Cold pool

Cold pools spread radially away from the rain event along the surface as a moving gust front. When the gust front passes, cold pools cause an increase in wind speed and a sudden drop in specific humidity and in air temperature. In large-eddy simulations, they reach 10 km in radius, whereas, in reality, they can become as large as 50–100 km in radius and last 2-3 h on average.{{Cite journal |last1=Wilbanks |first1=Matt C. |last2=Yuter |first2=Sandra E. |last3=Szoeke |first3=Simon P. de |last4=Brewer |first4=W. Alan |last5=Miller |first5=Matthew A. |last6=Hall |first6=Andrew M. |last7=Burleyson |first7=Casey D. |date=2015-09-01 |title=Near-Surface Density Currents Observed in the Southeast Pacific Stratocumulus-Topped Marine Boundary Layer |url=https://journals.ametsoc.org/view/journals/mwre/143/9/mwr-d-14-00359.1.xml |journal=Monthly Weather Review |language=EN |volume=143 |issue=9 |pages=3532–3555 |doi=10.1175/MWR-D-14-00359.1 |bibcode=2015MWRv..143.3532W |issn=1520-0493}}{{cite journal |last1=Zuidema |first1=Paquita |last2=Torri |first2=Giuseppe |last3=Muller |first3=Caroline |date=November 14, 2017 |title=A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment |journal=Surv Geophys |volume=38 |issue=6 |pages=1283–1305 |bibcode=2017SGeo...38.1283Z |doi=10.1007/s10712-017-9447-x |doi-access=free}}

File:Outflow boundary of storm approaching Tulsa - rocking animation.gif (light blue) created by the cold pool spreading from a thunderstorm (left).]]

Cold pools consist of a large-scale mass of cold air{{Cite book |last=Glickman |first=Todd S. |url=https://eref.uni-bayreuth.de/id/eprint/20659/ |title=Glossary of meteorology |date=2000 |publisher=American Meteorological Soc. |isbn=978-1-878220-34-9 |editor-last=Glickman |editor-first=Todd S. |location=Boston, Mass.}} surrounded by warmer air (according to the American Meteorological Society). Over the ocean, these masses of cold and dense surface air are mostly caused by cooling through evaporation of precipitation from shallow and thunderstorm clouds in unsaturated air. Evaporation of precipitation requires energy, which is used in the form of latent heat, making the air inside a cold pool denser than the environmental air. In addition, the falling rain drags the air around it.{{Cite journal |last1=Torri |first1=Giuseppe |last2=Kuang |first2=Zhiming |last3=Tian |first3=Yang |date=2015-03-28 |title=Mechanisms for convection triggering by cold pools |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GL063227 |journal=Geophysical Research Letters |language=en |volume=42 |issue=6 |pages=1943–1950 |doi=10.1002/2015GL063227 |arxiv=1511.02815 |bibcode=2015GeoRL..42.1943T |issn=0094-8276}}{{Cite journal |last1=Torri |first1=Giuseppe |last2=Kuang |first2=Zhiming |date=2017-09-01 |title=Corrigendum |journal=Journal of the Atmospheric Sciences |language=EN |volume=74 |issue=9 |pages=3125–3126 |doi=10.1175/JAS-D-17-0160.1 |bibcode=2017JAtS...74.3125T |issn=0022-4928|doi-access=free }} These effects accelerate the air mass towards the surface, leading to a rapid decrease in surface air temperature and generating a divergent flow that moves radially away from the location of the precipitation.{{Cite journal |last=Bastian |first=Kirsch |date=October 2022 |title=Illuminating convective cold pools with a dense station network |url=https://pure.mpg.de/pubman/faces/ViewItemOverviewPage.jsp?itemId=item_3432702 |doi=10.17617/2.3432702}} As the density current spreads horizontally outward, dry and cold air is injected into the boundary layer due to the penetrative downdrafts. This dries the central area of the cold pool, sometimes referred to as the cold pool wake, more than the edges. Cold pools spread radially away from the rain event along the surface as a moving gust front. When the gust front passes, cold pools cause an increase in wind speed and a sudden drop in specific humidity and in air temperature. The gust front is associated with wind enhancement and mechanical lifting of moist air.{{Cite journal |last1=Drager |first1=Aryeh J. |last2=Grant |first2=Leah D. |last3=van den Heever |first3=Susan C. |date=August 2020 |title=Cold Pool Responses to Changes in Soil Moisture |journal=Journal of Advances in Modeling Earth Systems |language=en |volume=12 |issue=8 |doi=10.1029/2019MS001922 |bibcode=2020JAMES..1201922D |issn=1942-2466|doi-access=free }}{{Cite journal |last1=Feng |first1=Zhe |last2=Hagos |first2=Samson |last3=Rowe |first3=Angela K. |last4=Burleyson |first4=Casey D. |last5=Martini |first5=Matus N. |last6=de Szoeke |first6=Simon P. |date=June 2015 |title=Mechanisms of convective cloud organization by cold pools over tropical warm ocean during the AMIE/DYNAMO field campaign |journal=Journal of Advances in Modeling Earth Systems |language=en |volume=7 |issue=2 |pages=357–381 |doi=10.1002/2014MS000384 |bibcode=2015JAMES...7..357F |issn=1942-2466|doi-access=free }} The cold pool gust front can therefore usually be identified as a mesoscale cloud arc.

