Boundary Layer Air Research Articles

Analysis of differences between thermodynamic and material boundary layer structure: Comparison of detection by ceilometer and microwave radiometer

Planetary boundary layer height (PBLH) is a key parameter for air quality prediction and boundary layer meteorology studies. The height of thermodynamic boundary layer (Hθ) and material boundary layer (HC) in different seasons are compared by using microwave radiometer (MWR) and ceilometer. There are considerable differences between Hθ and HC and the causes are further studied. We found that when the boundary layer is markedly stable at night, the MWR's algorithm used to estimate PBLH has a retrieval restriction and directly leads to a much higher Hθ than HC (1 to 2.5 km higher). This unrealistic PBLH can be identified and eliminated effectively by using temperature differences from the surface to 1 km. During periods of transportation and diffusion of aerosols, strong southerly winds transporting pollutants northward above Hθ or strong northerly winds throughout the boundary layer removing pollutants will cause a much higher HC than Hθ (0.5 to 2 km higher). However, when transportation occurs in the lower layer within Hθ, HC and Hθ are consistent. The aerosol layers is able to characterize the multiple-layered structure of the nocturnal boundary layer (NBL) and the evolution of the boundary layer by using backscatter. The stable boundary layer (SBL) ranges from 0.3 to 0.6 km at night. Compared with the idealized thermodynamic boundary layer, there isn't a distinct boundary between the SBL and the growing mixing layer (ML) after sunrise. The residual layer (RL) ranges from 0.5 to 1.5 km, it lasts 4 or 5 h after sunrise and shows up again within 2 h after sunset. The maximum detectable range of Doppler Lidar has a large backscatter gradient, so is strongly consistent with the RL at night and the ML in the daytime during a pollution event. The combined observation of microwave radiometer and ceilometer can well reveal the completed characteristics of boundary layer structures.

Transport From the Asian Summer Monsoon Anticyclone Over the Western Pacific

AbstractThe subseasonal scale dynamics of the Asian summer monsoon (ASM) upper troposphere (UT) anticyclone has been identified as a primary mechanism for convectively lofted Asian boundary layer air to leave the confinement of the anticyclone and impact the global UT and lower stratosphere (UTLS). This work quantifies eastward eddy shedding associated with the subseasonal scale oscillation of the anticyclone and associated chemical transport. Using reanalysis data together with satellite trace gas data, we examine the correlation between enhanced tropospheric trace gas species and the presence of a secondary anticyclone over the western Pacific Ocean. To diagnose the role of the western Pacific anticyclone (WPA) in the transport of ASM air into the global UTLS, transport pathways and transit times of air found within the WPA are analyzed using Lagrangian kinematic trajectories. Results show that about two thirds of the air within the WPA originates from, or is influenced by, the greater region of the Tibetan anticyclone. After leaving the WPA, air favors two pathways forward: eastward following the subtropical jet and southwestward following UTLS anticyclonic flow. Approximately 60% of the air parcels cross the tropopause into the lower stratosphere within 30 days, though most cross within a few days. These results highlight opportunities for investigating the impact of the ASM on global UTLS chemical and aerosol composition over the western Pacific via research aircraft.

Examination of the ocean as a source for atmospheric microplastics.

Global plastic litter pollution has been increasing alongside demand since plastic products gained commercial popularity in the 1930’s. Current plastic pollutant research has generally assumed that once plastics enter the ocean they are there to stay, retained permanently within the ocean currents, biota or sediment until eventual deposition on the sea floor or become washed up onto the beach. In contrast to this, we suggest it appears that some plastic particles could be leaving the sea and entering the atmosphere along with sea salt, bacteria, virus’ and algae. This occurs via the process of bubble burst ejection and wave action, for example from strong wind or sea state turbulence. In this manuscript we review evidence from the existing literature which is relevant to this theory and follow this with a pilot study which analyses microplastics (MP) in sea spray. Here we show first evidence of MP particles, analysed by μRaman, in marine boundary layer air samples on the French Atlantic coast during both onshore (average of 2.9MP/m3) and offshore (average of 9.6MP/m3) winds. Notably, during sampling, the convergence of sea breeze meant our samples were dominated by sea spray, increasing our capacity to sample MPs if they were released from the sea. Our results indicate a potential for MPs to be released from the marine environment into the atmosphere by sea-spray giving a globally extrapolated figure of 136000 ton/yr blowing on shore.

