Search

Geomorphic and sedimentologic data indicate that the climate of today's hyper-arid Atacama Desert (northern Chile) was more humid during the mid-Pliocene to Late Pliocene. The processes, however, leading to increased rainfall in this period are largely unknown. To uncover these processes we use both global and regional kilometre-scale model experiments for the mid-Pliocene (3.2 Ma). We found that the PMIP4–CMIP6 (Paleoclimate Modelling Intercomparison Project–Coupled Model Intercomparison Project) model CESM2 (Community Earth System Model 2) and the regional model WRF (Weather Research and Forecasting) used in our study simulate more rainfall in the Atacama Desert for the mid-Pliocene in accordance with proxy data, mainly due to stronger extreme rainfall events in winter. Case studies reveal that these extreme winter rainfall events during the mid-Pliocene are associated with strong moisture conveyor belts (MCBs) originating in the tropical eastern Pacific. For present-day conditions, in contrast, our simulations suggest that the moisture fluxes rather arise from the subtropical Pacific region and are much weaker. A clustering approach reveals systematic differences between the moisture fluxes in the present-day and mid-Pliocene climates, both in strength and origins. The two mid-Pliocene clusters representing tropical MCBs and occurring less than 1 d annually on average produce more rainfall in the hyper-arid core of the Atacama Desert south of 20∘ S than what is simulated for the entire present-day period. We thus conclude that MCBs are mainly responsible for enhanced rainfall during the mid-Pliocene. There is also a strong sea-surface temperature (SST) increase in the tropical eastern Pacific and along the Atacama coast for the mid-Pliocene. It suggests that a warmer ocean in combination with stronger mid-tropospheric troughs is beneficial for the development of MCBs leading to more extreme rainfall in a +3 ∘C warmer world like in the mid-Pliocene.

Land degradation is a cause of many social, economic, and environmental problems. Therefore identification and monitoring of high-risk areas for land degradation are necessary. Despite the importance of land degradation due to wind and water erosion in some areas of the world, the combined study of both types of erosion in the same area receives relatively little attention. The present study aims to create a land degradation map in terms of soil erosion caused by wind and water erosion of semi-dry land. We focus on the Lut watershed in Iran, encompassing the Lut Desert that is influenced by both monsoon rainfalls and dust storms. Dust sources are identified using MODIS satellite images with the help of four different indices to quantify uncertainty. The dust source maps are assessed with three machine learning algorithms encompassing the artificial neural network (ANN), random forest (RF), and flexible discriminant analysis (FDA) to map dust sources paired with soil erosion susceptibility due to water. We assess the accuracy of the maps from the machine learning results with the area under the curve (AUC) of the receiver operating characteristic (ROC) metric. The water and aeolian soil erosion maps are used to identify different classes of land degradation risks. The results show that 43 % of the watershed is prone to land degradation in terms of both aeolian and water erosion. Most regions (45 %) have a risk of water erosion and some regions (7 %) a risk of aeolian erosion. Only a small fraction (4 %) of the total area of the region had a low to very low susceptibility for land degradation. The results of this study underline the risk of land degradation for in an inhabited region in Iran. Future work should focus on land degradation associated with soil erosion from water and storms in larger regions to evaluate the risks also elsewhere.

A challenge of an energy system that nowadays more strongly depends on wind power generation is the spatial and temporal variability in winds. Nocturnal low-level jets (NLLJs) are typical wind phenomena defined as a maximum in the vertical profile of the horizontal wind speed. A NLLJ has typical core heights of 50–500 m a.g.l. (above ground level), which is in the height range of most modern wind turbines. This study presents NLLJ analyses based on new observations from Doppler wind lidars. The aim is to characterize the temporal and spatial variability in NLLJs on the mesoscale and to quantify their impacts on wind power generation. The data were collected during the Field Experiment on Submesoscale Spatio-Temporal Variability (FESSTVaL) campaign from June to August 2020 in Lindenberg and Falkenberg (Germany), located at about 6 km from each other. Both sites have seen NLLJs in about 70 % of the nights with half of them lasting for more than 3 h. Events longer than 6 h occurred more often simultaneously at both sites than shorter events, indicating the mesoscale character of very long NLLJs. Very short NLLJs of less than 1 h occurred more often in Lindenberg than Falkenberg, indicating more local influences on the wind profile. We discussed different meteorological mechanisms for NLLJ formation and linked NLLJ occurrences to synoptic weather patterns. There were positive and negative impacts of NLLJs on wind power that we quantified based on the observational data. NLLJs increased the mean power production by up to 80 % and were responsible for about 25 % of the power potential during the campaign. However, the stronger shear in the rotor layer during NLLJs can also have negative impacts. The impacts of NLLJs on wind power production depended on the relative height between the wind turbine and the core of the NLLJ. For instance, the mean increase in the estimated power production during NLLJ events was about 30 % higher for a turbine at 135 m a.g.l. compared to one at 94 m a.g.l. Our results imply that long NLLJs have an overall stronger impact on the total power production, while short events are primarily relevant as drivers for power ramps.

European countries are increasing their share of power production from wind and solar energy to mitigate climate change. Also the relative contributions from PV and wind power production in Europe change over time. At present, the installed capacity of photovoltaic (PV) power for all of Europe is smaller than that of wind power with a ratio of 3:4. Future scenarios for the installations of PV and wind power capacities for 2050 suggest that this ratio will substantially change. Namely, the PV power capacity might exceed the wind power capacity with a ratio of 3:2 to 2:1. We test the hypothesis that the weather dependency of anomalies in the PV plus wind power production will change in the future compared to today. Specifically, we examine which synoptic weather patterns are associated with anomalies in the PV plus wind power production for the present and future installed capacities in Europe. To that end, we developed a renewable energy model for the installed capacity of 2019 and 2050. This model allows us to simulate hourly PV and wind power production at 6 km horizontal resolution for all of Europe. We analyze the weather dependency of power-production anomalies by pairing our model output with results of the classification of weather patterns from the German Weather Service. Our results highlight similar weather patterns associated with positive anomalies in the hourly PV plus wind power production for the 2019 and 2050 installation, namely weather patterns with prevailing westerly winds. However, weather patterns associated with negative anomalies strongly change between the two installations. We also assess the dependency of the results on the duration of the production anomalies. Particularly for long production anomalies, the associated weather patterns are different for the 2019 and 2050 installation. One exception is the weather pattern Anticyclonic Southeasterly that is associated with the lowest 10-day power production in Europe for both 2019 and 2050. Regionally, weather patterns have different impacts on different regions in Europe, when comparing the associated patterns between the 2019 and 2050 installation. For instance, anomalously low power production differ for the Iberian peninsula and Southeastern Europe when the two installed capacities are compared. Taken together, our study gives a systematic overview on changes in the weather dependency of anomalies in the mix of PV and wind power between 2019 and 2050.

