Low-level Temperature Inversion Research Articles

A Comparative Study and Evaluation of Mixing-Height Estimation Based on Sodar-RASS, Ceilometer Data and Numerical Model Simulations

A comparative study and evaluation of mixing-layer height estimation was conducted, using data from remote sensing and in-situ instrumentation, radiosondes, synoptic analyses and model simulations. The data were collected during an experimental campaign conducted at the Athens International Airport, Greece, from 15 to 26 September 2007. Mixing-layer height from the sodar dataset was estimated taking into account the backscatter signal, temperature, Richardson number profiles and surface-based measurements, while for the ceilometer data, the optical attenuated aerosol backscatter intensity first derivative was utilized. Numerical simulations using the Penn State/NCAR MM5 numerical mesoscale model and the Weather Research and Forecast numerical model were also performed. Comparative results under different meteorological conditions (local flows, moderate to strong background flows) are presented and discussed. According to our results under moderate to strong winds the existing mechanical turbulence creates good signal conditions for the two remote systems leading to a good overall agreement between the two methodologies, while both models give reliable estimations of the mixing height. The sodar-RASS system is more suitable under low to moderate winds or when local flows are developed with weak stability, while the ceilometer system is more suitable for moderate to strong winds, which is associated with a homogeneous atmosphere and weaker low-level temperature inversions. In the models, the existing approach for atmospheric boundary-layer depth simulation usually provides higher compared to remote sensing values, especially during local flow events. An alternative approach for the estimation of mixing height by the models, the estimation and use of the diffusion coefficient profiles, is a promising methodology regarding the comparison with the sodar-RASS mixing-height estimations.

Synoptic-Scale Analysis of Freezing Rain Events in Montreal, Quebec, Canada

Abstract Freezing rain is a major environmental hazard that is especially common along the St. Lawrence River valley (SLRV) in southern Quebec, Canada. For large cities such as Montreal, severe events can have a devastating effect on people, property, and commerce. In this study, a composite analysis of 46 long-duration events for the period 1979–2008 is presented to identify key synoptic-scale structures and precursors of Montreal freezing rain events. Based on the observed structures of the 500-hPa heights, these events are manually partitioned into three types—west, central, and east—depending on the location and tilt of the 500-hPa trough axis. West events are characterized by a strong surface anticyclone downstream of Montreal, an inverted trough extending northward to the Great Lakes, and a quasi-stationary area of geostrophic frontogenesis located over Quebec. Central events are characterized by a cyclone–anticyclone couplet pattern, with a deeper surface trough extending into southern Ontario, and a strong stationary anticyclone over Quebec. East events are characterized by the passage of a transient well-defined cyclone, and a weaker downstream anticyclone. In all cases, cold northeasterly winds are channeled down the SLRV primarily by pressure-driven channeling. Northeasterly surface winds are associated with strong low-level temperature inversions within the SLRV. Additionally, west events tend to have a longer duration of weaker precipitation, while east events tend to have a shorter duration of more intense precipitation. The results of this study may aid forecasters in identifying and understanding the synoptic-scale structures and precursors to Montreal freezing rain events.

Unexpected High Winds in Northern New Jersey: A Downslope Windstorm in Modest Topography

