Observations of noctilucent clouds (NLC) from northwest Europe have been collected by a network of observers for almost 40 years. Previous analyses of the observations have found an apparent ...increasing frequency of occurrence, a 10–11 year modulation and evidence for 5‐day periodicity. Here we reexamine the observational data for NLC occurrence to test whether the observed variations can be explained by planetary wave activity in the middle atmosphere. Planetary wave amplitudes and phases in the lower mesosphere are derived from global meteorological assimilations from 1979–2000 and extrapolated to the mesopause. When the NLC observations are selected from a constant observing area, we find that there is no substantial trend in yearly NLC occurrence over the observation period, whereas the 10–11 year and 5‐day modulations remain significant. We find a strong correlation between the probability of observing NLC and the combined effects of stationary, 16‐day and 5‐day planetary waves at the NLC location. The most reasonable explanation for the correlation is that the probability of observing NLC depends on the strength of the wind from the north, which in turn depends on the amplitude and phase of the planetary waves. The influence of planetary waves on NLC occurrence may to a certain extent explain the 10–11 year periodicity in NLC. This possibility is a consequence of a strong correlation between the phase of the stationary planetary waves and the 10–11 year cycle of solar activity during the period 1979–2000.
Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and ...for different time periods. They were examined to estimate ensemble average, long‐term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions (omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between −1.6% and −2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Ångström's parameters α and β measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high‐altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of α as a function of AOD(500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated.
During the POLARCAT summer campaign in 2008, two episodes (2-5 July and 7-10 July 2008) occurred where low-pressure systems traveled from Siberia across the Arctic Ocean towards the North Pole. The ...two cyclones had extensive smoke plumes from Siberian forest fires and anthropogenic sources in East Asia embedded in their associated air masses, creating an excellent opportunity to use satellite and aircraft observations to validate the performance of atmospheric transport models in the Arctic, which is a challenging model domain due to numerical and other complications. Here we compare transport simulations of carbon monoxide (CO) from the Lagrangian transport model FLEXPART and the Eulerian chemical transport model TOMCAT with retrievals of total column CO from the IASI passive infrared sensor onboard the MetOp-A satellite. The main aspect of the comparison is how realistic horizontal and vertical structures are represented in the model simulations. Analysis of CALIPSO lidar curtains and in situ aircraft measurements provide further independent reference points to assess how reliable the model simulations are and what the main limitations are. The horizontal structure of mid-latitude pollution plumes agrees well between the IASI total column CO and the model simulations. However, finer-scale structures are too quickly diffused in the Eulerian model. Applying the IASI averaging kernels to the model data is essential for a meaningful comparison. Using aircraft data as a reference suggests that the satellite data are biased high, while TOMCAT is biased low. FLEXPART fits the aircraft data rather well, but due to added background concentrations the simulation is not independent from observations. The multi-data, multi-model approach allows separating the influences of meteorological fields, model realisation, and grid type on the plume structure. In addition to the very good agreement between simulated and observed total column CO fields, the results also highlight the difficulty to identify a data set that most realistically represents the actual pollution state of the Arctic atmosphere.
An overview on the data of columnar aerosol properties measured in Northern Europe is provided. Apart from the necessary data gathered in the Arctic, the knowledge of the aerosol loading in nearby ...areas (e.g. sub-Arctic) is of maximum interest to achieve a correct analysis of the Arctic aerosols and transport patterns. This work evaluates data from operational sites with sun photometer measurements belonging either to national or international networks (AERONET, GAW-PFR) and programs conducted in Scandinavia and Svalbard. We enumerate a list of sites, measurement type and periods together with observed aerosol properties. An evaluation and analysis of aerosol data was carried out with a review of previous results as well. Aerosol optical depth (AOD) and Ångström exponent (AE) are the current parameters with sufficient long-term records for a first evaluation of aerosol properties. AOD (500 nm) ranges from 0.08 to 0.10 in Arctic and sub-Arctic sites (Ny-Ålesund: 0.09; Andenes: 0.10; Sodankylä: 0.08), and it is somewhat higher in more populated areas in Southern Scandinavia (AOD about 0.10–0.12 at 500 nm). On the Norwegian coast, aerosols show larger mean size (AE = 1.2 at Andenes) than in Finland, with continental climate (AE = 1.5 at Sodankylä). Columnar particle size distributions and related parameters derived from inversion of sun/sky radiances were also investigated. This work makes special emphasis in the joint and collaborative effort of the various groups from different countries involved in this study. Part of the measurements presented here were involved in the IPY projects Polar-AOD and POLARCAT.
► Different AOD seasonality is found in Svalbard, northern and southern Scandinavia. ► In the European sub-Arctic region (about 65–70°N) the spring haze is not persistent. ► Sources in Eastern Europe produce a spring AOD peak over southern Scandinavia. ► Fine mode aerosols are predominant and their variations determine the seasonal aerosol variability. ► The coarse mode aerosol concentrations are very low except for coastal sites.
