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>

Cold Pool and Precipitation Responses to Aerosol Loading: Modulation by Dry Layers

2015 ◽  
Vol 72 (4) ◽  
pp. 1398-1408 ◽  
Author(s):  
Leah D. Grant ◽  
Susan C. van den Heever
Keyword(s):  
Cloud Droplet ◽  
Convective Precipitation ◽  
Cloud Base ◽  
Cold Pool ◽  
Moisture Profile ◽  
Positive Feedbacks ◽  
Low Level ◽  
Aerosol Loading ◽  
Cold Pools ◽  
Moving Storm

Abstract The relative sensitivity of midlatitude deep convective precipitation to aerosols and midlevel dry layers has been investigated in this study using high-resolution cloud-resolving model simulations. Nine simulations, including combinations of three moisture profiles and three aerosol number concentration profiles, were performed. Because of the veering wind profile of the initial sounding, the convection splits into a left-moving storm that is multicellular in nature and a right-moving storm, a supercell, which are analyzed separately. The results demonstrate that while changes to the moisture profile always induce larger changes in precipitation than do variations in aerosol concentrations, multicells are sensitive to aerosol perturbations whereas supercells are less so. The multicellular precipitation sensitivity arises through aerosol impacts on the cold pool forcing. It is shown that the altitude of the dry layer influences whether cold pools are stronger or weaker and hence whether precipitation increases or decreases with increasing aerosol concentrations. When the dry-layer altitude is located near cloud base, cloud droplet evaporation rates and hence latent cooling rates are greater with higher aerosol loading, which results in stronger low-level downdrafts and cold pools. However, when the dry-layer altitude is located higher above cloud base, the low-level downdrafts and cold pools are weaker with higher aerosol loading because of reduced raindrop evaporation rates. The changes to the cold pool strength initiate positive feedbacks that further modify the cold pool strength and subsequent precipitation totals. Aerosol impacts on deep convection are therefore found to be modulated by the altitude of the dry layer and to vary inversely with the storm organization.

Cold pool collisions as a triggering mechanism of convection

2020 ◽  
Author(s):  
Gustav Halvorsen ◽  
Bettina Meyer ◽  
Jan Härter
Keyword(s):  
Deep Convection ◽  
Radar Data ◽  
Spatial Distance ◽  
Cold Pool ◽  
Rain Event ◽  
Radar Images ◽  
Cold Pools ◽  
Object Based ◽  
Rain Events ◽  
Atmospheric Phenomena

<p>Cold pools are produced by rain evaporation from<br>convective thunderstorms and play an important role <br>in many atmospheric phenomena (e.g. transition to deep convection and convective self-aggregation). From observational<br>and numerical studies, it has been found that intersecting cold pools<br>increase the likelihood of triggering convection.<br>We test this hypothesis by combining observational<br>radar data from Darwin (Australia) with a simple conceptual model.</p><p>We identify precipitation objects in the radar data. It is assumed that each rain event produces a cold pool<br>that is initialized at the center of the precipitation cell. Cold pools are simulated with a stochastic surface growth model.<br>The spatial coordinate of each collision event is recorded. <br>Collectively these points take the shape of a Voronoi diagram. <br>According to our hypothesis, the probability of new rain events should decay with spatial distance to the Voronoi.</p><p>Our preliminary results suggest that rain events cluster in the<br>vicinity of the Voronoi with a higher frequency that one would expect if cold pool collisions did not stimulate convection. <br>To conclude, our findings suggest that dynamic collisions between cold pools increase the likelihood of convection in the surrounding area.<br>This work allows us to study the effect of cold pools from radar data, despite cold pools being invisible to the radar images,<br>using a simple object-based model of convective cold pools. </p>

