Improved mixing height monitoring through a combination of lidar and radon measurements

Surface-based radon (222Rn) measurements can be combined with lidar backscatter to obtain a higher quality time series of mixing height within the planetary boundary layer (PBL) than is possible from lidar alone, and a more quantitative measure of mixing height than is possible from only radon. The reason why lidar measurements are improved is that there are times when lidar signals are ambiguous, and reliably attributing the mixing height to the correct aerosol layer presents a challenge. By combining lidar with a mixing length scale derived from a time series of radon concentration, automated and robust attribution is possible during the morning transition. Radon measurements provide mixing information during the night, but concentrations also depend on the strength of surface emissions. After processing radon in combination with lidar, we obtain nightly measurements of radon emissions and are able to normalise the mixing length scale for changing emissions. After calibration with lidar, the radonderived equivalent mixing height agrees with other measures of mixing on daily and hourly timescales and is a potential method for studying intermittent mixing in nocturnal boundary layers.© 2013, Copernicus Publications

On the use of radon for quantifying the effects of atmospheric stability on urban emissions

Radon is increasingly being used as a tool for quantifying stability influences on urban pollutant concentrations. Bulk radon gradients are ideal for this purpose, since the vertical differencing substantially removes contributions from processes on timescales greater than diurnal and (assuming a constant radon source) gradients are directly related to the intensity of nocturnal mixing. More commonly, however, radon measurements are available only at a single height. In this study we argue that single-height radon observations should not be used quantitatively as an indicator of atmospheric stability without prior conditioning of the time series to remove contributions from larger-scale "non-local" processes. We outline a simple technique to obtain an approximation of the diurnal radon gradient signal from a single-height measurement time series, and use it to derive a four category classification scheme for atmospheric stability on a "whole night" basis. A selection of climatological and pollution observations in the Sydney region are then subdivided according to the radon-based scheme on an annual and seasonal basis. We compare the radon-based scheme against a commonly used Pasquill–Gifford (P–G) type stability classification and reveal that the most stable category in the P–G scheme is less selective of the strongly stable nights than the radon-based scheme; this lead to significant underestimation of pollutant concentrations on the most stable nights by the P–G scheme. Lastly, we applied the radon-based classification scheme to mixing height estimates calculated from the diurnal radon accumulation time series, which provided insight to the range of nocturnal mixing depths expected at the site for each of the stability classes. © 2015, Author(s)

Characterising terrestrial influences on Antarctic air masses using Radon-222 measurements at King George Island

We report on one year of high-precision direct hourly radon observations at King Sejong Station (King George Island) beginning in February 2013. Findings are compared with historic and ongoing radon measurements from other Antarctic sites. Monthly median concentrations reduced from 72 mBq m−3 in late-summer to 44 mBq m−3 in late winter and early spring. Monthly 10th percentiles, ranging from 29 to 49 mBq m−3, were typical of oceanic baseline values. Diurnal cycles were rarely evident and local influences were minor, consistent with regional radon flux estimates one tenth of the global average for ice-free land. The predominant fetch region for terrestrially influenced air masses was South America (47–53° S), with minor influences also attributed to aged Australian air masses and local sources. Plume dilution factors of 2.8–4.0 were estimated for the most terrestrially influenced (South American) air masses, and a seasonal cycle in terrestrial influence on tropospheric air descending at the pole was identified and characterised. © Author(s) 201

Using radon-222 to distinguish between vertical transport processes at Jungfraujoch

Trace gases measured at Jungfrajoch, a key baseline monitoring station in the Swiss Alps, are tranported from the surface to the alpine ridge by several different processes. On clear days with weak synoptic forcing, thermally-driven upslope mountain winds (anabatic winds) are prevalent. Using hourly radon–222 observations, which are often used to identify air of terrestrial origin, we used the shape of the diurnal cycle to sort days according to the strength of anabatic winds. Radon is ideal as an airmass tracer because it is emitted from soil at a relatively constant rate, it is chemically inert, and decays with a half-life of 3.8 days. Because of its short half-life, radon concentrations are much lower in the free troposphere than in boundary-layer air over land. For comparable radon concentrations, anabatic wind days at Jungfraujoch are different from non-anabatic days in terms of the average wind speed, humidity, air temperature anomalies, and trace species. As a consequence, future studies could be devised which focus on a subset of days, e.g. by excluding anabatic days, with the intention of choosing a set of days which can be more accurately simulated by a transport model. © Author(s) 2014

The vertical distribution of radon in clear and cloudy daytime terrestrial boundary layers

Radon ((222)Rn) is a powerful natural tracer of mixing and exchange processes in the atmospheric boundary layer. The authors present and discuss the main features of a unique dataset of 50 high-resolution vertical radon profiles up to 3500 m above ground level, obtained in clear and cloudy daytime terrestrial boundary layers over an inland rural site in Australia using an instrumented motorized research glider. It is demonstrated that boundary layer radon profiles frequently exhibit a complex layered structure as a result of mixing and exchange processes of varying strengths and extents working in clear and cloudy conditions within the context of the diurnal cycle and the synoptic meteorology. Normalized aircraft radon measurements are presented, revealing the characteristic structure and variability of three major classes of daytime boundary layer: 1) dry convective boundary layers, 2) mixed layers topped with residual layers, and 3) convective boundary layers topped with coupled nonprecipitating clouds. Robust and unambiguous signatures of important atmospheric processes in the boundary layer are identifiable in the radon profiles, including "top-down" mixing associated with entrainment in clear-sky cases and strongly enhanced venting and subcloud-layer mixing when substantial active cumulus are present. In poorly mixed conditions, radon gradients in the daytime atmospheric surface layer significantly exceed those predicted by Monin-Obukhov similarity theory. In two case studies, it is demonstrated for the first time that a sequence of vertical radon profiles measured over the course of a single day can consistently reproduce major structural features of the evolving boundary layer.© 2011, American Meteorological Society

Seasonal variability of the radon-222 flux density from the Southern Ocean derived from atmospheric radon-222 measurements at the Cape Grim baseline station in Tasmania.

