scholarly journals A cold pool perturbation scheme to improve convective initiation in convection‐permitting models

2021 ◽  
Author(s):  
Mirjam Hirt ◽  
George C. Craig
Keyword(s):  
Cold Pool ◽  
Perturbation Scheme ◽  
Convective Initiation

Cold pool driven convective initiation: How can we improve its representation in km-scale models?

2020 ◽  
Author(s):  
Mirjam Hirt ◽  
George Craig
Keyword(s):  
Positive Impact ◽  
Potential Temperature ◽  
Horizontal Wind ◽  
Cold Pool ◽  
Model Resolution ◽  
Convective Initiation ◽  
Cold Pools ◽  
Scale Models ◽  
Gust Fronts ◽  
Gust Front

<p>Cold pools are essential for organizing convection and play a particular role in convective initiation in the afternoon and evening. Both aspects are deficient in current convection-permitting models and a better representation of cold pools is likely necessary to overcome these deficiencies. In a recent investigation, we identified several sensitivities of cold pool driven convective initiation to model resolution within hectometer simulations. In particular, a causal graph analysis has revealed that the dominant impact of model resolution on convective initiation is due to too weak gust front vertical velocities. This implies that cold pool gust fronts in km-scale models are too weak to trigger sufficient new convection.</p><p>To address this deficiency, we develop a parameterization for the convection-permitting COSMO model to improve the representation of cold pool gust fronts. We use the potential temperature gradient to identify cold pool gust fronts and enhance vertical wind tendencies within these gust front regions.  Also, we perturb horizontal wind tendencies to yield 3d non-divergent perturbations.  This parameterization strengthens gust front circulations and thereby enhances cold pool driven convective initiation. Consequently, precipitation is amplified and becomes more organized in the afternoon and evening. This improves the diurnal cycle of precipitation and also has some positive impact on the spatial distribution as quantified by the fraction skill score. Furthermore, cold pools themselves are strengthened, which can further enhance the gust front circulations, giving rise to a feedback loop. </p>

Stochastic Parameterization of Processes Leading to Convective Initiation in Kilometer-Scale Models

2019 ◽  
Vol 147 (11) ◽  
pp. 3917-3934 ◽  
Author(s):  
Mirjam Hirt ◽  
Stephan Rasp ◽  
Ulrich Blahak ◽  
George C. Craig
Keyword(s):  
Boundary Layer ◽  
Stochastic Perturbation ◽  
Horizontal Wind ◽  
Perturbation Scheme ◽  
Convective Initiation ◽  
Scale Models ◽  
Physically Based ◽  
Boundary Layer Scheme ◽  
Original Scheme ◽  
Explicit Simulation

Abstract Kilometer-scale models allow for an explicit simulation of deep convective overturning but many subgrid processes that are crucial for convective initiation are still poorly represented. This leads to biases such as insufficient convection triggering and late peak of summertime convection. A physically based stochastic perturbation scheme (PSP) for subgrid processes has been proposed (Kober and Craig) that targets the coupling between subgrid turbulence and resolved convection. The first part of this study presents four modifications to this PSP scheme for subgrid turbulence: an autoregressive, continuously evolving random field; a limitation of the perturbations to the boundary layer that removes artificial convection at night; a mask that turns off perturbations in precipitating columns to retain coherent structures; and nondivergent wind perturbations that drastically increase the effectiveness of the vertical velocity perturbations. In a revised version, PSP2, the combined modifications retain the physically based coupling to the boundary layer scheme of the original scheme while removing undesirable side effects. This has the potential to improve predictions of convective initiation in kilometer-scale models while minimizing other biases. The second part of the study focuses on perturbations to account for convective initiation by subgrid orography. Here the mechanical lifting effect is modeled by introducing vertical and horizontal wind perturbations of an orographically induced gravity wave. The resulting perturbations lead to enhanced convective initiation over mountainous terrain. However, the total benefit of this scheme is unclear and we do not adopt the scheme in our revised configuration.

