View source for Physics:Cold-core low

{{Short description|Cyclone with an associated cold pool of air at high altitude}}
[[File:TUTTcellWestPac2007071012WV.jpg|thumb|right|Image of an upper tropospheric cyclonic vortex in the western North Pacific, a cold-core low]]
A '''cold-core low''', also known as an '''upper level low''' or '''cold-core cyclone''', is a [[Physics:Cyclone|cyclone]] aloft which has an associated cold pool of air residing at high altitude within the Earth's [[Earth:Troposphere|troposphere]], without a [[Earth:Weather front|frontal structure]]. It is a low pressure system that strengthens with height in accordance with the [[Earth:Thermal wind|thermal wind]] relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day. 

[[Earth:Severe weather|Severe weather]], such as [[Earth:Tornado|tornado]]es, can occur near the center of cold-core lows. Cold lows can help spawn [[Physics:Cyclone|cyclone]]s with significant [[Earth:Weather|weather]] impacts, such as [[Earth:Polar low|polar low]]s, and [[Physics:Kármán vortex street|Kármán vortices]]. Cold lows can lead directly to the development of tropical cyclones, owing to their associated cold pool of air aloft or by acting as additional [[Astronomy:Outflow (meteorology)|outflow]] channels to aid in further development.
[[File:Upper Level Low Jul 20 2012.jpg|thumb|right|An upper tropospheric cyclonic vortex in the western North Pacific, showing high level cloud tops being sucked in.]]

==Characteristics==
[[File:Thermal wind.jpg|thumb|left|The vertical variation of [[Physics:Geostrophic wind|geostrophic wind]] in a barotropic atmosphere (a) and in a baroclinic atmosphere (b). The blue portion of the surface denotes a cold region while the orange portion denotes a warm region.]]
Cold cyclones are stronger aloft than at the Earth's surface, or stronger in areas of the troposphere with lower pressures, per the thermal wind relationship and the [[Hypsometric equation|hypsometric equation]]. The hypsometric equation dictates that colder atmospheres have less room between pressure surfaces, which corresponds to the concept of lower atmospheric thickness, and the thermal wind relation indicates that in this situation, the wind increases with height.<ref>{{cite web|url=http://amsglossary.allenpress.com/glossary/browse?s=c&p=65|title=Cold low|date=June 2000|access-date=2010-05-02|publisher=American Meteorological Society|author=Glossary of Meteorology|archive-url=https://web.archive.org/web/20110514103120/http://amsglossary.allenpress.com/glossary/browse?s=c&p=65#|archive-date=2011-05-14|url-status=dead}}</ref><ref>{{cite book|title=Atmospheric Science: An Introductory Survey|author1=John M. Wallace |author2=Peter V. Hobbs |year=1977|publisher=Academic Press, Inc.|isbn=0-12-732950-1|pages=59,384–385}}</ref> It also means that an isolated pool of cold air aloft is associated with the feature. Because of its cold center, the center shows an area of thickness minimum. Since the temperature at any given radius is similar and parallel to the orientation of the pressure gradient, and therefore the [[Earth:Wind|wind]], it is barotropic in nature.<ref>{{cite web|url=http://amsglossary.allenpress.com/glossary/search?id=barotropic1|title=Barotropic|author=Glossary of Meteorology|date=June 2000|publisher=American Meteorological Society|access-date=2010-05-02|archive-url=https://web.archive.org/web/20110514065406/http://amsglossary.allenpress.com/glossary/search?id=barotropic1#|archive-date=2011-05-14|url-status=dead}}</ref> The movement of cold-core cyclones can be erratic since they are separated from the main belt of the [[Earth:Westerlies|Westerlies]], which would otherwise steer them progressively to the east.<ref>{{cite journal|url=http://met.psu.edu/~guishard/Aviva/public_html/Papers/SimpsonKonaLow.pdf|journal=Journal of Meteorology|date=February 1952|volume=9|title=Evolution of the Kona Storm, A Subtropical Cyclone|author=R. . Simpson|issue=1|access-date=2010-05-28|page=24|doi=10.1175/1520-0469(1952)009<0024:eotksa>2.0.co;2|bibcode=1952JAtS....9...24S}}{{dead link|date=August 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

