Stratiform Precipitation Region Research Articles

Comment on “Partitioning tropical oceanic convective and stratiform rains by draft strength” by David Atlas et al.

[1] The crux of this comment is to clarify attribution. If someone looks at a published graph and has a different interpretation from that of the original authors, that new interpretation should not be attributed to the original authors. [2] The references given by Atlas et al. [2000] regarding distinct convective and stratiform reflectivity Z to rain rate R relations attributed to Yuter and Houze [1997] are incorrect. The convective and stratiform Z-R relations labeled as those of Yuter and Houze in Table 3 (incorrectly referred to there as 1996) and mentioned in the text on pp. 2262, 2264, and 2265 of Atlas et al. [2000] do not appear anywhere in the Yuter and Houze [1997] paper. The Yuter and Houze [1997] study used two-dimensional (2-D) particle probe drop spectra collected by the National Center for Atmospheric Research Electra aircraft during the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) to compute Z and R values over 6 s intervals of flight track ( 1400 L sample volumes). These data were classified into convective and stratiform subsets using radar data obtained by the National Oceanic and Atmospheric Administration WP-3D airborne radar. Segments of the Electra flight track were categorized by noting when the Electra flew within regions of convective and stratiform precipitation classified according to a radar reflectivity texture algorithm based on the paper by Steiner et al. [1995]. The main conclusion of the Yuter and Houze [1997] study was that the populations of radarclassified convective and stratiform Z-R points overlapped in dbZ-log R space and did not form two statistically distinct populations. [3] The previous statements do not rule out the possibility that different physical processes can yield different drop size distributions (DSD) [e.g., Braun and Houze, 1994, pp. 2749–2750]. However, in the large areas of radar-classified convective and stratiform precipitation examined in this study, the physical processes mixed, exhibited natural variation, and were subject to sampling error to such a degree that the Z and Rvalues associated with the DSD samples in the convective and stratiform populations did not form distinct populations. A classification method that can truly distinguish between dominant precipitation growth by vapor deposition versus accretion (riming and collection/coalescence) would have the property of yielding distinct populations in lowrain-rate regions of dbZ-log R space where both processes are known to occur. 2. Follow-up

Observations and Parameterizations of Particle Size Distributions in Deep Tropical Cirrus and Stratiform Precipitating Clouds: Results from In Situ Observations in TRMM Field Campaigns

In this study, we report on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled. Most of the spirals began at cloud top, with temperatures (T) as low as -50 C, and ended at cloud base or below the melting layer (ML). The PSDs broadened from cloud top towards cloud base, with the largest particles increasing in size from several millimeters at cloud top to one centimeter or larger towards cloud base. Some continued growth was noted in the upper part of the ML. Concentrations of particles less than 1 mm in size decreased with decreasing height. The result was a consistent change in the PSDs in the vertical. Similarly, systematic changes in the size dependence of the particle cross-sectional area was noted with decreasing height. Aggregation-as ascertained from both the changes in the PSDs and evolution of particle habits as observed in high detail with the cloud particle imager (CPI) probe-was responsible for these trends. The PSDs were generally well-represented by gamma distributions of the form N = N0 gamma D microns e- lambda gamma D that were fitted to the PSDs over 1-km horizontal intervals throughout the spirals. The intercept (N0 gamma), slope (lambda gamma), and dispersion (microns) values were derived for each PSD. Exponential curves (N = N0e- lambdaD; micron = 0) were also fitted to the distributions. The lambda gamma values for given spirals varied systematically with temperature as did the values of lambda (exponential), and the data generally conformed to values found in previous studies involving exponential fits to size distributions in mid-latitude frontal and cirrus layers. Considerable variability often noted in the PSD properties during the loops of individual spirals was manifested primarily in large changes in N0 gamma and N0, but micron, lambda gamma and lambda remained fairly stable. Temperature is not found to be the sole factor controlling lambda gamma or lambda but is a primary one. Direct relationships were found between lambda gamma and N0 gamma or lambda gamma and micron for the gamma distributions and lambda and N0 for the exponential. The latter relationship was not found as distinctly in earlier studies; observed PSDs in this study had better fidelity with less scatter. The micron values changed monotonically with T over the range of temperatures and were directly related to N0 gamma or lambda gamma, thereby reducing the number of variables in the PSD functional equation to two. In the upper part of the ML, N0 and lambda continued to decrease, and in the lower part these values began to increase as the largest particles melted. We developed general expressions relating various bulk microphysical, radar, and radiative transfer-related variables to N0 gamma and lambda gamma, useful for both tropical and mid-latitude clouds. These relationships facilitate the specification of a number of bulk properties in cloud and climate models. The results presented in this paper apply best to temperatures between 0 and -40 C, for which the measured radar reflectivities fall in the range of 0 to 25 dBZe.

