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  • 1
    In: Meteorologische Zeitschrift, April 2011, Vol.20(2), pp.153-164
    Description: This paper addresses the question how to identify the best members of a convection-permitting numerical weather prediction ensemble. Two different metrics, a classical quadratic approach using conventional observations and a spatial approach based on radar derived precipitation estimates, are employed to identify the ensemble members closest to the observations and to separate good and bad ensemble members. The characteristics of the best member selections and their performance in different weather regimes are investigated and evaluated in the context of three potential applications. The results show clear differences between the different best member selections and based on their characteristics their use in different applications is suggested. The classical quadratic metric has a higher persistence and is thus well suited for synoptic-scale applications, while the spatial metric shows good correlations with a reference measure for precipitation for short lead times and is thus better suited for very short-range applications. German Die Arbeit beschäftigt sich mit der Identifizierung von besten Mitgliedern eines Konvektion auflösenden numerischen Wettervorhersagemodells. Zwei verschiedene Metriken, ein klassischer quadratischer Ansatz unter Verwendung konventioneller Beobachtungen und ein räumlicher Ansatz basierend auf Radardaten, werden verwendet, um die besten, d. h. die den Beobachtungen am nächsten liegenden Mitglieder eines Ensembles von Vorhersagen auszuwählen und gute und schlechte Mitglieder voneinander zu unterscheiden. Die Eigenschaften der Best Member Selections (BMS) und ihr Verhalten in unterschiedlichen Wetterregimes werden untersucht und in Hinblick auf drei verschiedene potentielle Anwendungen ausgewertet. Die Ergebnisse zeigen deutliche Unterschiede zwischen den verschiedenen BMS und abhängig von ihren Eigenschaften wird die Verwendung in unterschiedlichen Anwendungen empfohlen. Die klassische Metrik hat eine grössere Persistenz und ist darum gut für synoptisch-skalige Anwendungen geeignet, während die räumliche Metrik für kurze Leadtimes hohe Korrelationen mit dem Zielmaß für Niederschlag aufweist und sich somit besser für Kürzestfrist-Anwendungen eignet.
    Keywords: Apennines
    ISSN: 0941-2948
    E-ISSN: 16101227
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  • 2
    In: Geophysical Research Letters, 28 April 2015, Vol.42(8), pp.3056-3062
    Description: By directly comparing the results of a recent convection‐permitting upscale error growth study with simulations on coarser grids, we assessed the ability of two different convection schemes to represent convective uncertainty and propagate it upscale. It is found that the widely used Tiedtke convection scheme shows significantly reduced upscale error growth and hence leads to an overconfidence of the model. In contrast it is found that the stochastic convection scheme of Plant and Craig is able to better reproduce the high‐resolution results. Evidence is presented that this improved uncertainty evolution can be related to the representation of (unpredictable) convective variability near the grid scale. Upscale error growth is found to be sensitive to representation of convection A stochastic parametrization reproduces the behavior of a high‐resolution model
    Keywords: Predictability ; Upscale Error Growth ; Stochastic Parametrization ; Cumulus Parametrization
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 3
    In: Geophysical Research Letters, 28 March 2012, Vol.39(6), pp.n/a-n/a
    Description: Free tropospheric water vapor variability, measured by airborne lidar over Europe during summertime, is analyzed at altitudes from 2 km to 10 km. Horizontal structure functions of specific humidity were computed and show power‐law scaling between about 10 km to 100 km in range. The second‐order structure function shows scaling exponents equivalent to spectral slopes that vary from around 5/3 in the lower troposphere to 2 at upper levels. More specifically humidity smoothness typically increases with height, while intermittency decreases. A classification of the data according to whether the series occurred above or below the level of nearby convective cloud tops gives a separation of the scaling exponents in the two air masses. The results are consistent with a water vapor distribution determined at upper levels by a downscale cascade of variance by advective mixing, but increasingly influenced at lower levels by local injection of humidity by moist convection. Airborne lidar water vapor measurements show power law scaling from 10 to 100 km Two‐dimensional cross sections show properties depending on height level Levels influenced by convection have shallower slopes, higher intermittency
