A global climatology of polar lows investigated for local differences and wind-shear environments

Abstract

Abstract. Polar lows are intense mesoscale cyclones developing in marine polar air masses. This study presents a new global climatology of polar lows based on the ERA5 reanalysis for the years 1979–2020. Criteria for the detection of polar lows are derived based on a comparison of five polar-low archives with cyclones derived by a mesoscale tracking algorithm. The characteristics associated with polar lows are considered by the following criteria: (i) intense cyclone (large relative vorticity), (ii) mesoscale (small vortex diameter), and (iii) development in the marine polar air masses (a combination of low potential static stability and low potential temperature at the tropopause). Polar lows develop in all marine areas adjacent to sea ice or cold landmasses, mainly in the winter half year. The length and intensity of the season are regionally dependent. The highest density appears in the Nordic Seas. For all ocean sub-basins, forward-shear polar lows are the most common, whereas weak-shear polar lows and those propagating towards warmer environments are second and third most frequent, depending on the area. Reverse-shear polar lows and those propagating towards colder environments are rather seldom, especially in the Southern Ocean. Generally, polar lows share many characteristics across ocean basins and wind-shear categories. The most remarkable difference is that forward-shear polar lows often occur in a stronger vertical wind shear, whereas reverse-shear polar lows feature lower static stability. Hence, the contribution to a fast baroclinic growth rate is slightly different for the shear categories.

Insights into the relation between vertical cloud structure and dynamics of three polar lows: Observations from COMBLE

“…Moreover, the eye-like features of PL1 and PL3 are not as well developed as seen in the satellite images. The PL tracks inferred from the local maximum of AA 850 hPa smoothed relative vorticity 𝜉 smth,850 hPa (Stoll, 2022) are displayed in Figure 2a,d,g. The tracks start when 𝜉 smth,850 hPa exceeds 1.5 × 10 −4 s −1 -instead of 2 × 10 −4 s −1 used in Stoll (2022), to capture the early development stage of the PLs -and end at landfall.…”

“…The PL tracks inferred from the local maximum of AA 850 hPa smoothed relative vorticity 𝜉 smth,850 hPa (Stoll, 2022) are displayed in Figure 2a,d,g. The tracks start when 𝜉 smth,850 hPa exceeds 1.5 × 10 −4 s −1 -instead of 2 × 10 −4 s −1 used in Stoll (2022), to capture the early development stage of the PLs -and end at landfall. Compared with the satellite imagery (Figure 1), the AA tracks agree well.…”

“…For instance, Stoll (2022) finds that ∼95% of PLs have a minimum SLP below 1,010 hPa. The pressure anomaly p def and 𝜉 smth,850 hPa are better PL intensity indicators than absolute SLP values are (Stoll, 2022). p def is defined as SLP 110 km − SLP: the difference between the mean SLP in a 110 km radius circle and the SLP at the circle's center.…”

“…In particular, criterion (2) is quite scale dependent; Stoll et al (2018) used ECMWF Reanalysis-Interim (79 km grid resolution). We also analyze the previously mentioned 𝜉 smth,850 hPa (Stoll, 2022) as a parameter for PL intensity. A vortex for which 𝜉 smth,850 hPa exceeds 2 × 10 −4 s −1 is considered to have PL intensity (Stoll, 2022).…”

Subseasonal Predictions of Polar Low Activity Using a Hybrid Statistical‐Dynamical Approach

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Polar lows (PLs) are intense maritime mesocyclones that occur during the winter season over high-latitude oceans (Rasmussen & Turner, 2003) with average horizontal length scales on the order of 300 km and lifetimes around 20 hr (Stoll, 2022). PLs can serve as major threats to human life and property through their associated severe weather conditions, such as gale-force winds, large-amplitude oceanic waves (Rojo et al, 2019), and heavy snowfall (Harrold & Browning, 1969).

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