Self-aggregation conceptualized by cold pool organization

2020 ◽  
Author(s):  
Silas Boye Nissen ◽  
Jan O. Haerter
Keyword(s):  
Tropical Cyclones ◽  
Large Scale ◽  
Spatial Organization ◽  
Cold Pool ◽  
Mathematical Framework ◽  
Madden Julian Oscillation ◽  
Cold Pools ◽  
Convective Cells ◽  
Negatively Buoyant ◽  
Self Aggregation

<p>In radiative-convective equilibrium (RCE) simulations, self-aggregation is the spontaneous emergence of one or several long-lasting convective clusters from an apparently homogenous atmosphere (Wing, 2019). This phenomenon may implicate the formation of tropical cyclones (Wing et al., 2016; Muller et al., 2018) and large-scale events such as the Madden-Julian Oscillation (Arnold et al., 2015; Satoh et al., 2016; Khairoutdinov et al., 2018). However, it remains poorly understood how cold pools (CPs) contribute to self-aggregation. Using a suite of cloud-resolving numerical simulations, we link the life-cycle and the spatial organization of CPs to the evolution of self-aggregation. By tracking CPs, we determine the maximal CP radius R<sub>max</sub> ≈ 20 km and show that cloud-free regions exceeding such radii always grow indefinitely. Besides, we identify a minimum CP radius R<sub>min</sub> ≈ 8 km below which CPs are too cold, hence negatively buoyant, to initialize new convective cells. Finally, we suggest a simple mathematical framework that describes a mechanism, where cloud-free areas are likely to form when CPs have small R<sub>max</sub>, whereas large R<sub>max</sub> hampers cavity formation. Our findings imply that interactions between CPs crucially control the dynamics of self-aggregation, and known feedbacks may only be required in stabilizing the final, fully-aggregated state.</p>

Cold Pools and Their Influence on the Tropical Marine Boundary Layer

2017 ◽  
Vol 74 (4) ◽  
pp. 1149-1168 ◽  
Author(s):  
Simon P. de Szoeke ◽  
Eric D. Skyllingstad ◽  
Paquita Zuidema ◽  
Arunchandra S. Chandra
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Sensible Heat Flux ◽  
Surface Flux ◽  
Active Phase ◽  
Surface Fluxes ◽  
Cold Pool ◽  
Madden Julian Oscillation ◽  
Cold Pools ◽  
Central Indian Ocean

Abstract Cold pools dominate the surface temperature variability observed over the central Indian Ocean (0°, 80°E) for 2 months of research cruise observations in the Dynamics of the Madden–Julian Oscillation (DYNAMO) experiment in October–December 2011. Cold pool fronts are identified by a rapid drop of temperature. Air in cold pools is slightly drier than the boundary layer (BL). Consistent with previous studies, cold pools attain wet-bulb potential temperatures representative of saturated downdrafts originating from the lower midtroposphere. Wind and surface fluxes increase, and rain is most likely within the ~20-min cold pool front. Greatest integrated water vapor and liquid follow the front. Temperature and velocity fluctuations shorter than 6 min achieve 90% of the surface latent and sensible heat flux in cold pools. The temperature of the cold pools recovers in about 20 min, chiefly by mixing at the top of the shallow cold wake layer, rather than by surface flux. Analysis of conserved variables shows mean BL air is composed of 51% air entrained from the BL top (800 m), 22% saturated downdrafts, and 27% air at equilibrium with the ocean surface. The number of cold pools, and their contribution to the BL heat and moisture, nearly doubles in the convectively active phase compared to the suppressed phase of the Madden–Julian oscillation.

