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- The impact of offshore wind farms on the latent heat flux
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Abstract:  The impact of offshore wind farms on the latent heat fluxPlatis, Andreas; Büchau, Yann; Zuluaga, Sary; Bange, Jens
Meteorologische Zeitschrift, (2023), p. 261 - 277Abstract Unique airborne meteorological in-situ measurements have been used to study the impact of offshore wind farms on the latent heat flux in the marine boundary layer. 22 out of the total 42 flights collected during the WIPAFF project in the years 2016 and 2017 allowed for such an evaluation during various atmospheric conditions. The measurements revealed a significant increase of the vertical latent heat flux, mainly in the upward direction above the offshore wind farm cluster Amrumbank West, Nordsee Ost, Meerwind Süd/Ost or Godewind located in the German Bight. For thermally stable conditions, eight out of nine flights showed an enhanced turbulent latent heat flux downstream of the wind farms at hub height, with an increase of up to +70 W m−2 compared to the undisturbed flow. For flights during neutral thermal conditions, 10 out of 13 cases showed an increase, with the largest difference with respect to undisturbed flow of +600 W m−2 above the wind farm. Our observations suggest that the impact of offshore wind farms on the latent heat flux in the marine boundary layer is more frequent during stable conditions, but more prominent during neutral conditions and may trigger cloud development in the vicinity of the farm.
PubDate: Mon, 18 Sep 2023 00:00:00 +000
- Observed temperature trends in Germany: Current status and communication
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Abstract:  Observed temperature trends in Germany: Current status and communication toolsKaspar, Frank; Friedrich, Karsten; Imbery, Florian
Meteorologische Zeitschrift, (2023), p. 279 - 291Abstract Germany's national meteorological service (Deutscher Wetterdienst) is responsible for monitoring climate and climate change in Germany. Regional mean temperatures for Germany since 1881 are derived from the measurements of the station network on a monthly basis. Here, we discuss methodologies to assess the temperature trend, compare it with the global trends and present an overview on tools that are used for the communication of results. Consistent to the increase in global temperatures, the mean temperature in Germany also increased considerably, especially during the last 5 decades. The linear trend of the period 1881 to 2022 is approx. +1.7 °C, but the warming accelerated within the last decade. Especially in the period since 1971, the warming rate per decade was three times higher (0.38 °C/decade) than for the period since 1881 (0.12 °C/decade). The temperature increase in Germany has been faster than in the global mean (linear trend 1971 to 2022: 0.18 °C per decade; for 1881 to 2022: 0.08° per decade). However, the temperature increase in Germany is comparable to the temperature increase over the global land area (also 0.12 °C/decade since 1881). For the period since 1971 the increase is faster for Germany than for the global land area (0.29 °C/decade). As a result of the accelerated warming, most years within the last decade had annual mean temperatures above the linear trend line (1881–2022) and the decade 2011 to 2020 was +2 °C warmer in Germany than the first 30 years of the analysis period (1881–1910). The 30‑year period 1991 to 2020 was 1.5 °C warmer than the period 1881 to 1910. Several methods are used to communicate the results of the temperature analysis, mainly regular reports, internet portals and social media channels. The German Climate Atlas is a comprehensive interactive web-based tool that provides information on the observed climate in combination with information on model-based projections of future climate change for Germany.
