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  • Articles  (231)
  • 2010-2014  (231)
  • Physics  (231)
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  • 1
    Publication Date: 2014-12-13
    Description: The IMILAST project (‘Intercomparison of Mid-Latitude Storm Diagnostics’) was set up to compare low-level cyclone climatologies derived from a number of objective identification algorithms. This paper is a contribution to that effort where we determine the sensitivity of three key aspects of Northern Hemisphere cyclone behaviour [namely the number of cyclones, their intensity (defined here in terms of the central pressure) and their deepening rates] to specific features in the automatic cyclone identification. The sensitivity is assessed with respect to three such features which may be thought to influence the ultimate climatology produced (namely performance in areas of complicated orography, time of the detection of a cyclone, and the representation of rapidly propagating cyclones). We make use of 13 tracking methods in this analysis. We find that the filtering of cyclones in regions where the topography exceeds 1500 m can significantly change the total number of cyclones detected by a scheme, but has little impact on the cyclone intensity distribution. More dramatically, late identification of cyclones (simulated by the truncation of the first 12 hours of cyclone life cycle) leads to a large reduction in cyclone numbers over the both continents and oceans (up to 80 and 40%, respectively). Finally, the potential splitting of the trajectories at times of the fastest propagation has a negligible climatological effect on geographical distribution of cyclone numbers. Overall, it has been found that the averaged deepening rates and averaged cyclone central pressure are rather insensitive to the specifics of the tracking procedure, being more sensitive to the data set used (as shown in previous studies) and the geographical location of a cyclone. Keywords: cyclone identification, IMILAST, cyclone life cycle, rapidly intensifying cyclones, synoptic climatology, reanalysis (Published: 12 December 2014) Citation: Tellus A 2014, 66 , 24961, http://dx.doi.org/10.3402/tellusa.v66.24961 This publication is part of a Thematic Cluster entitled " Intercomparison of Mid-Latitude Storm Diagnostics ". Read the other papers from this thematic cluster here
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  • 2
    Publication Date: 2014-12-11
    Description: The impact of assimilating temperature, salinity, oxygen, phosphate and nitrate observations on marine ecosystem modelling is assessed. For this purpose, two 10-yr (1970–1979) reanalyses of the Baltic Sea are carried out using the ensemble optimal interpolation (EnOI) method and a coupled physical-biogeochemical model of the Baltic Sea. To evaluate the reanalyses, climatological data and available biogeochemical and physical in situ observations at monitoring stations are compared with results from simulations with and without data assimilation. In the first reanalysis, only observed temperature and salinity profiles are assimilated, whereas biogeochemical observations are unused. Although simulated temperature and salinity improve considerably as expected, the quality of simulated biogeochemical variables does not improve and deep water nitrate concentrations even worsen. This unexpected behaviour is explained by a lowering of the halocline in the Baltic proper due to the assimilation causing increased oxygen concentrations in the deep water and consequently altered nutrient fluxes. In the second reanalysis, both physical and biogeochemical observations are assimilated and good quality in all variables is found. Hence, we conclude that if a data assimilation method like the EnOI is applied, all available observations should be used to perform reanalyses of high quality for the Baltic Sea biogeochemical state estimates. Keywords: reanalysis, data assimilation, numerical modelling, Baltic Sea, biogeochemical simulation (Published: 10 December 2014) Citation: Tellus A 2014, 66 , 24908, http://dx.doi.org/10.3402/tellusa.v66.24908
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  • 3
    Publication Date: 2014-12-09
    Description: At the end of January 2012, a low-level cloud from partly ice-free Lake Ladoga caused very variable 2-m temperatures in Eastern Finland. The sensitivity of the High Resolution Limited Area Model (HIRLAM) to the lake surface conditions was tested in this winter anticyclonic situation. The lake appeared to be (incorrectly) totally covered by ice when the lake surface was described with its climatology. Both parametrisation of the lake surface state by using a lake model integrated to the NWP system and objective analysis based on satellite observations independently resulted in a correct description of the partly ice-free Lake Ladoga. In these cases, HIRLAM model forecasts were able to predict cloud formation and its movement as well as 2-m temperature variations in a realistic way. Three main conclusions were drawn. First, HIRLAM could predict the effect of Lake Ladoga on local weather, when the lake surface state was known. Second, the current parametrisation methods of air–surface interactions led to a reliable result in conditions where the different physical processes (local surface processes, radiation and turbulence) were not strong, but their combined effect was important. Third, these results encourage work for a better description of the lake surface state in NWP models by fully utilising satellite observations, combined with advanced lake parametrisation and data assimilation methods. Keywords: lake-effect, data assimilation, lake ice, NWP, cold outbreaks, stable boundary layer, lake–atmosphere interaction (Published: 8 December 2014) Citation: Tellus A 2014, 66 , 23929, http://dx.doi.org/10.3402/tellusa.v66.23929 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here
