scholarly journals Application of an improved horizontal diffusion scheme to a WFIP2 cold pool case

2019 ◽  
Author(s):  
R S Arthur ◽  
K A Lundquist
Keyword(s):  
Cold Pool ◽  
Horizontal Diffusion ◽  
Diffusion Scheme

Maintenance of a Mountain Valley Cold Pool: A Numerical Study

2006 ◽  
Vol 134 (8) ◽  
pp. 2266-2278 ◽  
Author(s):  
Brian J. Billings ◽  
Vanda Grubišić ◽  
Randolph D. Borys
Keyword(s):  
Snow Cover ◽  
Mesoscale Model ◽  
Numerical Study ◽  
Computational Cost ◽  
Horizontal Resolution ◽  
Cold Pool ◽  
Temperature Structure ◽  
Cold Air ◽  
Horizontal Diffusion ◽  
Terrain Following

Abstract A persistent cold-air pool in the Yampa Valley of northwestern Colorado was simulated with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The observed cold-air pool, which was identified by temperature measurements along a line of surface stations ascending the eastern side of the valley, remained in place throughout the day of 10 January 2004. The baseline simulation with horizontal resolution of 1 km, which is close to the resolution of operational regional mesoscale model forecasts, neither matched the strength of the observed cold-air pool nor retained the cold pool throughout the day. Varying the PBL parameterization, increasing the vertical resolution, and increasing the model spinup time did not significantly improve the results. However, the inclusion of snow cover, increased horizontal resolution, and an improved treatment of horizontal diffusion did have a sizable effect on the forecast quality. The snow cover in the baseline simulation was essential for preventing the diurnal heating from eroding the cold pool, but was only sufficient to produce a nearly isothermal temperature structure within the valley, largely because of an increased reflection of solar radiation. The increase of horizontal resolution to 333 and 111 m resulted in a stronger cold-air pool and its retention throughout the day. In addition to improving the resolution of flow features in steep terrain, resulting in, for example, less drainage out of the valley, the increase in horizontal resolution led to a better forecast because of a reduced magnitude of horizontal diffusion calculated along the terrain-following model surfaces. Calculating horizontal diffusion along the constant height levels had a beneficial impact on the quality of the simulations, producing effects similar to those achieved by increasing the horizontal resolution, but at a fraction of the computational cost.

scholarly journals Effects of Parameterized Diffusion on Simulated Hurricanes

2012 ◽  
Vol 69 (7) ◽  
pp. 2284-2299 ◽  
Author(s):  
Richard Rotunno ◽  
George H. Bryan
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Dominant Role ◽  
Control Factor ◽  
Vertical Diffusion ◽  
Maximum Wind ◽  
Gradient Wind ◽  
Horizontal Diffusion ◽  
Budget Analysis ◽  
Parameterized Model

Abstract In this study the authors analyze and interpret the effects of parameterized diffusion on the nearly steady axisymmetric numerical simulations of hurricanes presented in a recent study. In that study it was concluded that horizontal diffusion was the most important control factor for the maximum simulated hurricane intensity. Through budget analysis it is shown here that horizontal diffusion is a major contributor to the angular momentum budget in the boundary layer of the numerically simulated storms. Moreover, a new scale analysis recognizing the anisotropic nature of the parameterized model diffusion shows why the horizontal diffusion plays such a dominant role. A simple analytical model is developed that captures the essence of the effect. The role of vertical diffusion in the boundary layer in the aforementioned numerical simulations is more closely examined here. It is shown that the boundary layer in these simulations is consistent with known analytical solutions in that boundary layer depth increases and the amount of “overshoot” (maximum wind in excess of the gradient wind) decreases with increasing vertical diffusion. However, the maximum wind itself depends mainly on horizontal diffusion and is relatively insensitive to vertical diffusion; the overshoot variation with vertical viscosity mainly comes from changes in the gradient wind with vertical viscosity. The present considerations of parameterized diffusion allow a new contribution to the dialog in the literature on the meaning and interpretation of the Emanuel potential intensity theory.