The Gobi and the Taklamakan Desert are the two main dust source regions in East Asia. Extra-tropical cyclones are known as atmospheric driver for severe dust outbreaks in East Asia. Although previous studies show that dust storm frequency in Northern China have decreased associated with a decrease in near-surface wind speed, a severe dust storm occurred in mid-March 2021. This exceptional dust storm was driven by a Mongolian extra-tropical cyclone and had adverse socio-economic and health impacts. The aim of our study is to investigate the atmospheric dynamics, dust-aerosol contributions from the Gobi Desert and the Taklamakan Desert, as well as dust emission mechanisms involved in the event. We use ground-based observations from Chinese observational networks, satellite images from MODIS, data from ERA5 re-analysis, CAMS forecasts, and MERRA-2 re-analysis. The passage of the Mongolian cyclone first induced high dust-emitting winds along its cold front. The maximum wind speeds at 10m a.g.l. over the Gobi Desert exceeded the 99th percentile of the 30-year climatology (1992-2021) for March by around 6 ms−1 . The dust aerosols were emitted by these exceptionally strong near-surface winds and transported southeastwards along with the passage of the frontal system of the Mongolian cyclone from the afternoon of 14th March to the morning of 15th March 2021. Hence, high atmospheric PM10 concentrations were first recorded in Northern China on 15th March. As a consequence of the associated poor air quality caused by the high PM10 concentrations, 19 out of 218 stations recorded the lowest atmospheric visibility for March since the past 30 years. The passage of the Mongolian cyclone then led to a cold air intrusion into the Taklamakan Desert from the afternoon of the 15th onward, which was a few hours after the dust emissions in the Gobi Desert. The cold air intrusion with the associated near-surface temperature inversion was favourable for the formation of Nocturnal Low-Level Jets (NLLJs), which are known as an important mechanism for dust emissions in the dust source regions (e.g., East Asia and North Africa). By comparing the NLLJs from radiosonde observations and an automated detection algorithm applied to ERA5, stronger NLLJs were seen in the Taklamakan Desert in the mornings of 16, 17, and 18 March. The NLLJs breakdown during the morning hours led to sufficiently strong dust-emitting winds in this desert. Consequently, dust emissions are simulated in the mornings of 16, 17, and 18 March 2021 by both CAMS forecasts and MERRA-2 re-analysis. The impacts of the dust aerosols from the Taklamakan Desert were, however, limited to the west of China, supported by spatio-temporal distributions of station observations of the atmospheric PM10 concentrations and visibility.

Despite the importance of low-level jets (LLJ), their driving mechanisms are not well understood largely due to a shortage of suitable observational data. The classical description for LLJ follows the concept of inertial oscillations at night (NLLJ). Their development is associated with the nocturnal decoupling of winds from the surface friction due to the formation of a near-surface temperature inversion. However, LLJs have also been connected to convectively generated Cold Pools (CP) in kilometre-scale model. CPs are mesoscale areas of cool and dense air formed through convective downdrafts underneath precipitating clouds. Data from the Field Experiment on Submesoscale Spatio-Temporal Variability (FESSTVal) gave us the unique opportunity to test the hypothesis that LLJ formation is also connected to CP passages. We used measurements from three Doppler LIDAR instruments located about 6 km apart from each other, a microwave radiometer and radiosondes for atmospheric profiling, and a large and dense network of surface measurements for the CP detection. During the three-month long field experiment, about 4.7% of all identified LLJ profiles were connected to a CP event (CPLLJ). The average length of CPLLJs was almost two hours. The core of CPLLJ had a mean wind speed of 7 ms−1 and a mean height of 207 m. Using Doppler LIDAR also allowed us to look at wind gusts in the core of the CPLLJs. We measured wind gust of up to 17.5 ms−1 in their core, which exceeds the maximum gust of 15 ms−1 in NLLJs. Close to the surface, the wind speed differences between CPLLJs and NLLJs were even larger than in the core. Most measured CPLLJs appeared at the time of the passage of the CP front and lasted not long after the front has passed, with an interesting exception of a six-hour long CPLLJ during daytime on 29 June 2022. In wind and temperature profiles, we clearly see density currents reaching the experiment site paired with the appearance of strong LLJ profiles. After the passage of the CP front, relatively weaker LLJ profiles were seen. The measurements show that the CP favoured the development of a stably stratified near-surface layer. In a first moment, when the CP front reached the site, there was a mean cooling between the surface up to at least 400 m a.g.l.. After that, the layers bellow 200 m a.g.l. continued to cool, forming a temperature inversion, similar to what one would expect from nocturnal radiative cooling. Radiosondes indicate the typical daytime unstable conditions at the surface and a neutral stratification in the well-mixed boundary layer before the CP arrived. At the time of the CP passage, unstable stratification was seen over a deeper layer followed by the development of a stable stratification in the two hours after the front passed. These conditions led to the formation of a strong and long-lived CPLLJ during daytime. The observations from the FESSTVaL campaign gave first robust evidence that CPs can favour reduced frictional coupling of the wind field to the surface as a prerequisite for generating LLJs.

We present the first observational evidence for convectively generated cold pools (CP) as driving mechanism for low-level jets (LLJ). Our findings are based on a unique campaign dataset that allowed us to perform a systematic assessment of the process. During the three-month campaign in Germany, 4.7\% of all identified LLJ profiles were connected to a CP (CPLLJ). Most measured CPLLJs appeared with the CP front and lasted for up to two hours. Moreover, we have observed a CP favouring the formation of a several-hours long LLJ. In that case, a strong LLJ and cooling of the atmosphere between the surface and at least 400\,m a.g.l. were seen when the density current reached the measurement site. The development led to the formation of a near-surface temperature inversion during daytime as a prerequisite for the LLJ, not unlike the mechanism of nocturnal LLJs.

Geomorphic and sedimentologic data indicate that the climate of today’s hyper-arid Atacama Desert (Northern Chile) was more humid during the Mid-to Late Pliocene. The processes, however, leading to increased rainfall in this period are largely unknown. To uncover these processes we use both global and regional kilometre-scale model experiments for the mid-Pliocene (3.2 Ma BP). We found that the PMIP4-CMIP6 model (CESM2) and the regional model (WRF) used in our study simulate more rainfall in the Atacama Desert for the mid-Pliocene in accordance to proxy data, mainly due to stronger extreme rainfall events in winter. Case studies reveal that these extreme winter rainfall events during the mid-Pliocene are associated with strong moisture conveyer belts (MCBs) originating in the tropical East Pacific. For present-day conditions, in contrast, our simulations suggest that the moisture fluxes rather arise from the subtropical Pacific region and are much weaker. A clustering approach reveals systematic differences between the moisture fluxes in the present-day and mid-Pliocene climates, both in strength and origins. The two mid-Pliocene clusters representing tropical MCBs and occurring less than one day per year on average produce more rainfall in the hyper-arid core of the Atacama Desert south of 20° S than what is simulated for the entire present-day period. We thus conclude that MCBs are mainly responsible for enhanced rainfall during the mid-Pliocene. There is also a strong SST increase in the tropical East Pacific and along the Atacama coast for the mid-Pliocene. It suggests that a warmer ocean in combination with stronger mid-tropospheric troughs is beneficial for the development of MCBs leading to more extreme rainfall in a +3 K warmer world like in the mid-Pliocene.

Fundamental research addressing the challenges of weather forecasts and climate monitoring is required in order to steadily improve the quality of meteorological and climatological services. However, the potential of transferring results from this research into operations increases significantly if carried out in collaboration with national meteorological and hydrometeorological services (NMHS). Out of this motivation and experience, the German “Hans-Ertel-Centre for Weather Research” (HErZ) has been established in 2011 as a virtual competence centre. HErZ develops and organizes a lasting national consortium along common objectives between the German Meteorological Service (DWD), German universities and scientific institutes to accelerate innovation in DWD’s weather, climate and environmental services. HErZ carries out excellent research, strengthens university education and promotes early career scientists, ensures personnel and strategical development, and facilitates the establishment and success of new cooperation, also outside of Germany. Moreover, HErZ grants access to DWD’s infrastructure, including (global) observation data and prediction models. With the strong support of the German Federal Ministry of Digital and Transport, HErZ was able to overcome the difficulties within initiating a cooperation between partners of different institutional communities and established an unprecedented sustainable and national transdisciplinary network. The current research and objectives of HErZ cover the whole value chain expected from this year’s EMS Annual Meeting’s additional focus on “Connecting Communities”: Bringing together fundamental and applied research, observations with prediction models, research institutions with NHMS, and developers with end-users. This intertwined and progressive structure in science and communication ultimately results in acquiring value for society as a whole. Establishing and managing HErZ and its scientific, strategic and structural goals is challenging. The complex tasks of achieving excellence in science and long-term success require a suitable balance between demands and needs of all institutions involved as well as an independent and strong governance by national and international high-level experts. In the future, HErZ must respond to new societal challenges in a targeted but flexible manner with new topics and competences, national and international partners, and a strengthened dialog with stakeholders and users. With this contribution, we would like to share with the audience the establishment, structure and peculiarities of HErZ, the lessons learnt from a transdisciplinary consortium connecting NHMS, universities and research institutes, and our visions towards future seamless science and services.