The diurnal cycle can trigger convective self-aggregation 

2021 ◽  
Author(s):  
Gorm Gruner Jensen ◽  
Romain Fiévet ◽  
Jan O. Haerter
Keyword(s):  
Surface Temperature ◽  
Diurnal Cycle ◽  
Large Scale ◽  
Horizontal Resolution ◽  
Cold Pool ◽  
Atmospheric Sciences ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Spatio Temporal ◽  
Self Aggregation

<p>Convective self-aggregation (CSA) is an established modelling paradigm for large-scale thunderstorm clusters, as they form in mesoscale convective systems, the Madden-JulianOscillation or tropical cyclo-genesis [1]. The onset of CSA is characterized by the spontaneous formation of persistently dry patches with suppressed deep convective rainfall. Recently another type of spatio-temporal pattern formation was observed in simulations where the diurnal cycle was mimicked by a sinusoidally varying surface temperature [2]. This diurnal aggregation (DA) is characterized by clusters of intense rain that correlate negatively from one day to the next. </p><p>Here we demonstrate that the diurnal cycle can also act as a trigger of persistently dry patches resembling the early stages of CSA. When the surface temperature is held constant, CSA has been shown to occur within simulations of coarse horizontal model resolution, but not when the resolution was increased [3]. We show that, when a temporally periodic surface temperature forcing is imposed, persistently convection free patches occur even faster when the spatial resolution is increased. The failure to achieve CSA at high horizontal resolution has so far been attributed to the more pronounced cold pool effects at such resolution. In our simulations these cold pools in fact play a key role in promoting CSA. Our results have implications for the origin of persistent convective organization over continents and the sea — and point a path towards achieving such clustering under realistic conditions.</p><p><br>[1]  Christopher S Bretherton, Peter N Blossey, and Marat Khairoutdinov.  An energy-balance analysisof deep convective self-aggregation above uniform SST.Journal of the Atmospheric Sciences, 62(12):4273–4292, 2005.<br>[2]  J. O. Haerter, B. Meyer, and S. B. Nissen.  Diurnal self-aggregation.npj Climate and AtmosphericScience, 3:30, 2020.<br>[3]  Caroline  Muller  and  Sandrine  Bony.   What  favors  convective  aggregation  and  why?GeophysicalResearch Letters, 42(13):5626–5634, 2015.  doi:  https://doi.org/10.1002/2015GL064260.</p>

scholarly journals Synoptic-Scale Flow and Valley Cold Pool Evolution in the Western United States

2009 ◽  
Vol 24 (6) ◽  
pp. 1625-1643 ◽  
Author(s):  
Heather Dawn Reeves ◽  
David J. Stensrud
Keyword(s):  
United States ◽  
Large Scale ◽  
Numerical Models ◽  
Western United States ◽  
Cold Pool ◽  
Synoptic Scale ◽  
Temperature Changes ◽  
Cold Pools ◽  
Upper Level ◽  
Forecast Problem

Abstract Valley cold pools (VCPs), which are trapped, cold layers of air at the bottoms of basins or valleys, pose a significant problem for forecasters because they can lead to several forms of difficult-to-forecast and hazardous weather such as fog, freezing rain, or poor air quality. Numerical models have historically failed to routinely provide accurate guidance on the formation and demise of VCPs, making the forecast problem more challenging. In some case studies of persistent wintertime VCPs, there is a connection between the movement of upper-level waves and the timing of VCP formation and decay. Herein, a 3-yr climatology of persistent wintertime VCPs for five valleys and basins in the western United States is performed to see how often VCP formation and decay coincides with synoptic-scale (∼200–2000 km) wave motions. Valley cold pools are found to form most frequently as an upper-level ridge approaches the western United States and in response to strong midlevel warming. The VCPs usually last as long as the ridge is over the area and usually only end when a trough, and its associated midlevel cooling, move over the western United States. In fact, VCP strength appears to be almost entirely dictated by midlevel temperature changes, which suggests large-scale forcing is dominant for this type of VCP most of the time.

scholarly journals An oceanic pathway for Madden–Julian Oscillation influence on Maritime Continent Tropical Cyclones

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Karthik Balaguru ◽  
L. Ruby Leung ◽  
Samson M. Hagos ◽  
Sujith Krishnakumar
Keyword(s):  
Numerical Model ◽  
Tropical Cyclones ◽  
Large Scale ◽  
Sea Surface ◽  
Madden Julian Oscillation ◽  
Surface Cooling ◽  
Maritime Continent ◽  
Large Scale Circulation ◽  
Model Simulations ◽  
Sea Surface Cooling

