Polar Low Research Articles

In Situ Measurements of Surface Winds, Waves, and Sea State in Polar Lows Over the North Atlantic

AbstractPolar low (PL) storms are an important feature of the wintertime subsynoptic‐scale atmospheric circulation of middle‐ and higher‐latitude ocean areas. They can generate hazardous conditions impacting coastal and marine activities like fishing, transport, and oil extraction. However, there are few studies available of individual PL systems based on high‐resolution maritime surface data. Accordingly, the meteorological impacts of 29 PLs have been investigated for the 14 winters 1999–2013, using in situ measurements at eight stations in the Norwegian and North Seas. On average, the highest wind speed and significant wave height (SWH) occur following the minimum in sea level pressure of the PL, respectively, 1 and 3 hr after its passage. The strongest wind speed averages 17.1 m/s, and the highest peak SWH is 6.3 m, but these can reach 31 m/s and 11 m, respectively. PL characteristics of system horizontal extent, propagation speed, and the larger‐scale atmospheric circulation environment explain the large intercase differences. Large, multiple, and fast‐moving PLs within a meridional circulation environment appear to generate stronger near‐surface winds and higher waves than do small, single, and slow‐moving PLs within a zonal circulation. Multiple systems may have the largest impacts (e.g., SWH > 8 m), although a larger sample size is required to confirm this possibility. The impacts of PLs on sea surface temperature (SST) are quite small and are difficult to interpret separate from the background SST variation. The observed SST decrease may be mainly caused by the cold air outbreak within which the PL is embedded; indeed, a positive SST minus air temperature anomaly is found during the 24 hr preceding the passage of PL vortices, indicating enhanced low‐level atmospheric instability.

Environmental conditions for polar low formation and development over the Nordic Seas: study of January cases based on the Arctic System Reanalysis

The sparse observational network over the Arctic region makes severe storms such as polar lows (PLs) still hard to predict. To improve their forecasting and detection, it is of great importance to gain better understanding of their formation and development. Therefore, we have analyzed the environment of PLs at their genesis and mature stages using a set of parameters: the difference between sea surface and 500 hPa temperature (or at 2 m height), lapse rate (LR) and relative humidity below 850 hPa (RH), near-surface wind speed and geopotential height anomaly. We evaluate which of these conditions (or which combination) is(are) the most favourable for PL formation and persistence. The analysis was performed on 33 January cases over 12 years (2000–2011) using the Arctic System Reanalysis. The results showed that, for the cases with lower thermal instability during formation stage, LRs throughout the boundary layer were stronger and steeper; therefore, these PLs were fostering convective development. However, for few cases, it was noted that when convection decreased simultaneously with increased thermal stability, RH most of the times was above 90%. It was also noted that the higher amount of RH at lower levels during genesis stage promoted stronger winds at the maturity stage.

Evaluation of wind forecasts over Svalbard using the high-resolution Polar WRF with 3DVAR

ABSTRACTIn this study, the performance of wind forecasts over Svalbard, located between the Arctic Ocean and the Norwegian Sea, was evaluated using the Polar Weather Research and Forecasting (PWRF) model and three-dimensional variational data assimilation (DA) system. The forecasts of the analysis–forecast cycling experiment using the PWRF 3DVAR were compared with those of the cold start experiment using reanalysis as the initial condition. Three strong wind cases that occurred during January and February 2011–2012 were selected, where polar lows were generated on the east coast of Greenland and generated a wind speed above 20 m s−1 in Svalbard. The wind speed forecasts for both cycling and cold start experiments were similar to the highest 10-minute average wind speed for the last 1 hour (HAW). The average root mean square error (RMSE) of the forecasts in the cold start experiment from HAW was 3.78 m s−1 for three cases and was greater than that in the cycling experiment. The forecast performance in the cycling experiment was comparable to, or even better than, that in the cold start experiment, which implies that the cycling system with DA is more useful than the cold start system in forecasting polar weather to support research activities.

