Inversion Breakup Research Articles

Evaluation of Atmospheric Conditions Leading to a Fumigation Event.

During the afternoon of 30 January 2022, the Savannah River Site experienced unusual temperature conditions leading to a fumigation event that triggered safety alarms and caused considerable confusion about the cause of the event. Normally, it is assumed that fumigation events occur early in the day once surface heating has begun. While most fumigation events are related to the breakup of a nocturnal inversion, this event was related to synoptic atmospheric conditions, which provided a more unique scenario that led to the fumigation event. The unusual synoptic atmospheric conditions led to the downwash and fumigation of the elevated plume, causing a pollutant to mix rapidly to the surface. These conditions could have potentially harmed workers within the facility, since the plume was directed toward a building air intake system. We seek to outline the conditions that led to this unusual fumigation event and provide results of two-dimensional (2D) wind modeling of the event to characterize these conditions for future operational guidance of the facility air intake systems. This work sets the groundwork for future high-resolution modeling to explore the mechanisms and thresholds affecting fumigation on the facility-specific short distance scales and improve forecasting of non-standard fumigation events to protect human health.

Inversion breakup over different shapes of urban areas

Severe haze episodes and extreme heat events are both related to the strong inversion in the atmospheric boundary layer. Inversion breakup is the main mechanism to alleviate the accumulation of heat or pollutants, especially in urban areas. However, the physical process and turbulent flow structures of inversion breakup remain unclear. In this study, the characteristics of mean flow structures and turbulence statistics of inversion breakup at the two-dome stage were investigated by conducting reduced-scale water tank experiments. A two-layer density profile (i.e. a lower-level stable layer and an upper-level neutral layer) was created to simulate the surface-based inversion covered by a neutral residual layer. Various shapes of urban areas were tested. The results showed that the lower-level dome in the stable layer (Dome 1) and the upper-level dome in the neutral layer (Dome 2) are both substantially affected by the urban shape. In Dome 1, the convergent inflow is mainly along the angular bisector across the urban corners, whereas the divergent outflow is perpendicular to the edges. In Dome 2, the divergent outflow is on the same vertical plane with the convergent inflow (i.e. along the angular bisector). Furthermore, the turbulence production and momentum transfer are mainly induced by the thermal plume structures near the urban centre, indicating the strong influences of buoyancy. In divergent outflow regions, both shear and buoyancy play important roles.

Water tank modelling of variations in inversion breakup over a circular city

Air pollution and extreme heat waves are among the major environmental challenges faced by modern cities. Stable stratification and temperature inversion help to trap urban airborne pollutants and produce a strong urban heat island intensity. Such adverse effects can disappear when the buoyancy flows introduced by the city heat is sufficiently strong to break up the inversion. This paper investigates the possible mechanisms of the inversion breakup process and their effects. A lack of time-saving and cost-effective techniques with high temporal and spatial resolution in the field study has been the main limiting factor to a systematic investigation of the physical process of inversion breakup. In this study, water tank experiments were conducted to study the characteristics of the mean and turbulent flow structures of the inversion breakup process. Four cases with different background density profiles are simulated. Three stages of inversion breakup were identified, the two-dome stage, the wing-shrinking stage and the perforative-plume stage. During the two-dome stage, shear instability and buoyancy instability (Fig. 3d) are both important for heat and mass transfer between the lower-level dome in the stable layer near the surface and the upper-level dome in the neutral residual layer aloft. The vertical dimension of the lower-level dome begins to grow, whilst the horizontal extent shrinks at the wing-shrinking stage. Finally, in the perforative-plume stage, the stable layer is destroyed, and the plume extends from the surface to the top of the residual layer. The pollutants and heat dispersion are both enhanced significantly during this stage.

Field observations of the morning transition over a steep slope in a narrow alpine valley

This paper examines the morning transition period of the atmospheric boundary layer for convective summer days over steep topography. An experimental transect composed of two turbulence towers was installed in summer 2010 on a steep, west-facing slope of Val Ferret, in the Swiss Alps. The analysis focuses on valley-scale processes as well as near-surface turbulence for nine clear-sky days with thermally driven slope flows. At the valley scale, we show that shading from surrounding topography resulted in an inhomogeneous heating of the slope surface. Atmospheric profiles collected over the slope showed that subsidence warming was the main driver of the temperature inversion breakup in the central part of the valley. As a result, it took on average about 5 h after astronomical sunrise for downslope flows to be replaced by upslope/upvalley winds, with typical 2-m wind speeds of 1 and 2 m s−1, respectively. Near the surface, the slope flow layer went through a calm period with 2-m wind speeds below 1 m s−1 starting right after local sunrise. The duration of that quiescent period was highly variable, lasting between 1 and 40 min, possibly as a result of the presence or absence of large-scale, long-lived eddies feeding their energy downward to the surface. An analysis of the turbulence kinetic energy (TKE) budget for all days of interest showed that there was buoyant production of TKE throughout the full morning transition period as a result of the positive contribution from the along-slope buoyancy flux.