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  • 4
    Publication Date: 2014-12-06
    Description: This paper studies the question how the zonal mean potential vorticity ( PV ) distribution in potential temperature ( θ ) coordinates is established in the atmosphere by the interaction of diabatic processes (cross-isentropic transport of mass) with adiabatic dynamical processes (isentropic transport of mass and potential vorticity substance). As an aid in dissecting this interaction, a simplified model of the general circulation is constructed, which contains parametrisations of radiative transfer, wave drag and water cycle. This model reproduces the following four observed features of the atmosphere below 10 hPa: (1) a permanently present eastward subtropical jet, which in winter is separated from an eastward stratospheric jet by a zone (referred to as the ‘surf zone’), between θ =380 K and θ =550 K, where planetary wave drag reduces PV over the polar cap; (2) a stratospheric zonal wind reversal in spring or beginning of summer; (3) a tropical cold layer at 100 hPa, and (4) a realistic distribution of zonal mean cross-isentropic flow. The strength of the cross-isentropic flow depends on wave drag, latent heat release and the thermal inertia of both the atmosphere and the earth’s surface. Of special interest is the layer between θ =315 K and θ =370 K (the ‘Middleworld’), which lies in the troposphere in the tropics and in the stratosphere in the extratropics. Mass converges diabatically into this layer in the deep tropics, mainly due to latent heat release, and diverges out of this layer elsewhere due to radiation flux divergence. Meridional isentropic vorticity flux divergence in the tropical Middleworld, associated with the upper branch of the Hadley circulation, creates a region in the subtropics, at θ =350 K and adjacent isentropic levels, with a marked isentropic meridional PV -gradient, forming the isentropic dynamical tropopause. Keywords: potential vorticity, thermal inertia, diabatic circulation, zonal wind jets, tropopause, surf zone, planetary wave drag, radiation, water cycle (Published: 5 December 2014) Citation: Tellus A 2014, 66 , 24880, http://dx.doi.org/10.3402/tellusa.v66.24880 To access the supplementary material to this article, please see Supplementary files under Article Tools online.
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  • 5
    Publication Date: 2014-11-29
    Description: The role of the β -effect in the Rossby wave propagation mechanism is examined in the linearised shallow water equations directly in momentum–height variables, without recourse to potential vorticity (PV). Rigorous asymptotic expansion of the equations, with respect to the small non-dimensionalised β parameter, reveals in detail how the Coriolis force acting on the small ageostrophic terms translates the geostrophic leading-order solution to propagate westward in concert. This information cannot be obtained directly from the conventional PV perspective on the propagation mechanism. Furthermore, a comparison between the β -effect in planetary Rossby waves and the sloping-bottom effect in promoting topographic Rossby waves shows that the ageostrophic terms play different roles in the two cases. This is despite the fact that from the PV viewpoint whether the advection of mean PV gradient is set up by changes in planetary vorticity or by mean depth is inconsequential. Keywords: Rossby wave propagation, momentum-pressure perspective, role of ageostrophic flow, beta plane, sloping bottom (Published: 28 November 2014) Citation: Tellus A 2014, 66 , 22672, http://dx.doi.org/10.3402/tellusa.v66.22672
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  • 6
    Publication Date: 2014-11-29