AbstractWhile the Madden–Julian Oscillation (MJO) has been shown to affect tropical cyclones (TCs) worldwide through its modulation of large-scale circulation in the atmosphere, little or no role for the ocean has been identified to date in this influence of MJO on TCs. Using observations and numerical model simulations, we demonstrate that MJO events substantially impact TCs over the Maritime Continent (MC) region through an oceanic pathway. While propagating across the MC region, MJO events cause significant sea surface cooling with an area-averaged value of about 0.35 ± 0.12 °C. Hence, TCs over the MC region immediately following the passage of MJO events encounter considerably cooler sea surface temperatures. Consequently, the enthalpy fluxes under the storms are reduced and the intensification rates decrease by more than 50% on average. These results highlight an important role played by the ocean in facilitating MJO-induced sub-seasonal variability in TC activity over the MC region.

The winner takes it all - how long-lived raincells compete in cold pool-suppressed self-aggregation

2021 ◽  
Author(s):  
Ronja Gronemeyer
Keyword(s):  
Conceptual Model ◽  
Mass Parameter ◽  
Extreme Case ◽  
Cloud Base ◽  
Cold Pool ◽  
Large Area ◽  
Geophysical Research ◽  
Cold Pools ◽  
Rain Events ◽  
Self Aggregation

<p>Under radiative convective equilibrium (RCE), cloud populations simulated by large-eddy simulations (LES) can spontaneously segregate into cloudy and cloud-free subregions. This process is well-known as convective self-aggregation (CSA) [4]. But how initially randomly distributed raincells compete and merge until only one prevails, is not well-understood. We remove cold pools (CPs) in LES by suppressing the re-evaporation of rain, which leads to qualitatively different dynamics. This extreme case helps to understand the role of CPs in the formation of CSA and further has relevance when humidity is very high in the boundary layer, so very little rainfall evaporates.</p><p>When convection starts, patterns of high and low moisture develop, which increase in scale over time. In contrast to CSA with CPs, individual rain events and convection cells persist up to tens of hours in the course of this modified CSA [1]. For the long lasting individual rain clusters, we observe interesting oscillations in rain intensity and spatial extent. We define an algorithm, that tracks the tree-like merging behavior of initially many individual small raincells to a final, single, raincell of large area and precipitation yield. We conceptualize the LES behavior in a simple model, that assumes different rain events to compete for buoyancy. This hypothesis is justified when viewing rain events as linked to local maxima of relative humidity around cloud base. The clusters‘ dynamics seem to be dominated by merging with other events and ’stealing’ from smaller events, whereas splitting and emerging of new rain events seem neglectable after a build-up time. In each step, the conceptual model chooses two adjacent clusters. Initially, each cluster is attributed a ‘mass’ parameter of similar magnitude and a fraction (<em>p</em>) of the smaller ’mass’ (<em>m</em><sub>2</sub>) is transferred to the bigger event (<em>m</em><sub>1</sub>).</p><p><em>m</em><sub>1</sub><em><sup>new</sup></em> = <em>m</em><sub>1</sub> + <em>p</em>(<em>m</em><sub>1</sub> − <em>m</em><sub>2</sub>)<br><em>m</em><sub>2</sub><em><sup>new</sup></em> = <em>m</em><sub>2</sub> - <em>p</em>(<em>m</em><sub>1</sub> − <em>m</em><sub>2</sub>)</p><p>An event is removed, when its mass parameter is diminished to zero. In contrast to field based approaches [3], this approach implements discrete rich gets richer dynamics, to capture how individual cells grow. This conceptual model could be combined with existing models, where CP suppress the rain cells, but trigger new updrafts through the CP gust fronts [2]. Bringing together these two limits could further elucidate how CP dynamics can be made compatible with convective self-aggregation.</p><p>References:<br>[1] Nadir Jeevanjee and David M Romps. Convective self-aggregation, cold pools, and domain size. <em>Geophysical Research Letters</em>, 40(5):994–998, 2013.</p><p>[2] Silas Boye Nissen and Jan O. Haerter. How weakened cold pools open for convective self-aggregation, 2020, arXiv:1911.12849v3.</p><p>[3] Julia M. Windmiller and George C. Craig. Universality in the spatial evolution of self-aggregation of tropical convection. <em>Journal of the Atmospheric Sciences</em>, 76(6):1677 – 1696, 01 Jun. 2019.</p><p>[4] Allison A Wing, Kerry Emanuel, Christopher E Holloway, and Caroline Muller. Convective self-aggregation in numerical simulations: A review. In <em>Shallow Clouds, Water Vapor, Circulation, and Climate Sensitivity</em>, pages 1–25. Springer, 2017.</p>