Persistent inversion dynamics and wintertime PM10 air pollution in Alpine valleys

The present study investigates persistent inversions dynamics during a whole winter in Alpine valleys of the area of Grenoble (French Alps), and their relationship to PM10 air pollution episodes and synoptic scale meteorology. For this purpose, hourly time series from November to March of PM10 concentration measurements at the bottom of the valleys and of ground-based temperature data at different altitudes are used. A methodology is developed to quantify a simple estimate of the inversion strength from temperature profiles deduced from the ground-based observations. This estimate is shown to be equivalent to the boundary layer heat deficit. A criterion based on this estimate is proposed to identify persistent (more than 3 days) inversions.Persistent inversions are found to occur from November to February and span 35% of the time. It is shown that they are closely related to PM10 pollution episodes, the PM10 concentration increasing with the boundary layer stability as the inversion develops. Polluted episodes are primarily driven by persistent inversions and consequently, pollution is of fully local origin from November to February. In March local dynamics become less important and long-range transport can dominate.Persistent inversions occur systematically during a high-pressure regime, which first triggers a synoptic scale elevated inversion due to the advection of warm air masses in the mid-troposphere. In valleys, the sheltered boundary layer becomes decoupled from the free troposphere, which allows a ground-based inversion to intensify in the following days. An inversion layer of quasi-constant temperature gradient, greater than 5 K km−1, then forms up to an altitude of about 1600 m, close to the average elevation of the summits. If the episode is sufficiently long, a stagnation stage is reached during which daytime insolation produces a shallow convective surface layer which does not destroy the persistent inversion. The inversion break-up occurs rapidly, in less than a day, because of synoptic scale advection of cold air masses in the mid-troposphere when a low-pressure regime approaches. The end-of-episode decrease in PM10 concentration is due to precipitation combined with an increased ventilation inside the boundary-layer.

The Impact of the Temperature Inversion Breakup on the Exchange of Heat and Mass in an Idealized Valley: Sensitivity to the Radiative Forcing

Abstract The breakup of a nocturnal temperature inversion during daytime is studied in an idealized valley by means of high-resolution numerical simulations. Vertical fluxes of heat and mass are strongly reduced as long as an inversion is present; hence it is important to understand the mechanisms leading to its removal. In this study breakup times are determined as a function of the radiative forcing. Further, the effect of the nocturnal inversion on the vertical exchange of heat and mass is quantified. The Weather Research and Forecasting Model is applied to an idealized quasi-two-dimensional valley. The net shortwave radiation is specified by a sine function with amplitudes between 150 and 850 W m−2 during daytime and at zero during the night. The valley inversion is eroded within 5 h for the strongest forcing. A minimal amplitude of 450 W m−2 is required to reach the breakup, in which case the inversion is removed after 11 h. Depending on the forcing amplitude, between 10% and 57% of the energy provided by the surface sensible heat flux is exported out of the valley during the whole day. The ratio of exported energy to provided energy is approximately 1.6 times as large after the inversion is removed as before. More than 5 times the valley air mass is turned over in 12 h for the strongest forcing, whereas the mass is turned over only 1.3 times for 400 W m−2.

Temperature Inversion Breakup with Impacts on Air Quality in Urban Valleys Influenced by Topographic Shading

AbstractUrban valleys can experience serious air pollution problems as a combined result of their limited ventilation and the high emission of pollutants from the urban areas. Idealized simulations were analyzed to elucidate the breakup of an inversion layer in urban valleys subject to a strong low-level temperature inversion and topographic effects on surface heating such as topographic shading, as well as the associated air pollution transport mechanisms. The results indicate that the presence and evolution in time of the inversion layer and its interplay with an urban heat island within the valley strongly influence the venting of pollutants out of urban valleys. Three mechanisms of air pollution transport were identified. These are transport by upslope winds, transport by an urban heat island–induced circulation, and transport within a closed slope-flow circulation below an inversion layer.

