Record high solar irradiance in Western Europe during first COVID-19 lockdown largely due to unusual weather

Spring 2020 broke sunshine duration records across western Europe. The Netherlands recorded the highest surface irradiance since 1928, exceeding the previous extreme of 2011 by 13 %, and the diffuse fraction of the irradiance measured a record low percentage (38 %). The coinciding irradiance extreme and a reduction in anthropogenic pollution due to COVID-19 measures triggered the hypothesis that cleaner-than-usual air contributed to the record. Based on analyses of ground-based and satellite observations and experiments with a radiative transfer model, we estimate a 1.3 % (2.3 W m$^{-2}$) increase in surface irradiance with respect to the 2010-2019 mean due to a low median aerosol optical depth, and a 17.6 % (30.7 W m$^{-2}$) increase due to several exceptionally dry days and a very low cloud fraction overall. Our analyses show that the reduced aerosols and contrails due to the COVID-19 measures are far less important in the irradiance record than the dry and particularly cloud-free weather.Comment: 21 pages, 12 figures, submitted to Communications Earth and Environmen

State of the climate in 2018

In 2018, the dominant greenhouse gases released into Earth’s atmosphere—carbon dioxide, methane, and nitrous oxide—continued their increase. The annual global average carbon dioxide concentration at Earth’s surface was 407.4 ± 0.1 ppm, the highest in the modern instrumental record and in ice core records dating back 800 000 years. Combined, greenhouse gases and several halogenated gases contribute just over 3 W m−2 to radiative forcing and represent a nearly 43% increase since 1990. Carbon dioxide is responsible for about 65% of this radiative forcing. With a weak La Niña in early 2018 transitioning to a weak El Niño by the year’s end, the global surface (land and ocean) temperature was the fourth highest on record, with only 2015 through 2017 being warmer. Several European countries reported record high annual temperatures. There were also more high, and fewer low, temperature extremes than in nearly all of the 68-year extremes record. Madagascar recorded a record daily temperature of 40.5°C in Morondava in March, while South Korea set its record high of 41.0°C in August in Hongcheon. Nawabshah, Pakistan, recorded its highest temperature of 50.2°C, which may be a new daily world record for April. Globally, the annual lower troposphere temperature was third to seventh highest, depending on the dataset analyzed. The lower stratospheric temperature was approximately fifth lowest. The 2018 Arctic land surface temperature was 1.2°C above the 1981–2010 average, tying for third highest in the 118-year record, following 2016 and 2017. June’s Arctic snow cover extent was almost half of what it was 35 years ago. Across Greenland, however, regional summer temperatures were generally below or near average. Additionally, a satellite survey of 47 glaciers in Greenland indicated a net increase in area for the first time since records began in 1999. Increasing permafrost temperatures were reported at most observation sites in the Arctic, with the overall increase of 0.1°–0.2°C between 2017 and 2018 being comparable to the highest rate of warming ever observed in the region. On 17 March, Arctic sea ice extent marked the second smallest annual maximum in the 38-year record, larger than only 2017. The minimum extent in 2018 was reached on 19 September and again on 23 September, tying 2008 and 2010 for the sixth lowest extent on record. The 23 September date tied 1997 as the latest sea ice minimum date on record. First-year ice now dominates the ice cover, comprising 77% of the March 2018 ice pack compared to 55% during the 1980s. Because thinner, younger ice is more vulnerable to melting out in summer, this shift in sea ice age has contributed to the decreasing trend in minimum ice extent. Regionally, Bering Sea ice extent was at record lows for almost the entire 2017/18 ice season. For the Antarctic continent as a whole, 2018 was warmer than average. On the highest points of the Antarctic Plateau, the automatic weather station Relay (74°S) broke or tied six monthly temperature records throughout the year, with August breaking its record by nearly 8°C. However, cool conditions in the western Bellingshausen Sea and Amundsen Sea sector contributed to a low melt season overall for 2017/18. High SSTs contributed to low summer sea ice extent in the Ross and Weddell Seas in 2018, underpinning the second lowest Antarctic summer minimum sea ice extent on record. Despite conducive conditions for its formation, the ozone hole at its maximum extent in September was near the 2000–18 mean, likely due to an ongoing slow decline in stratospheric chlorine monoxide concentration. Across the oceans, globally averaged SST decreased slightly since the record El Niño year of 2016 but was still far above the climatological mean. On average, SST is increasing at a rate of 0.10° ± 0.01°C decade−1 since 1950. The warming appeared largest in the tropical Indian Ocean and smallest in the North Pacific. The deeper ocean continues to warm year after year. For the seventh consecutive year, global annual mean sea level became the highest in the 26-year record, rising to 81 mm above the 1993 average. As anticipated in a warming climate, the hydrological cycle over the ocean is accelerating: dry regions are becoming drier and wet regions rainier. Closer to the equator, 95 named tropical storms were observed during 2018, well above the 1981–2010 average of 82. Eleven tropical cyclones reached Saffir–Simpson scale Category 5 intensity. North Atlantic Major Hurricane Michael’s landfall intensity of 140 kt was the fourth strongest for any continental U.S. hurricane landfall in the 168-year record. Michael caused more than 30 fatalities and $25 billion (U.S. dollars) in damages. In the western North Pacific, Super Typhoon Mangkhut led to 160 fatalities and$6 billion (U.S. dollars) in damages across the Philippines, Hong Kong, Macau, mainland China, Guam, and the Northern Mariana Islands. Tropical Storm Son-Tinh was responsible for 170 fatalities in Vietnam and Laos. Nearly all the islands of Micronesia experienced at least moderate impacts from various tropical cyclones. Across land, many areas around the globe received copious precipitation, notable at different time scales. Rodrigues and Réunion Island near southern Africa each reported their third wettest year on record. In Hawaii, 1262 mm precipitation at Waipā Gardens (Kauai) on 14–15 April set a new U.S. record for 24-h precipitation. In Brazil, the city of Belo Horizonte received nearly 75 mm of rain in just 20 minutes, nearly half its monthly average. Globally, fire activity during 2018 was the lowest since the start of the record in 1997, with a combined burned area of about 500 million hectares. This reinforced the long-term downward trend in fire emissions driven by changes in land use in frequently burning savannas. However, wildfires burned 3.5 million hectares across the United States, well above the 2000–10 average of 2.7 million hectares. Combined, U.S. wildfire damages for the 2017 and 2018 wildfire seasons exceeded $40 billion (U.S. dollars)