Bimodality of hemispheric winter atmospheric variability via average flow tendencies and kernel EOFs

The topic of the existence of planetary winter circulation regimes has gone through a long debate. This article contributes to this debate by investigating nonlinearity in a 3-level quasi-geostrophic model and the Japanese JRA-55 reanalysis. The method uses averaged flow tendencies and kernel principal component (PC) analysis. Within two-dimensional (2D) kernel PCs the model reveals two fixed (or stationary) points. The probability density function (PDF) within this space is strongly bimodal where the modes match the regions of low tendencies in consistency with low-order conceptual models. The circulation regimes represent respectively zonal and blocked flows. Application to daily winter northern hemisphere sea level pressure and 500-hPa geopotential height yields strong bimodal PDFs. The modes represent respectively polar highs and lows with signatures of North Atlantic Oscillation. A clear climate change signal is observed showing a clear reduction (increase) of occurrence probability of polar high (low), translating into an increase of probability of zonal flow. Relation of the climate change signal to the polar amplification hypothesis is discussed.

Deep Convolutional Neural Networks Capabilities for Binary Classification of Polar Mesocyclones in Satellite Mosaics

Polar mesocyclones (MCs) are small marine atmospheric vortices. The class of intense MCs, called polar lows, are accompanied by extremely strong surface winds and heat fluxes and thus largely influencing deep ocean water formation in the polar regions. Accurate detection of polar mesocyclones in high-resolution satellite data, while challenging, is a time-consuming task, when performed manually. Existing algorithms for the automatic detection of polar mesocyclones are based on the conventional analysis of patterns of cloudiness and they involve different empirically defined thresholds of geophysical variables. As a result, various detection methods typically reveal very different results when applied to a single dataset. We develop a conceptually novel approach for the detection of MCs based on the use of deep convolutional neural networks (DCNNs). As a first step, we demonstrate that DCNN model is capable of performing binary classification of 500 × 500 km patches of satellite images regarding MC patterns presence in it. The training dataset is based on the reference database of MCs manually tracked in the Southern Hemisphere from satellite mosaics. We use a subset of this database with MC diameters falling in the range of 200–400 km. This dataset is further used for testing several different DCNN setups, specifically, DCNN built “from scratch”, DCNN based on VGG16 pre-trained weights also engaging the Transfer Learning technique, and DCNN based on VGG16 with Fine Tuning technique. Each of these networks is further applied to both infrared (IR) and a combination of infrared and water vapor (IR + WV) satellite imagery. The best skills (97% in terms of the binary classification accuracy score) is achieved with the model that averages the estimates of the ensemble of different DCNNs. The algorithm can be further extended to the automatic identification and tracking numerical scheme and applied to other atmospheric phenomena that are characterized by a distinct signature in satellite imagery.

Role of air-mass transformations in exchange between the Arctic and mid-latitudes

Pulses of warm and moist air from lower latitudes provide energy to the Arctic and form its main energy source outside of the summer months. These pulses can cause substantial surface warming and trigger ice melt. Air-mass transport in the opposite direction, away from the Arctic, leads to cold-air outbreaks. The outbreaks are often associated with cold extremes over continents, and extreme surface heat fluxes and occasional polar lows over oceans. Air masses advected across the strong Arctic-to-mid-latitude temperature gradient are rapidly transformed into colder and dryer or warmer and moister air masses by clouds, radiative and turbulent processes, particularly in the boundary layer. Phase changes from liquid to ice within boundary-layer clouds are critical in these air-mass transformations. The presence of liquid water determines the radiative effects of these clouds, whereas the presence of ice is crucial for subsequent cloud decay or dissipation, processes that are poorly represented in weather and climate models. We argue that a better understanding of how air masses are transformed on their way into and out of the Arctic is essential for improved prediction of weather and climate in the Arctic and mid-latitudes. Observational and modelling exercises should take an air-mass-following Lagrangian approach to attain these goals. Understanding the thermodynamics of air-mass transformations that occur in the atmosphere at the boundary between the Arctic and mid-latitudes is key to improving weather and climate predictions, according to a literature synthesis