Abstract This study presents the first evidence for the occurrence of a downslope windstorm in New Jersey. During the early morning hours of 4 January 2009, an unanticipated strong wind event was observed. Despite a zone forecast calling for winds less than 4 m s−1 issued 4 h prior to the event, winds up to 23 m s−1 were reported at High Point, New Jersey (elevation 550 m), with gusts to 30 m s−1 in its immediate lee (elevation 311 m). These winds were highly localized; a nearby Automated Surface Observing System (ASOS) station (Sussex, New Jersey, 12 km distant) reported calm winds between 0700 and 1000 UTC, just as the winds were peaking near High Point. High Point is the highest point in New Jersey, and is part of the quasi-two-dimensional Kittatinny Mountain extending from Pennsylvania into New York. This study tests the hypothesis that the topography of High Point, upon interacting with the local atmospheric stability and wind profiles, was sufficient to produce a downslope windstorm, thus causing these unusual winds. The results indicate that the presence of a sharp low-level temperature inversion in combination with a northwesterly low-level jet perpendicular to the ridge provided the key ingredients for the strong winds. Linear theory does not appear to explain the winds. Instead, prior studies incorporating nonlinearity predict a trapped lee wave or possibly a hydraulic jump, and model simulations suggest that High Point was indeed tall enough to generate such a wave along with rotors, although observations were not available to confirm this. Given sufficient model resolution, many aspects of this event were predictable. Similar windstorms have occurred before at High Point, but observations show that this event was the most amplified in recent years.

Coastal Atmospheric Circulation around an Idealized Cape during Wind-Driven Upwelling Studied from a Coupled Ocean–Atmosphere Model

Abstract The study analyzes atmospheric circulation around an idealized coastal cape during summertime upwelling-favorable wind conditions simulated by a mesoscale coupled ocean–atmosphere model. The domain resembles an eastern ocean boundary with a single cape protruding into the ocean in the center of a coastline. The model predicts the formation of an orographic wind intensification area on the lee side of the cape, extending a few hundred kilometers downstream and seaward. Imposed initial conditions do not contain a low-level temperature inversion, which nevertheless forms on the lee side of the cape during the simulation, and which is accompanied by high Froude numbers diagnosed in that area, suggesting the presence of the supercritical flow. Formation of such an inversion is likely caused by average easterly winds resulting on the lee side that bring warm air masses originating over land, as well as by air warming during adiabatic descent on the lee side of the topographic obstacle. Mountain leeside dynamics modulated by differential diurnal heating is thus suggested to dominate the wind regime in the studied case. The location of this wind feature and its strong diurnal variations correlate well with the development and evolution of the localized lee side trough over the coastal ocean. The vertical extent of the leeside trough is limited by the subsidence inversion aloft. Diurnal modulations of the ocean sea surface temperatures (SSTs) and surface depth-averaged ocean current on the lee side of the cape are found to strongly correlate with wind stress variations over the same area. Wind-driven coastal upwelling develops during the simulation and extends offshore about 50 km upwind of the cape. It widens twice as much on the lee side of the cape, where the coldest nearshore SSTs are found. The average wind stress–SST coupling in the 100-km coastal zone is strong for the region upwind of the cape, but is notably weaker for the downwind region, estimated from the 10-day-average fields. The study findings demonstrate that orographic and diurnal modulations of the near-surface atmospheric flow on the lee side of the cape notably affect the air–sea coupling on various temporal scales: weaker wind stress–SST coupling results for the long-term averages, while strong correlations are found on the diurnal scale.

Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Five sets of large-eddy simulations (LES) were performed to examine the characteristics of flows and pollutant dispersion in two-dimensional (2D) urban street canyons of unity building-height-to-street-width ratio in neutral, unstable, and stable thermal stratifications. The characteristic flows fall into the skimming flow regime for all the cases tested. The mean wind speed is increased and decreased, respectively, in unstable and stable conditions. Turbulence is enhanced in unstable conditions. Whereas, in stable conditions, the low-level temperature inversion weakens the recirculating flows forming another layer of stagnant air in the vicinity of the ground level. Unexpectedly, an increase in turbulence is found in the street canyon core in the slightly stable condition (Richardson number Rb=0.18). The turbulence promotion could be caused by the unique geometry of 2D street canyon in which the stable stratification slows down the primary recirculation. The rather stagnant flows in turn sharpen the roof-level vertical velocity gradient and deter the entrainment penetrating down to the ground level, leading to a substantial pollutant accumulation. While the pollutant tends to be well mixed in the street canyons in neutral and unstable conditions, a mildly improved pollutant removal in unstable conditions is observed because of the enhanced roof-level buoyancy-driven turbulence.