Atmospheric moisture transport and river runoff in the mid-latitudes

Hydrometeorological extremes such as floods and droughts are the result of anomalous transport of moisture in the atmosphere, impacting the land. Recent examples of these impacts are the devastating floods in south Louisiana (United States) in 2016 and drought over western Europe in 2018. In this thesis we enhance our understanding of the water cycle and its extremes in present and future climate. More specifically, transport of moisture in the atmosphere is studied and linked to precipitation and river runoff. This is done from a global modelling perspective, making use of reanalysis data, simulations from a global climate model, a moisture tracking tool, and a global hydrological model (all described in Chapter 2). Both hydrometeorological events as well as yearly averages and seasonal cycles are studied for three regions in the mid-latitudes; the Mississippi river basin in North America, and the Rhine river basin and Norway in Europe. In Chapter 3 we investigate atmospheric moisture transport and its relation to extreme precipitation over coastal Norway. We show a climatology (1979-2014) of cold season precipitation events based on the 99th percentile of daily precipitation from ERA-Interim reanalysis data, of which more than 85% is related to a so-called atmospheric river, an anomalous moisture flux. Characteristic patterns resulting in this anomalous moisture transport can be identified with significant time before the event (5 days), which is helpful to forecasting. However, the identification of large-scale patterns 5 days before the event is insufficient to predict the precise location of subsequent extreme precipitation. In Chapter 4 we explore precipitation and moisture transport over the Mississippi River basin under present (2002-2006) and future climate conditions (2094-2098; RCP4.5) using simulations from the global climate model EC-Earth. More specifically, we determine the moisture sources of the Mississippi basin, by tracking precipitation falling over the basin backward in time using the Eulerian tracking model WAM-2layers. We find that the most important continental moisture sources of the Mississippi basin are evaporation from the basin itself, and the area southwest of it, while the most relevant oceanic sources are the Gulf of Mexico/Caribbean and the Pacific. Of course, those sources vary per season. We conclude that the moisture sources of the Mississippi River basin in the future 1) enhance over the oceans in winter, resulting in more future winter precipitation, and 2) show a relative decline over terrestrial areas in summer, indicating that land surface properties will have relatively less impact on precipitation over the Mississippi River basin in the future. In Chapter 5 we move over the Atlantic back to Europe, where the extremely dry summers of 2003 and 2018 are studied with the ERA5 reanalysis dataset. Normally, evaporation over the Atlantic contributes to about half of the precipitation falling in the Rhine basin during summer. However, during the dry summers of 2003 and 2018 persistent blocking avoided moisture transport from the oceans towards the Rhine basin. Although this was both the case in 2003 and 2018, the normalized moisture sources in those years appear to be quite different, due to the slight different locations of the blocking systems. The large-scale circulation in 2018 was especially favorable for dry conditions over the Rhine, while in 2003 we find that local moisture recycling decreases because of the drying out of soils. The unique character of both extreme events indicate that hydrometeorological extremes should be investigated individually to enhance our understanding of these extremes, and there complex interaction from the larger-scale to the land-surface. To study the global hydrological cycle and its response to climate change, we rely on global climate models and global hydrological models. In Chapter 6 we assess and compare the benefits of an increased resolution of a global climate model (GCM; EC-Earth) and global hydrological model (GHM; W3RA) for the Rhine and Mississippi River basins. Increasing the