CAMP. Sodar records and stability calculations at CAMP indicate that building shadows may be a contributing factor. The building shadowing was simulated by a computer model and its effects were examined from 2 days of detailed vertical temperature profiles taken in the vicinity of CAMP. The vertical temperature structure was mapped both spatially and temporally as it pertains to the shadowed and unshadowed regions. Results show that shadowing at CAMP is quickly followed by the formation of a shadow surface-based inversion and a local rise in carbon monoxide concentrations. Strength of the inversion depends on the meteorology and surfa...

Evolution of the Nocturnal Inversion Layer at an Urban and Nonurban Location

The evolutionary cycle of the nocturnal radiation inversion layer, from formation through the time of erosion under fair weather summer conditions was investigated by time-series analyses of observations of inversion base and top heights, and inversion strength at an urban and a nonurban site in St. Louis, Missouri. The time-dependent behavior of each inversion parameter is presented from statistical and least-squares regression analyses. Differences of inversion evolution between these sites are discussed. A surface-based inversion generally formed before sunset at the nonurban site, and the growth of the inversion top height has been described quite well by (2KTt)½, where KT is the thermal eddy coefficient. The average time of formation of an elevated inversion layer at the urban site was 2.5 h after sunset. The height of the nocturnal urban boundary layer decreased after formation under steady wind conditions. The urban mixing height was consistently higher during the morning than at the nonurban site, although the difference diminished with time since the nonurban mixing height growth rate was greater. The slower growth rate of the urban mixing height was attributed primarily to advection of relatively cold air and lower mixing heights from the upwind nonurban environment. An overall rise of the inversion top height after sunrise was believed to be due to urban-induced upward motions, caused by low-level convergence that produced increasing mixing height growth rates. The average time for the inversion layer to erode completely was 4 h after sunrise at both sites.

Observations and Models of Simple Nocturnal Slope Flows

Abstract Measurements of simple nocturnal slope winds were taken on Rattlesnake Mountain, a nearly ideal two-dimensional ridge. Tower and tethered balloon instrumentation allowed the determination of the wind and temperature characteristics of the katabatic layer as well as the ambient conditions. Two cases were chosen for study; these were marked by well-defined surface-based temperature inversions and a low-level maximum in the downslope wind component. The downslope development of the slope flow could be determined from the tower measurements, and showed a progressive strengthening of the katabatic layer. Hydraulic models developed by Manins and Sawford (1979a) and Briggs (1981) gave useful estimates of drainage layer depths, but were not otherwise applicable. A simple numerical model that relates the eddy diffusivity to the local turbulent kinetic energy was found to give good agreement with the observed wind and temperature profiles of the slope flows.

Observations and Simulations of the Diurnal Variation of the Urban Heat Island Circulation and Associated Variations of the Ozone Distribution: A Case Study

Abstract The observed surface and upper air temperature and wind field patterns on 8 June 1976 in St. Louis, Missouri, were analyzed and compared with simulation results from a three-dimensional hydrodynamic model. An urban heat island (1–2°C temperature difference between the urban and rural regions) persisted during the day. The daytime temperature differential was relatively weak compared to that at night (∼5°C difference). In contrast, the urban heat island circulation was more intense during the day. This is thought to be due to the heating being distributed through a deeper layer. In the early evening, the heat island circulation dissipated due to the development of a surface-based inversion in the boundary layer. The highest concentrations of O3 at the surface were found in the zone of convergence associated with the urban heat island circulation immediately downwind of the center of the city. As the heat island circulation dissipated during the early evening, the area of high O3 concentration was ...

The observed generation and breaking of atmospheric internal gravity waves over the ocean

Observations are presented for internal gravity waves and their breaking at a height of 23.5 m over the ocean in surface-based inversions which are formed because of the advection of warm air over cold water. The spectral and cospectral analyses of velocity and temperature fluctuations were made to establish the characteristic features of the waves. Flow visualization photographs of smoke released during the breaking of a wave are also presented. Comparison between the turbulent energies present during and after breaking of a wave indicates enormous mixing and dispersion occurring during breaking.

Inversion Climatology at San José California

Month-to-month variations in the early morning surface-based and near-noon elevated inversions at San Jose, Calif., were determined from slow rise radiosondes launched during a four-year period. A high frequency of shallow, radiative, surface-based inversions were found in winter during the early morning hours, while during the same period in summer, a low frequency of deeper based inversions arose from a combination of radiative and subsidence processes. The frequency of elevated inversions in the hours near noon was lowest during fall and spring, while inversion bases were highest and thicknesses least during these periods.

Statistical Hypothesis Tests of Some Micrometeorological Observations

Abstract Chi-square goodness-of-fit is used to test the hypothesis that the medium scale of turbulence in the atmospheric surface layer is normally distributed. Coefficients of skewness and excess are computed from the data. If the data are not normal, these coefficients are used in Edgeworth's asymptotic expansion of Gram-Charlier series to determine an alternate probability density function. The observed data are then compared with the modified probability densities and the new chi-square values computed. Seventy percent of the data analyzed was either normal or approximately normal. The coefficient of skewness g1 has a good correlation with the chi-square values. Events with |g1| < 0.21 were normal to begin with and those with 0.21 < |g1| < 0.43 were approximately normal. Intermittency associated with the formation and breaking of internal gravity waves in surface-based inversions over water is thought to be the reason for the non-normality.

Vertical Temperature Structure During the 1966 Thanksgiving Week Air Pollution Episode in New York City

This paper presents time cross-sections of vertical temperature structure during the 1966 Thanksgiving week air pollution episode in New York City, based on 6-hourly soundings at Kennedy Airport. The analyses depict numerous inversions in the lower 10,000 ft of the atmosphere, including an interesting sequence of surface-based inversions and an outstanding inversion aloft. Diurnal and daily variations in the height of the mixing layer can also be seen. Some possible influences of the vertical temperature structure on SO2 concentrations in Manhattan are discussed, and it is suggested that exceptional peak SO2 concentrations were largely due to the fumigation process.