Afternoon Sea Breeze Research Articles

Prediction of air quality in Sydney, Australia as a function of forest fire load and weather using Bayesian statistics.

Smoke from Hazard Reduction Burns (HRBs) and wildfires contains pollutants that are harmful to human health. This includes particulate matter less than 2.5 μm in diameter (PM2.5), which affects human cardiovascular and respiratory systems and can lead to increased hospitalisations and premature deaths. Better models are needed to predict PM2.5 levels associated with HRBs so that agencies can properly assess smoke pollution risk and balance smoke risk with the wildfire mitigation benefits of HRBs. Given this need, our aim was to develop a probabilistic model of daily PM2.5 using Bayesian regression. We focused on the region around Sydney, Australia, which regularly has hazard reduction burning, wildfires and associated smoke. We developed two regional models (mean daily and maximum daily) from observed PM2.5, weather reanalysis and satellite fire hotspot data. The models predict that the worst PM2.5 in Sydney occurs when PM2.5 was high the previous day, there is low ventilation index (i.e. the product of wind speed and planetary boundary layer height), low temperature, west to northwest winds in the Blue Mountains, an afternoon sea breeze and large areas of HRBs are being conducted, particularly to the west and north of Sydney. A major benefit of our approach is that models are fast to run, require simple inputs and Bayesian predictions convey both predicted PM2.5 and associated prediction uncertainty. Future research could include the application of similar methods to other regions, collecting more data to improve model precision and developing Bayesian PM2.5 models for wildfires.

Composition of Clean Marine Air and Biogenic Influences on VOCs during the MUMBA Campaign

Volatile organic compounds (VOCs) are important precursors to the formation of ozone and fine particulate matter, the two pollutants of most concern in Sydney, Australia. Despite this importance, there are very few published measurements of ambient VOC concentrations in Australia. In this paper, we present mole fractions of several important VOCs measured during the campaign known as MUMBA (Measurements of Urban, Marine and Biogenic Air) in the Australian city of Wollongong (34°S). We particularly focus on measurements made during periods when clean marine air impacted the measurement site and on VOCs of biogenic origin. Typical unpolluted marine air mole fractions during austral summer 2012-2013 at latitude 34°S were established for CO2 (391.0 ± 0.6 ppm), CH4 (1760.1 ± 0.4 ppb), N2O (325.04 ± 0.08 ppb), CO (52.4 ± 1.7 ppb), O3 (20.5 ± 1.1 ppb), acetaldehyde (190 ± 40 ppt), acetone (260 ± 30 ppt), dimethyl sulphide (50 ± 10 ppt), benzene (20 ± 10 ppt), toluene (30 ± 20 ppt), C8H10 aromatics (23 ± 6 ppt) and C9H12 aromatics (36 ± 7 ppt). The MUMBA site was frequently influenced by VOCs of biogenic origin from a nearby strip of forested parkland to the east due to the dominant north-easterly afternoon sea breeze. VOCs from the more distant densely forested escarpment to the west also impacted the site, especially during two days of extreme heat and strong westerly winds. The relative amounts of different biogenic VOCs observed for these two biomes differed, with much larger increases of isoprene than of monoterpenes or methanol during the hot westerly winds from the escarpment than with cooler winds from the east. However, whether this was due to different vegetation types or was solely the result of the extreme temperatures is not entirely clear. We conclude that the clean marine air and biogenic signatures measured during the MUMBA campaign provide useful information about the typical abundance of several key VOCs and can be used to constrain chemical transport model simulations of the atmosphere in this poorly sampled region of the world.

Wind-driven diurnal temperature variability across a small bay and the spatial pattern of intertidal barnacle settlement

