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Esau I.N.,Nansen Environmental and Remote Sensing Center Bjerknes Center for Climate Research | Chernokulsky A.V.,Obukhov Institute for Atmospheric Physics
Izvestiya - Atmospheric and Ocean Physics | Year: 2015

Convective cloudiness in the Atlantic sector of the Arctic is considered as an atmospheric spatially self-organized convective field. Convective cloud development is usually studied as a local process reflecting the convective instability of the turbulent planetary boundary layer over a heated surface. The convective cloudiness has a different dynamical structure in high latitudes. Cloud development follows cold-air outbreaks into the areas with a relatively warm surface. As a result, the physical and morphological characteristics of clouds, such as the type of convective cloud, and their geographical localization are interrelated. It has been shown that marginal sea ice and coastal zones are the most frequently occupied by Cu hum, Cu med convective clouds, which are organized in convective rolls. Simultaneously, the open water marine areas are occupied by Cu cong, Cb, which are organized in convective cells. An intercomparison of cloud statistics using satellite data ISCCP and ground-based observations has revealed an inconsistency in the cloudiness trends in these data sources: convective cloudiness decreases in ISCCP data and increases in the groundbased observation data. In general, according to the stated hypothesis, the retreat of the sea-ice boundary may lead to an increase in the amount of convective clouds. © 2015, Pleiades Publishing, Ltd. Source

Gorbunov M.E.,Obukhov Institute for Atmospheric Physics | Lauritsen K.B.,Danish Meteorological Institute | Leroy S.S.,Harvard University
Radio Science | Year: 2010

We present the Wigner distribution function (WDF) as an alternative to radio holographic (RH) analysis in the interpretation of radio occultation (RO) observations of the Earth's atmosphere. RH analysis is widely used in RO retrieval to isolate signal from noise and to identify atmospheric multipath. The same task is performed by WDF which also maps a 1-D wave function to 2-D time-frequency phase space and which has maxima located at the ray manifold. Unlike the standard RH technique based on the spectrum analysis in small sliding apertures, WDF is given by a global integral transform, which allows for a higher resolution. We present a tomographic derivation of the WDF and discuss its properties. Examples of analysis of simulations and COSMIC RO data show that WDF allows for a much sharper localization of the details of bending angle profiles as compared to the standard RH analysis in sliding apertures. Both WDF and RH allow for identification of multivalued bending angle profiles arising in the presence of strong horizontal gradients and may introduce a negative bias into bending angle retrieval. Copyright 2010 by the American Geophysical Union. Source

Gorbunov M.E.,Obukhov Institute for Atmospheric Physics | Shmakov A.V.,Obukhov Institute for Atmospheric Physics | Leroy S.S.,Harvard University | Lauritsen K.B.,Danish Meteorological Institute
Journal of Atmospheric and Oceanic Technology | Year: 2011

A radio occultation data processing system (OCC) was developed for numerical weather prediction and climate benchmarking. The data processing algorithms use the well-established Fourier integral operator- based methods, which ensure a high accuracy of retrievals. The system as a whole, or in its parts, is currently used at the Global Navigation Satellite System Receiver for Atmospheric Sounding (GRAS) Satellite Application Facility at the Danish Meteorological Institute, German Weather Service, and Wegener Center for Climate and Global Change.Astatistical comparison of the inversions of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) data by the system herein, University Corporation for Atmospheric Research (UCAR) data products, and ECMWF analyses is presented. Forty days of 2007 and 2008 were processed (from 5 days in the middle of each season) for the comparison of OCC andECMWF, and 20 days of April 2009 were processed for the comparison of OCC, UCAR, and ECMWF. The OCC and UCAR inversions are consistent. For the tropics, the systematic difference between OCC and UCAR in the retrieved refractivity in the 2-30-km height interval does not exceed 0.1%; in particular, in the 9-25-km interval it does not exceed 0.03%. Below 1 km in the tropics the OCC - UCAR bias reaches 0.2%, which is explained by different cutoff and filtering schemes implemented in the two systems. The structure of the systematic OCC - ECMWF difference below 4 km changes in 2007, 2008, and 2009, which is explained by changes in the ECMWF analyses and assimilation schemes. It is estimated that in the 4-30-km height range the OCC occultation processing system obtains refractivities with a bias not exceeding 0.2%. The random error ranges from 0.3%-0.5% in the upper troposphere-lower stratosphere to about 2% below 4 km. The estimate of the bias below 4 km can currently be done with an accuracy of 0.5%-1% resulting from the structural uncertainty of the radio occultation (RO) data reflecting the insufficient knowledge of the atmospheric small-scale structures and instrumental errors. The OCC - UCAR bias is below the level of the structural uncertainty. © 2011 American Meteorological Society. Source

