A new bulk microphysical parameterization (BMP) has been developed for use with the Weather Research and Forecasting (WRF) Model or other mesoscale models. As compared with earlier single-moment BMPs, the new scheme incorporates a large number of improvements to both physical processes and computer coding, and it employs many techniques found in far more sophisticated spectral/bin schemes using lookup tables. Unlike any other BMP, the assumed snow size distribution depends on both ice water content and temperature and is represented as a sum of exponential and gamma distributions. Furthermore, snow assumes a nonspherical shape with a bulk density that varies inversely with diameter as found in observations and in contrast to nearly all other BMPs that assume spherical snow with constant density. The new scheme’s snow category was readily modified to match previous research in sensitivity experiments designed to test the sphericity and distribution shape characteristics. From analysis of four idea...

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Explicit Forecasts of Winter Precipitation Using an Improved Bulk Microphysics Scheme. Part II: Implementation of a New Snow Parameterization

The spatial resolution appropriate for the simulation of deep moist convection is addressed from a turbulence perspective. To provide a clear theoretical framework for the problem, techniques for simulating turbulent flows are reviewed, and the source of the subgrid terms in the Navier‐Stokes equation is clarified. For decades, cloud-resolving models have used large-eddy simulation (LES) techniques to parameterize the subgrid terms. A literature review suggests that the appropriateness of using traditional LES closures for this purpose has never been established. Furthermore, examination of the assumptions inherent in these closures suggests that grid spacing on the order of 100 m may be required for the performance of cloud models to be consistent with their design. Based on these arguments, numerical simulations of squall lines were conducted with grid spacings between 1 km and 125 m. The results reveal that simulations with 1-km grid spacing do not produce equivalent squallline structure and evolution as compared to the higher-resolution simulations. Details of the simulated squall lines that change as resolution is increased include precipitation amount, system phase speed, cloud depth, static stability values, the size of thunderstorm cells, and the organizational mode of convective overturning (e.g., upright towers versus sloped plumes). It is argued that the ability of the higher-resolution runs to become turbulent leads directly to the differences in evolution. There appear to be no systematic trends in specific fields as resolution is increased. For example, mean vertical velocity and rainwater values increase in magnitude with increasing resolution in some environments, but decrease with increasing resolution in other environments. The statistical properties of the simulated squall lines are still not converged between the 250- and 125-m runs. Several possible explanations for the lack of convergence are offered. Nevertheless, it is clear that simulations with O(1 km) grid spacing should not be used as benchmark or control solutions for resolution sensitivity studies. The simulations also support the contention that a minimum grid spacing of O(100 m) is required for traditional LES closures to perform appropriately for their design. Specifically, only simulations with 250- and 125-m grid spacing resolve an inertial subrange. In contrast, the 1-km simulations do not even reproduce the correct magnitude or scale of the spectral kinetic energy maximum. Furthermore, the 1-km simulations contain an unacceptably large amount of subgrid turbulence kinetic energy, and do not adequately resolve turbulent fluxes of total water. A guide to resolution requirements for the operational and research communities is proposed. The proposal is based primarily on the intended use of the model output. Even though simulations with O(1 km) grid spacing display behavior that is unacceptable for the model design, it is argued that these simulations can still provide valuable information to operational forecasters. For the research community, O(100 m) grid spacing is recommended for most applications, because a modeling system that is well founded should be desired for most purposes.

https://opensky.ucar.edu/islandora/object/articles%3A17616/datastream/PDF/view

Resolution Requirements for the Simulation of Deep Moist Convection

All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level a...

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A Baseline Climatology of Sounding-Derived Supercell and Tornado Forecast Parameters

A sample of 413 soundings in close proximity to tornadic and nontornadic supercells is examined. The soundings were obtained from hourly analyses generated by the 40-km Rapid Update Cycle-2 (RUC-2) analysis and forecast system. A comparison of 149 observed soundings and collocated RUC-2 soundings in regional supercell environments reveals that the RUC-2 model analyses were reasonably accurate through much of the troposphere. The largest error tendencies were in temperatures and mixing ratios near the surface, primarily in 1-h forecast soundings immediately prior to the standard rawinsonde launches around 1200 and 0000 UTC. Overall, the RUC-2 analysis soundings appear to be a reasonable proxy for observed soundings in supercell environments. Thermodynamic and vertical wind shear parameters derived from RUC-2 proximity soundings are evaluated for the following supercell and storm subsets: significantly tornadic supercells (54 soundings), weakly tornadic supercells (144 soundings), nontornadic supercells (215 soundings), and discrete nonsupercell storms (75 soundings). Findings presented herein are then compared to results from previous and ongoing proximity soundings studies. Most significantly, proximity soundings presented here reinforce the findings of previous studies in that vertical shear and moisture within 1 km of the ground can discriminate between nontornadic supercells and supercells producing tornadoes with F2 or greater damage. Parameters that combine measures of buoyancy, vertical shear, and low-level moisture show the strongest ability to discriminate between supercell classes.