Waste heat: the dominating root cause of current global warming

BackgroundPursuing GHG reductions by means of all resources and efforts has turned out no result to stop or even slow the global warming: the globe still gets warmer and warmer, especially in the recent years, at record-breaking rate almost each single year. Additionally, no definitive relationship has been found between the warming and the atmospheric GHG concentration. The link between them even in IPCC’s report lacks support and is unconvincing. All these imply that something else is responsible for the warming. On the other hand, huge amount of residual heat or waste heat from human activities has been poured into the climate system but has not been considered seriously in the context of global warming or climate change.ResultsThis article features deploying the basic principles of thermodynamics and applying a new model, Equivalent Climate Change Model, to analyse the currently available data on world energy consumption between 1965 and 2017, and to study the relation between the global warming and the waste heat entered the climate system. The results show that the temperature changes in air, oceans and land are definitively correlated to the respective heat allocated from the waste heat stream based on their specific heat capacities, with high certainty and reliability. The observed anomalies in air fall within a range of simulations at an equivalent climate change surface air boundary layer depth between 50 and 100 m (60 ~ 100 m in recent decades due to more establishments of high-rising heat discharging sources); the anomalies in oceans fall within a range of simulations at an equivalent climate change waters surface boundary layer depth between 0.10 and 0.20 m (0.125 ~ 0.20 m in recent decades); and the anomalies in land fall within a range of simulations at an equivalent climate change land surface boundary layer depth between 0.05 and 0.10 m (0.06 ~ 0.10 m in recent decades). The simulation results at the air layer depth of 70 m are almost the same as NASA’s Lowess smoothing trend. Forecast of future global warming based on this model under the scenario of business as usual indicates that the possible air temperature risings will be in the range of 0.68 ~ 1.13 °C in 2030 and 0.73 ~ 1.22 °C in 2040; the possible sea temperature risings will be in the range of 0.61 ~ 0.98 °C in 2030, 0.66 ~ 1.05 °C in 2040; and the possible land temperature risings will be in the range of 1.02 ~ 1.71 °C in 2030, 1.10 ~ 1.84 °C in 2040. However, if the energy conversion efficiency increased by 10% by 2030 and another 10% by 2040, then the possible air temperature risings would be in the range of 0.54 ~ 0.90 °C in 2030 and 0.44 ~ 0.73 °C in 2040; the possible sea temperature risings would be in the range of 0.49 ~ 0.78 °C in 2030, and 0.40 ~ 0.64 °C in 2040; and the possible land temperature risings would be in the range of 0.81 ~ 1.36 °C in 2030 and 0.66 ~ 1.11 °C in 2040. The observed global average air temperature changes and the Lowess Smoothing values in 2018 and 2019 fall within the range set by the air layer depth between 60 and 100 m, are consistent with the forecast under the scenario of business as usual, further confirms the reliability of this approach.ConclusionsGreenhouse gases are not the culprit of the current global warming, instead, huge amount of residual heat or waste heat discharged into the environment from human activities has dominated the warming (beside of solar irradiance and volcano eruptions). Pursuing GHG reductions is bound to be ineffective in preventing the globe from further warming but increases unnecessary burdens. Switching to 100% of surface renewable energies is the ideal solution to completely solve further warming problem. However, geotherm does cause global warming although it is a type of renewable energy. Increasing energy’s conversion efficiency can effectively help slow down the warming, it requires vast investment and will embrace breakthroughs in technologies. Changing human’s behavior individually and socially and retrofitting can decrease the energy consumption and the amount of heat entering the environment and thus help mitigate climate change and its impact in the most cost-effective way. Unlike the General Circulation Models that can only simulate the past air temperature changes with greater uncertainty, the Equivalent Climate Change Model can not only trace the past temperature changes in air, oceans and land, but also can predict the future changes in them, respectively, with high certainty and reliability.

