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Simple kinematic models for the environmental interaction of tropical cyclones in vertical wind shear

A major impediment to the intensity forecast of tropical cyclones (TCs) is believed to be associated with the interaction of TCs with dry environmental air. However, the conditions under which pronounced TC-environment interaction takes place are not well understood. As a step towards improving our understanding of this problem, we analyze here the flow topology of a TC immersed in an environment of vertical wind shear in an idealized, three-dimensional, convection-permitting numerical experiment. A set of distinct streamlines, the so-called manifolds, can be identified under the assumptions of steady and layer-wise horizontal flow. The manifolds are shown to divide the flow around the TC into distinct regions. The manifold structure in our numerical experiment is more complex than the well-known manifold structure of a non-divergent point vortex in uniform background flow. In particular, one manifold spirals inwards and ends in a limit cycle, a meso-scale dividing streamline encompassing the eyewall above the layer of strong inflow associated with surface friction and below the outflow layer in the upper troposphere. From the perspective of a steady and layer-wise horizontal flow model, the eyewall is well protected from the intrusion of environmental air. In order for the environmental air to intrude into the inner-core convection, time-dependent and/or vertical motions, which are prevalent in the TC inner-core, are necessary. Air with the highest values of moist-entropy resides within the limit cycle. This "moist envelope" is distorted considerably by the imposed vertical wind shear, and the shape of the moist envelope is closely related to the shape of the limit cycle. In a first approximation, the distribution of high- and low-θe air around the TC at low to mid-levels is governed by the stirring of convectively modified air by the steady, horizontal flow. Motivated by the results from the idealized numerical experiment, an analogue model based on a weakly divergent point vortex in background flow is formulated. The simple kinematic model captures the essence of many salient features of the manifold structure in the numerical experiment. A regime diagram representing realistic values of TC intensity and vertical wind shear can be constructed for the point-vortex model. The results indicate distinct scenarios of environmental interaction depending on the ratio of storm intensity and vertical-shear magnitude. Further implications of the new results derived from the manifold analysis for TCs in the real atmosphere are discussed.
Water uptake by biomass burning aerosol at sub- and supersaturated conditions: closure studies and implications for the role of organics

We investigate the CCN activity of freshly emitted biomass burning particles and their hygroscopic growth at a relative humidity (RH) of 85%. The particles were produced in the Mainz combustion laboratory by controlled burning of various wood types. The water uptake at sub- and supersaturations is parameterized by the hygroscopicity parameter, κ (c.f. Petters and Kreidenweis, 2007). For the wood burns, κ is low, generally around 0.06. The main emphasis of this study is a comparison of κ derived from measurements at sub- and supersaturated conditions (κG and κCCN), in order to see whether the water uptake at 85% RH can predict the CCN properties of the biomass burning particles. Differences in κGand κCCN can arise through solution non-idealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that κG and κCCN agree within experimental uncertainties (of around 30%) for particle sizes of 100 and 150 nm; only for 50 nm particles is κCCN larger than κG by a factor of 2. The magnitude of this difference and its dependence on particle size is consistent with the presence of surface active organic compounds. These compounds mainly facilitate the CCN activation of small particles, which form the most concentrated solution droplets at the point of activation. The 50 nm particles, however, are only activated at supersaturations higher than 1% and are therefore of minor importance as CCN in ambient clouds. By comparison with the actual chemical composition of the biomass burning particles, we estimate that the hygroscopicity of the water-soluble organic carbon (WSOC) fraction can be represented by a κWSOC value of approximately 0.2. The effective hygroscopicity of a typical wood burning particle can therefore be represented by a linear mixture of an inorganic component with κ &cong; 0.6, a WSOC component with κ &cong; 0.2, and an insoluble component with κ = 0.
EVAPORATION: a new vapour pressure estimation methodfor organic molecules including non-additivity and intramolecular interactions