scholarly journals Impacts of Evaporation from Raindrops on Tropical Cyclones. Part II: Features of Rainbands and Asymmetric Structure

2010 ◽  
Vol 67 (1) ◽  
pp. 84-96 ◽  
Author(s):  
Masahiro Sawada ◽  
Toshiki Iwasaki
Keyword(s):  
Evaporative Cooling ◽  
Heavy Precipitation ◽  
Normal Direction ◽  
Cold Pool ◽  
Asymmetric Structure ◽  
Moist Air ◽  
Low Level ◽  
Cold Pools ◽  
Asymmetric Flows ◽  
Convective Cells

Abstract In this study, the impacts of evaporative cooling from raindrops on a tropical cyclone (TC) are examined using cloud-resolving simulations under an idealized condition. Part I of this study showed that evaporative cooling greatly increases the kinetic energy of a TC and its size because rainbands provide a large amount of condensation heating outside the eyewall. Part II investigates characteristics of simulated rainbands in detail. Rainbands are actively formed, even outside the eyewall, in the experiment including evaporative cooling, whereas they are absent in the experiment without evaporative cooling. Rainbands propagate in the counterclockwise and radially outward direction, and such behaviors are closely related to cold pools. New convective cells are successively generated at the upstream end of a cold pool, which is referred to here as the upstream development. The upstream development organizes spiral-shaped rainbands along a low-level streamline that is azimuthally averaged and propagates them radially outward. Asymmetric flows from azimuthally averaged low-level wind advance cold pool fronts in the normal direction to rainbands, which are referred to here as cross-band propagation. The cross-band propagation deflects the movement of each cell away from the low-level streamlines and rotates it in the counterclockwise direction. Cross-band propagation plays an essential role in the maintenance of rainbands. Advancement of cold pool fronts lifts up the warm and moist air mass slantwise and induces heavy precipitation. Evaporative cooling from raindrops induces downdrafts and gives feedback to the enhancement of cold pools.

scholarly journals How weakened cold pools open for convective self-aggregation

2021 ◽  
Author(s):  
Silas Boye Nissen ◽  
Jan O. Haerter
Keyword(s):  
Crucial Role ◽  
Generation Time ◽  
Spatial Organization ◽  
Large Eddy Simulations ◽  
Cell Generation ◽  
Cold Pools ◽  
Large Eddy ◽  
Simulated Rain ◽  
Self Aggregation ◽  
Large Domains

<p>In radiative-convective equilibrium (RCE) simulations, convective self-aggregation (CSA) is the spontaneous organization into segregated cloudy and cloud-free regions. Evidence exists for how CSA is stabilized, but how it arises favorably on large domains is not settled. Using large-eddy simulations (LES), we link the spatial organization emerging from the interaction of cold pools (CPs) to CSA. We systematically weaken simulated rain evaporation to reduce maximal CP radii, <em>R</em><sub>max</sub>, and find reducing <em>R</em><sub>max</sub> causes CSA to occur earlier. We further identify a typical rain cell generation time and a minimum radius, <em>R</em><sub>min</sub>, around a given rain cell, within which the formation of subsequent rain cells is suppressed. Incorporating <em>R</em><sub>min</sub> and <em>R</em><sub>max</sub>, we propose a toy model that captures how CSA arises earlier on large domains: when two CPs of radii <em>r</em><sub><em>i</em>,<em>j </em></sub>∈ [<em>R</em><sub>min</sub>, <em>R</em><sub>max</sub>] collide, they form a new convective event. These findings imply that CPs play a crucial role in RCE simulations by preventing the onset of CSA.</p>

