scholarly journals Low-level Mesoscale and Cloud-scale Interactions Promoting Deep Convection Initiation

2021 ◽  
Author(s):  
James N. Marquis ◽  
Adam C. Varble ◽  
Paul Robinson ◽  
T. Connor. Nelson ◽  
Katja Friedrich
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Surface Wind ◽  
Horizontal Wind ◽  
Moist Convection ◽  
Surface Precipitation ◽  
Low Level ◽  
Deep Moist Convection ◽  
Convection Initiation

AbstractData from scanning radars, radiosondes, and vertical profilers deployed during three field campaigns are analyzed to study interactions between cloud-scale updrafts associated with initiating deep moist convection and the surrounding environment. Three cases are analyzed in which the radar networks permitted dual-Doppler wind retrievals in clear air preceding and during the onset of surface precipitation. These observations capture the evolution of: i) the mesoscale and boundary layer flow, and ii) low-level updrafts associated with deep moist convection initiation (CI) events yielding sustained or short-lived precipitating storms.The elimination of convective inhibition did not distinguish between sustained and unsustained CI events, though the vertical distribution of convective available potential energy may have played a role. The clearest signal differentiating the initiation of sustained versus unsustained precipitating deep convection was the depth of the low-level horizontal wind convergence associated with the mesoscale flow feature triggering CI, a sharp surface wind shift boundary or orographic upslope flow. The depth of the boundary layer relative to the height of the LFC failed to be a consistent indicator of CI potential. Widths of the earliest detectable low-level updrafts associated with sustained precipitating deep convection were ~3-5 km, larger than updrafts associated with surrounding boundary layer turbulence (~1-3-km wide). It is hypothesized that updrafts of this larger size are important for initiating cells to survive the destructive effects of buoyancy dilution via entrainment.

scholarly journals Radiosonde observations of environments supporting deep moist convection initiation during RELAMPAGO-CACTI

2020 ◽  
Author(s):  
T. Connor Nelson ◽  
James Marquis ◽  
Adam Varble ◽  
Katja Friedrich
Keyword(s):  
Complex Terrain ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Environmental Parameters ◽  
Convective Precipitation ◽  
Moist Convection ◽  
Spatiotemporal Resolution ◽  
Close Proximity ◽  
Deep Moist Convection ◽  
Convection Initiation

AbstractThe Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) and Cloud, Aerosol, and Complex Terrain Interactions (CACTI) projects deployed a high-spatiotemporal-resolution radiosonde network to examine environments supporting deep convection in the complex terrain of central Argentina. This study aims to characterize atmospheric profiles most representative of the near-cloud environment (in time and space) to identify the mesoscale ingredients affecting storm initiation and growth. Spatiotemporal autocorrelation analysis of the soundings reveals that there is considerable environmental heterogeneity, with boundary layer thermodynamic and kinematic fields becoming statistically uncorrelated on scales of 1–2 hr and 30 km. Using this as guidance, we examine a variety of environmental parameters derived from soundings collected within close proximity (30 km and 30 min in space and time) of 44 events over 9 days where the atmosphere either: 1) supported the initiation of sustained precipitating convection, 2) yielded weak and short-lived precipitating convection, or 3) produced no precipitating convection in disagreement with numerical forecasts from convection-allowing models (i.e., Null events). There are large statistical differences between the Null event environments and those supporting any convective precipitation. Null event profiles contained larger convective available potential energy, but had low free tropospheric relative humidity, higher freezing levels, and evidence of limited horizontal convergence near the terrain at low levels that likely suppressed deep convective growth. We also present evidence from the radiosonde and satellite measurements that flow-terrain interactions may yield gravity wave activity that affects CI outcome.