Peligrosidad de heladas por inversión térmica en la huerta de Murcia

<p>En el Sureste peninsular son frecuentes las situaciones anticiclónicas alimentadas en<br />invierno por advecciones de aire polares/árticas continentales. Bajo estas condiciones, las<br />cuencas y valles fluviales intrabéticos son bastante proclives al desarrollo de inversiones<br />térmicas. Un claro ejemplo lo constituye la Vega Media del Segura en los meses de invierno.<br />En el presente artículo se analizan tales factores y las situaciones de inversión térmica más<br />importantes registradas en esta Vega durante el período 2000-2013. Como complemento al<br />estudio, se realiza una propuesta de cartografía de peligrosidad de heladas en la Huerta de<br />Murcia para los diferentes meses invernales.<br />Palabras claves: Inversión térmica, anticiclón,</p>

Effects of Urbanization on the Temperature Inversion Breakup in a Mountain Valley with Implications for Air Quality

AbstractMany cities located in valleys with limited ventilation experience serious air pollution problems. The ventilation of an urban valley can be limited not only by orographic barriers, but also by urban heat island–induced circulations and/or the capping effect of temperature inversions. Furthermore, land-use/-cover changes caused by urbanization alter the dynamics of temperature inversions and urban heat islands, thereby affecting air quality in an urban valley. By means of idealized numerical simulations, it is shown that in a mountain valley subject to temperature inversions urbanization can have an important influence on air quality through effects on the inversion breakup. Depending on the urban area fraction in the simulations, the breakup time changes, the cross-valley wind system can evolve from a confined to an open system during the daytime, the slope winds can be reversed by the interplay between the urban heat island and the temperature inversion, and the breakup pattern can migrate from one dominated by the growth of the convective boundary layer to one also involving the removal of mass from the valley floor by the upslope winds. The analysis suggests that the influence of urbanization on the air quality of an urban valley may lead to contrasting and possibly counterintuitive effects when considering temperature inversions. More urban land does not necessarily imply worse air quality, even when considering that the amount of pollutants emitted grows with increased urbanization.

The Influence of Air Temperature Inversions on Snowmelt and Glacier Mass Balance Simulations, Ammassalik Island, Southeast Greenland

Abstract In many applications, a realistic description of air temperature inversions is essential for accurate snow and glacier ice melt, and glacier mass-balance simulations. A physically based snow evolution modeling system (SnowModel) was used to simulate 8 yr (1998/99–2005/06) of snow accumulation and snow and glacier ice ablation from numerous small coastal marginal glaciers on the SW part of Ammassalik Island in SE Greenland. These glaciers are regularly influenced by inversions and sea breezes associated with the adjacent relatively low temperature and frequently ice-choked fjords and ocean. To account for the influence of these inversions on the spatiotemporal variation of air temperature and snow and glacier melt rates, temperature inversion routines were added to MircoMet, the meteorological distribution submodel used in SnowModel. The inversions were observed and modeled to occur during 84% of the simulation period. Modeled inversions were defined not to occur during days with strong winds and high precipitation rates because of the potential of inversion breakup. Field observations showed inversions to extend from sea level to approximately 300 m MSL, and this inversion level was prescribed in the model simulations. Simulations with and without the inversion routines were compared. The inversion model produced air temperature distributions with warmer lower-elevation areas and cooler higher-elevation areas than without inversion routines because of the use of cold sea-breeze-based temperature data from underneath the inversion. This yielded an up to 2 weeks earlier snowmelt in the lower areas and up to 1–3 weeks later snowmelt in the higher-elevation areas of the simulation domain. Averaged mean annual modeled surface mass balance for all glaciers (mainly located above the inversion layer) was −720 ± 620 mm w.eq. yr−1 (w.eq. is water equivalent) for inversion simulations, and −880 ± 620 mm w.eq. yr−1 without the inversion routines, a difference of 160 mm w.eq. yr−1. The annual glacier loss for the two simulations was 50.7 × 106 and 64.4 × 106 m3 yr−1 for all glaciers—a difference of ∼21%. The average equilibrium line altitude (ELA) for all glaciers in the simulation domain was located at 875 and 900 m MSL for simulations with or without inversion routines, respectively.