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>

scholarly journals The Role of Cold Pools in Tropical Oceanic Convective Systems

2018 ◽  
Vol 75 (8) ◽  
pp. 2615-2634 ◽  
Author(s):  
Leah D. Grant ◽  
Todd P. Lane ◽  
Susan C. van den Heever
Keyword(s):  
Cloud Base ◽  
Cold Pool ◽  
Convective Systems ◽  
Global Circulation ◽  
Cold Pools ◽  
Mesoscale Structure ◽  
Sensitivity Tests ◽  
Passive Tracers ◽  
Negative Buoyancy

Abstract The processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particular, cold pools are known to influence the structure and maintenance of midlatitude systems via Rotunno–Klemp–Weisman (RKW) theory, but cold pools may interact differently with tropical convection because of differences in cold pool strength and environmental shear. In this study, the role of cold pools in organized oceanic tropical convective systems is investigated, including their influence on system intensity, mesoscale structure, and propagation. To accomplish this goal, high-resolution idealized simulations are performed for two different systems that are embedded within a weakly sheared cloud population approaching radiative–convective equilibrium. The cold pools are altered by changing evaporation rates below cloud base in a series of sensitivity tests. The simulations demonstrate surprising findings: when cold pools are weakened, the convective systems become more intense. However, their propagation speeds and mesoscale structure are largely unaffected by the cold pool changes. Passive tracers introduced into the cold pools indicate that the convection intensifies when cold pools are weakened because cold pool air is entrained into updrafts, thereby reducing updraft intensity via the cold pools’ initial negative buoyancy. Gravity waves, rather than cold pools, appear to be the important modulators of system propagation and mesoscale structure. These results reconfirm that RKW theory does not fully explain the behavior of tropical oceanic convective systems, even those that otherwise appear consistent with RKW thinking.

Impact of Graupel Parameterization Schemes on Idealized Bow Echo Simulations

2013 ◽  
Vol 141 (4) ◽  
pp. 1241-1262 ◽  
Author(s):  
Rebecca D. Adams-Selin ◽  
Susan C. van den Heever ◽  
Richard H. Johnson
Keyword(s):  
Complete Removal ◽  
Cold Pool ◽  
Size Distributions ◽  
Pressure Gradients ◽  
Cooling Rates ◽  
Cold Pools ◽  
Highly Sensitive ◽  
Bow Echo ◽  
Mean Size ◽  
Microphysical Processes

Abstract The effect of changes in microphysical cooling rates on bow echo development and longevity are examined through changes to graupel parameterization in the Advanced Research Weather Research and Forecasting Model (ARW-WRF). Multiple simulations are performed that test the sensitivity to different graupel size distributions as well as the complete removal of graupel. It is found that size distributions with larger and denser, but fewer, graupel hydrometeors result in a weaker cold pool due to reduced microphysical cooling rates. This yields weaker midlevel (3–6 km) buoyancy and pressure perturbations, a later onset of more elevated rear inflow, and a weaker convective updraft. The convective updraft is also slower to tilt rearward, and thus bowing occurs later. Graupel size distributions with more numerous, smaller, and lighter hydrometeors result in larger microphysical cooling rates, stronger cold pools, more intense midlevel buoyancy and pressure gradients, and earlier onset of surface-based rear inflow; these systems develop bowing segments earlier. A sensitivity test with fast-falling but small graupel hydrometeors revealed that small mean size and slow fall speed both contribute to the strong cooling rates. Simulations entirely without graupel are initially weaker, because of limited contributions from cooling by melting of the slowly falling snow. However, over the next hour increased rates of melting snow result in an increasingly more intense system with new bowing. Results of the study indicate that the development of a bow echo is highly sensitive to microphysical processes, which presents a challenge to the prediction of these severe weather phenomena.

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.