Solar radiation received at the Earth's surface (Rs) is comprised of two components, the direct radiation (Rd) and the diffuse radiation (Rf). Rd, the direct beam from the sun, is essential for concentrated solar power generation. Rf, scattered by atmospheric molecules, aerosols, or cloud droplets, has a fertilization effect on plant photosynthesis. But how Rd and Rf change diurnally is largely unknown owing to the lack of long-term measurements. Taking advantage of 22 years of homogeneous hourly surface observations over China, this study documents the climatological means and evolutions in the diurnal cycles of Rd and Rf since 1993, with an emphasis on their implications for solar power and agricultural production. Over the solar energy resource region, we observe a loss of Rd which is relatively large near sunrise and sunset at low solar elevation angles when the sunrays pass through the atmosphere on a longer pathway. However, the concentrated Rd energy covering an average 10-hr period around noon during a day is relatively unaffected. Over the agricultural crop resource region, the large amounts of clouds and aerosols scattering more of the incoming light result in Rf taking the main proportion of Rs during the whole day. Rf resources and their fertilization effect in the main crop region of China further enhances since 1993 over almost all hours of the day. Key Points: - The loss of direct radiation over China since 1993 is relatively large at sunrise and sunset with little effect on solar power generation - The diffuse component dominates solar radiation normally near sunrise and sunset, but for the whole day over the main sown area of China - The diffuse fraction is further enhanced in the main sown area of China over almost all hours of the day since 1993

The Atacama Desert in Northern Chile is considered to be the driest warm desert on Earth. However, its hyper-aridity was punctuated by more humid phases in the past. A prominent paleoclimate period of increased rainfall is the warm Pliocene (ca. 5.3 to 2.6 Ma before present). The processes leading to this rainfall increase are not well understood. While recent studies propose regions Southwest or East of the Atacama Desert as moisture sources for enhanced rainfall, the tropical South-eastern Pacific has so far been overlooked as a potential source. The goal of this study is to identify potential key drivers for the increased rainfall in the Pliocene. With this aim, we have downscaled PMIP4-CMIP6 mid-Pliocene (3.2Ma) and historical (1985-2014) experiments using the regional climate model WRF with a spatial resolution of 10 km. The mid-Pliocene WRF simulation exhibits increased mean annual rainfall in the hyper-arid core of the Atacama Desert when compared with the historical period consistent with paleo-records. This increase can be attributed to frequent intense rainfall events in austral winter during the mid-Pliocene, often associated with strong upper-level moisture conveyer belts (MCBs) originating in the tropical Southeast Pacific in front of mid-tropospheric troughs. In present-day climate, such MCBs are much weaker and mostly originate from the relatively dry subtropics producing less moisture advection. Our clustering of upper-level moisture fluxes uncovers systematic differences between MCBs in the mid-Pliocene and the present-day climate: for the mid-Pliocene we found clusters of strong MCBs from the tropical Pacific, that do not occur under present-day conditions. This is due to stronger troughs off the Atacama Desert paired with warmer sea-surface temperatures in the tropical Eastern Pacific and along the northwest coast of South America during the mid-Pliocene, which are favourable for the development of MCBs. The winter rainfall amount associated with MCBs in the mid-Pliocene far exceeds the present-day total rainfall amount. We therefore conclude that tropical MCBs are a key driver for increased rainfall in the Atacama Desert and should be assessed for future climate. This will require kilometre-scale simulations to resolve the dynamical processes involved.

Photovoltaic power production strongly depends on meteorological conditions. Influencing factors are the air temperature and wind speed at the photovoltaic module, as well as changes in irradiance caused by humidity, clouds and aerosols. Here we analyse the global photovoltaic (PV) power potential and compare the changes therefore between a historical time period (1985 to 2014) and possible futures (SSP1-2.6, SSP2-4.5 and SSP5-8.5) at the end of the 21st century (2071-2100). To that end, we use an ensemble of more than 12 CMIP6 models with interactive aerosol schemes and estimate the potential photovoltaic power generation following Crook et al. (2011). We calculate the change in the PV power potential associated with surface irradiance, temperature, and wind speed between the future and the past, and divide the change in PV power generation into contributions from changes in temperature, cloud cover, humidity, aerosols and wind speed.Our results point to a global decrease in future PV power potentials due to the rising air temperatures and the associated increase of humidity by -1.2% to more than -3.5% depending on the future scenario. The contribution from changes in cloud cover and aerosols have heterogeneous spatial patterns, with typically stronger influences in SSP5-8.5 compared to SSP1-2.6, e.g. for the clouds. The contribution by clouds is mostly positive, but has a negative effect for PV power potential in the pole regions, over India, and equatorial oceans. The models also show a large spread in their contributions from clouds, especially over Europe and North America. The future change in the contributions from aerosols has beneficial effects for PV power production over Europe and China in all scenarios assesses, but over South-America, Australia and Africa, the contributions from aerosols are positive in SSP1-2.6 but negative in SSP5-8.5 owing to the different behaviour in anthropogenic aerosols and of natural desert-dust emissions. Our study highlights the impacts of CMIP6 model uncertainty in the future development of aerosol burden, the magnitude of future warming, and the unclear response of clouds to warming on estimating future PV power potentials.

Atmospheric stability is a measure of atmospheric status which determines whether thermodynamically perturbed air will rise, sink, or be neutral. Atmospheric stability has a major impact on the evolution of wind turbine wakes and thus on the yield and performance of offshore wind parks. For estimations of wind park power output and for improving analyses of offshore wind park wakes, a crucial parameter was found to be profiles of atmospheric temperature and stability metrics. Atmospheric temperature profiles can be measured in-situ by balloon-borne sensors, but also estimated from the ground using remote sensing observations.Ground-based microwave radiometer (MWR) units operating in the 22-30 GHz and 50-60 GHz bands are commonly used to estimate atmospheric temperature and humidity profiles. A handful of MWR profiling types are nowadays available as off-the-shelf commercial products, and a MWR network is currently being established in the framework of EUMETNET E-PROFILE programme (Rüfenacht et al., 2021). This presentation reviews the stability metrics useful for monitoring wind park performances and provides a quantitative assessment of the value of MWR observations to estimate these stability metrics from near surface, either over land or ocean. Results from three different MWR instruments, representing the most common available on the market, will be presented, as obtained during at least three field experiments, both onshore and offshore.This contribution presents the main outcomes of the Radiometry and Atmospheric Profiling (RAP) scoping study, carried in the framework of the COST Action PROBE (https://www.probe-cost.eu/) and funded by Carbon Trust and the partner companies of the Off-shore Wind Accelerator (OWA) program: (in alphabetical order) EnBW, Equinor, Orsted, RWE, Scottish Power Renewables, Shell, SSE Renewables, Total Energies, Vattenfall.

