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Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments

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Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments Patrick Johannes Stoll1, Thomas Spengler2, Annick Terpstra2, and Rune Grand Graversen1,3 1Department of Physics and Technology, Arctic University of Norway, Tromsø, Norway 2Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway 3Norwegian Meteorological Institute, Norway Correspondence: Patrick Johannes Stoll ([email protected]) Abstract. Polar lows are intense mesoscale cyclones that tensification of polar lows and propose instead that spirali- develop in polar marine air masses. Motivated by the large form clouds are associated with a warm seclusion process. variety of their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use 5 self-organising maps to classify polar lows. The method is applied to 370 polar lows in the North-East Atlantic, which 1 Introduction 35 were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered in the STARS Polar lows (PLs) are intense mesoscale cyclones with a typ- dataset by the Norwegian Meteorological Institute. ERA-5 ical diameter of 200 - 500 km and a short lifetime of 6 - 36 h 10 reproduces most of the STARS polar lows. that develop in marine cold-air outbreaks during the extended We identify five different polar-low configurations which winter season (e.g Rasmussen and Turner, 2003; Yanase are characterised by the vertical wind shear vector, the et al., 2004; Claud et al., 2004; Renfrew, 2015; Rojo et al., 40 change of the horizontal-wind vector with height, relative to 2015). Despite numerical weather-prediction models being the propagation direction. Four categories feature a strong capable of simulating these systems, operational forecasts 15 shear with different orientations of the shear vector, whereas still have have issues predicting the exact location and inten- the fifth category contains conditions with weak shear. This sity of PLs (e.g. Kristjánsson et al., 2011; Føre et al., 2012; confirms the relevance of a previously identified categorisa- Stoll et al., 2020). Several paradigms have been proposed to 45 tion into forward and reverse-shear polar lows. We expand describe PL development, ranging from baroclinic instabil- the categorisation with right and left-shear polar lows that ity (e.g. Harrold and Browning, 1969; Reed, 1979; Reed and 20 propagate towards colder and warmer environments, respec- Duncan, 1987) to symmetric hurricane-like growth (e.g. Ras- tively. mussen, 1979; Emanuel and Rotunno, 1989). The multitude For the strong-shear categories, the shear vector organises of paradigms demonstrates that our dynamical interpretation 50 the moist-baroclinic dynamics of the systems. This is appar- of these systems is still deficient (Jonassen et al., 2020). To ent in the low-pressure anomaly tilting with height against further alleviate this shortcoming, we present a classification 25 the shear vector, and the main updrafts occurring along the of PLs by their structure and sub-synoptic environment to warm front located in the forward-left direction relative to identify the relevance of the proposed paradigms. the shear vector. These main updrafts contribute to the inten- The proposed PL paradigms often stem from the differ- 55 sification through latent-heat release and are typically asso- ent cloud structures (Forbes and Lottes, 1985; Rojo et al., ciated with comma-shaped clouds. 2015) and sub-synoptic environments (Duncan, 1978; Terp- 30 Polar low situations with a weak shear, that often feature stra et al., 2016). From the 1980s, the PL spectrum was spirali-form clouds, occur mainly at decaying stages of the thought to range from pure baroclinic PLs with a comma- development. We thus find no evidence for hurricane-like in- shaped cloud structure to convective systems with a spirali- 60 form cloud signature like hurricanes (p.157 Rasmussen and Turner, 2003). The latter types usually were argued to be in- 2 Patrick Stoll: Polar Lows – Moist Baroclinic Cyclones voked either conditional instability of the second kind (CISK cannot address the hurricane-like part of the PL spectrum. To Charney and Eliassen, 1964; Ooyama, 1964) or by wind- alleviate this shortcoming, we categorise PLs based on their induced surface heat exchange (WISHE Emanuel, 1986). sub-synoptic environment using self-organising maps (SOM, However, most PLs appear as hybrids between the extremes see Section 2.5). The thereby identified meteorological con- 5 of this spectrum (Bracegirdle and Gray, 2008). figurations reveal the underlying PL intensification mecha- 60 Using idealised simulations to map the sensitivities of cy- nisms, allowing us to investigate the following research ques- clone development in the PL spectrum, Yanase and Niino tions: (2007) found that cyclones intensify fastest in an environ- ment with high baroclincity where latent heat release sup- – What are the archetypal meteorological conditions dur- 10 ports the development. As neither dry baroclinic nor pure ing PL development? CISK modes grow fast enough to explain the rapid inten- sification of PLs, the growth is most likely associated with – Can the existing PL classification be confirmed or 65 moist baroclinic instability (Sardie and Warner, 1983; Terp- should it be revised? stra et al., 2015). – What are the pertinent intensification mechanisms? 