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  • 551.5  (80)
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  • 2020-2022  (104)
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  • 11
    Publikationsdatum: 2021-10-12
    Beschreibung: Surfaces exposed to atmospheric cold temperature and humid environments are prone to ice accretion. Airplanes, electrical power transmission cables, and wind turbines are typical examples for which icing has to be considered. The measurement of the resulting ice shapes is a challenging process. While macroscopic characteristics of the ice geometry can be observed using photography and optical scanning techniques, microscopic measurements are difficult to conduct because grooved surface partially occludes the geometry of chasms. To overcome this optical inaccessibility, we propose a method to carry out detailed high-resolution measurements of the accretion surface with micro-computed tomography. This approach provides a unique visualization of the empty spaces in the feather region. The information obtained by this technique can improve the understanding of ice accretion physics and its computational modeling.
    Schlagwort(e): 551.5 ; atmospheric icing ; ice feather ; icing wind tunnel ; micro-computed tomography
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 12
    Publikationsdatum: 2021-10-12
    Beschreibung: Gravity waves (GWs) are important for coupling the mesosphere to the lower atmosphere during sudden stratospheric warmings (SSWs). Here, a minor SSW is internally generated in a simulation with the upper-atmosphere configuration of the ICOsahedral Nonhydrostatic model. At a horizontal resolution of 20 km the simulation uses no GW drag parameterizations but resolves large fractions of the GW spectrum explicitly, including orographic and nonorographic sources. Consistent with previous studies, the simulated zonal-mean stratospheric warming is accompanied by zonal-mean mesospheric cooling. During the course of the SSW the mesospheric GW momentum flux (GWMF) turns from mainly westward to mainly eastward. Waves of large phase speed (40–80 m s −1) dominate the eastward GWMF during the peak phase of the warming. The GWMF is strongest along the polar night jet axis. Parameterizations of GWs usually assume straight upward propagation, but this assumption is often not satisfied. In the case studied here, a substantial amount of the GWMF is significantly displaced horizontally between the source region and the dissipation region, implying that the local impact of GWs on the mesosphere does not need to be above their local transmission through the stratosphere. The simulation produces significant vertically misaligned anomalies between the stratosphere and mesosphere. Observations by the Microwave Limb Sounder confirm the poleward tilt with height of the polar night jet and horizontal displacements between mesospheric cooling and stratospheric warming patterns. Thus, lateral GW propagation may be required to explain the middle-atmosphere temperature evolution in SSW events with significant zonally asymmetric anomalies.
    Schlagwort(e): 551.5 ; Sudden Stratospheric Warming ; Gravity wave propagation ; Zonal asymmetries ; High-resolution climate model ; Microwave Limb Sounder ; Tilt of polar night jet
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 13
    Publikationsdatum: 2021-10-12
    Beschreibung: Geopotential height measurements from the Aura Microwave Limb Sounder between 9- and 97-km altitudes during 2004–2018 are used to examine long-period (3–20 days) wave activity during the Northern Hemisphere winter and spring, with the primary focus on the response of normal mode Rossby waves in the middle atmosphere to sudden stratospheric warmings (SSWs). Unusually large westward propagating waves with Zonal Wave Number 1 and period ∼10 days are observed at 55° latitude at the stratopause height (∼48 km) and above following final warmings of 2016, 2015, and 2005. In each case, large-amplitude waves are observed for the duration of two to three wave cycles. Characteristics of the waves are in conformity with the second antisymmetric Rossby normal mode of Zonal Wave Number 1, or the quasi-10-day wave. The growth rate of the waves is significantly greater than the classical normal mode in the upper stratosphere (approximately 30–50 km) where instability conditions are met, indicating the amplification or excitation of the waves in that region. The response of the quasi-10-day wave during midwinter SSWs, and also during the spring transition without an SSW, is not as obvious as the wave response during final warmings. The results suggest that not only the occurrence of SSW but also the seasonal timing of SSW is an important factor for the transient variability of the quasi-10-day wave in the middle atmosphere.