Nocturnal low-Level Jets (NLLJ) are maxima in the vertical profile of the horizontal wind speed in the lowest hundreds of meters of the troposphere. NLLJ impacts have been noted in many research fields and applications, e.g., air traffic, forest fire propagation, aerosol transport, and wind power production. The present study assesses NLLJs in ERA5 reanalysis of ECMWF in comparison to the predecessor product ERA-Interim and a novel observational dataset for Lindenberg (Germany) developed by DWD. The observational reference data is a composite of measurements for the years 2014 to 2018 from a meteorological tower limited to levels below 100 m a.g.l. and from a Doppler Wind Lidar for data above 100 m. The NLLJs are evaluated in three (six) hourly data from ERA5 (ERA-Interim) against the observations by applying an automated detection algorithm for NLLJs. Specifically, the NLLJ frequency of occurrence, core height and wind speed are statistically analysed and compared. The results for 2014 so far show an improvement in the NLLJ occurrence in the most recent reanalysis, with probability of detection (POD) of 49% (33%) and false alarm rate (FAR) of 27% (42%) for ERA5 (ERA-Interim). Both ERA5 and ERA-Interim qualitatively reproduce the month-to-month differences in the NLLJ numbers. We see also an improvement of the NLLJ core wind speed in ERA5 compared to ERA-Interim, with mean differences of +0.5 m/s (+1.6 m/s) for ERA5 (ERA-Interim) relative to the composite data. The core of NLLJs is too high in both reanalyses, with mean differences of 63m (27m) for ERA5 (ERA-Interim). Furthermore, we compared ERA5 against the composite data for all five available years. The POD and FAR are similar to 2014, with no clear period of the year with a better performance. The year with the best reproduction of NLLJs was 2018, with a POD of 52% and FAR of 27%, and March was the best month, with POD 67% of and FAR of 27%. The mean wind speed difference in the NLLJ core assessed across all years was only 0.01 m/s lower in the ERA5 re-analyses compared to the observation, while the mean core height was overestimated by 69 m. Taken together, we see an improvement of the wind profile and the representation of NLLJs in ERA5 compared to ERA-Interim, although further work is needed for representing the NLLJ heights and frequency of occurrence with a higher precision. Ongoing work includes the development of a better understanding of the spatiotemporal differences in the occurrence and properties of NLLJs in Europe and beyond.

The Atacama Desert should be a region prone to wind erosion and dust events given the barren landscape and lack of moisture but it rarely sees strong dust events. Despite this perception, there is no literature quantifying the frequency of dust events in the Atacama and analysing the underlying meteorological mechanisms responsible for dust storms in the region. To this end, we analysed surface observations spanning 37 years (1984-2021) to compile wind climatology and systematically assess the dust events in the Atacama Desert. We used the Met Office Integrated Data Archive System (MIDAS) Land and Marine Surface Stations Data. Based on the present weather codes for nine Chilean stations present in the data, frequency of dust events was computed. The Atacama Desert has a very low annual mean dust event frequency of 0.6% despite its hyper-aridity. The station at Chañaral records the highest number of dust events, accounting for 68% of all recorded dust observations. There is no perceptible seasonal pattern but there is a strong diurnal cycle in the dust activity in the Atacama as most dust events occur between 1500 and 1800 local time. Threshold wind speeds, defined as the minimum wind speed required for 5, 25 and 50% of dust event frequency, are estimated for all stations except at Vallenar. The thresholds allow us to determine the lowest winds capable of emitting dust from the surface and also the highest wind speeds required, and the differences between the thresholds is indicative of the changes in soil conditions such as soil moisture or land use. For 5% of dust event frequency, the lowest threshold wind speed of 6-8 ms-1is estimated at Chañaral and for 50% of dust event frequency, 10-12 ms-1at Desierto de Atacama. Higher threshold wind speeds are observed for stations north of 26°S than the stations south of 26°S. It implies that dust particles can be mobilized by weaker near-surface wind speeds in the South than in the North of the Atacama. Seasonal variations in the threshold wind speed are not strongly pronounced in stations where sufficiently many observations are available. Ongoing work includes incorporating additional datasets for filling in identified data gaps in MIDAS and for allowing a spatially broader analysis of dust-emitting winds across the Atacama Desert.

The research area Climate Monitoring and Diagnostics of the Hans-Ertel-Centre for Weather Research advances the fundamental science basis for energy meteorology and translates the physical knowledge into stakeholder-relevant information for a climate neutral energy system in Europe. We show recent successes building on a kilometre-scale reanalysis dataset for Europe that was developed by the HErZ research area at the Universities of Cologne and Bonn, and produced by the German Weather Service. Paired with contemporary models for renewable power production, we carry out transdisciplinary research across meteorology and economics to answer urgent questions for the energy transition from currently primarily fossil fuels to a future energy system that relies more on renewables energy sources. Our research follows a seamless approach ranging from the assessment of weather and climate processes for wind and photovoltaic power production via economic system modelling and price estimates to communicating our research results to stakeholders in industry and politics, engaging in public outreach on national level, and teaching the next generation about our HErZ research. Scientific results from our works highlight (1) balancing potentials of natural variability and extremes in the production of wind and photovoltaic power across Europe, (2) distinct synoptic-scale weather patterns for dark doldrums with a possibility of compensating power shortfalls with sufficient storage capacities for a secure power supply, (3) nocturnal low-level jets having both beneficial and adverse impacts on wind power production, and (4) different biases in gridded irradiance datasets for estimating photovoltaic power production in comparison to on-site measurements. The research area closely collaborates with the Institute of Energy Economics at the University of Cologne (EWI). This cooperation informs the research agenda from an energy system perspective and facilitates the rapid transfer of research results into policy-relevant work, such as e.g. a contribution to EWI's report for the "dena Study Integrated Energy Transition" (dena, 2021). The dena Study, commissioned by the German Energy Agency (dena), has been widely received in the German policy debate, and it raised substantial public awareness e.g. by way of press releases and interviews for major newspapers. Future directions of the fruitful collaboration will involve blending climate change scenarios into the work of the research team for knowing possible extreme weather impacts on a redesigned energy system before they occur.

In electricity grids with high shares of solar and wind, storage technologies, such as electric batteries, are suitable to complement renewable generation. Renewable generation is unequally distributed both temporally and spatially. Storage can offset the temporal volatility of wind and photovoltaic (PV) by shifting generation or load if it they do not coincide temporally. In Germany, the power system is additionally affected by a spatial mismatch between load and renewable generation. While wind power generation is predominantly located in the North of Germany, demand is centred in the more densely populated West and South. Due to insufficient transmission capacity, North-South transmission lines are frequently overloaded in situations with high wind generation. As a result, wind generation is curtailed while fossil fired generation is used to supply loads south of the transmission bottleneck, thus jeopardizing German efforts to increase renewable generation and reducing greenhouse gases. Installing storage capacities can address this spatial mismatch to achieve a more balanced transmission line utilization, thus increasing overall transmission grid utilization, decreasing curtailment and reducing the use of fossil-fuel combustion. To do so, the allocation of storage within the electricity grid is critical. To investigate the relationship between storage, renewable generation, and the transmission grid, we employ a linear optimization model that allows to analyse optimal allocation of storage and generation technologies within Germany. Our model simulations use high resolution reanalysis meteorological data from the COSMO-REA6 data set developed by the Hans-Ertel-Centre for Weather Research, Climate Monitoring and Diagnostics, and a detailed depiction of electricity grid constraints of Germany. We find that an allocation of storage close to grid bottlenecks is optimal to facilitate wind generation replacing fossil fired generation, thus making efficient use of renewable generation. If grid bottlenecks are not accounted for in the storage allocation, storage cannot be used to relieve grid congestion. Consequently, we find an increase of up to 10% in the overall system operational costs and higher curtailment rates for wind and solar power compared to having storage installed at optimal locations.