    Description: The meridional structures of stratospheric and tropospheric planetary wave variability (PWV) over the Northern Hemisphere (NH) extratropics were investigated and compared using reanalysis data. By performing the spherical double Fourier series expansion of geopotential height data, the horizontal structures of PWV at each vertical level could be examined in the two-dimensional (2D) wavenumber (zonal and meridional wavenumbers) space. Comparing the amplitudes of wave components during the last three decades, the results suggested that the structures of PWV in the NH troposphere significantly differ from the stratospheric counterparts. The PWV in the troposphere shows multiple meridional wave-like structures, most pronounced for the meridional dipole; while in contrast, PWV in the stratosphere mainly shows large-scale zonal wave patterns, dominated by zonal waves 1 and 2, and have little wave-like fluctuation in the latitudinal direction. The dominant patterns of the NH PWV also show contrasting features of meridional structure between the stratosphere and the troposphere. As represented in the 2D wavenumber space, the leading two empirical orthogonal functions of PWV in the stratosphere largely exhibit the zonal wave 1 pattern, while those in the troposphere clearly show meridional wave-like structures and are dominated by the dipole. The refractive index was derived based on the zonal mean basic state to qualitatively interpret the observational findings. The results suggested that the basic state in the NH troposphere is much more favourable for latitudinally propagating stationary waves than the stratosphere. The difference in meridional structure between stratospheric and tropospheric planetary waves can be well captured in a linear baroclinic model with the observed zonal mean basic state. Furthermore, both theoretical and modelling analyses demonstrated that the fact that zonal wave patterns are preferred in the NH stratosphere may be partly attributable to the vertical curvature of the stratospheric zonal mean basic state. Keywords: planetary waves, stratosphere, troposphere, spherical double Fourier series, two-dimensional wavenumber (Published: 28 November 2014) Citation: Tellus A 2014, 66 , 25303, http://dx.doi.org/10.3402/tellusa.v66.25303
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  • 7
    Publication Date: 2014-11-29
    Description: Orographic winds near a 914 m high mountain in Southwest-Iceland are explored using unique observations made aloft with a small remotely piloted aircraft, as well as with traditional observations and high-resolution atmospheric simulations. There was an inversion well above mountain top level at about 2 km with weak winds below. Observed winds in the lee of the mountain were indicative of flow locally enhanced by wave activity aloft. Winds descended along the lee slope with a prevailing direction away from the mountain. They were relatively strong and gusty at the surface close to the mountain, with a maximum at low levels, and weakening and becoming more diffuse a short distance further downstream. The winds weakened further aloft, with a minimum on average near mountain top level. This situation is reproduced in a high-resolution atmospheric simulation forced with atmospheric analysis as well as with the observed lee-side profiles of wind and temperature below 1.4 km. Without the additional observations consisting of the lee-side profiles, the model fails to reproduce the winds aloft as well as at the surface in a region in the lee of the mountain, as was also the case for the operational numerical models at that time. A sensitivity simulation indicates that this poor performance is a result of the poorly captured strength and sharpness of the inversion aloft. The study illustrates, firstly, that even at very low wind speed, in a close to neutral low-level flow, gravity waves may still be a dominating feature of the flow. Secondly, the study presents an example of the usefulness of lee-side atmospheric profiles, retrieved by simple model aircraft, for improving numerical simulations and short-term weather forecasting in the vicinity of mountains. Thirdly, the study confirms the sensitivity of downslope flow to only moderate change in the sharpness of an upstream inversion. Keywords: downslope flow, complex orography, small remotely piloted aircraft, observational nudging (Published: 28 November 2014) Citation: Tellus A 2014, 66 , 25421, http://dx.doi.org/10.3402/tellusa.v66.25421
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  • 8
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    Co-Action Publishing
    In: Tellus A
    Publication Date: 2014-11-25
    Description: Accurate estimates of error variances in numerical analyses and forecasts (i.e. difference between analysis or forecast fields and nature on the resolved scales) are critical for the evaluation of forecasting systems, the tuning of data assimilation (DA) systems and the proper initialisation of ensemble forecasts. Errors in observations and the difficulty in their estimation, the fact that estimates of analysis errors derived via DA schemes, are influenced by the same assumptions as those used to create the analysis fields themselves, and the presumed but unknown correlation between analysis and forecast errors make the problem difficult. In this paper, an approach is introduced for the unbiased estimation of analysis and forecast errors. The method is independent of any assumption or tuning parameter used in DA schemes. The method combines information from differences between forecast and analysis fields (‘perceived forecast errors’) with prior knowledge regarding the time evolution of (1) forecast error variance and (2) correlation between errors in analyses and forecasts. The quality of the error estimates, given the validity of the prior relationships, depends on the sample size of independent measurements of perceived errors. In a simulated forecast environment, the method is demonstrated