An objective global climatology of polar lows based on reanalysis data

Here we present an objective global climatology of polar lows. In order to obtain objective detection criteria, the efficacy of several parameters for separating polar lows from other cyclones has been compared. The comparison and the climatology are based on the ERA‐Interim reanalysis from 1979 to 2016 and the high‐resolution Arctic System Reanalysis from 2000 to 2012. The most effective parameters in separating polar lows from other extratropical cyclones were found to be the difference between the sea‐level pressure at the centre of the low and its surroundings, the difference in the potential temperature between the sea surface and the 500 hPa level, and the tropopause wind speed poleward of the system. Other parameters often used to identify polar lows, such as the 10 m wind speed and the temperature difference between the sea surface and the 700 hPa level, were found to be less effective. The climatologies reveal that polar lows occur in all marine basins at high latitudes, but with high occurrence density in the vicinity of the sea‐ice edge and coastal zones. The regions showing the highest degree of polar‐low activity are the Denmark Strait and the Nordic Seas, especially for the most intense polar lows. In the North Atlantic and Pacific, the main polar‐low season ranges from November to March. In the Southern Hemisphere, polar lows are mainly detected between 50 and 65°S from April to October, indicating that this hemisphere compared to its northern counterpart has a two months longer, but less intense, polar‐low season. No significant hemispheric long‐term trends are observed, although some regions, such as the Denmark Strait and the Nordic Seas, experience significant downward and upward trends in polar lows, respectively, over the last decades. For intense polar lows, a significant declining trend has been observed for the Northern Hemisphere.

Modification of Polar Low Development by Orography and Sea Ice

ABSTRACTThe life cycles of intense high-latitude mesoscale cyclones and polar lows are strongly shaped by their ambient environments. This study focuses on the influence of the orography of Svalbard and the sea ice cover in the Norwegian and Barents Seas on polar low development. We investigate two typical polar lows that formed near Svalbard during northerly cold-air outbreaks. Each case is simulated using the Met Office Unified Model with convection-permitting grid spacing. A series of sensitivity experiments is conducted with an artificially changed land mask, orography, and sea ice distribution. We find that Svalbard acts to block stably stratified air from the ice-covered Arctic Ocean, and as an additional source of low-level cyclonic vorticity aiding polar low genesis and intensification. A decrease in sea ice cover west of Svalbard results in a moderate intensification of the polar lows, particularly for the more convectively driven case, while an increase in the sea ice cover significantly hinders their development. These experiments exemplify that polar mesoscale cyclones in the northeast Atlantic can withstand large perturbations in the surface conditions (such as the removal of Svalbard) and still develop to sufficient intensity to be labeled as polar lows. However, there is a sensitivity to Svalbard’s orography and surrounding sea ice cover, illustrated by a clear modulation of polar low genesis and development.

Control of Storminess over the Pacific and North America by Circulation Regimes

Circulation regimes are identified from a cluster analysis applied to 5-day means of the combined anomalies of 500 hPa geopotential height and the 250 hPa zonal wind over the extended Pacific-North America region obtained from ERA-Interim reanalyses. Five regimes are identified: the Arctic Low (AL), Pacific wavetrain (WT), Alaskan Ridge (AR) and Pacific Trough (PT) and Arctic High (AH). Regime dependent anomalies of the frequency of atmospheric rivers and measures of the storm tracks (850 hPa covariance between high pass filtered meridional wind and temperature and 300 hPa variance of high pass filtered meridional wind) are shown to be significant. The circulation regimes provide statistically significant modulation of observed precipitation (obtained from the Multi-Source Weighted-Ensemble data set). We examine regime-specific anomalies, normalized anomalies and the occurrence of extreme values of precipitation. We identify regions in which the ratio of frequency of occurrence of extremes (5th and 95th percentile) to the climatological frequency in each regime is significantly high. Precipitation is decreased over the Pacific Northwest in the AL regime, but is increased over the central and Eastern US, with a large increase in the probability of extreme precipitation, enhanced atmospheric river occurrence and stronger storm tracks. In the WT regime, Pacific atmospheric rivers are shifted towards Alaska, accompanied by a decrease in storm track magnitude over the Eastern North Pacific. The AR regime is characterized by an increase in moisture flux into the Southwest, increases in extreme precipitation and atmospheric rivers and storm track related heat transport. Increases in precipitation, and atmospheric rivers along the Southwest coast are noted in the PT regime, along with increased precipitation over Texas and Florida. The Northeastern region experiences increases in extremely dry conditions. Significant drought conditions occur over the Midwest US. Significant increases in precipitation over a broad region of the Northwest occur during the AH regime.