Peering into the internal structure of cold pools and their interactions with a dense observational network

2021 ◽  
Author(s):  
Cathy Hohenegger ◽  
Jaemyeong Seo ◽  
Hannes Nevermann ◽  
Bastian Kirsch ◽  
Nima Shokri ◽  
...  
Keyword(s):  
Internal Structure ◽  
Land Surface ◽  
Surface Fluxes ◽  
Cold Pool ◽  
Convective Clouds ◽  
Cold Pools ◽  
Soil Sensors ◽  
Modelling Studies ◽  
Surface Network ◽  
Shed Light

<p>Melting and evaporation of hydrometeors in and below convective clouds generates cold, dense air that falls through the atmospheric column and spreads at the surface like a density current, the cold pool. In modelling studies, the importance of cold pools in controlling the lifecycle of convection has often been emphasized, being through their organization of the cloud field or through their sheer deepening of the convection. Larger, longer-lived cold pools benefit convection, but little is actually known on the size and internal structure of cold pools from observations as the majority of cold pools are too small to be captured by the operational surface network.  One aim of the field campaign FESSTVaL was to peer into the internal structure of cold pools and their interactions with the underlying land surface by deploying a dense network of surface observations. This network consisted of 80 self-designed cold pool loggers, 19 weather stations and 83 soil sensors deployed in an area of 15 km around Lindenberg. FESSTVaL took place from 17 May to 27 August 2021.</p> <p>In principle, cold pool characteristics are affected both by the atmospheric state, which fuels cold pools through melting and evaporation of hydrometeors, and the land surface, which acts to destroy cold pools through friction and warming by surface fluxes. In this talk, the measurements collected during FESSTVaL will be used to shed light on these interactions.  We are particularly interested to assess how homogeneous the internal structure of cold pools is and whether heterogeneities of the land surface imprint themselves on this internal structure. The results will be compared to available model simulations.</p>

FESST@HH 2020: Ein dichtes Messnetz als Lupe für die Struktur konvektiver Cold Pools

2021 ◽  
Author(s):  
Bastian Kirsch ◽  
Cathy Hohenegger ◽  
Daniel Klocke ◽  
Felix Ament
Keyword(s):  
Cold Pool ◽  
Cold Pools ◽  
O 10

<p>Cold Pools sind mesoskalige Gebiete kalter und dichter Luftmassen, die durch Verdunstung von Hydrometeoren unterhalb regnender Wolken entstehen. Während die kalte Luft absinkt und sich als Dichteströmung an der Erdoberfläche ausbreitet, löst sie durch Hebung an ihrer Vorderseite häufig neue Konvektion aus oder forciert den Übergang von flacher zu tiefer Konvektion. Viele modellbasierte Arbeiten belegen die Bedeutung von Cold Pools für die Organisation von Konvektion. Operationelle Messnetze mit einer typischen Maschenweite von 25 km hingegen sind blind für sub-mesoskalige (O(100) m — O(10) km) Prozesse wie Cold Pools und erlauben somit weder die Untersuchung noch die Validierung ihrer raum-zeitlichen Struktur.</p> <p>Im Rahmen der Messkampagne FESST@HH wurde von Juni bis August 2020 im Großraum Hamburg (50 km × 35 km) ein dichtes Netz bestehend aus 103 meteorologischen Messstationen betrieben. Das Rückgrat des Messnetzes bildeten 82 eigens für diesen Zweck entwickelte und gebaute APOLLO-Stationen (Autonomous cold POoL LOgger), die Lufttemperatur und -druck mit trägheitsarmen Sensoren in sekündlicher Auflösung messen. Das Netzwerk wurde mit 21 Wetterstationen ergänzt, die zusätzlich Luftfeuchte, Windgeschwindigkeit und Niederschlag in 10-sekündiger Auflösung aufzeichnen und auf kommerziellen Sensoren basieren. Ein besonderes Merkmal von FESST@HH ist, dass die Durchführung der Kampagne während der COVID19-Pandemie nur durch eine große Zahl Freiwilliger ermöglicht wurde, die kurzfristig Messstandorte bereitgestellt und die Betreuung der Instrumente unterstützt haben.</p> <p>Wir präsentieren die neuartigen Messinstrumente und den Datensatz der FESST@HH-Kampagne (DOI: 10.25592/UHHFDM.8966). Ein Fallbeispiel zeigt, dass das dichte Messnetz in der Lage ist sowohl die horizontale Heterogenität des Temperaturfeldes innerhalb eines Cold Pools als auch seine Größe und Ausbreitungsgeschwindigkeit während verschiedener Phasen des Lebenszyklus abzubilden. Darüber hinaus erlauben die Messungen einen neuen Blick auf weitere Quellen sub-mesoskaliger Variabilität wie die nächtliche städtische Wärmeinsel und die Variation turbulenter Temperaturfluktuationen als Ausdruck charakteristischer Standorteigenschaften.</p>