Arctic Inversion Strength in Climate Models

Recent work indicates that climate models have a positive bias in the strength of the wintertime low-level temperature inversion over the high-latitude Northern Hemisphere. It has been argued this bias leads to underestimates of the Arctic’s surface temperature response to anthropogenic forcing. Here the bias in inversion strength is revisited. The spatial distribution of low-level stability is found to be bimodal in climate models and observational reanalysis products, with low-level inversions represented by a stable primary mode over the interior Arctic Ocean and adjacent continents, and a secondary unstable mode over the Atlantic Ocean. Averaging over these differing conditions is detrimental to understanding the origins of the inversion strength bias. While nearly all of the 21 models examined overestimate the area-average inversion strength, conditionally sampling the two modes shows about half the models are biased because of the relative partitioning of the modes and half because of biases within the stable mode.

Atmospheric inversion strength over polar oceans in winter regulated by sea ice

Low-level temperature inversions are a common feature of the wintertime troposphere in the Arctic and Antarctic. Inversion strength plays an important role in regulating atmospheric processes including air pollution, ozone destruction, cloud formation, and negative longwave feedback mechanisms that shape polar climate response to anthropogenic forcing. The Atmospheric Infrared Sounder (AIRS) instrument provides reliable measures of spatial patterns in mean wintertime inversion strength when compared with available radiosonde observations and reanalysis products. Here, we examine the influence of sea ice concentration on inversion strength in the Arctic and Antarctic. Correlation of inversion strength with mean annual sea ice concentration, likely a surrogate for the effective thermal conductivity of the wintertime ice pack, yields strong, linear relationships in the Arctic (r = 0.88) and Antarctic (r = 0.86). We find a substantially greater (stronger) linear relationship between sea ice concentration and surface air temperature than with temperature at 850 hPa, lending credence to the idea that sea ice controls inversion strength through modulation of surface heat fluxes. As such, declines in sea ice in either hemisphere may imply weaker mean inversions in the future. Comparison of mean inversion strength in AIRS and global climate models (GCMs) suggests that many GCMs poorly characterize mean inversion strength at high latitudes.

The Seasonal Atmospheric Response to Projected Arctic Sea Ice Loss in the Late Twenty-First Century

Abstract The authors investigate the atmospheric response to projected Arctic sea ice loss at the end of the twenty-first century using an atmospheric general circulation model (GCM) coupled to a land surface model. The response was obtained from two 60-yr integrations: one with a repeating seasonal cycle of specified sea ice conditions for the late twentieth century (1980–99) and one with that of sea ice conditions for the late twenty-first century (2080–99). In both integrations, a repeating seasonal cycle of SSTs for 1980–99 was prescribed to isolate the impact of projected future sea ice loss. Note that greenhouse gas concentrations remained fixed at 1980–99 levels in both sets of experiments. The twentieth- and twenty-first-century sea ice (and SST) conditions were obtained from ensemble mean integrations of a coupled GCM under historical forcing and Special Report on Emissions Scenarios (SRES) A1B scenario forcing, respectively. The loss of Arctic sea ice is greatest in summer and fall, yet the response of the net surface energy budget over the Arctic Ocean is largest in winter. Air temperature and precipitation responses also maximize in winter, both over the Arctic Ocean and over the adjacent high-latitude continents. Snow depths increase over Siberia and northern Canada because of the enhanced winter precipitation. Atmospheric warming over the high-latitude continents is mainly confined to the boundary layer (below ∼850 hPa) and to regions with a strong low-level temperature inversion. Enhanced warm air advection by submonthly transient motions is the primary mechanism for the terrestrial warming. A significant large-scale atmospheric circulation response is found during winter, with a baroclinic (equivalent barotropic) vertical structure over the Arctic in November–December (January–March). This response resembles the negative phase of the North Atlantic Oscillation in February only. Comparison with the fully coupled model reveals that Arctic sea ice loss accounts for most of the seasonal, spatial, and vertical structure of the high-latitude warming response to greenhouse gas forcing at the end of the twenty-first century.