Comments on “Preconditioning Deep Convection with Cumulus Congestus”

2013 ◽  
Vol 70 (11) ◽  
pp. 3689-3690 ◽  
Author(s):  
David M. Schultz
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Moisture Flux ◽  
Moisture Convergence ◽  
Moist Convection ◽  
Moisture Flux Convergence ◽  
Deep Moist Convection ◽  
Cumulus Congestus ◽  
Convection Initiation ◽  
The Tropics

Abstract The hypothesis that cumulus congestus clouds in the tropics moisten dry layers above the boundary layer and promote the formation of deep moist convection was tested by Hohenegger and Stevens. This comment asks whether their hypothesis is also true for cumulus congestus clouds and deep moist convection in the midlatitudes. This comment also requests clarification on how their expression for moisture convergence is calculated and used in their article, especially in light of previous studies showing that moisture flux convergence is a less-than-adequate diagnostic for convection initiation and that deep moist convection requires sufficient lift and instability, in addition to sufficient moisture.

Quasi-idealized numerical simulations of processes involved in orogenic convection initiation over the Sierras de Córdoba mountains

2021 ◽  
Author(s):  
Christopher A. Davis
Keyword(s):  
Numerical Simulations ◽  
Mountain Range ◽  
Moist Convection ◽  
Low Level ◽  
Physical Mechanisms ◽  
Convergence Line ◽  
Synoptic Conditions ◽  
Deep Moist Convection ◽  
Outflow Boundary ◽  
Convection Initiation

Abstract The Sierras de Córdoba (SDC) mountain range in Argentina is a hotspot of deep moist convection initiation (CI). Radar climatology indicates that 44% of daytime CI events that occur near the SDC in spring and summer seasons and that are not associated with the passage of a cold front or an outflow boundary involve a northerly LLJ, and these events tend to preferentially occur over the southeast quadrant of the main ridge of the SDC. To investigate the physical mechanisms acting to cause CI, idealized convection-permitting numerical simulations with a horizontal grid spacing of 1 km were conducted using CM1. The sounding used for initializing the model featured a strong northerly LLJ, with synoptic conditions resembling those in a previously postulated conceptual model of CI over the region, making it a canonical case study. Differential heating of the mountain caused by solar insolation in conjunction with the low-level northerly flow sets up a convergence line on the eastern slopes of the SDC. The southern portion of this line experiences significant reduction in convective inhibition, and CI occurs over the SDC southeast quadrant. Thesimulated storm soon acquires supercellular characteristics, as observed. Additional simulations with varying LLJ strength also show CI over the southeast quadrant. A simulation without background flow generated convergence over the ridgeline, with widespread CI across the entire ridgeline. A simulation with mid- and upper-tropospheric westerlies removed indicates that CI is minimally influenced by gravity waves. We conclude that the low-level jet is sufficient to focus convection initiation over the southeast quadrant of the ridge.

Practical Predictability of the 20 May 2013 Tornadic Thunderstorm Event in Oklahoma: Sensitivity to Synoptic Timing and Topographical Influence

2015 ◽  
Vol 143 (8) ◽  
pp. 2973-2997 ◽  
Author(s):  
Yunji Zhang ◽  
Fuqing Zhang ◽  
David J. Stensrud ◽  
Zhiyong Meng
Keyword(s):  
Boundary Layer ◽  
Vertical Mixing ◽  
Vertical Wind Shear ◽  
Mesoscale Convective System ◽  
Convective System ◽  
Moist Convection ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Deep Moist Convection ◽  
Simulation Time

Abstract The practical predictability of severe convective thunderstorms during the 20 May 2013 severe weather event that produced the catastrophic enhanced Fujita scale 5 (EF-5) tornado in Moore, Oklahoma, was explored using ensembles of convective-permitting model simulations. The sensitivity of initiation and the subsequent organization and intensity of the thunderstorms to small yet realistic uncertainties in boundary layer and topographical influence within a few hours preceding the thunderstorm event was examined. It was found that small shifts in either simulation time or terrain configuration led to considerable differences in the atmospheric conditions within the boundary layer. Small shifts in simulation time led to changes in low-level moisture and instability, primarily through the vertical distribution of moisture within the boundary layer due to vertical mixing during the diurnal cycle as well as advection by low-level jets, thereby influencing convection initiation. Small shifts in terrain led to changes in the wind field, low-level vertical wind shear, and storm-relative environmental helicity, altering locally enhanced convergence that may trigger convection. After initiation, an upscale growth of errors resulting from deep moist convection led to large forecast uncertainties in the timing, intensity, structure, and organization of the developing mesoscale convective system and its embedded supercells.