scholarly journals Evaluation of Forecasts of a Convectively Generated Bore Using an Intensively Observed Case Study from PECAN

2018 ◽  
Vol 146 (9) ◽  
pp. 3097-3122 ◽  
Author(s):  
Aaron Johnson ◽  
Xuguang Wang ◽  
Kevin R. Haghi ◽  
David B. Parsons
Keyword(s):  
Cross Sections ◽  
Turbulent Mixing ◽  
Weather Research And Forecasting ◽  
Cold Pool ◽  
Model Errors ◽  
Near Surface ◽  
Cold Pools ◽  
Convection Model

Abstract This paper presents a case study from an intensive observing period (IOP) during the Plains Elevated Convection at Night (PECAN) field experiment that was focused on a bore generated by nocturnal convection. Observations from PECAN IOP 25 on 11 July 2015 are used to evaluate the performance of high-resolution Weather Research and Forecasting Model forecasts, initialized using the Gridpoint Statistical Interpolation (GSI)-based ensemble Kalman filter. The focus is on understanding model errors and sensitivities in order to guide forecast improvements for bores associated with nocturnal convection. Model simulations of the bore amplitude are compared against eight retrieved vertical cross sections through the bore during the IOP. Sensitivities of forecasts to microphysics and planetary boundary layer (PBL) parameterizations are also investigated. Forecasts initialized before the bore pulls away from the convection show a more realistic bore than forecasts initialized later from analyses of the bore itself, in part due to the smoothing of the existing bore in the ensemble mean. Experiments show that the different microphysics schemes impact the quality of the simulations with unrealistically weak cold pools and bores with the Thompson and Morrison microphysics schemes, cold pools too strong with the WDM6 and more accurate with the WSM6 schemes. Most PBL schemes produced a realistic bore response to the cold pool, with the exception of the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme, which creates too much turbulent mixing atop the bore. A new method of objectively estimating the depth of the near-surface stable layer corresponding to a simple two-layer model is also introduced, and the impacts of turbulent mixing on this estimate are discussed.

scholarly journals Exploring the use of a column model for the characterization of microphysical processes in warm rain: results from a homogeneous rainshaft model

2007 ◽  
Vol 10 ◽  
pp. 145-152 ◽  
Author(s):  
O. P. Prat ◽  
A. P. Barros
Keyword(s):  
Field Experiments ◽  
Ground Surface ◽  
Cloud Base ◽  
Column Model ◽  
Raindrop Size Distribution ◽  
State Boundary ◽  
Raindrop Size ◽  
Microphysical Processes ◽  
Rain Events ◽  
Rain Rates

Abstract. A study of the evolution of raindrop spectra (raindrop size distribution, DSD) between cloud base and the ground surface was conducted using a column model of stochastic coalescense-breakup dynamics. Numerical results show that, under steady-state boundary conditions (i.e. constant rainfall rate and DSD at the top of the rainshaft), the equilibrium DSD is achieved only for high rain rates produced by midlevel or higher clouds and after long simulation times (~30 min or greater). Because these conditions are not typical of most rainfall, the results suggest that the theoretical equilibrium DSD might not be attainable for the duration of individual rain events, and thus DSD observations from field experiments should be analyzed conditional on the specific storm environment under which they were obtained.

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