As Europe strives for a decarbonization of electricity supply, many countries proclaim ambitious targets for capacity additions of wind and photovoltaic (PV) power. By increasing renewable capacities, countries seek to reduce their dependency on fossil fired electricity generation. For instance, Germany recently announced new capacity targets that mean a triplication of wind and PV power capacities while at the same time aiming for a full coal exit until 2030. However, the potential of renewable energies is not equally distributed over the European continent. Due to higher irradiance levels with decreasing latitude, PV power achieves higher full load hours and thus higher generation per installed capacity in southern Europe. Wind power reaches higher capacity factors close to shorelines, where near-surface wind speeds are higher.In light of the unequal distribution of PV and wind power generation potentials, national capacity targets may not lead to optimal utilization of capacities, i.e. maximum renewable generation. We analyse the effects of such national capacity targets for the case of PV generation. To do so, we compare two scenarios: A scenario with PV generation capacity distributed over Europe according to national political targets and a scenario with a distribution of newly installed capacities chosen such that it optimizes the renewable power potentials. Next to PV generation potentials, our assessment considers the European power grid, because line capacities limit the transmission and hence the trading between different countries. Line limits can have a negative impact on the overall power generation since renewable generation has to be curtailed if power can neither be locally used nor transmitted to another region. We employ a linear optimization-based electricity market model with inter-European transmission constraints and hourly resolution weather data for 35 historic weather years (1982-2016). We analyse the scenarios if terms of differences in power generation and curtailment, greenhouse gas emissions, and system operational cost. We pay special attention to extreme weather situations and the effects of a sub-optimal PV power allocation on security of supply.

In mid-March 2021, one of the strongest dust storms of the last decades hit East Asia with adverse impacts on socio-economic activities, and loss of life. The dust storm was associated with a Mongolian extra-tropical cyclone. In this study, we assess the atmospheric dynamics involved in the temporal evolution of this event and quantify contributions from the main dust source regions, namely the Gobi Desert and the Taklamakan Desert. We use the ground-based observations from Chinese observational networks, satellite images from MODIS satellite data, and model data from ERA5 and CAMS of ECMWF. Our results highlight that dust aerosols advected far towards the East primarily originated in the Gobi Desert, where anomalously strong dust-emitting winds were associated with the passage of a cold front embedded in the extra-tropical cyclone over Mongolia. The subsequently formed dust cloud of several hundred kilometers in extent led to sharp increases in the concentration of particulate matter that cause severe deterioration of the air quality across Chinese cities. Also, the Taklamakan Desert contributed to the event, but to a lesser degree than the Gobi, measured by the spatial influence across China. In the Taklamakan Desert, strong low-level easterly winds occurred with the cold air intrusion building up a substantial regional dust burden through the morning breakdown of nocturnal low-level jets. The subsequent directional change in synoptic-scale winds allowed an eastward transport of the dust-laden air from the Taklamakan Desert and increased the dust burden over Western China. Indeed the largest dust burden was seen in the West indicating that the Taklamakan Desert contributed to the dust outbreak, although dust from the Gobi Desert affected larger areas. The dust outbreak has led to record-high hourly concentrations of particulate matter in both the western and eastern cities in China based on data of the past seven years. Our results provide the first evidence for the relative contributions of dust aerosols from different East Asian dust source regions to this unusual event. It remains to be investigated how such dust outbreaks change in a warming world.

Wind and photovoltaic (PV) power production vary across time and space. We aim at a first systematic assessment of anomalies in PV and wind power production associated with different synoptic-scale weather patterns with a kilometre-scale resolution for all of Europe. To that end, we have developed the University of Cologne’s Renewable Energy Model (UoC-REM). UoC-REM simulates the hourly PV and wind power production using the reanalysis data set COSMO-REA6 with a horizontal resolution of 6 km. The installed capacities of PV and wind power for 2050 are taken from gridded scenario data. The output of UoC-REM is paired with data from a classification of 29 synoptic weather patterns identified and provided by the German Weather Service. Our results reveal substantial spatio-temporal differences in PV and wind power production depending on the weather pattern. We group the PV and wind power production from individual weather patterns into three groups to facilitate a composite analysis underlining similarities for the PV and wind power production across the weather patterns. These are (1) the group with anomalously high wind power production that almost always produces above average total production, primarily associated with patterns related to westerly winds, (2) the group of moderate PV plus wind power production with weather patterns causing mostly mild anomalies in both PV and wind power, but also ‘dark doldrums’ with simultaneously low wind and PV power production during blocking high pressure systems, e.g., South-Shifted Westerly, and (3) the group with high PV power production paired with below average wind power that is mostly associated with anti-cyclonic weather patterns. We identify that the lowest 10-day production event of PV plus wind power occurs during the pattern Anticyclonic South-Easterly from Group 3, with a reduction by 41% compared to the average hourly total production for Europe. On the contrary, the highest 10-day production event occurs during the pattern Cyclonic North-Westerly from Group 1, with an increase by 18%. Our results suggest that identifying the weather pattern can be used as a quick estimate of the overall production anomaly in PV and wind power production. It would allow to monitor and issue warnings of weather conditions that pose a risk to a future energy system that relies on more weather-dependent renewable sources, without the need to perform operational simulations of the expected power production. Future work will focus on an in-depth analysis of extreme events in renewable power production considering the full spatial and temporal resolution of the new dataset of UoC-REM.

Differences in female/male plant metabolism for dioecious species are poorly understood but relevant if the economic exploitation of such resources is the objective. In this work, the composition o...

The increasing use of wind and solar power requires interventions to balance the associated variability in energy production. One option to reduce the costly interventions is to exploit the natural de-correlation of wind and irradiance. This study characterises the balancing potential of the natual variability in wind and photovoltaic energy production within and across eleven European countries. We use 20 years of highly resolved meteorological data from a post-processed regional reanalysis with a 6 km horizontal grid to calculate daily photovoltaic and wind power. Our results suggest a country-dependent reduction of the day-to-day variability in energy production by 29%–42% due to installing both PV and wind power capacities, compared to wind power only. The optimized photovoltaic to photovoltaic plus wind power generation ratios are 45–57% for maximizing balancing effects associated with the changing weather. We further identify on less than 10% of the days a simultaneous occurrence of extremes in photovoltaic and wind power across European countries. The cross-border balancing potentials for the extremes in renewable energy production are therefore high due to the spatio-temporal differences of the local weather.

Doppler wind lidars (DWLs) have increasingly been used over the last decade to derive the mean wind in the atmospheric boundary layer. DWLs allow the determination of wind vector profiles with high vertical resolution and provide an alternative to classic meteorological tower observations. They also receive signals from higher altitudes than a tower and can be set up flexibly in any power-supplied location. In this work, we address the question of whether and how wind gusts can be derived from DWL observations. The characterization of wind gusts is one central goal of the Field Experiment on Sub-Mesoscale Spatio-Temporal Variability in Lindenberg (FESSTVaL). Obtaining wind gusts from a DWL is not trivial because a monostatic DWL provides only a radial velocity per line-of-sight, i.e., only one component of a three-dimensional vector, and measurements in at least three linearly independent directions are required to derive the wind vector. Performing them sequentially limits the achievable time resolution, while wind gusts are short-lived phenomena. This study compares different DWL configurations in terms of their potential to derive wind gusts. For this purpose, we develop a new wind retrieval method that is applicable to different scanning configurations and various time resolutions. We test eight configurations with StreamLine DWL systems from Halo Photonics and evaluate gust peaks and mean wind over 10 minutes at 90 m a.g.l. against a sonic anemometer at the meteorological tower in Falkenberg, Germany. The best performing configuration for retrieving wind gusts proves to be a fast continuous scanning mode (CSM) that completes a full observation cycle within 3.4 s. During this time interval, about eleven radial Doppler velocities are measured, which are then used to retrieve single gusts. The fast CSM configuration was successfully operated over a three-month period in summer 2020. The CSM paired with our new retrieval technique provides gust peaks that compare well to classic sonic anemometer measurements from the meteorological tower.

