Representation of atmospheric blocking in the new global non-hydrostatic weather prediction model ICON

International audienceThe correct depiction of atmospheric blocking still poses a key challenge for current numerical weather prediction (NWP) and climate models. This study evaluates the representation of blocking in the new global ICOsahedral Non-hydrostatic NWP and climate model ICON and links model mean state biases to observed blocking deviations. Blocking is identified using both an anomaly and a flow reversal approach in an eight member ensemble of 15-year AMIP-type ICON simulations and verified against ERA Interim reanalyses. Either approach demonstrates a good representation of annual blocking frequencies in ICON. Deviations emerge when considering individual seasons. In the anomaly framework, enhanced blocking occurrence in the mid-latitude Pacific domain during winter and spring and a marked underestimation of blocking in the Euro-Atlantic region are found during summer. Moreover, this approach indicates a general underestimation of blocking at higher latitudes. The flow reversal index reveals the often reported underestimation of blocking in the Euro-Atlantic region during winter. Furthermore, increased blocking activity in the Pacific and Greenland region during spring and decreased blocking occurrence at high latitudes in summer are found. Focusing on the anomaly approach, we assess how the model mean state influences blocking identification. A systematically higher tropopause, forced by a cold bias in the lower stratosphere, reduces diagnosed blocking frequencies at higher latitudes especially during summer. This goes along with a reduction in blocking size, duration, and intensity. While confirming an overall good representation of blocking in ICON, this study demonstrates how mean state biases can crucially affect the identification of blocking and that blocking deviations have to be interpreted with caution as they are highly dependent on the exact diagnostic used

Representation of blocking anticyclones in the new numerical weather prediction model ICON

Atmospheric blocking describes an important part of the weather variability inherent to the mid-latitudes imposing an anomalous strong, quasi stationary anticyclonic circulation on the upper-level flow. Blocking leads to a reversal of the customary upper-level westerly flow and deflects the jet stream as well as migratory weather systems north and southward. Owing to the long persistence and anomalous flow regime, blocking is associated with severe heat waves in summer and cold spells in winter. Despite its vast impact on regional weather and society, average skills in predicting blocking initiation and continuation are observed across many numerical weather prediction models. In this study, the representation of atmospheric blocking in the new general circulation model ICON is investigated by means of a blocking climatology, an analysis of blocking characteristics and a mean state assessment. To this end, a Northern Hemisphere blocking climatology is derived by identifying blocking as regions of anomalous low vertically averaged potential vorticity (PV) values directly below the tropopause. Three 5-year model runs forced with sea surface temperatures and sea ice fields from the ERA-Interim reanalysis for the period of 2001 to 2005 are integrated (AMIP setup). Results from increasing horizontal resolutions (80, 40 and 20 km) are considered and compared to ERA-Interim observations for the period of 1979 to 2014. Three preferred regions of blocking activity are identified, two at the end of the Atlantic and Pacific storm track and a third over northern Russia. Good agreement of the spatial blocking patterns is found, especially in the core region of blocking activity. Main regions of deviation are confined to the mid-latitudes ranging from the Pacific to the Atlantic ocean. Statistics show that blocking duration is overestimated, while blocking size is systematically underestimated in ICON. The atmospheric mean state of the troposphere is generally simulated well, whereas large biases emerge in the stratosphere. A cool bias in the lower stratosphere leads to a PV dipole which is responsible for globally lifting the tropopause north of 50° N. A potential link between the Madden-Julian oscillation and the PV distribution in the Pacific region is suggested, leading to a shift of the customary Pacific PV trough. The strong mean state bias as well as the overestimated duration of blocking in the low resolution configuration are mainly responsible for the mediocre blocking representation in the mid-latitude Pacific sector. An improved mean state with increasing horizontal resolution is observed in the Pacific, resulting in better blocking depiction. However, in the Atlantic sector, no improvement of the 20 km run over the intermediate configuration is found. This points to the importance of different processes in blocking representation such as orographic or diabatic effects in the Atlantic sector

Systematic assessment of the diabatic processes that modify low-level potential vorticity in extratropical cyclones