Cloud-to-Ground Lightning in Linear Mesoscale Convective Systems

Recently, three distinct archetypes for midlatitude linear mesoscale convective systems (MCSs) have been identified: those comprising convective lines with trailing stratiform precipitation (TS), leading stratiform precipitation (LS), and parallel stratiform precipitation (PS). While cloud-to-ground (CG) lightning in TS MCSs has received a great deal of study in recent years, linear MCSs exhibiting leading or parallel stratiform precipitation have received comparatively little attention. This paper documents the arrangement and frequency of CG lightning for four LS and four PS MCSs that occurred during May 1996 and May 1997. On average, the LS cases bore some similarity to previously studied TS MCSs; they produced predominantly positive CG lightning (1CG) early in their lifetimes (concomitant with convective precipitation) and late in their lifetimes (concomitant with stratiform precipitation). In contrast with previously published results for MCSs in general, the LS MCSs in the present study overall had relatively low negative CG (2CG) flash densities, yielding higher percentages of 1CGs than other MCS archetypes. While the PS systems had higher average flash densities than the LS systems, they did not exhibit prolonged 1CG modes early in their lifetimes, and they produced a lower overall fraction of 1CGs. Notably, the presence of line-parallel flow may render a somewhat unique electrical character in PS systems. In particular, the PS MCSs in this study exhibited enhanced CG lightning in the vicinity of decaying convective cells that were embedded within their stratiform precipitation regions.

Reply [to “Comment on ‘‘Spider’ lightning in intracloud and positive cloud‐to‐ground flashes’ by Vladislav Mazur, Xuan‐Min Shao, and Paul R. Krehbiel”

Journal of Geophysical Research: AtmospheresVolume 105, Issue D6 p. 7401-7402 CommentariesFree Access Reply [to “Comment on ‘‘Spider’ lightning in intracloud and positive cloud-to-ground flashes’ by Vladislav Mazur, Xuan-Min Shao, and Paul R. Krehbiel”] Vladislav Mazur, Vladislav MazurSearch for more papers by this author Vladislav Mazur, Vladislav MazurSearch for more papers by this author First published: 01 March 2000 https://doi.org/10.1029/2000JD900021Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Boccippio, D. J., E. R. Williams, S. J. Heckman, W. A. Lyons, I. T. Baker, R. Boldi, Sprites, ELF transients, and positive ground strokes, Science, 269, 1088– 1091, 1995. Engholm, C. D., E. R. Williams, R. M. Dole, Meteorological and electrical conditions associated with positive cloud-to-ground lightning, Mon. Weather Rev., 118, 470– 487, 1990. Marshall, T. C., Comment on “'Spider' lightning in intracloud and positive cloud-to-ground flashes' by V. Mazur, X.-M. Shao, and P. R. Krehbiel,J. Geophys. Res., 105(D6), 2000. Marshall, T. C., W. D. Rust, Two types of vertical electrical structures in stratiform precipitation regions of mesoscale convective systems, Bull. Am. Meteorol. Soc., 74, 2159– 2170, 1993. Marshall, T. C., W. D. Rust, W. P. Winn, K. E. Gilbert, The electrical structure in two thunderstorms anvil clouds, J. Geophys. Res., 94, 2171– 2181, 1989. Mazur, V., X.-M. Shao, P. R. Krehbiel, ”Spider“ lightning in intracloud and positive cloud-to-ground flashes, J. Geophys. Res., 10319, 811– 19822, 1998. Rutledge, S. A., E. R. Williams, W. A. Petersen, Lightning and electrical structure of mesoscale convective systems, Atmos. Res., 29, 27– 53, 1993. Schuur, T. J., B. F. Smull, W. D. Rust, T. C. Marshall, Electrical and kinematic structure of the stratiform precipitation region trailing an Oklahoma squall line, J. Atmos. Sci., 45, 825– 841, 1991. Citing Literature Volume105, IssueD627 March 2000Pages 7401-7402 ReferencesRelatedInformation

