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

2021 ◽  
Author(s):  
Patrick Johannes Stoll
Keyword(s):  
Wind Shear ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Static Stability ◽  
Relative Vorticity ◽  
Polar Lows ◽  
Air Masses ◽  
Winter Half ◽  
Polar Low ◽  
Marine Areas

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 ERA-5 reanalysis for the years 1979–2020. Criteria for the detection of polar lows are derived based on a comparison of six polar-low archives with cyclones derived by a mesoscale tracking algorithm. The characteristics associated with polar lows are considered by the criteria: (i) intense cyclone: large relative vorticity, (ii) mesoscale: small vortex diameter, and (iii) development in the marine polar air masses: combination of low dry-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 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, PLs share many characteristics across ocean basins and wind-shear categories. The most remarkable difference is that forward-shear polar lows are often occurring in 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.

scholarly journals Polar Lows - Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments

2021 ◽  
Author(s):  
Patrick Stoll ◽  
Thomas Spengler ◽  
Rune Grand Graversen
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Horizontal Wind ◽  
Polar Lows ◽  
Air Masses ◽  
Strong Shear ◽  
Polar Low ◽  
Pressure Anomaly ◽  
Marine Air

<p>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. </p><p>We identify five different polar-low configurations which are characterised by the vertical wind shear vector, the change of 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.</p><p>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.</p><p>Polar low situations with a weak shear, that 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.</p>

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

2017 ◽  
Vol 145 (6) ◽  
pp. 2375-2383 ◽  
Author(s):  
Julia Smirnova ◽  
Pavel Golubkin
Keyword(s):  
Strong Dependence ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Static Stability ◽  
Relative Vorticity ◽  
Reference List ◽  
Near Surface ◽  
Polar Lows ◽  
Polar Low ◽  
Definition Of

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.

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

2020 ◽  
Author(s):  
Patrick Johannes Stoll ◽  
Thomas Spengler ◽  
Annick Terpstra ◽  
Rune Grand Graversen
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Polar Lows ◽  
North East ◽  
The North ◽  
Strong Shear ◽  
Polar Low ◽  
Mesoscale Cyclones

Abstract. Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety in 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-East Atlantic, which were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered by the Norwegian Meteorological Institute in the STARS dataset. ERA-5 reproduces 93 % of the STARS polar lows. We identify five different polar-low configurations, which are characterised by the vertical wind shear vector 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. The orientation of the vertical-shear vector for the strong shear categories determines the dynamics of the systems, confirming the relevance of the previously identified categorisation into forward and reverse-shear polar lows. In addition, we expand the categorisation with right and left-shear polar lows that propagate towards colder and warmer environments, respectively. Polar lows in the four strong shear categories feature an up-shear tilt in the vertical, typical for the intensification through moist baroclinic processes. As weak-shear conditions mainly occur at the mature or lysis stage of polar lows, we find no evidence for hurricane-like development and propose that spirali-form PLs are most likely associated with a warm seclusion process.

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

2021 ◽  
Vol 2 (1) ◽  
pp. 19-36
Author(s):  
Patrick Johannes Stoll ◽  
Thomas Spengler ◽  
Annick Terpstra ◽  
Rune Grand Graversen
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Horizontal Wind ◽  
Polar Lows ◽  
The North ◽  
Strong Shear ◽  
Polar Low ◽  
Pressure Anomaly ◽  
Mesoscale Cyclones

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.

The Intensity- and Size-Dependent Response of Tropical Cyclones to Increasing Vertical Wind Shear

2021 ◽  
Author(s):  
Peter M. Finocchio ◽  
Rosimar Rios-Berrios
Keyword(s):  
Wind Field ◽  
Wind Shear ◽  
Diabatic Heating ◽  
Inner Core ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Rapid Intensification ◽  
Vertical Coupling ◽  
Tangential Wind

AbstractThis study describes a set of idealized simulations in which westerly vertical wind shear increases from 3 to 15 m s−1 at different stages in the lifecycle of an intensifying tropical cyclone (TC). The TC response to increasing shear depends on the intensity and size of the TC’s tangential wind field when shear starts to increase. For a weak tropical storm, increasing shear decouples the vortex and prevents intensification. For Category 1 and stronger storms, increasing shear causes a period of weakening during which vortex tilt increases by 10–30 km before the TCs reach a near-steady Category 1–3 intensity at the end of the simulations. TCs exposed to increasing shear during or just after rapid intensification tend to weaken the most. Backward trajectories reveal a lateral ventilation pathway between 8–11 km altitude that is capable of reducing equivalent potential temperature in the inner core of these TCs by nearly 2°C. In addition, these TCs exhibit large reductions in diabatic heating inside the radius of maximum winds (RMW) and lower-entropy air parcels entering downshear updrafts from the boundary layer, which further contributes to their substantial weakening. The TCs exposed to increasing shear after rapid intensification and an expansion of the outer wind field reach the strongest near-steady intensity long after the shear increases because of strong vertical coupling that prevents the development of large vortex tilt, resistance to lateral ventilation through a deep layer of the middle troposphere, and robust diabatic heating within the RMW.