Diabatic processes significantly affect the development and structure of extratropical cyclones. Previous studies quantified the dynamical relevance of selected diabatic processes by studying their influence on potential vorticity (PV) in individual cyclones. However, a more general assessment of the relevance of all PV-modifying processes in a larger ensemble of cyclones is currently missing. Based on a series of twelve 35 d model simulations using the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts, this study systematically quantifies the diabatic modification of positive and negative low-level PV anomalies along the cold front, warm front, and in the center of 288 rapidly intensifying extratropical cyclones. Diabatic PV modification is assessed by accumulating PV tendencies associated with each parametrized process along 15 h backward trajectories. The primary processes that modify PV typically remain temporally consistent during cyclone intensification. However, a pronounced case-to-case variability is found when comparing the most important processes across individual cyclones. Along the cold front, PV is primarily generated by condensation in half of the investigated cyclones in the cold season (October to March). For most of the remaining cyclones, convection or long-wave radiative cooling is the most important process. Similar results are found in the warm season (April to September); however, the fraction of cyclones with PV generation by convection as the most important process is reduced. Negative PV west of the cold front is primarily produced by turbulent mixing of momentum, long-wave radiative heating, or turbulent mixing of temperature. The positive PV anomaly at the warm front is most often primarily generated by condensation in the cold season and by turbulent mixing of momentum in the warm season. Convection is the most important process only in a few cyclones. Negative PV along the warm front is primarily produced by long-wave radiative heating, turbulent mixing of temperature, or melting of snow in the cold season. Turbulent mixing of temperature becomes the primary process in the warm season, followed by melting of snow and turbulent mixing of momentum. The positive PV anomaly in the cyclone center is primarily produced by condensation in most cyclones, with only few cases primarily associated with turbulent mixing or convection. A composite analysis further reveals that cyclones primarily associated with PV generation by convection exhibit a negative air–surface temperature difference in the warm sector, which promotes a heat flux directed into the atmosphere. These cyclones generally occur over warm ocean currents in the cold season. On the other hand, cyclones that occur in a significantly colder environment are often associated with a positive air–surface temperature difference in the warm sector, leading to PV generation by long-wave radiative cooling. Finally, long-wave radiative heating due to a negative air–surface temperature difference in the cold sector produces negative PV along the cold and warm front, in particular in the cold season.ISSN:2698-4016ISSN:2698-400

The ERA5 Extreme Seasons Explorer as a Basis for Research at the Weather and Climate Interface

Meteorological extremes on the seasonal time scale have received increased attention due to their relevance for society and economy. A recently developed approach to identify seasonal extremes is applied here to ERA5 reanalyses from 1950 to 2020 to identify hot and cold, wet and dry, and stormy and calm extreme seasons globally. The approach consists of (i) fitting a statistical model to seasonal mean values (of temperature, precipitation, and wind speed) at each grid point, (ii) selecting a local return period threshold above which seasonal mean values are deemed extreme, and (iii) forming spatially coherent extreme season objects. The paper introduces the ERA5 extreme season explorer, an open-access web portal enabling researchers to visualize and download ex-treme season objects of any of the six types in their region of interest, for further investigating their underlying dynamics, statistical properties, and impacts. To illustrate the potential of our extreme season objects, we first discuss the top 10 cold winters in ERA5 globally and then focus on an unusual triple-compound extreme season in winter 1953/54 in Europe, which was simultaneously extremely cold, dry, and calm. We show that detailed analysis of weather system dynamics, includ-ing cyclones, blocks, jets, and Rossby waves, provides important insight into the processes leading to extreme seasons. In summary, this study presents for the first time a catalogue of objectively identified extreme seasons in the last decades, shows exemplarily how large-scale dynamics can lead to such seasons, and with the help of the explorer supports the community in accelerating research in this important area at the interface of weather and climate dynamics.ISSN:0003-0007ISSN:1520-047

Quantifying the role of individual diabatic processes for the formation of PV anomalies in a North Pacific cyclone

ISSN:0035-9009ISSN:1477-870

Modification of Potential Vorticity near the Tropopause by Nonconservative Processes in the ECMWF Model

ISSN:0022-4928ISSN:1520-046