resolution of a GCM (1.125° to 0.25°) results in an improved precipitation budget over the Rhine basin, attributed to a more realistic large-scale circulation. These improvements with increased resolution are not found for the Mississippi basin, possibly because precipitation is strongly dependent on the representation of still unresolved convective processes. The (improved) monthly-averaged precipitation from the GCM is reflected in (improved) monthly-averaged actual evaporation and discharge from the GHM, although an increase in resolution in the GHM does not lead to significant changes in discharge. A straightforward resolution increase in the GHM is thus most likely not the best method to improve discharge predictions, which emphasizes the need for better representation of processes and improved parameterizations that go hand in hand with resolution increase in a GHM. This work contributed to a better understanding of the water cycle and its extremes in the mid-latitudes, from a modelling perspective. To further enhance our knowledge on atmospheric moisture transport and the impact on land, combined efforts from climate science and the fields of meteorology and hydrology are needed

Datasets used for the study: An evaluation of the importance of spatial resolution in a global climate and hydrological model based on the Rhine and Mississippi basin

This repository contains the observational data and the parameter fields for the hydrological model which is used in the study by Imme Benedict et al., 2018: An evaluation of the importance of spatial resolution in a global climate and hydrological model based on the Rhine and Mississippi basin, Hydrology and Earth System Science. More specific, this repository contains the following datasets: • Precipitation over Europe from E-OBS: E-OBS dataset version 12.0 at 0.25° from 1985 until 2015 (30 years). We acknowledge the E-OBS dataset from the EU-FP6 project ENSEMBLES () and the data providers in the ECA&D project (). Haylock, M.R., N. Hofstra, A.M.G. Klein Tank, E.J. Klok, P.D. Jones, M. New: A European daily high-resolution gridded dataset of surface temperature and precipitation. J. Geophys. Res (Atmospheres), 113, D20119, , 2008 • Precipitation over the USA from CPC: The Climate Prediciton Center (CPC) 0.25° Daily US Unified Gauge-Based precipitation dataset version 1.0 is used from 1985-2015 (30 years). Higgins, R.W., Shi, W., Yarosh, E., Joyce, R. Improved United States Precipitation Quality Control System and Analysis, ATLAS No. 7, NCEP/Climate Prediction Center, Camp Springs, USA, 40 pp., 2000. • Actual evaporation from GLEAM: Global Land Evaporation: the Amsterdam Methodology (GLEAM) dataset version 3.0a from 1985 until 2015 (30 years) Martens, B., Miralles, D.G., Lievens, H., van der Schalie, R., de Jeu, R.A.M., Fernández-Prieto, D., Beck, H.E., Dorigo, W.A., and Verhoest, N.E.C.: GLEAM v3: satellite-based land evaporation and root-zone soil moisture, Geoscientific Model Development, 10, 1903–1925, , 2017. Miralles, D.G., Holmes, T.R.H., de Jeu, R.A.M., Gash, J.H., Meesters, A.G.C.A., Dolman, A.J.: Global land-surface evaporation estimated from satellite-based observations, Hydrology and Earth System Sciences, 15, 453–469, , 2011. • Observed discharge at Lobith and Vicksburg from GRDC: Daily discharge data for the Rhine at Lobith and the Mississippi at Vicksburg are obtained from the Global Runoff Data Center (GRDC) from 1985 until 2015 (30 years). GRDC: GRDC in the Bundesanstalt fuer Gewaesserkunde, Tech. rep., Koblenz, Germany, available at: , last access: 12-03-2019, 2007. • Parameter fields for the two resolutions (0.5° and 0.05°) hydrological model W3RA and routing module wflow: Extra information on the model parameters is provided in the README file in the w3ra_parameters folder • Shapefiles Rhine and Mississippi basin The files in this folder called wribasin contain the shapefile for the Rhine and Mississippi basin which is used to determine the basin averages. The input data for the hydrological model and the output of the hydrological model are not stored in this repository. This data is available upon request to the author ([email protected] or [email protected]

