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Blessed Weather

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  1. On the Generation and Maintenance of the 2012/13 Sudden Stratospheric Warming Authors: Fen Xu, X. San Liang Published: July 2017 Abstract: Using a newly developed analysis tool, multiscale window transform (MWT), and the MWT-based localized multiscale energetics analysis, the 2012/13 sudden stratospheric warming (SSW) is diagnosed for an understanding of the underlying dynamics. The fields are first reconstructed onto three scale windows: that is, mean window, sudden warming window or SSW window, and synoptic window. According to the reconstructions, the major warming period may be divided into three stages: namely, the stages of rapid warming, maintenance, and decay, each with different mechanisms. It is found that the explosive growth of temperature in the rapid warming stage (28 December–10 January) results from the collaboration of a strong poleward heat flux and canonical transfers through baroclinic instabilities in the polar region, which extract available potential energy (APE) from the mean-scale reservoir. In the course, a portion of the acquired APE is converted to and stored in the SSW-scale kinetic energy (KE), leading to a reversal of the polar night jet. In the stage of maintenance (11–25 January), the mechanism is completely different: First the previously converted energy stored in the SSW-scale KE is converted back, and, most importantly, in this time a strong barotropic instability happens over Alaska–Canada, which extracts the mean-scale KE to maintain the high temperature, while the mean-scale KE is mostly from the lower atmosphere, in conformity with the classical paradigm of mean flow–wave interaction with the upward-propagating planetary waves. This study provides an example that a warming may be generated in different stages through distinctly different mechanisms. Link to full paper:
  2. Blessed Weather

    2018 Atlantic Hurricane Season

    For the record, a summary of the season from Phil Klotzbach:
  3. Blessed Weather

    *** 33andrain -- Happy Anniversary!! ***

    I'm lost for words. Thank you Pat & Geoff for creating such a great forum and extending such a warm welcome to us Brits. I'm very proud to have played just a tiny part in the development of the forum by helping build the Research Portal with David @Bring Back 1962-63 and Zac @Snowy Hibbo. Onwards and upwards for 33andrain! Malcolm.
  4. Blessed Weather

    [Global] Stratospheric Discussion and Forecasting

    It’s always interesting when a couple of very knowledgeable and well respected members of 33 appear to have differing views, effectively about the extent/nature of coupling between troposphere and stratosphere and the impact on surface conditions (weather). Here’s a quote from the latest blog from Dr. Judah Cohen published 19th Nov: "The plot of Wave Activity Flux (WAFz) or poleward heat transport shows a relatively robust pulse of energy for the upcoming week. This relatively strong pulse of WAFz is predicted to disturb the PV causing it to stretch and briefly split into two pieces this week with one piece in Western Siberia and the other in Eastern Canada. It is my opinion that the sister lobe of the PV in Eastern Canada is related to the record cold temperatures predicted for Southeastern Canada and the Northeastern US this week." And this tweet from Anthony Masiello on 19th Nov: "The splitting vortex this week and the cold shot are the effects of tropospheric causes. The split vortex didn't cause the cold shot. I realize this is a lost cause." Please note that I have no reason to think the AM tweet was referring to the JC blog – just that I found the apparent differing views interesting. For what's it's worth, my own view aligns with AM that atm it’s the trop calling the shots and it’s not the PV downwelling and dictating the tropospheric pattern (if that is indeed what's being commented on). Nevertheless a close pattern match exists up through the atmosphere. Here’s the GFS height anomalies through the trop & strat at 500hPa, 100hPa, 50hPa and 10hPa for today, 21st Nov: Looking forward and GFS suggesting further robust Wave 1 perturbation of the PV as we move into early December: Wave 1 activity as a GIF: Again this won’t be the killer punch, but as with last year, the ongoing Wave 1 disturbance of the PV can help hold back its development. We don't want the PV to be the dominant player, propagating downwards and driving a strong mid-latitude westerly jet stream. It’s also been found that these early disturbances can be pre-conditioning pulses that soften up the PV for the ‘big one’ that can follow around 20 days later. GFS PV Zonal Mean Zonal Wind forecast shows the resulting dip in wind strength: ECM 30th Nov) and GFS (6th Dec) Geopotential & Temp charts show the PV remains displaced (typical of Wave 1 attack): As we move through December of great interest will be developments in the tropics (MJO) to generate the sequence of events (ripple effect) of tropical > extra-tropical > troposphere > stratosphere interaction that will hopefully drive the knock-out punch to the PV later in December. In my view it was the record breaking high amplitude Phase 6/7 MJO event that caused the demise of the PV last year. So this year my interest will be to look out for (much simplified): MJO in Phase 7 = high level blocking (Scandi High / Alaskan High / Aleutian Low combo would be good) = Wave 2 resulting in a split Vortex event Global Wind Oscillation (GWO) moving through Phase 4 into Phase 5 = increasing Frictional Torque leading to increasing Mountain Torque A spike in East Asian Mountain Torque = Himalayan and Mongolian Altai mountain ranges in particular = generation of upward wave propagation into the stratosphere. So plenty of interest in the stratosphere over the coming weeks, the outcome of which will have a major impact on how the winter plays out. Source of charts:
  5. The key role of background sea surface temperature over the cold tongue in asymmetric responses of the Arctic stratosphere to El Niño–Southern Oscillation Authors: Fei Xie, Xin Zhou, Jianping Li, Cheng Sun, Juan Feng and Xuan Ma Published: Nov 2018 Abstract: The response of the Arctic stratosphere to El Niño activity is strong but the response to La Niña activity is relatively weak. The asymmetric responses of Arctic stratosphere to El Niño and La Niña events are thought to be caused by asymmetric El Niño–Southern Oscillation (ENSO) teleconnections. Here, we suggest that the background sea surface temperature (SST) over cold tongue of tropical eastern Pacific may be an important contributor to the asymmetric ENSO teleconnections. The atmosphere is very sensitive to tropical SST variations in the range of 26 °C–30 °C. During El Niño events, the background SST over cold tongue plus El Niño SST anomalies typically falls into the range. Under these conditions, the atmospheric response to El Niño SST anomalies is strong. During La Niña events, the background SST plus La Niña SST anomalies is typically below the range, which leads to a weak response of the atmosphere to SST anomalies. The proposed mechanism is well supported by simulations. Link to full paper:
  6. Blessed Weather