Impacts of spectrally resolved irradiance on photolysis frequency calculations within a forest canopy

Although photolysis frequencies are wavelength-dependent and the dependence varies among chemical species, previous canopy radiative transfer models did not consider more than three broad bands (ultraviolet, photosynthetically active radiation (PAR), and near-infrared). In this study, high spectral resolution and wavelength-dependent idealized leaf optical properties allow us to determine the disposition of the light spectrum within a mixed deciduous forest canopy. Four radiative transfer approaches of varying complexity are applied to obtain vertical profiles of spectral actinic flux. Broad-band radiation measurements made above and below a mixed deciduous forest provide the necessary information to verify the fidelity of each radiative transfer approach. Model comparison results indicate that the Beer–Lambert scheme gives less total actinic flux, while the other three schemes give similar actinic flux profiles. Spectral actinic flux profiles are used to calculate in-canopy photolysis for different chemical species and to assess the importance of in-canopy photochemistry in modifying biogenic volatile organic compounds transported to the overlying atmospheric boundary layer. We find that, depending on the time of day and chemical species, percent errors in photolysis frequencies incurred by using a common in-canopy approximation based on weighting by relative PAR profiles can be as high as ± 50% in lower regions of the canopy, or 10–20% in daily canopy integrated photolysis frequency. Results obtained using a one-dimensional photochemical model suggest that choice of canopy radiative transfer scheme can have substantial impacts on in-canopy chemical reactions and concentrations in the overlying atmospheric boundary layer air. Such effects caused in-canopy gas concentration differences ranging from 8% for ozone and 35% for hydroxyl radical to 77% for nitrate radical.

Investigating microclimate effects in an oasis-desert interaction zone

To investigate oasis-desert microclimate effects, we performed a series of numerical simulations in an idealized oasis-desert system based on an improved computational fluid dynamics (CFD) model for simulating atmospheric boundary layer flows, air temperature and humidity. Numerical simulations were designed based on the hydrometeorological observations obtained during the HiWATER-MUSOEXE (Heihe Watershed Allied Telemetry Experimental Research, Multi-Scale Observation Experiment on Evapotranspiration over heterogeneous land surfaces) campaign. The results are summarized as follows: (1) Oasis-desert interactions are significantly affected by background wind conditions. We observed the oasis-desert local circulation under calm background wind conditions and the oasis thermal internal boundary layer under low wind speed conditions induced by hydrothermal contracts. These interactions will disappear when the background wind speed is sufficiently high, and there is only an oasis dynamic internal boundary layer caused by the aerodynamic roughness length contrast. (2) Oasis-desert interactions lead to a series of microclimate effects, including the oasis cold-wet island effect, air humidity inversion effect within the surrounding desert and oasis wind shield effect, which are important for the stability and sustainability of the oases-desert ecosystem. (3) The hydrothermal conditions due to the difference between the oasis and desert, the vegetation fraction and distribution patterns impact the oasis-desert microclimate effects. The intensity of oasis-desert interactions increases with the land surface temperature (LST) difference in the oasis-desert. The oasis-desert interactions are gradually strengthened with the increase of the vegetation fraction within the oasis. Integrated ecological and economic benefits of the oasis, the oasis vegetation pattern, which includes the croplands and shelterbelts staggered within the oasis and the shelterbelts surrounding the outside, is beneficial to limiting the loss of water vapor and preventing sandstorms from the oasis. The findings of the current study improve the fundamental understanding of the microclimate and provide implications for maintaining the sustainability of oasis-desert ecosystems.