We present EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature, Intramolecular, and Non-additivity effects), a method to predict (subcooled) liquid pure compound vapour pressure p0 of organic molecules that requires only molecular structure as input. The method is applicable to zero-, mono- and polyfunctional molecules. A simple formula to describe log10p0(T) is employed, that takes into account both a wide temperature dependence and the non-additivity of functional groups. In order to match the recent data on functionalised diacids an empirical modification to the method was introduced. Contributions due to carbon skeleton, functional groups, and intramolecular interaction between groups are included. Molecules typically originating from oxidation of biogenic molecules are within the scope of this method: aldehydes, ketones, alcohols, ethers, esters, nitrates, acids, peroxides, hydroperoxides, peroxy acyl nitrates and peracids. Therefore the method is especially suited to describe compounds forming secondary organic aerosol (SOA).
The detection of post-monsoon tropospheric ozone variability over south Asia using IASI data

The ozone (O3) variability over south Asia during the 2008 post-monsoon season has been assessed using measurements from the MetOP-A/IASI instrument and O3 profiles retrieved with the SOftware for a Fast Retrieval of IASI Data (SOFRID). The information content study and error analyses carried out in this paper show that IASI Level 1 data can be used to retrieve tropospheric O3 columns (TOC, surface-225 hPa) and UTLS columns (225–70 hPa) with errors smaller than 20%. Validation with global radiosonde O3 profiles obtained during a period of 6 months show the excellent agreement between IASI and radiosonde for the UTLS with correlation coefficient R > 0.91 and good agreement in the troposphere with correlation coefficient R > 0.74. For both the UTLS and the troposphere Relative Standard Deviations (RSD) are lower than 23%. Comparison with in-situ measurements from the MOZAIC program around Hyderabad demonstrates that IASI is able to capture the TOC inter and intra-seasonal variability in central India. Nevertheless, the agreement is mitigated by the fact that the smoothing of the true O3 profiles by the retrieval results in a reduction of the TOC variability detected by IASI relative to the variability observed by in situ instruments. The post-monsoon temporal variability of the vertical profile of O3 around Hyderabad has been investigated with MOZAIC observations. These observations from airborne instruments show that tropospheric O3 is steadily elevated during most of the studied period with the exception of two sharp drops following the crossing of tropical storms over India. Lagrangian simulations with the FLEXPART model indicate that elevated O3 concentrations in the middle troposphere near Hyderabad are associated with the transport of UTLS air-masses that have followed the Subtropical Westerly Jet (SWJ) and subsided over northern India together with boundary layer polluted air-masses transported from the Indo-gangetic plain by the north-easterly trades. Low O3 concentrations result from the uplift and westward transport of pristine air-masses from the marine boundary layer of the Bay of Bengal by tropical storms. In order to extend the analysis of tropospheric O3 variability to the whole of south Asia, we have used IASI-SOFRID O3 data. We show that IASI O3 data around Hyderabad were able to capture the fast variability revealed by MOZAIC. Furthermore, their spatio-temporal coverage demonstrates that the behaviour of tropospheric O3 observed near Hyderabad extended over most of central and south India and part of the Bay of Bengal. This result highlights the ability of the IASI sensor to capture fast changes in chemical composition related to dramatic tropical weather conditions.
The observation of chemiluminescent NiO* emissions in the laboratory and in the night airglow

The recent finding of an orange spectral feature in OSIRIS/Odin spectra of the night airglow near 87 km has raised interest in the origin of the emission. The feature was positively identified as the chemiluminescent FeO* emission where the iron is of meteoric origin. Since the meteorite source of atomic metals in the mesosphere contains both iron and nickel, with Ni being typically 6 % of Fe, it is expected that faint emissions involving Ni should also be present in the night airglow. The present study summarizes the laboratory observations of chemiluminescent NiO* emissions and includes a search for the NiO* signature in the night airglow. A very faint previously unidentified "continuum" extending longwave of 440 nm has been detected in the night airglow spectra obtained with two space-borne limb viewing instruments. Through a comparison with laboratory spectra this continuum is identified as arising from the NiO* emission. The altitude profile of the new airglow emission has also been measured. The similarity of the altitude profiles of the FeO* and NiO* emissions also suggests the emission is NiO as both can originate from reaction of the metal atoms with mesospheric ozone. The observed NiO* to FeO* ratio exhibits considerable variability; possible causes of this observed variation are briefly discussed.
Source attribution of Bornean air masses by back trajectory analysis during the OP3 project

