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Conrad L. Ziegler

Abstract

The hydrometeor content and thermal fields in a thunderstorm are estimated from a three-dimensional kinematic cloud model employing Doppler wind fields and parameterized microphysical processes. The sensitivity of the cloud model calculations to variation of the microphysical parameterization is determined by comparing results of model runs with modified parameterizations to the results of a standard or control model run with complete warm/cold cloud microphysics. Changes of certain calculated or specified model parameters and alternate exclusion or inclusion of the ice phase modulate extreme values of precipitation content. Differences between the model solutions, which result from altering the balance between predominant precipitation processes, are traced through analysis of model output to some major change of the precipitation accretion mechanism. The largest differences in maximum retrieved graupel/hail content and radar reflectivity associate with the parameterizations which fix the graupel/hail distribution intercept parameter or accomplish the riming of supercooled cloud droplets by graupel/hail of a fixed high density. A reduction of the collection efficiency of cloud droplets by snow crystals, while significantly weakening the Bergeron precipitation process, has negligible impact on either the content or riming growth of graupel/hail. The sensitivity of precipitation content to variation of model parameters such as CCN concentration and dispersion of the cloud droplet distribution is relatively weak because the balance among predominant precipitation processes is not significantly altered. Exclusion of the ice phase results in a cooling of up to 2°C in the main updraft region. The major impact on cloud dynamical forcing by presence of the ice phase is a reduction of negative buoyancy in the upper half of the main updraft region.

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Conrad L. Ziegler

Abstract

The air flow in convective storms, the force that regulate the flow, and the processes that produce hydrometeors of various kinds are all being studied intensively by meteorologists using Doppler radar observations. The research reported here proceeds from the observed motion through accompanying thermodynamic and micro-physical processes to the analysis of hydrometer content and thermal fields in thunderstorms. A three-dimensional numerical kinematic cloud model employing Doppler wind fields is developed and used to diagnose temperature and mixing ratios within a thunderstorm. The microphysical parameterization includes stochastic coalescence effects in warm clouds as well as well- and variable-density dry hail growth.

Known fields from a dynamically simulated cloud are used to establish the accuracy of the retrieval scheme. Real data tests indicate good agreement between retrieved and observed radar reflectivities, qualitative dynamic consistency between observed winds and retrieved buoyancies, and the model's ability to partition liquid and solid hydrometeors. A modification of weighting factors in the variational multiple Doppler analysis changes the vertical motion field and improves the verification of retrieved reflectivities. This analysis sensitivity emphasizes the great influence of the input wind field on retrieved thunderstorm variables.

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Conrad L. Ziegler

Abstract

A diabatic Lagrangian analysis (DLA) technique for deriving potential temperature, water vapor and cloud water mixing ratios, and virtual buoyancy from three-dimensional time-dependent Doppler radar wind and reflectivity fields in storms is presented. The DLA method proceeds from heat and water substance conservation along discrete air trajectories via microphysical diabatic heating/cooling and simple damping and surface flux parameterizations in a parcel-following ground-relative reference frame to thermodynamic fields on a regular grid of trajectory endpoints at a common analysis time. Rain and graupel precipitation size distributions are parameterized from observed reflectivity at discrete Lagrangian points to simplify the cloud model–based microphysically driven heating and cooling rate calculations. The DLA approximates the precipitation size distributions from reflectivity assuming conventional inverse exponential size distributions and prescribed input intercept parameter values based on the output of a mature simulated storm. The DLA is demonstrated via an observing system simulation experiment (OSSE), and its analysis compares favorably with the known output buoyancy and water substance fields in the simulated storm case. The DLA-analyzed thermal–solenoidal horizontal vorticity tendency is of comparable magnitude to the corresponding modeled solenoidal vorticity tendency. A test application of the DLA to a radar-observed storm is presented in a companion paper (Part II).

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Conrad L. Ziegler

Abstract

A new diabatic Lagrangian analysis (DLA) technique that derives predicted fields of potential temperature, water vapor and cloud water mixing ratios, and virtual buoyancy from three-dimensional, time-dependent wind and reflectivity fields (see Part I) is applied to the radar-observed 9 June 2009 supercell storm during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The DLA diagnoses fields of rain and graupel content from radar reflectivity and predicts the evolution of analysis variables following radar-inferred air trajectories in the evolving storm with application of the diagnosed precipitation fields to calculate Lagrangian-frame microphysical processes. Simple damping and surface flux terms and initialization of trajectories from heterogeneous, parametric mesoscale analysis fields are also included in the predictive Lagrangian calculations. The DLA output compares favorably with observations of surface in situ temperature and water vapor mixing ratio and accumulated rainfall from a catchment rain gauge in the 9 June 2009 storm.

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Edward A. Brandes
and
Conrad L. Ziegler

Abstract

The vorticity dynamics of a lower-tropospheric mesovortex within a mature mesoscale convective system (MCS) is investigated with observations from a special sounding network and with dual-Doppler radar measurements. The data show that the vorticity distribution and tendency were dictated by the mesoscale downdraft, which formed within the storm's trailing stratiform region, and not by the mesoscale updraft. At midlevels, preexisting vertical vorticity was amplified by convergence as environmental air overtook the storm from the rear and funneled into the evaporatively cooled mesoscale downdraft.

