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  • American Meteorological Society  (32)
  • Lohmann, Ulrike  (32)
  • 1
    Online Resource
    Online Resource
    A Comparison of Surface Observations and ECHAM4-GCM Experiments and Its Relevance to the Indirect Aerosol Effect
    American Meteorological Society ; 2001
    In:  Journal of Climate Vol. 14, No. 6 ( 2001-03), p. 1078-1091
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 14, No. 6 ( 2001-03), p. 1078-1091
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/1520-0442(2001)014〈1078:ACOSOA〉2.0.CO;2
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2001
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 2
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    Optical Properties of Aerosol Particles over the Northeast Pacific
    American Meteorological Society ; 2005
    In:  Journal of Applied Meteorology Vol. 44, No. 8 ( 2005-08-01), p. 1206-1220
    Details
    In: Journal of Applied Meteorology, American Meteorological Society, Vol. 44, No. 8 ( 2005-08-01), p. 1206-1220
    Abstract: In July 2002, atmospheric aerosol measurements were conducted over the northeast Pacific Ocean as part of the Subarctic Ecosystem Response to Iron Enhancement Study (SERIES). The following aerosol quantities were measured: particle number size distribution, particle scattering and backscattering coefficients at three wavelengths, particle absorption coefficient at one wavelength, and size-segregated particle chemical composition. Using Mie theory to calculate the aerosol particle scattering and absorption coefficients from the size distribution and chemical measurements, closure with the measured optical coefficients is not attained. Discrepancies between the calculated and measured scattering and backscattering coefficients are largely a result of the fact that the nephelometer measures scattering only between 7° and 170°. Over 90% of the total scattering and 50% of the backscattering in this study was not measured by the nephelometer because of the missing forward-scattering (0°–7°) and backscattering (170°–180°) segments of the phase function. During this study the majority of the total scattering and backscattering in the marine boundary layer of this region was a result of coarse particles consisting almost entirely of sea salt.
    Type of Medium: Online Resource
    ISSN: 1520-0450 , 0894-8763
    URL: Article
    DOI: 10.1175/JAM2267.1
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2005
    detail.hit.zdb_id: 242493-9
    detail.hit.zdb_id: 2027356-3
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  • 3
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    Local and Remote Impacts of Aerosol Climate Forcing on Tropical Precipitation*
    American Meteorological Society ; 2005
    In:  Journal of Climate Vol. 18, No. 22 ( 2005-11-15), p. 4621-4636
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 18, No. 22 ( 2005-11-15), p. 4621-4636
    Abstract: Mechanisms that determine the direct and indirect effects of aerosols on the tropical climate involve moist dynamical processes and have local and remote impacts on regional tropical precipitation. These mechanisms are examined in a climate model of intermediate complexity [quasi-equilibrium tropical circulation model (QTCM)] forced by prescribed aerosol forcing, which is obtained from a general circulation model (ECHAM4). The aerosol reflection is the dominant aerosol forcing, while the aerosol absorption has complex but much weaker influences on the regional tropical precipitation based on the ECHAM4 aerosol forcing. The local effect associated with aerosols contributes negative precipitation anomalies over convective regions by affecting the net energy flux into the atmospheric column. This net energy flux is controlled by the radiative forcing at the top of the atmosphere on time scales where surface heat flux is near equilibrium, balancing anomalous solar radiation by evaporation, longwave radiation, and sensible heat. Considering the aerosol absorption effect alone, the associated precipitation anomalies are slightly negative but small when surface heat fluxes are near equilibrium. Two effects found in global warming, the upped-ante mechanism and the anomalous gross moist stability mechanism, occur with opposite sign in the aerosol case. Both act as remote effects via the widespread cold tropospheric temperature anomalies induced by the aerosol forcing. In the upped-ante mechanism in global warming, a warm troposphere increases the low-level moisture “ante” required for convection, creating spatially varying moisture anomalies that disfavor precipitation on those margins of convective zones where the mean flow imports air from nonconvective regions. In the aerosol case here, a cool troposphere preferentially decreases moisture in convective regions, creating positive precipitation anomalies at inflow margins. In the anomalous gross moist stability mechanism for the aerosol case, the decrease in moisture in convective regions acts to enhance the gross moist stability, so convection and the associated precipitation are reduced. The partitioning between the aerosol local and remote effects on regional tropical precipitation differs spatially. Over convective regions that have high aerosol concentration, such as the South American region, the aerosol local effect contributes more negative precipitation anomalies than the anomalous gross moist stability mechanism in the QTCM simulations. On the other hand, the remote effect is more important over convective regions with small aerosol concentrations, such as the western Pacific Maritime Continent. Remote effects of midlatitude aerosol forcing have a substantial contribution to tropical anomalies.