Unusually Long Duration, Multiple-Doppler Radar Observations of a Front in a Convective Boundary Layer

2007 ◽  
Vol 135 (1) ◽  
pp. 93-117 ◽  
Author(s):  
John R. Stonitsch ◽  
Paul M. Markowski
Keyword(s):  
Boundary Layer ◽  
Density Gradient ◽  
Doppler Radar ◽  
Deep Convection ◽  
Three Dimensional ◽  
Horizontal Wind ◽  
Normal Density ◽  
Wind Fields ◽  
Convective Boundary ◽  
Convection Initiation

Abstract Dual-Doppler observations acquired by a network of mobile radars deployed in the Oklahoma panhandle on 3 June 2002 are used to document the kinematic structure and evolution of a front. The data were collected during the International H2O Project on a mission to study the initiation of deep convection. Synchronized scanning allowed for the synthesis of three-dimensional wind fields for nearly 5.5 h of the 1557–0000 UTC period. The front initially moved southward as a cold front, stalled, and later retreated northward as a warm front. Deep convection failed to be initiated along the front. In situ thermodynamic measurements obtained by a mobile mesonet were used to document changes in the density gradient at the surface. This paper examines the relationships among the changes in baroclinity, the thermally direct frontal circulation, updraft intensity, alongfront updraft variability, and the intensity of vortices along the front. Increases in the front-normal density gradient tended to be associated with increases in the thermally direct frontal circulation, as expected. Increases in the front-normal density gradient were also associated with an increase in the tilt of the frontal updraft as well as an increase in the contiguity of the updraft along the front, termed the “slabularity.” During periods when the front-normal density gradient and associated thermally direct frontal circulation were weak, the kinematic fields were dominated by boundary layer convection and the slabularity of the front was reduced. Intensification of the front-normal density gradient was accompanied by an increase in the horizontal wind shear and the intensity of vortices that were observed along the front. The vortices modulated the vertical velocity field along the front and therefore the slabularity, too. Thus, although the slabularity was a strong function of the strength of the thermally direct frontal circulation, the slabularity appeared to be modified by vortices in complex ways. Possible implications of the observations for convection initiation are also discussed, particularly with respect to updraft tilt and slabularity.

scholarly journals Formation and Quasi-Periodic Behavior of Outer Spiral Rainbands in a Numerically Simulated Tropical Cyclone*

2012 ◽  
Vol 69 (3) ◽  
pp. 997-1020 ◽  
Author(s):  
Qingqing Li ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Periodic Oscillation ◽  
Quasi Periodic Oscillation ◽  
Periodic Behavior ◽  
Thermodynamic Conditions ◽  
Spiral Rainbands ◽  
Convection Initiation