Cold pools end when their features can no longer be distinguished from the large-scale flow, i.e., when their signature in meteorological variables is within the background variability of the environment. The temperature in the interior of a cold pool usually recovers faster than the temperature at the edge, since air from above the boundary layer is entrained into the cold pool wake. Additionally, the moisture recovers much more quickly than the temperature.

Cold pools are ubiquitous both over land and ocean.{{Cite journal |last=Zipser |first=E. J. |date=1977-12-01 |title=Mesoscale and Convective–Scale Downdrafts as Distinct Components of Squall-Line Structure |journal=Monthly Weather Review |language=EN |volume=105 |issue=12 |pages=1568–1589 |doi=10.1175/1520-0493(1977)105<1568:MACDAD>2.0.CO;2 |bibcode=1977MWRv..105.1568Z |issn=1520-0493|doi-access=free }} Nearly all shallow clouds in the trade-wind region that produce precipitation rates larger than 1 mm/h are associated with cold pools.{{Cite journal |last1=Zuidema |first1=Paquita |last2=Li |first2=Zhujun |last3=Hill |first3=Reginald J. |last4=Bariteau |first4=Ludovic |last5=Rilling |first5=Bob |last6=Fairall |first6=Chris |last7=Brewer |first7=W. Alan |last8=Albrecht |first8=Bruce |last9=Hare |first9=Jeff |date=2012-01-01 |title=On Trade Wind Cumulus Cold Pools |url=https://journals.ametsoc.org/view/journals/atsc/69/1/jas-d-11-0143.1.xml |journal=Journal of the Atmospheric Sciences |language=EN |volume=69 |issue=1 |pages=258–280 |doi=10.1175/JAS-D-11-0143.1 |bibcode=2012JAtS...69..258Z |issn=0022-4928}}

Cold pool characteristics differ depending on the depth of the parent convection (deep or shallow). The properties of cold pools formed in the trades (characterized by shallow convection) were observed to vary significantly from properties of cold pools formed from tropical deep convection. For example, they are associated with temperature drops that are on average 2 K weaker, and they experience less drying and smaller wind speed enhancement. The trade wind region is mainly drier and is characterized by subsiding motion that caps the growth of convection and maintains clouds shallow.

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Cold pools are found to trigger secondary convection at their edges, through the combination of mechanical and thermodynamic lifting. Thermodynamic convective triggering is due to enhanced water vapor and virtual temperature in the gust front; it is prevalent in regions of deep convection with low vertical shear over the ocean. Mechanical convective triggering also occurs at the cold pool edges when the spreading velocity is high enough. Collisions between multiple cold pools can trigger the formation of a new thunderstorm event, and thereby form a new cold pool.

