Entrainment Of Free Tropospheric Air Research Articles

Response of the boundary layer clouds to the surface forcings: A case study of western India

Clouds atop atmospheric boundary layer over an urban location have been studied using a ground-based Raman LIDAR (RL), supported by radiosonde, INSAT-3D & Meteosat-7 geostationary satellite, and ERA5 reanalysis datasets. The low-level clouds over Ahmedabad (23.02°N, 72.57°E) have been formed from the outflow of a large-scale convergence zone extending from the East-West Indian region. The persistence of these clouds throughout the day has been aided by the transport of moisture to cloud heights by turbulent updrafts. The cloud cover broke off with the entrainment of free tropospheric air with the deepening of the boundary layer, disrupting the surface moisture supply. The consecutive two days (10 and 11 June 2016) observations from the RL showed a unique sensitivity of these clouds to the diurnal variation of the atmospheric boundary layer due to the strong coupling between the clouds and the boundary layer. Ground-based LIDARs provide a better platform for studying the low-level boundary layer clouds and their sensitivity to surface forces due to high accuracy and high temporal resolution. Thus, increasing ground-based observations of such boundary layer clouds is essential for a better understanding of climate sensitivity.

Examining relationships between entrainment-driven scalar dissimilarity and surface energy balance underclosure in a semiarid valley

A surface energy budget underclosure, equivalent to an undersampling of sensible and latent heat fluxes at the surface, is commonly observed in the daytime convective boundary layer over land. The two mechanisms currently held responsible for such underclosure are, on one hand, entrainment-induced reduction of heat fluxes due to low-frequency circulations spanning the depth of the convective boundary layer, and on the other hand, horizontal advection effects induced by surface horizontal heterogeneity. Given the involvement of both heat fluxes in the budget equation, it is expected that the turbulent perturbations of variables involved in these fluxes (potential temperature θ′ and specific humidity q′) contain the signature of both mechanisms. Here, we assess to what extent non-local entrainment effects contribute to the budget underclosure in complex terrain using a quadrant analysis and multiresolution flux decomposition approach.We use data collected in the semi-arid Owens Valley, CA, USA, during the Terrain-Induced Rotor Experiment (T-REX) campaign. The budget components are sampled near the surface of the valley floor and the adjacent slope, with turbulence flux measurements, pyranometers, pyrgeometers, as well as ground heat flux measurements. We found that the budget underclosure was within the range of underclosures reported in past studies, with a systematically poorer closure over the valley slope. Although quadrant analysis revealed a dominance of scalar-similar events, associated with convective boundary layer overturning, a high degree of disagreement was found between the scalar-dissimilar quadrants which represent the counter-gradient motions. Specifically, those turbulent motions associated with entrainment of free tropospheric air to the surface, were found to be of minor importance in degrading scalar similarity. Instead, we postulate that local horizontal advection and hectometre-scale land-use heterogeneity in the vicinity of both the valley floor and slope sites, are the major factors in controlling the relationship between the budget underclosure and scalar similarity.

Is oxidation of atmospheric mercury controlled by different mechanisms in the polluted continental boundary layer vs. remote marine boundary layer?

Deposition of atmospheric mercury is of global concern, primarily due to health effects associated with efficient bioaccumulation of mercury in marine food webs. Although oxidation of gaseous elementary mercury (GEM), the major fraction of atmospheric mercury, is a critical stage in regulating atmospheric mercury deposition efficiency, this oxidation is currently not well-characterized, limiting modeling-based assessments of mercury in the environment. Based on a previous study, we hypothesized that the oxidation of GEM is predominantly controlled by multistep bromine- and chlorine-induced oxidation (MBCO) in the remote marine boundary layer (RMBL), and by photochemical smog oxidants, primarily ozone (O3) and hydroxyl radical (OH), in the polluted continental boundary layer (PCBL). To test this hypothesis, we used the following analyses: (i) application of a newly developed criterion to evaluate the gaseous oxidized mercury (GOM)–O3 association based on previous studies in the RMBL and PCBL; (ii) measurement-based box simulations of GEM oxidation in the RMBL and at a PCBL site; and (iii) measurement-based analysis of photochemical oxidation vs. other processes which potentially influence GOM. Our model simulations indicated that the MBCO mechanism can reproduce GOM levels in the RMBL, but not in the PCBL. Our data analysis suggested the important role of photochemical smog oxidants in GEM oxidation in the PCBL, potentially masked by the effect of relative humidity and entrainment of free tropospheric air.