Numerical Simulations of the Impacts of the Saharan Air Layer on Atlantic Tropical Cyclone Development

Abstract In this study, the role of the Saharan air layer (SAL) is investigated in the development and intensification of tropical cyclones (TCs) via modifying environmental stability and moisture, using multisensor satellite data, long-term TC track and intensity records, dust data, and numerical simulations with a state-of-the-art Weather Research and Forecasting model (WRF). The long-term relationship between dust and Atlantic TC activity shows that dust aerosols are negatively associated with hurricane activity in the Atlantic basin, especially with the major hurricanes in the western Atlantic region. Numerical simulations with the WRF for specific cases during the NASA African Monsoon Multidisciplinary Analyses (NAMMA) experiment show that, when vertical temperature and humidity profiles from the Atmospheric Infrared Sounder (AIRS) were assimilated into the model, detailed features of the warm and dry SAL, including the entrainment of dry air wrapping around the developing vortex, are well simulated. Active tropical disturbances are found along the southern edge of the SAL. The simulations show an example where the dry and warm air of the SAL intruded into the core of a developing cyclone, suppressing convection and causing a spin down of the vortical circulation. The cyclone eventually weakened. To separate the contributions from the warm temperature and dry air associated with the SAL, two additional simulations were performed, one assimilating only AIRS temperature information (AIRST) and one assimilating only AIRS humidity information (AIRSH) while keeping all other conditions the same. The AIRST experiments show almost the same simulations as the full AIRS assimilation experiments, whereas the AIRSH is close to the non-AIRS simulation. This is likely due to the thermal structure of the SAL leading to low-level temperature inversion and increased stability and vertical wind shear. These analyses suggest that dry air entrainment and the enhanced vertical wind shear may play the direct roles in leading to the TC suppression. On the other hand, the warm SAL temperature may play the indirect effects by enhancing vertical wind shear; increasing evaporative cooling; and initiating mesoscale downdrafts, which bring dry air from the upper troposphere to the lower levels.

Process-Oriented Analysis of Environmental Conditions Associated with Precipitation Fog Events in the New York City Region

Abstract An analysis of the environmental conditions associated with precipitation fog events is presented using 20 yr of historical observations taken in a region centered on New York, New York. The objective is to determine the preferred weather scenarios and identify physical processes influencing the formation of fog during precipitation. Salient synoptic-scale features are identified using NCEP–NCAR reanalyses. Local environmental parameters, such as wind speed and direction, temperature, and humidity, are analyzed using surface observations, while the vertical structure of the lower atmosphere is examined using available rawinsonde data. The analysis reveals that precipitation fog mostly occurs as a result of the gradual lowering of cloud bases as continuous light rain or light drizzle is observed. Such scenarios occur under various synoptic weather patterns in areas characterized by large-scale uplift, differential temperature advection, and positive moisture advection. Precipitation fog onset typically occurs with winds from the northeast at inland locations and onshore flow at coastal locations, with flows from the south to southwest aloft. A majority of the cases showed the presence of a sharp low-level temperature inversion resulting from differential temperature advection or through the interaction of warm air flowing over a cold surface in onshore flow conditions. This suggests a common scenario of fog formation under moistening conditions resulting from precipitation evaporating into colder air near the surface. A smaller number of events formed with cooling of the near-saturated or saturated air. Evidence is also presented of the possible role of shear-induced turbulent mixing in the production of supersaturation and fog formation during precipitation.