Abstract The formation and quasi-periodic behavior of outer spiral rainbands in a tropical cyclone simulated in the cloud-resolving tropical cyclone model version 4 (TCM4) are analyzed. The outer spiral rainbands in the simulation are preferably initiated near the 60-km radius, or roughly about 3 times the radius of maximum wind (RMW). After initiation, they generally propagate radially outward with a mean speed of about 5 m s−1. They are reinitiated quasi-periodically with a period between 22 and 26 h in the simulation. The inner spiral rainbands, which form within a radius of about 3 times the RMW, are characterized by the convectively coupled vortex Rossby waves (VRWs), but the formation of outer spiral rainbands (i.e., rainbands formed outside a radius of about 3 times the RMW) is much more complicated. It is shown that outer spiral rainbands are triggered by the inner-rainband remnants immediately outside the rapid filamentation zone and inertial instability in the upper troposphere. The preferred radial location of initiation of outer spiral rainbands is understood as a balance between the suppression of deep convection by rapid filamentation and the favorable dynamical and thermodynamic conditions for initiation of deep convection. The quasi-periodic occurrence of outer spiral rainbands is found to be associated with the boundary layer recovery from the effect of convective downdrafts and the consumption of convective available potential energy (CAPE) by convection in the previous outer spiral rainbands. Specifically, once convection is initiated and organized in the form of outer spiral rainbands, it will produce strong downdrafts and consume CAPE. These effects weaken convection near its initiation location. As the rainband propagates outward farther, the boundary layer air near the original location of convection initiation takes about 10 h to recover by extracting energy from the underlying ocean. Convection and thus new outer spiral rainbands will be initiated near a radius of about 3 times the RMW. This will be followed by a similar outward propagation and the subsequent boundary layer recovery, leading to a quasi-periodic occurrence of outer spiral rainbands. In response to the quasi-periodic appearance of outer spiral rainbands, the storm intensity experiences a similar quasi-periodic oscillation with its intensity or intensification rate starting to decrease after about 4 h of the initiation of an outer spiral rainband. The results provide an alternative explanation or one of the mechanisms that are responsible for the quasi-periodic (quasi-diurnal) variation in the intensity and in the area of outflow-layer cloud canopy of observed tropical cyclones.

Mechanisms for Quasi-Stationary Behavior in Simulated Heavy-Rain-Producing Convective Systems

2009 ◽  
Vol 66 (6) ◽  
pp. 1543-1568 ◽  
Author(s):  
Russ S. Schumacher
Keyword(s):  
Deep Convection ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Cold Pool ◽  
Convective System ◽  
Moist Convection ◽  
Initial State ◽  
Low Level ◽  
Convective Systems ◽  
Deep Moist Convection

Abstract In this study, idealized numerical simulations are used to identify the processes responsible for initiating, organizing, and maintaining quasi-stationary convective systems that produce locally extreme rainfall amounts. Of particular interest are those convective systems that have been observed to occur near mesoscale convective vortices (MCVs) and other midlevel circulations. To simulate the lifting associated with such circulations, a low-level momentum forcing is applied to an initial state that is representative of observed extreme rain events. The initial vertical wind profile includes a sharp reversal of the vertical wind shear with height, indicative of observed low-level jets. Deep moist convection initiates within the region of mesoscale lifting, and the resulting convective system replicates many of the features of observed systems. The low-level thermodynamic environment is nearly saturated, which is not conducive to the production of a strong surface cold pool; yet the convection quickly organizes into a back-building line. It is shown that a nearly stationary convectively generated low-level gravity wave is responsible for the linear organization, which continues for several hours. New convective cells repeatedly form on the southwest end of the line and move to the northeast, resulting in large local rainfall amounts. In the later stages of the simulated convective system, a cold pool does develop, but its interaction with the strong reverse shear at low levels is not optimized for the maintenance of deep convection along its edge. A series of sensitivity experiments shows some of the effects of hydrometeor evaporation and melting, planetary rotation, and the imposed mesoscale forcing.

Numerical Simulations of Interactions between Gravity Waves and Deep Moist Convection

2005 ◽  
Vol 62 (5) ◽  
pp. 1480-1496 ◽  
Author(s):  
Zachary A. Eitzen ◽  
David A. Randall
Keyword(s):  
Gravity Waves ◽  
Deep Convection ◽  
Velocity Versus ◽  
Moist Convection ◽  
Vertical Grid ◽  
Wave Energy Flux ◽  
Deep Moist Convection ◽  
Vertically Propagating ◽  
The Tropics ◽  
The Waves

Abstract This study uses a numerical model to simulate deep convection both in the Tropics over the ocean and the midlatitudes over land. The vertical grid that was used extends into the stratosphere, allowing for the simultaneous examination of the convection and the vertically propagating gravity waves that it generates. A large number of trajectories are used to evaluate the behavior of tracers in the troposphere, and it is found that the tracers can be segregated into different types based upon their position in a diagram of normalized vertical velocity versus displacement. Conditional sampling is also used to identify updrafts in the troposphere and calculate their contribution to the kinetic energy budget of the troposphere. In addition, Fourier analysis is used to characterize the waves in the stratosphere; it was found that the waves simulated in this study have similarities to those observed and simulated by other researchers. Finally, this study examines the wave energy flux as a means to provide a link between the tropospheric behavior of the convection and the strength of the waves in the stratosphere.