Synoptic-scale controls of fog and low-cloud variability in the Namib Desert

Abstract. Fog is a defining characteristic of the climate of the Namib Desert, and its water and nutrient input are important for local ecosystems. In part due to sparse observation data, the local mechanisms that lead to fog occurrence in the Namib are not yet fully understood, and to date, potential synoptic-scale controls have not been investigated. In this study, a recently established 14-year data set of satellite observations of fog and low clouds in the central Namib is analyzed in conjunction with reanalysis data in order to identify synoptic-scale patterns associated with fog and low-cloud variability in the central Namib during two seasons with different spatial fog occurrence patterns. It is found that during both seasons, mean sea level pressure and geopotential height at 500 hPa differ markedly between fog/low-cloud and clear days, with patterns indicating the presence of synoptic-scale disturbances on fog and low-cloud days. These regularly occurring disturbances increase the probability of fog and low-cloud occurrence in the central Namib in two main ways: (1) an anomalously dry free troposphere in the coastal region of the Namib leads to stronger longwave cooling of the marine boundary layer, increasing low-cloud cover, especially over the ocean where the anomaly is strongest; (2) local wind systems are modulated, leading to an onshore anomaly of marine boundary-layer air masses. This is consistent with air mass back trajectories and a principal component analysis of spatial wind patterns that point to advected marine boundary-layer air masses on fog and low-cloud days, whereas subsiding continental air masses dominate on clear days. Large-scale free-tropospheric moisture transport into southern Africa seems to be a key factor modulating the onshore advection of marine boundary-layer air masses during April, May, and June, as the associated increase in greenhouse gas warming and thus surface heating are observed to contribute to a continental heat low anomaly. A statistical model is trained to discriminate between fog/low-cloud and clear days based on information on large-scale dynamics. The model accurately predicts fog and low-cloud days, illustrating the importance of large-scale pressure modulation and advective processes. It can be concluded that regional fog in the Namib is predominantly of an advective nature and that fog and low-cloud cover is effectively maintained by increased cloud-top radiative cooling. Seasonally different manifestations of synoptic-scale disturbances act to modify its day-to-day variability and the balance of mechanisms leading to its formation and maintenance. The results are the basis for a new conceptual model of the synoptic-scale mechanisms that control fog and low-cloud variability in the Namib Desert and will guide future studies of coastal fog regimes.

Impact of land surface physics in WRF on the simulation of sea breeze circulation over southeast coast of India

In this work, the impact of land surface physics in simulating the sea and land breeze circulation along the south east coast of India near Kalpakkam is studied using the Weather Research and Forecasting (WRF) model for emergency response application. Simulations were conducted with three different land surface models (LSMs)—5-Layer soil thermal diffusion scheme (5-Layer), Noah land surface model (Noah) and Noah Multi-Physics land surface model (Noah-MP) for different days in summer and southwest monsoon. Observations from meteorological towers, automated weather stations, sonic anemometers and GPS Radiosonde profiles were used for comparison. Results indicate that the Noah-MP followed by Noah schemes produce more realistic simulations of the sea breeze circulation in terms of intensity, onset time, duration, horizontal and vertical extents in most of the studied cases compared to the 5-Layer scheme. Results indicate that the 5-Layer model grossly overestimated air temperature, heat flux and boundary layer height relative to Noah and Noah-MP schemes. The Noah-MP scheme better represented the land-surface feedback in terms of soil temperature, skin temperature and surface energy fluxes leading to more realistic simulations of sea breeze and boundary layer characteristics.

Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super Typhoon Rammasun (2014)

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of Typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500–850-hPa environmental winds, and (b) midlevel moisture and ventilation. We then describe how these differences impact on the evolving convective organization, storm structure, and the timing of RI. As expected, ascent, diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI. The evolution of vortex cloudiness from this member is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We present evidence that this structure, and hence the enhanced diabatic heating, is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member: (a) inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) with minimal ventilation, undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; and (c) this process seems related to a critical down-shear tilt of the vortex from midlevels, and the vertical phase-locking of the circulation over up-shear quadrants. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger vertical wind shear, initially resulting in poor vertical coherence of the circulation, lesser moisture and larger ventilation. Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.