Accuracy of Boundary Layer Temperature Profiles Retrieved With Multifrequency Multiangle Microwave Radiometry

The potential of a ground-based microwave temperature profiler to combine full tropospheric profiling with high-resolution profiling of the boundary layer is investigated. For that purpose, statistical retrieval algorithms that incorporate observations from different elevation angles and frequencies are derived from long-term radiosonde data. A simulation study shows the potential to significantly improve the retrieval performance in the lowest kilometer by combining angular information from relatively opaque channels with zenith-only information from more transparent channels. Observations by a state-of-the-art radiometer employed during the International Lindenberg campaign for assessment of humidity and cloud profiling systems and its impact on High-resolution modeling (LAUNCH) in Lindenberg, Germany, are used for an experimental evaluation with observations from a 99-m mast and radiosondes. The comparison not only reveals the high accuracy achieved by combining angular and spectral observations (overall, less than 1 K below 1.5 km), but also emphasizes the need for a realistic description of radiometer noise within the algorithm. The capability of the profiler to observe the height and strength of low-level temperature inversions is highlighted.

A flow regime diagram for forecasting lee waves, rotors and downslope winds

AbstractThe influence of a strong low‐level temperature inversion on the occurrence of lee waves, rotors and hydraulic jumps has been investigated using high resolution numerical model simulations. The aim of the work is to develop tools for forecasting hazardous winds downstream of mountains. Two‐dimensional simulations were conducted for a range of inversion heights and strengths and a fixed hill shape; lee waves, rotors and hydraulic jumps were found to occur. The flow type depends largely on the ratio of mountain height to inversion height and the upstream Froude number. A flow regime diagram based on these two parameters has been constructed and suggests that rotors could be forecast using upstream profiles, which are generally readily available from numerical weather prediction models.The applicability of the regime diagram for two‐dimensional flow to flows over real terrain has been tested using three‐dimensional simulations of flows over East Falkland, South Atlantic, under a range of upstream conditions. The flow type is found to be determined largely by the upstream profiles of wind and temperature, and the maximum height of orography directly upstream, indicating that the flow regime diagram can be used to predict flow type downstream of such terrain. Various three‐dimensional flow phenomena occur, such as flow channelling through gaps, and could be taken into account to improve the information available from the regime diagram. Copyright © 2006 Royal Meteorological Society.

The performance of the Hadley Centre Climate Model (HadCM3) in high southern latitudes

An assessment of mean atmospheric and oceanic data from a 100-year segment of a pre-industrial control run of version 3 of the Hadley Centre climate model is presented. The model output has been verified against in situ measurements from expeditions, data from the research stations and mean fields from the 15 year re-analysis project of the European Centre for Medium-range Weather Forecasts (ECMWF). The wave number 3 pattern of the mean sea-level pressure (MSLP) and 500-hPa height fields are handled reasonably well, but the climatological troughs over the Bellingshausen Sea and at 130 °E are too deep in winter by about 9 hPa at the surface. This is a result of positive sea-surface temperature (SST) errors over the tropical, eastern sides of the major ocean basins. These overly deep surface troughs result in the Antarctic coastal easterlies being too strong along the coast of Marie Byrd Land and much of the coast of East Antarctica. The circumpolar trough is too deep in summer by about 1.5 hPa and is located several degrees too far north in winter. Near-surface air temperatures over the interior of the Antarctic are in error by several degrees where the model has incorrect orographic height. The low-level temperature inversion is too strong over the Antarctic plateau. Precipitation minus evaporation over the interior of the Antarctic is slightly too low. The maximum in sea ice extent and the phase of the semi-annual oscillation (SAO) both lag the best available verification data by about 1 month. Copyright  2006 Royal Meteorological Society.