Over land the effect of aerosols and surface fluxes are of high importance for the triggering of new convection.{{Cite journal |last1=Schlemmer |first1=Linda |last2=Hohenegger |first2=Cathy |date=January 2016 |title=Modifications of the atmospheric moisture field as a result of cold-pool dynamics |url=https://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.2625 |journal=Quarterly Journal of the Royal Meteorological Society |language=en |volume=142 |issue=694 |pages=30–42 |doi=10.1002/qj.2625 |bibcode=2016QJRMS.142...30S |issn=0035-9009|url-access=subscription }}

An important role of cold pools can be found in the evolution of convective aggregation. Cold pools tend to homogenize the convection and moisture fields through the divergence of near-surface air, and can therefore act to oppose convective self-aggregation.{{Cite journal |last1=Jeevanjee |first1=Nadir |last2=Romps |first2=David M. |date=2013-03-16 |title=Convective self-aggregation, cold pools, and domain size |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/grl.50204 |journal=Geophysical Research Letters |language=en |volume=40 |issue=5 |pages=994–998 |doi=10.1002/grl.50204 |bibcode=2013GeoRL..40..994J |osti=1171623 |issn=0094-8276}} This can be the case for deep {{Cite journal |last1=Muller |first1=Caroline |last2=Bony |first2=Sandrine |date=2015-07-16 |title=What favors convective aggregation and why? |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GL064260 |journal=Geophysical Research Letters |language=en |volume=42 |issue=13 |pages=5626–5634 |doi=10.1002/2015GL064260 |bibcode=2015GeoRL..42.5626M |issn=0094-8276}} and shallow convection.{{Cite journal |last1=Narenpitak |first1=Pornampai |last2=Kazil |first2=Jan |last3=Yamaguchi |first3=Takanobu |last4=Quinn |first4=Patricia K. |last5=Feingold |first5=Graham |date=January 2023 |title=The Sugar-To-Flower Shallow Cumulus Transition Under the Influences of Diel Cycle and Free-Tropospheric Mineral Dust |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022MS003228 |journal=Journal of Advances in Modeling Earth Systems |language=en |volume=15 |issue=1 |doi=10.1029/2022MS003228 |bibcode=2023JAMES..1503228N |issn=1942-2466}} Cold pools have been pointed to as the source of the domain-dependence of cloud resolving models to initiate convective self-aggregation.{{cite journal |last1 = Nissen |first1 = Silas Boye |last2 = Haerter| first2 = Jan O.|title=Circling in on Convective Self-Aggregation |journal=Journal of Geophysical Research: Atmospheres |date=September 24, 2021 |volume=126 |issue = 20 |pages = e2021JD035331 |doi=10.1029/2021JD035331 |pmid = 35864905 |pmc = 9285845 |arxiv=1911.12849 |bibcode = 2021JGRD..12635331N |s2cid = 244074835 }} Convective self-aggregation can only occur at domains larger than ~200km. In larger domains, cold pools would dissipate before they could travel far enough to inhibit aggregation of convection. There is a competition between the homogenizing effect of cold pools and the inflow of moisture from the dry regions into a convectively aggregated cell, which is thought to contribute to edge-intensified convection.{{Cite journal |last1=Windmiller |first1=J. M. |last2=Hohenegger |first2=C. |date=December 2019 |title=Convection On the Edge |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019MS001820 |journal=Journal of Advances in Modeling Earth Systems |language=en |volume=11 |issue=12 |pages=3959–3972 |doi=10.1029/2019MS001820 |bibcode=2019JAMES..11.3959W |issn=1942-2466|hdl=21.11116/0000-0004-5169-B |hdl-access=free }}