Scavenging of Sub-Micron to Micron-Sized Microbial Aerosols during Simulated Rainfall

The processes removing aerosols from the atmosphere during rainfall are generically referred to as scavenging. Scavenging influences aerosol distributions in the atmosphere, with consequent effects on cloud properties, radiative forcing, and human health. In this study, we investigated the below-cloud scavenging process, specifically focusing on the scavenging of 0.2 to 2 µm-sized microbial aerosols by populations of water drops with average diameters of 3.0 and 3.6 mm. Rainfall was simulated in convective boundary layer air masses by dispensing the water drops from a 55 m bridge and collecting them at ground level. Particles and microbial cells scavenged by the water drops were visualized, enumerated, and sized using scanning electron and epifluorescence microscopy. Aerosolized particles and DNA-containing microbial cells of 2 µm diameter were scavenged at efficiencies similar to those reported previously in empirical studies; however, the efficiencies derived for smaller aerosols were significantly higher (one to three orders of magnitude) than those predicted by microphysical modeling. Application of the derived scavenging efficiencies to cell data from rainfall implies that, on average, approximately 50 to 70% of the 1 µm microbial cells in the precipitation originated from within the cloud. Further study of submicron to micron-sized aerosol scavenging over a broader raindrop size distribution would improve fundamental understanding of the scavenging process and the capacity to estimate (bio)aerosol abundances in the source cloud through analysis of rainfall.

Atmospheric visibility variation over global land surface during 1973–2012: Influence of meteorological factors and effect of aerosol, cloud on ABL evolution

Atmospheric visibility (Vis) is used to evaluate air pollution over global land surface during 1973–2012. Over global more populated areas with major anthropogenic emissions and hygroscopic aerosol species, weaker surface wind is generally associated with more severe air pollution and Vis degradation, while an increase of relative humidity (RH) generally favors more efficient secondary formation and hygroscopic growth of aerosol and Vis impairment. The allocation of meteorological factor with the major aerosol component over global different areas can influence the formation and variation of aerosol and cloud, and their effects on the atmospheric boundary layer (ABL) evolution and air pollution. Over East Asia, India and Southeast Asia, the air pollution with a growing concern with Vis decrease can be associated to an increase in anthropogenic emission and a synchronous stabilization of ABL (characterized by a weakened, less variable surface wind and a decreased diurnal temperature range, DTR). Both the increase of aerosol loading and cloud cover could (i) increase daily minimum air temperature versus decrease daily maximum air temperature and result in a decrease of DTR, (ii) reduce solar radiation reaching the surface, stabilize the ABL and aggravate air pollution. While Vis increase over Europe could be related to more effective emission control and an intensification of surface wind. Also significantly, a close relationship is identified between RH and DTR; RH explains about 90% of the variance of DTR over Europe in summer; a drier weather is corresponding to a greater surface incoming short-wave flux and a greater DTR.

Multi-decadal surface ozone trends at globally distributed remote locations

Extracting globally representative trend information from lower tropospheric ozone observations is extremely difficult due to the highly variable distribution and interannual variability of ozone, and the ongoing shift of ozone precursor emissions from high latitudes to low latitudes. Here we report surface ozone trends at 27 globally distributed remote locations (20 in the Northern Hemisphere, 7 in the Southern Hemisphere), focusing on continuous time series that extend from the present back to at least 1995. While these sites are only representative of less than 25% of the global surface area, this analysis provides a range of regional long-term ozone trends for the evaluation of global chemistry-climate models. Trends are based on monthly mean ozone anomalies, and all sites have at least 20 years of data, which improves the likelihood that a robust trend value is due to changes in ozone precursor emissions and/or forced climate change rather than naturally occurring climate variability. Since 1995, the Northern Hemisphere sites are nearly evenly split between positive and negative ozone trends, while 5 of 7 Southern Hemisphere sites have positive trends. Positive trends are in the range of 0.5–2 ppbv decade–1, with ozone increasing at Mauna Loa by roughly 50% since the late 1950s. Two high elevation Alpine sites, discussed by previous assessments, exhibit decreasing ozone trends in contrast to the positive trend observed by IAGOS commercial aircraft in the European lower free-troposphere. The Alpine sites frequently sample polluted European boundary layer air, especially in summer, and can only be representative of lower free tropospheric ozone if the data are carefully filtered to avoid boundary layer air. The highly variable ozone trends at these 27 surface sites are not necessarily indicative of free tropospheric trends, which have been overwhelmingly positive since the mid-1990s, as shown by recent studies of ozonesonde and aircraft observations.