A new perspective of the physical processes associated with the clear-sky greenhouse effect over high latitudes

The physical processes associated with the clear-sky greenhouse effect in the presence of water vapor are examined by including surface emissivity in the greenhouse effect formulation, and by introducing a new way to partition physical processes of the greenhouse effect. In this new framework, it is found that the clear-sky greenhouse effect is governed by three physical processes associated with (1) the temperature contrast between the surface and the atmosphere, (2) the interaction between the surface emissivity and the temperature contrast, and (3) the surface emissivity. The importance of the three physical processes is assessed by computing their vertical and spectral variations for the subarctic winter and summer standard atmosphere using the radiation model MODTRAN3 (Moderate Resolution Transmittance code Version 3). The results show that the process associated with the temperature contrast between the surface and the atmosphere dominates over the other two processes in magnitude. The magnitude of this process has substantial variations in the spectral region of 1250 to 1880 cm−1 and in the far infrared region. Due to the low-level temperature inversion over the subarctic winter, there exists a negative contribution to the greenhouse trapping. The seasonal variations are, however, dominated by the processes associated with the interaction between the surface emissivity and the temperature contrast as well as the surface emissivity itself. The magnitudes of these two physical processes contributing to the greenhouse trapping over the subarctic winter are about 7 to 10 times of those over the subarctic summer, whereas the magnitude of the processes associated with the temperature contrast in the subarctic summer is only about 2 times of that in the subarctic winter.

Detection and Analysis of Clear-Sky, Low-Level Atmospheric Temperature Inversions with MODIS

The near-surface atmosphere of the polar region is characterized by temperature inversions throughout most of the year. However, radiosonde data are sparse, and numerical weather prediction models have relatively poor vertical resolution for boundary layer studies. A method is developed for detecting and estimating the characteristics of clear-sky, low-level temperature inversions using the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites. The method is based on an empirical relationship between the inversion strength, defined as the temperature difference across the inversion, or depth, defined as the altitude difference, and the difference between brightness temperatures in the 7.2- mm water vapor and 11-mm infrared window bands. Results indicate that inversion strength can be estimated unbiasedly with a root-mean-square error (rmse) of 28‐38C and an R2 of 0.80‐0.97. Inversion depth can be estimated with an rmse of 130‐250 m and an R2 of 0.62‐0.82. With MODIS, temperature inversions can be observed at a spatial resolution as high as 1k m 2 and a temporal sampling of up to 14 times per day, providing an opportunity for detailed studies of the spatial distribution and temporal evolution of the high-latitude boundary layer.

Extreme Winter Warming Events over the Mackenzie Basin: Dynamic and Thermodynamic Contributions.

The Mackenzie River of Canada is one of the great rivers of the world. Its basin is characterized by a highly variable topography and its climate is subject to many important cold-region phenomena. Over the last few decades, the Mackenzie Basin has also been experiencing a pronounced winter warming. In this study, a number of extreme basin warming events and related processes during the winter are investigated using surface and rawinsonde data. By documenting these events, we have found that the basin warming is mainly associated with low pressure systems in and near the basin, and with extratropical and subpolar high pressure systems in the vicinity of the basin. The dynamic and thermodynamic contributions (due to atmospheric circulations, topography, low-level temperature inversion and their interactions) to the basin warming are also examined. It is shown that basin warming events occur through the horizontal advection of warm air from west and south of the basin, and through adiabatic descent induced by both the topography and the low and/or high pressure system, particularly when the low-level temperature inversion occurs.

The Interactions among Cyclone Dynamics, Vertical Thermodynamic Structure, and Cloud Radiative Forcing in the North Atlantic Summertime Storm Track

Abstract This study investigates the changes in the vertical thermodynamic structure of the troposphere associated with the passage of cyclones and the corresponding influence on cloud radiative forcing (CRF). The focus is the synoptic-scale evolution of lower-tropospheric sounding structure in the extratropical North Atlantic during July 1985–89. The type and amount of low-level cloud cover, which has a strong impact on total CRF in this region, is sensitive to these variations. Composite profiles at ocean weather stations L and C are constructed for soundings with low-level temperature inversions, soundings with surface-based inversions, and soundings without inversions, separately for conditions of large-scale subsidence and ascent. In addition, collocated European Centre for Medium-Range Weather Forecasts analyzed fields and Earth Radiation Budget Experiment CRF data are averaged for each composite sounding. These composites correspond to distinct dynamical regimes associated with the movement of cycl...