An evaluation of East Asian Extratropical cyclones in CMIP5 models and their response to greenhouse warming

2020 ◽  
Author(s):  
Jaeyeon Lee ◽  
Jaeyoung Hwang ◽  
Seok-Woo Son ◽  
John Gyakum
Keyword(s):  
East Asia ◽  
East Asian ◽  
Vertical Wind Shear ◽  
Coupled Model ◽  
Static Stability ◽  
Relative Vorticity ◽  
Cmip5 Models ◽  
Leeward Side ◽  
Extratropical Cyclones ◽  
The Tibetan Plateau

<p>The extratropical cyclones (ETCs) over East Asia and their possible future changes are evaluated using the Coupled Model Intercomparison Project phase 5 (CMIP5) models. The East Asian ETCs are identified using an automated tracking algorithm applied to the 850-hPa relative vorticity field for both reference data (ERA-Interim reanalysis data) and model data. The CMIP5 models well capture the spatial distribution of East Asian ETC properties, although significant biases are present around the high-topography regions. Based on the individual model biases, Best 5 models are selected and used for examining the future changes of East Asian ETCs. In future climate, Best 5 shows declined cyclogenesis in the leeward side of the Tibetan Plateau, which is partly responsible for the decreased ETC frequency over the western North Pacific. The intensity of individual ETCs is also projected to decrease in a warm climate. These changes could be attributed to the combined effect of increased static stability and decreased vertical wind shear in East Asia, which means reduced local baroclinicity. It is also found that CMIP6 models have smaller bias than Best 5 CMIP5 models, indicating that the result documented in this study may change in quantity when newly-available CMIP6 models are utilized.</p>

Observations of a Squall Line and Its Near Environment Using High-Frequency Rawinsonde Launches during VORTEX2

2010 ◽  
Vol 138 (11) ◽  
pp. 4076-4097 ◽  
Author(s):  
George H. Bryan ◽  
Matthew D. Parker
Keyword(s):  
Wind Shear ◽  
Deep Layer ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Stratiform Region ◽  
Gust Front

Abstract Rawinsonde data were collected before and during passage of a squall line in Oklahoma on 15 May 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Nine soundings were released within 3 h, allowing for unprecedented analysis of the squall line’s internal structure and nearby environment. Four soundings were released in the prestorm environment and they document the following features: low-level cooling associated with the reduction of solar isolation by a cirrus anvil; abrupt warming (1.5 K in 30 min) above the boundary layer, which is probably attributable to a gravity wave; increases in both low-level and deep-layer vertical wind shear within 100 km of the squall line; and evidence of ascent extending at least 75 km ahead of the squall line. The next sounding was released ∼5 km ahead of the squall line’s gust front; it documented a moist absolutely unstable layer within a 2-km-deep layer of ascent, with vertical air velocity of approximately 6 m s−1. Another sounding was released after the gust front passed but before precipitation began; this sounding showed the cold pool to be ∼4 km deep, with a cold pool intensity C ≈ 35 m s−1, even though this sounding was located only 8 km behind the surface gust front. The final three soundings were released in the trailing stratiform region of the squall line, and they showed typical features such as: “onion”-shaped soundings, nearly uniform equivalent potential temperature over a deep layer, and an elevated rear inflow jet. The cold pool was 4.7 km deep in the trailing stratiform region, and extended ∼1 km above the melting level, suggesting that sublimation was a contributor to cold pool development. A mesoscale analysis of the sounding data shows an upshear tilt to the squall line, which is consistent with the cold pool intensity C being much larger than a measure of environmental vertical wind shear ΔU. This dataset should be useful for evaluating cloud-scale numerical model simulations and analytic theory, but the authors argue that additional observations of this type should be collected in future field projects.

scholarly journals Further examination of the thermodynamic modification of the inflow layer of tropical cyclones by vertical wind shear

2013 ◽  
Vol 13 (1) ◽  
pp. 327-346 ◽  
Author(s):  
M. Riemer ◽  
M. T. Montgomery ◽  
M. E. Nicholls
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Numerical Experiments ◽  
Structural Changes ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Experimental Setup ◽  
Power Machine ◽  
The Impact

