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Steven A. Rutledge

Abstract

A three-dimensional kinematic cloud model has been used to study the precipitation processes within an intense, narrow cold-frontal rainband (NCFR). A triple-Doppler radar analysis has provided the necessary kinematic flow field. The leading edge of the advancing cold air was viewed as a density current, which contained a well-defined and intense rotor circulation. Observed and predicted local precipitation rates were in excess of 200 mm h−1. The model indicated that heavy precipitation formed through riming, associated with the development of graupel. Coalescence growth at temperatures above 0°C was also important. A parameterization of the Hallett-Mossop ice multiplication process was included in the model. Copious amounts of small ice crystals were produced by this mechanism, but the model results were insensitive to their presence. The rather high temperatures associated with the region splinters formed (−3°to −8°C), and the circulation pattern, prevented their growth to hydrometeor sizes.

The model output was used to diagnose the two-dimensional frontogenesis equation for the cross-front potential temperature gradient. Diabatic processes were found to be important to the maintenance of the cross-front temperature gradient despite strong frontolysis associated with tilting. Heating associated with condensation immediately ahead of the density current and cooling from evaporation immediately behind were found to be important in maintaining the density contrast across the front, and therefore the rainband itself. Subsidence warming in the descending branch of the rotor effectively displaced the cold air to a position behind the wind shift line. This particular distribution of diabatic heating processes, including melting, is considered essential to the maintenance of the intense circulations pattern in this NCFR when viewed in light of the recent theoretical studies discussed by Moncrieff.

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Steven A. Rutledge

Abstract

Wind and thermodynamic data analyzed in the startiform region of a tropical squall line have been combined with a kinematic, three-dimensional cloud model to study the precipitation processes in this region. The flux of condensate into the stratiform region from the convective region has been parameterized by specifying vertical profiles of cloud water, cloud ice, snow and graupel at the boundary between these two regions, through the use of a one-dimensional time-dependent cumulus model.

A standard case, in which all four forms of condensate stream into the stratiform (anvil) is studied in detail. The graupel entering this region rapidly removes snow advected into the anvil from the cells as well as snow produced by the mesoscale updraft. The snow produced by the mesoscale updraft actively contributes to this precipitation by providing mass for the graupel particles to feed upon. A series of sensitivity studies are discussed which reveal two distinct regions: a precipitation region due to fallout from the convective cells followed by a horizontally homogeneous region where the precipitation is produced by the mesoscale updraft. The anvil region is nearly entirely below water saturation which eliminates mixed-phase growth processes in this region and hence allows ice particles to grow only by deposition and collection. We conclude that the condensate produced by the mesoscale updraft is an important source of precipitation and is largely responsible for the extensive region of stratiform precipitation to the rear of the convective line.

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Steven A. Rutledge
and
Walter A. Petersen

Abstract

This study presents further evidence in support of an in situ, noninductive charging mechanism as the process likely responsible for significant electrification of the trailing stratiform regions of mesoscale convective systems (MCSs). In contrast to previous studies of MCS electrification that have investigated observations of radar reflectivity and cloud-to-ground lightning in the horizontal (e.g., Orville et al.; Rutledge et al.), here the relationship between the location and occurrence of cloud-to-ground lightning in the stratiform regions of midlatitude and tropical MCSs and the vertical profile of radar reflectivity are examined. The vertical profile of radar reflectivity at elevations above the 0°C level is used as a proxy for the amount of mass present in the mixed-phase region of the stratiform clouds, which in turn is related to the generation of charge through a noninductive charging mechanism.

To further explore the relationship between radar reflectivity, mixed-phase microphysics, and in situ charging by means of a noninductive mechanism, we present calculations with a simple one-dimensional model used to diagnose the presence of supercooled liquid water between the 0° and −20°C levels in the stratiform region. We use the model to contrast two cases: 1) a case in which reflectivities greater than 15 dBZ existed above the 0°C level in the stratiform clouds, cloud-to-ground lightning occurred, and moderate amounts of supercooled liquid water were present in the stratiform region (as inferred from the model results); 2) a case where no lightning was observed in the stratiform region, reflectivities above the 0°C level were less than 15 dBZ, and very little supercooled water was present (as inferred from the model results). Based on observations in several MCSs, we show that the number of cloud-to-ground lightning flashes in the stratiform region is highly correlated with the vertical radar reflectivity profile.

