Tropospheric aerosol detection over Southern Ireland using a backscatter Raman lidar

Lidar is an optical remote sensing instrument that can measure atmospheric parameters. A Raman lidar instrument (UCLID) was established at University College Cork to contribute to the European lidar network, EARLINET. System performance tests were carried out to ensure strict data quality assurance for submission to the EARLINET database. Procedures include: overlap correction, telecover test, Rayleigh test and zero bin test. Raman backscatter coefficients, extinction coefficients and lidar ratio were measured from April 2010 to May 2011 and February 2012 to June 2012. Statistical analysis of the profiles over these periods provided new information about the typical atmospheric scenarios over Southern Ireland in terms of aerosol load in the lower troposphere, the planetary boundary layer (PBL) height, aerosol optical density (AOD) at 532 nm and lidar ratio values. The arithmetic average of the PBL height was found to be 608 ± 138 m with a median of 615 m, while average AOD at 532 nm for clean marine air masses was 0.119 ± 0.023 and for polluted air masses was 0.170 ± 0.036. The lidar ratio showed a seasonal dependence with lower values found in winter and autumn (20 ± 5 sr) and higher during spring and winter (30 ± 12 sr). Detection of volcanic particles from the eruption of the volcano Eyjafjallajökull in Iceland was measured between 21 April and 7 May 2010. The backscatter coefficient of the ash layer varied between 2.5 Mm-1sr-1 and 3.5 Mm-1sr-1, and estimation of the AOD at 532 nm was found to be between 0.090 and 0.215. Several aerosol loads due to Saharan dust particles were detected in Spring 2011 and 2012. Lidar ratio of the dust layers were determine to be between 45 and 77 sr and AOD at 532 nm during the dust events range between 0.84 to 0.494.

Contribution of the North Atlantic subtropical high to regional climate model (RCM) skill in simulating southeastern United States summer precipitation

© 2014, Springer-Verlag Berlin Heidelberg.This study assesses the skill of advanced regional climate models (RCMs) in simulating southeastern United States (SE US) summer precipitation and explores the physical mechanisms responsible for the simulation skill at a process level. Analysis of the RCM output for the North American Regional Climate Change Assessment Program indicates that the RCM simulations of summer precipitation show the largest biases and a remarkable spread over the SE US compared to other regions in the contiguous US. The causes of such a spread are investigated by performing simulations using the Weather Research and Forecasting (WRF) model, a next-generation RCM developed by the US National Center for Atmospheric Research. The results show that the simulated biases in SE US summer precipitation are due mainly to the misrepresentation of the modeled North Atlantic subtropical high (NASH) western ridge. In the WRF simulations, the NASH western ridge shifts 7° northwestward when compared to that in the reanalysis ensemble, leading to a dry bias in the simulated summer precipitation according to the relationship between the NASH western ridge and summer precipitation over the southeast. Experiments utilizing the four dimensional data assimilation technique further suggest that the improved representation of the circulation patterns (i.e., wind fields) associated with the NASH western ridge substantially reduces the bias in the simulated SE US summer precipitation. Our analysis of circulation dynamics indicates that the NASH western ridge in the WRF simulations is significantly influenced by the simulated planetary boundary layer (PBL) processes over the Gulf of Mexico. Specifically, a decrease (increase) in the simulated PBL height tends to stabilize (destabilize) the lower troposphere over the Gulf of Mexico, and thus inhibits (favors) the onset and/or development of convection. Such changes in tropical convection induce a tropical–extratropical teleconnection pattern, which modulates the circulation along the NASH western ridge in the WRF simulations and contributes to the modeled precipitation biases over the SE US. In conclusion, our study demonstrates that the NASH western ridge is an important factor responsible for the RCM skill in simulating SE US summer precipitation. Furthermore, the improvements in the PBL parameterizations for the Gulf of Mexico might help advance RCM skill in representing the NASH western ridge circulation and summer precipitation over the SE US.