/pdf/close-proximity-soundings-within-supercell-environments-1bx6enprf4.pdf

Close Proximity Soundings within Supercell Environments Obtained from the Rapid Update Cycle

/pdf/a-description-of-the-advanced-research-wrf-model-version-4-1167arlp4w.pdf

A Description of the Advanced Research WRF Model Version 4

Despite the long-surmised importance of the hook echo and rear-flank downdraft (RFD) in tornadogenesis, only a paucity of direct observations have been obtained at the surface within hook echoes and RFDs. In this paper, in situ surface observations within hook echoes and RFDs are analyzed. These “mobile mesonet” data have unprecedented horizontal spatial resolution and were obtained from the Verifications of the Origins of Rotation in Tornadoes Experiment (VORTEX) and additional field experiments conducted since the conclusion of VORTEX. The surface thermodynamic characteristics of hook echoes and RFDs associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and RFDs are favorable (or unfavorable) for tornadogenesis. Tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy, potential buoyancy (as measured by the convective available potential energy), and equivalent potential temperature in the R...

http://www.geosci.sfsu.edu/Geosciences/classes/m835/pdfs/Markowskietal.pdf

Direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells

This work reports on the sensitivity of accumulated precipitation to the microphysical parameterization in simulations of deep convective storms using a three-dimensional, nonhydrostatic cloud model with a simple liquid‐ice microphysics scheme. Various intercept parameters from an assumed Marshall‐Palmer exponential size distribution are tested along with two particle densities for the hail/graupel (qh) category. These variations allow testing of unique qh distributions that have been observed and documented in previous literature. Tests are conducted for a single thermodynamic profile and three idealized wind shear profiles. The amount of accumulated precipitation at the ground is very sensitive to the way the qh category is parameterized. Distributions characterized by larger intercepts and/or smaller particle density have a smaller mass-weighted mean terminal fall velocity and produce smaller qh mixing ratios spread over a larger area. For example, for a qh category weighted toward graupel, only a fourth as much precipitation accumulates on the ground over 2 h (and none is hail) compared to a qh category weighted toward large hail (with baseball-sized stones common). The inherent uncertainty within the qh distribution for this simple cloud-scale three-class ice microphysics scheme suggests limited usefulness in the forecasting of ground-accumulated precipitation and damaging hail.

Precipitation Uncertainty Due to Variations in Precipitation Particle Parameters within a Simple Microphysics Scheme

This note updates a previous study that utilized a baseline climatology of soundings associated with large hail, significant tornadoes, and 10 or more cloud-to-ground lightning flashes from 1992. Expanding on the earlier analysis, it is shown that three modified forecast parameters have more value in distinguishing between environments that favor significant tornadoes and those that favor large hail but no significant tornadoes, in the climatological data. These parameters are storm-relative helicity in the 0–1-km layer adjacent to the ground, energy–helicity index computed from this measure of helicity, and the convective available potential energy that accrues from the surface to 3 km above ground level. In addition, this note provides caveats regarding the interpretation of the climatological findings.

Refined Supercell and Tornado Forecast Parameters

This paper describes the Verification of the Origins of Rotation in Tornadoes Experiment planned for 1994 and 1995 to evaluate a set of hypotheses pertaining to tornadogenesis and tornado dynamics. Observations of state variables will be obtained from five mobile mesonet vehicles, four mobile ballooning laboratories, three movie photography teams, portable Doppler radar teams, two in situ tornado instruments deployment teams, and the T-28 and National Atmospheric and Oceanic Administration P-3 aircraft. In addition, extensive use will be made of the new generation of observing systems, including the WSR-88D Doppler radars, demonstration wind profiler network, and National Weather Service rawinsondes.

Erik N. Rasmussen

Papers

A Baseline Climatology of Sounding-Derived Supercell and Tornado Forecast Parameters

Direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells

Precipitation Uncertainty Due to Variations in Precipitation Particle Parameters within a Simple Microphysics Scheme

Refined Supercell and Tornado Forecast Parameters

Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX