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Matthew S. Gilmore
,
Jerry M. Straka
, and
Erik N. Rasmussen

Abstract

Weisman and Klemp suggested that their liquid-only, deep convective storm experiments should be repeated with a liquid-ice microphysics scheme to determine if the solutions are qualitatively the same. Using a three-dimensional, nonhydrostatic cloud model, such results are compared between three microphysics schemes: the “Kessler” liquid-only scheme (used by Weisman and Klemp), a Lin–Farley–Orville-like scheme with liquid and ice parameterization (Li), and the same Lin–Farley–Orville-like microphysics scheme but with only liquid processes turned on (Lr). Convection is simulated using a single thermodynamic profile and a variety of shear profiles. The shear profiles are represented by five idealized half-circle wind hodographs with arc lengths (U s ) of 20, 25, 30, 40, and 50 m s−1. The precipitation, cold pool characteristics, and storm evolution produced by the different schemes are compared.

The Kessler scheme produces similar accumulated precipitation over 2 h compared to Lr for all shear regimes. Although Kessler's rain evaporation rate is 1.5–1.8 times faster in the lower troposphere, rain production is also faster via accretion and autoconversion of cloud water. In addition, nearly ∼40% more accumulated precipitation occurs in Li compared to Lr. This can be attributed primarily to increased precipitation production rates and enhanced low-level precipitation fluxes in Li for all shear regimes. Differences in the amount of precipitation reaching ground and the low-level cooling rates also cause differences in storm cold pools.

For the U s = 25 shear regime, microphysics cases with colder low-level outflow are shown to be associated with temporarily weaker (Li) or shorter-lived (Kessler) supercells as compared to cases with warmer outflow (Lr). This is consistent with a previous study showing that the cold pool has a greater relative impact on the storm updraft compared to dynamic forcing for weaker shear.

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Mark A. Askelson
,
Jean-Pierre Aubagnac
, and
Jerry M. Straka

Abstract

Spatial objective analysis is routinely performed in several applications that utilize radar data. Because of their relative simplicity and computational efficiency, one-pass distance-dependent weighted-average (DDWA) schemes that utilize either the Cressman or the Barnes filter are often used in these applications. The DDWA schemes that have traditionally been used do not, however, directly account for two fundamental characteristics of radar data. These are 1) the spacing of radar data depends on direction and 2) radar data density systematically decreases with increasing range.

A DDWA scheme based on an adaptation of the Barnes filter is proposed. This scheme, termed the adaptive Barnes (A-B) scheme, explicitly takes into account radar data properties 1 and 2 above. Both theoretical and experimental investigations indicate that two attributes of the A-B scheme, direction-splitting and automatic adaptation to data density, may facilitate the preservation of the maximum amount of meaningful information possible within the confines of one-pass DDWA schemes.

It is shown that in the idealized situation of infinite, continuous data and for an analysis in rectangular-Cartesian coordinates, a direction-splitting scheme does not induce phase shifts if the weight function is even in each direction. Moreover, for radar data that are infinite, collected at regular radial, azimuthal, and elevational increments, and collocated with analysis points, the direction-splitting design of the A-B filter removes gradients in the analysis weights. This is a beneficial attribute when considering the treatment of gradient information of rectangular Cartesian data by an analysis system because then postanalysis gradients equal the analysis of gradients. The direction-splitting design of the A-B filter is unable, however, to circumvent the impact of the varying physical distances between adjacent measurements that are inherent to the spherical coordinate system of ground-based weather radars. Because of this, even with the direction-splitting design of the A-B filter postanalysis gradients do not equal the analysis of gradients.

Ringing in the response function of a one-dimensional Barnes filter is illustrated. The negative impact of data windows on the main lobe of the response function is found to decrease rapidly as the window is widened relative to the weight function. Unless an analysis point is near a data boundary, in which case both ringing and phase shifting will adversely affect the analysis, window effects are unlikely to be significant in applications of the A-B filter to radar data.

The A-B filter has potential drawbacks, the most significant of which is misinterpretations owing to the use of the A-B filter without comprehension of its direction- and range-dependent response function. Despite its drawbacks, the A-B filter has the potential to improve analyses owing to the aforementioned attributes and thus to aid research efforts in areas such as multiple-Doppler wind analyses, pseudo-dual-Doppler analyses, and retrieval studies.

