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Valery Melnikov and Jerry M. Straka

Abstract

A novel method of retrieving the mean axis ratio (width/length) and standard deviation of orientation angles (σθ, which is called herein the intensity of fluttering) of ice cloud particles from polarimetric radar data is described. The method is based on measurements of differential reflectivity Z DR and the copolar correlation coefficient in cloud areas with Z DR > 4 dB. In three analyzed cases, the values of the retrieved axis ratio were in an interval from 0.15 to 0.4 and σθ found in an interval from 2° to 20°. The latter values indicate that the particles experienced light to moderate fluttering. Ambiguities in the retrievals because of uncertainties in the bulk ice density of the particles and possible presence of columnar crystals are considered. The retrieval method is applicable for centimeter-wavelength radars; the analyzed data were collected with the dual-polarization S-band Weather Surveillance Radar-1988 Doppler (WSR-88D).

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Katharine M. Kanak and Jerry M. Straka

Abstract

Photographic documentation of a rare and enigmatic reticular cloud formation that occurred in conjunction with a thunderstorm outflow anvil on 4 June 1995 at 2230 UTC at Norman, Oklahoma, is presented. A National Weather Service vertical sounding, taken within 1 h of the occurrence of the formation at Norman, is also presented. Possible formation mechanisms for these unusual cloud features are discussed.

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Paul M. Markowski and Jerry M. Straka

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The authors document some of the unusual rotating updrafts (one of which produced a tornado) that developed over central Oklahoma on 28 October 1998 in an environment of strong (1.8 × 10−2 s−1) low-level (0–3 km) mean shear. The maximum convective available potential energy (including virtual temperature effects) a “storm” could have realized was approximately 300 J kg−1; however, most of the storms probably realized less than 100 J kg−1. Average (maximum) parcel virtual temperature excesses were estimated to be 0.4–1.2 K (1.8–2.8 K). Echo tops were measured from less than 5 km to 11.2 km above ground level (AGL), although visual observations and radar data suggested echoes that extended above approximately 5–6 km AGL were not associated with significantly buoyant cloud elements. Radar characteristics of many of the storms were similar to supercell storms (e.g., weak echo regions, echo overhang, velocity couplets, hook echoes), as were some of the visual characteristics near cloud base (e.g., wall clouds, rain-free bases, and striated low-level updrafts); however, visual characteristics in middle to upper portions of the storms were not characteristic of typical severe storms, supercells, or previously documented “minisupercells.” Furthermore, the buoyancy realized by the updrafts was estimated to be considerably less than environments associated with the aforementioned minisupercells.

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Jerry M. Straka and John R. Anderson

Abstract

The minimal aliasing local spectral (LS) method is a numerical technique that embodies features of both finite-difference (FD) and spectral transform (ST) methods. Anderson first described this method in the context of the one-dimensional advection-diffusion equation. In the current paper, we describe the extension of the LS method to multidimensions. First, we review the one-dimensional version of the LS method from a more rigorous view. In addition, we describe interpolation, differentiation, and dealiasing fitters for the LS method based on Lagrange polynomials. Without the dealiasing filters, this version of the LS method collapses to a standard high-order Taylor series FD scheme. When filter lengths span the integration domain and the dealiasing stage is retained, the LS method becomes an ST method, as described by Anderson. Issues concerning the implementation of the LS method in multidimensions are also discussed. These issues include the form of the high-resolution grid, the implementation of the interpolation stage, and the implementation of the dealiasing stage. Then, we test the LS method with a two-dimensional nonlinear density current problem using idealized boundary conditions. Comparisons are made with a high-resolution reference solution from a reference model, as well as with solutions from a high-order FD model. Results from simulations of the test problem demonstrate that the LS method is more accurate than high-order FD schemes at coarse grid resolutions, and as accurate at finer grid resolutions. Furthermore, the results show that solutions from LS models are more robust than solutions from FD models. After this, we show that dealiasing the nonlinear advection tendencies plays an important role in the success of the LS method, especially for simulations with sharp boundaries that are marginally resolved. For adequately resolved flows, dealiasing does not necessarily improve solutions for the short-term integrations that are presented. However, aliasing errors still must be controlled to prevent a catastrophic buildup of energy at the smallest resolvable wavelengths. Finally, the LS method is tested using open lateral boundary conditions. As the LS method is a higher-order scheme, special treatment of the vertical and lateral boundaries is required. One possibility is to use lower-order versions of the LS method as boundaries are approached, and outflow conditions at the lateral boundaries. This simple treatment results in solutions that compare very favorably to the reference solution of the test problem.

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Jerry M. Straka and Erik N. Rasmussen

Abstract

Prognostic equations are proposed for use in gridpoint models for the purpose of providing Lagrangian information without the need for computing Lagrangian trajectories. The information provided by the proposed methods might lead to improved representations of microphysical conversion processes. For example, the proposed methods could help improve the timing and location of the onset of precipitation in cloud models.