It has been shown that specific mesoscale cloud organization patterns{{Cite journal |last1=Stevens |first1=Bjorn |last2=Bony |first2=Sandrine |last3=Brogniez |first3=Hélène |last4=Hentgen |first4=Laureline |last5=Hohenegger |first5=Cathy |last6=Kiemle |first6=Christoph |last7=L'Ecuyer |first7=Tristan S. |last8=Naumann |first8=Ann Kristin |last9=Schulz |first9=Hauke |last10=Siebesma |first10=Pier A. |last11=Vial |first11=Jessica |last12=Winker |first12=Dave M. |last13=Zuidema |first13=Paquita |date=January 2020 |title=Sugar, gravel, fish and flowers: Mesoscale cloud patterns in the trade winds |url=https://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.3662 |journal=Quarterly Journal of the Royal Meteorological Society |language=en |volume=146 |issue=726 |pages=141–152 |doi=10.1002/qj.3662 |bibcode=2020QJRMS.146..141S |issn=0035-9009|hdl=21.11116/0000-0003-9F41-1 |hdl-access=free }} in the trade-wind region are associated with different occurrence frequencies and properties of cold pools. This is due to the different environmental conditions, as well as rain and cloud properties, associated with different patterns.{{Cite journal |last1=Schulz |first1=Hauke |last2=Eastman |first2=Ryan |last3=Stevens |first3=Bjorn |date=2021-09-16 |title=Characterization and Evolution of Organized Shallow Convection in the Downstream North Atlantic Trades |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JD034575 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=126 |issue=17 |doi=10.1029/2021JD034575 |bibcode=2021JGRD..12634575S |issn=2169-897X|hdl=21.11116/0000-0007-B12C-0 |hdl-access=free }} It is not yet clear how important cold pools are for maintaining or initiating patterns. Cold pools may also aid in the transition from shallow to deep convection.{{Cite journal |last1=Schlemmer |first1=Linda |last2=Hohenegger |first2=Cathy |date=2014-08-01 |title=The Formation of Wider and Deeper Clouds as a Result of Cold-Pool Dynamics |url=https://journals.ametsoc.org/view/journals/atsc/71/8/jas-d-13-0170.1.xml |journal=Journal of the Atmospheric Sciences |language=EN |volume=71 |issue=8 |pages=2842–2858 |doi=10.1175/JAS-D-13-0170.1 |bibcode=2014JAtS...71.2842S |issn=0022-4928|hdl=11858/00-001M-0000-0019-12BC-4 |hdl-access=free }}

Cold pools are expected to have an influence on the cloud cover through their effects on both triggering and suppressing cloudiness. However, this is still an open topic of research. In the trade wind region, this is especially important because the cloud cover greatly contributes to the planetary albedo.{{Cite journal |last=Bony |first=Sandrine |date=2005 |title=Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models |url=http://doi.wiley.com/10.1029/2005GL023851 |journal=Geophysical Research Letters |language=en |volume=32 |issue=20 |doi=10.1029/2005GL023851 |bibcode=2005GeoRL..3220806B |issn=0094-8276}} Challenges in representing and observing microphysical processes, wind shear effects, and the recovery of cold pools, hinder the understanding of the relationship between cold pools and cloud cover.