Impact of polar lows on synoptic scale variability of Atlantic inflow in the Fram Strait

The Atlantic inflow in the Fram Strait (78°50′N) has synoptic scale variability based on an array of moorings over the period of 1998–2010. The synoptic scale variability of Atlantic inflow, whose significant cycle is 3–16 d, occurs mainly in winter and spring (from January to April) and is related with polar lows in the Barents Sea. On the synoptic scale, the enhancement (weakening) of Atlantic inflow in the Fram Strait is accompanied by less (more) polar lows in the Barents Sea. Wind stress curl induced by polar lows in the Barents Sea causes Ekman-transport, leads to decrease of sea surface height in the Barents Sea, due to geostrophic adjustment, further induces a cyclonic circulation anomaly around the Barents Sea, and causes the weakening of the Atlantic inflow in the Fram Strait. Our results highlight the importance of polar lows in forcing the Atlantic inflow in the Fram Strait and can help us to further understand the effect of Atlantic warm water on the change of the Arctic Ocean.

Polar Mesoscale Cyclone Climatology for the Nordic Seas Based on ERA-Interim

AbstractPolar mesoscale cyclones (PMCs) are automatically detected and tracked over the Nordic seas using the Melbourne University algorithm applied to ERA-Interim. The novelty of this study lies in the length of the dataset (1979–2014), using PMC tracks to infer relationships to large-scale flow patterns, and elucidating the sensitivity to different selection criteria when defining PMCs and polar lows and their genesis environments. The angle between the ambient mean and thermal wind is used to distinguish two different PMC genesis environments. The forward shear environment (thermal and mean wind have the same direction) features typical baroclinic conditions with a temperature gradient at the surface and a strong jet stream at the tropopause. The reverse shear environment (thermal and mean wind have opposite directions) features an occluded cyclone with a barotropic structure throughout the entire troposphere and a low-level jet. In contrast to previous studies, PMC occurrence features neither a significant trend nor a significant link with the North Atlantic Oscillation and the Scandinavian blocking (SB), though the SB negative pattern seems to promote reverse shear PMC genesis. The sea ice extent in the Nordic seas is not associated with overall changes in PMC occurrence but influences the genesis location. Selected cold air outbreak indices and the temperature difference between the sea surface and 500 hPa (SST − T500) show no robust link with PMC occurrence, but the characteristics of forward shear PMCs and their synoptic environments are sensitive to the choice of the SST − T500 threshold.

On weather limitations for safe marine operations in the Barents Sea

The Barents Sea wave conditions and the weather conditions that often accompany fully developed polar lows are presented in this paper. The studies on the extreme waves showed that the extreme waves in the Barents Sea decreases as we move further north. The Barents Sea wave conditions are found to be more accessible for marine operations in the summer months compared to the North Sea and the Norwegian Sea wave conditions. However, the quality of actual weather forecasts and the contribution of long periodic swells to the total sea in the Barents Sea could negate the longer weather windows observed in the area. Further, the weather conditions that often accompany fully developed polar lows represent limitations to marine operations in the Barents Sea. It was found that the significant wave height in polar lows could be up to 9 m. In addition, snow and ice accretion in polar lows are deemed operation and safety hazards. Overall, marine operation may not be carried out within the period when a polar low is likely to occur.