Sistema convectivo de mesoescala observado no dia 12 de dezembro de 1992 durante TOGA-COARE: Estrutura cinemática e transporte de quantidade de movimento

No dia 12/12/92, durante o experimento de campo TOGA-COARE (Tropical Ocean Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment) uma missão foi efetuada, detectando uma região de convecção a NE da zona chamada de "Intensive Flux Array " (IFA: 1ºN-4ºS, 151-158ºE). Essa região caracterizava-se por apresentar uma circulação ciclônica nos baixos níveis e um fluxo divergente de NE em altitude, caracterizando a atividade convectiva do sistema. A análise das imagens no canal infravermelho do satélite GMS-4 indicava uma discreta propagação do sistema em direção a sudoeste. Os dois aviões WP-3D da "National Oceanic and Atmospheric Administration" (NOAA), utilizando a técnica de varredura FAST (Fore/Aft Scanning Technique), descreveram trajetórias paralelas coordenadas, primeiramente ao longo da linha convectiva e em seguida dentro da região estratiforme do sistema. Os dados de radares Doppler coletados durante essa missão mostravam a existência de células convectivas moderadas, localizadas dentro de uma área de precipitação estratiforme, e células convectivas mais intensas localizadas a sudoeste do sistema. A combinação das porções paralelas das trajetórias de cada avião foi utilizada para restituir o campo de vento 3D, usando a técnica variacional EODD (Extended Overdetermined Dual-Doppler). O campo de vento 3D, derivado dos dados de radar, serviu para estudar as características do sistema observado e para examinar a estrutura dos transportes de quantidade de movimento operado pelo sistema. A estrutura cinemática do sistema mostrou características clássicas, com um fluxo ascendente na frente do sistema que transporta o ar das camadas inferiores para o alto e um fluxo descendente atrás da linha convectiva. Os resultados para os transportes de quantidade de movimento também mostraram características similares a outros casos estudados anteriormente, tais como: (i) o transporte vertical de quantidade de movimento horizontal perpendicular à linha convectiva é amplificador do gradiente, resultando no aumento do seu cisalhamento vertical; (ii) para a componente paralela (v), a divergência vertical do fluxo v reduziu o seu cisalhamento vertical, gerando um transporte redutor do gradiente.

A Mesovortex within a Near-Equatorial Mesoscale Convective System during TOGA COARE

Abstract Airborne Doppler radar observations are used to investigate the internal structure of a midlevel mesovortex that developed within the rear part of the stratiform precipitation region of a mature-to-decaying mesoscale convective system. This system, composed of several convective elements, occurred on 13 December 1992, on the eastern side of the intensive flux array of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, near the equator (2°S). The mesovortex contained several features that were previously observed in other tropical and midlatitude systems. Radar observations show that the closed wind circulation at midlevels had a horizontal dimension of ∼150 km and was coincident with a marked rear inflow at the central rear of the stratiform region where a notch pattern clearly identified the rear dry air intrusion. It was associated with positive vertical vorticity (anticyclonic for the Southern Hemisphere) and could be classified as a cooling-induced vortex. Maxi...

Kinematic characteristics of air flowing into and out of precipitating convection over the west Pacific warm pool: An airborne Doppler radar survey

AbstractAir flowing into and out of 25 cloud systems over the west Pacific warm pool was sampled by Doppler radars on board two NOAA WP‐3D aircraft and the NCAR Electra aircraft during the Tropical Ocean/Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE). In convective cells, updraught inflows and downdraught outflows tended to be positioned adjacent to each other at sharp updraught‐downdraught interfaces sloping at various angles and directions. The updraught inflows were sometimes shallow and sometimes deep, often extending well above the boundary layer. Downdraught outflow magnitudes were usually stronger than updraught inflows, and the horizontal directions of the inflow and outflow were frequently offset from each other at angles of ˜|90°|.In stratiform precipitation regions, the dominant airflow features were descending mid‐level inflows, located at the bases of anvils and often extending into the interior of the precipitation system. These stratiform inflows originated at levels between 5 and 10 km and descended to about 3 km. A few extended down to the surface. Stratiform inflows were typically strong and sandwiched between weaker stratiform outflows at upper and lower altitudes. The stratiform‐region inflow and outflow directions were frequently offset by ˜|90°| in the horizontal. The mid‐level inflow currents entered the stratiform regions horizontally from a direction similar to that of the large‐scale ambient wind.The mid‐level stratiform inflow tended to occur in the same vertical plane as the convective downdraught outflow, although the mid‐level stratiform inflow appeared to be slowed by horizontal pressure gradients before connecting with the convective downdraught outflow. In other respects, the mesoscale systems differed from a simplified two‐dimensional model of airflow. The systems moved primarily by discrete propagation, and updraughts and downdraughts tended to have orthogonal horizontal relative airflow.