Karsisto P.,University of Helsinki | Karsisto P.,Finnish Meteorological Institute | Fortelius C.,Finnish Meteorological Institute | Demuzere M.,Catholic University of Leuven | And 6 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2016

The performance of three urban land-surface models, run in off-line mode, with their default external parameters, is evaluated for two distinctly different sites in Helsinki: Torni and Kumpula. The former is a dense city-centre site with 22% vegetation, while the latter is a suburban site with over 50% vegetation. At both locations the models are compared against sensible and latent heat fluxes measured using the eddy covariance technique, along with snow depth observations. The cold climate experienced by the city causes strong seasonal variations that include snow cover and stable atmospheric conditions. Most of the time the three models are able to account for the differences between the study areas as well as the seasonal and diurnal variability of the energy balance components. However, the performances are not systematic across the modelled components, seasons and surface types. The net all-wave radiation is well simulated, with the greatest uncertainties related to snow-melt timing, when the fraction of snow cover has a key role, particularly in determining the surface albedo. For the turbulent fluxes, more variation between the models is seen which can partly be explained by the different methods in their calculation and partly by surface parameter values. For the sensible heat flux, simulation of wintertime values was the main problem, which also leads to issues in predicting near-surface stabilities particularly at the dense city-centre site. All models have the most difficulties in simulating latent heat flux. This study particularly emphasizes that improvements are needed in the parametrization of anthropogenic heat flux and thermal parameters in winter, snow cover in spring, and evapotranspiration, in order to improve the surface energy balance modelling in cold-climate cities. © 2016 Royal Meteorological Society. Source

Stjernberg A.-C.E.,Norwegian Institute For Air Research | Stjernberg A.-C.E.,University of Stockholm | Skorokhod A.,Obukhov Institute for Atmospheric Physics | Paris J.D.,Laboratorie Des Science Du Climat Et Of Lenvironnment | And 3 more authors.
Tellus, Series B: Chemical and Physical Meteorology | Year: 2012

Siberia with its large area covered with boreal forests, wetlands and tundra is believed to be an important sink for ozone via dry deposition and reactions with biogenic volatile organic compounds (BVOCs) emitted by the forests. To study the importance of deposition of ozone in Siberia, we analyse measurements of ozone mixing ratios taken along the Trans-Siberian railway by train, air-borne measurements andpoint measurements at the Zotino station. For all data, we ran the Lagrangian particle dispersion model FLEXPART in backward mode for 20 d, which yields the so-called potential emission sensitivity (PES) fields. These fields give a quantitative measure of where andhow strongly the sampledair masses have been in contact with the surface and hence possible influenced by surface fluxes. These fields are further statistically analysed to identify source andsink regions that are influencing the observedozone. Results show that the source regions for the surface ozone in Siberia are located at lower latitudes: the regions around the Mediterranean Sea, the Middle East, Kazakhstan andChina. Low ozone mixing ratios are associated to transport from North West Russia, the Arctic region, andthe Pacific Ocean. By calculating PES values for both a passive tracer without consideration of removal processes and for an ozone-like tracer where dry deposition processes are included, we are able to quantify the ozone loss occurring en route to the receptor. Strong correlations between low ozone concentrations andthe spatially integratedfootprints from FLEXPART, especially during the period summer to autumn, indicate the importance of the Siberian forests as a sink for tropospheric ozone. © 2012 A.-C. Engvall Stjernberg et al. Source

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