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Edward R. Mansell

Abstract

In two-moment bulk microphysics schemes, the practice of using different weighted fall velocities for the various moments is known to lead to artificial growth in reflectivity values for fast-falling particles, particularly at the downward leading edge of a precipitation column. Two simple correction schemes that prevent these artifacts while still allowing some effects of size sorting are presented. The corrections are obtained by comparing particle number concentrations that result from two or three different sedimentation calculations. The corrections do not conserve particle number concentrations but do prevent spurious reflectivity growth automatically without the need to place ad hoc limits on mean particle size.

Multimoment bulk microphysics schemes often have used inconsistent variables in terms of the appropriate advection equation (e.g., mass mixing ratio and particle number concentration). A brief review of consistent advection and turbulent mixing for such variables is presented to provide clarification.

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Edward R. Mansell

Abstract

A set of observing system simulation experiments (OSSEs) demonstrates the potential benefit from ensemble Kalman filter (EnKF) assimilation of total lightning flash mapping data. Synthetic lightning data were generated to mimic the Geostationary Lightning Mapper (GLM) instrument that is planned for the Geostationary Operational Environmental Satellite-R series (GOES-R) platform. The truth simulation was conducted using multimoment bulk microphysics, explicit electrification mechanisms, and a branched lightning parameterization to produce 2-min-averaged synthetic pseudo-GLM observations at 8-km GLM resolution and at a hypothetical 1-km resolution.

The OSSEs use either perfect (two-moment bulk) or imperfect (single-moment, graupel only) microphysics. One OSSE with perfect microphysics included the same electrification physics as the truth simulation to generate lightning flash rates and flash-extent densities (FED). The other OSSEs used linear relationships between flash rate and graupel echo volume as the observation operator. The assimilation of FED at 8-km horizontal resolution can effectively modulate the convection simulated at 1-km horizontal resolution by sharpening the location of reflectivity echoes and the spatial location probability of convective updrafts. Tests with zero flash rates show that the lightning assimilation can help to limit spurious deep convection, as well. Pseudo-GLM observations at 1 km further sharpen the analyses of location (updraft and reflectivity) of the relatively simple storm structure.

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Edward R. Mansell
and
Conrad L. Ziegler

Abstract

The effects of cloud condensation nuclei (CCN) concentrations are found to strongly affect the microphysical and electrical evolution of a numerically simulated small multicell storm. The simulations reproduce the well-known effects of updraft invigoration and delay of precipitation formation as increasing CCN from low to intermediate concentrations causes droplet sizes to decrease. Peak updrafts increased from 16 m s−1 at the lowest CCN to a maximum of 21–22 m s−1 at moderate CCN, where condensation latent heating is maximized. The transition from low to high CCN first maximizes warm-rain production before switching over to the ice process as the dominant precipitation mechanism. Average graupel density stays fairly high and constant at lower CCN, but then drops monotonically at higher CCN concentration, although high CCN also foster the appearance of small regions of larger, high-density graupel with high simulated radar reflectivity.

Graupel production increases monotonically as CCN concentration rises from 50 to about 2000 cm−3. The lightning response is relatively weak until the Hallett–Mossop rime-splintering ice multiplication becomes more active at CCN > 700 cm−3. At very high CCN concentrations (>2000 cm−3), graupel production decreases slowly, but lightning activity drops dramatically when the parameterization of Hallett–Mossop rime-splintering ice multiplication is based on the number of large cloud droplets collected by graupel. Conversely, lightning activity remains steady at extremely high CCN concentration when the Hallett–Mossop parameterization is based simply on the rate of rime mass accumulation. The results lend support to the aerosol hypothesis as applied to lightning production, whereby greater CCN concentration tends to lead to greater lightning activity, but with a large sensitivity to ice multiplication.