    [Global] Teleconnections: A Technical Discussion

    Michael Ventrice Nov 5th:
  7. Sudden Stratospheric Warmings – developing a new classification based on vertical depth, applying theory to a SSW in 2018, and assessing predictability of a cold air outbreak following this SSW Author: L. van Galen Published: August 2018 Abstract: n this study a new classification has been developed to classify Sudden Stratospheric Warmings (SSWs) based on their vertical depth. This new classification was developed because the depth of a SSW has been found to be important for the magnitude of tropospheric impact (Gerber et al., 2009; Palmeiro et al., 2015), and the official SSW classification does not tell anything about the vertical extent of a SSW. The new classification adapted from a previously developed classification of Kramer (2016; hereafter referred to as K16)). It prescribes that the zonal mean zonal wind between 60N and 70N should reverse over a depth of at least 80 hPa between 10 and 100 hPa for at least two days in a 5-day period. This classification was termed a ‘Deep Stratospheric Warming’ (DSW). In the stratosphere, the new DSW classification was compared to both the SSW-classification and the K16-classification. However, the K16-classification included events that were too weak to be considered important for tropospheric impact. Coupled to the fact that K-16-events did not contain any information about the vertical depth of SSWs, this classification was not taken into account in subsequent analyses. Compared to the official SSW classification, it appeared that DSWs were less frequent; whereas SSWs occurred about 6 times per decade, DSWs were found to occur only 4 times per decade. Furthermore, the SSW and DSW classifications were connected: most DSWs occurred a few days to weeks after the SSW date. This was explained by the behavior of SSWs and DSWs: first a warming at 10 hPa took place (the SSW date). After that, the wind reversal extended downward in an irregular fashion, during which after some SSWs at some point the wind reversal extended sufficiently far to the lower stratosphere to cause a DSW to be classified. Connected to the differences in classification date, the DSWs were rarely located close to the moment of rapid warming in the stratosphere. Thus, unlike SSWs, the DSWs did not show the ‘sudden warming’ behavior. Rather, (most of) the DSWs seemed to be a result warm anomalies in the mid-stratosphere that developed sometime during or after the SSW and subsequently downwelled on a timescale of about a week. In terms of upper tropospheric impact, the DSW classification resulted in a similar response compared to the SSW classification: anomalously high temperatures near the pole and a southward displaced jet stream. Furthermore, the jet stream was found to become slightly more meridionally oriented in the midlatitudes and slightly more zonally oriented in the subtropics, but this signal was fairly weak. The main difference between the DSW and SSW was that the DSW effects were mostly stronger in magnitude than the SSW effects. Thus, a DSW results in stronger upper tropospheric impacts compared to a SSW. A similar story was found in terms of surface impacts. Both the SSW and DSW classifications resulted in positive pressure anomalies over the pole and generally negative pressure anomalies in the midlatitudes (indicative of a negative Arctic Oscillation); but the anomalies were more pronounced after a DSW. The surface temperature response after both a SSW and DSW was chaotic, though, showing that the DSW classification did not result in a stronger or more structured surface temperature response. Finally, a thermodynamic perspective was presented to explain why the DSW classification resulted in a stronger tropospheric response compared to the SSW classification method (only in terms of a -AO and a weaker jet stream in the midlatitudes). The cause was speculated to be that the warming accompanied by a DSW extended further to the upper troposphere. This is important for two reasons, being: 1) A deeper warming causes the pressure surfaces to be less tilted from the midlatitudes to the pole, resulting in a more pronounced weakening of midlatitude westerlies, including the polar jet stream. 2) A warming that extends more towards the troposphere exerts more pressure on the underlying air, because the density of air closer to the troposphere is higher. This results in a higher surface pressure. ` In short, the new DSW classification enables to better assess the zonal mean timing and intensity of tropospheric impact compared to the SSW classification. However, no specific conclusions could be drawn about the zonally asymmetric effects of DSWs. Link to full paper: Credit goes to @sebastiaan1973 for finding this presentation - thank you.
  8. Blessed Weather