scholarly journals Observations of the temporal variability in aerosol properties and their relationships to meteorology in the summer monsoonal South China Sea/East Sea: the scale-dependent role of monsoonal flows, the Madden–Julian Oscillation, tropical cyclones, squall lines and cold pools

2015 ◽  
Vol 15 (4) ◽  
pp. 1745-1768 ◽  
Author(s):  
J. S. Reid ◽  
N. D. Lagrosas ◽  
H. H. Jonsson ◽  
E. A. Reid ◽  
W. R. Sessions ◽  
...  
Keyword(s):  
Boundary Layer ◽  
South China Sea ◽  
Wind Shear ◽  
Large Scale ◽  
Aerosol Particles ◽  
Vertical Wind Shear ◽  
Sea Salt ◽  
Madden Julian Oscillation ◽  
Cold Pools ◽  
Squall Lines

Abstract. In a joint NRL/Manila Observatory mission, as part of the Seven SouthEast Asian Studies program (7-SEAS), a 2-week, late September 2011 research cruise in the northern Palawan archipelago was undertaken to observe the nature of southwest monsoonal aerosol particles in the South China Sea/East Sea (SCS/ES) and Sulu Sea region. Previous analyses suggested this region as a receptor for biomass burning from Borneo and Sumatra for boundary layer air entering the monsoonal trough. Anthropogenic pollution and biofuel emissions are also ubiquitous, as is heavy shipping traffic. Here, we provide an overview of the regional environment during the cruise, a time series of key aerosol and meteorological parameters, and their interrelationships. Overall, this cruise provides a narrative of the processes that control regional aerosol loadings and their possible feedbacks with clouds and precipitation. While 2011 was a moderate El Niño–Southern Oscillation (ENSO) La Niña year, higher burning activity and lower precipitation was more typical of neutral conditions. The large-scale aerosol environment was modulated by the Madden–Julian Oscillation (MJO) and its associated tropical cyclone (TC) activity in a manner consistent with the conceptual analysis performed by Reid et al. (2012). Advancement of the MJO from phase 3 to 6 with accompanying cyclogenesis during the cruise period strengthened flow patterns in the SCS/ES that modulated aerosol life cycle. TC inflow arms of significant convection sometimes span from Sumatra to Luzon, resulting in very low particle concentrations (minimum condensation nuclei CN < 150 cm−3, non-sea-salt PM2.5 < 1 μg m−3). However, elevated carbon monoxide levels were occasionally observed suggesting passage of polluted air masses whose aerosol particles had been rained out. Conversely, two drier periods occurred with higher aerosol particle concentrations originating from Borneo and Southern Sumatra (CN > 3000 cm−3 and non-sea-salt PM2.5 10–25 μg m−3). These cases corresponded with two different mechanisms of convection suppression: lower free-tropospheric dry-air intrusion from the Indian Ocean, and large-scale TC-induced subsidence. Veering vertical wind shear also resulted in aerosol transport into this region being mainly in the marine boundary layer (MBL), although lower free troposphere transport was possible on the western sides of Sumatra and Borneo. At the hourly time scale, particle concentrations were observed to be modulated by integer factors through convection and associated cold pools. Geostationary satellite observations suggest that convection often takes the form of squall lines, which are bowed up to 500 km across the monsoonal flow and 50 km wide. These squall lines, initiated by cold pools from large thunderstorms and likely sustained by a veering vertical wind shear and aforementioned mid-troposphere dry layers, propagated over 1500 km across the entirety of the SCS/ES, effectively cutting large swaths of MBL aerosol particles out of the region. Our conclusion is that while large-scale flow patterns are very important in modulating convection, and hence in allowing long-range transport of smoke and pollution, more short-lived phenomena can modulate cloud condensation nuclei (CCN) concentrations in the region, resulting in pockets of clean and polluted MBL air. This will no doubt complicate large scale comparisons of aerosol–cloud interaction.