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.

Diffusion Experiments with a Global Discontinuous Galerkin Shallow-Water Model

2009 ◽  
Vol 137 (10) ◽  
pp. 3339-3350 ◽  
Author(s):  
Ramachandran D. Nair
Keyword(s):  
Shallow Water ◽  
Discontinuous Galerkin ◽  
Water Model ◽  
Order System ◽  
Small Scale ◽  
Shallow Water Model ◽  
First Order ◽  
Advection Diffusion Equation ◽  
Diffusion Scheme ◽  
Cubed Sphere

Abstract A second-order diffusion scheme is developed for the discontinuous Galerkin (DG) global shallow-water model. The shallow-water equations are discretized on the cubed sphere tiled with quadrilateral elements relying on a nonorthogonal curvilinear coordinate system. In the viscous shallow-water model the diffusion terms (viscous fluxes) are approximated with two different approaches: 1) the element-wise localized discretization without considering the interelement contributions and 2) the discretization based on the local discontinuous Galerkin (LDG) method. In the LDG formulation the advection–diffusion equation is solved as a first-order system. All of the curvature terms resulting from the cubed-sphere geometry are incorporated into the first-order system. The effectiveness of each diffusion scheme is studied using the standard shallow-water test cases. The approach of element-wise localized discretization of the diffusion term is easy to implement but found to be less effective, and with relatively high diffusion coefficients, it can adversely affect the solution. The shallow-water tests show that the LDG scheme converges monotonically and that the rate of convergence is dependent on the coefficient of diffusion. Also the LDG scheme successfully eliminates small-scale noise, and the simulated results are smooth and comparable to the reference solution.

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.

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.

scholarly journals A Unified Convection Scheme (UNICON). Part I: Formulation

2014 ◽  
Vol 71 (11) ◽  
pp. 3902-3930 ◽  
Author(s):  
Sungsu Park
Keyword(s):  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Horizontal Resolution ◽  
Vertical Transport ◽  
Convective Precipitation ◽  
Cold Pool ◽  
Quasi Equilibrium ◽  
Convection Scheme ◽  
Time Step ◽  
Turbulent Eddies

Abstract The author develops a unified convection scheme (UNICON) that parameterizes relative (i.e., with respect to the grid-mean vertical flow) subgrid vertical transport by nonlocal asymmetric turbulent eddies. UNICON is a process-based model of subgrid convective plumes and mesoscale organized flow without relying on any quasi-equilibrium assumptions such as convective available potential energy (CAPE) or convective inhibition (CIN) closures. In combination with a relative subgrid vertical transport scheme by local symmetric turbulent eddies and a grid-scale advection scheme, UNICON simulates vertical transport of water species and conservative scalars without double counting at any horizontal resolution. UNICON simulates all dry–moist, forced–free, and shallow–deep convection within a single framework in a seamless, consistent, and unified way. It diagnoses the vertical profiles of the macrophysics (fractional area, plume radius, and number density) as well as the microphysics (production and evaporation rates of convective precipitation) and the dynamics (mass flux and vertical velocity) of multiple convective updraft and downdraft plumes. UNICON also prognoses subgrid cold pool and mesoscale organized flow within the planetary boundary layer (PBL) that is forced by evaporation of convective precipitation and accompanying convective downdrafts but damped by surface flux and entrainment at the PBL top. The combined subgrid parameterization of diagnostic convective updraft and downdraft plumes, prognostic subgrid mesoscale organized flow, and the feedback among them remedies the weakness of conventional quasi-steady diagnostic plume models—the lack of plume memory across the time step—allowing UNICON to successfully simulate various transitional phenomena associated with convection (e.g., the diurnal cycle of precipitation and the Madden–Julian oscillation).

Sign in / Sign up

Export Citation Format

Share Document