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Alexandre O. Fierro
and
Edward R. Mansell

Abstract

This study investigates relationships between storm-scale properties and the electrification and lightning of two simulations of an intensifying idealized tropical cyclone (TC) using the cloud-resolving Collaborative Model for Multiscale Atmospheric Simulation (COMMAS). To produce an intensifying storm, an initial weak TC is subjected to a linear increase in sea surface temperature.

As the TC intensifies, lightning flash rates increase in both the inner core (r ≤ 100 km) and outer region (100 < r ≤ 300 km). As time progresses, lightning in the outer region gradually decreases, while the inner-core lightning remains relatively steady. Bootstrapped correlation statistics using 1000 random samples between the pressure trace and time series of lightning rates shows a statistically significant negative correlation between inner-core lightning and TC intensification. Lightning rates in the outer bands were found to lag minimum surface pressure by 12 h.

The increases in lightning in both the inner core and outer region coincided well with increases in 0.5 g kg−1 graupel and 5 m s−1 updraft volumes in each respective region. Correlation statistics with selected kinematic and microphysical variables known to be associated with lightning in thunderstorms, such as the ice water path, integrated updraft volume, and graupel volume, revealed that their increase in the inner core indicated an ongoing deepening, similar to the lightning. Trends in these proxy variables in the outer bands were also found to lag TC intensification by 12 h.

Overall, the best linear relationships with lightning in either the inner core or the outer region were obtained with the 0.5 g kg−1 graupel volume and total graupel mass.

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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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Alexandre O. Fierro
and
Edward R. Mansell

Abstract

Relationships between intensity fluctuations, cloud microphysics, lightning variations, and electrical structures within idealized tropical cyclones are investigated with the cloud-resolving Collaborative Model for Multiscale Atmospheric Simulation (COMMAS). An initial strong tropical cyclone is subjected to either steady-state control conditions (CTRL), increased wind shear (SHEAR), or a reduction in sea surface temperature (SST).

In CTRL, nearly all the lightning (>95%) occurred in the outer region (100 < r ≤ 300 km) and was overall very episodic in the inner core (r ≤ 100 km), consistent with observations. The inner-core updrafts were weaker and experienced greater depletion of cloud water by warm rain processes, which, in contrast to the deeper updrafts in the rainband convection, reduced the mixed-phase cloud depth and confined the bulk of the charging and lightning initiations to lower levels.

Notably, larger flash rates were produced in the asymmetric inner core of the SHEAR case, with the majority of the flashes located in the downshear left quadrant, consistent with prior observational works. In contrast to CTRL, the more vigorous inner-core convection in SHEAR resulted in the formation of a prominent negative charge region and enhanced production of negative ground flashes.

With a nearly identical filling rate as SHEAR, the introduction of cooler sea surface temperature in the SST case caused lightning activity to fade rapidly in both the inner core and rainbands.

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Edward R. Mansell
,
Conrad L. Ziegler
, and
Donald R. MacGorman

Abstract

Lightning observations have been assimilated into a mesoscale model for improvement of forecast initial conditions. Data are used from the National Lightning Detection Network (cloud-to-ground lightning detection) and a Lightning Mapping Array (total lightning detection) that was installed in western Kansas–eastern Colorado. The assimilation method uses lightning as a proxy for the presence or absence of deep convection. During assimilation, lightning data are used to control the Kain–Fritsch (KF) convection parameterization scheme. The KF scheme can be forced to try to produce convection where lightning indicated storms, and, conversely, can optionally be prevented from producing spurious convection where no lightning was observed. Up to 1 g kg−1 of water vapor may be added to the boundary layer when the KF convection is too weak. The method does not employ any lightning–rainfall relationships, but rather allows the KF scheme to generate heating and cooling rates from its modeled convection. The method could therefore easily be used for real-time assimilation of any source of lightning observations. For the case study, the lightning assimilation was successful in generating cold pools that were present in the surface observations at initialization of the forecast. The resulting forecast showed considerably more skill than the control forecast, especially in the first few hours as convection was triggered by the propagation of the cold pool boundary.