Large-scale flow patterns associated with extreme precipitation and atmospheric rivers over Norway

A climatology of extreme cold season precipitation events in Norway from 1979 to 2014 is presented, based on the 99th percentile of the 24-h accumulated precipitation. Three regions, termed north, west, and south are identified, each exhibiting a unique seasonal distribution. There is a proclivity for events to occur during the positive phase of the NAO. The result is statistically significant at the 95th percentile for the north and west regions. An overarching hypothesis of this work is that anomalous moisture flux, or so-called atmospheric rivers (ARs), are integral to extreme precipitation events during the Norwegian cold season. An objective analysis of the integrated vapor transport illustrates that more than 85% of the events are associated with ARs. An empirical orthogonal function and fuzzy cluster technique is used to identify the large-scale weather patterns conducive to the moisture flux and extreme precipitation. Five days before the event and for each of the three regions, two patterns are found. The first represents an intense, southward-shifted jet with a southwest-northeast orientation. The second identifies a weak, northward-shifted, zonal jet. As the event approaches, regional differences become more apparent. The distinctive flow pattern conducive to orographically enhanced precipitation emerges in the two clusters for each region. For the north and west regions, this entails primarily zonal flow impinging upon the south-north-orientated topography, the difference being the latitude of the strong flow. In contrast, the south region exhibits a significant southerly component to the flow

Teaching a Weather Forecasting Class in the 2020s

We report on redesigning the undergraduate course in synoptic meteorology and weather forecasting at Wageningen University (the Netherlands) to meet the current-day requirements for operational forecasters. Weather strongly affects human activities through its impact on transportation, energy demand planning, and personal safety, especially in the case of weather extremes. Numerical weather prediction (NWP) models have developed rapidly in recent decades, with reasonably high scores, even on the regional scale. The amount of available NWP model output has sharply increased. Hence, the role and value of the operational weather forecaster has evolved into the role of information selector, data quality manager, storyteller, and product developer for specific customers. To support this evolution, we need new academic training methods and tools at the bachelor's level. Here, we present a renewed education strategy for our weather forecasting class, called Atmospheric Practical, including redefined learning outcomes, student activities, and assessments. In addition to teaching the interpretation of weather maps, we underline the need for twenty-first-century skills like dealing with open data, data handling, and data analysis. These skills are taught using Jupyter Python Notebooks as the leading analysis tool. Moreover, we introduce assignments about communication skills and forecast product development as we aim to benefit from the internationalization of the classroom. Finally, we share the teaching material presented in this paper for the benefit of the community

The benefits of spatial resolution increase in global simulations of the hydrological cycle evaluated for the Rhine and Mississippi basins