to reproduce the true analysis and forecast error within predicted error bounds. The method is then applied to forecasts from four leading numerical weather prediction centres to assess the performance of their corresponding DA and modelling systems. Error variance estimates are qualitatively consistent with earlier studies regarding the performance of the forecast systems compared. The estimated correlation between forecast and analysis errors is found to be a useful diagnostic of the performance of observing and DA systems. In case of significant model-related errors, a methodology to decompose initial value and model-related forecast errors is also proposed and successfully demonstrated. Keywords: uncertainty of analysis, forecast verification, estimation methods, data assimilation, ensemble forecasts (Published: 24 November 2014) Citation: Tellus A 2014, 66 , 21767, http://dx.doi.org/10.3402/tellusa.v66.21767
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  • 9
    Publication Date: 2014-11-19
    Description: A fully coupled regional climate system model (CNRM-RCSM4) dedicated to the Mediterranean region is described and evaluated using a multidecadal hindcast simulation (1980–2012) driven by global atmosphere and ocean reanalysis. CNRM-RCSM4 includes the regional representation of the atmosphere (ALADIN-Climate model), land surface (ISBA model), rivers (TRIP model) and the ocean (NEMOMED8 model), with a daily coupling by the OASIS coupler. This model aims to reproduce the regional climate system with as few constraints as possible: there is no surface salinity, temperature relaxation, or flux correction; the Black Sea budget is parameterised and river runoffs (except for the Nile) are fully coupled. The atmospheric component of CNRM-RCSM4 is evaluated in a companion paper; here, we focus on the air–sea fluxes, river discharges, surface ocean characteristics, deep water formation phenomena and the Mediterranean thermohaline circulation. Long-term stability, mean seasonal cycle, interannual variability and decadal trends are evaluated using basin-scale climatologies and in-situ measurements when available. We demonstrate that the simulation shows overall good behaviour in agreement with state-of-the-art Mediterranean RCSMs. An overestimation of the shortwave radiation and latent heat loss as well as a cold Sea Surface Temperature (SST) bias and a slight trend in the bottom layers are the primary current deficiencies. Further, CNRM-RCSM4 shows high skill in reproducing the interannual to decadal variability for air–sea fluxes, river runoffs, sea surface temperature and salinity as well as open-sea deep convection, including a realistic simulation of the Eastern Mediterranean Transient. We conclude that CNRM-RCSM4 is a mature modelling tool allowing the climate variability of the Mediterranean regional climate system to be studied and understood. It is used in hindcast and scenario modes in the HyMeX and Med-CORDEX programs. Keywords: Mediterranean Sea, regional climate system model, air-sea fluxes, hindcast simulation, interannual variability (Published: 18 November 2014) Citation: Tellus A 2014, 66 , 23967, http://dx.doi.org/10.3402/tellusa.v66.23967
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  • 10
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    Co-Action Publishing
    In: Tellus A
    Publication Date: 2014-11-15
    Description: An observation impact is an estimate of the forecast error reduction by assimilating observations with numerical model forecasts. This study compares the sampling errors of the observation impact statistics (OBIS) of July 2011 and January 2012 using two methods. One method uses the random error under the assumption that the samples are independent, and the other method uses the error with lag correlation under the assumption that the samples are correlated with each other. The OBIS are obtained using the forecast sensitivity to observation (FSO) tool in the Korea Meteorological Administration (KMA) unified model (UM). To verify the self-correlation of the observation impact data, the lag correlations of the observation impact data at 00 UTC in the Northern Hemisphere (NH) summer months (June, July and August 2011) and winter months (December 2011 and January and February 2012) are calculated. The self-correlation approaches zero at 6 days for the summer, whereas it approaches zero at 4 days for the winter, which implies that the observation impact data are serially correlated. The sampling error considering lag correlation is larger than the random error for NH summer and winter. While the random sampling error is approximately 12–13% of the approximation error, the sampling error considering the lag correlation is approximately half of the approximation error of the OBIS. The sampling error that considers the lag correlation of the OBIS is more appropriate for representing the uncertainty in the OBIS because the OBIS at different times are correlated. Keywords: forecast sensitivity to observation, observation impact statistics, sampling error (Published: 14 November 2014) Citation: Tellus A 2014, 66 , 25435, http://dx.doi.org/10.3402/tellusa.v66.25435
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