Cold-core low

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Upper-level low
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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

Indian oceans, it is called a subtropical cyclone
. Cloud cover and rainfall mainly occurs with these systems during the day.

Severe weather, such as tornadoes, can occur near the center of cold-core lows. Cold lows can help spawn cyclones with significant weather impacts, such as polar lows, and 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 outflow channels to aid in further development.

An upper tropospheric cyclonic vortex in the western North Pacific, showing high level cloud tops being sucked in.

Characteristics

See also: Thermal wind
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

barotropic in nature.[3] The movement of cold-core cyclones can be erratic since they are separated from the main belt of the Westerlies, which would otherwise steer them progressively to the east.[4]

Most

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.[5] The development of severe weather, particularly tornadoes, can occur near the center of these systems over land during any season of the year.[6] During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach −45 °C (−49 °F) move over open waters, deep convection forms which allows polar low development to become possible.[7]

Importance to cyclones within the subtropics and mid-latitudes

November 4, 1995 Kona Low
See also: Nor'easter and Subtropical cyclone

Kona lows, most extratropical cyclones, and 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 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 monsoon rains during the wet season.[8]

East coast lows form near and east of where a cold core low interacts with a

La Niña years.[11]

Importance to tropical cyclones

See also: Tropical cyclone and Tropical cyclogenesis
Peaks of activity worldwide

The summer tropical upper tropospheric trough in the

La Niña events.[13]

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 jet stream emanating from the developing tropical disturbance/cyclone.[14][15] 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 tropical upper tropospheric trough.[16] Tropical cyclone movement can also be influenced by TUTT cells within 1,700 kilometres (1,100 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 Westerlies normally dominate.[17]

Normally, an ocean temperature of 26.5 °C (79.7 °F) spanning through a depth of at least 50 metres (160 ft) is one of the six requirements needed to maintain the special

relative humidity, the required lapse rate is 9.8 °C/km (29 °F/mi).[19] A recent example of a tropical cyclone that maintained itself over cooler waters was Alex of the 2016 Atlantic hurricane season, which became a hurricane over waters at only 20 °C (68 °F).[20]

At the 500 hPa level, the air temperature averages −7 °C (18 °F) within the tropics, but air in the tropics is normally dry at this level, giving the air room to wet-bulb, or cool as it moistens, to a more favorable temperature that can then support convection. A wet-bulb temperature at 500 hPa in a tropical atmosphere of −13.2 °C (8.2 °F) is required to initiate convection if the water temperature is 26.5 °C (79.7 °F), and this temperature requirement increases or decreases proportionally by 1 °C in the sea surface temperature for each 1 °C change at 500 hpa. Under a cold cyclone, 500 hPa temperatures can fall as low as −30 °C (−22 °F), which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the troposphere, roughly at the 500 hPa level, is normally a requirement for development. However, when dry air is found at the same height, temperatures at 500 hPa need to be even colder as dry atmospheres require a greater lapse rate for instability than moist atmospheres.[21][22] At heights near the tropopause, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was −77 °C (−132 °F).[23]

See also

References

  1. ^ Glossary of Meteorology (June 2000). "Cold low". American Meteorological Society. Archived from the original on 2011-05-14. Retrieved 2010-05-02.
  2. ISBN 0-12-732950-1
    .
  3. ^ Glossary of Meteorology (June 2000). "Barotropic". American Meteorological Society. Archived from the original on 2011-05-14. Retrieved 2010-05-02.
  4. doi:10.1175/1520-0469(1952)009<0024:eotksa>2.0.co;2. Retrieved 2010-05-28.[permanent dead link
    ]
  5. ^ JetStream (2010-01-05). "Glossary: C's". National Weather Service. Retrieved 2010-05-28.
  6. doi:10.1175/WAF967.1
    .
  7. ISBN 978-0-521-62430-5
    .
  8. ISBN 978-0-7923-1346-5
    . Retrieved on 2009-02-29.
  9. ^ Storm-E (2007). "Nor'easters". Center For Educational Technologies. Archived from the original on 2007-06-26. Retrieved 2008-01-22.
  10. ISBN 978-0-521-48440-4
    .
  11. ^ Paul Graham (12 February 2008). "Upper cold pool causes storms in the southeast". Weatherzone.
  12. University of Hawaii
    . Retrieved 2009-12-23.
  13. ISSN 1520-0493
    .
  14. ^ Clark Evans (January 5, 2006). "Favorable trough interactions on tropical cyclones". Flhurricane.com. Archived from the original on October 17, 2006. Retrieved 2006-10-20.
  15. doi:10.1175/1520-0493(2001)129<2570:ACSOTI>2.0.CO;2
    .
  16. ^ Joint Typhoon Warning Center (2010). "2.5 Upper Tropospheric Cyclonic Vortices". United States Navy. Retrieved 2009-04-24.
  17. doi:10.1175/2009WAF2222181.1
    .
  18. Chris Landsea (2011). "Subject: A15) How do tropical cyclones form?". Hurricane Research Division
    . Retrieved 2011-01-27.
  19. ^ Kushnir, Yochanan (2000). "The Climate System". Columbia University. Retrieved 24 September 2010.
  20. ^ Richard Pasch (January 14, 2016). Hurricane Alex Discussion Number 4 (Report). Miami, Florida: National Hurricane Center. Retrieved January 14, 2016.
  21. ^ John M. Wallace; Peter V. Hobbs (1977). Atmospheric Science: An Introductory Survey. Academic Press, Inc. pp. 76–77.
  22. Chris Landsea (2000). "Climate Variability of Tropical Cyclones: Past, Present and Future". Storms. Atlantic Oceanographic and Meteorological Laboratory
    . pp. 220–41. Retrieved 2006-10-19.
  23. ^ Dian J. Gaffen-Seidel; Rebecca J. Ross; James K. Angell (November 2000). "Climatological characteristics of the tropical tropopause as revealed by radiosondes". National Oceanic and Atmospheric Administration Air Resources Laboratory. Archived from the original on May 8, 2006. Retrieved 2006-10-19.
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