Characteristics of extratropical cyclones and precursors to windstorms in Northern Europe

Laurila, Terhi K.; Gregow, Hilppa; Cornér, Joona; Sinclair, Victoria A.

doi:10.5194/wcd-2021-44

Cited by 2 publications

(2 citation statements)

References 40 publications

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“…In autumn, the diurnal cycle of BL height is weaker than spring or summer. One potential reason is that synoptic-scale weather patterns and thermal advection may have a stronger influence on the BL height in Autumn compared to other times of year as strong extra-tropical cyclones are more common in autumn than spring or summer (Laurila et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

Boundary-layer height and surface stability at SMEAR II, Hyytiälä, Finland in ERA5 and observations

Sinclair

Ritvanen

Urbancic

et al. 2021

Preprint

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Abstract. The planetary boundary layer (BL) height and stratification are key parameters in determining the exchange of heat, momentum, moisture and trace gases between the surface and the free troposphere. Numerous different methods have been used to quantify the BL height and these methods have been applied to a wide variety of observational data sets obtained from different instruments and to numerical model output. We investigate the BL height at the Hyytiälä SMEAR II station in southern Finland diagnosed from radiosonde observations, a microwave radiometer (MWR) and ERA5 reanalysis. Four different algorithms are used to diagnose the BL height from the radiosondes. The diagnosed BL height is sensitive to the method used and the level of agreement, and the sign of systematic bias, between the 4 different methods depends on the surface-layer stability. For example, for very unstable situations, the median BL height diagnosed from the radiosondes varies from 600 m to 1500 m depending on which method is applied. Good agreement between the BL height in ERA5 and diagnosed from the radiosondes using Richardson number-based methods is found for almost all stability classes, suggesting that ERA5 has adequate vertical resolution near the surface to resolve the BL structure. However, ERA5 overestimates the BL height in very stable conditions highlighting the on-going challenge for numerical models to correctly resolve the stable BL. Furthermore, ERA5 BL height differs most from the radiosondes at 18 UTC suggesting ERA5 does not resolve the evening transition correctly. This study has also shown that BL height estimates from the MWR are reliable in unstable situations but often are inaccurate under stable conditions when, in comparison to ERA5 BL heights, they are much deeper. The errors in the MWR BL height estimates originate from the limitations of the manufacturers algorithm for stable conditions and also the mis-identification of the type of BL. A climatology of the annual and diurnal cycle of BL height and observed surface layer stability was created. The shallowest (353 m) monthly median BL height occurs in February and the deepest (576 m) in June. In winter there is no diurnal cycle in BL height, unstable BLs are rare yet so are very stable BLs. The shallowest BLs occur at night in spring and summer and very stable conditions are most common at night in the warm season. Finally, using ERA5 gridded data we determined that the BL height observed at Hyytiälä is representative of most land areas in southern and central Finland. However, the spatial variability of the BL height is largest during daytime in summer reducing the area over which BL height observations from Hyytiälä would be representative of.

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Section: Discussionmentioning

confidence: 99%

Boundary-layer height and surface stability at SMEAR II, Hyytiälä, Finland in ERA5 and observations

Sinclair

Ritvanen

Urbancic

et al. 2021

Preprint

Self Cite

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“…Finland is a country located in the end of North Atlantic storm track (e.g. Wernli and Schwierz, 2006), and the strongest ETCs affecting northern Europe tend to originate and occur over the Norwegian Sea and the Barents Sea (Laurila et al, 2021).…”

Section: Variability Of Compound Events and The Effects Of Large-scale Circulationmentioning

confidence: 99%

Characteristics of joint heavy precipitation and high sea level events on the Finnish coast in 1961–2020

Rantanen

Jylhä

Särkkä

et al. 2021

Preprint

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Abstract. Simultaneous heavy precipitation with high sea level can lead to more severe flooding than if the hazards occur individually. In recent years, the significance of compound flooding has gained attention in many coastal areas, but studies on compound flooding in Finland are still lacking. In this paper, we investigate the co-occurrence of heavy precipitation and high sea level events (hereafter compound events) on the Finnish coast, because the simultaneous occurrence of these events can be considered as an indicator for compound flooding. We use daily sea level observations from tide gauges and gridded precipitation data to extract the days when both sea level and precipitation are extreme on the same day. Reanalysis data is used to derive composite maps of the compound events, and these maps are then qualitatively compared to the maps of non-compound events (i.e. when only one of the two variables is extreme alone). Finally, the dependence of the compound events on monthly indices of different atmospheric circulation patterns such as North Atlantic Oscillation is studied with correlation analysis. Our results show that the occurrence of the compound events is mostly controlled by sea level variability, with climatologically the most compound events taking place in autumn and winter. In addition to a large inter-annual variability of the compound events, statistically significant increasing trends were observed at some tide gauges, especially in the Bothnian Bay. The composite maps of meteorological variables indicate that the compound events are associated with strong extratropical cyclones which bring moisture and push the storm surge towards the coast. In average, the compound events are associated with stronger winds near the tide gauges than those events when only sea level or precipitation is extreme alone. Finally, we found that a negative phase of Scandinavian pattern (i.e. upper-level through over Scandinavia) is the most favorable atmospheric circulation pattern for the occurrence of compound events, in particular during autumn.

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Characteristics of extratropical cyclones and precursors to windstorms in Northern Europe

Cited by 2 publications

References 40 publications

Boundary-layer height and surface stability at SMEAR II, Hyytiälä, Finland in ERA5 and observations

Boundary-layer height and surface stability at SMEAR II, Hyytiälä, Finland in ERA5 and observations

Characteristics of joint heavy precipitation and high sea level events on the Finnish coast in 1961–2020

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