Evaluating the conservation of energy variables in simulations of deep moist convection

2021 ◽  
Author(s):  
John M. Peters ◽  
Daniel R. Chavas
Keyword(s):  
Heat Capacity ◽  
Numerical Models ◽  
Deep Convection ◽  
Squall Line ◽  
Moist Convection ◽  
Conservation Of Energy ◽  
Deep Moist Convection ◽  
Static Energy ◽  
Hydrostatic Balance ◽  
Latent Heats

AbstractIt is often assumed in parcel theory calculations, numerical models, and cumulus parameterizations that moist static energy (MSE) is adiabatically conserved. However, the adiabatic conservation of MSE is only approximate because of the assumption of hydrostatic balance. Two alternative variables are evaluated here: MSE −IB and MSE +KE, wherein IB is the path integral of buoyancy (B) and KE is kinetic energy. Both of these variables relax the hydrostatic assumption and are more precisely conserved than MSE. This article quantifies the errors that result from assuming that the aforementioned variables are conserved in large eddy simulations (LES) of both disorganized and organized deep convection. Results show that both MSE −IB and MSE +KE better predict quantities along trajectories than MSE alone. MSE −IB is better conserved in isolated deep convection, whereas MSE −IB and MSE +KE perform comparably in squall line simulations. These results are explained by differences between the pressure perturbation behavior of squall lines and isolated convection. Errors in updraft B diagnoses are universally minimized when MSE−IB is assumed to be adiabatically conserved, but only when moisture dependencies of heat capacity and temperature dependency of latent heating are accounted for. When less accurate latent heat and heat capacity formulae were used, MSE−IB yielded poorer B predictions than MSE due to compensating errors. Our results suggest that various applications would benefit from using either MSE −IB or MSE +KE instead of MSE with properly formulated heat capacities and latent heats.

scholarly journals Size-Resolved Evaluation of Simulated Deep Tropical Convection

2018 ◽  
Vol 146 (7) ◽  
pp. 2161-2182 ◽  
Author(s):  
Fabian Senf ◽  
Daniel Klocke ◽  
Matthias Brueck
Keyword(s):  
Deep Convection ◽  
Winter Season ◽  
Power Laws ◽  
Moist Convection ◽  
Modeling Framework ◽  
Western Atlantic ◽  
Spatial Coupling ◽  
Wide Range ◽  
Realistic Representation ◽  
Deep Moist Convection

Abstract Deep moist convection is an inherently multiscale phenomenon with organization processes coupling convective elements to larger-scale structures. A realistic representation of the tropical dynamics demands a simulation framework that is capable of representing physical processes across a wide range of scales. Therefore, storm-resolving numerical simulations at 2.4 km have been performed covering the tropical Atlantic and neighboring parts for 2 months. The simulated cloud fields are combined with infrared geostationary satellite observations, and their realism is assessed with the help of object-based evaluation methods. It is shown that the simulations are able to develop a well-defined intertropical convergence zone. However, marine convective activity measured by the cold cloud coverage is considerably underestimated, especially for the winter season and the western Atlantic. The spatial coupling across the resolved scales leads to simulated cloud number size distributions that follow power laws similar to the observations, with slopes steeper in winter than summer and slopes steeper over ocean than over land. The simulated slopes are, however, too steep, indicating too many small and too few large tropical cloud cells. It is also discussed that the number of larger cells is less influenced by multiday variability of environmental conditions. Despite the identified deficits, the analyzed simulations highlight the great potential of this modeling framework for process-based studies of tropical deep convection.

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