    Schlagwort(e): 551.5 ; sudden stratospheric warming ; Rossby waves ; planetary waves ; quasi-10-day wave ; stratosphere ; mesosphere
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 14
    Publikationsdatum: 2021-10-12
    Beschreibung: A realistic simulation of the atmospheric boundary layer (ABL) depends on an accurate representation of the land–atmosphere coupling. Land surface temperature (LST) plays an important role in this context and the assimilation of LST can lead to improved estimates of the boundary layer and its processes. We assimilated synthetic satellite LST retrievals derived from a nature run as truth into a fully coupled, state-of-the-art land–atmosphere numeric weather prediction model. As assimilation system a local ensemble transform Kalman filter was used and the control vector was augmented by the soil temperature and humidity. To evaluate the concept of the augmented control vector, two-day case-studies with different control vector settings were conducted for clear-sky periods in March and August 2017. These experiments with hourly LST assimilation were validated against the nature run and overall, the RMSE of atmospheric and soil temperature of the first-guess (and analysis) were reduced. The temperature estimate of the ABL was particularly improved during daytime as was the estimate of the soil temperature during the whole diurnal cycle. The best impact of LST assimilation on the soil and the ABL was achieved with the augmented control vector. Through the coupling between the soil and the atmosphere, the assimilation of LST can have a positive impact on the temperature forecast of the ABL even after 15 hr because of the memory of the soil. These encouraging results motivate further work towards the assimilation of real satellite LST retrievals.
    Schlagwort(e): 551.5 ; data assimilation ; land–atmosphere coupling ; land surface temperature ; LETKF
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 15
    Publikationsdatum: 2021-10-12
    Beschreibung: Vortex streets formed in the stratocumulus-capped wake of mountainous islands are the atmospheric analogues of the classic Kármán vortex street observed in laboratory flows past bluff bodies. The quantitative analysis of these mesoscale unsteady atmospheric flows has been hampered by the lack of satellite wind retrievals of sufficiently high spatial and temporal resolution. Taking advantage of the cutting-edge Advanced Baseline Imager, we derived kilometer-scale cloud-motion winds at 5-min frequency for a vortex street in the lee of Guadalupe Island imaged by Geostationary Operational Environmental Satellite-16. Combined with Moderate Resolution Imaging Spectroradiometer data, the geostationary imagery also provided accurate stereo cloud-top heights. The time series of geostationary winds, supplemented with snapshots of ocean surface winds from the Advanced Scatterometer, allowed us to capture the wake oscillations and measure vortex shedding dynamics. The retrievals revealed a markedly asymmetric vortex decay, with cyclonic eddies having larger peak vorticities than anticyclonic eddies at the same downstream location. Drawing on the vast knowledge accumulated about laboratory bluff body flows, we argue that the asymmetric island wake arises from the combined effects of Earth's rotation and Guadalupe's nonaxisymmetric shape resembling an inclined flat plate at low angle of attack. However, numerical simulations will need to establish whether or not the selective destabilization of the shallow atmospheric anticyclonic eddies is caused by the same mechanisms that destabilize the deep columnar anticyclones of laboratory flows, such as three-dimensional vertical perturbations due to centrifugal or elliptical instabilities.
    Schlagwort(e): 551.5 ; cloud-motion winds ; vortex street ; GOES-R ; ASCAT ; Karman ; satellite winds
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 16
    Publikationsdatum: 2021-10-11
    Beschreibung: Peatlands are lands with a peat layer at the surface, containing a large proportion of organic carbon. Such lands cover ≈1 000 000 km2 in Europe, which is almost 10% of the total surface area. In many countries, peatlands have been artificially drained over centuries, leading to not only enormous emissions of CO2 but also soil subsidence, mobilization of nutrients, higher flood risks, and loss of biodiversity. These problems can largely be solved by stopping drainage and rewetting the land. Wet peatlands do not release CO2, can potentially sequester carbon, help to improve water quality, provide habitat for rare and threatened biodiversity, and can still be used for production of biomass (“paludiculture”). Wisely adjusted land use on peatlands can substantially contribute to low-emission goals and further benefits for farmers, the economy, society, and the environment.
    Schlagwort(e): 553.21 ; climate protection ; ecosystem services ; greenhouse gas emissions ; paludiculture ; rewetting
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 17
    Publikationsdatum: 2021-10-11
    Beschreibung: The El Niño phase of the El Niño Southern Oscillation (ENSO) is typically associated with below-average cool-season rainfall in southeastern Australia (SEA). However, there is also large case-to-case variability on monthly time-scales. Despite recent progress in understanding the links between remote climate drivers and this variability, the underlying dynamical processes are not fully understood. This reanalysis-based study aims to advance the dynamical understanding by quantifying the contribution of midlatitude weather systems to monthly precipitation anomalies over SEA during the austral winter–spring season. A k-means clustering reveals four rainfall anomaly patterns with above-average rainfall (Cluster 1), below-average rainfall (Cluster 2), above-average rainfall along the East Coast (Cluster 3) and along the South Coast (Cluster 4). Cluster 2 occurs most frequently during El Niño, which highlights the general suppression of SEA rainfall during these events. However, the remaining three clusters with local above-average rainfall are found in ∼52% of all El Niño months. Changes of weather system frequency determine the respective rainfall anomaly pattern. Results indicate significantly more cut-off lows and warm conveyor belts (WCBs) over SEA in El Niño Cluster 1 and significantly fewer in El Niño Cluster 2. In El Niño Cluster 3, enhanced blocking south of Australia favours cut-off lows leading to increased rainfall along the East Coast. Positive rainfall anomalies along the South Coast in El Niño Cluster 4 are associated with frontal rainfall due to an equatorward shift of the midlatitude storm track. Most of the rainfall is produced by WCBs and cut-off lows but the contributions strongly vary between the clusters. In all clusters, rainfall anomalies result from changes in rainfall frequency more than in rainfall intensity. Backward trajectories of WCB and cut-off low rainfall highlight the importance of moist air masses from the Coral Sea and the northwest coast of Australia during wet months.