Most [[Earth:Cloud|cloud]] cover and precipitation in association with cold lows occurs during the daylight hours as sunlight warms the Earth's surface, destabilizing the atmosphere and causing upward vertical motion.<ref>{{cite web|url=http://www.srh.noaa.gov/jetstream/append/glossary_c.htm|author=JetStream|publisher=[[Earth:National Weather Service|National Weather Service]]|title=Glossary: C's|date=2010-01-05|access-date=2010-05-28}}</ref> The development of severe weather, particularly tornadoes, can occur near the center of these systems over land during any season of the year.<ref>{{cite journal|author=Jonathan M. Davies|pages=1051–1062|title=Tornadoes with Cold Core 500-mb Lows|journal=Weather and Forecasting|volume=21|issue=6|date=December 2006|bibcode=2006WtFor..21.1051D|doi=10.1175/WAF967.1|doi-access=free}}</ref> During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach {{convert|-45|C|F}} move over open waters, deep convection forms which allows [[Earth:Polar low|polar low]] development to become possible.<ref>{{cite book|url=https://books.google.com/books?id=-tBa1DWYoDIC&pg=PA227|title=Polar lows: mesoscale weather systems in the polar regions|page=224|author1=Erik A. Rasmussen |author2=John Turner |year=2003|publisher=Cambridge University Press|isbn=978-0-521-62430-5}}</ref>

==Importance to cyclones within the subtropics and mid-latitudes==
[[File:KonaLowNovember041995IR.jpg|thumb|right|November 4, 1995 [[Earth:Kona storm|Kona Low]]]]
Kona lows, most extratropical cyclones, and [[Physics:Upper tropospheric cyclonic vortex|tropical upper tropospheric cyclones]] are cold core lows. In the eastern half of the north Pacific ocean and north Indian ocean, the formation of a weak circulation underneath a [[Physics:Upper tropospheric cyclonic vortex|mid to upper-tropospheric low]] which has cut off from the main belt of the westerlies during the cold season (winter) is called a subtropical cyclone. In the case of the north Indian ocean, the formation of this type of vortex leads to the onset of [[Earth:Monsoon|monsoon]] rains during the wet season.<ref name="Hast">S. Hastenrath (1991). [https://books.google.com/books?id=_UwvYopNedMC&dq=subtropical+cyclones+indian+ocean&pg=PA244 Climate Dynamics of the Tropics.] Springer, pp 244. {{ISBN|978-0-7923-1346-5}}. Retrieved on 2009-02-29.</ref>

East coast lows form near and east of where a cold core low interacts with a [[Physics:Sea surface temperature|sea surface temperature]] (SST) gradient along the east coast of [[Earth:Continent|continent]]s, such as [[Earth:Asia|Asia]], [[Earth:North America|North America]], southern [[Earth:Africa|Africa]], and {{wipe|Australia}} within a pre-existing area of high pressure. Those offshore the East Coast of the United States, which normally form in the months between October and April, are termed nor'easters.<ref name="disc1">{{cite web|author=Storm-E|publisher=Center For Educational Technologies|title=Nor'easters|year=2007|access-date=2008-01-22|url=http://www3.cet.edu/weather2/h17.html|archive-url=https://web.archive.org/web/20070626044252/http://www3.cet.edu/weather2/h17.html#|archive-date=2007-06-26|url-status=dead}}</ref> Not initially associated with weather fronts, they form in tandem with blocking [[Earth:Anticyclone|anticyclone]]s at higher latitudes, which leads to a slow poleward movement up the SST gradient. East coast lows can persist for up to a week. Some East coast lows develop rapidly, becoming meteorological bombs. There is a 4.5&nbsp;year cycle in the formation of East coast lows near Australia, in years in transition between warm (El Niño) and cold (La Niña) ENSO years. These systems can develop strong wind, heavy rains, waves over {{convert|10|m|ft}} in height, and [[Physics:Eye (cyclone)|eye]] features on [[Engineering:Weather satellite|weather satellite]] imagery.<ref>{{cite book|url=https://books.google.com/books?id=jyD4sBCg11EC&pg=PA43|title=Climate process & change|author=Edward Arndt Bryant|pages=43–45|year=1997|publisher=Cambridge University Press|isbn=978-0-521-48440-4}}</ref> Upper-level lows are usually the cause of heavy rainfall and storms in southeastern Australia, particularly during [[Earth:La Niña|La Niña]] years.<ref>{{cite web|url=https://www.weatherzone.com.au/news/upper-cold-pool-causes-storms-in-the-southeast/8608|title=Upper cold pool causes storms in the southeast|author=Paul Graham|date=12 February 2008|publisher=Weatherzone}}</ref>