Interactions between Physics Diversity and Multiscale Initial Condition Perturbations for Storm-Scale Ensemble Forecasting

2020 ◽  
Vol 148 (8) ◽  
pp. 3549-3565
Author(s):  
Aaron Johnson ◽  
Xuguang Wang
Keyword(s):  
Spatial Scale ◽  
Lead Time ◽  
Cold Pool ◽  
Small Scale ◽  
Lead Times ◽  
Synoptic Scale ◽  
Convective Initiation ◽  
Forecast Performance ◽  
Forecast Lead Time

Abstract This study investigates impacts on convection-permitting ensemble forecast performance of different methods of generating the ensemble IC perturbations in the context of simultaneous physics diversity among the ensemble members. A total of 10 convectively active cases are selected for a systematic comparison of different methods of perturbing IC perturbations in 10-member convection-permitting ensembles, both with and without physics diversity. These IC perturbation methods include simple downscaling of coarse perturbations from a global model (LARGE), perturbations generated with ensemble data assimilation directly on the multiscale domain (MULTI), and perturbations generated using each method with small scales filtered out as a control. MULTI was found to be significantly more skillful than LARGE at early lead times in all ensemble physics configurations, with the advantage of MULTI gradually decreasing with increasing forecast lead time. The advantage of MULTI, relative to LARGE, was reduced but not eliminated by the presence of physics diversity because of the extra ensemble spread that the physics diversity provided. The advantage of MULTI, relative to LARGE, was also reduced by filtering the IC perturbations to a commonly resolved spatial scale in both ensembles, which highlights the importance of flow-dependent small-scale (<~10 m) IC perturbations in the ensemble design. The importance of the physics diversity, relative to the IC perturbation method, depended on the spatial scale of interest, forecast lead time, and the meteorological characteristics of the forecast case. Such meteorological characteristics include the strength of synoptic-scale forcing, the role of cold pool interactions, and the occurrence of convective initiation or dissipation.

scholarly journals Wave Disturbances and Their Role in the Maintenance, Structure, and Evolution of a Mesoscale Convection System

2019 ◽  
Vol 77 (1) ◽  
pp. 51-77 ◽  
Author(s):  
Shushi Zhang ◽  
David B. Parsons ◽  
Yuan Wang
Keyword(s):  
Great Plains ◽  
Wave Energy ◽  
Deep Convection ◽  
Inversion Layer ◽  
Mesoscale Convective System ◽  
Vertical Shear ◽  
Cold Pool ◽  
Convective System ◽  
Convective Initiation ◽  
Mesoscale Convective

Abstract This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

scholarly journals Cold‐pool‐driven convective initiation: using causal graph analysis to determine what convection‐permitting models are missing

2020 ◽  
Vol 146 (730) ◽  
pp. 2205-2227 ◽  
Author(s):  
Mirjam Hirt ◽  
George C. Craig ◽  
Sophia A. K. Schäfer ◽  
Julien Savre ◽  
Rieke Heinze
Keyword(s):  
Cold Pool ◽  
Graph Analysis ◽  
Convective Initiation ◽  
Causal Graph

A WEAKLY NONLINEAR INSTABILITY OF MISCIBLE DISPLACEMENTS IN A RECTILINEAR POROUS MEDIA FLOW

1994 ◽  
Vol 04 (05) ◽  
pp. 1319-1328 ◽  
Author(s):  
WILLIAM B. ZIMMERMAN
Keyword(s):  
Linear Theory ◽  
Landau Equation ◽  
Separation Of Variables ◽  
Finite Amplitude ◽  
Linear Stability Theory ◽  
Porous Media Flow ◽  
Perturbation Scheme ◽  
Viscous Effects ◽  
Regular Perturbation ◽  
Displacement Front

The linear stability theory of Tan & Homsy [1986] is extended to include the effects of weak nonlinear coupling between mass flux and viscous effects when the viscous fingers grow from a slowly diffusing, nearly flat displacement front. A regular perturbation scheme combined with a similarity-separation of variables technique leads to a Landau equation for the amplitude of the disturbance. The Landau constant has a simple pole for a given wavenumber within the linear theory cutoff wavenumber for growth. An argument is given that this pole leads to pairing of fingers while the instability remains small. Comparison of the length scale of the pole of the Landau constant with experimental measurements of finger scale shows good agreement where plausibly finite-amplitude effects might come into play, but with the linear theory otherwise.

scholarly journals Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

2021 ◽  
Author(s):  
Lena Pfister ◽  
Karl Lapo ◽  
Larry Mahrt ◽  
Christoph K. Thomas
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Side Wall ◽  
Cold Pool ◽  
Continuous Data ◽  
Valley Bottom ◽  
Near Surface ◽  
Cold Air ◽  
Stable Boundary ◽  
Surface Temperatures