Atmospheric composition affects the radiative balance of the Earth through the creation of greenhouse gases and the formation of aerosols. The latter interact with incoming solar radiation, both directly and indirectly through their effects on cloud formation and lifetime. The tropics have a major influence on incoming sunlight however the tropical atmosphere is poorly characterised, especially outside Amazonia. The origins of air masses influencing a measurement site in a protected rainforest in Borneo, South East Asia, were assessed and the likely sources of a range of trace gases and particles were determined. This was conducted by interpreting in situ measurements made at the site in the context of ECMWF backwards air mass trajectories. Two different but complementary methods were employed to interpret the data: comparison of periods classified by cluster analysis of trajectories, and inspection of the dependence of mean measured values on geographical history of trajectories. Sources of aerosol particles, carbon monoxide and halocarbons were assessed. The likely source influences include: terrestrial organic biogenic emissions; long range transport of anthropogenic emissions; biomass burning; sulphurous emissions from marine phytoplankton, with a possible contribution from volcanoes; marine production of inorganic mineral aerosol; and marine production of halocarbons. Aerosol sub- and super-saturated water affinity was found to be dependent on source (and therefore composition), with more hygroscopic aerosol and higher numbers of cloud condensation nuclei measured in air masses of marine origin. The prevailing sector during the majority of measurements was south-easterly, which is from the direction of the coast closest to the site, with a significant influence inland from the south-west. This analysis shows that marine and terrestrial air masses have different dominant chemical sources. Comparison with the AMAZE-08 project in the Amazon basin shows Bornean composition to arise from a different, more complex mixture of sources. In particular sulphate loadings are much greater than in Amazonia which is likely to mainly be the result of the marine influence on the site. This suggests that the significant region of the tropics made up of island networks is not well represented by extrapolation from measurements made in the Amazon. In addition, it is likely that there were no periods where the site was influenced only by the rainforest, with even the most pristine inland periods showing some evidence of non-rainforest aerosol. This is in contrast to Amazonia which experienced periods dominated by rainforest emissions.
Re-analysis of tropospheric sulfate aerosol and ozone for the period 1980–2005 using the aerosol-chemistry-climate model ECHAM5-HAMMOZ

Understanding historical trends of trace gas and aerosol distributions in the troposphere is essential to evaluate the efficiency of existing strategies to reduce air pollution and to design more efficient future air quality and climate policies. We performed coupled photochemistry and aerosol microphysics simulations for the period 1980–2005 using the aerosol-chemistry-climate model ECHAM5-HAMMOZ, to assess our understanding of long-term changes and inter-annual variability of the chemical composition of the troposphere, and in particular of ozone and sulfate concentrations, for which long-term surface observations are available. In order to separate the impact of the anthropogenic emissions and natural variability on atmospheric chemistry, we compare two model experiments, driven by the same ECMWF re-analysis data, but with varying and constant anthropogenic emissions, respectively. Our model analysis indicates an increase of ca. 1 ppbv (0.055 ± 0.002 ppbv yr−1) in global average surface O3 concentrations due to anthropogenic emissions, but this trend is largely masked by the larger O3 anomalies due to the variability of meteorology and natural emissions. The changes in meteorology (not including stratospheric variations) and natural emissions account for the 75 % of the total variability of global average surface O3 concentrations. Regionally, annual mean surface O3 concentrations increased by 1.3 and 1.6 ppbv over Europe and North America, respectively, despite the large anthropogenic emission reductions between 1980 and 2005. A comparison of winter and summer O3 trends with measurements shows a qualitative agreement, except in North America, where our model erroneously computed a positive trend. Simulated O3 increases of more than 4 ppbv in East Asia and 5 ppbv in South Asia can not be corroborated with long-term observations. Global average sulfate surface concentrations are largely controlled by anthropogenic emissions. Globally natural emissions are an important driver determining AOD variations. Regionally, AOD decreased by 28 % over Europe, while it increased by 19 % and 26 % in East and South Asia. The global radiative perturbation calculated in our model for the period 1980–2005 was rather small (0.05 W m−2 for O3 and 0.02 W m−2 for total aerosol direct effect), but larger perturbations ranging from −0.54 to 1.26 W m−2 are estimated in those regions where anthropogenic emissions largely varied.
The Eyjafjöll explosive volcanic eruption from a microwave weather radar perspective