A challenge of an energy system, that nowadays more strongly depends on wind power generation, is the spatial and temporal variability of winds. Nocturnal low-level jets (NLLJ) are typical wind phenomena defined as a maximum in the vertical profile of the horizontal wind speed. A NLLJ has typical core heights of 50–500 m above ground level (a.g.l.), which is in the height range of most modern wind turbines. This study presents NLLJ analyses based on new observations from Doppler wind LiDARs. The aim is to characterize the temporal and spatial variability of NLLJs on the mesoscale and to quantify their impacts on wind power generation. The data was collected during the Field Experiment on Submesoscale Spatio-Temporal Variability (FESSTVaL) campaign from June to August 2020 in Lindenberg and Falkenberg (Germany), located at about 6 km from each other. Both sites have seen NLLJs in about 70 % of the nights with half of them lasting for more than 3 hours. Events longer than 6 hours occurred more often simultaneously at both sites than shorter events, indicating the mesoscale character of very long NLLJs. Very short NLLJs of less than one hour occurred more often in Lindenberg than Falkenberg, indicating more local influences on the wind profile. We discussed different meteorological mechanisms for NLLJ formation and linked NLLJ occurrences to synoptic weather patterns. There were positive and negative impacts of NLLJs on wind power that we quantified based on the observational data. While NLLJs increased the mean power production by up to 80 %, the stronger shear in the rotor layer during NLLJs had also negative impacts. The impacts of NLLJs on wind power production depended on the relative height between the wind turbine and the core of the NLLJ. For instance, the mean increase of the estimated power production during NLLJ events was about 30 % higher for a turbine at 135 m a.g.l. compared to one at 94 m a.g.l.. Our results imply that long NLLJs have an overall stronger impact on the total power production, while short events are primarily relevant as driver for power ramps.

Dust storm genesis is one of several high-impact weather phenomena that may result from Rossby wave breaking (RWB). The wave propagation into low latitudes over North Africa facilitates instability through upper-level divergence and the reduction of the static stability ahead of the cyclonic side of the wave train, and therefore modulates the low-level tropospheric conditions in dust source areas.The analysis of three dust storm case studies that strongly impacted the Iberian Peninsula and another one impacting the Cape Verde Islands indicates that a double RWB process within the Polar Jet (PJ) stream over the eastern North Atlantic and western Europe is a common large-scale upper-level precursor to the formation of the large North African dust storms and subsequent transport of dust (Orza et al, 2020; Dhital et al, 2020).The synoptic and larger subsynoptic scale features were analyzed using ERA-Interim reanalysis while mesoscale features were studied by high-resolution WRF-CHEM simulations. There are substantial differences between the case studies in the location and geometry of upper- and low-level features following the first break (Dhital et al, 2020; 2021). However, in all case studies (1) RWB within the PJ stream is observed; (2) there is a substantial poleward expansion of the upper-level subtropical ridge; and (3) the secondary RWB is amplified by nonlinear wave reflection from the first break.Preliminary results from a larger number of cases study indicate the relevance of secondary RWB in the PJ stream. The dynamics may also result in cutoff lows and amplified low-PV ridging accompanying cyclogenesis and heatwaves, respectively, which emphasizes the need to improve our understanding of the involved processes.- Dhital S, ML Kaplan, JAG Orza, S Fiedler (2020). Atmospheric dynamics of a Saharan dust outbreak over Mindelo, Cape Verde Islands, preceded by Rossby wave breaking: Multiscale observational analyses and simulations. J. Geophys. Res. Atmos. 125, e2020JD032975.- Dhital S, ML Kaplan, JAG Orza, S Fiedler (2021). Poleward Transport of African Dust to the Iberian Peninsula Organized by a Barrier Jet and Hydraulic Jumps: Observations and High-Resolution Simulation Analyses. Atmos. Environ. 261, 118574.- Orza JAG, S Dhital, S Fiedler, ML Kaplan (2020). Large Scale Upper-level Precursors for Dust Storm Formation over North Africa and Poleward Transport to the Iberian Peninsula. Part I: An Observational Analysis. Atmos. Environ. 237, 117688.

The evolution of wind gusts is difficult to observe as gusts are short-lived and small-scale phenomena. They occur with certain weather configurations (e.g. fronts, cold pools) and may already differ very locally. The question arises if individual gust observations can be taken as representative of their surroundings or if significant differences can already be apparent on the meso-gamma scale (2-20 km). Within the Field Experiment on Sub-Mesoscale Spatio-Temporal Variability in Lindenberg (FESSTVaL) different phenomena in the atmospheric boundary layer are studied with a variety of measurement instruments. This involved installing three StreamLine DWL systems from Halo Photonics at a distance of 6 km apart from each other. DWLs allow the retrieval of wind vector profiles and offer an alternative to classic meteorological tower observations, since they can be flexibly deployed at any electrified site. However, short-lived gusts are more difficult to capture than a persistent mean wind. A wind vector has to be obtained from different radial velocity measurements that are made sequentially, which limits the achievable temporal resolution. Therefore, we have developed a new retrieval method for deriving wind measurements that is suitable for different scan configurations and different time resolutions respectively different numbers of radial velocities. A fast continuous scanning mode (CSM), that completes a full observation cycle within 3.4 seconds and measures about eleven radial Doppler velocities is a suitable DWL configuration for deriving wind gusts, as shown by comparisons with measurements of a sonic anemometer at 90.3 m a.g.l. on the meteorological tower in Falkenberg. The fast CSM configuration was operated on the DWLs during the summer months 2021 at the three different sites. Their surrounding area is predominantly flat farmland, minimizing topographic impacts. This set-up allows us to observe the spatial-temporal evolution of gusts at the meso-gamma scale. Examples will be presented that illustrate the variability of wind gusts as observed during FESSTVaL.

Atmospheric stability is a measure of atmospheric status which determines whether thermodynamically perturbed air will rise, sink, or be neutral. Atmospheric stability has a major impact on the evolution of wind turbine wakes and thus on the yield and performance of offshore wind parks. For estimations of wind park power output and for improving analyses of offshore wind park wakes, a crucial parameter was found to be profiles of atmospheric temperature and stability metrics. Atmospheric temperature profiles can be measured in-situ by balloon-borne sensors, but also estimated from the ground using radiometric observations. This presentation reviews the stability metrics useful for monitoring wind park performances and provides a quantitative assessment of the value of microwave radiometer (MWR) observations to estimate these stability metrics from near surface, either over land or ocean. Results from three different MWR instruments, representing the most common available on the market, and at least three field experiments will be presented. This work has been funded by Carbon Trust and the partner companies of the Off-shore Wind Accelerator program: (in alphabetical order) EnBW, Equinor, Orsted, RWE, Scottish Power Renewables, Shell, SSE Renewables, Total Energies, Vattenfall.