Observations of near‐surface wind and temperature structures and their variations with topography and latitude in East Antarctica

The first multiyear surface meteorological observations over Dome A, the highest ice feature in the entire Antarctica continent, are analyzed to understand the surface wind, temperature, and stability climatology over Dome A and how it differs from the surface climatology at two lower‐latitude/lower‐elevation sites along similar longitude in East Antarctica. The climatology is also compared with that over Dome C. In contrast to the surface winds at lower sites, where moderate to strong northeasterly winds prevail with a distinct diurnal oscillation in wind speed in response to the diurnal change in katabatic forcing, summertime surface winds over Dome A are very weak, are variable in direction, and show little diurnal variation. Although both temperature and temperature gradient oscillate diurnally, the gradient over Dome A remained positive all day long, indicating a persistent surface inversion, while at the two lower sites, as well as over Dome C, sufficient insolation leads to the breakup of inversion and the development of a convective boundary layer in the afternoon. Wavelet analysis of near‐surface stability revealed that besides the strong diurnal signal, the near‐surface stability also exhibits annual, semiannual, and interseasonal (period ∼50 days) oscillations at all locations. These oscillations in near‐surface stability are linked to the same peaks in the 500‐hPa geopotential height spectra and therefore are believed to be caused by variations of synoptic conditions.

Summertime Ultrafine Particles in Urban and Industrial Air: Aitken and Nucleation Mode Particle Events

The main objectives of this study are to develop a systematic approach for the identification and classification of ultrafine particle (UFP) events and to analyze the events for implications of sources and meteorological conditions conducive to elevated UFP number concentrations. UFP events are prolonged periods with elevated UFP number concentrations. Particle number size distributions, gas pollutants, and meteorological parameters were concurrently measured during the summers of 2003 to 2005 in Detroit, Michigan, USA. Among the 74 identified UFP events, 40 (54%) are Aitken mode particle events and 34 (46%) are nucleation mode particle events. Correlation results show that 65 out of the 74 UFP events were associated with plumes of combustion sources, including all the Aitken mode events and 25 out of the 34 nucleation mode events. These in-plume particle events were positively correlated with elevated NO, CO, particle surface area, and occasionally high levels of SO2. The remaining 9 nucleation mode events, however, showed no such correlations, and hence referred to as secondary nucleation mode particle events. These secondary nucleation events occurred under relatively clean (i.e., low preexisting aerosols) and sunny conditions shortly after the breakup of nocturnal inversion and during midday. Overall, the results indicate that motor vehicles and industrial plumes are the major sources of elevated UFPs in urban/industrial air. Under favorable conditions, atmospheric secondary nucleation can occur in rather polluted urban/industrial air and become a major contributor of UFPs. The formation mechanisms, hence chemical composition, are likely different between the in-plume and secondary UFPs. Therefore, exposure assessments to ambient UFPs need to take into account the contributions from both types of particles.

Parameterization of Inversion Breakup in Idealized Valleys. Part II: Thermodynamic Model

AbstractA simple thermodynamic parameterization based on a modified version of the Whiteman and McKee inversion destruction model is presented to simulate the evolution of vertical temperature structure during the inversion breakup period in idealized valleys under clear, undisturbed weather conditions. The proposed method adopts simplified semiempirical parameterizations of radiation and surface energy budgets at the valley floor and sidewalls and an empirical scheme for the partitioning of energy in the valley atmosphere, eliminating the need for selecting arbitrary values for the adjustable model parameters. The model accurately simulates the changes with time of the height of the inversion top and the depth of the convective boundary layer during the breakup of nocturnal temperature inversions in a wide range of valley topography. The theoretical estimates were validated and compared with dynamical model predictions and actual measurements. Because of its simplicity and its fair agreement with observations, the proposed method may be useful in applications in boundary layer, air pollution, and complex terrain meteorology. It is recognized that more work is necessary before the validity of the suggested procedure can be fully established.

Parameterization of Inversion Breakup in Idealized Valleys. Part I: The Adjustable Model Parameters

AbstractThe factors that affect the atmospheric energy budget approach used in the thermodynamic valley inversion destruction model of Whiteman and McKee are investigated theoretically. The height at which the sinking inversion top meets the rising convective boundary layer to destroy valley inversions can be uniquely determined by the topographic characteristics of the valley and an adjustable model parameter, relating to the fraction of sensible heat flux going to convective boundary layer growth, through a simple parabolic relationship. The time required to break a temperature inversion can be expressed with very good approximation as a simple power-law function of the topographic parameters and the fraction of extraterrestrial solar flux that is partitioned to sensible heat flux in the valley atmosphere. The theoretical estimates compare very favorably to predictions from the bulk thermodynamic model of Whiteman and McKee. A new approach to handle time-dependent sensible heat fluxes is outlined. The paper ends with recommendations for future research.