File:Nasa-worldview-modis-cloud-holes.jpg

From satellite images, cold pools can be identified as mesoscale arcs of clouds surrounding clear-sky areas or stratiform decks. Common detection methods rely on measurements of strong and abrupt surface temperature drops or the onset of strong rain rates.{{Cite journal |last1=Young |first1=George S. |last2=Perugini |first2=Samuel M. |last3=Fairall |first3=C. W. |date=1995-01-01 |title=Convective Wakes in the Equatorial Western Pacific during TOGA |journal=Monthly Weather Review |language=EN |volume=123 |issue=1 |pages=110–123 |doi=10.1175/1520-0493(1995)123<0110:CWITEW>2.0.CO;2 |bibcode=1995MWRv..123..110Y |issn=1520-0493|doi-access=free }} Cold pools can also be identified from changes in the depth of the atmospheric mixed layer,{{Cite journal |last1=Touzé-Peiffer |first1=Ludovic |last2=Vogel |first2=Raphaela |last3=Rochetin |first3=Nicolas |date=2022-05-01 |title=Cold Pools Observed during EUREC4A: Detection and Characterization from Atmospheric Soundings |url=https://journals.ametsoc.org/view/journals/apme/61/5/JAMC-D-21-0048.1.xml |journal=Journal of Applied Meteorology and Climatology |language=EN |volume=61 |issue=5 |pages=593–610 |doi=10.1175/JAMC-D-21-0048.1 |issn=1558-8424}} or from synthetic aperture radar images.{{cite journal |last1=Brilouet |first1=P.-E. |last2=Bouniol |first2=D. |last3=Couvreux |first3=F. |last4=Ayet |first4=A. |last5=Granero-Belinchon |first5=C. |last6=Lothon |first6=M. |last7=Mouche |first7=A. |title=Trade Wind Boundary Layer Turbulence and Shallow Precipitating Convection: New Insights Combining SAR Images, Satellite Brightness Temperature, and Airborne In Situ Measurements |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL102180 |journal=Geophysical Research Letters |language=en |doi=10.1029/2022GL102180 |date=28 January 2023|volume=50 |issue=2 |bibcode=2023GeoRL..5002180B }}

Cold pools have been studied from observations taken during several field campaigns. For example during the Rain in Cumulus over the Ocean campaign (RICO{{Cite journal |last1=Rauber |first1=Robert |last2=Heikes |first2=Brian |last3=al |first3=et |date=2007-01-01 |title=Rain in Shallow Cumulus Over the Ocean: The RICO Campaign |url=https://digitalcommons.uri.edu/gsofacpubs/427 |journal=Graduate School of Oceanography Faculty Publications |volume=88 |issue=12 |pages=1912–1928 |doi=10.1175/BAMS-88-12-1912|hdl=11858/00-001M-0000-000E-71B0-F |hdl-access=free }}) in the eastern Caribbean between December 2004 and January 2005, during the Dynamics of the Madden–Julian Oscillation experiment (DYNAMO) in the Indian Ocean, from the Barbados Cloud Observatory over 12 years{{Cite journal |last1=Stevens |first1=Bjorn |last2=Farrell |first2=David |last3=Hirsch |first3=Lutz |last4=Jansen |first4=Friedhelm |last5=Nuijens |first5=Louise |last6=Serikov |first6=Ilya |last7=Brügmann |first7=Björn |last8=Forde |first8=Marvin |last9=Linne |first9=Holger |last10=Lonitz |first10=Katrin |last11=Prospero |first11=Joseph M. |date=2016-05-01 |title=The Barbados Cloud Observatory: Anchoring Investigations of Clouds and Circulation on the Edge of the ITCZ |url=https://journals.ametsoc.org/view/journals/bams/97/5/bams-d-14-00247.1.xml |journal=Bulletin of the American Meteorological Society |language=EN |volume=97 |issue=5 |pages=787–801 |doi=10.1175/BAMS-D-14-00247.1 |bibcode=2016BAMS...97..787S |issn=0003-0007|hdl=11858/00-001M-0000-0024-2595-B |hdl-access=free }} and during EUREC4A,{{Cite journal |last1=Stevens |first1=Bjorn |last2=Bony |first2=Sandrine |last3=Farrell |first3=David |last4=Ament |first4=Felix |last5=Blyth |first5=Alan |last6=Fairall |first6=Christopher |last7=Karstensen |first7=Johannes |last8=Quinn |first8=Patricia K. |last9=Speich |first9=Sabrina |last10=Acquistapace |first10=Claudia |last11=Aemisegger |first11=Franziska |last12=Albright |first12=Anna Lea |last13=Bellenger |first13=Hugo |last14=Bodenschatz |first14=Eberhard |last15=Caesar |first15=Kathy-Ann |date=2021-08-25 |title=EUREC4A |url=https://essd.copernicus.org/articles/13/4067/2021/ |journal=Earth System Science Data |language=English |volume=13 |issue=8 |pages=4067–4119 |doi=10.5194/essd-13-4067-2021 |doi-access=free |issn=1866-3508|hdl=20.500.11850/504422 |hdl-access=free }} and from dense station networks (FESSTVaL, FESST@HH), to name some examples.

