Characterizing the dispersive state of convective boundary layers for applied dispersion modeling

Estimates from semiempirical models that characterize surface heat flux, mixing depth, and profiles of temperature, wind, and turbulence are compared with observations from atmospheric field studies conducted in Colorado, Illinois, Indiana, and Minnesota. Sodar observations are compared with tower measurements at the Colorado site, for wind and turbulence profiles. The median surface heat flux, as calculated using surface-layer flux-profile relationships and an energy budget model, was consistently overestimated by 20 to 80%. Several mixing-depth models were evaluated: (1) integration of the hourly surface heat flux and friction velocity, (2) solving for the time rate of change of profiles of virtual potential temperature, and (3) an interpolation scheme used by the U.S. Environmental Protection Agency in regulatory dispersion models. For the late afternoon, 80 to 90% of the estimates from the first and third models were within 40% of the observed values. For the morning hours after sunrise, all were less accurate. Temperature estimates from surface-layer flux-profile relationships compared well with observations within the mixed layer, but were too low for the inversion layer aloft. Wind profiles were derived using surface-layer flux-profile relationships, a windprofile power-law based on Pasquill stability category, and sodar measurements. The sodar measurements were superior to both types of model estimates. Turbulence profiles were derived from sodar measurements and from semiempirical similarity relationships based on mixing depth and Obukhov length. The scatter in the comparisons with the sodar observations is twice that seen in the comparisons with empirical profile relationships. Overall, it appears that uncertainty of as low as 20 to 30% in the characterization of the diffusion meteorology is the exception rather than the rule.On assignment from the National Oceanic and Atmospheric Administration, U. S. Department of Commerce.Disclaimer: Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.     

Unique isoprene oxidation products demonstrate chlorine atom chemistry occurs in the Houston,Texas urban area

As part of the 2000 Texas Air Quality Study (TexAQS), we studied the isoprene oxidation process under ambient conditions to discern the presence of chlorine atom (Cl) chemistry in the Houston, Texas urban area. By measuring chloromethylbutenone (CMBO) and an isomer of chloromethylbutenal (CMBA), we clearly observed sixteen episodes of active Cl chemistry during the 24-day experiment. Estimated median Cl concentration during each of these episodes was between the detection limit of ~10² atoms cm⁻³ and 50 _{- 30} ^{+ 70} ×10⁴ 50_{ - 30}^{ + 70} \times {10^4} atoms cm⁻³. Cl concentration during all the episodes averaged 7.6 _{- 2.0} ^{+ 4.7} ×10⁴ 7.6_{ - 2.0}^{ + 4.7} \times {10^4} atoms cm⁻³ and thus amounted to less than 3% of the OH concentration during the same periods. During the episodes, the fraction of oxidation chemistry initiated by Cl ranged from 3–43% and was strongly dependent on the quantity and type of hydrocarbons present in the atmosphere. Because of its intermittent presence and low concentration, Cl is not a broadly influential oxidant in the Houston, Texas urban area.     

Effect of cold-air advection on internal boundary-layer development over warm oceanic currents

Field experiments have shown that an internal boundary layer (IBL) can develop when the wind blows perpendicularly from the cold-water side of a warm oceanic current to the warmer side. The height of this IBL downwind from the thermal front as computed by an equation originally developed by Venkatram is in excellent agreement with observations.     

Simulating the 1999 Capbreton canyon turbidity current with a Cellular Automata model

A numerical model has been developed for the simulation of turbidity currents driven by nonuniform, non cohesive sediment and flowing over a complex three dimensional submarine topography. The model is based on an alternative approach known as Cellular Automata paradigm. The model is validated by comparing a simulation with a reported field-scale event. The chosen case is a turbidity current which occurred in Capbreton Canyon and was initiated by a storm in December 1999. Using data from recent oceanographic cruises, the deposit of the event has been precisely described, which constrain values of model parameters. The model simulates the 1999 turbidity current over the actual canyon topography and related turbidite using three different types of particle. The model successfully simulates areas of erosion and deposition in the canyon. It predicts the vertical and longitudinal grain size evolution, and shows that the fining-up sequence can be deposited by several phases of deposition and erosion related to the current energetic variation during its evolution. This result could explain the presence of intrabed contacts or the frequent lack of facies in Bouma sequences.     

Seasonal and inter-annual relationships between vegetation and climate in central New Mexico, USA

Linear correlations between seasonal and inter-annual measures of meteorological variables and normalized difference vegetation index (NDVI) are calculated at six nearby yet distinct vegetation communities in semi-arid New Mexico, USA Monsoon season (June–September) precipitation shows considerable positive correlation with NDVI values from the contemporaneous summer, following spring, and following summer. Non-monsoon precipitation (October–May), temperature, and wind display both positive and negative correlations with NDVI values. These meteorological variables influence NDVI variability at different seasons and time lags. Thus vegetation responds to short-term climate variability in complex ways and serves as a source of memory for the climate system.     

Patterns of resource use by livestock during and after drought on the commons of Namaqualand, South Africa

The aim of this study was to determine the effect of drought on grazing patterns of livestock in a communal rangeland in Namaqualand. Water points were the foci of almost all grazing routes even after the drought, and the areas away from the water points were less heavily utilized. The mountains could be regarded as key resource areas for livestock since the grazing intensity was higher in the elevated areas of the rangeland than on the surrounding low lying areas during and immediately after drought. Spatial patterns of resource use by livestock on the commons did not differ during the drought and post-drought period. The lengths of the daily grazing routes were similar for both periods. This may be a consequence of the weakness of animals during the drought when forage was scarce which prevented them from walking further. The size of home ranges of the herds did not change during the two monitoring periods. We attribute this to social reasons because herders try to avoid the mixing of herds. In the commons of Namaqualand, herders are also spatially constrained and this restricts their options to adapt to drought, and hence many animals died.