Temperature variability under different wind conditions and its association with the spatial pattern of settlement of three intertidal barnacles – the chthamaloids Jehlius cirratus and Notochthamalus scabrosus, and the balanoid Notobalanus flosculus – were studied across Cartagena Bay, located in the upwelling region of central Chile. During days of strong winds, the diurnal signal in surface temperature at the protected end of the bay (site CTGN) was attenuated and decoupled from the northern sites (ECIM and PCHC) which are directly exposed to wind forcing, suggesting that wind intensity drives shifts in the relative importance of physical transport processes across the bay. Overall, the mean settlement rates of both chthamaloids were higher at PCHC, whereas N. flosculus settled at higher rates in CTGN. Under strong wind conditions, settlement rates of both chthamaloids decreased at the northern sites, while the settlement of N. flosculus reached minima at all three sites. Moreover, the effect of wind stress on the spatial pattern of settlement across the bay differed between species. A significant and positive correlation between the spatial heterogeneity of settlement and maximum daily wind stress – used as a metric for the intensity of the afternoon sea breeze – was found only for J. cirratus. It is concluded that daily changes in wind stress have a strong effect on the spatial pattern of diurnal temperature fluctuations, and on the spatial pattern of barnacle settlement around the bay. Such association emerges from the effect of wind on near-shore circulation and its differential modulation of thermal structure around an open embayment and, by extension, on the patterns of larval transport and onshore delivery to sites located at extremes of the bay, which probably is common in other bays with similar characteristics.

Ambient Thinking: Or, Sweating over Theory

Ambient Thinking: Or, Sweating over Theory

Diurnal Cycle of Convection during the 2004 North American Monsoon Experiment

Abstract The diurnal cycle of summer monsoon convection in the coastal, mountainous region of northwestern Mexico is investigated using data from the 2004 North American Monsoon Experiment (NAME). Data from a special sounding network consisting of research and operational sites have been quality controlled and combined with surface, wind profiler, and pibal observations to create a gridded dataset over the NAME domain. This study concentrates on results from the interior portion of the NAME sounding network, where gridded analysis fields are independent of model data. Special attention is given to surface and pibal observations along the western slope of the Sierra Madre Occidental (SMO) in order to obtain an optimal analysis of the diurnally varying slope flows. Results show a prominent sea-breeze–land-breeze cycle along the western slopes of the SMO. There is a deep return flow above the afternoon sea breeze as a consequence of the elevated SMO immediately to the east. The upslope flow along the western slope of the SMO is delayed until late morning, likely in response to early morning low clouds over the SMO crest and reduced morning insolation over the west-facing slopes. The diurnal cycle of the net radiative heating rate is characterized by a net cooling during most of the daytime except for net heating in the lower and upper troposphere at midday. The diurnal cycle of the apparent heat source Q1 minus the radiative heating rate QR (providing a measure of net condensational heating) and the apparent moisture sink Q2 over the SMO is indicative of shallow convection around noon, deep convection at 1800 LT, evolving to stratiform precipitation by midnight, consistent with the radar-observed diurnal evolution of precipitation over this coastal mountainous region as well as the typical evolution of tropical convective systems across a wide range of spatial and temporal scales. Convection over the Gulf of California is strikingly different from that over land, namely, heating and moistening are confined principally to the lower troposphere below 700 hPa, peaking during the nighttime hours.

Multisensor Estimation of Mixing Heights over a Coastal City

AbstractAn airborne microwave temperature profiler (MTP) was deployed during the Texas 2000 Air Quality Study (TexAQS-2000) to make measurements of boundary layer thermal structure. An objective technique was developed and tested for estimating the mixed layer (ML) height from the MTP vertical temperature profiles. The technique identifies the ML height as a threshold increase of potential temperature from its minimum value within the boundary layer. To calibrate the technique and evaluate the usefulness of this approach, coincident estimates from radiosondes, radar wind profilers, an aerosol backscatter lidar, and in situ aircraft measurements were compared with each other and with the MTP. Relative biases among all instruments were generally less than 50 m, and the agreement between MTP ML height estimates and other estimates was at least as good as the agreement among the other estimates. The ML height estimates from the MTP and other instruments are utilized to determine the spatial and temporal evolution of ML height in the Houston, Texas, area on 1 September 2000. An elevated temperature inversion was present, so ML growth was inhibited until early afternoon. In the afternoon, large spatial variations in ML height developed across the Houston area. The highest ML heights, well over 2 km, were observed to the north of Houston, while downwind of Galveston Bay and within the late afternoon sea breeze ML heights were much lower. The spatial variations that were found away from the immediate influence of coastal circulations were unexpected, and multiple independent ML height estimates were essential for documenting this feature.