==Importance to tropical cyclones==
[[File:WorldwideTCpeaks.gif|thumb|right|Peaks of activity worldwide]]
The summer tropical upper tropospheric trough in the [[Earth:Southern Hemisphere|Southern Hemisphere]] lies over the trade wind region of the east central Pacific and can cause [[Earth:Tropical cyclogenesis|tropical cyclogenesis]] offshore Central America.<ref>{{cite web|url=http://www.soest.hawaii.edu/Library/Sadler.html|title=The Upper Tropospheric Circulation Over the Global Tropics|author=James Sadler|date=November 1975|access-date=2009-12-23|publisher=[[Organization:University of Hawaii|University of Hawaii]]}}</ref> In the western Pacific, [[Physics:Upper tropospheric cyclonic vortex|tropical upper tropospheric low]]s are the main cause for the few tropical cyclones which develop north of the 20th parallel north and east of the 160th meridian east during La Niña events.<ref>{{cite journal|title=Eastern Hemisphere Tropical Cyclones of 1996|author1=Mark A. Lander |author2=Eric J. Trehubenko |author3=Charles P. Guard |volume=127|issue=6|journal=Monthly Weather Review|date=June 1999|issn=1520-0493|pages=1274|doi=10.1175/1520-0493(1999)127<1274:EHTCO>2.0.CO;2|bibcode = 1999MWRv..127.1274L |doi-access=free}}</ref>

Trailing upper cyclones and upper troughs can cause additional outflow channels and aid in the intensification process of tropical cyclones. Developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow [[Earth:Jet stream|jet stream]] emanating from the developing tropical disturbance/cyclone.<ref name="CLARK">{{cite web | author = Clark Evans | url = http://flhurricane.com/cyclone/showflat.php?Cat=0&Number=64429&an=0&page=0 | title = Favorable trough interactions on tropical cyclones | publisher = Flhurricane.com | date = January 5, 2006 | access-date = 2006-10-20 | archive-url = https://web.archive.org/web/20061017001243/http://flhurricane.com/cyclone/showflat.php?Cat=0&Number=64429&an=0&page=0 | archive-date = October 17, 2006 | url-status = dead }}</ref><ref name="AMSPAPER">{{cite journal |author1=Deborah Hanley |author2=John Molinari |author3=Daniel Keyser | title = A Composite Study of the Interactions between Tropical Cyclones and Upper-Tropospheric Troughs |date=October 2001 | journal = Monthly Weather Review | publisher = American Meteorological Society | volume = 129 | issue = 10 | pages = 2570–84 | doi = 10.1175/1520-0493(2001)129<2570:ACSOTI>2.0.CO;2 | bibcode=2001MWRv..129.2570H| doi-access = free }}</ref> In the western North Pacific, there are strong reciprocal relationships between the areas of formative tropical cyclones and that of the lower tropospheric monsoon troughs and the [[Earth:Tropical upper tropospheric trough|tropical upper tropospheric trough]].<ref name="JTWC">{{cite web|author=Joint Typhoon Warning Center|url=http://www.nrlmry.navy.mil/~chu/chap2/se205.htm|publisher=United States Navy|year=2010|access-date=2009-04-24|title=2.5 Upper Tropospheric Cyclonic Vortices|author-link=Joint Typhoon Warning Center}}</ref> Tropical cyclone movement can also be influenced by TUTT cells within {{convert|1700|km|mi}} of their position, which can lead to non-climatological tracks, such as eastward movement within the tropics or westward movement in an area where the [[Earth:Westerlies|Westerlies]] normally dominate.<ref>{{cite journal|title=A Conceptual Model for the Influence of TUTT Cells on Tropical Cyclone Motion in the Northwest Pacific Ocean|author1=Jason E. Patla |author2=Duane Stevens |author3=Gary M. Barnes |journal=Weather and Forecasting|date=October 2009|volume=24|issue=5|pages=1215–1235|doi=10.1175/2009WAF2222181.1|bibcode = 2009WtFor..24.1215P |doi-access=free}}</ref>