Wind flow and photochemical air pollution in Thessaloniki, Greece. Part I: Simulations with the European Zooming Model

The European Zooming Model (EZM) is applied to simulate the wind flow and the pollutant transport and transformation in the Greater Thessaloniki area (GTA) during a day characterised by weak synoptic forcing and a pronounced sea breeze circulation. Two chemical reaction mechanisms of different complexities, KOREM and EMEP, are used for these simulations. The evolution of the wind field and particularly the development of the sea/land breeze flow are investigated in relation to ozone concentration patterns. Comparison of model results to observations at a large number of locations showed the considerable skill of the model in predicting the major mesoscale features affecting the GTA: the sea breeze onset time and development, its inland penetration as well as its intensity. According to both observations and model results, ozone levels are kept at moderate levels. They are decisively affected by the break-up of the night-time inversion and the onset of the sea breeze. The compact mechanism KOREM is very sensitive to the VOC/NO x emission ratio. When this ratio is close to unity, i.e. inside urban areas, KOREM performs very well proving its appropriateness for regulatory use in such environments. The more sophisticated EMEP mechanism underestimates urban ozone levels but performs very well in rural areas remaining the most reliable choice for regional scale applications.

Extensive dust outbreaks following the morning inversion breakup in the Taklimakan Desert

Extensive dust outbreaks occurred on the late morning of 26 March 2004 in the Taklimakan Desert. An image by the satellite AQUA shows the dust outbreaks as an enormous dust storm extending as far as 1000 km in the direction of east and west across the desert. A ground‐based lidar was successful in observing the dust storm. Values of the backscattering ratio from near the ground surface to an altitude of 6 km above sea level rapidly increased as soon as the dust storm covered the lidar observation site. Numerical experiments including a diffusion experiment revealed that strong westerly winds appeared at the surface level following the morning inversion breakup and then induced the dust outbreaks. The nocturnal inversion layer prevented the strong westerly winds from spreading to the surface level until the breakup. Notable nocturnal inversion is considered to be easily formed in the Taklimakan Desert because of its deep basin topography which can accumulate cold air masses. These results show the stability of the planetary boundary layer to be an extremely important factor in dust outbreaks especially in basin deserts, in addition to the intensity of surface winds. This implies that the occurrence frequency of dust storms in the desert has a diurnal cycle.

Temperature And Wind Velocity Oscillations Along a Gentle Slope During Sea-Breeze Events

The flow structure on a gentle slope at Vallon d’Ol in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment. The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60--90 min oscillation period) and several days (about 120–180 min oscillation period) during the whole observing period. Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.In contrast to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published to our knowledge, probably because of temperature inversion break-up that inhibits upslope winds. The present paper shows that cold air advection by a sea breeze generates a mesoscale horizontal temperature gradient, and hence baroclinicity in the atmosphere, which then allows low-frequency oscillations, similar to a katabatic flow. An expression for the oscillation period is derived that accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period is compared to the measurements, and good agreement is found. The statistical analysis of the wind flow at Vallon d’Ol shows a dominant north-easterly to easterly flow pattern for nighttime oscillations and a dominant south-westerly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results that show that the air drains off the mountain along the maximum slope direction, which in the studied case is oriented south-west to north-east.

Evaluation of Real-Time High-Resolution MM5 Predictions over the Great Lakes Region