A synoptic climatology of tropospheric ozone episodes in Sydney, Australia

AbstractConcentrations of tropospheric ozone often exceed Australian air quality goals in Sydney during summer. However, features in the occurrence of ozone in Sydney are yet to be fully explained. Meteorological conditions associated with ozone episodes in Sydney are caused by complex interactions between synoptic and meso‐scale processes. This paper discusses the meteorological influences behind ozone pollution episodes in Sydney.A synoptic climatology of ozone episodes in Sydney was generated using multivariate statistical techniques, including principal component analysis (PCA) and a two‐stage cluster analysis, to classify days into meteorologically homogeneous synoptic categories. Surface and upper air meteorological data for warm months (Oct–Mar) over a 10‐year period were used as input into the statistical analyses. Eleven synoptic categories were identified in Sydney during the warm season and ozone concentrations associated with each of the synoptic categories were investigated. One synoptic category was found to be associated almost exclusively with high pollution concentrations. High ozone concentrations were found to be associated with a high‐pressure system located in the middle to eastern Tasman Sea producing light northwesterly gradient winds, an afternoon sea breeze, high afternoon temperatures, a shallow mixing height at the coast and warming aloft during the day.Over 90% of all days exceeding current air quality goals for ozone in Sydney fell within the synoptic category associated with the highest ozone concentrations. It is envisaged that results from this research will be useful to Australian regulatory bodies from both a forecast point of view and for the siting of future ozone precursor sources in Sydney and surrounding regions. Copyright © 2006 Royal Meteorological Society

A 7-Yr Climatological Study of Land Breezes over the Florida Spaceport

Abstract Seven years of wind and temperature data from a high-resolution network of 44 towers at the Kennedy Space Center and Cape Canaveral Air Force Station were used to develop an objective method for identifying land breezes, which are defined as seaward-moving wind shift lines in this study. The favored meteorological conditions for land breezes consisted of surface high pressure in the vicinity of the Florida peninsula, mainly clear skies, and light synoptic onshore flow and/or the occurrence of a sea breeze during the afternoon preceding a land breeze. The land breeze characteristics are examined for two events occurring under different weather regimes—one with light synoptic onshore flow and no daytime sea breeze, and another following a daytime sea breeze under a prevailing offshore flow. Land breezes were found to occur over east-central Florida in all months of the year and had varied onset times and circulation depths. Land breezes were most common in the spring and summer months and least common in the winter. The average onset times were ∼4–5 h after sunset from May to July and ∼6.5–8 h after sunset from October to January. Land breezes typically moved from the west or southwest during the spring and summer, from the northwest in the autumn, and nearly equally from all directions in the winter. Shallow land breezes (<150-m depth) were typically not associated with the afternoon sea breeze and behaved like density currents, exhibiting the largest temperature decreases and latest onset times. Deep land breezes (>150-m depth) were most often preceded by an afternoon sea breeze, had the smallest horizontal temperature gradients, and experienced a mean onset time that is 4 h earlier than that of shallow land breezes.

The Impact of Anthropogenic Land-Cover Change on the Florida Peninsula Sea Breezes and Warm Season Sensible Weather

During the twentieth century, the natural landscape of the Florida peninsula was transformed extensively by agriculture, urbanization, and the diversion of surface water features. The purpose of this paper is to present a numerical modeling study in which the possible impacts of this transformation on the warm season climate of the region were investigated. For three separate July‐August periods (1973, 1989, and 1994), a pair of simulations was performed with the Regional Atmospheric Modeling System. Within each pair, the simulations differed only in the specification of land-cover class. The two different classes were specified using highly detailed datasets that were constructed to represent pre-1900 natural land cover and 1993 land-use patterns, thus capturing the landscape transformation within each pair of simulations. When the pre-1900 natural cover was replaced with the 1993 land-use dataset, the simulated spatial patterns of the surface sensible and latent heat flux were altered significantly, resulting in changes in the structure and strength of climatologically persistent, surface-forced mesoscale circulations—particularly the afternoon seabreeze fronts. This mechanism was associated with marked changes in the spatial distribution of convective rainfall totals over the peninsula. When averaged over the model domain, this redistribution was reflected as an overall decrease in the 2-month precipitation total. In addition, the domain average of the diurnal cycle of 2-m temperature was amplified, with a noted increase in the daytime maximum. These results were consistent among all three simulated periods, and largely unchanged when subjected to a number of model sensitivity factors. Furthermore, the model results are in reasonable agreement with an analysis of observational data that indicates decreasing regional precipitation and increasing daytime maximum temperature during the twentieth century. These results could have important implications for water resource and land-use management issues in south Florida, including efforts to restore and preserve the natural hydroclimate of the Everglades ecosystem. This study also provides more evidence for the need to consider anthropogenic land-cover change when evaluating climate trends.