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Daniel T. Dawson II
,
Edward R. Mansell
, and
Matthew R. Kumjian

Abstract

Several recent studies have implicated vertical wind shear in producing steady-state size sorting of a distribution of hydrometeors falling at their terminal velocity, which varies as a function of hydrometeor diameter. In particular, this mechanism has been invoked to explain both the strength and storm-relative orientation of the commonly observed differential reflectivity (Z DR) arc in supercell thunderstorms. However, the actual role of the shear has not been fully clarified. In this study, a simple analytical model is used to show that the fundamental source of size sorting is the storm-relative wind field itself and, in particular, its mean taken over the depth of the sorting layer. Wind shear is only strictly required for producing sustained size sorting in the special but common case of a precipitation source having a motion that lies on the hodograph (such as with the environmental winds at the source level). In supercells, the precipitation source (the rotating updraft) does not necessarily move with the winds at any level. It is shown that this off-hodograph propagation and the associated storm-relative mean wind is responsible for the positive correlation of size-sorting observables (such as Z DR) and storm-relative helicity that has been noted in previous work.

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Edward R. Mansell
,
Conrad L. Ziegler
, and
Eric C. Bruning

Abstract

Electrification and lightning are simulated for a small continental multicell storm. The results are consistent with observations and thus provide additional understanding of the charging processes and evolution of this storm. The first six observed lightning flashes were all negative cloud-to-ground (CG) flashes, after which intracloud (IC) flashes also occurred between middle and upper levels of the storm. The model simulation reproduces the basic evolution of lightning from low and middle levels to upper levels. The observed lightning indicated an initial charge structure of at least an inverted dipole (negative charge above positive). The simulations show that noninductive charge separation higher in the storm can enhance the main negative charge sufficiently to produce negative CG flashes before upper-level IC flashes commence. The result is a “bottom-heavy” tripole charge structure with midlevel negative charge and a lower positive charge region that is more significant than the upper positive region, in contrast to the traditional tripole structure that has a less significant lower positive charge region. Additionally, the occurrence of cloud-to-ground lightning is not necessarily a result of excess net charge carried by the storm, but it is primarily caused by the local potential imbalance between the lowest charge regions.

The two-moment microphysics scheme used for this study predicted mass mixing ratio and number concentration of cloud droplets, rain, ice crystals, snow, and graupel. Bulk particle density of graupel was also predicted, which allows a single category to represent a greater range of particle characteristics. (An additional hail category is available but was not needed for the present study.) The prediction of hydrometeor number concentration is particularly critical for charge separation at higher temperatures (−5° < T < −20°C) in the mixed phase region, where ice crystals are produced by rime fracturing (Hallett–Mossop process) and by splintering of freezing drops. Cloud droplet concentration prediction also affected the rates of inductive charge separation between graupel and droplets.

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Matthew D. Brothers
,
Eric C. Bruning
, and
Edward R. Mansell

Abstract

Large-eddy-resolving simulations using the Collaborative Model for Multiscale Atmospheric Simulation (COMMAS), which contains microphysical charging and branched-lightning parameterizations, produce much more complex net charge structures than conventionally visualized from previous observations, simulations, and conceptual diagrams. Many processes contribute to the hydrometeor charge budget within a thunderstorm, including advection, hydrometeor differential sedimentation, subgrid turbulent mixing and diffusion, ion drift, microphysical separation, and the attachment of ion charge deposited by the lightning channel. The lightning deposition, sedimentation, and noninductive charging tendencies contribute the most overall charge at relatively large scales, while the advection tendency, from resolved turbulence, provides the most “texture” at small scales to the net charge density near the updraft region of the storm. The scale separation increases for stronger storm simulations. In aggregate, lightning deposition and sedimentation resemble the smoother distribution of the electric potential, while evidence suggests individual flashes could be responding to the fine texture in the net charge. The clear scale separation between the advection and other net charge tendencies suggest the charge advection is most capable of providing net charge texture; however, a clear-cut causality is not obtained from this study.

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