Inviscid Interactions Between Wake Vortices and Shear Layers

Aircraft trailing vortices can be influenced significantly by atmospheric conditions such as crosswind, turbulence, and stratification. According to the NASA 1994 and 1995 field measurement program in Memphis, Tennessee, the descending aircraft wake vortices could stall or be deflected at the top of low-level temperature inversions that usually produce pronounced shear zones. Numerical simulations of vortex/shear interactions with ground effects have been performed by several groups. Burnham used a series of evenly spaced line vortices at a particular altitude to model the ground shear layer of the cross- wind. He found that the wind shear was swept up around the downwind vortex and caused the downwind vortex to move upward, and claimed that the effect was actually produced by the vertical gradient in the wind shear rather than by the wind shear directly, because uniformly distributed wind-shear vortices would have no effect on the trailing vortex vertical motion. Recently, Proctor et al. numerically tested the effects of narrow shear zones on the behavior of the vortex pair, motivated by the observation of the Memphis field data. The shear-layer sensitivity tests indicated that the downwind vortex was more sensitive and deflected to a higher altitude than its upwind counterpart. The downstream vortex contained vorticity of opposite sign to that of the shear. There was no detectable preference for the downwind vortex (or upwind vortex) to weaken (or strengthen) at a greater rate.

Adverse health effects of air pollutants in a nonsmoking population

Utah Valley has provided an interesting and unique opportunity to evaluate the health effects of respirable particulate air pollution (PM 10). Residents of this valley are predominantly nonsmoking members of the Church of Jesus Christ of Latter-day Saints (Mormons). The area has moderately high average PM 10 levels with periods of highly elevated PM 10 concentrations due to local emissions being trapped in a stagnant air mass near the valley floor during low-level temperature inversion episodes. Due to a labor dispute, there was intermittent operation of the single largest pollution source, an old integrated steel mill. Levels of other common pollutants including sulfur dioxide, ozone, and acidic aerosol are relatively low. Studies specific to Utah Valley have observed that elevated PM 10 concentrations are associated with: (1) decreased lung function; (2) increased incidence of respiratory symptoms; (3) increased school absenteeism; (4) increased respiratory hospital admissions; and (5) increased mortality, especially respiratory and cardiovascular mortality.

Low-Level Temperature Inversions of the Eurasian Arctic and Comparisons with Soviet Drifting Station Data

Seasonal and regional variations in characteristics of the Arctic low-level temperature inversion are examined using up to 12 years of twice-daily rawinsonde data from 31 inland and coastal sites of the Eurasian Arctic and a total of nearly six station years of data from three Soviet drifting stations near the North Pole. The frequency of inversions, the median inversion depth, and the temperature difference across the inversion layer increase from the Norwegian Sea eastward toward the Laptev and East Siberian seas. This effect is most pronounced in winter and autumn, and reflects proximity to oceanic influences and synoptic activity, possibly enhanced by a gradient in cloud cover. East of Novaya Zemlya during winter, inversions are found in over 95% of all soundings and tend to be surface based. For all locations, however, inversions tend to he most intense during winter due to the large deficit in surface net radiation. The strongest inversions are found over eastern Siberia, and reflect the effects of local topography. The frequency of inversions is lowest during summer, but is still >50% at all locations. Most summer inversions are elevated, and are much weaker than their winter counterparts. Data from the drifting stations reveal an inversion in every sounding from December to April. The minimum frequency of 85% occurs during August. While the median inversion depth is over 1200 m during March, it decreases to approximately 400 m during August, with median temperature differences across the inversion layer of 12.6° and 2.8°C, respectively. The median depth of the summertime mixed layer below inversions at the drifting stations ranges from 300 to 400 m. Seasonal changes in these inversion characteristics show a strong relationship with seasonal changes in cloud cover.