Factors affecting suspended‐solids concentrations in South San Francisco Bay, California

Measurements of suspended‐solids concentration (SSC) were made at two depths at three sites in South San Francisco Bay (South Bay) to determine the factors that affect SSC. Twenty‐eight segments of reliable and continuous SSC time series data longer than 14 days were collected from late 1991 or 1992 through September 1993. Spectral analysis and singular spectrum analysis were used to relate these data segments to time series of several potential forcing factors, including diurnal and semidiurnal tides, the spring‐neap tidal cycle, wind shear, freshwater runoff, and longitudinal density differences. SSC is greatest during summer, when a landward wind shear is applied to South Bay by the afternoon sea breeze. About one half the variance of SSC is caused by the spring‐neap cycle, and SSC lags the spring‐neap cycle by about 2 days. Relatively short duration of slack water limits the duration of deposition of suspended solids and consolidation of newly deposited bed sediment during the tidal cycle, so suspended solids accumulate in the water column as a spring tide is approached and slowly deposit as a neap tide is approached. Perturbations in SSC caused by wind and local runoff from winter storms during the study period were usually much smaller than SSC variations caused by the spring‐neap cycle. Variations of SSC at the study sites at tidal timescales are tidally forced, and nonlinear physical processes are significant. Advective transport dominates during spring tides, when water with higher SSC due to wind wave resuspension is advected to the main channel from shallow water, but, during neap tides, advective transport is less significant. The findings of this and other studies indicate that the tidally averaged transport of suspended solids responds to seasonal variations of wind shear in South Bay.

Airborne Bacteria in the Atmospheric Surface Layer: Temporal Distribution above a Grass Seed Field

Temporal airborne bacterial concentrations and meteorological conditions were measured above a grass seed field in the Willamette River Valley, near Corvallis, Oreg., in the summer of 1993. The concentration of airborne bacteria had a maximum of 1,368.5 CFU/m(sup3), with a coefficient of variation of 90.5% and a mean of 121.3 CFU/m(sup3). The lowest concentration of bacteria occurred during the predawn hours, with an average of 32.2 CFU/m(sup3), while sunrise and early evening hours had the highest averages (164.7 and 158.1 CFU/m(sup3), respectively). The concentrations of bacteria in the atmosphere varied greatly, with a maximum difference between two 2-min samples of 1,995 CFU/m(sup3). The concentrations of bacteria in the atmosphere could be divided into five time periods during the day that were thought to be related to the local diurnal sea breeze and Pacific Coast monsoon weather conditions as follows: (i) the nighttime minimum concentration, i.e., 2300 to 0600 h; (ii) the sunrise peak concentration, i.e., 0600 to 0800 h; (iii) the midday accumulating concentration, i.e., 0800 to 1515 h; (iv) the late-afternoon sea breeze trough concentration, i.e., 1515 to 1700 h; and (v) the evening decrease to the nighttime minimum concentration, i.e., 1700 to 2300 h. The sunrise peak concentration (period ii) is thought to be a relatively general phenomenon dependent on ground heating by the sun, while the afternoon trough concentration is thought to be a relatively local phenomenon dependent on the afternoon sea breeze. Meteorological conditions are thought to be an important regulating influence on airborne bacterial concentrations in the outdoor atmosphere in the Willamette River Valley.