Despite the implication of aerosols for radiative forcing, there are di erences in aerosol estimates from both, observations and models. This study quanti es di erences between current estimates of aerosol optical depth (AOD) to address two questions: (1) How well do we know the large-scale spatio-temporal pattern of present-day AOD across state-of-the-art data? (2) Has the representation of AOD improved across phases of aerosol-climate model intercomparison projects? To answer these questions, we analyze spatio-temporal patterns of the present-day monthly mean AOD from 94 global datasets. The data is taken from eight satellite retrievals, four aerosol-climate model intercomparison projects, two global reanalyses, one operational ensemble product, one climatology and one merged satellite product covering periods between 1998 and 2019. The evaluation includes new satellite data from SLSTR and aerosol-climate models of CMIP6 and AeroCom-III. The comprehensive data assessment allows us to evaluate the performance of individual products and models concerning di erent spatial and temporal aspects. Our assessment is based on metrics for a detailed investigation with respect to di erent spatio-temporal characteristics of AOD.Our results highlight spatio-temporal di erences in AOD across datasets, were the performance of individual data sets varies with respect to the di erent spatio-temporal metrics assessed. Global mean AOD of individual satellites ranges between -11% to +17% around a satellite mean of 0.14. The ensemble means from the aerosol-climate model intercomparison projects fall within the satellite range, but individual models can di er considerably. Reanalyses and climalologies are typically closer to the satellite mean than aerosol-climate models. No systematic improvement from earlier to later phases of CMIP and AeroCom is found, although some regional biases have been reduced. Compared to the satellite and reanalysis data, all aerosol-climate ensemble means tend to overestimate AOD along extra-tropical storm tracks and underestimate AOD in regions of high aerosol load in South America, South Africa, India, and Southeast Asia. The identi ed di erences may be used to guide further e orts to improve satellite retrievals and model simulations for aerosols. In addition, the uncertainty in observed AOD implies that a model evaluation based on a single satellite product might draw biased conclusions. This underlines the need for continued e orts to improve both model and satellite estimates of AOD to facilitate a better understanding of aerosol e ects in the Earth system. At the same time, our analysis suggests that an assimilation of multiple satellite products for AOD would be bene cial to account for observational uncertainty.Reference: Vogel, A., Alessa, G., Scheele, R., Weber, L., Dubovik, O., North, P., Fiedler, S. (2022). Uncertainty in aerosol optical depth from modern aerosol-climate models, reanalyses, and satellite products. Journal of Geophysical Research: Atmospheres, 127, e2021JD035483. https://doi.org/10.1029/2021JD035483

Accurate irradiance data is necessary for model estimates of expected photovoltaic (PV) power production. Such data is freely available from reanalysis and satellite products with a high temporal and spatial resolution, including locations without ground-based measurements. Gridded irradiance data is therefore used for the characterization of solar resources at specific locations and larger areas, e.g. by power system modellers. Past assessments of irradiance data for PV modelling often relied on the evaluation of global horizontal irradiance (QGHI). However, the direct and diffuse irradiance components as well as differences in seasonal characteristics can strongly affect the PV capacity factors (C) potentially leading to larger biases in C than for QGHI. We therefore systematically assess differences in QGHI, direct and diffuse horizontal irradiance (Qdir and Qdif) and quantify the subsequent bias propagation from individual radiation components to C in a contemporary PV power model. Our PV model simulations use seven different gridded irradiance data sets, namely ERA5, COSMO-REA6, COSMO-REA6pp, COSMO-REA2, CAMS radiation service, SARAH-2 and CERES Syn1Deg. All data sets provide Qdir and Qdif as separate time series spanning seven to 43 years and with a temporal resolution of 15 minutes to one hour. The results are compared against seven years of simulations based on reference measurements from 30 weather stations of the German Weather Service. We compute metrics characterizing biases in seasonal and annual spatial means, day-to-day variability and extremes in C, considering single stations and a simulated PV fleet. Our results highlight biases of -1.4 % (COSMO-REA6) to +8.2 % (ERA5) in annual and spatial means of C at single stations across Germany, while the bias in QGHI is -3 % for COSMO-REA6 to +3.6 % for ERA5. We also show the bias on days of very low PV production, relevant for extreme event analysis: The days within the lowest ten percent of daily PV production in a PV fleet show a bias of +70.2 % in ERA5, while it is only +4 % in the post-processed COSMO-REA6 data (COSMO-REA6pp). SARAH-2 and COSMO-REA6pp outperform the other products for many metrics, but also cause some biases in C. For instance, SARAH-2 yields good results in summer, but overestimates C in winter by 16 % averaged across all stations. COSMO-REA6pp represents day-to-day variability in C of a simulated PV fleet very well and has a relatively small bias of +0.5 % in the annual spatial means, but this is partly due to compensating biases from individual stations. Our results suggest that gridded irradiance data should be used with caution for site assessments and should ideally be complemented by local measurements. For power system modellers, our results may provide guidance for the quantification of uncertainties caused by gridded irradiance data. Reference:Kenny, D., and Fiedler, S., in press, Which gridded irradiance data is best for modelling photovoltaic power production in Germany?, Solar Energy.

The spatial distribution of anthropogenic aerosols was substantially different in the 1970s and 2000s. In the 1970s, a maximum in anthropogenic aerosol optical depth (τa) was situated over the North Atlantic, North America, and Europe. By the 2000s, the τa maximum has shifted to East Asia. That difference in the global pattern of τa has implications for the regional radiative effects and the regional climate. Here, we show how the differences in the global pattern of τa favour changes in the North Atlantic sea-surface temperature (NASST). To this end, we perform several hundred years of fully coupled atmosphere-ocean simulations with the CMIP6 model MPI-ESM1.2. Our simulations use observationally informed anthropogenic aerosol optical properties and a prescribed effect on the cloud albedo. All our simulations are sufficiently long to eliminate the impact of model-internal variability on the results.Contrasting the output of the simulation using τa of the 1970s against the output from the pre-industrial control without τa largely shows the expected decrease in NASSTs, which is due to the surface cooling by the aerosol effects on radiation and clouds. Surprisingly, we also see an unusual warming response in the eastern sub-polar gyre - the same region that became known as the North Atlantic Warming Hole (NAWH). The described NASST responses reversed as we go from the anthropogenic aerosols of the 1970s to the 2000s, namely a relative decrease of the NASST in the region of the NAWH and an increase away from it. Analysing the atmosphere and ocean dynamics in the model output reveals that these NASST responses are driven by changes in the circulation of the model. The signal in the NAWH is explained by the oceanic meridional heat convergence that is primarily attributed to the gyre heat transport and not the overturning circulation that is often in focus of research. Our diagnostics also show a latitudinal shift of the jet stream over the North Atlantic being closer to the equator in the 1970s than in the 2000s, which is consistent with the response of the sub-polar gyre via the coupling by the wind stress at the ocean surface. Our results imply that the regional reduction in anthropogenic aerosols over the North Atlantic between the 1970s and 2000s favoured (1) a stronger NAWH cooling signal, (2) a stronger Arctic amplification measured by the NASST, and (3) a latitudinal shift of the mean position of the jet stream. Sensitivity tests with the model indicate that more than two-thirds of these signals are associated with aerosol effects on clouds. It highlights the need to better constraint aerosol-cloud interactions for an assessment of circulation responses to changes in atmospheric composition. Reference:Fiedler, S., and Putrasahan, D., 2021: How does the North Atlantic SST pattern respond to anthropogenic aerosols in the 1970s and 2000s? Geophysical Research Letters, 48, e2020GL092142. https://doi.org/10.1029/2020GL092142.