The Nature of Climate Change- equivalent Climate Change Model’s Application in Decoding the Root Cause of Global Warming

Scrutiny and analysis of various energy applications show that the energy conversion to useful work or new products has been systematically inefficient. The global energy’s total effective conversion efficiency is estimated only about 20% and about 80% of the energy has been discharged into the environment. It is this energy that leads to the unbalance of the climate system’s energy budget balance and causes the global warming.
 This article presents a simple equivalent climate change model to track the past global warming and to predict the future change trend at the global scale. The model comprises of an equivalent climate change surface air boundary layer, an equivalent climate change land surface boundary layer and an equivalent climate change seawaters surface boundary layer. It produces unique definitive relationships between the temperature changes and the heat entered the air, waters and land. The model can also be used to forecast future non-renewable energy consumption needed to keep the temperature rising under Paris Accord.
 Analysis of currently available data by using this model confirms that temperature changes in air, seawaters and land closely correlate to the amount of heat discharged into the climate system from human activities. NASA and NOAA’s observations of temperature anomalies for the surface air, sea surface and land surface are well consistent with the temperature changes calculated by this model, especially the calculated results at the depth of 70 meters of the surface air boundary layer and NASA’s Lowess Smoothing trend are very close.
 Flaring intensifies global warming. Reducing use of fossil fuels, nuclear and geothermal energies, developing surface renewable energies and increasing energy’s total effective conversion efficiency and thus reducing the amount of residual/waste energy are the paths to effectively and efficiently control global warming.

One year of aerosol refractive index measurement from a coastal Antarctic site

Abstract. Though the environmental conditions of the Weddell Sea region and Dronning Maud Land are still relatively stable compared to the fast-changing Antarctic Peninsula, we may suspect pronounced effects of global climate change for the near future (Thompson et al., 2011). Reducing the uncertainties in climate change modeling requires a better understanding of the aerosol optical properties, and for this we need accurate data on the aerosol refractive index (RI). Due to the remoteness of Antarctica only very few RI data are available from this region (Hogan et al., 1979; Virkkula et al., 2006; Shepherd et al., 2018). We calculate the real refractive index of natural atmospheric aerosols from number size distribution measurements at the German coastal Antarctic station Neumayer III. Given the high average scattering albedo of 0.992 (Weller et al., 2013), we assumed that the imaginary part of the RI is zero. Our method uses the overlapping size range (particle diameter D between 120 and 340 nm) of a scanning mobility particle sizer (SMPS), which sizes the particles by their electrical mobility, and a laser aerosol spectrometer (LAS), which sizes the particles by their optical scattering signal at the 633 nm wavelength. Based on almost a complete year of measurement, the average effective refractive index (RIeff, as we call our retrieved RI because of the used assumptions) for the dry aerosol particles turned out to be 1.44 with a standard deviation of 0.08, in a good agreement with the RI value of 1.47, which we derived from the chemical composition of bulk aerosol sampling measurements. At Neumayer the aerosol shows a pronounced seasonal pattern in both number concentration and chemical composition. Despite this, the variability of the monthly averaged RIeff values remained between 1.40 and 1.50. Compared to the annual mean, two austral winter months (July and September) showed slightly but significantly increased values (1.50 and 1.47, respectively). The size dependency of the RIeff could be determined from time-averaged LAS and SMPS number size distributions measured between December 2017 and January 2018. Here we calculated RIeff for four different particle size ranges and observed a slight decrease from 1.47 (D range 116–168 nm) to 1.37 (D range 346–478 nm). We find no significant dependence of the derived RIeff values on the wind direction. Thus we conclude that RIeff is largely independent of the general weather situation, roughly classified as (i) advection of marine boundary layer air masses during easterly winds caused by passing cyclones in contrast to (ii) air mass transport from continental Antarctica under southern katabatic winds. Neumayer, the only relevant contamination source, is located 1.5 km north of the air chemistry observatory, where the measurements were performed. Given that northerly winds are almost absent, the potential impact of local contamination is minimized in general. Indeed our data show no impact of local contamination on RIeff. Just in one case a temporary high-contamination episode with diesel engines operating right next to the measurement site resulted in an unusual high RIeff of 1.59, probably caused by the high black carbon content of the exhaust fumes. To conclude, our study revealed largely constant RIeff values throughout the year without any sign of seasonality. Therefore, it seems reasonable to use a single, constant RIeff value of 1.44 for modeling optical properties of natural, coastal Antarctic sub-micrometer aerosol.

