The Climatology, Meteorology, and Boundary Layer Structure of Marine Cold Air Outbreaks in Both Hemispheres*

Fletcher, Jennifer K.; Mason, Shannon; Jakob, Christian

doi:10.1175/jcli-d-15-0268.1

Cited by 114 publications

(219 citation statements)

References 41 publications

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“…The fast build-up and the overall transient nature of the peaks of the events indicate that they are associated with passages of baroclinic waves. This result is consistent with the findings of Fletcher et al (2016), who concluded that strong MCAOs occur in the cold air sector of mid-latitude cyclones. Additionally, found that about 80 percent of MCAOs in the Pacific sector of the Southern Ocean are induced by the flow induced by the passages of synoptic-scale cyclones.…”

Section: Summary and Discussionsupporting

confidence: 93%

“…Fletcher et al (2016) used 3 K as a lower threshold for moderate MCAO events, using air temperatures at the 800 hPa level. applied five thresholds ranging from 0 K to 8 K, yielding different intensity categories.…”

Section: Methodsmentioning

confidence: 99%

“…Using two reanalysis products, the correlations between wind speeds and MCAOs are calculated for two marginal seas in the North Atlantic: the Labrador Sea and the Greenland-Iceland-Norwegian (GIN) Seas. These regions are known to experience a high frequency of MCAOs during winter (Kolstad et al, 2009;Fletcher et al, 2016). They are also areas in which surface water masses are transformed as a result of interactions with the atmosphere and sea-ice (Dickson et al, 1996;Marshall and Schott, 1999).…”

Section: Introductionmentioning

confidence: 99%

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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: Summary and Discussionsupporting

confidence: 93%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Kolstad¹

2017

Preprint

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“…We use the UK Met Office Large Eddy Model (LEM, Gray et al, 2001) to investigate the influence of large-scale subsidence on mixed-phase marine Sc cloud microphysics. The set up is the same as that used by Young et al (2017), whose study gives further details on the model itself.…”

Section: Model Set-upmentioning

confidence: 99%

“…Regions of high surface pressure are often found upstream of CAOs in the European Arctic (Walsh et al, 2001;Fletcher et al, 2016). In the high Arctic (≥ 80 • N, over sea ice), such regions contribute towards reduced cloud fractions (Kay and Gettelman, 2009;Stramler et al, 2011;Morrison et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Relating large-scale subsidence to convection development in Arctic mixed-phase marine stratocumulus

2018

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Abstract. Large-scale subsidence, associated with highpressure systems, is often imposed in large-eddy simulation (LES) models to maintain the height of boundary layer (BL) clouds. Previous studies have considered the influence of subsidence on warm liquid clouds in subtropical regions; however, the relationship between subsidence and mixedphase cloud microphysics has not specifically been studied. For the first time, we investigate how widespread subsidence associated with synoptic-scale meteorological features can affect the microphysics of Arctic mixed-phase marine stratocumulus (Sc) clouds. Modelled with LES, four idealised scenarios -a stable Sc, varied droplet (N drop ) or ice (N ice ) number concentrations, and a warming surface (representing motion southwards) -were subjected to different levels of subsidence to investigate the cloud microphysical response. We find strong sensitivities to large-scale subsidence, indicating that high-pressure systems in the ocean-exposed Arctic regions have the potential to generate turbulence and changes in cloud microphysics in any resident BL mixedphase clouds.Increased cloud convection is modelled with increased subsidence, driven by longwave radiative cooling at cloud top and rain evaporative cooling and latent heating from snow growth below cloud. Subsidence strengthens the BL temperature inversion, thus reducing entrainment and allowing the liquid-and ice-water paths (LWPs, IWPs) to increase. Through increased cloud-top radiative cooling and subsequent convective overturning, precipitation production is enhanced: rain particle number concentrations (N rain ), incloud rain mass production rates, and below-cloud evaporation rates increase with increased subsidence.Ice number concentrations (N ice ) play an important role, as greater concentrations suppress the liquid phase; therefore, N ice acts to mediate the strength of turbulent overturning promoted by increased subsidence. With a warming surface, a lack of -or low -subsidence allows for rapid BL turbulent kinetic energy (TKE) coupling, leading to a heterogeneous cloud layer, cloud-top ascent, and cumuli formation below the Sc cloud. In these scenarios, higher levels of subsidence act to stabilise the Sc layer, where the combination of these two forcings counteract one another to produce a stable, yet dynamic, cloud layer.

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Re‐examining the roles of surface heat flux and latent heat release in a “hurricane‐like” polar low over the Barents Sea

2016

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Polar lows are intense mesoscale cyclones that occur at high latitudes in both hemispheres during winter. Their sometimes evidently convective nature, fueled by strong surface fluxes and with cloud-free centers, have led to some polar lows being referred to as "arctic hurricanes." Idealized studies have shown that intensification by hurricane development mechanisms is theoretically possible in polar winter atmospheres, but the lack of observations and realistic simulations of actual polar lows have made it difficult to ascertain if this occurs in reality. Here the roles of surface heat fluxes and latent heat release in the development of a Barents Sea polar low, which in its cloud structures showed some similarities to hurricanes, are studied with an ensemble of sensitivity experiments, where latent heating and/or surface fluxes of sensible and latent heat were switched off before the polar low peaked in intensity. To ensure that the polar lows in the sensitivity runs did not track too far away from the actual environmental conditions, a technique known as spectral nudging was applied. This was shown to be crucial for enabling comparisons between the different model runs. The results presented here show that (1) no intensification occurred during the mature, postbaroclinic stage of the simulated polar low; (2) surface heat fluxes, i.e., air-sea interaction, were crucial processes both in order to attain the polar low's peak intensity during the baroclinic stage and to maintain its strength in the mature stage; and (3) latent heat release played a less important role than surface fluxes in both stages.

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The Climatology, Meteorology, and Boundary Layer Structure of Marine Cold Air Outbreaks in Both Hemispheres*

Cited by 114 publications

References 41 publications

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

Relating large-scale subsidence to convection development in Arctic mixed-phase marine stratocumulus

Re‐examining the roles of surface heat flux and latent heat release in a “hurricane‐like” polar low over the Barents Sea

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