PubDate: Mon, 18 Sep 2023 00:00:00 +000
- Meteorological and air quality measurements in a city region with complex
terrain: influence of meteorological phenomena on urban climate-
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Abstract:  Meteorological and air quality measurements in a city region with complex terrain: influence of meteorological phenomena on urban climateSamad, Abdul; Kiseleva, Olga; Holst, Christopher Claus; Wegener, Robert; Kossmann, Meinolf; Meusel, Gregor; Fiehn, Alina; Erbertseder, Thilo; Becker, Ralf; Roiger, Anke; Stanislawsky, Peter; Klemp, Dieter; Emeis, Stefan; Kalthoff, Norbert; Vogt, Ulrich
Meteorologische Zeitschrift, (2023), p. 293 - 315Abstract On 8 and 9 July 2018 extensive observations were conducted under fair-weather conditions in the German city of Stuttgart and its surroundings. This intensive observation period, part of the four weeks Urban Climate Under Change (UC)2 campaign, intended to provide a comprehensive data set to understand the complex interactions of thermally induced wind systems, vertical turbulent mixing and air pollutant concentration distribution in the atmospheric boundary layer of the city. Stuttgart has a very special and complex topography with a city center located in a basin surrounded by 250 to 300 m higher hills influencing the wind and flow system, reducing the wind speed, and causing inhibited dispersion of air pollutants. Cold air flows from the surrounding plains can penetrate the urban areas and influence the urban climate including the air quality. For investigating these effects with a focus on urban climate, combinations of different measurement platforms and techniques were used, such as in situ stationary and mobile measurements with cars, vertical profiling by means of tethered balloons, radiosondes, a drone, and aircraft observations, remote sensing devices and satellite-based instruments. Numerous atmospheric processes in an urban area regarding boundary layer evolution, inversion, local wind systems, urban heat island, etc. were observed. Some important findings are: Temperature observations provide local information about the warmest areas in the city and about the city and its surroundings. The urban heat island effect was evident from the results of stationary and mobile air temperature measurements as the higher air temperature was measured in the Stuttgart basin compared to its surroundings. Considerable spatio-temporal differences concerning the wind (speed and direction), turbulence and the convective boundary depth are evident. Lower wind speeds were observed during the nighttime and the main wind direction in the Stuttgart valley was measured to be southwest, which carried cold air from the hillsides into the city and pollutants to the windward side of the city into the Neckar valley. The low wind speed favored the accumulation of pollutants in a shallow nocturnal boundary layer close to the surface. During the day, the overall pollutant concentration was reduced by vertical convective mixing. The vertical profile measurements have shown that the applied techniques provided a good overview to understand the vertical characteristics of meteorological parameters and pollutants as well as the stability of the atmosphere and extent of the urban boundary layer. It also showed that the extent of atmospheric mixing determines the dispersion, dilution and mixing of emitted pollutants. Finally, the additional comprehensive air-chemical observations (surface and satellite based) allow an understanding of the diurnal cycle of air pollutants in the atmospheric boundary layer of the city of Stuttgart. Satellite-based observations from Sentinel‑5P/TROPOMI have shown their potential for mapping urban pollution islands and urban pollution plumes even in cities with a complex terrain like Stuttgart. These observations assisted to obtain a comprehensive data set intended for the validation of a novel urban climate model, PALM‑4U.
PubDate: Mon, 18 Sep 2023 00:00:00 +000
- Contrasting temporal dynamics of methane and carbon dioxide emissions from
a eutrophic reservoir detected by eddy covariance measurements-
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Abstract:  Contrasting temporal dynamics of methane and carbon dioxide emissions from a eutrophic reservoir detected by eddy covariance measurementsSpank, Uwe; Bernhofer, Christian; Mauder, Matthias; Keller, Philipp S.; Koschorreck, Matthias
Meteorologische Zeitschrift, (2023), p. 317 - 342Abstract Inland waters are an import source of greenhouse gases for the atmosphere. In particular, the emissions of methane from lakes and reservoirs are suspected of almost offsetting the terrestrial carbon sink. However, the estimates found in the literature are subject to large uncertainties due to both missing data and methodological limitations. In particular, there is a gap of observations in the temperate climate zone, and data are especially scarce from eutrophic waters despite the fact that the emissions increase with the degree of eutrophication. We present data from a eutrophic reservoir in the temperate climate zone, measured continuously with a floating eddy-covariance system. Data from two seasons are analyzed, both starting and ending with spring and autumn mixing, respectively, and including the complete period of summer stratification. During the spring and summer months, clear diurnal patterns for the carbon dioxide fluxes were detected in the eddy covariance data reflecting the interplay between photosynthesis and respiration. However, this daylight-driven oscillation weakened with the onset of autumn and disappeared at the end. In contrast to the carbon dioxide fluxes, the methane fluxes did not show any daytime dynamics. Notwithstanding, distinct seasonal patterns with increasing CH4 emissions over summer appeared. The carbon dioxide balance was about −9.8 and −71.0 g C m−2, in the first and second season, respectively. Thus, based on the eddy covariance measurements, the reservoir was a carbon dioxide sink in both study periods. However, the difference between the two seasons indicates a distinct inter-annual variability. The seasonal methane emissions were 24.0 g C m−2 and 23.2 g C m−2, respectively. Accordingly, the seasonal carbon budget resulted in 15.1 g C m−2 and −47.8 g C m−2, respectively, meaning the reservoir was a carbon source in the first and a sink in the second study period. However, considering the significant higher warming potential of methane and transposing these emissions into carbon dioxide equivalents, the reservoir contributed to the greenhouse potential of the atmosphere in both study periods.