Atmospheric deposition of methanol over the Atlantic Ocean

In the troposphere, methanol (CH3OH) is present ubiquitously and second in abundance among organic gases after methane. In the surface ocean, methanol represents a supply of energy and carbon for marine microbes. Here we report direct measurements of air-sea methanol transfer along a similar to 10,000-km north-south transect of the Atlantic. The flux of methanol was consistently from the atmosphere to the ocean. Constrained by the aerodynamic limit and measured rate of air-sea sensible heat exchange, methanol transfer resembles a one-way depositional process, which suggests dissolved methanol concentrations near the water surface that are lower than what were measured at similar to 5 m depth, for reasons currently unknown. We estimate the global oceanic uptake of methanol and examine the lifetimes of this compound in the lower atmosphere and upper ocean with respect to gas exchange. We also constrain the molecular diffusional resistance above the ocean surface-an important term for improving air-sea gas exchange models.

Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide

Volatile halogenated organic compounds containing bromine and iodine, which are naturally produced in the ocean, are involved in ozone depletion in both the troposphere and stratosphere. Three prominent compounds transporting large amounts of marine halogens into the atmosphere are bromoform (CHBr3), dibromomethane (CH2Br2) and methyl iodide (CH3I). The input of marine halogens to the stratosphere has been estimated from observations and modelling studies using low-resolution oceanic emission scenarios derived from top-down approaches. In order to improve emission inventory estimates, we calculate data-based high resolution global sea-to-air flux estimates of these compounds from surface observations within the HalOcAt (Halocarbons in the Ocean and Atmosphere) database (https://halocat.geomar.de/). Global maps of marine and atmospheric surface concentrations are derived from the data which are divided into coastal, shelf and open ocean regions. Considering physical and biogeochemical characteristics of ocean and atmosphere, the open ocean water and atmosphere data are classified into 21 regions. The available data are interpolated onto a 1 degrees x 1 degrees grid while missing grid values are interpolated with latitudinal and longitudinal dependent regression techniques reflecting the compounds' distributions. With the generated surface concentration climatologies for the ocean and atmosphere, global sea-to-air concentration gradients and sea-to-air fluxes are calculated. Based on these calculations we estimate a total global flux of 1.5/2.5 Gmol Br yr(-1) for CHBr3, 0.78/0.98 Gmol Br yr(-1) for CH2Br2 and 1.24/1.45 Gmol Br yr(-1) for CH3I (robust fit/ordinary least squares regression techniques). Contrary to recent studies, negative fluxes occur in each sea-to-air flux climatology, mainly in the Arctic and Antarctic regions. "Hot spots" for global polybromomethane emissions are located in the equatorial region, whereas methyl iodide emissions are enhanced in the subtropical gyre regions. Inter-annual and seasonal variation is contained within our flux calculations for all three compounds. Compared to earlier studies, our global fluxes are at the lower end of estimates, especially for bromoform. An under-representation of coastal emissions and of extreme events in our estimate might explain the mismatch between our bottom-up emission estimate and top-down approaches.

Technical Note: Ensuring consistent, global measurements of short-lived halocarbon gases in the ocean and atmosphere

Very short-lived halocarbons are significant sources of reactive halogen in the marine boundary layer, and likely in the upper troposphere and lower stratosphere. Quantifying ambient concentrations in the surface ocean and atmosphere is essential for understanding the atmospheric impact of these trace gas fluxes. Despite the body of literature increasing substantially over recent years, calibration issues complicate the comparison of results and limit the utility of building larger-scale databases that would enable further development of the science (e.g. sea-air flux quantification, model validation, etc.). With this in mind, thirty-one scientists from both atmospheric and oceanic halocarbon communities in eight nations gathered in London in February 2008 to discuss the scientific issues and plan an international effort toward developing common calibration scales (http://tinyurl.com/c9cg58). Here, we discuss the outputs from this meeting, suggest the compounds that should be targeted initially, identify opportunities for beginning calibration and comparison efforts, and make recommendations for ways to improve the comparability of previous and future measurements.