To study the global hydrological cycle and its response to a changing climate, we rely on global climate models (GCMs) and global hydrological models (GHMs). The spatial resolution of these models is restricted by computational resources and therefore limits the processes and level of detail that can be resolved. Increase in computer power therefore permits increase in resolution, but it is an open question where this resolution is invested best: in the GCM or GHM. In this study, we evaluated the benefits of increased resolution, without modifying the representation of physical processes in the models. By doing so, we can evaluate the benefits of resolution alone.We assess and compare the benefits of an increased resolution for a GCM and a GHM for two basins with long observational records: the Rhine and Mississippi basins. Increasing the resolution of a GCM (1.125 to 0.25°) results in an improved precipitation budget over the Rhine basin, attributed to a more realistic large-scale circulation. These improvements with increased resolution are not found for the Mississippi basin, possibly because precipitation is strongly dependent on the representation of still unresolved convective processes. Increasing the resolution of the GCM improved the simulations of the monthly-averaged discharge for the Rhine, but did not improve the representation of extreme streamflow events. For the Mississippi basin, no substantial differences in precipitation and discharge were found with the higher-resolution GCM and GHM. Increasing the resolution of parameters describing vegetation and orography in the high-resolution GHM (from 0.5 to 0.05°) shows no significant differences in discharge for both basins. A straightforward resolution increase in the GHM is thus most likely not the best method to improve discharge predictions, which emphasizes the need for better representation of processes and improved parameterizations that go hand in hand with resolution increase in a GHM.</p

Anomalous moisture sources of the Rhine basin during the extremely dry summers of 2003 and 2018

Droughts can be studied from an atmospheric perspective by analysing large-scale dynamics and thermodynamics, and from a hydrological perspective by analysing interaction of precipitation, evaporation, soil moisture and temperature at the land-surface. Here, we study it from both perspectives, and assess the moisture (evaporative) sources of precipitation in the Rhine basin during the exceptionally dry summers of 2003 and 2018. We use ERA5 re-analysis data (1979–2018) and the Eulerian moisture tracking model WAM-2layers in order to determine the moisture sources of the Rhine basin. During an average summer, these evaporative sources are mostly located over the Atlantic Ocean, and there is a large contribution from continental evaporation, mostly from regions west of the Rhine basin. Both in 2003 and 2018 the absolute moisture source contribution declined over the ocean. In both years the anomalous moisture fluxes over the boundaries of the Rhine basin are mainly a result of anomalous wind and not because of anomalous moisture advection by the mean wind. Due to high pressure (blocking) over Europe, moisture is transported from the ocean with anticyclonic flow around the Rhine basin, but not into the basin. In 2018, unlike 2003, moisture is transported from the east towards the basin as a result of the anticyclonic flow around the Scandinavian blocking. The large-scale synoptic situation during the summer of 2018 was exceptional, and very favourable for dry conditions over the Rhine basin. Although blocking also occurred in 2003, the exact synoptic conditions were less favourable to dryness over the Rhine basin. In 2003 however, the recycling of moisture within the basin was much lower than the climatology and 2018, especially in August, possibly indicating the drying out of the soil resulting in the second heatwave in August 2003. To conclude, although the summer of 2003 and 2018 were both exceptionally dry, their characteristics in terms of moisture sources and recycling, and thereby their dependence on the large-scale circulation and land-atmosphere interactions, were found to be very different. It is therefore imperative that droughts are also studied as individual events to advance understanding of complex interactions between the large-scale atmospheric processes and the land surface.</p

Decline in terrestrial moisture sources of the mississippi river basin in a future climate

Assessment of the impact of climate change on water resources over land requires knowledge on the origin of the precipitation and changes therein toward the future. We determine the origin of precipitation over the Mississippi River basin (MRB) using high-resolution (~25 km) climate model simulations for present and future climate (RCP4.5). Moisture resulting in precipitation over the MRB is tracked back in time using Eulerian offline moisture tracking, in order to find out from where this water originally evaporated (i.e., the moisture sources). We find that the most important continental moisture sources are the MRB itself and the area southwest of the basin. The two most relevant oceanic sources are the Gulf of Mexico/Caribbean and the Pacific. The distribution of sources varies per season, with more recycling of moisture within the basin during summer and more transport of moisture from the ocean toward the basin in winter. In future winters, we find an increase in moisture source from the oceans (related to higher sea surface temperatures), resulting in more precipitation over the MRB. In future summers, we find an approximately 5% decrease in moisture source from the basin itself, while the decrease in precipitation is smaller (i.e., lower recycling ratios). The results here are based on one climate model, and we do not study low-frequency climate variability. We conclude that Mis-sissippi’s moisture sources will become less local in a future climate, with more water originating from the oceans.</p