Analysis of a Small, Vigorous Mesoscale Convective System in a Low-Shear Environment. Part II: Evolution of the Stratiform Precipitation and Mesoscale Flows

Abstract The evolution of the mesoscale flow and precipitation distribution are investigated for a small mesoscale convective system (MCS) that evolved in a nearly barotropic environment exhibiting moderate instability and weak wind shear. Observations primarily from a single Doppler radar detail the growth of the MCS from the merger of several clusters and lines of vigorous convective cells into a mature state consisting of a weaker convective line trailed by an expanding stratiform precipitation region. Analysis of radar reflectivity reveals that this stratiform region formed in situ in the presence of weak mesoscale updraft as decaying convective cores coalesced, rather than through rearward advection of ice particles directly from the convective region. In the absence of sufficient low-level shear, the MCS collapsed rapidly as it assumed the structure of the archetypal convective line and trailing stratiform precipitation region. Velocity–azimuth displays reveal mesoscale updrafts of about 70 cm s−1 d...

A Numerical Study of the Along-Line Variability of a Frontal Squall Line during PRE-STORM

Despite considerable progress in the understanding of two-dimensional structures of squall lines, little attention has been paid to the along-line variability of these convective systems. In this study, the roles of meso- and larger-scale circulations in the generation of along-line variability of squall lines are investigated, using an 18-h prediction of a frontal squall line that occurred on 26–27 June 1985 during PRE-STORM (Preliminary Regional Experiment for Stormscale Operational Research Meteorology). It is shown that the Canadian regional finite-element (RFE) model reproduces reasonably well a number of surface and vertical circulation structures of the squall system, as verified against available network observations. These include the initiation, propagation, and dissipation of the squall system, surface pressure perturbations, and cold outflow boundaries; a midlevel mesolow and an upper-level mesohigh; a front-to-rear (FTR) ascending flow overlying an intense rear-to-front (RTF) flow; and a leading convective line followed by stratiform precipitation regions. It is found that across-line circulations at the northern segment of the squall line differ significantly from those at its southern segment, including the different types of precipitation, the absence of the RTF flow and midlevel mesolow, and the early dissipation of organized convection in the northern part. The along-line variability of the squall’s circulations results primarily from the interaction of convectively generated perturbations with a midlevel baroclinic trough. The large-scale trough provides an extensive RTF flow component in the southern portion of the squall system and an FTR flow component in the north, whereas the midlevel mesolow tends to enhance the RTF flow to the south and the FTR flow to the north of the mesolow during the mature to decaying stages. The along-line variability of the squall’s circulations appears to be partly responsible for the generation of different weather conditions along the line, such as the development of an upper-level stratiform region in the southern segment and a midlevel cloud region in the northern portion of the squall line.

The Genesis of Hurricane Guillermo: TEXMEX Analyses and a Modeling Study

The transformation of a mesoscale convective system into Hurricane Guillermo was captured by aircraft and Doppler wind data during the Tropical Experiment in Mexico. The early phase of the system evolves in a way very similar to previously documented mesoscale convective systems, with a midlevel mesocyclone developing in the stratiform precipitation region. More unusually, the cyclone extends to low altitudes: A weak cyclone is discernable even in the 300-m altitude wind field. After another day of evolution, a small, surface-based warm-core cyclone is observed to develop within the relatively cold air associated with the mesocyclone aloft. This mesocyclone develops into a hurricane over the subsequent day. A nonhydrostatic, axisymmetric numerical model is used to explore the evolution of the initially cold-core, midlevel vortex into a tropical cyclone. A mesoscale, midlevel “showerhead” is switched on in an initially quiescent, tropical atmosphere overlying a warm ocean surface. Evaporation of the falling rain cools the lower troposphere and leads to the spinup of a midlevel vortex, while divergent outflow develops near the surface. After some time, the midlevel vortex expands downward toward the boundary layer, and later a warm-core, surface-flux-driven cyclone develops within it. Experiments with the model show that both the cyclone, with its associated cold anomaly, and the relatively humid air associated with the evaporatively cooled mesoscale cyclone are important for the subsequent development of the warm-core system. The simulations also suggest that, for the warm-core development to occur, the stratiform rain must last long enough to drive the midlevel vortex down to the boundary layer. The authors present a simple argument for why this must be so and suggest that this process would be significantly impeded by the presence of background vertical wind shear.