Modeling the diurnal mixed layer

The numerical model presented here is based on the one‐dimensional equations for momentum, heat, salt, and turbulent kinetic energy integrated over the diurnal mixed layer. Two noteworthy features are the inclusion of the energetics of billowing due to Kelvin‐Helmholtz instability at the base of the mixed layer, and a closure hypothesis which retains average mixed‐layer turbulent kinetic energy as an explicit variable, rather than assuming it to be a fixed proportion of external energy input. The importance of both these features is illustrated in the application of the model to the data for 1 day which included several mixing processes: a fairly calm morning with uninterrupted solar heating, followed by rapid mixed‐layer deepening during a strong afternoon sea breeze, and finally a relatively cool night with little wind. The data contain rapidly changing meteorological inputs as well as evidence of strong billow formation and collapse and thus provide an ideal test of the importance of both billow energetics and the temporal effects in the turbulent kinetic energy budget. The model does not account for processes occurring below the mixed layer nor for the effects of horizontal advection. Within these constraints, model results agree well with measured temperatures and represent an improvement over the results of a slab model without billow energetics.

The diurnal mixed layer1

Fine‐scale measurements and temperature gradient microstructure data are used to describe the diurnal energetics of the mixed layer in the Wellington Reservoir in Western Australia. The data covered a morning period of solar heating, a period of severe wind mixing induced by an afternoon sea breeze, and a period of pure penetrative convection extending through most of the night. The Wedderburn number (W) ranged from a value of 0.02 at the start of the heating phase to a value of about 10 at the end of the convective period. During the intermediate strong wind period the value of W remained about 0.1; the surface wind stress induced a strong tilt of the isotherms at the base of the mixed layer. The other main parameter, the ratio of the Monin‐Obukhov length to the depth of the mixed layer, ranged from about 5.0 during the morning down to 0.005 at the end of the convective period. The full range of mixed‐layer deepening processes could thus be analyzed.A simple one‐dimensional integral mixed‐layer model was applied to assess the importance of the temporal terms in diurnal simulations and to verify the values of the energy conversion efficiencies used in such models. Overall, the model performed extremely well; however, the results showed the need to develop parameterizations for the energetics of billowing induced by large shears across the base of the mixed layer and for the mixing accompanying upwelling.

Uncertainties Associated with the Estimation of Mass Balances and Gaussian Parameters from Atmospheric Tracer Studies

Data resulting from two atmospheric tracer experiments in the land-sea breeze winds in Los Angeles, CA are used to compare the observed and released amounts of tracer (a mass balance). The mass balance calculation indicated that essentially all of the tracer transported to sea during the land breeze was transported back across the shore during the subsequent sea breeze. A methodology for calculating a mass balance and the associated uncertainties is presented. The experimental and calculation procedures presented allowed mass balance estimates with less uncertainty than is present in individual measurements of concentration or mixing height. Similarly, a methodology for calculating dispersion parameters for the gaussian plume model from tracer data is discussed and applied to the results of two atmospheric tracer studies conducted during the afternoon sea breeze in the Santa Barbara Channel of California. The method presented involves the integral definitions of the statistical quantities. By considering only tracer concentrations greater than 10% of the maximum concentration, and by considering sufficiently many data points, the uncertainty associated with the parameter estimation was again less than the relative uncertainties in any individual data point. These studies were primarily designed to relate the uncertainties in estimates of mass balances and in estimations of gaussian parameters to the uncertainties inherent within field data.

Observations of Nighttime Winds Using Pilot Balloons in Anderson Creek Valley, Geysers, California

Nighttime drainage or downslope winds along the east-facing slope of Anderson Creek Valley located in the Geysers area of northern California are examined using pilot balloons as air parcel tracers. Observations made over four nights show a persistent pattern of winds. Before sunset strong westerly winds blow into the valley. These winds are believed due to the late afternoon sea breeze. Drainage winds become fully developed within 2 h after sunset. The drainage wind has an average speed of approx.3 m s/sup -1/ regardless of the speed of the ambient wind. The drainage layer thickness grows at a rate of approx.80 m per kilometer of downwind distance. Balloons in the drainage current quickly spread throughout the shallow, sidewall canyon containing Anderson Creek. The transition from the local valley wind system to the free, ambient wind occurs at about ridge height, i.e., /sub 5/00 m above the valley basin, for weak ambient winds, and at /sub 3/00 m above the valley basin for strong ambient winds.