    Schlagwort(e): 551.5 ; backward trajectories ; clustering ; El Niño ; rainfall decomposition ; rainfall origin ; rainfall variability ; southeastern Australia ; synoptic weather systems
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 18
    Publikationsdatum: 2021-10-11
    Beschreibung: A pronounced warm anomaly occurred at the Peruvian coast in early 2017. This “Coastal Niño” caused heavy rainfalls, leading to flooding in Peru and Ecuador. At the same time, neutral conditions prevailed in the equatorial Pacific. Using observational sea surface temperature data sets and an ocean reanalysis product for the time period 1900 to 2010, previous similar events are investigated. Eighteen coastal warming events without corresponding equatorial Pacific warming are identified. Further analysis shows, however, that only four of these events are not connected to the central equatorial Pacific. All other periods of strong coastal warm anomalies are directly followed or preceded by El Niño-like conditions. The “stand-alone” coastal warming events are characterized by comparatively low equatorial heat content. We thus hypothesize that the depleted heat content in the equatorial Pacific in the wake of the strong 2015/2016 El Niño prevented the warming to spread westward in 2017.
    Schlagwort(e): 551.5 ; El Nino ; Coastal Warming ; Tropical Pacific
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 19
    Publikationsdatum: 2021-10-11
    Beschreibung: Abstract Evaporation—a key process for water exchange between soil and atmosphere—is controlled by internal water fluxes and surface vapor fluxes. Recent studies demonstrated that the dynamics of the water flow in corners determine the time behavior of the evaporation rate. The internal water flux of the porous media is often described by capillary flow assuming complete wetting. Particularly, the crucial influence of partial wetting, that is, the nonlinear contact angle dependency of the capillary flow has been neglected so far. The focus of the paper is to demonstrate that SiO2-surfaces can exhibit contact angles of about 40°. This reduces the internal capillary flow by 1 order of magnitude compared to complete wetting. First, we derived the contact angle by inverse modeling. We conducted a series of evaporation experiments in a 2-D square lattice microstructure connected by lognormal distributed throats. We used an explicit analytical power series solution of the single square capillary model. A contact angle of 38° ± 1° was derived. Second, we directly measured the contact angle of the Si-SiO2 wafer using the Drop Shape Analyzer Krüss 100 and obtained an averaged contact angle of 42° ± 2°. The results support the single square capillary model as an appropriate model for the description of the evaporation process in an ideal square capillary.
    Schlagwort(e): 550.724 ; 551.5 ; evaporation ; experiments
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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  • 20
    Publikationsdatum: 2021-10-11
    Beschreibung: Coupled climate models participating in the CMIP5 (Coupled Model Intercomparison Project Phase 5) exhibit a large intermodel spread in the representation of long-term trends in soil moisture and snow in response to anthropogenic climate change. We evaluate long-term (January 1861 to December 2099) water storage trends from 21 CMIP5 models against observed trends in terrestrial water storage (TWS) obtained from 14 years (April 2002 to August 2016) of the GRACE (Gravity Recovery And Climate Experiment) satellite mission. This is complicated due to the incomplete representation of TWS in CMIP5 models and interannual climate variability masking long-term trends in observations. We thus evaluate first the spread in projected trends among CMIP5 models and identify regions of broad model consensus. Second, we assess the extent to which these projected trends are already present during the historical period (January 1861 to August 2016) and thus potentially detectable in observational records available today. Third, we quantify the degree to which 14-year tendencies can be expected to represent long-term trends, finding that regional long-term trends start to emerge from interannual variations after just 14 years while stable global trend patterns are detectable after 30 years. We classify regions of strong model consensus into areas where (1) climate-related TWS changes are supported by the direction of GRACE trends, (2) mismatch of trends hints at possible model deficits, (3) the short observation time span and/or anthropogenic influences prevent reliable conclusions about long-term wetting or drying. We thereby demonstrate the value of satellite observations of water storage to further constrain the response of the terrestrial water cycle to climate change.
    Schlagwort(e): 551.5 ; GRACE ; CMIP5 ; water storage trends
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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