Statistical modelling of roadside and urban background ultrafine and accumulation mode particle number concentrations using generalized additive models

Quantification of the exposure of urban residents to ultrafine particle number concentrations (UFP) is challenging due to its high spatial and temporal variability. Hence, statistical models, e.g. generalized additive models (GAM), may be used to estimate time series or spatial characteristics of UFP. The GAM approach allows the representation of non-linear relations of a response variable with explanatory variables without the need to pre-define model functions. Up to now, GAMs were usually fitted to UFP data from a single site or from mobile measurement campaigns with limited temporal coverage. In this study, GAMs were used to determine UFP, accumulation mode particle (ACC) and total number concentration (TNC) at five urban sites in the cities of Leipzig and Dresden, Germany for the period 2011–2013. As explanatory variables, reanalysis data sets of meteorological quantities, urban geometry and traffic volume data were evaluated. Variables causing concurvity, which is the equivalent to collinearity in non-linear model approaches, were neglected to guarantee the interpretability of the final models. The models were then validated in a ten-fold cross-validation approach. The final models contained smooth functions for the building surface fraction, planetary boundary layer height, traffic volume, air temperature, wind direction, atmospheric pressure, relative humidity, global radiation and precipitation. Adjusted coefficients of determination (R2adj.) for the final models were R2adj. = 0.44 for UFP, R2adj. = 0.51 for ACC and R2adj. = 0.48 for TNC. Coefficients of determination of the cross-validation were in a similar range (0.44 for UFP, 0.51 for ACC, 0.49 for TNC). Finally, our study shows that GAMs are able to represent important processes that contribute to the particle number concentration from the smooth functions, i.e. emission, dilution, nucleation, deposition and long-range transport.

Changes in Surface Ozone Concentration during Precipitation

Changes in ozone concentration during precipitation are studied using data of ozone monitoring in the surface air layer in Tomsk and at Karadag. It is found that these changes can be either positive or negative. The largest jumps in ozone content are observed during frontal precipitations. During air mass precipitation, the sign and magnitude of the changes are determined by the diurnal behavior of ozone concentration. Analysis showed that the increase in the ozone concentration during precipitation and ozone increase in the diurnal behavior are time-coincident in 59% of cases in Tomsk and in 63% of cases at the Karadag. The decreasing wave of ozone concentration in the diurnal behavior coincides in time with ozone decrease during precipitation even more often, in 85% of cases in Tomsk and in 79% of cases at Karadag. Based on data of aircraft sensing, it is shown that ozone descent from the boundary air layer occurs in a number cases when the temperature stratification during precipitation changes to neutral.

Relationship between pedestrian-level outdoor thermal comfort and building morphology in a high-density city

Urban compact buildings impose large frictional drag on boundary-layer air flow and create stagnant air within the building environment. It results in exacerbating the street-level outdoor thermal comfort (OTC). It is common to perform in-situ measurements of the OTC in different urban forms and to study their relationship. However, it is impossible to do so from a planning perspective because of the absence of physical planned urban forms. Our objective was to quantify the thermal environment and OTC in different planned complex urban forms by well-validated numerical models. We coupled a computational fluid dynamics (CFD) model to an OTC (Rayman) model to study the OTC. The κ–ω SST turbulent model was adopted for the CFD simulations, with accuracy of the turbulent model validated by wind tunnel measurements. The highest calculated air temperature within the street canyon of a planned bulky urban form could reach more than 5 °C higher than the surrounding environment. Within the tested urban forms, our coupled model predicted mean radiant temperature comparable with measurements in the literature. The model also produced sensible street-level physiologically equivalent temperatures (PETs) when comparing with those listed in the human thermal sensation categories. In the morning, the predicted PETs within all the urban forms were lower than that in open areas, which indicated that the shading of buildings effectively reduced the PET increase due to solar irradiance. At noon, increases in PETs by more than 10 °C relative to the morning situation indicated that when the buildings acted as heat sources after insolation absorption, increase in the air temperature at the street intersection and in the street canyon made an important contribution to the receiver PETs. The reduction in building lengths and increase in the low-level porosity were the most effective ways to reduce the heat stress at the pedestrian-level.