Effects of Scavenging, Entrainment, and Aqueous Chemistry on Peroxides and Formaldehyde in Deep Convective Outflow Over the Central and Southeast United States.

Deep convective transport of gaseous precursors to ozone (O3) and aerosols to the upper troposphere is affected by liquid phase and mixed-phase scavenging, entrainment of free tropospheric air and aqueous chemistry. The contributions of these processes are examined using aircraft measurements obtained in storm inflow and outflow during the 2012 Deep Convective Clouds and Chemistry (DC3) experiment combined with high-resolution (dx ≤ 3 km) WRF-Chem simulations of a severe storm, an air mass storm, and a mesoscale convective system (MCS). The simulation results for the MCS suggest that formaldehyde (CH2O) is not retained in ice when cloud water freezes, in agreement with previous studies of the severe storm. By analyzing WRF-Chem trajectories, the effects of scavenging, entrainment, and aqueous chemistry on outflow mixing ratios of CH2O, methyl hydroperoxide (CH3OOH), and hydrogen peroxide (H2O2) are quantified. Liquid phase microphysical scavenging was the dominant process reducing CH2O and H2O2 outflow mixing ratios in all three storms. Aqueous chemistry did not significantly affect outflow mixing ratios of all three species. In the severe storm and MCS, the higher than expected reductions in CH3OOH mixing ratios in the storm cores were primarily due to entrainment of low-background CH3OOH. In the air mass storm, lower CH3OOH and H2O2 scavenging efficiencies (SEs) than in the MCS were partly due to entrainment of higher background CH3OOH and H2O2. Overestimated rain and hail production in WRF-Chem reduces the confidence in ice retention fraction values determined for the peroxides and CH2O.

Current understanding of the driving mechanisms for spatiotemporal variations of atmospheric speciated mercury: a review

Abstract. Atmospheric mercury (Hg) is a global pollutant and thought to be the main source of mercury in oceanic and remote terrestrial systems, where it becomes methylated and bioavailable; hence, atmospheric mercury pollution has global consequences for both human and ecosystem health. Understanding of spatial and temporal variations of atmospheric speciated mercury can advance our knowledge of mercury cycling in various environments. This review summarized spatiotemporal variations of total gaseous mercury or gaseous elemental mercury (TGM/GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) in various environments including oceans, continents, high elevation, the free troposphere, and low to high latitudes. In the marine boundary layer (MBL), the oxidation of GEM was generally thought to drive the diurnal and seasonal variations of TGM/GEM and GOM in most oceanic regions, leading to lower GEM and higher GOM from noon to afternoon and higher GEM during winter and higher GOM during spring–summer. At continental sites, the driving mechanisms of TGM/GEM diurnal patterns included surface and local emissions, boundary layer dynamics, GEM oxidation, and for high-elevation sites mountain–valley winds, while oxidation of GEM and entrainment of free tropospheric air appeared to control the diurnal patterns of GOM. No pronounced diurnal variation was found for Tekran measured PBM at MBL and continental sites. Seasonal variations in TGM/GEM at continental sites were attributed to increased winter combustion and summertime surface emissions, and monsoons in Asia, while those in GOM were controlled by GEM oxidation, free tropospheric transport, anthropogenic emissions, and wet deposition. Increased PBM at continental sites during winter was primarily due to local/regional coal and wood combustion emissions. Long-term TGM measurements from the MBL and continental sites indicated an overall declining trend. Limited measurements suggested TGM/GEM increasing from the Southern Hemisphere (SH) to the Northern Hemisphere (NH) due largely to the vast majority of mercury emissions in the NH, and the latitudinal gradient was insignificant in summer probably as a result of stronger meridional mixing. Aircraft measurements showed no significant vertical variation in GEM over the field campaign regions; however, depletion of GEM was observed in stratospherically influenced air masses. In examining the remaining questions and issues, recommendations for future research needs were provided, and among them is the most imminent need for GOM speciation measurements and fundamental understanding of multiphase redox kinetics.