Vessel stability in polar low situations: case study for semi-submersible drilling rigs

ABSTRACTThe additional snow and ice loads that a typical sixth generation semi-submersible in polar lows may experience are estimated and the effects on the intact stability of the semi-submersible are analysed. The additional snow loads could be very significant if the snow accrete on all the horizontal surfaces of the semi-submersible, and could have more impact on the initial stability of the semi-submersible when in the survival from operating condition. Ice loads from sea spray icing may not exceed 300 tons in any of the semi-submersible loading conditions, except when the icing event extends to the main deck of the semi-submersible. The resulting asymmetric ice loads will lead to increase heeling of the semi-submersible, with maximum heeling occurring when the icing event extends to the main deck. Generally, the additional snow and ice loads in polar lows are deemed to represent operation and safety challenges.

Southern Ocean mesocyclones and polar lows from manually tracked satellite mosaics

AbstractA new reference data set of mesocyclone activity over the Southern Ocean has been developed from the manual analysis of high‐resolution infrared satellite mosaics for winter 2004. Of the total 1735 mesocyclones which were identified and analyzed, about three quarters were classified as being “polar lows” (i.e., intense systems; see Rasmussen and Turner, 2003). The data set includes mesocyclone track, size, associated cloud vortex type, and background synoptic conditions. Maxima in track density were observed over the Bellingshausen Sea and around East Antarctica and are highly correlated with cyclogenesis regions. A comparison against QuikSCAT and reanalyses wind characteristics shows that the reanalyses, while capturing mesocyclone events, tend to considerably underestimate their wind speed (by up to 10 m s−1). This mesocyclone data set is available as a reference for further analysis of mesocyclones and for the evaluation and development of cyclone‐tracking algorithms.

North Atlantic polar lows and weather regimes: do current links persist in a warmer climate?

AbstractPolar low development over the North Atlantic under a warmer climate is assessed using simulations of an atmospheric–oceanic coupled general circulation model, specifically with regards to the evolution of large‐scale atmospheric variability described by weather regimes, and their links with lower‐layer static stability. While a relationship has been identified for the present climate, under a warmer climate, polar low favorable conditions are expected to occur less often, and the large‐scale circulation variability appears to have reduced influence on stability, and thus, on polar low occurrence.

Improvements in Atmospheric Water Vapor Content Retrievals Over Open Oceans From Satellite Passive Microwave Radiometers

The study is dedicated to the development of an improved algorithm for integrated water vapor content (WVC) retrieval from the advanced microwave sounding radiometer 2 (AMSR2) measurement data over open ocean areas. The algorithm is based on physical modeling of the brightness temperature (BT) of the upwelling radiation of the atmosphere – ocean system. The BT inversion is carried out with neural networks (NNs), trained on an ensemble of BTs, calculated using the dataset of the atmospheric and oceanic parameters. The improvement in the WVC retrieval algorithm as compared to previously developed one is associated, first, with the usage of the new ocean emissivity empirical model and, second, with the exclusion of the vertically polarized BT from NNs inputs. The new model was incorporated into the BT geophysical model and a new set of NNs coefficients was obtained for WVC algorithm. Though the results of the numerical experiment demonstrate higher retrieval accuracy for the old version of the NN model, real-valued validation with global positioning system (GPS) WVC shows better performance of the new NN model with the excluded vertically polarized AMSR2 BT from the NNs algorithm inputs. The advantages of the new algorithm are the most remarkable under conditions of high winds. This is confirmed by the validation based on the database of GPS WVC measurements under extreme winds. The improved WVC retrieval algorithm is mostly beneficial for the studies of extreme weather events such as polar lows, extratropical and tropical cyclones, associated with high winds.

Higher ocean wind speeds during marine cold air outbreaks

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 MCAOs 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 as defined here can persist for 50 days or more.