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Bard A. Zajac
and
Steven A. Rutledge

Abstract

The spatial and temporal distributions of cloud-to-ground lightning are examined over the contiguous United States from 1995 to 1999 using data from the National Lightning Detection Network. Annual flash density, annual lightning days, cumulative frequency distributions of daily flash counts, and annual and summertime diurnal distributions of lightning are documented. The spatial, annual, and summertime diurnal distributions of positive and negative polarity cloud-to-ground lightning are also documented. Over the same five-year period, the production of positive and negative lightning is examined over two case study areas located in the north-central United States and along the Gulf Coast, centered on Sioux Falls, South Dakota, and Fort Rucker, Alabama, respectively. Case studies include radar–lightning analyses of significant lightning episodes from 1996.

Maximum flash densities and lightning days are found over coastal regions of the southeastern United States. Other prominent maxima are seen over parts of the southern Rocky Mountains and adjacent High Plains. Cumulative frequency distributions indicate that throughout the contiguous United States roughly 10% of the days with lightning accounted for 50% of lightning production. The majority of lightning was produced during summer (June–August) throughout the contiguous United States, except over the south-central United States and along and near the Pacific coast. Summertime lightning activity over the western and eastern United States exhibited a diurnal cycle with maximum frequencies in the afternoon to early evening. Over the central United States, summertime lightning activity was complex with significant longitudinal variations in daily activity and a tendency to occur at night.

Over most of the contiguous United States, a larger fraction of negative lightning was produced during summer than positive lightning, and the diurnal cycle of positive lightning lagged the diurnal cycle of negative lightning by up to two hours during summer. The main exception to these behaviors occurred over an area in the north-central United States extending from the Colorado–Kansas border to western Minnesota. Over this area, positive lightning peaked during midsummer versus late summer for negative lightning, and the diurnal cycle of positive lightning also peaked up to several hours prior to the maximum in the diurnal cycle of negative lightning during summer. In addition, this area was characterized by maxima in the percentage of positive lightning and positive mean peak current. The maximum in the percentage of positive lightning over the north-central United States was caused by a dramatic increase in positive flash density to the east of the Rocky Mountains and a local minimum in negative flash density over the area described above.

Results from the Sioux Falls case study indicate that positive lightning was produced primarily during summer in the hours around sunset by isolated storms and convective lines in various stages of mesoscale convective system (MCS) development. These convective events usually contained one or more storms that were characterized by predominantly positive lightning, high positive flash rate, and large positive peak currents. Negative lightning activity was produced later in the summer and throughout the night by more mature convective systems arranged in lines or clusters. Over Fort Rucker, positive and negative lightning was produced throughout the year, by diurnally forced storms during the warm season and by MCSs with areally extensive stratiform regions during the cold season. Diurnally forced storms (MCSs) were characterized by a low (high) percentage of positive lightning.

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Brenda A. Dolan
and
Steven A. Rutledge

Abstract

Polarimetric Doppler radars provide valuable information about the kinematic and microphysical structure of storms. However, in-depth analysis using radar products, such as Doppler-derived wind vectors and hydrometeor identification, has been difficult to achieve in (near) real time, mainly because of the large volumes of data generated by these radars, lack of quick access to these data, and the challenge of applying quality-control measures in real time. This study focuses on modifying and automating several radar-analysis and quality-control algorithms currently used in postprocessing and merging the resulting data from several radars into an integrated analysis and display in (near) real time. Although the method was developed for a specific network of four Doppler radars: two Weather Surveillance Radar-1988 Doppler (WSR-88D) radars (KFTG and KCYS) and two Colorado State University (CSU) research radars [Pawnee and CSU–University of Chicago–Illinois State Water Survey (CSU–CHILL)], the software is easily adaptable to any radar platform or network of radars. The software includes code to synthesize radial velocities to obtain three-dimensional wind vectors and includes algorithms for automatic quality control of the raw polarimetric data, hydrometeor identification, and rainfall rate. The software was successfully tested during the summers of 2004 and 2005 at the CSU–CHILL radar facility, ingesting data from the four-radar network. The display software allows users the ability to view mosaics of reflectivity, wind vectors, and rain rates, to zoom in and out of radar features easily, to create vertical cross sections, to contour data, and to archive data in real time. Despite the lag time of approximately 10 min, the software proved invaluable for diagnosing areas of intense rainfall, hail, strong updrafts, and other features such as mesocyclones and convergence lines. A case study is presented to demonstrate the utility of the software.