Vertical vorticity within the stratiform region also increased by the twisting of horizontal vorticity associated with a backing wind. The horizontal vorticity vector pointed opposite the velocity vector and tipped into positive vertical vorticity when the Row encountered the mesoscale downdraft. At lower-middle storm levels the twisted vorticity was subsequently amplified by convergence along with preexisting vertical vorticity. Below 2 km, vorticity decreased largely in response to horizontal divergence.

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Conrad L. Ziegler
and
Carl E. Hane

Abstract

This study presents analyses of data collected in the vicinity of a cloud-free dryline that occurred in western Oklahoma on 24 May 1989. Observations reveal sharp contrasts across the quasi-stationary, north-south dryline during midafternoon. Of greatest significance is a pronounced gradient of virtual potential temperature, although horizontal convergence and vorticity also maximize at the dryline.

The environment of the 24 May dryline is dominated by vertical mixing that maintains a convective boundary layer (CBL) on both sides of the dryline. The dryline resembles a “mixing zone” containing varying proportions of hot, dry air to the west side and warm, moist air from the lowest 200 m within 10 km to the east of the dryline. The mixing zone slopes eastward from the surface dryline location, then becomes a quasi-horizontal elevated moist layer above the CBL east of the dryline. Saturation-point analysis indicates that the mixing zone is characterized by a single mixing-line structure defined by the respective quasi-homogeneous air masses on either side of the dryline.

Dynamical analysis reveals that near-surface westerly flow is accelerated upward and over relatively cool air above the surface by an elevated low pressure region at the dryline. Flow accelerations are nonhydrostatic at the dryline, while the flow is in hydrostatic balance both to the west and to the east of the dryline. Magnitudes of the inertial, pressure, and Coriolis accelerations are comparable to the east of the dryline, implying a considerable ageostrophic flow component as well as a quasigeostrophic linkage between the low-level jet and the west-east horizontal pressure gradient.

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Conrad L. Ziegler
and
Erik N. Rasmussen

Abstract

The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 field projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression.

The results of the present study confirm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lack of deep convergence in cloud-free regions. The results document the modulating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to leaving the mesoscale updraft to form deep convection. By relaxing the overly restrictive assumptions of parcel theory, it is suggested that a modification of proximity soundings to account for mesoscale lift and westerly wind shear effects can improve the diagnosis of the mesoscale dryline environment and the prediction of convective initiation at the dryline. Conversely, proximity environmental soundings, taken by themselves with consideration of CAPE and convective inhibition values according to parcel theory but neglecting vertical boundary layer circulations, are found to have less prognostic value than is conventionally assumed.

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Peter S. Ray
and
Conrad Ziegler

Abstract

A technique to remove the ambiguity in Doppler mean velocity estimates is described. The technique assumes that along a radial, or portion of a radial, the velocity estimates are quasi-uniformly distributed about the mean. If the data do not meet this criterion, the velocities are adjusted such that they are distributed about the mean.

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Michael S. Buban
and
Conrad L. Ziegler

Abstract

Motivated by high-resolution observations of small-scale atmospheric vortices along near-surface boundaries, this study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries. The series of dry, constant potential temperature simulations are initialized with a north–south-oriented constant-vorticity shear zone and north–south periodic boundary conditions.

In all simulations, the shear zones develop wavelike perturbations that eventually roll up into discrete vortices. These vortices have features resembling those observed in many laboratory and numerical studies (i.e., instabilities developed into elliptical cores connected by vorticity braids that precess and contain pressure minima in their centers). To assess the instability mechanism, the results are compared to linear theory. Excellent agreement is found between predictions from linear theory for the wavenumber of maximum growth as a function of shear zone width and growth rate as a function of shear zone vorticity, suggesting to a very good first approximation, horizontal shearing instability (HSI) is responsible for the growth of initial small perturbations. It is also found that predictions of linear theory tend to extend well into the nonlinear regime.

Finally, preferred regions of cumulus formation are assessed by including moisture in four simulations. Maximum updrafts and simulated cumuli tend to form along the periphery of cores and/or along the braided regions adjacent to the cores. Because of the important modulating effect of misocyclone development via HSI and subsequent moisture transport, cumulus spacing and size/depth are also dependent on the shear zone width and vorticity.

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Michael S. Buban
and
Conrad L. Ziegler

Abstract

This study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries in the presence of horizontal virtual density gradients. The series of dry simulations are initialized to contain a north–south-oriented potential temperature gradient collocated with a constant-vorticity shear zone and employ north–south periodic boundary conditions. In all simulations, the shear zones frontogenetically collapse as wavelike perturbations develop that eventually roll up into discrete vortices. Convergence associated with the developing solenoidally forced secondary vertical circulation induces an accumulative shear zone contraction, which in turn increases the vertical vorticity of both the shear zone and the intensifying vortices, owing primarily to stretching that is partially offset by tilting of the vertical vorticity into the horizontal by the secondary circulation. The simulated vortices bear strong morphological resemblance to vortices reported in many earlier laboratory and numerical studies. To assess hypothesized baroclinic effects on the instability mechanism, the present results are compared to a previous study of barotropic horizontal shearing instability (HSI). Linear theory has been modified for the baroclinic cases by introducing a parametric model of frontal contraction, according to which the growth rate expressions incorporate model-prescribed, continuously varying shear zone widths. This modified parametric model is found to provide excellent agreement with the growth rates computed from the present simulations, suggesting that HSI can be extended to the baroclinic shear zone cases to a very good approximation over a range of near-surface boundary types.

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