    Keywords: Scaling ; Structure Function ; Troposphere ; Water Vapor Variability
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 4
    In: Geophysical Research Letters, 28 June 2016, Vol.43(12), pp.6612-6619
    Description: The Plant‐Craig (PC) stochastic convective parameterization scheme is implemented into the National Center for Atmospheric Research Community Atmosphere Model version 5 (CAM5) to couple with the Zhang‐McFarlane deterministic convection scheme. To evaluate its impact on tropical precipitation simulation, two experiments are conducted: one with the standard CAM5 and the other with the stochastic scheme incorporated. Results show that the PC stochastic parameterization decreases the frequency of weak precipitation and increases the frequency of strong precipitation, resulting in better agreement with observations. The most striking improvement is in the probability distribution function (PDF) of precipitation intensity, with the well‐known too‐much‐drizzle problem in CAM5 largely eliminated. In the global tropical belt, the precipitation intensity PDF from the simulation agrees remarkably well with that of Tropical Rainfall Measuring Mission observations. The stochastic scheme also yields a similar magnitude of intraseasonal variability of precipitation to observations and improves the simulation of the eastward propagating intraseasonal signals of precipitation and zonal wind. Application of a stochastic convection parameterization in a global climate model Impacts of the stochastic parameterization on the simulation of tropical precipitation Too light but too frequent precipitation in CAM5 has been largely eliminated
    Keywords: Stochastic Parameterization ; Convection ; Tropical Precipitation ; Cam5
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 5
    In: Meteorologische Zeitschrift, April 2011, Vol.20(2), pp.145-151
    Description: Forecast uncertainty of convective precipitation is influenced by all scales, but in different ways in different meteorological situations. Forecasts of the high resolution ensemble prediction system COSMO-DE-EPS of Deutscher Wetterdienst (DWD) are used to examine the dominant sources of uncertainty of convective precipitation. A validation with radar data using traditional as well as spatial verification measures highlights differences in precipitation forecast performance in differing weather regimes. When the forecast uncertainty can primarily be associated with local, small-scale processes individual members run with the same variation of the physical parameterisation driven by different global models outperform all other ensemble members. In contrast when the precipitation is governed by the large-scale flow all ensemble members perform similarly. Application of the convective adjustment time scale confirms this separation and shows a regime-dependent forecast uncertainty of convective precipitation. German Generell wird die Vorhersagbarkeit konvektiven Niederschlags von Prozessen auf allen Skalen beeinflusst. Im Einzelfall hängt diese aber entscheidend von der meteorologischen Strömungssituation ab. Verschiedene Ursachen der Vorhersagbarkeit, bzw. der Ungenauigkeit der Vorhersage, lassen sich mit Vorhersagen des konvektions-auflösenden Ensemble-Vorhersage-Systems COSMO-DE-EPS des Deutschen Wetterdienstes (DWD) untersuchen. Eine Überprüufung der Vorhersagequalität mit traditionellen und räumlichen Qualitätsmaßen angewandt auf Radarbeobachtungen verdeutlicht die unterschiedliche Güte der Niederschlagsprognosen während verschiedener meteorologischer Strömungssituationen. Falls die Vorhersagbarkeit in erster Linie von klein-skaligen Prozessen beeinflusst wird, haben diejenigen Vorhersagen eine höhere Qualität, die mit derselben physikalischen Störung aber verschiedenen globalen Modellen angetrieben werden. Falls der Niederschlag von der synoptisch-skaligen Strömung dominiert wird, zeigen alle Vorhersagen eine ähnliche Qualität. Die Anwendung der konvektiven Zeitskala bestätigt diese Unterteilung und zeigt die strömungsabhängige Vorhersagbarkeit des konvektiven Niederschlags.