PubDate: Mon, 18 Sep 2023 00:00:00 +000
- Analysis of the different faces of a nocturnal urban heat island
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Abstract:  Analysis of the different faces of a nocturnal urban heat islandGross, Günter
Meteorologische Zeitschrift, (2023), p. 343 - 349Abstract The measurements of an urban-rural observation network in Hannover obtained over almost four years were used to analyse a canopy layer urban heat island (UHIUCL). Especially during the summer months, the UHIUCL was pronounced on numerous nights with maximum values of more than 6 K. We have demonstrated that the choice of rural reference station is important to describe a UHIUCL in detail. The time evolution of UHIUCL differed greatly from night to night and four main types could be identified with comparable frequencies of occurrence. The difference between these types was the appearance of the maximum urban heat island intensity during very different times of the night. The main drivers for the development of a specific type were the rural winds and rural temperature changes caused by turbulent mixing within the near-surface inversion.
PubDate: Mon, 18 Sep 2023 00:00:00 +000
- Linkages between Arctic and Mid-Latitude Weather and Climate: Unraveling
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Abstract:  Linkages between Arctic and Mid-Latitude Weather and Climate: Unraveling the Impact of Changing Sea Ice and Sea Surface Temperatures during WinterJaiser, Ralf; Akperov, M.; Timazhev, A.; Romanowsky, E.; Handorf, D.; Mokhov, I.I.
Meteorologische Zeitschrift, (2023), p. 173 - 194Abstract The study addresses the question, if observed changes in terms of Arctic-midlatitude linkages during winter are driven by Arctic Sea ice decline alone or if the increase of global sea surface temperatures plays an additional role. We compare atmosphere-only model experiments with ECHAM6 to ERA-Interim Reanalysis data. The model sensitivity experiment is implemented as a set of four combinations of sea ice and sea surface temperature boundary conditions. Atmospheric circulation regimes are determined and evaluated in terms of their cyclone and blocking characteristics and changes in frequency during winter. As a prerequisite, ECHAM6 reproduces general features of circulation regimes very well. Tropospheric changes induced by the change of boundary conditions are revealed and further impacts on the large-scale circulation up into the stratosphere are investigated. In early winter, the observed increase of atmospheric blocking in the region between Scandinavia and the Urals are primarily related to the changes in sea surface temperatures. During late winter, we find a weakened polar stratospheric vortex in the reanalysis that further impacts the troposphere. In the model sensitivity study a climatologically weakened polar vortex occurs only if sea ice is reduced and sea surface temperatures are increased together. This response is delayed compared to the reanalysis. The tropospheric response during late winter is inconclusive in the model, which is potentially related to the weak and delayed response in the stratosphere. The model experiments do not reproduce the connection between early and late winter as interpreted from the reanalysis. Potentially explaining this mismatch, we identify a discrepancy of ECHAM6 to reproduce the weakening of the stratospheric polar vortex through blocking induced upward propagation of planetary waves.