Air-Sea Exchange of Marine Trace Gases

Many of the reactive trace gases detected in the atmosphere are both emitted from and deposited to the global oceans via exchange across the air–sea interface. The resistance to transfer through both air and water phases is highly sensitive to physical drivers (waves, bubbles, films, etc.), which can either enhance or suppress the rate of diffusion. In addition to outlining the fundamental processes controlling the air–sea gas exchange, the authors discuss these drivers, describe the existing parameterizations used to predict transfer velocities, and summarize the novel techniques for measuring in situ exchange rates. They review trace gases that influence climate via radiative forcing (greenhouse gases), those that can alter the oxidative capacity of the atmosphere (nitrogen- and sulfur-containing gases), and those that impact ozone levels (organohalogens), both in the troposphere and stratosphere. They review the known biological and chemical routes of production and destruction within the water column for these gases, whether the ocean acts as a source or sink, and whether temporal and spatial variations in saturation anomalies are observed. A current estimate of the marine contribution to the total atmospheric flux of these gases, which often highlights the significance of the oceans in biogeochemical cycling of trace gases, is provided, and how air–sea gas fluxes may change in the future is briefly assessed.

Microbial acetone oxidation in coastal seawater

Acetone is an important oxygenated volatile organic compound (OVOC) in the troposphere where it influences the oxidizing capacity of the atmosphere. However, the air-sea flux is not well quantified, in part due to a lack of knowledge regarding which processes control oceanic concentrations, and, specifically whether microbial oxidation to CO2 represents a significant loss process. We demonstrate that 14C labeled acetone can be used to determine microbial oxidation to 14CO2. Linear microbial rates of acetone oxidation to CO2 were observed for between 0.75-3.5 h at a seasonally eutrophic coastal station located in the western English Channel (L4). A kinetic experiment in summer at station L4 gave a Vmax of 4.1 pmol L-1 h-1, with a Km constant of 54 pM. We then used this technique to obtain microbial acetone loss rates ranging between 1.2 and 42 pmol L-1 h-1.(monthly averages) over an annual cycle at L4, with maximum rates observed during winter months. The biological turnover time of acetone (in situ concentration divided by microbial oxidation rate) in surface waters varied from ~3 days in February 2011, when in situ concentrations were 3 ± 1 nM, to >240 days in June 2011, when concentrations were more than twofold higher at 7.5 ± 0.7 nM. These relatively low marine microbial acetone oxidation rates, when normalized to in situ concentrations, suggest that marine microbes preferentially utilize other OVOCs such as methanol and acetaldehyde.

Studies on the Characteristics of Vertical Circulation and Equatorial Waves using Indian MST Radar

The aim of the present study is to understand the characteristics and properties of different wave modes and the vertical circulation pattern in the troposphere and lower stratosphere over Indian region using data obtained from the Indian Mesosphere-Stratosphere Troposphere (MST) radar, National Center for Environmental Prediction/National Centres of Atmospheric Research (NCEP/NCAR) reanalysed data and radiosonde observations.Studies on the vertical motion in monsoon Hadley circulation are carried out and the results are discussed . From the analysis of MST radar data, an overall picture of vertical motion of air over Indian region is explained and noted that there exists sinking motion both during winter and summer. Besides, the study shows that there is an anomalous northerly wind in the troposphere over the southern peninsular region during southwest monsoon season.The outcome of the study on intrusion of mid-latitude upper tropospheric trough and associated synoptic-scale vertical velocity over the tropical Indian latitudes are reported and discussed . It shows that there is interaction between north Indian latitudes and tropical easterly region, when there is an eastward movement of Western Disturbance across the country. It explains the strengthening of westerlies and a change of winter westerlies into easterlies in the tropical troposphere and lower stratosphere. The divergence field computed over the MST radar station shows intensification in the downward motion in association with the synoptic systems of the northwest Indian region.