    [Global] Stratospheric Discussion and Forecasting

    Hi Guys - fyi: "In the top panel image of Figure 6.14, we see the single high-low structure that exists between the Gulf of Alaska and Northern Europe centered roughly along the 60°N latitude. The thick black line highlights the wave structure. Atmospheric scientists refer to this single high-low structure as a planetary wave-1 pattern, since a single wave (one ridge and one trough) straddles the entire planet at that latitude. At 60°N, the wavelength of this wave-1 is 20,000 kilometers. In the bottom panel image, we see two highs and two lows in a double high-low structure extending around the northern high latitudes. It is again centered roughly on 60°N. We refer to this as a planetary wave-2, with a wavelength of 10,000 kilometers at 60°N." Source:
  9. Blessed Weather

    [Global] Stratospheric Discussion and Forecasting

    Hi sebastiaan. Welcome to this side of the 'pond'. That is indeed a very interesting paper you've summarised in your post. I'm not sure if you've taken a look around the Research Portal on 33andrain (follow the 33 University tab on the navigation bar) but it would be great to add this one to the library. Would you mind if I did that on your behalf? Malcolm.
  10. The changing impact of El Niño on US winter temperatures Authors: Jin‐Yi Yu, Yuhao Zou, Seon Tae Kim, Tong Lee Published: Aug 2012 Abstract: In this study, evidence is presented from statistical analyses, numerical model experiments, and case studies to show that the impact on US winter temperatures is different for the different types of El Niño. While the conventional Eastern‐Pacific El Niño affects winter temperatures primarily over the Great Lakes, Northeast, and Southwest US, the largest impact from Central‐Pacific El Niño is on temperatures in the northwestern and southeastern US. The recent shift to a greater frequency of occurrence of the Central‐Pacific type has made the Northwest and Southeast regions of the US most influenced by El Niño. It is shown that the different impacts result from differing wave train responses in the atmosphere to the sea surface temperature anomalies associated with the two types of El Niño. Link to full paper:
  11. Snow-atmosphere coupling and its impact on temperature variability and extremes over North America Authors: G. T. Diro, L. Sushama, O. Huziy Published: July 2017 Abstract: The impact of snow-atmosphere coupling on climate variability and extremes over North America is investigated using modeling experiments with the fifth generation Canadian Regional Climate Model (CRCM5). To this end, two CRCM5 simulations driven by ERA-Interim reanalysis for the 1981–2010 period are performed, where snow cover and depth are prescribed (uncoupled) in one simulation while they evolve interactively (coupled) during model integration in the second one. Results indicate systematic influence of snow cover and snow depth variability on the inter-annual variability of soil and air temperatures during winter and spring seasons. Inter-annual variability of air temperature is larger in the coupled simulation, with snow cover and depth variability accounting for 40–60% of winter temperature variability over the Mid-west, Northern Great Plains and over the Canadian Prairies. The contribution of snow variability reaches even more than 70% during spring and the regions of high snow-temperature coupling extend north of the boreal forests. The dominant process contributing to the snow-atmosphere coupling is the albedo effect in winter, while the hydrological effect controls the coupling in spring. Snow cover/depth variability at different locations is also found to affect extremes. For instance, variability of cold-spell characteristics is sensitive to snow cover/depth variation over the Mid-west and Northern Great Plains, whereas, warm-spell variability is sensitive to snow variation primarily in regions with climatologically extensive snow cover such as northeast Canada and the Rockies. Furthermore, snow-atmosphere interactions appear to have contributed to enhancing the number of cold spell days during the 2002 spring, which is the coldest recorded during the study period, by over 50%, over western North America. Additional results also provide useful information on the importance of the interactions of snow with large-scale mode of variability in modulating temperature extreme characteristics. Link to full paper:
  12. Snow–Atmosphere Coupling Strength. Part I: Effect of Model Biases Authors: Li Xu, Paul Dirmeyer Published: April 2013 Abstract: Snow–atmosphere coupling strength, the degree to which the atmosphere (temperature and precipitation) responds to underlying snow anomalies, is investigated using the Community Climate System Model (CCSM) with realistic snow information obtained from satellite and data assimilation. The coupling strength is quantified using seasonal simulations initialized in late boreal winter with realistic initial snow states or forced with realistic large-scale snow anomalies, including both snow cover fraction observed by remote sensing and snow water equivalent from land data assimilation. Errors due to deficiencies in the land model snow scheme and precipitation biases in the atmospheric model are mitigated by prescribing realistic snow states. The spatial and temporal distributions of strong snow–atmosphere coupling in this model are revealed to track the continental snow cover edge poleward during the ablation period in spring, with secondary maxima after snowmelt. Compared with prescribed “perfect” snow simulations, the free-running CCSM captures major regions of strong snow–atmosphere coupling strength, with only minor departures in magnitude, but showing uneven biases over the Northern Hemisphere. Signals of strong coupling to air temperature are found to propagate vertically into the troposphere, at least up to 500 hPa over the coupling “cold spots.” The main mechanism for this vertical propagation is found to be longwave radiation and condensation heating. Link to full paper:
  13. Blessed Weather

