Jerome D. Fast

Meteorological factors associated with inhomogeneous ozone concentrations within the Mexico City basin

Meteorological processes associated with inhomogeneous ozone concentrations over Mexico City are examined by using observations from a recent field campaign and a mesoscale dynamics and dispersion modeling system. During this 4-week field campaign, meteorological measurements of the spatial flow structure within the Mexico City basin were obtained for the first time. A mesoscale model that employs four-dimensional data assimilation is used to create analyses that describe the boundary layer characteristics and local and regional circulations in the vicinity of Mexico City. The mesoscale analyses are then used to drive a Lagrangian particle dispersion model to simulate pollutant transport and diffusion. The resemblance between the calculated particle concentration fields and the observed spatial ozone patterns indicates that the mesoscale analyses, based on the model and the observed profiles of wind, temperature, and humidity, captured the main flow features responsible for the inhomogeneous ozone concentrations within the basin. The highest particle concentrations usually occurred in the vicinity of the peak ozone concentrations during the afternoon. The observations and mesoscale analyses provided evidence that the circulations are highly complex, and relatively weak upper level synoptic systems had an impact on the local and regional thermally driven flows in the area. In addition to horizontal advection and vertical diffusion, vertical wind shears, recirculation patterns associated with venting and entrainment processes, and mean vertical motions due to convergence within the basin also played an important role in producing the spatial variations in the near-surface particle distributions. The contribution of emissions from the previous day was found to be relatively minor for the periods examined in this study, even though the nocturnal wind speeds were light, suggesting that the high ozone concentrations are not due to multiday accumulation of pollutants.

A Case Study of the Great Plains Low-Level Jet Using Wind Profiler Network Data and a High-Resolution Mesoscale Model

Abstract A detailed case study of one complete episode of a typical summertime Great Plains low-level jet (LLJ) using data collected by the NOAA wind profiler demonstration network is presented. The high temporal and spatial resolution of the data from the profiler network permits a much more detailed picture of the Great Plains LLJ than is possible from previous studies of this phenomenon. A three-dimensional mesoscale numerical model is also used to simulate the episode and to provide information on the physical mechanisms responsible for the initiation, evolution, maintenance, and decay of the LLJ. The position and width of the jet core, as well as the diurnal variation of wind speed and direction inside the jet core are well predicted by the model. The analysis and modeling suggest that the diurnal oscillation of horizontal pressure gradient over sloping terrain is secondary to the inertial oscillation mechanism resulting from the release of frictional constraint in the evening and throughout the nigh...

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.

The IMADA-AVER Boundary Layer Experiment in the Mexico City Area

Abstract A boundary layer field experiment in the Mexico City basin during the period 24 February–22 March 1997 is described. A total of six sites were instrumented. At four of the sites, 915-MHz radar wind profilers were deployed and radiosondes were released five times per day. Two of these sites also had sodars collocated with the profilers. Radiosondes were released twice per day at a fifth site to the south of the basin, and rawinsondes were flown from another location to the northeast of the city three times per day. Mixed layers grew to depths of 2500–3500 m, with a rapid period of growth beginning shortly before noon and lasting for several hours. Significant differences between the mixed-layer temperatures in the basin and outside the basin were observed. Three thermally and topographically driven flow patterns were observed that are consistent with previously hypothesized topographical and thermal forcing mechanisms. Despite these features, the circulation patterns in the basin important for the...

Correlation of secondary organic aerosol with odd oxygen in Mexico City

> 0.9. The dependence of the observed proportionality onthe gas-phase hydrocarbon profile is discussed. Theobservationally-based correlation between oxygenatedorganic aerosol mass and odd oxygen may provide insightinto poorly understood secondary organic aerosolproduction mechanisms by leveraging knowledge of gas-phase ozone production chemistry. These results suggestthat global and regional models may be able to use theobserved proportionality to estimate SOA as a co-product ofmodeled O

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.

The Effect of Heterogeneous Soil Moisture on a Summer Baroclinic Circulation in the Central United States

Abstract Thermally induced circulations, similar to sea breezes, may be established in the presence of horizontal gradients in soil moisture, soil type, vegetation, or snow cover. The expense of extensive observational networks and the relatively small-scale circulations involved has made examining these circulations very difficult. Recent numerical studies have indicated that sharp gradients in soil or vegetation properties may induce mesoscale circulations in the absence of synoptic forcing. The current study employed a three-dimensional, hydrostatic mesoscale model to evaluate the effects of horizontally heterogeneous soil moisture and soil type on the passage of a summer cold front in the central United States. Grid-scale condensation, precipitation, latent heat release, and cumulus conviction are not accounted for in this model; moisture was affected only by advection, diffusion, and evaporation. Numerical simulations demonstrated that evaporation of soil moisture significantly affected the boundary ...

The evolution of the boundary layer and its effect on air chemistry in the Phoenix area

During a 4-week period in May and June of 1998, meteorological and chemical measurements were made as part of a field campaign carried out in the Phoenix area. Data from the field campaign provide the first detailed measurements of the properties of the convective boundary layer in this area and of the effects of these properties on ozone levels. The meteorological and chemical measurements have been combined with results from a set of meteorological, particle, and chemistry models to study ozone production, transport, and mixing in the vicinity of Phoenix. Good agreement between the simulations and observations was obtained, and the results have been used to illustrate several important factors affecting ozone patterns in the region. Heating of the higher terrain north and east of Phoenix regularly produced thermally driven circulations from the south and southwest through most of the boundary layer during the afternoon, carrying the urban ozone plume to the northeast. The combination of deep mixed layers and moderate winds aloft provided good ventilation of the Phoenix area on most days so that multiday buildups of locally produced ozone did not appear to contribute significantly to ozone levels during the study period. Sensitivity simulations determined that 20 to 40% of the afternoon surface ozone mixing ratios (corresponding to 15 to 35 ppb) were due to vertical mixing processes that entrained reservoirs of ozone into the growing convective boundary layer. The model results also indicated that ozone production in the region is volatile organic compound limited.

Nocturnal Low-Level Jet in a Mountain Basin Complex. Part I: Evolution and Effects on Local Flows

A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north‐south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s 21 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.

Pseudovertical Temperature Profiles and the Urban Heat Island Measured by a Temperature Datalogger Network in Phoenix, Arizona

Abstract As part of an air-quality field campaign conducted in Phoenix, Arizona, during the summer of 2001, a network of temperature dataloggers and surface meteorological stations was deployed across the metropolitan area for a 61-day period. The majority of the dataloggers were deployed along two intersecting lines across the city to quantify characteristics of the urban heat island (UHI). To obtain pseudovertical temperature profiles, some of the instrumentation was also deployed along a mountain slope that rose to 480 m above the valley floor. The instrumentation along the mountain slope provided a reasonable approximation of the vertical temperature profile of the free atmosphere over the valley center during the night and a few hours after sunrise. Mean differences of 0.63 and 0.92 K and standard deviations of 1.33 and 1.45 K were obtained when compared with the in situ radiosonde and remote radio acoustic sounding system measurements, respectively. The vertical temperature gradients associated with...