A Lagrangian perspective on cold pool collisions

2020 ◽  
Author(s):  
Giuseppe Torri ◽  
Zhiming Kuang
Keyword(s):  
Boundary Layer ◽  
Life Cycle ◽  
Spatial Scales ◽  
Mesoscale Convective Systems ◽  
Cold Pool ◽  
Convective Systems ◽  
Cold Pools ◽  
Mesoscale Convective ◽  
Temporal And Spatial ◽  
Convective Cells

&lt;p&gt;Collisions represent one of the most important processes through which cold pools&amp;#8212;essential boundary layer features of precipitating systems&amp;#8212;help to organize convection. For example, by colliding with one another, expanding cold pools can trigger new convective cells, a process that has been argued to be important to explain the deepening of convection and the maintenance of mesoscale convective systems for many hours. In spite of their role, collisions are an understudied process, and many aspects remain to be fully clarified. In order to quantify the importance of collisions on the life cycle of cold pools, we will present some results based on a combination of numerical simulations in radiative-convective equilibrium and a Lagrangian cold pool tracking algorithm. First, we will discuss how the Lagrangian algorithm can be used to estimate that the median time of the first collision for the simulated cold pools is under 10 minutes. We will then show that cold pools are significantly deformed by collisions and lose their circular shape already at the very early stages of their life cycle. Finally, we will present results suggesting that cold pools appear to be clustered, and we will provide some estimates of the associated temporal and spatial scales.&lt;/p&gt;

scholarly journals The Boreal Summer Madden–Julian Oscillation and Moist Convective Morphology over the Maritime Continent

2020 ◽  
Vol 77 (2) ◽  
pp. 647-667 ◽  
Author(s):  
Benjamin A. Toms ◽  
Susan C. van den Heever ◽  
Emily M. Riley Dellaripa ◽  
Stephen M. Saleeby ◽  
Eric D. Maloney
Keyword(s):  
Cell Morphology ◽  
Large Scale ◽  
Convective Cell ◽  
Boreal Summer ◽  
Madden Julian Oscillation ◽  
Maritime Continent ◽  
Intermediate Phases ◽  
Cloud Resolving Model ◽  
Radiative Feedbacks ◽  
Convective Cells

Abstract While the boreal summer Madden–Julian oscillation (MJO) is commonly defined as a planetary-scale disturbance, the convective elements that constitute its cloud dipole exhibit pronounced variability in their morphology. We therefore investigate the relationship between the intraseasonal cloud anomaly of the MJO and the convective elements that populate its interior by simulating a boreal summer MJO event over the Maritime Continent using a cloud-resolving model. A progressive relationship between convective cell morphology and the MJO within the convectively enhanced region of the MJO was identified and characterized as follows: anomalously long-lasting cells in the initial phases, followed by an increased number of cells in the intermediate phases, progressing into more expansive cells in the terminal phases. A progressive relationship does not seem to exist within the convectively suppressed region of the MJO within the simulated domain, however. Within the convectively enhanced region of the MJO, the progressive relationship is partially explained by the evolution of bulk atmospheric characteristics, such as instability and wind shear. Positive midlevel moisture anomalies coincide with anomalously long-lasting convective cells, which is hypothesized to further cascade into an increase in convective cell volume, although variability in the number of convective cells seems to be related to an unidentified variable. This intraseasonal relationship between convective cell morphology and the boreal summer MJO within the Maritime Continent may have broader implications for the large-scale structure and evolution of the MJO, related to both convective moistening and cloud-radiative feedbacks.

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