    Type of Medium: Online Resource
    ISSN: 1520-0442 , 0894-8755
    URL: Article
    DOI: 10.1175/JCLI3554.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2005
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 4
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    A Case Study in Modeling Low-Lying Inversions and Stratocumulus Cloud Cover in the Bay of Biscay
    American Meteorological Society ; 2014
    In:  Weather and Forecasting Vol. 29, No. 2 ( 2014-04-01), p. 289-304
    Details
    In: Weather and Forecasting, American Meteorological Society, Vol. 29, No. 2 ( 2014-04-01), p. 289-304
    Abstract: Many regional forecasting models struggle to simulate low-lying strong temperature inversions. To understand this apparent deficit for forecast improvements, a case study of a strong inversion occurring in the Bay of Biscay on 27 January 2003 is conducted. The event was characterized by extensive stratocumulus cloud cover beneath an extensive high pressure system in combination with a particularly strong inversion of 10–12 K at an altitude of 500–800 m. Simulations were performed at 2- and 12-km horizontal resolutions, with 60 vertical levels (13 levels within the first 1000 m), and with lead times of 12–72 h. The simulations were validated using in situ radiosonde and satellite data. Besides large-scale subsidence, turbulent vertical mixing is a key dynamical process for the formation of nocturnal inversions. Sensitivities to parameters for vertical mixing (the minimum threshold for eddy diffusivity and the turbulence length scale) are investigated. Results presented herein show the planetary boundary layer (PBL) profiles to be very sensitive to the minimum threshold applied for eddy diffusivity, whereas little sensitivity with respect to the turbulence length-scale parameter was found. PBL moisture and potential temperature θ profiles for hindcasts between 24- and 72-h lead times at both resolutions were adequately simulated. In simulations with an adequate representation of the vertical turbulent exchange, realistic cloud cover was simulated, while too high values of the aforementioned threshold produced a strong underestimation of the cloud cover. These results indicate that a realistic simulation of strong inversions and their associated cloud cover is feasible, provided the vertical turbulent exchange is adequately represented.
    Type of Medium: Online Resource
    ISSN: 0882-8156 , 1520-0434
    URL: Article
    DOI: 10.1175/WAF-D-13-00039.1
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2014
    detail.hit.zdb_id: 2025194-4
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  • 5
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    Aerosol Influence on Mixed-Phase Clouds in CAM-Oslo
    American Meteorological Society ; 2008
    In:  Journal of the Atmospheric Sciences Vol. 65, No. 10 ( 2008-10), p. 3214-3230
    Details
    In: Journal of the Atmospheric Sciences, American Meteorological Society, Vol. 65, No. 10 ( 2008-10), p. 3214-3230
    Abstract: A new treatment of mixed-phase cloud microphysics has been implemented in the general circulation model, Community Atmosphere Model (CAM)-Oslo, which combines the NCAR CAM2.0.1 and a detailed aerosol module. The new treatment takes into account the aerosol influence on ice phase initiation in stratiform clouds with temperatures between 0° and −40°C. Both supersaturation and cloud ice fraction, that is, the fraction of cloud ice compared to the total cloud water in a given grid box, are now determined based on a physical reasoning in which not only temperature but also the ambient aerosol concentration play a role. Included in the improved microphysics treatment is also a continuity equation for ice crystal number concentration. Ice crystal sources are heterogeneous and homogeneous freezing processes and ice multiplication. Sink terms are collection processes and precipitation formation, that is, melting and sublimation. Instead of using an idealized ice nuclei concentration for the heterogeneous freezing processes, a common approach in global models, the freezing processes are here dependent on the ability of the ambient aerosols to act as ice nuclei. Additionally, the processes are dependent on the cloud droplet number concentration and hence the aerosols’ ability to act as cloud condensation nuclei. Sensitivity simulations based on the new microphysical treatment of mixed-phase clouds are presented for both preindustrial and present-day aerosol emissions. Freezing efficiency is found to be highly sensitive to the amount of sulphuric acid available for ice nuclei coating. In the simulations, the interaction of anthropogenic aerosols and freezing mechanisms causes a warming of the earth–atmosphere system, counteracting the cooling effect of aerosols influencing warm clouds. The authors find that this reduction of the total aerosol indirect effect amounts to 50%–90% for the specific assumptions on aerosol properties used in this study. However, many microphysical processes in mixed-phase clouds are still poorly understood and the results must be interpreted with that in mind.