Abstract. Recent work has developed a new framework for the impact of vertical wind shear on the intensity evolution of tropical cyclones. A focus of this framework is on the frustration of the tropical cyclone's power machine by shear-induced, persistent downdrafts that flush relatively cool and dry (lower equivalent potential temperature, θe) air into the storm's inflow layer. These previous results have been based on idealised numerical experiments for which we have deliberately chosen a simple set of physical parameterisations. Before efforts are undertaken to test the proposed framework with real atmospheric data, we assess here the robustness of our previous results in a more realistic and representative experimental setup by surveying and diagnosing five additional numerical experiments. The modifications of the experimental setup comprise the values of the exchange coefficients of surface heat and momentum fluxes, the inclusion of experiments with ice microphysics, and the consideration of weaker, but still mature tropical cyclones. In all experiments, the depression of the inflow layer θe values is significant and all tropical cyclones exhibit the same general structural changes when interacting with the imposed vertical wind shear. Tropical cyclones in which strong downdrafts occur more frequently exhibit a more pronounced depression of inflow layer θe outside of the eyewall in our experiments. The magnitude of the θe depression underneath the eyewall early after shear is imposed in our experiments correlates well with the magnitude of the ensuing weakening of the respective tropical cyclone. Based on the evidence presented, it is concluded that the newly proposed framework is a robust description of intensity modification in our suite of experiments.

Misocyclone Characteristics along Florida Gust Fronts during CaPE

2005 ◽  
Vol 133 (11) ◽  
pp. 3345-3367 ◽  
Author(s):  
Katja Friedrich ◽  
David E. Kingsmill ◽  
Carl R. Young
Keyword(s):  
Wind Shear ◽  
Doppler Radar ◽  
Vertical Wind Shear ◽  
Static Stability ◽  
Vertical Wind ◽  
Horizontal Wind ◽  
Gust Fronts ◽  
Convection Initiation ◽  
The Relationship ◽  
Gust Front

Abstract Multiple-Doppler radar and rawinsonde data are used to examine misocyclone characteristics along gust fronts observed during the Convection and Precipitation/Electrification (CaPE) project in Florida. The objective of this study is to investigate the observational representativeness of previous numerical simulations of misocyclones by employing a consistent analysis strategy to 11 gust fronts observed in the same region. The investigation focuses on the intensity range of misocyclones and their organization along gust fronts; the relationship between misocyclone intensity and horizontal wind shear, vertical wind shear, and static stability; and the relationship between misocyclones and convection initiation. The intensity of misocyclones, as indicated by the maximum values of vertical vorticity, varied from 2.8 × 10−3 to 13.9 × 10−3 s−1, although all but one case exhibited values less than 6.4 × 10−3 s−1. Organized misocyclone patterns were only found along small segments of gust fronts. Within those segments misocyclones were spaced between 3 and 7 km. Results show that the intensity of misocyclones was most closely related to the strength of horizontal wind shear across the gust front. The relationship between misocyclone intensity and vertical wind shear and static stability was not as clear. Although convection was initiated along the gust front in 7 of the 11 cases, those regions were not collocated with or in close proximity to misocyclones.

scholarly journals Estimating the Maximum Vertical Velocity at the Leading Edge of a Density Current

2020 ◽  
Vol 77 (11) ◽  
pp. 3683-3700
Author(s):  
Dylan W. Reif ◽  
Howard B. Bluestein ◽  
Tammy M. Weckwerth ◽  
Zachary B. Wienhoff ◽  
Manda B. Chasteen
Keyword(s):  
Vertical Velocity ◽  
Wind Shear ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Mesoscale Convective System ◽  
Leading Edge ◽  
Density Current ◽  
Convective System ◽  
Density Currents ◽  
Spatiotemporal Resolution

AbstractThe maximum upward vertical velocity at the leading edge of a density current is commonly <10 m s−1. Studies of the vertical velocity, however, are relatively few, in part owing to the dearth of high-spatiotemporal-resolution observations. During the Plains Elevated Convection At Night (PECAN) field project, a mobile Doppler lidar measured a maximum vertical velocity of 13 m s−1 at the leading edge of a density current created by a mesoscale convective system during the night of 15 July 2015. Two other vertically pointing instruments recorded 8 m s−1 vertical velocities at the leading edge of the density current on the same night. This study describes the structure of the density current and attempts to estimate the maximum vertical velocity at their leading edges using the following properties: the density current depth, the slope of its head, and its perturbation potential temperature. The method is then be applied to estimate the maximum vertical velocity at the leading edge of density currents using idealized numerical simulations conducted in neutral and stable atmospheres with resting base states and in neutral and stable atmospheres with vertical wind shear. After testing this method on idealized simulations, this method is then used to estimate the vertical velocity at the leading edge of density currents documented in several previous studies. It was found that the maximum vertical velocity can be estimated to within 10%–15% of the observed or simulated maximum vertical velocity and indirectly accounts for parameters including environmental wind shear and static stability.

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