Comparing Polar Lows in Atmospheric Reanalyses: Arctic System Reanalysis versus ERA-Interim

Abstract Representation of polar lows in the new high-resolution Arctic System Reanalysis (ASR) was for the first time assessed and compared to that in the ERA-Interim. Substantial improvements were found in the 850-hPa relative vorticity and near-surface wind speed information. The latter was found to be in close agreement with satellite-derived estimates. Representation of polar lows from a widely used selective list in ERA-Interim and ASR was estimated as 48% and 89%, respectively. The proportion of polar lows represented in ASR is substantially higher than reported for other reanalyses in previous studies. Verifications were found to be sensitive to the polar low reference list used, and to the definition of a polar low. As found, when a more complete polar low list from a recent satellite-derived climatology was used, the proportion of represented events decreased to 26% and 66% for ERA-Interim and ASR, respectively. Variations in polar low representation in reanalyses were also observed in different regions, with the highest proportion resolved in the Norwegian Sea. Strong dependence between polar low sizes and their representation in ERA-Interim was found. In the case of ASR, polar low representation remains constant in the size range of 200–500 km and slightly decreases only for the smallest systems with diameters less than 200 km. Usage of the strict threshold of 43 K for the atmospheric static stability criterion was found to exclude a considerable number of otherwise well-represented polar lows.

Synoptic characteristics of heavy snowfalls at Busan of Korea caused by polar lows over the East/Japan Sea

The results of the present study prove that snowfall occurred due to the polar low (PL) in the Korean Peninsula and six cases of snowfall exceeding a snow depth of 2 cm over the past 16 years in Busan, South Korea. A strong northwesterly air current with a cold outbreak at the lower level passed through the Korean Peninsula and penetrated into the East/Japan Sea causing the generation and characteristics of a PL. However, a northeasterly air current due to a synoptic low (SL) in East Japan approached the east coast via the East/Japan Sea, which generated a wind field with mesoscale cyclonic circulation. In the center of this cyclone, a strong positive vorticity region was revealed from the lower level to the upper level. The air temperature in the center of the PL was warmer than the surrounding areas at the lower level. As the PL developed and the air temperature decreased, a rapid tropopause drop followed due to the effect of the cold core along with the cutoff low at the mid-level or the higher level. As a result, the stratification became more unstable. The PL moved into Busan as the cold core at the upper level rapidly moved to the lower latitudes, which formed an unstable region around Busan. The PL decayed because the cutoff low, the cold core, and the positive vorticity region at the upper level quickly moved to the east, thereby causing the stratification to stabilize. Also, because the approach to the Japanese Archipelago caused an increase in surface friction, the original structure could no longer be maintained.

Sensitivity of an apparently hurricane‐like polar low to sea‐surface temperature

Polar lows are intense, small‐scale cyclones in the high latitudes. Typically, polar lows are initiated through baroclinic processes, but they sometimes evolve into a post‐baroclinic mature stage where air–sea interaction becomes more important. In this stage some polar lows have developed hurricane‐like cloud structures, and idealized axisymmetric hurricane models have indicated that air–sea interaction‐fuelled pressure drops of up to 50 hPa are theoretically possible in polar environments. Here we study a polar low that formed in an extreme marine cold air outbreak over the Barents Sea and which had cloud structures with similarities to hurricanes. Using a high‐resolution weather model, we artificially modified the sea‐surface temperatures (SSTs) to assess if the polar low was close to developing into a true hurricane‐like system, with air–sea interaction processes leading to intensification in the post‐baroclinic, mature phase of its life cycle. The polar low simulations with SSTs augmented by 2–6 K produced more intense mature phases than the control experiment (with unmodified SSTs). The intensity of the polar low in the last of these, which it must be pointed out was unrealistic with an SST increase of 6 K, surpassed the intensity in the earlier baroclinic phase. The experiment where the SSTs were reduced by 2 K did not produce a much weaker polar low than the control run. Broadly speaking, our experiments suggest that in this case large and unrealistic SST increases would be needed for apparently hurricane‐like polar lows to exhibit true hurricane‐like behaviour, but nevertheless air–sea interaction did drive a more intense and prolonged mature phase.