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Charlotte A. DeMott
and
Steven A. Rutledge

Abstract

Radar data collected by the 5-cm MIT radar, which was deployed aboard the R/V Vickers during the intensive observing period of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, are partitioned into convective and stratiform Cartesian grid columns. The vertical structure of convective echo is examined through the use of two variables: echo top height and the height of the 30-dBZ reflectivity contour. The first of these variables has traditionally been used to describe the vertical structure characteristics of convection, and the second has recently been linked to internal microphysical properties and lightning.

Histograms of the relative frequency of convective-only echo top heights and 30-dBZ contour heights were constructed for the three cruises of the Vickers, with each cruise experiencing different phases of the intraseasonal oscillation (ISO). Cruise 1, which was dominated by the convectively “inactive” phase of the ISO was characterized by the highest frequency of shallow convection (based on echo top heights), whereas cruise 2, which was dominated by a particularly well-defined passage of the convective phase of the ISO, exhibited the tallest echo top heights. Cruise 3 convection was influenced by moderate westerly surface winds characteristic of postwesterly wind burst conditions, and convection was of intermediate height.

When viewed as a function of “internal” vertical structure (i.e., 30-dBZ contour height), the frequency distributions vary less from cruise to cruise, with cruises 1 and 2 having nearly identical distributions of convective 30-dBZ contour heights. Furthermore, when the contribution to convective rainfall is examined as a function of 30-dBZ contour height, it is seen that relatively more rain fell from vertically “intense” convection (i.e., convection with tall 30-dBZ contours) during cruises 1 and 3 than during cruise 2. Instantaneous correlations between rainfall rate and radar echo height were highly scattered about a mean value of about 0.55, whereas rainfall rate and 30-dBZ contour height correlations peaked at about 0.8 and exhibited much less scatter.

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Charlotte A. DeMott
and
Steven A. Rutledge

Abstract

The temporal variability of western Pacific warm pool convection, especially its vertical structure, is examined in this study. Distributions of convective echo top heights and 30-dBZ contour heights have been produced from shipboard radar data collected during Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Elevation and suppression of convective heights was primarily influenced by the phase of the intraseasonal oscillation (ISO), with heights being suppressed during convectively inactive and westerly wind burst (WWB) phases of the ISO. Echo top heights were greatest during the convective phases and post-WWB phases of the ISO. However, at least some very deep convection was always present within the area observed by radar, indicating that local conditions were favorable for deep convection, even when the large-scale environment was not capable of supporting widespread deep convection. In addition to the ISO, echo top and 30-dBZ contour heights were also influenced over shorter timescales by intrusions of dry subtropical air into the COARE Intensive Flux Array (IFA). Periods of convective suppression were also accompanied by upper-tropospheric drying.

Convective diabatic heating profiles, computed from a combination of surface radar and sounding data, reveal that the shape of the monthly mean heating profiles varied over the four-month intensive observing period. Maximum heating occurred at the highest elevations during the convectively inactive phases of the ISO, and at the lowest elevations during the convectively active phases of the ISO. These variations are qualitatively consistent with higher (lower) convective available potential energy values and higher (lower) 30-dBZ contour heights above the freezing level during the strong surface easterly (westerly) phase of the ISO. Factors leading to widespread convective suppression despite the presence of a high environmental CAPE are also discussed.

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Walter A. Petersen
and
Steven A. Rutledge

Abstract

Observation of the vertical profile of precipitation over the global Tropics is a key objective of the Tropical Rainfall Measuring Mission (TRMM) because this information is central to obtaining vertical profiles of latent heating. This study combines both TRMM precipitation radar (PR) and Lightning Imaging Sensor (LIS) data to examine “wet-season” vertical structures of tropical precipitation across a broad spectrum of locations in the global Tropics. TRMM-PR reflectivity data (2A25 algorithm) were utilized to produce seasonal mean three-dimensional relative frequency histograms and precipitation ice water contents over grid boxes of approximately 5°–10° in latitude and longitude. The reflectivity histograms and ice water contents were then combined with LIS lightning flash densities and 2A25 mean rainfall rates to examine regional relationships between precipitation vertical structure, precipitation processes, and lightning production.