    Keywords: Apennines
    ISSN: 0941-2948
    E-ISSN: 16101227
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  • 6
    Language: English
    In: Quarterly Journal of the Royal Meteorological Society, January 2014, Vol.140(679), pp.480-490
    Description: Predictability of convective precipitation depends on the interaction between synoptic forcing and local‐scale flow characteristics. In order to assess different predictability levels it is desirable to objectively determine the dominant process in a given meteorological situation. Such a measure is given by the convective adjustment time‐scale , a physically based quantity that distinguishes between strong and weak synoptically forced precipitation regimes. By employing the convective adjustment time‐scale diagnostic, forecasts of the convection‐permitting COSMO‐DE ensemble prediction system available for a total of 88 days in summer 2009 are examined. Based on the normalized ensemble spread of hourly precipitation rates, it is shown that the practical predictability of total precipitation is higher during strong large‐scale forcing than during weak forcing. Likewise, the forecast skill, determined using two deterministic scores, is higher during strong than during weak forcing conditions. Different predictability levels of convective precipitation can be revealed by examining distinct sub‐ensembles depending on their source of uncertainty. The impact of variations in the boundary conditions of the driving global models used in the ensemble system is quite insensitive to the prevailing flow regime, while the impact of physics perturbations representing the model error is clearly weather regime dependent, exhibiting a strong contribution only during weakly forced conditions. Then convective precipitation turns out to be especially sensitive to variations in the physics parametrization even at forecast lead times of 12 to 18 hours during the main convective period in the afternoon. Two case‐studies exemplifying the strong and weak forcing regimes are shown, to illustrate how forecast skill varies and the different ensemble members cluster as the precipitation event evolves.
    Keywords: Cosmo Model ; Limited‐Area Ensemble Prediction System ; Sources Of Uncertainty ; Flow Dependence ; Radar Data ; Verification
    ISSN: 0035-9009
    E-ISSN: 1477-870X
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  • 7
    In: Nature, 2002, Vol.415(6873), p.774
    Description: The atmospheric heat transport on Earth from the Equator to the poles is largely carried out by the mid-latitude storms. However, there is no satisfactory theory to describe this fundamental feature of the Earth's climate. Previous studies have characterized the poleward heat transport as a diffusion by eddies of specified horizontal length and velocity scales, but there is little agreement as to what those scales should be. Here we propose instead to regard the baroclinic zone--the zone of strong temperature gradients and active eddies--as a heat engine which generates eddy kinetic energy by transporting heat from a warmer to a colder region. This view leads to a new velocity scale, which we have tested along with previously proposed length and velocity scales, using numerical climate simulations in which the eddy properties have been varied by changing forcing and boundary conditions. The experiments show that the eddy velocity varies in accordance with the new scale, while the size of the eddies varies with the well-known Rhines beta -scale. Our results not only give new insight into atmospheric eddy heat transport, but also allow simple estimates of the intensities of mid-latitude storms, which have hitherto only been possible with expensive general circulation models.
    Keywords: Eddy Conduction ; Thermodynamics ; Eddy Kinetic Energy ; Climate ; Baroclinic Motion ; Ocean-Atmosphere System ; Atmospheric Circulation ; Temperature Differences ; Storms ; Heat Transport ; Meridional Heat Transport ; General Circulation Models ; Mechanics and Thermodynamics (551.513.1) ; Structure, Mechanics and Thermodynamics ; Meteorology ; Rhines Beta -Scale;
    ISSN: 0028-0836
    E-ISSN: 1476-4687
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  • 8
    Language: English
    In: Monthly Weather Review, 10/2014, Vol.142(10), pp.3781-3808
    Description: An idealized convective test bed for the local ensemble transform Kalman filter (LETKF) is set up to perform storm-scale data assimilation of simulated Doppler radar observations. Convective systems with lifetimes exceeding 6 h are triggered in a doubly periodic domain. Perfect-model experiments are used to investigate the limited predictability in precipitation forecasts by comparing analysis schemes that resolve different length scales. Starting from a high-resolution reference scheme with 8-km covariance localization and observations with 2-km resolution on a 5-min cycle, an experimental hierarchy is set up by successively choosing a larger covariance localization radius of 32 km, observations that are horizontally averaged by a factor of 4, a coarser resolution in the calculation of the analysis weights, and a cycling interval of 20 min. After 3 h of assimilation, the high-resolution analysis scheme is clearly superior to the configurations with coarser scales in terms of RMS error and field-oriented measures. The difference is associated with the observation resolution and a larger localization radius required for filter convergence with coarse observations. The high-resolution analysis leads to better forecasts for the first hour, but after 3 hours, the forecast quality of the schemes is indistinguishable. The more rapid error growth in forecasts from the high-resolution analysis appears to be associated with a limited predictability of the small scales, but also with gravity wave noise and spurious convective cells. The latter suggests that the field is in some sense less balanced, or less consistent with the model dynamics, than in the coarser-resolution analysis.