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Matthew S. Gilmore
,
Jerry M. Straka
, and
Erik N. Rasmussen

Abstract

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.

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Matthew S. Van Den Broeke
,
Jerry M. Straka
, and
Erik N. Rasmussen

Abstract

Preliminary schematics of polarimetric signatures at low levels in southern plains classic supercells are developed for pretornado, tornado, and tornado demise times from a small collection of cases, most of which are cyclic tornado producers. Characteristic signatures and patterns are identified for the reflectivity factor (Z HH), the differential reflectivity (Z DR), the correlation coefficient (ρ hv), and the specific differential phase (K DP). Signatures likely related to an ongoing tornado are also discussed. Major findings in Z HH at tornado times include “wings” of higher values often extending away from the updraft region, a stronger gradient on the west side of the echo appendage, and a local maximum at the storm location favorable for tornadogenesis. Increasing cyclonic curvature of the hook-echo region was noted through the tornado life cycle. The Z DR tended to indicate hail shafts most commonly at tornado times, with the highest storm values typically located along the storm’s forward flank throughout the tornado life cycle. A Z DR minimum often occurred at the tornado-favorable location, while low Z DR occasionally trailed the tornado region. Storm-minimum ρ hv typically occurred at the tornado-favorable location at tornado times and in hail shafts or heavy rain areas at other times. Another region of low correlation was the storm updraft, while the highest storm correlation was typically found in the downwind light-precipitation shield. The K DP typically exhibited a storm-core temporal maximum at tornado times, with the highest storm values in regions of hail and heavy rain and the lowest values in the downwind light-precipitation region. Values in the tornado-favorable region were typically near zero and sometimes strongly negative.

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Donald R. MacGorman
,
Jerry M. Straka
, and
Conrad L. Ziegler

Abstract

A new lightning parameterization has been developed to enable cloud models to simulate the location and structure of individual lightning flashes more realistically. To do this, three aspects of previous parameterizations have been modified: 1) To account for subgrid-scale variations, the initiation point is chosen randomly from among grid points at which the electric field magnitude is above a threshold value, instead of being assigned always to the grid point having the maximum electric field magnitude. 2) The threshold value for initiation can either be constant, as in previous parameterizations, or can vary with height to allow different flash initiation hypotheses to be tested. 3) Instead of stopping at larger ambient electric field magnitudes, extensive flash development can continue in regions having a weak ambient electric field but a substantial charge density. This behavior is based on lightning observations and conceptual models of lightning physics. However, like previous parameterizations for cloud models, the new parameterization attempts to mimic only the gross structure of flashes, not the detailed development of lightning channels, the physics of which is only poorly understood. Though the choice of parameter values affects the dimensions of a flash, the qualitative features of simulated flash structure are similar to those of observed lightning as long as the parameter values are consistent with the larger electric field magnitudes measured in storms and with simulated charge densities produced over reasonably large regions. Initial simulations show that, by permitting development of flashes in regions of substantial charge density and weak ambient electric field, the new parameterization produces flash structure much like that of observed flashes, as would be expected from the inferred correlation between observed horizontal lightning structure and thunderstorm charge.

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Jerry M. Straka
,
Dusan S. Zrnić
, and
Alexander V. Ryzhkov

Abstract

A new synthesis of information forming the foundation for rule-based systems to deduce dominant bulk hydrometeor types and amounts using polarimetric radar data is presented. The information is valid for a 10-cm wavelength and consists of relations that are based on an extensive list of previous and recent observational and modeling studies of polarimetric signatures of hydrometeors. The relations are expressed as boundaries and thresholds in a space of polarimetric radar variables. Thus, the foundation is laid out for identification of hydrometeor types (species), estimation of characteristics of hydrometeor species (size, concentrations, etc.), and quantification of bulk hydrometeor contents (amounts). A fuzzy classification algorithm that builds upon this foundation will be discussed in a forthcoming paper.

