Weather and climate dynamics 

About: Weather and climate dynamics is an academic journal published by Copernicus GmbH. The journal publishes majorly in the area(s): Climatology & Environmental science. It has an ISSN identifier of 2698-4016. It is also open access. Over the lifetime, 79 publications have been published receiving 178 citations. The journal is also known as: An interactive open-access journal of the European Geosciences Union.

Atmospheric blocking and weather extremes over the Euro-Atlantic sector – a review

Abstract: Abstract. The physical understanding and timely prediction of extreme weather events are of enormous importance to society due to their associated impacts. In this article, we highlight several types of weather extremes occurring in Europe in connection with a particular atmospheric flow pattern, known as atmospheric blocking. This flow pattern effectively blocks the prevailing westerly large-scale atmospheric flow, resulting in changing flow anomalies in the vicinity of the blocking system and persistent conditions in the immediate region of its occurrence. Blocking systems are long-lasting, quasi-stationary and self-sustaining systems that occur frequently over certain regions. Their presence and characteristics have an impact on the predictability of weather extremes and can thus be used as potential indicators. The phasing between the surface and the upper-level blocking anomalies is of major importance for the development of the extreme event. In summer, heat waves and droughts form below the blocking anticyclone primarily via large-scale subsidence that leads to cloud-free skies and, thus, persistent shortwave radiative warming of the ground. In winter, cold waves that occur during atmospheric blocking are normally observed downstream or south of these systems. Here, meridional advection of cold air masses from higher latitudes plays a decisive role. Depending on their location, blocking systems also may lead to a shift in the storm track, which influences the occurrence of wind and precipitation anomalies. Due to these multifaceted linkages, compound events are often observed in conjunction with blocking conditions. In addition to the aforementioned relations, the predictability of extreme events associated with blocking and links to climate change are assessed. Finally, current knowledge gaps and pertinent research perspectives for the future are discussed.

A climate-change attribution retrospective of some impactful weather extremes of 2021

Abstract: Abstract. The IPCC report AR6 indicates a general consensus that anthropogenic climate change is modifying frequency and intensity of class of extreme events such as cold spells, heatwaves, storms or floods. A different point of view is to investigate whether a specific extreme event would have been possible in the absence of climate change, or whether climate change may have affected its specific characteristics. Here, we address this question by performing an attribution of some major extreme events that occurred in 2021 over Europe and North America: the winter storm Filomena, the French Spring cold spell, the Westphalia Floods, the Mediterranean summer heatwave, the hurricane Ida, the Po Valley tornadoes outbreak, the medicane Apollo and the late autumn Scandinavian cold-spell. We focus on the role of the atmospheric circulation associated with the events and its likelihood in present (factual world) and past climate conditions (counterfactual world) – defined using the ERA5 dataset 1950 to present. We use an analogs-based methodology whose aim is to find the most similar sea-level pressure patterns to the target events in the factual and counterfactual worlds and compute significant shifts in probability, persistence, predictability and seasonality of the patterns. We also diagnose whether in the present climate the analogs of the studied events lead to warmer/cooler or dryer/wetter conditions than in the past. We find that most of the events are significantly modified in present climate with respect to the past, because of changes in position, persistence and seasonality of cyclonic/anticyclonic patterns. Two of the events, storm Filomena and Medicane Apollo, appears to be a black swan of the atmospheric circulation, with analogs of bad quality. Our approach, complementary to the statistical methods already available in the community, warns that the role of the atmospheric circulation should be taken into account when performing attribution studies.

Future changes in North Atlantic winter cyclones in CESM-LE – Part 1: Cyclone intensity, potential vorticity anomalies, and horizontal wind speed