Normally, an ocean temperature of 26.5&nbsp;°C (79.7&nbsp;°F) spanning through a depth of at least {{convert|50|m|ft}} is one of the six requirements needed to maintain the special [[Earth:Mesocyclone|mesocyclone]] that is the tropical cyclone.<ref name="A15">{{cite web|author=Chris Landsea|url=http://www.aoml.noaa.gov/hrd/tcfaq/A15.html|title=Subject: A15) How do tropical cyclones form?|access-date=2011-01-27|year=2011|publisher=Hurricane Research Division|author-link=Chris Landsea}}</ref> Cooler air temperatures at a higher altitude (e.g., at the 500&nbsp;hPa level, or 5.9&nbsp;km) can lead to tropical cyclogenesis at lower water temperatures than normal, as a certain [[Lapse rate|lapse rate]] is required to force the atmosphere to be [[Earth:Baroclinic instability|unstable]] enough for convection. In a moist atmosphere, this lapse rate is 6.5&nbsp;°C/km (19&nbsp;°F/mi), while in an atmosphere with less than 100% [[Physics:Relative humidity|relative humidity]], the required lapse rate is 9.8&nbsp;°C/km (29&nbsp;°F/mi).<ref name="TCS EESC">{{cite web|year=2000|last=Kushnir|first=Yochanan|title=The Climate System|url=http://eesc.columbia.edu/courses/ees/climate/lectures/atm_phys.html|publisher=[[Organization:Columbia University|Columbia University]]|access-date=24 September 2010}}</ref> A recent example of a [[Physics:Tropical cyclone|tropical cyclone]] that maintained itself over cooler waters was Alex of the 2016 Atlantic hurricane season, which became a hurricane over waters at only {{convert|20|C|F|abbr=on}}.<ref>{{cite report|author=Richard Pasch|publisher=National Hurricane Center|date=January 14, 2016|access-date=January 14, 2016|title=Hurricane Alex Discussion Number 4|url=http://www.nhc.noaa.gov/archive/2016/al01/al012016.discus.004.shtml|location=Miami, Florida}}</ref>

At the 500&nbsp;hPa level, the air temperature averages −7&nbsp;°C (18&nbsp;°F) within the tropics, but air in the tropics is normally dry at this level, giving the air room to [[Physics:Wet-bulb temperature|wet-bulb]], or cool as it moistens, to a more favorable temperature that can then support convection. A wet-bulb temperature at 500&nbsp;hPa in a tropical atmosphere of {{convert|-13.2|C|F}} is required to initiate convection if the water temperature is {{convert|26.5|C|F}}, and this temperature requirement increases or decreases proportionally by 1&nbsp;°C in the sea surface temperature for each 1&nbsp;°C change at 500&nbsp;hpa. Under a cold cyclone, 500&nbsp;hPa temperatures can fall as low as {{convert|-30|C|F}}, which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the [[Earth:Troposphere|troposphere]], roughly at the 500&nbsp;hPa level, is normally a requirement for development. However, when dry air is found at the same height, temperatures at 500&nbsp;hPa need to be even colder as dry atmospheres require a greater lapse rate for instability than moist atmospheres.<ref>{{cite book|title=Atmospheric Science: An Introductory Survey|author1=John M. Wallace |author2=Peter V. Hobbs |pages=76–77|publisher=Academic Press, Inc|year=1977}}</ref><ref name="LANDSEACLI">{{Cite web| author = Chris Landsea | url = http://www.aoml.noaa.gov/hrd/Landsea/climvari/index.html | title = Climate Variability of Tropical Cyclones: Past, Present and Future | year = 2000 | work = Storms | pages = 220–41 | access-date = 2006-10-19 | publisher = [[Organization:Atlantic Oceanographic and Meteorological Laboratory|Atlantic Oceanographic and Meteorological Laboratory]]| author-link = Chris Landsea }}</ref> At heights near the [[Earth:Tropopause|tropopause]], the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was −77&nbsp;°C (−132&nbsp;°F).<ref name="UAIRtropics">{{Cite web|author1=Dian J. Gaffen-Seidel |author2=Rebecca J. Ross |author3=James K. Angell | title = Climatological characteristics of the tropical tropopause as revealed by radiosondes | url = http://www.aero.jussieu.fr/~sparc/SPARC2000_new/OralSess2/D_Gaffen/GaffenHtml/Abs_Gaffen.html | publisher = [[Organization:National Oceanic and Atmospheric Administration|National Oceanic and Atmospheric Administration Air Resources Laboratory]] |date = November 2000| access-date = 2006-10-19 |archive-url = https://web.archive.org/web/20060508184913/http://www.aero.jussieu.fr/~sparc/SPARC2000_new/OralSess2/D_Gaffen/GaffenHtml/Abs_Gaffen.html |archive-date = May 8, 2006}}</ref>

==See also==
*[[Physics:Cut-off low|Cut-off low]]
*[[Earth:Cold pool|Cold pool]]

==References==
{{reflist|2}}
{{Cyclones}}

[[Category:Meteorological phenomena]]
[[Category:Tropical cyclone meteorology]]
[[Category:Types of cyclone|Cold core low]]
[[Category:Vortices]]

{{Sourceattribution|Cold-core low}}