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  • Author or Editor: Jerry M. Straka x
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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
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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Jerry M. Straka
,
Katharine M. Kanak
, and
Matthew S. Gilmore

Abstract

This paper presents a mathematical explanation for the nonconservation of total number concentration Nt of hydrometeors for the continuous collection growth process, for which Nt physically should be conserved for selected one- and two-moment bulk parameterization schemes. Where possible, physical explanations are proposed. The assumption of a constant no in scheme A is physically inconsistent with the continuous collection growth process, as is the assumption of a constant Dn for scheme B. Scheme E also is nonconservative, but it seems this result is not because of a physically inconsistent specification; rather the solution scheme’s equations simply do not satisfy Nt conservation and Nt does not come into the derivation. Even scheme F, which perfectly conserves Nt , does not preserve the distribution shape in comparison with a bin model.

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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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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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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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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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Jelena Andrić
,
Matthew R. Kumjian
,
Dušan S. Zrnić
,
Jerry M. Straka
, and
Valery M. Melnikov

Abstract

Polarimetric radar observations above the melting layer in winter storms reveal enhanced differential reflectivity Z DR and specific differential phase shift K DP, collocated with reduced copolar correlation coefficient ρ hv; these signatures often appear as isolated “pockets.” High-resolution RHIs and vertical profiles of polarimetric variables were analyzed for a winter storm that occurred in Oklahoma on 27 January 2009, observed with the polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) in Norman. The Z DR maximum and ρ hv minimum are located within the temperature range between −10° and −15°C, whereas the K DP maximum is located just below the Z DR maximum. These signatures are coincident with reflectivity factor ZH that increases toward the ground. A simple kinematical, one-dimensional, two-moment bulk microphysical model is developed and coupled with electromagnetic scattering calculations to explain the nature of the observed polarimetric signature. The microphysics model includes nucleation, deposition, and aggregation and considers only ice-phase hydrometeors. Vertical profiles of the polarimetric radar variables (ZH , Z DR, K DP, and ρ hv) were calculated using the output from the microphysical model. The base model run reproduces the general profile and magnitude of the observed ZH and ρ hv and the correct shape (but not magnitude) of Z DR and K DP. Several sensitivity experiments were conducted to determine if the modeled signatures of all variables can match the observed ones. The model was incapable of matching both the observed magnitude and shape of all polarimetric variables, however. This implies that some processes not included in the model (such as secondary ice generation) are important in producing the signature.

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

Abstract

One- and two-moment parameterizations are integrated over hydrometeor diameters D(0, ∞) for vapor diffusion and the continuous collection growth processes. For the conditions specified, the total number concentration of collector particles should be conserved. To address the problem, the gamma distribution function is used for the spectral density function. Predicted variables can include total mixing ratio q, total number concentration Nt , and characteristic diameter Dn (inverse of the distribution slope λ). In all of the cases, the slope intercept no is diagnosed or specified. The popular one- and two-moment methods that are explored include the one-moment method in which q is predicted, no is specified, and Nt and Dn are diagnosed; the one-moment method in which q is predicted, Dn is specified, and Nt and no are diagnosed; the two-moment method in which q and Dn are predicted and Nt and no are diagnosed; and the two-moment method in which q and Nt are predicted and no and Dn are diagnosed. It is demonstrated for the processes examined that all of the schemes 1) fail to conserve Nt for the collector particles when Nt should be conserved and 2) have other unphysical attributes, except for the two-moment method in which q and Nt are predicted. In recent years there has been a dramatic increase in the use of more-sophisticated microphysical parameterizations in cloud, mesoscale, and climate models, and it is increasingly important for a model user to be cognizant of the strengths and weaknesses of the parameterizations in complex models.

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