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Sonia G. Lasher-Trapp
,
Charles A. Knight
, and
Jerry M. Straka

Abstract

The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.

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Daniel E. Johnson
,
Pao K. Wang
, and
Jerry M. Straka

Abstract

The Wisconsin Dynamical-Microphysical Model is used in two simulations of the 2 August 1981 supercell that passed through the Cooperative Convective Precipitation Experiment in southeastern Montana. The first simulation uses liquid water-only microphysics and is denoted as the liquid water model (LWM). The second includes both liquid water and ice microphysics and is designated as the hail category model (HCM). Results from the two simulations show that the inclusion of ice significantly alters the dynamics, kinematics, thermodynamics, and distributions of water in the storm, especially at the lower levels. Supercell features such as a rotating intense updraft, bounded weak-echo region, large forward overhanging anvil, and hooklike structure in the low-level rainwater field are present in both simulations. These features are generally more pronounced, however, and have a longer lifetime in the HCM.

Hail embryo and graupel particles make up more than 85% of the total hail mass during the steady-state phase in the HCM. Many of these particles are advected into the anvil regions away from the updraft and sublimate slowly. As a result, distributions of graupel and hail in the HCM cover a more extensive but less concentrated region than do the distributions of rainwater in the LWM. Heavier more localized precipitation in the LWM results in a stronger low-level downdraft and a faster-moving gust front than in the HCM. The LWM gust front propagates ahead of the low-level updraft, cutting off the warm, moist, low-level easterly flow into the storm that leads to complete dissipation of the cloud by the end of the 150-min simulation period. Conversely, less concentrated precipitation failing to the surface in the HCM results in a weaker downdraft and a slower-moving gust front. The gust front propagates with the low-level updraft, thus allowing the storm to remain in a quasi-steady state for the final 80 min of the simulation. Overall, there is slightly more total surface precipitation in the HCM due to the larger areal coverage of precipitation and slower movement of the storm.

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Jerry M. Straka
,
Erik N. Rasmussen
, and
Sherman E. Fredrickson

Abstract

A mobile weather observing system (mobile mesonet) was designed to augment existing meteorological networks in the study of severe local storms and other mesoscale weather phenomena in conjunction with the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). Fifteen mobile mesonet units were built, each consisting of meteorological instruments mounted on standard automobiles. for high temporal and spatial resolution observations. While the most accurate measurements are possible from stationary mobile mesonet vehicles, accurate observations also are possible from moving vehicles. The mobile mesonet instruments measure pressure (600–1100 mb), temperature (−33° to 48°C), relative humidity (0%–100%), and wind direction and speed (0°–360° and 0–60 m s−1). Onboard each vehicle, a Global Positioning System (GPS) receiver and a flux-gate compass obtain universal time, vehicle location (latitude, longitude, altitude), and vehicle heading and speed. A standard laptop computer stores data, computes derived variables, and provides real-time data display. Instrument compatibility with the Oklahoma Mesonet allows for high-quality instrument calibration and maintenance.

The purpose of this paper is to provide a technical overview of the mobile mesonet system. The rationale for choice of instrumentation and justification for method of exposure are discussed. The performance of the mobile mesonet is demonstrated with two examples of data collected during VORTEX-1994 and comparisons with data from an Oklahoma Mesenet site.

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Leigh G. Orf
,
John R. Anderson
, and
Jerry M. Straka

Abstract

A parameter study of colliding microburst outflows is performed using a high-resolution three-dimensional model. The colliding microburst pairs me simulated in a domain of 18 km × 16 km × 4.25 km with 50-m resolution. Microburst pairs are examined in varying space and time separations, and the authors find that for certain geometries strong elevated wind fields are generated from the interactions between outflows. For a narrow range of space-time geometries, this elevated wind field is extremely divergent. An examination of the F-factor aircraft hazard parameter reveals that both the divergent wind fields and microburst downdraft cores are regions of danger to jet aircraft. Trajectory analysis reveals that the air composing the elevated jets can be traced back to the shallow outflow formed beneath each microburst core. An analysis of the parcel kinetic energy budget indicates that the pressure domes beneath and between the microbursts are the primary mechanisms for directing energy into the elevated jets.

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