Cold pools have been studied using Large Eddy Simulations{{Cite journal |last1=Seifert |first1=A. |last2=Heus |first2=T. |date=2013-06-10 |title=Large-eddy simulation of organized precipitating trade wind cumulus clouds |url=https://acp.copernicus.org/articles/13/5631/2013/ |journal=Atmospheric Chemistry and Physics |language=English |volume=13 |issue=11 |pages=5631–5645 |doi=10.5194/acp-13-5631-2013 |doi-access=free |bibcode=2013ACP....13.5631S |issn=1680-7316|hdl=11858/00-001M-0000-0013-ADAE-8 |hdl-access=free }}{{Cite journal |last1=Hirt |first1=Mirjam |last2=Craig |first2=George C. |last3=Schäfer |first3=Sophia A. K. |last4=Savre |first4=Julien |last5=Heinze |first5=Rieke |date=July 2020 |title=Cold-pool-driven convective initiation: using causal graph analysis to determine what convection-permitting models are missing |url=https://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.3788 |journal=Quarterly Journal of the Royal Meteorological Society |language=en |volume=146 |issue=730 |pages=2205–2227 |doi=10.1002/qj.3788 |bibcode=2020QJRMS.146.2205H |issn=0035-9009}} and Cloud Resolving Models{{Cite journal |last1=Chandra |first1=Arunchandra S. |last2=Zuidema |first2=Paquita |last3=Krueger |first3=Steven |last4=Kochanski |first4=Adam |last5=de Szoeke |first5=Simon P. |last6=Zhang |first6=Jianhao |date=2018-10-27 |title=Moisture Distributions in Tropical Cold Pools From Equatorial Indian Ocean Observations and Cloud-Resolving Simulations |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JD028634 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=123 |issue=20 |doi=10.1029/2018JD028634 |bibcode=2018JGRD..12311445C |issn=2169-897X}} over smaller domains. Models reproduce the water vapor rings seen in observations, but may overestimate the moisture content of these rings. Difficulties in modeling cold pools arise in the representation of turbulent mixing and microphysics, which occur at the very small scales. The boundary conditions at the edges of the model domain must also be considered and impact the properties of simulated cold pools.{{Cite journal |last1=Li |first1=Zhujun |last2=Zuidema |first2=Paquita |last3=Zhu |first3=Ping |date=2014-08-01 |title=Simulated Convective Invigoration Processes at Trade Wind Cumulus Cold Pool Boundaries |url=https://journals.ametsoc.org/view/journals/atsc/71/8/jas-d-13-0184.1.xml |journal=Journal of the Atmospheric Sciences |language=EN |volume=71 |issue=8 |pages=2823–2841 |doi=10.1175/JAS-D-13-0184.1 |bibcode=2014JAtS...71.2823L |issn=0022-4928}}{{Cite journal |last1=Li |first1=Zhujun |last2=Zuidema |first2=Paquita |last3=Zhu |first3=Ping |last4=Morrison |first4=Hugh |date=2015-09-01 |title=The Sensitivity of Simulated Shallow Cumulus Convection and Cold Pools to Microphysics |url=https://journals.ametsoc.org/view/journals/atsc/72/9/jas-d-14-0099.1.xml |journal=Journal of the Atmospheric Sciences |language=EN |volume=72 |issue=9 |pages=3340–3355 |doi=10.1175/JAS-D-14-0099.1 |bibcode=2015JAtS...72.3340L |issn=0022-4928}}

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• Cold drop

Category:Severe weather and convection

Category:Meteorological phenomena

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