PubDate: Mon, 11 Sep 2023 00:00:00 +000
- Influence of data uncertainty on cold season threshold-based climate
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Abstract:  Influence of data uncertainty on cold season threshold-based climate indicesBell, Louisa Marie; Schlünzen, K. Heinke; Sieck, Kevin
Meteorologische Zeitschrift, (2023), p. 195 - 206Abstract Climate indices are used to reduce the complex climate system and its changes to simple measures. The data basis – whether observational data or climate model data – to which the climate indices are applied, is usually subject to uncertainties. For threshold-based climate indices, the data uncertainty influences the threshold value, and, hence, the uncertainty can influence the values for the climate index. What the actual impacts of these uncertainties are on threshold-based climate indices is examined in this paper. The focus is not only on the climate model uncertainty, but also on the observational data uncertainty. The general sensitivity of each of the chosen climate indices to arbitrary changes in the threshold is studied. This shows a higher sensitivity of indices assessing extremes (ice days, heavy precipitation days) to changes in the threshold than indices that integrate a quantity over a given time interval (coldsum, consecutive days). For assessing an ensemble of climate model data with respect to their ability to reproduce the index values for current climate, the reference data uncertainty is applied to the chosen threshold-based climate indices by changing their threshold value by its corresponding uncertainty. It is shown that the climate model uncertainty can be within the range of the reference data uncertainty. When using threshold-based climate indices to assess changes in future climate periods, uncertainties should always be taken into account and ideally corrected in an appropriate way. This is especially important for indices that assess extremes.
PubDate: Mon, 11 Sep 2023 00:00:00 +000
- Coastal horizontal wind speed gradients in the North Sea based on
observations and ERA5 reanalysis data-
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Abstract:  Coastal horizontal wind speed gradients in the North Sea based on observations and ERA5 reanalysis dataCañadillas, Beatriz; Wang, Shuhan; Ahlert, Yasmin; Djath, Bughsin'; Barekzai, Mares; Foreman, Richard; Lampert, Astrid
Meteorologische Zeitschrift, (2023), p. 207 - 228Abstract The transition from land to sea affects the wind field in coastal regions. From the perspective of near-coastal offshore wind farms, the coastal transition complicates the task of energy resource assessment by, for example, introducing non-homogeneity into the free wind field. To help elucidate the matter, we quantify the average horizontal wind speed gradients at progressively increasing distances from the German coast using two years of hourly ERA5 reanalysis data, and further describe the dependence of wind speed gradients on the measurement height, atmospheric stability, and season. A vertical wind lidar located on Norderney Island near the German mainland acts as our observational reference for the ERA5 data, where a good agreement ( R 2 = 0 . 9 3 $R^2 =\nobreak 0.93$ ) is found despite the relatively coarse ERA5 data resolution. Interestingly, the comparison of lidar data with the higher-resolution Weather Research and Forecasting (WRF) mesoscale model yields good but relatively weaker agreement ( R 2 = 0 . 8 5 $R^2 =\nobreak 0.85$ ). The ERA5 data reveal that, for flow over the North Sea originating from the German mainland from the south, the wind speed at 10 m (110 m) above sea level increases by 30 % (20 %) some 80 km from the coast on average, and by 5 % at larger heights. An increased stratification increases the horizontal wind speed gradient at 10 m above sea level but decreases it at 110 m. Case studies using satellite and flight measurements are first analyzed to help reveal some of the underlying mechanisms governing horizontal wind speed gradients, including cases of decreasing wind speed with increasing distance from the coast, in which stable flow of warm air over the colder sea leads to an overall deceleration of the flow. The accuracy of offshore resource assessment appears to profit from utilising the horizontal wind speed gradient information contained in ERA5 reanalysis data.