Abstract: Abstract. Strong low-level winds associated with extratropical cyclones can have substantial impacts on society. The wind intensity and the spatial distribution of wind maxima may change in a warming climate; however, the involved changes in cyclone structure and dynamics are not entirely clear. Here, such structural changes of strong North Atlantic cyclones in a warmer climate close to the end of the current century are investigated with storm-relative composites based on Community Earth System Model Large Ensemble (CESM-LE) simulations. Furthermore, a piecewise potential vorticity inversion is applied to associate such changes in low-level winds to changes in potential vorticity (PV) anomalies at different levels. Projected changes in cyclone intensity are generally rather small. However, using cyclone-relative composites, we identify an extended wind footprint southeast of the center of strong cyclones, where the wind speed tends to intensify in a warmer climate. Both an amplified low-level PV anomaly driven by enhanced diabatic heating and a dipole change in upper-level PV anomalies contribute to this wind intensification. On the contrary, wind changes associated with lower- and upper-level PV anomalies mostly compensate for each other upstream of the cyclone center. Wind changes at upper levels are dominated by changes in upper-level PV anomalies and the background flow. Altogether, our results indicate that a complex interaction of enhanced diabatic heating and altered non-linear upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones.

Trends in the tropospheric general circulation from 1979 to 2022

Abstract: Abstract. Atmospheric general circulation changes from March 1979 to February 2022 are examined using the ERA5 reanalysis. Maps of linear trends and time series for specific areas are presented. Attention is concentrated on monthly, seasonal and annual means, but shorter-timescale variability is also considered, including extremes. Changes in near-tropopause winds are the main focus, but related changes in temperature, wind and other variables throughout the troposphere are discussed. Middle- and upper-tropospheric warming is larger in the subtropics and outer tropics than in the deep tropics, except over the Pacific. This is linked with a strengthening and meridional expansion of the tropical easterlies that has received little previous attention. The change occurs predominantly over the first half of the period. Warming over several mid-latitude and subtropical land areas comes close to matching the large warming of the Arctic, in some seasons at least. Westerly upper-level winds in general weaken over the Arctic in winter but strengthen in northern middle latitudes, contrary to arguments based on circulation changes due solely to amplified Arctic warming. The jet-stream region over the eastern North Atlantic and western Europe shifts southward. Westerlies strengthen in a band stretching south-eastwards from the tropical western Pacific to southern Australia, as well as in the polar-jet-stream region that surrounds Antarctica. Extreme jet-stream winds increase over the North Atlantic. Net kinetic energy also increases, mostly associated with sub-monthly variability along the mid-latitude storm tracks and over the tropical Pacific. Available potential energy changes less. Geopotential height shows a distinct pattern of change in stationary long-wave structures. There are increases in surface pressure over the North Pacific and southern mid-latitudes and decreases over the Arctic Ocean and offshore of Antarctica. Several comparisons are made between ERA5 and the JRA-55 reanalysis and between ERA5 and the observations it assimilated. They show reassuring agreement, but some regional differences require further investigation.

Differentiating lightning in winter and summer with characteristics of the wind field and mass field

Abstract: Abstract. Lightning in winter (December–January–February, DJF) is rare compared to lightning in summer (June–July–August, JJA) in central Europe north of the Alps. The conventional explanation attributes the scarcity of lightning in winter to seasonally low values of variables that create favorable conditions in summer. Here we systematically examine whether different meteorological processes are at play in winter. We use cluster analysis and principal component analysis and find physically meaningful groups in ERA5 atmospheric reanalysis data and lightning data for northern Germany. Two thunderstorm types emerged: wind-field thunderstorms and CAPE (convective available potential energy) thunderstorms. Wind-field thunderstorms are characterized by increased wind speeds, high cloud shear, large dissipation of kinetic energy in the boundary layer, and moderate temperatures. Clouds are close to the ground, and a relatively large fraction of the clouds are warmer than −10 ∘C. CAPE thunderstorms are characterized by increased convective available potential energy (CAPE), the presence of convective inhibition (CIN), high temperatures, and accompanying large amounts of water vapor. Large amounts of cloud-physics variables related to charge separation such as ice particles or cloud base height further differentiate both wind-field thunderstorms and CAPE thunderstorms. Lightning in winter originates in wind-field thunderstorms, whereas lightning in summer originates mostly in CAPE thunderstorms and only a small fraction in wind-field thunderstorms. Consequently, typical weather situations of wind-field thunderstorms in the study area in northern Germany are strong westerlies with embedded cyclones. For CAPE thunderstorms, the area is typically on the anticyclonic side of a southwesterly jet.