    Keywords: Data Assimilation ; Meteorology ; Storms ; Studies;
    ISSN: 0027-0644
    E-ISSN: 1520-0493
    Source: CrossRef
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  • 9
    Language: English
    In: Journal of the Atmospheric Sciences, 2016, Vol.73(7), p.2893(19)
    Description: Stochastic perturbations allow for the representation of small-scale variability due to unresolved physical processes. However, the properties of this variability depend on model resolution and weather regime. A physically based method is presented for introducing stochastic perturbations into kilometer-scale atmospheric models that explicitly account for these dependencies. The amplitude of the perturbations is based on information obtained from the modelEs subgrid turbulence parameterization, while the spatial and temporal correlations are based on physical length and time scales of the turbulent motions. The stochastic perturbations lead to triggering of additional convective cells and improved precipitation amounts in simulations of two days with weak synoptic forcing of convection but different amounts of precipitation. The perturbations had little impact in a third case study, where precipitation was mainly associated with a cold front. In contrast, an unphysical version of the scheme with constant perturbation amplitude performed poorly since there was no perturbation amplitude that would give improved amounts of precipitation during the day without generating spurious convection at other times.
    Keywords: Perturbations (Astrophysics) – Research ; Weather Forecasting – Analysis
    ISSN: 0022-4928
    E-ISSN: 15200469
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  • 10
    Language: English
    In: Monthly Weather Review, 03/2015, Vol.143(3), pp.813-827
    Description: The growth of small-amplitude, spatially uncorrelated perturbations has been studied in a weather forecast of a 4-day period in the summer of 2007, using a large domain covering Europe and the eastern Atlantic and with explicitly resolved deep convection. The error growth follows the three-stage conceptual model of Zhang et al., with rapid initial growth (e-folding time about 0.5 h) on all scales, relaxing over about 20 h to a slow growth of the large-scale perturbations (e-folding time 12 h). The initial growth was confined to precipitating regions, with a faster growth rate where conditional instability was large. Growth in these regions saturated within 3-10 h, continuing for the longest where the precipitation rate was large. While the initial growth was mainly in the divergent part of the flow, the eventual slow growth on large scales was more in the rotational component. Spectral decomposition of the disturbance energy showed that the rapid growth in precipitating regions projected onto all Fourier components; however, the amplitude at saturation was too small to initiate the subsequent large-scale growth. Visualization of the disturbance energy showed it to expand outward from the precipitating regions at a speed corresponding to a deep tropospheric gravity wave. These results suggest a physical picture of error growth with a rapidly growing disturbance to the vertical mass transport in precipitating regions that spreads to the radius of deformation while undergoing geostrophic adjustment, eventually creating a balanced perturbation that continues to grow through baroclinic instability.
    Keywords: Numerical Simulations ; Gravity Waves ; Baroclinic Instability ; Convection Development ; Precipitation ; Mass Transport ; Instability ; Weather Forecasting ; Ane, Europe ; AE, Atlantic ; Forecasting (551.509.1/.5) ; Extratropical Cyclones ; Mesoscale Processes ; Cloud Resolving Models;
    ISSN: 0027-0644
    E-ISSN: 1520-0493
    Source: CrossRef
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