AbstractIn the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

Mixing of chlorophyll from the Middle Atlantic Bight cold pool into the Gulf Stream at Cape Hatteras in July 1993

1996 ◽  
Vol 101 (C9) ◽  
pp. 20579-20593 ◽  
Author(s):  
A. Michelle Wood ◽  
Nelson D. Sherry ◽  
Adriana Huyer
Keyword(s):  
Gulf Stream ◽  
Cold Pool ◽  
Middle Atlantic Bight ◽  
Middle Atlantic ◽  
Cape Hatteras

scholarly journals Evolution and Vertical Structure of an Undular Bore Observed on 20 June 2015 during PECAN

2017 ◽  
Vol 145 (9) ◽  
pp. 3775-3794 ◽  
Author(s):  
Dana Mueller ◽  
Bart Geerts ◽  
Zhien Wang ◽  
Min Deng ◽  
Coltin Grasmick
Keyword(s):  
Life Cycle ◽  
University Of Wyoming ◽  
Cold Pool ◽  
Undular Bore ◽  
Wave Trapping ◽  
Hydraulic Theory ◽  
The University ◽  
Second Wave ◽  
Scorer Parameter ◽  
Equal Amplitude

This study documents the evolution of an impressive, largely undular bore triggered by an MCS-generated density current on 20 June 2015, observed as part of the Plains Elevated Convection at Night (PECAN) experiment. The University of Wyoming King Air with profiling nadir- and zenith-viewing lidars sampled the south-bound bore from the time the first bore wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s−1 and turbulence in the wave’s wake were encountered, through the early dissipative stage in which the leading wave began to lose amplitude and speed. Through most of the bore’s life cycle, its second wave had a higher or equal amplitude relative to the leading wave. Striking roll clouds formed in wave crests and wave energy was detected to about 5 km AGL. The upstream environment indicates a negative Scorer parameter region due to flow reversal at midlevels, providing a wave trapping mechanism. The observed bore strength of 2.4–2.9 and speed of 15–16 m s−1 agree well with values predicted from hydraulic theory. Surface and profiling measurements collected later in the bore’s life cycle, just after sunrise, indicate a transition to a soliton.

Radial lift on a suspended finite-sized sphere due to fluid inertia for low-Reynolds-number flow through a cylinder

2013 ◽  
Vol 722 ◽  
pp. 159-186 ◽  
Author(s):  
Sukalyan Bhattacharya ◽  
Dil K. Gurung ◽  
Shahin Navardi
Keyword(s):  
Reynolds Number ◽  
Particle Diameter ◽  
Axial Direction ◽  
Perturbation Scheme ◽  
Fluid Inertia ◽  
Regular Perturbation ◽  
Cross Sectional ◽  
First Order ◽  
Reynolds Number Flow ◽  
Freely Suspended

AbstractThis article describes the radial drift of a suspended sphere in a cylinder-bound Poiseuille flow where the Reynolds number is small but finite. Unlike past studies, it considers a circular narrow conduit whose cross-sectional diameter is only $1. 5$–$6$ times the particle diameter. Thus, the analysis quantifies the effect of fluid inertia on the radial motion of the particle in the channel when the flow field is significantly influenced by the presence of the suspended body. To this end, the hydrodynamic fields are expanded as a series in Reynolds number, and a set of hierarchical equations for different orders of the expansion is derived. Accordingly, the zeroth-order fields in Reynolds number satisfy the Stokes equation, which is accurately solved in the presence of the spherical particle and the cylindrical conduit. Then, recognizing that in narrow vessels Stokesian scattered fields from the sphere decrease exponentially in the axial direction, a simpler regular perturbation scheme is used to quantify the first-order inertial correction to hydrodynamic quantities. Consequently, it is possible to obtain two results. First, the sphere is assumed to follow the axial motion of a freely suspended sphere in a Stokesian condition, and the radial lift force on it due to the presence of fluid inertia is evaluated. Then, the approximate motion is determined for a freely suspended body on which net hydrodynamic force including first-order inertial lift is zero. The results agree well with the available experimental results. Thus, this study along with the measured data would precisely describe particle dynamics inside narrow tubes.

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