The sub-glacial Eyjafjöll explosive volcanic eruptions of April and May 2010 are analyzed and quantitatively interpreted by using ground-based weather radar data and the Volcanic Ash Radar Retrieval (VARR) technique. The Eyjafjöll eruptions have been continuously monitored by the Keflavík C-band weather radar, located at a distance of about 155 km from the volcano vent. Considering that the Eyjafjöll volcano is approximately 20 km from the Atlantic Ocean and that the northerly winds stretched the plume toward the mainland Europe, weather radars are the only means to provide an estimate of the total ejected tephra. The VARR methodology is summarized and applied to available radar time series to estimate the plume maximum height, ash particle category, ash volume, ash fallout and ash concentration every 5 min near the vent. Estimates of the discharge rate of eruption, based on the retrieved ash plume top height, are provided together with an evaluation of the total erupted mass and volume. Deposited ash at ground is also retrieved from radar data by empirically reconstructing the vertical profile of radar reflectivity and estimating the near-surface ash fallout. Radar-based retrieval results cannot be compared with ground measurements, due to the lack of the latter, but further demonstrate the unique contribution of these remote sensing products to the understating and modelling of explosive volcanic ash eruptions.
Use of satellite erythemal UV products in analysing the global UV changes

Long term changes in solar UV radiation affect global bio-geochemistry and climate. The satellite-based dataset of TOMS (Total Ozone Monitoring System) and OMI (Ozone Monitoring Instrument) of erythemal UV product was applied for the first time to estimate the long-term ultraviolet (UV) changes at the global scale. The analysis of the uncertainty related to the different input information is presented. OMI and GOME-2 (Global Ozone Monitoring Experiment-2) products were compared in order to analyse the differences in the global UV distribution and their effect on the linear trend estimation. The results showed that the differences in the inputs (mainly surface albedo and aerosol information) used in the retrieval, affect significantly the UV change calculation, pointing out the importance of using a consistent dataset when calculating long term UV changes. The areas where these differences played a major role were identified using global maps of monthly UV changes. Despite the uncertainties, significant positive UV changes (ranging from 0 to about 5 %/decade) were observed, with higher values in the Southern Hemisphere at mid-latitudes during spring-summer, where the largest ozone decrease was observed.
Biological residues define the ice nucleation properties of soil dust

Soil dust is a major driver of ice nucleation in clouds leading to precipitation. It consists largely of mineral particles with a small fraction of organic matter constituted mainly of remains of micro-organisms that participated in degrading plant debris before their own decay. Some micro-organisms have been shown to be much better ice nuclei than the most efficient soil mineral. Yet, current aerosol schemes in global climate models do not consider a difference between soil dust and mineral dust in terms of ice nucleation activity. Here, we show that particles from the clay and silt size fraction of four different soils naturally associated with 0.7 to 11.8 % organic carbon (w/w) can have up to four orders of magnitude more ice nucleation sites per unit mass active in the immersion freezing mode at −12 °C than montmorillonite, the nucleation properties of which are often used to represent those of mineral dusts in modelling studies. Most of this activity was lost after heat treatment. Removal of biological residues reduced ice nucleation activity to, or below that of montmorillonite. Desert soils, inherently low in organic content, are a large natural source of dust in the atmosphere. In contrast, agricultural land use is concentrated on fertile soils with much larger organic matter contents than found in deserts. It is currently estimated that the contribution of agricultural soils to the global dust burden is less than 20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere of a very different and much more potent kind than mineral dusts.