Studies on the Characteristics of Tropical Tropopause

Department of Atmospheric Sciences, School of Marine Sciences, Cochin University of Science And Technology

Tropical Mesoscale Convective Systems and Associated Energetics : Observational and Modeling Studies

The main purpose of the thesis is to improve the state of knowledge and understanding of the physical structure of the TMCS and its short range prediction. The present study principally addresses the fine structure, dynamics and microphysics of severe convective storms.The structure and dynamics of the Tropical cloud clusters over Indian region is not well understood. The observational cases discussed in the thesis are limited to the temperature and humidity observations. We propose a mesoscale observational network along with all the available Doppler radars and other conventional and non—conventional observations. Simultaneous observations with DWR, VHF and UHF radars of the same cloud system will provide new insight into the dynamics and microphysics of the clouds. More cases have to be studied in detail to obtain climatology of the storm type passing over tropical Indian region. These observational data sets provide wide variety of information to be assimilated to the mesoscale data assimilation system and can be used to force CSRM.The gravity wave generation and stratosphere troposphere exchange (STE) processes associated with convection gained a great deal of attention to modem science and meteorologist. Round the clock observations using VHF and UHF radars along with supplementary data sets like DWR, satellite, GPS/Radiosondes, meteorological rockets and aircrafl observations is needed to explore the role of convection and associated energetics in detail.

Southern Hemispheric Features and their Teleconnection with Indian Summer Monsoon

This thesis entitled southern hemispheric features and their Teleconnection with indian summer monsoon.Southern hemisphere is entirely distinct from the northern hemisphere in many aspects, which is well reflected in atmospheric and oceanic properties.The thesis consists of eight chapters, in which the first chapter contains an overview of southern hemisphere. In this chapter, variability in southern hemisphere is described along with Indian summer monsoon and its teleconnection. The different types of data sets used and various methodologies adopted in the present thesis were described in Chapter 2. The period of climate shift and the magnitude of anomalies after the climate shift, which extended from troposphere to stratopause level, were investigated in detail and presented in chapter 3. Chapter 4 depicts the recent trend and variability in southern stratosphere. The higher order variability during various months and the frequency of extremity is included in this chapter.Climatology of divergence and convergence after the documented shift is reported in chapter 5.Southern extratropical connection to Indian summer monsoon through the modulation of SAM is presented in Chapter 6.Chapter 7 deals with the modulation of SAM‐Monsoon link through North Atlantic Oscillation.

Spatio-temporal Variability of Atmospheric Ozone over Indian Subcontinent and its Relation to Meteorological Parameters

Ozone present in the atmosphere not only absorbs the biologically harmful ultraviolet radiation but also is an important ingredient of the climate system. The radiative absorption properties of ozone make it a determining factor in the structure of the atmosphere. Ozone in the troposphere has many negative impacts on humans and other living beings. Another significant aspect is the absorption of outgoing infrared radiation by ozone thus acting as a greenhouse gas. The variability of ozone in the atmosphere involves many interconnections with the incoming and outgoing radiation, temperature circulation etc. Hence ozone forms an important part of chemistry-climate as well as radiative transfer models. This aspect also makes the quantification of ozone more important. The discovery of Antarctic ozone hole and the role of anthropogenic activities in causing it made it possible to plan and implement necessary preventive measures. Continuous monitoring of ozone is also necessary to identify the effect of these preventive steps. The reactions involving the formation and destruction of ozone are influenced significantly by the temperature fluctuations of the atmosphere. On the other hand the variations in ozone can change the temperature structure of the atmosphere. Indian subcontinent is a region having large weather and climate variability which is evident from the large interannual variability of monsoon system over the region. Nearly half of Indian region comprises the tropical region. Most of ozone is formed in the tropical region and transported to higher latitudes. The formation and transport of ozone can be influenced by changes in solar radiation and various atmospheric circulation features. Besides industrial activities and vehicular traffic is more due to its large population. This may give rise to an increase in the production of tropospheric ozone which is greenhouse gas. Hence it becomes necessary to monitor the atmospheric ozone over this region. This study probes into the spatial distribution and temporal evolution of ozone over Indian subcontinent and discusses the contributing atmospheric parameters.