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Jerry M. Straka and Edward R. Mansell

Abstract

A single-moment bulk microphysics scheme with multiple ice precipitation categories is described. It has 2 liquid hydrometeor categories (cloud droplets and rain) and 10 ice categories that are characterized by habit, size, and density—two ice crystal habits (column and plate), rimed cloud ice, snow (ice crystal aggregates), three categories of graupel with different densities and intercepts, frozen drops, small hail, and large hail. The concept of riming history is implemented for conversions among the graupel and frozen drops categories. The multiple precipitation ice categories allow a range of particle densities and fall velocities for simulating a variety of convective storms with minimal parameter tuning. The scheme is applied to two cases—an idealized continental multicell storm that demonstrates the ice precipitation process, and a small Florida maritime storm in which the warm rain process is important.

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Erik N. Rasmussen and Jerry M. Straka

Abstract

It is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by the amount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper-level flow of the anvil. This paper presents the results of a climatological analysis of soundings associated with three types of isolated supercells having distinctive precipitation distributions, the so-called classic, low-precipitation (LP), and high-precipitation (HP) storms. It is shown that storm-relative flow at 9–10 km above the ground is strongest in the environments of LP storms, and relatively weak in the environments of HP storms, with classic storms occurring in environments with intermediate magnitudes of upper storm-relative flow. It is plausible that comparatively strong flow in the anvil-bearing levels of LP storms transports hydrometeors far enough from the updraft that they are relatively unlikely to be reingested into the updraft, leading to greatly diminished precipitation formation in the updraft itself. Conversely, the weak upper flow near HP storms apparently allows a relatively large number of hydrometeors to return to the updraft, leading to the generation of relatively large amounts of precipitation in the updraft. It also is apparent that thermodynamic factors such as convective available potential energy, low-level mixing ratio, and mean relative humidity are of lesser importance in determining storm type from a climatological perspective, although important variations in humidity may not be well sampled in this study. This climatological analysis does not directly evaluate the stated hypothesis; however, the findings do indicate that further modeling and microphysical observations are warranted.

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Jerry M. Straka and Matthew S. Gilmore

Abstract

This note documents the results of more exact parameterizations for continuous-collection growth and evaporation against simpler traditional ones. Although the main focus is on improving research models, the research results also apply to high-resolution forecast models because the use of lookup tables can make the proposed evaporation, terminal velocity, and collection parameterizations as fast as or faster than proposed ones. It is shown that the older method of ignoring oblate-like distortions of shapes in raindrops, truncated at a maximum diameter of 8 mm, gives a solution like that including oblate-like distortions but only because of two large errors that nearly cancel. The biggest differences from the solutions using oblate-like distortions in shape arise from parameterizations that incorporate more exact approximations (e.g., sweep-out diameter) that are not combined with appropriately more exact approximations for other variables dependent on diameter (e.g., terminal velocity).

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Matthew S. Gilmore and Jerry M. Straka

Abstract

The simplified version of the Berry and Reinhardt parameterization used for initiating rain from cloud droplets is presented and is compared with 12 other versions of itself from the literature. Many of the versions that appear to be different from each other can be brought into agreement with the original parameterization by making the same assumptions: a mean diameter based upon mass or volume and distribution shape parameters chosen to give the same cloud mass relative variance as the original Berry and Reinhardt parameterization. However, there are differences in how authors have chosen to parameterize the cloud number concentration sink and rain number concentration source, and those choices, along with model limitations, have important impacts on rain development within the scheme. These differences among versions are shown to have important time-integrated feedbacks upon the developing initial rain distribution. Three of 12 implementations of the bulk scheme are shown to be able to reproduce the original Berry and Reinhardt bin-model solutions very well, and about 6 of 12 do poorly.

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Jerry M. Straka and John R. Anderson

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In the first part of this paper, the characteristics of microburst-producing storms are examined with a three-dimensional cloud model using soundings from the Cooperative Huntsville Meteorological Experiment (COHMEX). With a grid resolution of 500 m, it is shown that the general characteristics of observed vertical velocities, vertical draft sizes, water contents, radar reflectivities, and surface outflow strengths can be simulated. In addition, observed microburst precursors such as midlevel convergence and descending precipitation cores can also be simulated. Using a grid resolution of 250 m, the observed structure of a particularly well-documented storm on 20 July 1986 during COHMEX is simulated, including a hail shaft 1–2 km wide that descended to the ground.

In the second part of this paper, the influence of microphysical processes in the production of low-level downdrafts in simulated COHMEX storms is investigated. It is shown that low-level downdrafts are in some cases stronger and deeper in simulations made with the ice phase than in simulations made without the ice phase. These differences are due, in part, to the additional cooling associated with the melting of ice, and are consistent with findings of several other recent studies of low-level downdraft production in deep convective storms.

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