    [Global] Stratospheric Discussion and Forecasting

    Lots of Twitter activity these past few days on the potential (or not) of some disruption to the Strat PV (SPV) as we move into November. A couple to add to those above: Simon Lee 29 Oct: Jason Furtado 30 Oct: The stratosphere and troposphere are currently decoupled and this morning's Berlin output suggests that in the forecast period it's not the stratosphere's zonal westerly winds descending, but rather the influence of the troposphere pattern pushing up into the lower stratosphere: The ECM forecast blocking pattern is looking favourable for Wave 1 disruption to the SPV with suggestions of elongation and displacement off the Pole towards the Siberian side late in the forecast. Charts for ECM 500hPa, 100hPa and 10hPa for 8th/9th Nov: Wave activity and amplitude forecast (30th Oct GFS 06z run): Wave charts: http:// Further info in the paper Blocking precursors to stratospheric sudden warming events. Illustrative charts: Displacement v Split blocking patterns: Displacement v Split Wave activity: Link to paper in Research Portal: http://
  14. Snow–atmosphere coupling in the Northern Hemisphere Authors: Gina R. Henderson, Yannick Peings, Jason C. Furtado & Paul J. Kushner Published: Oct 2018 Abstract: Local and remote impacts of seasonal snow cover on atmospheric circulation have been explored extensively, with observational and modelling efforts focusing on how Eurasian autumn snow-cover variability potentially drives Northern Hemisphere atmospheric circulation via the generation of deep, planetary-scale atmospheric waves. Despite climate modelling advances, models remain challenged to reproduce the proposed sequence of processes by which snow cover can influence the atmosphere, calling into question the robustness of this coupling. Here, we summarize the current level of understanding of snow–atmosphere coupling, and the implications of this interaction under future climate change. Projected patterns of snow-cover variability and altered stratospheric conditions suggest a need for new model experiments to isolate the effect of projected changes in snow on the atmosphere. Link to paper: (This paper is currently behind a paywall. Please use 'reply' below if you know of a free-to-view copy). Alternatively a full copy is available for temporary download from here: 10.1038@s41558-018-0295-6.pdf
  15. Blessed Weather

    [Global] Teleconnections: A Technical Discussion

    Many thanks griteater. I didn't include the following research paper in my post, but very relevant to your chart: Westerly Wind Events in the Tropical Pacific and their Influence on the Coupled Ocean‐Atmosphere System: A Review "Based on the examination of 10 years of 10-meter winds from the ECMWF analyses, Haarten proposed a subjective classification based on large-scale aspects of the circulation associated with periods of WWEs. According to her classification, nine typical patterns can represent the near-surface flow during 90% of the synoptic westerly wind variability. A single cyclone or a series of cyclones and several different types of cross-equatorial flow are the major components of the patterns." Link to paper:‐atmosphere-system-a-review/