Polar lows – moist-baroclinic cyclones developing in four different vertical wind shear environments

Abstract

Abstract. Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety of their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use self-organising maps to classify polar lows. The method is applied to 370 polar lows in the north-eastern Atlantic, which were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered in the STARS dataset by the Norwegian Meteorological Institute. ERA-5 reproduces most of the STARS polar lows. We identify five different polar-low configurations which are characterised by the vertical wind shear vector, the change in the horizontal-wind vector with height, relative to the propagation direction. Four categories feature a strong shear with different orientations of the shear vector, whereas the fifth category contains conditions with weak shear. This confirms the relevance of a previously identified categorisation into forward- and reverse-shear polar lows. We expand the categorisation with right- and left-shear polar lows that propagate towards colder and warmer environments, respectively. For the strong-shear categories, the shear vector organises the moist-baroclinic dynamics of the systems. This is apparent in the low-pressure anomaly tilting with height against the shear vector and the main updrafts occurring along the warm front located in the forward-left direction relative to the shear vector. These main updrafts contribute to the intensification through latent heat release and are typically associated with comma-shaped clouds. Polar-low situations with a weak shear, which often feature spirali-form clouds, occur mainly at decaying stages of the development. We thus find no evidence for hurricane-like intensification of polar lows and propose instead that spirali-form clouds are associated with a warm seclusion process.

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

“…This is different from Stoll et al (2021) who analyze PLs from the perspective of their propagation direction and hence compare the shear vector to the propagation of the PLs. However, Stoll et al (2021) argue that the mean tropospheric wind provides a good estimate of the propagation direction of PLs, and hence both methods are expected to give similar results. Differences in the propagation direction and the orientation of the mean tropospheric wind vector can occur in situations of https://doi.org/10.5194/wcd-2021-60 Preprint.…”

“…The vertical wind shear is derived as in Stoll et al (2021), but within a distance of 250 km, instead of 500 km. Both radii were tested and the results are robust, but the smaller radius appears to better represent the local environment.…”

“…The vertical wind-shear angle is computed between the tropospheric mean wind vector and the wind-shear vector, as in Terpstra et al (2016). This is different from Stoll et al (2021) who analyze PLs from the perspective of their propagation direction and hence compare the shear vector to the propagation of the PLs. However, Stoll et al (2021) argue that the mean tropospheric wind provides a good estimate of the propagation direction of PLs, and hence both methods are expected to give similar results.…”

“…Different to Stoll et al (2021), the upper threshold for the weak shear category is set to 1.0 instead of 1.5×10 −3 s −1 , since the purposes are different. Stoll et al (2021) demonstrated differences among the strong shear categories, whereas this study aims to identify the locations in which the shear types occur. However, the results of both studies are robust for the other threshold.…”

“…The vortex diameter is estimated by computing the diameter of a circle with the same area as the vortex area, which is the area surrounding the vorticity maxima with ξ smth,850hP a exceeding 10×10 −5 s −1 as described in Watanabe et al (2016) but with thresholds in accordance to Stoll et al (2021). It should be noted that the vortex diameter depends to some degree on the threshold and the smoothness of the vorticity field.…”