On the selection of perturbations for thermal boundary layer control

The convective instability of the natural convection boundary layers of air (Pr = 0.7) in the laminar-to-turbulent transition regime (Ra = 8.7 × 107–1.1 × 109) is investigated by stability analysis in the framework of direct numerical simulations. To understand the spatial and temporal evolution of the convective instability of the thermal boundary layers, small-amplitude random-mode numerical perturbations are first introduced into the boundary condition of the boundary layer flow. The prescribed full spectral perturbations (i.e., white noise) are mostly damped out immediately by a limited upstream boundary layer. A low-frequency band is initially distinct in the upstream near the leading edge but decays spatially as the instability propagates downstream. In contrast, a high-frequency band emerges to finally become the most dominant frequency band in the thermal boundary layer transition regime. To obtain further insights into the nature of the established high-frequency band, single-mode perturbations of various frequencies are then introduced into the boundary layer near the leading edge. It is found that a single-mode perturbation at the peak frequency within the high-frequency band excites the maximum response of the thermal boundary layer, suggesting that the peak frequency is in fact the characteristic frequency or resonance frequency of the thermal boundary layer. The dimensionless form of the dependence of the characteristic frequency on Ra is then found to be fc = 0.07Ra2/3. The single-mode perturbation numerical experiments also revealed the propagation speed of convective instability waves, which was significantly greater than the convection speed of the thermal boundary layer. The smaller the Ra, the larger the difference between the two propagation speeds. A semi-analytical scaling of the wave propagation speed in the form csc ∼ Ra1/2y1/2Pr was derived (y denoting the streamwise location of the boundary layer), providing a predictive correlation that can be used for thermal boundary layer control.

Assessing the Influence of Convective Downdrafts and Surface Enthalpy Fluxes on Tropical Cyclone Intensity Change in Moderate Vertical Wind Shear

AbstractThe thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TCs) were investigated using dropsonde observations collected from 1996 to 2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 h following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.

Interactions between a Nocturnal MCS and the Stable Boundary Layer as Observed by an Airborne Compact Raman Lidar during PECAN

Abstract Small-scale variations within the low-level outflow and inflow of an MCS can either support or deter the upscale growth and maintenance of the MCS. However, these small-scale variations, in particular in the thermodynamics (temperature and humidity), remain poorly understood, due to a lack of detailed measurements. The compact Raman lidar (CRL) deployed on the University of Wyoming King Air aircraft directly sampled temperature and water vapor profiles at unprecedented vertical and along-track resolutions along the southern margin of a series of mature nocturnal MCSs traveling along a frontal boundary on 1 July 2015 during the Plains Elevated Convection at Night (PECAN) campaign. Here, the capability of the airborne CRL to document interactions between the MCS inflow and outflow currents is illustrated. The CRL reveals the well-defined boundary of a cooler current. This is interpreted as the frontal boundary sharpened by convectively induced cold pools, in particular by the outflow boundary of the downstream MCS. In one CRL transect, the frontal/outflow boundary appeared as a distinct two-layer structure of moisture and aerosols formed by moist stable boundary layer air advected above the boundary. The second transect, one hour later, reveals a single sloping boundary. In both cases, the lofting of the moist stably stratified air over the boundary favors MCS maintenance, through enhanced elevated CAPE and reduced CIN. The CRL data are sufficiently resolved to reveal Kelvin–Helmholtz (KH) billows and the vertical structure of the outflow boundary, which in this case behaved as a density current rather than an undular bore.