Satellite Observations of an Unusual Cloud Formation near the Tropopause

Abstract This paper describes observations of a field of deep and regular cloud formations that spans several hundreds of kilometers at the top of a midlatitude frontal system in the North Pacific storm track. Space-based imagery of the event from active and passive measurements reveals smooth, clearly defined cloud lobes approximately 10 km across and 2–4 km deep that resemble upside-down mammatus. These observations, together with theoretical arguments and prior modeling work, suggest that the lobes were part of a deepening turbulent mixed layer that formed as a consequence of strong cloud-top radiative cooling. Over the course of a day, the cloud-top formation evolved to leave behind a sheet of cumuliform cirrus that stretched hundreds of kilometers across. The potential is for such clouds to facilitate mixing across the tropopause, much as cloud-top cooling drives the entrainment of free-tropospheric air into stratocumulus-topped boundary layers.

Transport and chemistry of formaldehyde by mesoscale convective systems in West Africa during AMMA 2006

In situ measurements of formaldehyde (CH2O) onboard four European research aircraft in August 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) experiment in West Africa are used (1) to examine the redistribution of CH2O by mesoscale convective systems (MCS) in the tropical upper troposphere (UT), (2) to evaluate the scavenging efficiency (SE) of CH2O by MCS and (3) to quantify the impact of CH2O on UT photooxidant production downwind of MCS. The intercomparison of CH2O measurements is first tested, providing a unique and consistent 3‐D‐spatially resolved CH2O database in background and convective conditions. While carbon monoxide (CO) is vertically uplifted by deep convection up to 12 km, CH2O is also affected by cloud processing as seen from its ratio relative to CO with altitude. A new observation‐based model is established to quantify the SE of CH2O. This model shows that convective entrainment of free tropospheric air cannot be neglected since it contributes to 40% of the convective UT air. For the 4 studied MCS, SE shows a large variability within a 4% to 39% range at a relative standard deviation of 30%, which is consistent with MCS features. A time‐dependent photochemical box model is applied to convective UT air. After convection, 60% of CH2O is due to its photochemical production rather than to its direct transport. Model results indicate that CH2O directly injected by convection does not impact ozone and HOx production in the tropical UT of West Africa. NOx and anthropogenic hydrocarbon precursors dominate the secondary production of CH2O, ozone and HOx.

New particle formation and growth in relation to vertical mixing and chemical species during ABC-EAREX2005

As a part of ABC-EAREX2005 (Atmospheric Brown Cloud-East Asia Regional Experiment 2005), particle number size distributions, gaseous species, and meteorological parameters were measured at Gosan Superstation (33.17°N, 126.10°E, 70 m ASL) on Jeju Island in South Korea from March 13 to 30, 2005. Four particle formation and growth events that occurred on 14, 19, 25, and 30 March were discussed in detail. They took place on a synoptic scale under a high pressure system with intervals of 5–6 days. New particle formation and growth processes were very sensitive to the level of precursor gases and preexisting particles. SO 2 was likely to be the dominant precursor gas. Vertical mixing also played a key role by diluting preexisting particles. The fastest growth event occurred on 25 March with the entrainment of free tropospheric air, which must have been influenced by an anthropogenic plume. The particle growth rates ranged from 1.97 to 5.81 nm h − 1 and the condensation sink varied between 1.5 ∙ 10 − 3 and 8.4 ∙ 10 − 2 s − 1.