Abstract Real-time high-resolution mesoscale predictions using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) over the Great Lakes region are evaluated for the 2002/03 winter and 2003 summer seasons using surface and upper-air observations, with a focus on near-surface and boundary layer properties that are important for applications such as air quality and fire weather predictions. The summer season predictions produce a cold bias in maximum daily temperature and a warm bias in minimum temperature that together lead to a good prediction of daily mean temperature but a smaller-than-observed diurnal temperature cycle. In winter, the predicted near-surface temperatures are lower both day and night, yielding good agreement with the observed amplitude of the diurnal temperature cycle but relatively large cold bias in daily mean temperature. The predicted temperatures in the boundary layer are also systematically lower than the observed temperatures in the two seasons. The cold bias is consistent with the wetter-than-observed lower atmosphere in the model prediction, which in turn can be attributed to an inadequate specification of soil moisture. In both seasons, the model produced substantially more precipitation in all categories, especially in the heavy precipitation category, and the overprediction is primarily associated with more widespread area coverage in the model prediction. The chances of producing a false precipitation forecast are substantially higher than missing an observed precipitation event. Small systematic errors are found in the predictions of low-level winds, but above the boundary layer, the predicted winds are predominantly from the west, while the observed winds are from the west-northwest. The model is able to capture the general development and evolution of the lake–land breezes in areas surrounding Lake Michigan during summer, although errors exist in the strengths of the breezes and the timing of their transition. Predicted early morning inversions are slightly stronger than observed in winter and weaker than observed in summer. The weak summer morning inversion results in a rapid inversion breakup followed by an earlier growth of a mixed layer after sunrise. Despite the head start, the predicted mixed-layer heights in late afternoon are lower than those observed, suggesting that either the predicted surface sensible heat flux may be too low or the boundary layer flux divergence may be too high. Decreasing horizontal grid spacing from 12 to 4 km results in little improvement in the predictions of near-surface and boundary layer properties except for precipitation, for which the model bias is significantly reduced by the increase in horizontal resolution. The cold and wet biases and errors in inversion strengths and mixed-layer development call for extra caution when using products from mesoscale forecasts in applications such as air pollution and fire weather prediction.

TAPM: a practical approach to prognostic meteorological and air pollution modelling

Air pollution predictions for environmental impact assessments are usually generated by Gaussian plume/puff models driven by observationally-based meteorological inputs. An alternative approach is to use prognostic meteorological and air pollution models, which are better founded than the Gaussian approach, but tend to require much expertise and time to set up and large computing resources to run. This paper provides an overview of the science and verification of The Air Pollution Model (TAPM), which was designed to apply the best science in an easy-to-use and fast-to-run model. TAPM is a PC-based, nestable, prognostic meteorological and air pollution model (with photochemistry) driven by a Graphical User Interface, and is a viable tool for year-long simulations. Datasets of the important inputs needed for meteorological simulations (such as terrain and land-use data and synoptic analyses) accompany the model, allowing quick GUI configuration of the model for any region. We present results from the application of TAPM for urban and coastal areas in Australia, for two United States tracer experiments (Kincaid and Indianapolis) used for international model inter-comparison, and for point source dispersion in wind-tunnel building wakes. The meteorological results show that TAPM performs well in coastal, inland, and complex terrain, in sub-tropical to mid-latitude conditions, for both case studies and year-long simulations. The pollution results show that TAPM performs well for a range of important phenomena such as nocturnal inversion break-up fumigation, convective dispersion, shoreline fumigation, building wakes, and general dispersion in complex rural and urban conditions. In particular, the TAPM performance is very good for the prediction of extreme pollution statistics, important for environmental impact assessments, for both non-reactive (tracer) and reactive (nitrogen dioxide, ozone and particulate) pollutants for a variety of sources (e.g. industrial stacks and surface or urban emissions).

Inversion Breakup in Small Rocky Mountain and Alpine Basins

Comparisons are made between the postsunrise breakup of temperature inversions in two similar closed basins in very different climate settings, one in the eastern Alps and one in the Rocky Mountains. The small, high-altitude, limestone sinkholes have both experienced extreme temperature minima below −50°C and both develop strong nighttime inversions. On undisturbed clear nights, temperature inversions reach to 120-m heights in both sinkholes but are much stronger in the drier Rocky Mountain basin (24 vs 13 K). Inversion destruction takes place 2.6–3 h after sunrise in these basins and is accomplished primarily by subsidence warming associated with the removal of air from the base of the inversion by the upslope flows that develop over heated sidewalls. A conceptual model of this destruction is presented, emphasizing the asymmetry of the boundary layer development around the basin and the effects of solar shading by the surrounding ridgeline. Differences in inversion strengths and postsunrise heating rates between the two basins are caused by differences in the surface energy budget, with drier soil and a higher sensible heat flux in the Rocky Mountain sinkhole. Inversions in the small basins break up more quickly following sunrise than for previously studied valleys. The pattern of inversion breakup in the non-snow-covered basins is the same as that reported in snow-covered Colorado valleys. The similar breakup patterns in valleys and basins suggest that along-valley wind systems play no role in the breakups, since the small basins have no along-valley wind system.