    Type of Medium: Online Resource
    ISSN: 0022-4928 , 1520-0469
    URL: Article
    DOI: 10.1175/2008JAS2430.1
    RVK:
    UA 4800
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2008
    detail.hit.zdb_id: 218351-1
    detail.hit.zdb_id: 2025890-2
    SSG: 16,13
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  • 6
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    Experimental Study of Collection Efficiencies between Submicron Aerosols and Cloud Droplets
    American Meteorological Society ; 2011
    In:  Journal of the Atmospheric Sciences Vol. 68, No. 9 ( 2011-09-01), p. 1853-1864
    Details
    In: Journal of the Atmospheric Sciences, American Meteorological Society, Vol. 68, No. 9 ( 2011-09-01), p. 1853-1864
    Abstract: Collection efficiency E experiments for aerosol particles scavenged by cloud droplets were carried out in the newly built Collision Ice Nucleation Chamber (CLINCH). Pure water droplets having radii between 12.8 and 20.0 μm were allowed to fall freely and to collide in a laminar flow with lithium metaborate particles having radii between 0.05 and 0.33 μm. At the bottom of the chamber, the droplets and the particles captured were collected using a cup impactor. The collected solution was analyzed for the scavenged aerosol mass by inductively coupled plasma mass spectrometry. Evaporation of droplets was taken into account since the relative humidity inside the chamber was below 100%, resulting in final theoretical droplet sizes between 4.2 and 17.6 μm. The resulting experimental measurements were compared with theoretical values to see their correlation. The authors found an experimental trend similar to theory, as well as the “Greenfield gap” at the particle radius of 0.24 μm (E = 0.038) for the smallest cloud droplet size investigated in this study. The experimental values of collection efficiency found herein agree with those from theory within one order of magnitude, similar to previous studies reported in the literature.
    Type of Medium: Online Resource
    ISSN: 0022-4928 , 1520-0469
    URL: Article
    DOI: 10.1175/JAS-D-11-012.1
    RVK:
    UA 4800
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2011
    detail.hit.zdb_id: 218351-1
    detail.hit.zdb_id: 2025890-2
    SSG: 16,13
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  • 7
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    Application and Comparison of Robust Linear Regression Methods for Trend Estimation
    American Meteorological Society ; 2009
    In:  Journal of Applied Meteorology and Climatology Vol. 48, No. 9 ( 2009-09-01), p. 1961-1970
    Details
    In: Journal of Applied Meteorology and Climatology, American Meteorological Society, Vol. 48, No. 9 ( 2009-09-01), p. 1961-1970
    Abstract: In this study, robust parametric regression methods are applied to temperature and precipitation time series in Switzerland and the trend results are compared with trends from classical least squares (LS) regression and nonparametric approaches. It is found that in individual time series statistically outlying observations are present that influence the LS trend estimate severely. In some cases, these outlying observations lead to an over-/underestimation of the trends or even to a trend masking. In comparison with the classical LS method and standard nonparametric techniques, the use of robust methods yields more reliable trend estimations and outlier detection.