Nocturnal Low-Level Jets (NLLJ) are maxima in vertical profiles of the horizontal wind speed in the lowest hundreds of meters of the troposphere. NLLJs therefore influence the winds at typical rotor heights. However, due to rare measurements with a sufficient precision and resolution, the occurrence frequency and spatio-temporal characteristics of NLLJs on the mesoscale are still poorly understood. The present work uses new measurements of wind profiles for June to August 2020 from Doppler wind lidars that were installed as a part of the Field Experiment on Submesoscale Spatio-Temporal Variability (FESTVaL) campaign, in Lindenberg and Falkenberg (Germany), at about 6 km of distance from each other. The aim of our NLLJ assessment is to characterize their mesoscale properties and evaluate their potential impacts on wind power production. The vertical profiles of the 10-minute mean winds from the lidar measurements were statistically analysed using automated detection tools for NLLJs. These allowed the determination of the frequency of occurrence, height and wind speed in the core of NLLJs as well as the vertical wind shear with a high temporal resolution. First, we intercompared the results from the two sites in order to analyse the temporal and spatial variability of NLLJs on the mesoscale. Our automatic detection identified NLLJs in about 64% to 74% of the summer nights in 2020, showing that they were a common phenomenon during that summer. About half of the NLLJ events were longer than 3 hours, with Lindenberg having more often shorter events of less than 1 hour. If very long NLLJ events (> 6 hours) occurred, they typically affected both places simultaneously, an indicative of their mesoscale character. Our results further suggest that very long NLLJ events are generated by the classical inertial oscillations, influenced by a large-scale horizontal pressure gradient and intermittent turbulent mixing, while shorter NLLJ events are more strongly dependent on local conditions or are driven by shorter-living density currents. Regarding their potential impact on wind turbines, we simulated wind power production for two different turbine types of different height and capacity. Both simulations indicate that NLLJs clearly increase the power production compared to nights without NLLJs. The quantitative NLLJ impacts on power production strongly depend on the height of the wind turbines: during NLLJ events the average wind production was 80% higher for a hub height of 135 m and only 53% higher for 94 m. At the same time, NLLJs increased the wind shear across the rotor layer. Extreme shear in the rotor layer was often associated with NLLJs, with 37% of all NLLJs leading to extreme shear and 48% of all extreme shear cases being caused by NLLJs. We infer from our assessment that particularly long NLLJ events strong influence wind power production, while shorter NLLJs can increase the temporal and spatial variability in power production, causing power ramps.

Despite the implication of aerosols for the radiation budget, there are persistent differences in data for the aerosol optical depth (τ) for 1998–2019. This study presents a comprehensive evaluation of the large‐scale spatio‐temporal patterns of mid‐visible τ from modern data sets. In total, we assessed 94 different global data sets from eight satellite retrievals, four aerosol‐climate model ensembles, one operational ensemble product, two reanalyses, one climatology and one merged satellite product. We include the new satellite data SLSTR and aerosol‐climate simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Aerosol Comparisons between Observations and Models Phase 3 (AeroCom‐III). Our intercomparison highlights model differences and observational uncertainty. Spatial mean τ for 60°N – 60°S ranges from 0.124 to 0.164 for individual satellites, with a mean of 0.14. Averaged τ from aerosol‐climate model ensembles fall within this satellite range, but individual models do not. Our assessment suggests no systematic improvement compared to CMIP5 and AeroCom‐I. Although some regional biases have been reduced, τ from both CMIP6 and AeroCom‐III are for instance substantially larger along extra‐tropical storm tracks compared to the satellite products. The considerable uncertainty in observed τ implies that a model evaluation based on a single satellite product might draw biased conclusions. This underlines the need for continued efforts to improve both model and satellite estimates of τ, for example, through measurement campaigns in areas of particularly uncertain satellite estimates identified in this study, to facilitate a better understanding of aerosol effects in the Earth system., Plain Language Summary: Aerosols are known to affect atmospheric processes. For instance, particles emitted during dust storms, biomass burning and anthropogenic activities affect air quality and influence the climate through effects on solar radiation and clouds. Although many studies address such aerosol effects, there is a persistent difference in current estimates of the amount of aerosols in the atmosphere across observations and complex climate models. This study documents the data differences for aerosol amounts, including new estimates from climate‐model simulations and satellite products. We quantify considerable differences across aerosol amount estimates as well as regional and seasonal variations of extended and new data. Further, this study addresses the question to what extent complex climate models have improved over the past decades in light of observational uncertainty., Key Points: Present‐day patterns in aerosol optical depth differ substantially between 94 modern global data sets. The range in spatial means from individual satellites is −11% to +17% of the multi‐satellite mean. Spatial means from climate model intercomparison projects fall within the satellite range but strong regional differences are identified., Hans‐Ertel‐Center for Weather Research, Collaborative Research Centre 1211, Max‐Planck‐Institute for Meteorology

Highlights • Broad analysis of bias propagation with seven irradiance data sets in PV model. • We identify seasonal and regional biases in irradiance and PV power. • There is no single data set performing best in all metrics for means and variability. Abstract Model estimates of expected photovoltaic (PV) power production rely on accurate irradiance data. Reanalysis and satellite products freely provide irradiance data with a high temporal and spatial resolution including locations for which no ground-based measurements are available. We assess differences in such gridded irradiance data and quantify the subsequent bias propagation from individual radiation components to capacity factors in a contemporary PV model. PV power production is simulated based on four reanalysis (ERA5, COSMO-REA6, COSMO-REA6pp, COSMO-REA2) and three satellite products (CAMS, SARAH-2, CERES Syn1Deg). The results are compared against simulations using measurements from 30 weather stations of the German Weather Service. We compute metrics characterizing biases in seasonal and annual means, day-to-day variability and extremes in PV power. Our results highlight a bias of −1.4% (COSMO-REA6) to ＋8.2% (ERA5) in annual and spatial means of PV power production for Germany. No single data set is best in all metrics, although SARAH-2 and the postprocessed COSMO-REA6 data (COSMO-REA6pp) outperform the other products for many metrics. SARAH-2 yields good results in summer, but overestimates PV output in winter by 16% averaged across all stations. COSMO-REA6pp represents day-to-day variability in the PV power production of a simulated PV fleet best and has a particularly small bias of 0.5% in annual means. This is at least in parts due to compensating biases in local and seasonal means. Our results imply that gridded irradiance data should be used with caution for site assessments and ideally be complemented by local measurements.

In the Field Experiment on Sub-Mesoscale Spatio-Temporal Variability in Lindenberg (FESSTVaL, www.fesstval.de) various phenomena in the atmospheric boundary layer are investigated. One goal is to detect wind gusts from measurements of a Doppler wind lidar (DWL). DWL’s allow the determination of wind vector profiles with a high vertical resolution (∼ 30 m) and therefore are an attractive alternative to metorological towers.However, obtaining wind gusts from DWL measurements is not trivial because a monostatic lidar provides only a radial velocity, i.e., only one component of a three-dimensional vector per individual beam. Measurements in at least three linearly independent directions are therefore necessary to derive the wind vector. These must be performed sequentially, which prolongs the time interval for determining the wind vector and therefore limits the time resolution of the derived wind vector. In order to retrieve wind gusts, wind maxima of a few seconds, one needs to operate the instrument in a quick scanning mode. In this presentation, we show results from different scanning modes and discuss the method for retrieving wind gusts. We tested various configurations with respect to their ability to detect gusts and mean winds at the Boundary Layer Field Site in Falkenberg in autumn 2019. The DWL configurations that measure different lines-of-sight with rapid temporal repetitions have a lower signal-to-noise ratio (SNR) but the highest chance of detecting gusts.We have developed a new retrieval method that skips prior SNR filtering and instead iteratively removes a fixed number of measurements that do not match a least-squares-fit. The least-squares-fit is then recalculated on the reduced set of measurements, and if necessary, this step is repeated. With appropriate retrieval steps and iteration criteria, our results suggest that prior filtering can be omitted.We present the results of our new retrieval for eight different DWL configurations consisting of double-beam swinging, step-stare modes, and continuous-scanning modes. The evaluation is done by a comparison of the minimum, maximum and mean wind speed at 90 m a.g.l. against the reference measurements of a sonic anemometer that is located nearby. Ongoing work is addressing further comparison of our retrieved wind variables with unmanned aerial vehicles from the FESSTVaL campaign in summer 2020.