Mesoscale Variability in the Summer Arctic Boundary Layer

Observations from the summer Arctic Ocean Experiment 2001 (AOE-2001) are analysed with a focus on the interactions between mesoscale and boundary-layer dynamics. Wavelet analyses of surface-pressure variations show daylong periods with different characteristics, some featuring episodes of pronounced high-frequency surface-pressure variability, here hypothesized to be caused by trapped gravity waves. These episodes are accompanied by enhanced boundary-layer turbulence and an enhanced spectral gap, but with only minor influence on the surface stress. During these episodes, mesoscale phenomena were often encountered and usually identified as front-like features in the boundary layer, with a peak in drizzle followed by changing temperature. These phenomena resemble synoptic fronts, though they are generally shallow, shorter-lasting, have no signs of frontal clouds, and do not imply a change in air mass. Based on this analysis, we hypothesize that the root cause of the episodes with high-frequency surface-pressure variance are shallow, mesoscale fronts moving across the pack ice. They may be formed due to local-to-regional horizontal contrasts, for example, between air with different lifetimes over the Arctic or with perturbations in the cloud field causing differential cooling of the boundary layer. Thermal contrasts sharpen as the air is transported with the mean flow. The propagating mesoscale fronts excite gravity waves, which affect the boundary-layer turbulence and also seem to favour entrainment of free tropospheric air into the boundary layer.

Measurements of stable carbon isotopic composition of ethane and propane over the western North Pacific and eastern Indian Ocean: A useful indicator of atmospheric transport process

Stable carbon isotopic composition of ethane and propane over the western North Pacific and eastern Indian Ocean between 31°N and 26°S was investigated from February through March 2004. The isotopic composition of ethane ranged from −28 to −18‰ and showed a gradual increase from north to south. Conversely, that of propane was between −31 and −24‰; it showed no systematic latitudinal variation. Investigation of the ethane/propane ratio indicates that ethane and propane that originated from northern mid-latitude countries in the eastern part of Eurasia were both transported into the western North Pacific region. However, the results of the isotopic analyses indicate the contribution of oceanic emission to the atmospheric propane during transport, although that contribution can not be discerned for ethane. A ship based stationary observation conducted in the western equatorial North Pacific showed that the isotopic composition of ethane varied from −25 to −19‰ and showed clear systematic diurnal variation: propane ranged between −32 to −26‰ and no such isotopic diurnal signal was observed. The diurnal variation for ethane is explained by entrainment of free tropospheric air, whereas the variation for propane was influenced by oceanic emissions as well as the entrainment. The contribution of oceanic emissions to the atmospheric propane inventory was considered from our isotopic observation. Isotopic composition of dissolved propane is estimated to be less than −38‰, and the contribution up to 79% was calculated when the isotopic composition of dissolved propane is assumed to be −40‰. Our study demonstrates that isotopic analysis can be more useful than ratio-based analysis to improve our present understanding of transport processes, especially for impact of the oceanic emissions on the atmospheric distribution of low level C2–C5 non-methane hydrocarbons such as propane in the remote marine atmosphere.

Vertical and horizontal distributions of the aerosol number concentration and size distribution over the northern Indian Ocean

Airborne measurements of the aerosol number concentration and size distribution were conducted over the northern Indian Ocean during the Indian Ocean Experiment (INDOEX) in February–March 1999. Vertical profiles of the aerosol number concentration demonstrate elevated concentrations of nucleation mode particles in a layer between 8 and 12.5 km altitude. By using a novel combination of back trajectory information and cloud top temperatures retrieved from satellite images, it is shown that these particles most likely originated from the outflow of large convective clouds. Accumulation mode particles observed at these altitudes show indications for cloud processing. The aerosol size distributions observed in the layer between 4 and 8 km altitude show characteristics of an aged aerosol. In this layer the accumulation mode particle number concentration presents a minimum, a low variability, and very few particles larger than 0.7 μm diameter. The aerosol in the marine boundary layer can be characterized by high number concentrations of submicron and accumulation mode particles, which gradually decrease with distance from the Indian subcontinent. The particle loss rate is equivalent to 10–16 cm−3 per hour. This decrease takes place over the whole size distribution and can therefore not be explained by coagulation, but is likely due to precipitation and entrainment of free tropospheric air.