    Type of Medium: Online Resource
    ISSN: 1558-8432 , 1558-8424
    URL: Article
    DOI: 10.1175/2009JAMC1851.1
    RVK:
    UA 4547
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2009
    detail.hit.zdb_id: 2227779-1
    detail.hit.zdb_id: 2227759-6
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  • 8
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    The Dynamical Tropopause Location as a Potential Predictor for North Atlantic Tropical Cyclone Activity
    American Meteorological Society ; 2023
    In:  Journal of Climate Vol. 36, No. 8 ( 2023-04-15), p. 2515-2533
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 36, No. 8 ( 2023-04-15), p. 2515-2533
    Abstract: Tropical cyclones are among the most devastating natural phenomena that can cause severe damage when undergoing landfall. In the wake of the poorly forecast 2013 North Atlantic hurricane season, Rossby wave breaking on the 350-K isentropic surface has been linked to tropical cyclone activity measured by the accumulated cyclone energy (ACE). Here, ERA5 data and HURDAT2 tropical cyclone data are used to argue that the latitude of the 2 potential vorticity unit (PVU; 1 PVU = 10 −6 K kg −1 m 2 s −1 ) contour on the 360-K isentropic surface in the western North Atlantic is linked to changes in vertical wind shear and relative humidity during the month of September. A more equatorward position of the 2-PVU contour is shown to be linked to an increase in vertical wind shear and a reduction in relative humidity, as manifested in an increased ventilation index, in the tropical western North Atlantic during September. The more equatorward position is further linked to a reduction in the number of named storms, storm and hurricane days, hurricane lifetime, and number of tropical cyclones making landfall. Changes in genesis location are shown to be of importance for the changes in landfall frequency and hurricane lifetime. In summary, the 2-PVU contour latitude in the western North Atlantic can, therefore, potentially be used as a predictor in seasonal and subseasonal forecasting. Significance Statement Forecasts for the North Atlantic hurricane season are operationally produced. Their aim is to predict the number of tropical cyclones and their total energy throughout the season. This study proposes to include the tropopause latitude in these forecasts, as it is shown to be linked to vertical wind shear and midtropospheric relative humidity in the western tropical North Atlantic. The tropopause latitude is thereby linked to the number of tropical cyclones, their lifetime, and the total energy throughout the season. This link is particularly strong during September.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-22-0479.1
    RVK:
    UA 4548
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2023
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 9
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    GCM Aerosol Radiative Effects Using Geographically Varying Aerosol Sizes Deduced from AERONET Measurements
    American Meteorological Society ; 2003
    In:  Journal of the Atmospheric Sciences Vol. 60, No. 22 ( 2003-11), p. 2747-2764
    Details
    In: Journal of the Atmospheric Sciences, American Meteorological Society, Vol. 60, No. 22 ( 2003-11), p. 2747-2764
    Type of Medium: Online Resource
    ISSN: 0022-4928 , 1520-0469
    URL: Article
    DOI: 10.1175/1520-0469(2003)060〈2747:GAREUG〉2.0.CO;2
    RVK:
    UA 4800
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2003
    detail.hit.zdb_id: 218351-1
    detail.hit.zdb_id: 2025890-2
    SSG: 16,13
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  • 10
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    Formation and Development of Orographic Mixed-Phase Clouds
    American Meteorological Society ; 2017
    In:  Journal of the Atmospheric Sciences Vol. 74, No. 11 ( 2017-11-01), p. 3703-3724
    Details
    In: Journal of the Atmospheric Sciences, American Meteorological Society, Vol. 74, No. 11 ( 2017-11-01), p. 3703-3724
    Abstract: Orographic forcing can stabilize mixed-phase clouds (MPCs), which are thermodynamically unstable owing to the different saturation vapor pressure over liquid water and ice. This study presents simulations of MPCs in orographically complex terrain over the Alpine ridge with the regional model COSMO using a horizontal resolution of 1 km. Two case studies provide insights into the formation of Alpine MPCs. Trajectory studies show that the majority of the air parcels lifted by more than 600 m are predominantly in the liquid phase even if they originate from glaciated clouds. The interplay between lifted and advected air parcels is crucial for the occurrence of MPCs. Within a sensitivity study, the orography is reduced to 80%, which changed both the total barrier height and steepness. The changes in total water path (TWP), liquid water path (LWP), and ice water path (IWP) vary in sign and strength as the affected precipitation does. LWP can experience changes up to 500% resulting in a transformation from an ice-dominated MPC to a liquid-dominated MPC. In further simulations with increased steepness and maintained surface height at Jungfraujoch, TWP experiences a reduction between 25% and 40% during different time periods, which results in reduced precipitation by around 30%. An accurate representation of the steepness and the height of mountains in models is crucial for the formation and development of MPCs.
    Type of Medium: Online Resource
    ISSN: 0022-4928 , 1520-0469
    URL: Article
    DOI: 10.1175/JAS-D-16-0348.1
    RVK:
    UA 4800
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2017
    detail.hit.zdb_id: 218351-1
    detail.hit.zdb_id: 2025890-2
    SSG: 16,13
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