Measurements of Raindrop Size Distributions over the Pacific Warm Pool and Implications forZ–RRelations

Raindrop images obtained on research flights of the NCAR Electra aircraft in the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) are analyzed. The drop size distributions, based on the images obtained in 6-s samples (about 750 m of flight track), are used to calculate both radar reflectivity Z and rain rate R. Airborne radar data from the NOAA P-3 aircraft flying in coordination with the Electra are used to categorize the particle-image data according to whether the drop images were obtained in regions of convective or stratiform precipitation. Within stratiform precipitation, the same rain rate could be produced by a drop spectrum dominated by numerous small drops (lower reflectivity) or by a few large drops (higher reflectivity). The reflectivity values varied by as much as 9 dB for a given rain rate. Reflectivity data from the airborne radar and flight-level data reveal that the stratiform regions often contain fallstreaks of about 0.1–2 km in horizontal dimension. The fallstreaks are associated with large-drop spectra and local maxima in reflectivity up to approximately 40 dBZ and in rain rates up to 25 mm h−1. The fallstreaks extend downward from the melting band and bend with the low-level wind shear, but do not usually reach the surface. Thus, although relatively more uniform than convective regions, stratiform regions can be variable in reflectivity and rain rate at fine spatial scales in both the horizontal and vertical directions. Stratiform regions are therefore best characterized by their ensemble properties rather than the values of individual high-resolution measurements. The variability of stratiform drop size spectra arises primarily from the occurrence of fallstreaks and the discontinuous nature of regions favoring aggregation of snow crystals, and it implies that Z–R distributions associated with convective and stratiform precipitation are not statistically distinct. Thus, separate Z–R relations for convective and stratiform precipitation are not justified, and techniques to distinguish between convective and stratiform precipitation based solely on the characteristics of drop size distributions are not likely to be accurate. The variability of the drop size spectra in tropical precipitation makes an exponential fit to the Z–R relation sensitive to the spatial scale over which Z and R are determined. This sensitivity can be avoided by using a probability-matched Z–R relation. The probability-matched Z–R relation for all the raindrop image data from the Electra collected between altitudes of 2.7 and 3.3 km in TOGA COARE is similar to the Z–R relation obtained at the sea surface in the Global Atmospheric Research Program Atlantic Tropical Experiment.

Momentum Budget of a Squall Line with Trailing Stratiform Precipitation: Calculations with a High-Resolution Numerical Model

Abstract In this paper, the authors investigate the momentum budget of a squall line with trailing stratiform precipitation by examining how the momentum balance varies with respect to the storm's internal structure. In particular, the authors determine differences between the momentum budgets of the convective and stratiform precipitation regions, which are physically distinct parts of the storm. The results from a high-resolution nonhydrostatic numerical simulation of the two-dimensional segment of the 10–11 June 1985 PRE-STORM squall line are used. The momentum equation is averaged over a 300-km-wide large-scale area for time periods of 1 h. On the 1-h timescale, the convective-scale temporal variations of horizontal and vertical velocities are nearly uncorrelated, and thus their contribution to the momentum flux is negligible. The remaining standing-eddy and mean-flow circulations account for the momentum flux on this timescale. The combination of the standing eddy and mean flow behave almost exactly ...