Impact of a simulated severe local storm on the redistribution of sulfur dioxide

A series of numerical experiments in a two‐dimensional framework have been carried out in order to investigate the impact of a severe summertime, midlatitude storm on the SO2‐S(IV)‐S(VI) transport and oxidation processes by comparing results of numerical simulations with and without consideration of aqueous phase chemistry. The cloud‐transport‐only runs showed considerable dilution of boundary layer air with free tropospheric air in the cloud, such that in its upper reaches and anvil parts only 10% or less originated from the boundary layer. A substantial fraction of the SO2 transported upward into the cloud area is dissolved in the liquid phase particles. As a consequence, about 50% of the SO2 that has gone up in the cloud is removed by precipitation processes under the chosen conditions. Below‐cloud scavenging was likewise an efficient process. Most importantly, the uptake of SO2 in falling cloud drops strongly hinders the transfer of SO2 to the upper reaches of the cloud. Transport of SO2 from the boundary layer to the upper troposphere was therefore a rather inefficient process. Another result of our simulation is that only a small amount of dissolved SO2 is converted into S(VI) during the storm simulation period (1 hour). This is explained by the fact that we assumed H2O2 to be present only in the boundary layer at concentrations about 10 times lower than those of SO2, together with the higher solubility of H2O2. However, because of the large entrainment of free tropospheric air into the cloud, the H2O2 in the free troposphere would be much more efficient in oxidizing S(IV). A number of sensitivity calculations which we conducted, assuming different chemical makeups of the boundary layer did not change the main conclusions of our study.

The effects of entrainment on the growth of droplets in continental cumulus clouds

In this paper data are presented from aircraft passes through continental cumulus clouds obtained during the Cooperative Convective Precipitation Experiment. The effect of evaporation of liquid water from the cloud following the entrainment of free tropospheric air is to totally remove a fraction of the droplets of all sizes rather than partially evaporate most of the droplets. Supersaturations in the cloud are found on average to be considerably higher than those predicted for an adiabatic cloud and the supersaturation is not closely tied to the vertical wind. These observations are found to have a number of important consequences for cloud droplet growth: 1. The average lifetime of individual droplets is considerably less than the age of the cloud and is not expected to exceed about five minutes. This has important consequences for the oxidation of SO2 to sulphate. 2. Statistically favoured droplets that avoid evaporation are observed to grow much larger than average droplets and to sizes larger than would occur at the same altitude in an adiabatic cloud.

The effects of entrainment on the growth of droplets in continental cumulus clouds

AbstractIn this paper data are presented from aircraft passes through continental cumulus clouds obtained during the Cooperative Convective Precipitation Experiment. The effect of evaporation of liquid water from the cloud following the entrainment of free tropospheric air is to totally remove a fraction of the droplets of all sizes rather than partially evaporate most of the droplets. Supersaturations in the cloud are found on average to be considerably higher than those predicted for an adiabatic cloud and the supersaturation is not closely tied to the vertical wind.These observations are found to have a number of important consequences for cloud droplet growth: 1. The average lifetime of individual droplets is considerably less than the age of the cloud and is not expected to exceed about five minutes. This has important consequences for the oxidation of SO2 to sulphate. 2. Statistically favoured droplets that avoid evaporation are observed to grow much larger than average droplets and to sizes larger than would occur at the same altitude in an adiabatic cloud.