‘Sudden Stratospheric Warming and its Influence on the Tropical Atmosphere'

The stratospheric warming events are categorized into major and minor warming depending on the temperature increase in the polar stratosphere. The warming is called a ‘major’, when the polar temperature increases poleward from 60 degree latitude and followed by a reversal in the zonal wind at 10 hPa (~32 Km). Usually major warming events are associated with the displacement of polar vortex from high to mid latitudes or the splitting of vortices in to two. The warming is called a "Minor", when the polar temperature increases more than 25 degree in a period of a week or less, at any stratospheric level with less intensified easterly wind anomalies. The stratospheric warmings generated during the transition period of winter to spring are called final warmings. The warming events observed in the early winter period (November to early December) over Canadian region are called Canadian warmings. There is strong interaction between stratosphere and troposphere during SSW period over high and low latitudes regions. The thesis consists of 7 chapters

Diurnal Structure of Monsoon Boundary Layer

The characteristics of the monsoon boundary layer are imperative to understand in the perception of the tropical regions. The southwest monsoon is associated with a strong wind in the lower troposphere near 1.5 km and is referred to as Low Level Jet stream (LLJ). The boundary layer structure associated with the LLJ during monsoon can be studied using L-band Ultra High Frequency (UHF) radar. This L-band wind profiler-commonly referred as lower atmospheric wind profiler (LAWP), was installed at NARL, Gadanki. Zonal, meridional and vertical wind components are used to understand the diurnal variation of the wind in the Atmospheric Boundary Layer (ABL) and associated features. From the analysis during non rainy days of the southwest monsoon, it is found that the LLJ has maximum strength during the early morning hours at lower level and the height increases as day progresses. The vertical wind shows the transfer of momentum from the LLJ towards the surface, indicating the sinking motion during the daytime. Vertical gradient of the wind shear shows the intensity of clear air turbulence is moderate and no severe clear air turbulence is noticed during the monsoon period

A new perspective on Southern hemisphere storm-tracks.

A detailed view of Southern Hemisphere storm tracks is obtained based on the application of filtered variance and modern feature-tracking techniques to a wide range of 45-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. It has been checked that the conclusions drawn in this study are valid even if data from only the satellite era are used. The emphasis of the paper is on the winter season, but results for the four seasons are also discussed. Both upper- and lower-tropospheric fields are used. The tracking analysis focuses on systems that last longer than 2 days and are mobile (move more than 1000 km). Many of the results support previous ideas about the storm tracks, but some new insights are also obtained. In the summer there is a rather circular, strong, deep high-latitude storm track. In winter the high-latitude storm track is more asymmetric with a spiral from the Atlantic and Indian Oceans in toward Antarctica and a subtropical jet–related lower-latitude storm track over the Pacific, again tending to spiral poleward. At all times of the year, maximum storm activity in the higher-latitude storm track is in the Atlantic and Indian Ocean regions. In the winter upper troposphere, the relative importance of, and interplay between, the subtropical and subpolar storm tracks is discussed. The genesis, lysis, and growth rate of lower-tropospheric winter cyclones together lead to a vivid picture of their behavior that is summarized as a set of overlapping plates, each composed of cyclone life cycles. Systems in each plate appear to feed the genesis in the next plate through downstream development in the upper-troposphere spiral storm track. In the lee of the Andes in South America, there is cyclogenesis associated with the subtropical jet and also, poleward of this, cyclogenesis largely associated with system decay on the upslope and regeneration on the downslope. The genesis and lysis of cyclones and anticyclones have a definite spatial relationship with each other and with the Andes. At 500 hPa, their relative longitudinal positions are consistent with vortex-stretching ideas for simple flow over a large-scale mountain. Cyclonic systems near Antarctica have generally spiraled in from lower latitudes. However, cyclogenesis associated with mobile cyclones occurs around the Antarctic coast with an interesting genesis maximum over the sea ice near 150°E. The South Pacific storm track emerges clearly from the tracking as a coherent deep feature spiraling from Australia to southern South America. A feature of the summer season is the genesis of eastward-moving cyclonic systems near the tropic of Capricorn off Brazil, in the central Pacific and, to a lesser extent, off Madagascar, followed by movement along the southwest flanks of the subtropical anticyclones and contribution to the “convergence zone” cloud bands seen in these regions.