Sprite observations above the U.S. High Plains in relation to their parent thunderstorm systems

Transient luminous events (sprites, blue jets, elves) above large mesoscale convective systems (MCSs) over the U.S. High Plains have been routinely monitored from the Yucca Ridge Field Station near Fort Collins, Colorado using ground‐based low‐light video systems. We analyzed 36 sprites above the Nebraska MCS of August 6, 1994. The results lend further support to the hypothesis that sprites are almost uniquely associated with positive cloud‐to‐ground (+CG) lightning flashes. Sprite‐associated +CGs also averaged substantially larger peak currents than the remaining +CG population (81 kA versus 30 kA in this storm system). There is some evidence that sprite‐associated +CGs also have higher stroke multiplicity. This study yields no evidence of sprites associated with negative CG events. In the central United States an additional requirement appears to be that the parent MCS has a contiguous radar reflectivity area exceeding 20–25,000 km2. The majority of the sprites occur above the large stratiform precipitation region and not the high‐reflectivity convective core of the MCS. Triangulation of a limited number of paired images (from September 7, 1994) suggests that the sprite is generally centered within 50 km of the parent +CG. Assuming the +CG provides the range, single‐image photogrammetric analyses provide estimates of the maximum vertical extent of the sprites. For this storm the sprite tops averaged 77 km with a maximum of 88 km. The bases averaged 50 km but with a few sprite tendrils extending as low as 31 km.

Comments on “The Impact of the Ice Phase and Radiation on a Midlatitude Squall Line System”

Abstract This comment addresses two conclusions arising from a modeling study of Chin: that the contribution of mesoscale stratiform areas to large-scale heat and moisture budgets at midlatitudes is small compared to that associated with deep convection and that longwave radiative processes are the cause of the transition zone. A review of the literature and a comparison to another simulated squall line reaffirm the long-standing result that mesoscale stratiform precipitation regions often contribute significantly to the large-scale heating and moistening and demonstrate that longwave radiation is just one of the many factors that modify the kinematic and micro-physical processes that form the transition zone.

A squall-like narrow cold-frontal rainband diagnosed by combined thermodynamic and cloud microphysical retrieval

Doppler radar, airborne, and sounding data are used to describe the structure of an exceptionally vigorous narrow cold-frontal rainband (NCFR). A combined thermodynamic and cloud microphysical retrieval technique is used to derive the pressure perturbations and buoyancy in balance with the flow field in the vicinity of the NCFR. The NCFR was triggered by a well-defined cold front that penetrated beneath a deep layer of conditionally unstable prefrontal air. The rainband, which persisted for ∼ 6 h, was located over the Gulf Stream, just off the mid-Atlantic Coast of the U.S.A. It consisted of a series of short-lived cells with updraft maxima between 5 and 15 m s −1 and tops around 12 km. The updraft associated with the NCFR was driven primarily by buoyancy in the middle troposphere, which explains its convective nature. Trailing behind the NCFR was a broad region of stratiform precipitation. Pressure perturbations in the NCFR and the trailing stratiform region were produced mainly by a combination of frontal convergence and buoyancy opposition. The pressure perturbations surrounding the cells within the rainband were due primarily to strong updrafts in the presence of wind shear parallel to the rainband. The same dynamic and cloud microphysical processes that operate in stratiform regions behind some squall lines were present in this NCFR. However, in contrast to a squall line with a trailing stratiform region, the low-level cold pool that penetrated beneath the NCFR was maintained primarily by strong cold advection, rather than by evaporational cooling in a shallow rear-to-front flow. The preponderance of advective over diabatic cooling can be used as a criterion to diagnostically differentiate between NCFRs and squall lines.

Melting and freezing in a mesoscale convective system

AbstractMicrophysical and thermodynamic fields retrieved from dual‐Doppler synthesized winds are used to examine the distribution of melting and freezing rates within a midlatitude squall line with trailing stratiform precipitation. Previously, the effect of the heating/cooling processes associated with the release/consumption of the latent heat of fusion have emphasized the melting layer in the stratiform precipitation region, where the layer is typically marked by a radar bright band. The retrieved fields reveal the melting and freezing in the convective as well as stratiform region. Two‐dimensional retrieved fields indicate that the strong melting effect is not confined to the stratiform region, but extends across the entire breadth of the storm. The peak rates of cooling in the stratiform precipitation region are −2 to −3 K h−1, in general agreement with previous studies. In contrast, cooling rates in the convective precipitation region reach −14 K h−1. The heating by freezing is concentrated in a vertical column associated with the convective updraughts, with peak heating rates of 8 K h−1 just above the 0 °C level. For the mature stage of the storm, the convective region is found to account for 56% of the total cooling by melting, and 87% of the total heating by freezing within the squall line.A one‐dimensional retrieval model is applied to mean vertical‐velocity profiles to retrieve area‐mean profiles of the melting and freezing rates. In the convective region, average cooling rates of −10 K h−1 are found. The strong cooling by melting is expected to exert a strong influence on the development of convective downdraughts, the cold pool, and the gust front. The heating by freezing occurs through a deep layer, with a peak average heating rate of about 4 K h−1 just above the 0 °C level. In the stratiform precipitation region, a melting rate of about −2 K h−1 produced a near 0 °C isothermal layer approximately 500 m deep, similar to isothermal layers observed in stratiform precipitation areas associated with fronts.