Proper operation of control systems is essential for achieving good economic results and reducing control effort. The paper is focused on presenting a new application of a well-known concept. The ...main scope of the paper is a practical presentation of obtaining a minimum process performance index by means of known statistical tools. This is achievable by appropriate selection of the correction value for set-point and the width of the time window of a statistical algorithm. The proposed novel algorithm was successfully implemented in the pilot neutralization process. On one hand, the proposed algorithm is a corrector of the statistical properties of the control error, and, on the other one, of a set point of the control system. Index Terms--Nonlinear control systems, PI control, Process control, Error analysis, Chemical industry.
The validation of ozone profiles retrieved by satellite instruments through comparison with data from ground-based instruments is important to monitor the evolution of the satellite instrument, to ...assist algorithm development and to allow multi-mission trend analyses. In this study we compare ozone profiles derived from GOMOS night-time observations with measurements from lidar, microwave radiometer and balloon sonde. Collocated pairs are analysed for dependence on several geophysical and instrument observational parameters. Validation results are presented for the operational ESA level 2 data (GOMOS version 5.00) obtained during nearly seven years of observations and a comparison using a smaller dataset from the previous processor (version 4.02) is also included. The profiles obtained from dark limb measurements (solar zenith angle >107°) when the provided processing flag is properly considered match the ground-based measurements within ±2 percent over the altitude range 20 to 40 km. Outside this range, the pairs start to deviate more and there is a latitudinal dependence: in the polar region where there is a higher amount of straylight contamination, differences start to occur lower in the mesosphere than in the tropics, whereas for the lower part of the stratosphere the opposite happens: the profiles in the tropics reach less far down as the signal reduces faster because of the higher altitude at which the maximum ozone concentration is found compared to the mid and polar latitudes. Also the bias is shifting from mostly negative in the polar region to more positive in the tropics Profiles measured under "twilight" conditions are often matching the ground-based measurements very well, but care has to be taken in all cases when dealing with "straylight" contaminated profiles. For the selection criteria applied here (data within 800 km, 3 degrees in equivalent latitude, 20 h (5 h above 50 km) and a relative ozone error in the GOMOS data of 20% or less), no dependence was found on stellar magnitude, star temperature, nor the azimuth angle of the line of sight. No evidence of a temporal trend was seen either in the bias or frequency of outliers, but a comparison applying less strict data selection criteria might show differently.
The Eyjafjallajökull ash that crossed over Spain and Portugal on 6–12 May 2010 has been monitored by a set of operational sun photometer sites within AERONET-RIMA and satellite sensors. The sun ...photometer observations (aerosol optical depth, coarse mode concentrations) and ash products from IASI and SEVIRI satellite sensors, together with FLEXPART simulations of particle transport, allow identifying the volcanic aerosols. The aerosol columnar properties derived from inversions were investigated, indicating specific properties, especially regarding the absorption. The single scattering albedo was high (0.95 at 440 nm) and nearly wavelength independent, although with slight decrease with wavelength. Other parameters, like the fine mode fraction of the volume size distributions (0.20–0.80) or the portion of spherical particles (15–90%), were very variable among the sites and indicated that the various ash clouds were inhomogeneous with respect to particle size and shape.
► Ground-based sun photometer and satellite data confirm ash over the Iberian Peninsula on 6–12 May. ► Volcanic ash optical properties, especially regarding absorption, are different from other aerosol types. ► Mean size and shape were highly variable depending on the site and the dates.
Accuracy requirements for aerosol optical depth (AOD) in polar regions are much more stringent than those usually encountered in established sun photometer networks, while comparability of data from ...different archive centres is a further important issue. Therefore, two intercomparison campaigns were held during spring 2006 at Ny-Ålesund (Svalbard) and autumn 2008 at Izaña (Tenerife) within the framework of the IPY POLAR-AOD project, with the participation of various research institutions routinely employing different instrument models at Arctic and Antarctic stations. As reported here, a common algorithm was used for data analysis with the aim of minimizing a large part of the discrepancies affecting the previous studies. During the Ny-Ålesund campaign, spectral values of AOD derived from measurements taken with different instruments were found to agree, presenting at both 500nm and 870nm wavelengths average values of root mean square difference (RMSD) and standard deviation of the difference (SDD) equal to 0.003. Correspondingly, the mean bias difference (MBD) varied mainly between −0.003 and +0.003 at 500nm, and between −0.004 and +0.003 at 870nm. During the Izaña campaign, which was also intended as an intercalibration opportunity, RMSD and SDD values were estimated to be equal to 0.002 for both channels on average, with MBD ranging between −0.004 and +0.004 at 500nm and between −0.002 and +0.003 at 870nm. RMSD and SDD values for Ångström exponent α were estimated equal to 0.06 during the Ny-Ålesund campaign and 0.39 at Izaña. The results confirmed that sun photometry is a valid technique for aerosol monitoring in the pristine atmospheric turbidity conditions usually observed at high latitudes.
► Two sun photometer comparison campaigns were conducted for the POLAR-AOD project. ► AOD mean bias differences were found to be within ±0.004 at all wavelengths. ► Ångström exponent α was estimated to be within 0.06 and 0.39 for the two campaigns.
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