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

• Main Author: Attinger, Roman
• Other Authors: Grams, Christian M., Wernli, Heini, Riboldi, Jacopo, Keller, Julia
• Language: English
• Published: Institute for Atmospheric and Climate Science (IAC), ETH Zurich 2016
• Subjects: atmospheric blocking; Model evaluation; Pacific; Sea ice
• Online Access: https://hdl.handle.net/20.500.11850/361283; https://doi.org/10.3929/ethz-b-000361283

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.