Diagnosis of hydrometeor profiles from area‐mean vertical‐velocity data

AbstractA simple one‐dimensional microphysical retrieval model is developed for estimating vertical profiles of liquid and frozen hydrometeor mixing ratios from observed vertical profiles of area‐mean vertical velocity in regions of convective and/or stratiform precipitation. The mean vertical‐velocity profiles can be obtained from Doppler radar (single and dual) or other means. The one‐dimensional results are shown to be in good agreement with two‐dimensional microphysical fields from a previous study. Sensitivity tests are performed.

Synthetic Dual-Doppler Analysis of Mesoscale Convective Systems

The synthetic dual-Doppler (SDD) analysis technique is applied to the 6–7 May 1985 mesoscale convective system (MCS) that occurred during the Oklahoma-Kansas Preliminary Regional Experiment-STORM Central. This system had a cyclonic mesoscale circulation in its stratiform precipitation region. The SDD analyses are compared to corresponding actual dual-Doppler analyses. The sensitivity of the former to various parameters is discussed. The SDD analysis technique is also applied to an MCS that passed by the NEXRAD (Next Generation Weather Radar) facility in Norman, Oklahoma, on 13 June 1989. Analyses of both the leading line of convective cells and the trailing stratiform precipitation area we presented; the salient features were similar to those found in dual-Doppler analyses of other systems. There was a mesoscale cyclonic circulation present at low and midlevels in the stratiform precipitation area. Vertical wind profiles obtained from the SDD analysis are compared to those obtained front velocity-azimuth display (VAD) analyses and from a conventional sounding. Vertical profiles of divergence and vertical velocity that were determined from VAD analyses indicated upward motion ahead of the convective line, and sinking motion and rising motion in the lower and upper troposphere, respectively, in the stratiform precipitation area behind the leading line. Rising motion and sinking motion in the lower and upper troposphere, respectively, were indicated in the vicinity of the mesoscale cyclonic circulation.

Doppler Radar Analysis of Vortices over the Mountains of Taiwan

Abstract Small-scale cyclonic vortices, which were embedded within a larger, mesoscale area of cyclonically curved flow, are documented over the mountains of Taiwan during the Taiwan Area Mesoscale Experiment. The vortices are visualized in dual-Doppler radar analyses of the wind field in the region of stratiform precipitation of a decaying mesoscale convective system. Each vortex was approximately 20 km across and had a lifetime on the order of an hour. The vortices formed and decayed during a 4-h period over Taiwan's Central Mountain Range and propagated with the flow associated with the mesoscale area of cyclonically curved flow in which they were embedded. The vortices extended through a deep layer in the troposphere, but were strongest near the mountaintop level, and decayed in intensity with height. Circumstantial evidence is presented that the vortices may have formed through the interaction of the mesoscale area of cyclonic flow with a mountain ridge.

Two Types of Vertical Electrical Structures in Stratiform Precipitation Regions of Mesoscale Convective Systems

Electric field (E) soundings in the stratiform regions and transition zones of mesoscale convective systems (MCSs) are reported. Most of the E soundings were made during the 1991 Cooperative Oklahoma Profiler Studies (COPS-91). Multiple E soundings were made in several MCSs. All of the E soundings collected here can be grouped into one of two types. Within each type the soundings and the inferred charge structures are remarkably similar from one place in an MCS to another and from one MCS to another. The charge regions inferred from the E soundings are hundreds of meters thick and have charge densities up to 5.3 nC m−3. Typically, the maximum E in the soundings is about 100 kV m−1 Here, E soundings from three classes of MCSs are discussed. The bow-echo MCSs have simpler vertical charge structures with four main charge regions, while squall-line MCSs and predominantly stratiform MCSs have five main charge regions. In all of the E soundings there is a substantial region of charge and a large E at or near 0°C.