Analysis of the reflectivity vertical structure histograms and lightning flash density data reveals that 1) relative to tropical continental locations, wet-season isolated tropical oceanic locations exhibit relatively little spatial (and in some instances seasonal) variability in vertical structure across the global Tropics; 2) coastal locations and areas located within 500–1000 km of a continent exhibit considerable seasonal and spatial variability in mean vertical structure, often resembling “continental” profiles or falling intermediate to that of tropical continental and isolated oceanic regimes; and 3) interior tropical continental locations exhibit marked variability in vertical structure both spatially and seasonally, exhibiting a continuum of characteristics ranging from a near-isolated oceanic profile observed over the central Amazon and India to a more robust continental profile observed over regions such as the Congo and Florida. Examination of regional and seasonal mean conditional instability for a small but representative subset of the geographic locations suggests that tropospheric thermodynamic structure likely plays a significant role in the regional characteristics of precipitation vertical structure and associated lightning flash density.

In general, the largest systematic variability in precipitation vertical structure observed between all of the locations examined occurred above the freezing level. It is important that subfreezing temperature variability in the vertical reflectivity structures was well reflected in the seasonal mean lightning flash densities and ice water contents diagnosed for each location. In turn, systematically larger rainfall rates were observed on a pixel-by-pixel basis in locations with larger precipitation ice water content and lightning flash density. These results delineate, in a regional sense, the relative importance of mixed-phase precipitation production across the global Tropics.

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Patrick C. Kennedy
and
Steven A. Rutledge

Abstract

During the afternoon and evening hours of 23 May 1991 a hail-producing multicellular severe thunderstorm developed near Denver, Colorado, and tracked eastward for more than 100 km. Along this path, hailstone diameters of 2–7 cm (0.75–2.75 in.) were reported at several points. The storm was observed by both the CSU-CHILL (CHL) and NCAR Mile High (MUR) 10-cm Doppler radars. The general echo morphology evolved by way of cyclic, discrete new cell formation near an outflow boundary moving ahead of the storm's forward flank. As this new cell growth occurred, the shape of the storm's most intense core also evolved in a periodic fashion. On four separate occasions these cores briefly assumed a bow shape with peak reflectivity values of 65–70 dBZ. The evolution of one such bow echo was examined by a series of six CHL-MHR dual-Doppler analyses. The resultant airflow patterns suggested that the core reflectivity structure was deformed into the bowlike configuration by updraft-induced flow field perturbations. During the period covered by the dual-Doppler analyses, dual polarization measurements made by the CSU-CHILL radar were used to infer hail characteristics by placing the differential reflectivity (Z DR) and zero-lag cross correlation between horizontally and vertically polarized echoes [ρHV(0)] observations in the context of the synthesized wind fields. These polarimetric data suggest that the areal coverage of the hail and the diameter of the hailstones both maximized during the bow-echo phase.

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Steven A. Rutledge
and
Donald R. MacGorman

Abstract

As part of the field program for the Oklahoma–Kansas PRE-STORM Project conducted in May–June 1985, a network of electromagnetic direction-finders was deployed to locate and detect the polarity of cloud-to-ground (CG) lighting flashes associated with Mesoscale Convective Systems (MCSs). We present an analysis of such data for the 10–11 June MCS. This storm consisted of a line of convective cells trailed by an 80 km wide stratiform precipitation region. Data from the lightning strike locating network, along with both conventional and Doppler radar data, are analyzed over a significant portion of the storm's lifetime to examine the relationship between the storm precipitation structure and the position and polarity of the lighting activity. The majority of the negative CG activity is located in the convective precipitation region. The frequency of negative CG activity is highest around the period of most intense convective rainfall. Positive CG activity is mainly confined to the trailing stratiform region, and there is a correlation between the areally integrated stratiform precipitation and the frequency of positive CG flashes. We propose that the occurrence of positive CG flashes in the trailing stratiform region is a result of the reward advection of positive charge on small ice particles from the upper levels of the convective cells by the storm relative winds. However, charging of hydrometeors may occur within the stratiform region and contribute to the positive space charge. Candidate charging mechanism are discussed.

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