PubDate: Mon, 11 Sep 2023 00:00:00 +000
- Evaluating Bunkers' storm motion of hail-producing supercells and their
storm-relative helicity in Germany-
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Abstract:  Evaluating Bunkers' storm motion of hail-producing supercells and their storm-relative helicity in GermanyTonn, Mathis; Wilhelm, Jannik; Kunz, Michael
Meteorologische Zeitschrift, (2023), p. 229 - 243Abstract This paper presents a statistical analysis of the motion of hail-producing supercells in Germany based on data from a radar-based cell detection and tracking algorithm and a mesocyclone detection algorithm. The parameterization of supercell motion by Bunkers et al. (2000), originally developed using storm data from the United States, is evaluated regarding its applicability in Central Europe, where storm environments have other dynamic and thermodynamic characteristics owing to different geographical features. As a first step, the motion of 354 observed supercells in the warm season (April to September) 2013–2016 is compared to the motion obtained with the original parameterization. The cells are classified as right-moving or left-moving supercells due to their motion direction with regard to the vertical wind shear of the environment, which is calculated using high-resolution model analyses. Afterwards, the accuracy of the parameterization is checked for both motion classes, as well as for classifications according to the lifetime, track length, and severity proxies of the cells. Clear differences between observed and parameterized motion are obtained for all categories, calling for an adjustment of the parameterization in a second step. This adjusted parameterization improves the storm motion estimation for most of the storm categories. A better storm motion estimation improves the calculation of storm-relative helicity, enabling a more reliable nowcasting and forecasting of supercell potential.
PubDate: Mon, 11 Sep 2023 00:00:00 +000
- Actual versus geostrophic wind: statistics from 12‑year measurements at
the 280 m high Hamburg Weather Mast-
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Abstract:  Actual versus geostrophic wind: statistics from 12‑year measurements at the 280 m high Hamburg Weather MastBrümmer, Burghard
Meteorologische Zeitschrift, (2023), p. 245 - 258Abstract The geostrophic wind, representing the horizontal pressure gradient as driving force of motion, is often taken as a first guess of the actual wind. There is, however no fixed relation between them but depends on various influencing factors. How good is this first guess i.e. to which extent do these factors change the actual versus geostrophic wind relation' A 12‑year set of six-hourly (00, 06, 12, 18 UT) geostrophic ( U g $U_{\text{g}}$ ) wind data taken from the ERA-Interim model and actual ( U $U$ ) wind data measured at five levels (10, 50, 110, 175, 250 m) at the Hamburg Weather Mast, Germany, is used to statistically study the dependence of the speed ratio U ∕ U g $U/U_{\text{g}}$ and the angle difference α g - α $\alpha_{\text{g}}-\nobreak\alpha$ on various influence parameters: height z $z$ above ground, geostrophic wind speed U g $U_{\text{g}}$ , thermal wind U therm $U_{\text{therm}}$ , surface roughness z 0 $z_{0}$ , and day-night stratification differences. The actual wind has a Weibull-like frequency distribution (FD) with systematically changing parameters from lower to upper levels. In contrast, the U g $U_{\text{g}}$ ‑FD has the same constant Weibull-like shape at all levels. This does not imply that U g $U_{\text{g}}$ is constant with height, but that the various configurations of thermal wind almost balance. The U therm $U_{\text{therm}}$ ‑FD itself is Weibull-like. The all-times FDs of U ∕ U g $U/U_{\text{g}}$ ( α g - α $\alpha_{\text{g}}-\nobreak\alpha$ ) peak at 0.25 (47°) at 10 m and gradually increase (decrease) to 0.77 (17°) at 250 m. The U ∕ U g $U/U_{\text{g}}$ ratio decreases systematically with increasing U g $U_{\text{g}}$ towards height-staggered asymptotic limits for U g > 3 0 m/s $U_{\text{g}}>\nobreak 30\,\text{m/s}$ . With respect to the thermal wind influence, cold-air advection (CAA) causes on average 1–1.5 m/s larger U values accompanied with 12° less wind turning between 10 m and 250 m than warm-air advection (WAA) for the same low-level U g $U_{\text{g}}$ forcing. The z 0 $z_{0}$ values around the Hamburg Weather Mast vary between 0.3 and 1.1 m and lead to 0.1 differences in the U ∕ U g $U/U_{\text{g}}$ ratio. Vertical stratification has the largest impact on U ∕ U g $U/U_{\text{g}}$ and α g - α $\alpha_{\text{g}}-\nobreak\alpha$ . Stable stratification developing during n...
PubDate: Mon, 11 Sep 2023 00:00:00 +000
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