We present and discuss an estimate of the radon-222 (radon) flux density from the Southern Ocean and its seasonal variability. The flux estimate was based on selected hourly atmospheric radon concentration measurements in the Baseline wind sector of the Cape Grim station in Tasmania (40°41’S, 144°41’22”E) from 2001 to 2008. The aim of the selection process was to define a subset of hourly radon observations corresponding to oceanic air parcels that were least perturbed by land emissions and exhibiting minimal exchange with the free troposphere, so that an equilibrium could be assumed between the measured radon concentrations and their oceanic source. The initial dataset included all events in the Cape Grim Baseline sector, defined solely by local wind direction (190º< Ө <280º). In this set, more than 75% of the hourly radon observations were already below 100 mBq m-3. We demonstrate that the strongest perturbation due to land emissions occurs at the beginning and end of each baseline period, with the duration of these transitional periods being 24 and 12 hours, respectively. Additional statistically significant terrestrial radon emissions and near-shore influences were identified and quantified by analysing special features of 10-day back trajectories, calculated using the HYSPLIT model, and the associated radon distributions. We show that the Australian terrestrial influence leads to an upward shift of the corresponding radon distributions, with the converse being true for Antarctic terrestrial influence. Statistically significant near-shore influences were attributed to horizontal radon gradients extending from the coast over the ocean, south from the Australian mainland and north from the Antarctic sea ice boundary. Progressive application of the selection criteria contracts radon distributions of the resulting subsets, with the higher percentile concentrations undergoing the most pronounced reductions. For example, the concentration ranges of 90% of baseline observations (ie. from the 5th to 95th percentiles) were 483, 165, 126, 90, and 88 mBq m-3 for the five major radon sub-sets considered with progressively stringent selection criteria. The progressive reduction in concentration range for each category confirms the efficacy of the selections made, since the narrower the concentration range, the more homogenous the radon source probed by the observations. We also found that the more that the terrestrial influences could be reduced, the more clearly revealed was the seasonal variability in flux estimates. The ratio of summer to winter median radon concentrations increases from 1.13 for all baseline observations, to 1.6 for the final subset of baseline observations (considered to include only those radon observations corresponding to air parcels most closely in equilibrium with their oceanic radon source). The final dataset included 900 hourly radon observations, about 3% of all baseline events recorded in the 2001-2008 period. The marine boundary layer heights required for the flux estimates were derived from an ECMWF operational model reanalysis on a 1.5° grid. We compared the reanalysis for 1998 with mini-lidar measurements gathered at Cape Grim and found that on average the lidar estimates were approximately 11% lower. Assuming that the application of our stringent selection criteria also excluded meteorological events such as frontal passage and significant boundary layer venting by deep active cumulus, we employed an entrainment velocity typical of the Southern Ocean marine boundary layer in the study region (0.004 ms-1 ±0.002 ms-1). The flux density means calculated using the above assumptions, expressed in units of mBq m-2s-1 were 0.27 ±0.10, 0.30 ±0.11, and 0.19 ±0.07 for the 2001-2008 composite year, winter, and summer, respectively, with the stated uncertainties resulting from uncertainties in the estimates of marine boundary layer heights at Cape Grim, and of the assumed entrainment velocity.European Geosciences Unio

Forced Gravity Waves and the Tropospheric Response to Convection

We present theoretical work directed toward improving our understanding of the mesoscale influence of deep convection on its tropospheric environment through forced gravity waves. From the linear, hydrostatic, non-rotating, incompressible equations, we find a two-dimensional analytical solution to prescribed heating in a stratified atmosphere, which is upwardly radiating from the troposphere when the domain lid is sufficiently high. We interrogate the spatial and temporal sensitivity of both the vertical velocity and potential temperature to different heating functions, considering both the near-field and remote responses to steady and pulsed heating. We find that the mesoscale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves, which are in turn dependent upon the temporal and spatial structure of the source, and the assumed stratification. We find a 50% reduction in tropospherically averaged vertical velocity when moving from a trapped (i.e. low lid) to upwardly-radiating (i.e. high lid) solution, but even with maximal upward radiation, we still observe significant tropospheric vertical velocities in the far-field 4 hours after heating ends. We quantify the errors associated with coarsening a 10 km wide heating to a 100 km grid (in the way a General Circulation Model (GCM) would), observing a 20% reduction in vertical velocity. The implications of these results for the parameterisation of convection in low-resolution numerical models are quantified and it is shown that the smoothing of heating over a grid-box leads to significant in grid-box tendencies, due to the erroneous rate of transfer of compensating subsidence to neighbouring regions. Further, we explore a simple time-dependent heating parameterisation that minimises error in a parent GCM grid box, albeit at the expense of increased error in the neighbourhood