15 Furthermore, hurricane-like PLs rarely resemble the struc- ture of hurricanes; instead they feature asymmetric updrafts typical of baroclinic development with latent heating not 2 Methods playing the dominant role (Føre et al., 2012; Kolstad et al., 2016; Kolstad and Bracegirdle, 2017). The PLs that appear 2.1 Polar-low list 20 hurricane-like in their mature stage appear to be initiated by baroclinic instability (e.g. Nordeng and Rasmussen, 1992; This study is based on the Rojo list (Rojo et al., 2019), a mod- 70 Føre et al., 2012). ified version of the STARS (Sea Surface Temperature and Several categorisations of PLs were proposed to shed light Altimeter Synergy for Improved Forecasting of Polar Lows) on the development pathways of PLs (e.g. Duncan, 1978; dataset. This list includes the location and time of PLs de- 25 Businger and Reed, 1989; Rasmussen and Turner, 2003; tected from AVHRR satellite images over the North-East At- Bracegirdle and Gray, 2008; Terpstra et al., 2016). Duncan lantic that were listed in the STARS dataset by the Norwe- 75 (1978) suggested a categorisation based on the vertical wind- gian Meteorological Institute between November 1999 and shear angle of the PL environment, defined as the angle be- March 2019 (Noer and Lien, 2010). The STARS dataset has tween the tropospheric mean wind vector and the thermal been used for several previous PL studies (e.g. Laffineur 30 wind vector. Situations where the vectors point in the same et al., 2014; Zappa et al., 2014; Rojo et al., 2015; Terpstra (opposite) direction are referred to as forward (reverse) shear et al., 2016; Smirnova and Golubkin, 2017; Stoll et al., 2018). 80 conditions. PLs have been found to occur in both forward- The advantage of the Rojo list compared to the STARS shear (e.g. Reed and Blier, 1986; Hewson et al., 2000) dataset is that it contains considerably more PL cases. While and reverse-shear environments (e.g. Reed, 1979; Bond and the STARS dataset only includes the major PL centre for 35 Shapiro, 1991; Nordeng and Rasmussen, 1992), where both events of multiple PL developments, the Rojo list includes types of PLs occur approximately equally often (Terpstra the location of all individual PL centres (Rojo et al., 2015). 85 et al., 2016). Thus, the Rojo list includes 420 PL centres, which are associ- PLs in forward-shear environments develop similar to typ- ated with the 262 PL events in the STARS database of which ical mid-latitude cyclones in a deep-baroclinic zone with an 183 PL events feature a single PL centre and the remaining 40 associated upper-level jet. They have the cold air to the left 79 events have 2 - 4 PL centres per event. In addition, the with respect to their direction of propagation and are mainly Rojo list classifies the cloud morphology for each detected 90 propagating eastward (Terpstra et al., 2016). PLs in reverse- PL time step. shear environments, on the other hand, often develop in the As the Rojo list includes individual time steps of each vicinity of an occluded low-pressure system and are charac- PL when AVHRR satellite images were available, the time 45 terised by a low-level jet. They have the cold air to the right interval between observations is irregular and ranges from with respect to their propagation direction and are mainly 30 minutes and up to 12 hours. This also implies that the list 95 propagating southward. Reverse-shear PLs also feature con- in many cases lacks the genesis and lysis time of some PLs. siderably higher surface heat fluxes and a lower static sta- bility in the troposphere, expressed by a larger temperature 2.2 Polar-low tracks in ERA-5 50 contrast between the temperature at the sea surface and at 500 hPa (Terpstra et al., 2016). We use the European Centre for Medium-Range Weather However, assigning PL environments solely based on two Forecasts (ECMWF) state-of-the-art reanalysis version 5 types of shear conditions might be insufficient to characterise (ERA-5 Hersbach and Dee, 2016) to track PLs and anal- 100 all PLs types. The shear angle cannot distinguish between yse the atmospheric environment. The ability of ERA-5 to 55 baroclinically- and convectively-driven systems, hence it simulate PLs has not yet been investigated, though some Patrick Stoll: Polar Lows – Moist Baroclinic Cyclones 3 studies have shown that atmospheric models with a com- latter occurs if the location of the vorticity maximum moves parable horizontal resolution to ERA-5 are capable to pro- within an area of high vorticity, e.g., a frontal zone.
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