A global climatology of favourable conditions for polar lows

Kolstad, Erik W.

doi:10.1002/qj.888

Cited by 62 publications

(73 citation statements)

References 48 publications

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“…They pose a direct hazard to human activities in that they set up a large energy imbalance which can drive extreme weather. For instance, MCAOs constitute a favourable environment for the formation of polar lows (Kolstad, 2011), intense mesoscale cyclones that form over open ocean at high latitudes (Rasmussen and Turner, 2003). Even when no polar lows form, strong surface wind speeds can occur along Arctic fronts separating cold air masses from warmer air further south (Grønås and Skeie, 1999).…”

Section: Introductionmentioning

confidence: 99%

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Kolstad¹

2017

Preprint

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Marine cold air outbreaks (MCAOs) are large-scale events in which cold air masses are advected over open ocean. It is well-known that these events are linked to the formation of polar lows and other mesoscale phenomena associated with high wind speeds, and that they therefore in some cases represent a hazard to maritime activities. However, it is still unknown whether MCAOs are generally conducive to higher wind speeds than normal. Here this is investigated by comparing ocean near-surface wind speeds during MCAOs in atmospheric reanalysis products with different horizontal grid spacings, along with two case studies using a convection-permitting numerical weather prediction model. The study regions are the Labrador Sea and the Greenland-Iceland-Norwegian (GIN) Seas, where MCAOs have been shown to be important for air-sea interaction and deep water formation. One of the main findings is that wind speeds during the strongest MCAO events are higher than normal and higher than wind speeds during less severe events. Limited evidence from the case studies suggests that reanalyses with grid spacings of more than 50 km underestimate winds driven by the large ocean-atmosphere energy fluxes during MCAOs. The peak times of MCAO usually occur when baroclinic waves pass over the regions. Therefore, the strong wind episodes during MCAOs generally last for just a few days. However, MCAOs can persist for 50 days or more.

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Section: Introductionmentioning

confidence: 99%

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Kolstad¹

2017

Preprint

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“…These two conditions correspond to the two parameters, marine cold-air outbreaks associated with low-level static stability and tropopause level as an indicator of upper-level forcing, assessed by Kolstad (2011). Also, these two conditions may correspond to the condition proposed by Grøna˚s and Kvamstø (1995), namely, a height difference of less than 1000 m between the tropopause, defined as the 2 PVU surface, and the top of the convective mixing layer below.…”

Section: Favourable Conditions For Pl Developmentmentioning

confidence: 85%

Roles of an upper-level cold vortex and low-level baroclinicity in the development of polar lows over the Sea of Japan

Shimada¹,

Wada²,

Yamazaki

et al. 2014

Tellus A: Dynamic Meteorology and Oceanography

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A B S T R A C TThe relationship between a synoptic-scale, upper-level cold vortex and a meso-a scale polar low (PL) over the Sea of Japan, and the role played by low-level baroclinicity in the development of the PL were investigated. We examined three PLs from the perspective of potential vorticity (PV) and energy budgets, using mesoscale analysis data with a horizontal resolution of Â11 km. During the development stage of each PLs, a meso-a scale upperlevel PV (UPV) anomaly, which intrudes to around the 700Á600 hPa level and which is just a part of a synopticscale PV anomaly accompanying the cold vortex, moved to the west side of a low-level PL. The PL was located in a wide region of low static stability in the lower troposphere. These conditions suggested that the synoptic-scale cold vortex, accompanied by a cold dome and the different scales of UPV anomalies, played a role in creating a favourable environment for the PL development. One of the common characteristics of the PL cases studied here was that, in contrast to the low-level PL, the upper-level disturbance associated with the meso-a scale UPV anomaly showed no development at all. This is likely because of the mobile UPV anomaly cut off from its source. Low-level baroclinicity, which varied with time, was an important factor in differentiating the PL shape and the energy sources necessary for PL development. Furthermore, the strength of baroclinicity, through its enhancement of mesoscale fronts and convection, affected diabatic processes, as well as baroclinic development.

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“…Kolstad, 2006Kolstad, , 2011. Empirical data suggest that the temperature difference between the surface (i.e.…”

Section: Static Stability and Upper-level Conditionsmentioning

confidence: 99%

“…well inside the polar air mass. In the Northern Hemisphere, the most favoured regions for polar low formation are over the northernmost extent of the warm ocean currents (Gulf Stream, Kuroshio) and in regions with frequent MCAOs, such as over the Labrador, Irminger, Norwegian and Barents Seas, as well as near Japan and over the Sea of Okhotsk (Kolstad, 2011). Recently, substantial changes in polar low frequency in a future warmer climate have been suggested (Kolstad and Bracegirdle, 2008;Zahn and von Storch, 2010), drawing attention to the importance of a better understanding of this intriguing phenomenon.…”

Section: Introductionmentioning

confidence: 99%

Orographic influence of east Greenland on a polar low over the Denmark Strait

Kristjánsson

Þorsteinsson

Kolstad

et al. 2011

Quart J Royal Meteoro Soc

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We present a numerical study of a polar low which hit western Iceland in January 2007, with heavy snowfall and mean wind speeds exceeding 20 m s −1 in several locations. The operational models at the time captured the polar low formation rather well, but there was a large spread in their predictions of the subsequent evolution and track of the polar low. The objective of this study is to investigate possible orographic forcing from Greenland as a trigger for the polar low development. In addition to an analysis of surface observations and satellite imagery, sensitivity studies using HIRLAM were carried out with various degradations of Greenland's orography, as well as with modifications to the sea-surface temperature (SST), surface roughness and the data assimilation scheme. Despite the presence of an upper-level trough and weak static stability in all the simulations, the polar low development was found to be very sensitive to the presence of the high mountains of eastern Greenland. Whereas the control run captured well the main features of the polar low, simulations with parts of east Greenland's orography removed gave a southward-displaced polar low which moved rapidly eastward, resulting in substantially underestimated nearsurface winds and snowfall amounts. Setting the orographic heights over all of Greenland to zero led to the complete disappearance of the polar low. On the other hand, artificially increasing the SST by 4 K in the Denmark Strait, reducing the orographic roughness or replacing the four-dimensional variational assimilation scheme (4D-Var) by 3D-Var had only a small effect on the polar low. We suggest that hitherto unreported interactions between the high mountains of east Greenland and polar low development over the Denmark Strait may be more important for polar low formation than katabatic flow from valleys in east Greenland that was highlighted in earlier studies.

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A global climatology of favourable conditions for polar lows

Cited by 62 publications

References 48 publications

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Roles of an upper-level cold vortex and low-level baroclinicity in the development of polar lows over the Sea of Japan

Orographic influence of east Greenland on a polar low over the Denmark Strait

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