Nucleation Processes in Deep Convection Simulated by a Cloud-System-Resolving Model with Double-Moment Bulk Microphysics

2007 ◽  
Vol 64 (3) ◽  
pp. 738-761 ◽  
Author(s):  
Vaughan T. J. Phillips ◽  
Leo J. Donner ◽  
Stephen T. Garner
Keyword(s):  
Deep Convection ◽  
Phase Region ◽  
Mixed Phase ◽  
Cloud Droplets ◽  
Ice Concentration ◽  
Double Moment ◽  
Preferential Evaporation ◽  
Cloud Ice ◽  
Homogeneous Freezing ◽  
Mixed Phase Region

Abstract A novel type of limited double-moment scheme for bulk microphysics is presented here for cloud-system-resolving models (CSRMs). It predicts the average size of cloud droplets and crystals, which is important for representing the radiative impact of clouds on the climate system. In this new scheme, there are interactive components for ice nuclei (IN) and cloud condensation nuclei (CCN). For cloud ice, the processes of primary ice nucleation, Hallett–Mossop (HM) multiplication of ice particles (secondary ice production), and homogeneous freezing of aerosols and droplets provide the source of ice number. The preferential evaporation of smaller droplets during homogeneous freezing of cloud liquid is represented for the first time. Primary and secondary (i.e., in cloud) droplet nucleation are also represented, by predicting the supersaturation as a function of the vertical velocity and local properties of cloud liquid. A linearized scheme predicts the supersaturation, explicitly predicting rates of condensation and vapor deposition onto liquid (cloud liquid, rain) and ice (cloud ice, snow, graupel) species. The predicted supersaturation becomes the input for most nucleation processes, including homogeneous aerosol freezing and secondary droplet activation. Comparison of the scheme with available aircraft and satellite data is performed for two cases of deep convection over the tropical western Pacific Ocean. Sensitivity tests are performed with respect to a range of nucleation processes. The HM process of ice particle multiplication has an important impact on the domain-wide ice concentration in the lower half of the mixed-phase region, especially when a lack of upper-level cirrus suppresses homogeneous freezing. Homogeneous freezing of droplets and, especially, aerosols is found to be the key control on number and sizes of cloud particles in the simulated cloud ensemble. Preferential evaporation of smaller droplets during homogeneous freezing produces a major impact on ice concentrations aloft. Aerosols originating from the remote free troposphere become activated in deep convective updrafts and produce most of the supercooled cloud droplets that freeze homogeneously aloft. Homogeneous aerosol freezing is found to occur only in widespread regions of weak ascent while homogeneous droplet freezing is restricted to deep convective updrafts. This means that homogeneous aerosol freezing can produce many more crystals than homogeneous droplet freezing, if conditions in the upper troposphere are favorable. These competing mechanisms of homogeneous freezing determine the overall response of the ice concentration to environmental CCN concentrations in the simulated cloud ensemble. The corresponding sensitivity with respect to environmental IN concentrations is much lower. Nevertheless, when extremely high concentrations of IN are applied, that are typical for plumes of desert dust, the supercooled cloud liquid is completely eliminated in the upper half of the mixed phase region. This shuts down the process of homogeneous droplet freezing.

scholarly journals Spin excitations in [La[sub 1−x]Ca[sub x]MnO[sub 3]] in the mixed-phase region

2002 ◽  
Vol 91 (10) ◽  
pp. 7511 ◽  
Author(s):  
L. Stumpe ◽  
B. Kirby ◽  
H. Kaiser ◽  
J. J. Rhyne ◽  
J. F. Mitchell
Keyword(s):  
Phase Region ◽  
Mixed Phase ◽  
Spin Excitations ◽  
Mixed Phase Region

Phase diagrams for impurity-doped VO2

1976 ◽  
Vol 54 (1) ◽  
pp. 1-8 ◽  
Author(s):  
J. M. Reyes ◽  
S. L. Segel ◽  
M. Sayer
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Phase Diagrams ◽  
Phase Region ◽  
Mixed Phase ◽  
Phase Boundaries ◽  
Temperature Changes ◽  
Impurity Doped ◽  
T Phase ◽  
Mixed Phase Region

The general form of the phase diagrams for impurity-doped VO2 is considered and shown to include mixed phase boundary regions. Phase diagrams for Cr and Al-doped VO2 are established on the basis of the temperature dependence of the 51V nuclear magnetic resonance (NMR) signal. In Cr-doped VO2, the (M2) phase is shown to exist over a smaller temperature–concentration range than reported previously and in Al-doped VO2, two intermediate single phases (M2) and (M3) are found. The (T) phase which has been previously suggested to be a definite phase having triclinic symmetry is shown to be a mixed phase region in which the contributions from the components change with temperature. Changes in NMR signal intensity at the phase boundaries show that half the V sites have paired spins in both the(M2) and (M3) phases.

scholarly journals Comparison between Low-Flash and Non-Lightning-Producing Convective Areas within a Mature Mesoscale Convective System

2011 ◽  
Vol 26 (4) ◽  
pp. 468-486 ◽  
Author(s):  
Jennifer L. Palucki ◽  
Michael I. Biggerstaff ◽  
Donald R. MacGorman ◽  
Terry Schuur
Keyword(s):  
Vertical Motion ◽  
Mesoscale Convective System ◽  
Phase Region ◽  
Life Cycles ◽  
Radar Reflectivity ◽  
Mixed Phase ◽  
Convective System ◽  
Lightning Flash ◽  
Mesoscale Convective ◽  
Mixed Phase Region

Abstract Two small multicellular convective areas within a larger mesoscale convective system that occurred on 20 June 2004 were examined to assess vertical motion, radar reflectivity, and dual-polarimetric signatures between flash and non-flash-producing convection. Both of the convective areas had similar life cycles and general structures. Yet, one case produced two flashes, one of which may have been a cloud-to-ground flash, while the other convective area produced no flashes. The non-lightning-producing case had a higher peak reflectivity up to 6 km. Hence, if a reflectivity-based threshold were used as a precursor to lightning, it would have yielded misleading results. The peak upward motion in the mixed-phase region for both cases was 8 m s−1 or less. However, the lightning-producing storm contained a wider region where the updraft exceeded 5 m s−1. Consistent with the broader updraft region, the lightning-producing case exhibited a distinct graupel signature over a broader region than the non-lightning-producing convection. Slight differences in vertical velocity affected the quantity of graupel present in the mixed-phase region, thereby providing the subtle differences in polarimetric signatures that were associated with lightning activity. If the results here are generally applicable, then graupel volume may be a better precursor to a lightning flash than radar reflectivity. With the dual-polarimetric upgrade to the national observing radar network, it should be possible to better distinguish between lightning- and non-lightning-producing areas in weak convective systems that pose a potential safety hazard to the public.

scholarly journals Cloud-to-Ground Lightning Production in Strongly Forced, Low-Instability Convective Lines Associated with Damaging Wind

2005 ◽  
Vol 20 (4) ◽  
pp. 517-530 ◽  
Author(s):  
Matthew S. van den Broeke ◽  
David M. Schultz ◽  
Robert H. Johns ◽  
Jeffry S. Evans ◽  
John E. Hales
Keyword(s):  
Phase Region ◽  
Mixed Phase ◽  
Second Phase ◽  
Eastern United States ◽  
Southern Plains ◽  
Cloud To Ground Lightning ◽  
Strong Surface ◽  
Lifting Condensation Level ◽  
Mixed Phase Region ◽  
Cg Lightning

Abstract During 9–11 November 1998 and 9–10 March 2002, two similar convective lines moved across the central and eastern United States. Both convective lines initiated over the southern plains along strong surface-based cold fronts in moderately unstable environments. Both lines were initially associated with cloud-to-ground (CG) lightning, as detected by the National Lightning Detection Network, and both events met the criteria to be classified as derechos, producing swaths of widespread damaging wind. After moving into areas of marginal, if any, instability over the upper Midwest, CG lightning production ceased or nearly ceased, although the damaging winds continued. The 9 March 2002 line experienced a second phase of frequent CG lightning farther east over the mid-Atlantic states. Analysis of these two events shows that the production of CG lightning was sensitive to the occurrence and vertical distribution of instability. Periods with frequent CG lightning were associated with sufficient instability within the lower mixed-phase region of the cloud (i.e., the temperature range approximately between −10° and −20°C), a lifting condensation level warmer than −10°C, and an equilibrium level colder than −20°C. Periods with little or no CG lightning possessed limited, if any, instability in the lower mixed-phase region. The current Storm Prediction Center guidelines for forecasting these convective lines are presented.

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