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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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Robert Cifelli and Steven A. Rutledge

Abstract

Wind profiler data wore used to determine the vertical motion structure in four tropical mcsoscale convective systems (MCSs), which occurred during the Down Under Doppler and Electricity Experiment (DUNDEE) near Darwin. Northern Territory, Australia. Three of the MCSs occurred during the monsoon-break convective regime and one occurred during the monsoon regime. In the break regime cases (each with a leading convective and trailing stratiform region structure), the wind profiler sampled low-level convective cells on the leading edge of the convective region, trailed by deeper updrafts of comparable magnitude. Surface rainfall measurements from a network of raingauges showed two comparable peaks in rainfall intensity that roughly corresponded to the passage of low-level and deep convective updraft (71%–80% of the system total rainfall was associated with the passage of the convective line). In the stratiform region, the profiler data showed generally weak vertical drafts (<1 m s−1) and the presence of both mesoscale upward and downward motion (17%–28% of the system total rainfall was associated with the passage of the stratiform region). Deep subsidence in the transition zone located between the convective and stratiform regions was also documented in each of the break regime cases. Composite vertical motion profiles in different regions of the break MCSs were constructed and the salient features of the profiles are described. The composite vertical motion profiles are compared to similar profiles from different graphical regions.

The evolution of the monsoon MCS was different from the break regime cases. This system was characterized by a series of convective updrafts embedded in stratiform cloud.

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Brenda Dolan and Steven A. Rutledge

Abstract

Although much work has been done at S band to automatically identify hydrometeors by using polarimetric radar, several challenges are presented when adapting such algorithms to X band. At X band, attenuation and non-Rayleigh scattering can pose significant problems. This study seeks to develop a hydrometeor identification (HID) algorithm for X band based on theoretical simulations using the T-matrix scattering model of seven different hydrometeor types: rain, drizzle, aggregates, pristine ice crystals, low-density graupel, high-density graupel, and vertical ice. Hail and mixed-phase hydrometeors are excluded for the purposes of this study. Non-Rayleigh scattering effects are explored by comparison with S-band simulations. Variable ranges based on the theoretical simulations are used to create one-dimensional fuzzy-logic membership beta functions that form the basis of the new X-band HID. The theory-based X-band HID is applied to a case from the Collaborative Adaptive Sensing of the Atmosphere (CASA) Integrated Project 1 (IP1) network of X-band radars, and comparisons are made with similar S-band hydrometeor identification algorithms applied to data from the S-band polarimetric Next Generation Weather Radar (NEXRAD) prototype radar, KOUN. The X-band HID shows promise for illustrating bulk hydrometeor types and qualitatively agrees with analysis from KOUN. A simple reflectivity- and temperature-only HID is also applied to both KOUN and CASA IP1 data to reveal benefits of the polarimetric-based HID algorithms, especially in the classification of ice hydrometeors and oriented ice crystals.

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Steven A. Rutledge and V. Chandrasekar

Abstract

Great strides have been made over the past decades in educating radar meteorologists. These advances appear to be loosely associated with the arrival of new hardware in the field, for example, Doppler radars followed by polarimetric radars. Many radar meteorologists received a substantial portion of their early training through participation in field programs utilizing this new hardware. In this study, a brief look at the evolution of radar education will first be offered, followed by an assessment of the current state of this field. Finally, a view of the future will be offered. Future educational thrusts in radar meteorology will take full advantage of Internet technology, allowing radar systems to be brought into remote classrooms in a “virtual” sense. This study is purposely limited to meteorological radar and is focused on graduate-level education.

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Brenda Dolan and Steven A. Rutledge

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Data from the Collaborative Adaptive Sensing of the Atmosphere (CASA) Integrated Project I (IP1) network of polarimetric X-band radars are used to observe a convective storm. A fuzzy logic hydrometeor identification algorithm is employed to study microphysical processes. Dual-Doppler techniques are used to analyze the 3D wind field. The scanning strategy, sensitivity, and low-level scanning focus of the radars are investigated for influencing bulk hydrometeor identification and dual-Doppler wind retrievals. Comparisons are made with the nearby S-band polarimetric Next Generation Weather Radar (NEXRAD) prototype radar (KOUN), for consistency. Lightning data are used as an independent indicator of storm evolution for comparison with radar observations.

A new methodology for retrieving the vertical wind utilizing upward and variational integration techniques is employed and shown to illustrate trends in mean wind, with particularly good results at low levels. IP1 observations of a case on 10 June 2007 show the development of the updraft, subsequent graupel echo volume evolution, and a descending downdraft preceded by significant graupel in the midlevels, with updraft and graupel volumes leading the onset of lightning. Many of these trends are corroborated by KOUN. The high temporal resolution of three minutes and near-ground sampling provided by IP1 is integral to resolving up- and downdrafts, as well as hydrometeor evolution. IP1 coverage of the upper levels is diminished compared to KOUN, impacting the quality of the dual-Doppler derived vertical winds and ice echo volumes, although the low-level coverage helps to mitigate some errors. However, IP1 coverage of the low- to midlevels is demonstrated to be comparable or better than coverage by KOUN for this storm location.

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Weixin Xu and Steven A. Rutledge

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This study uses Dynamics of the Madden–Julian Oscillation (DYNAMO) shipborne [Research Vessel (R/V) Roger Revelle] radar and Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) datasets to investigate MJO-associated convective systems in specific organizational modes [mesoscale convective system (MCS) versus sub-MCS and linear versus nonlinear]. The Revelle radar sampled many “climatological” aspects of MJO convection as indicated by comparison with the long-term TRMM PR statistics, including areal-mean rainfall (6–7 mm day−1), convective intensity, rainfall contributions from different morphologies, and their variations with MJO phase. Nonlinear sub-MCSs were present 70% of the time but contributed just around 20% of the total rainfall. In contrast, linear and nonlinear MCSs were present 10% of the time but contributed 20% and 50%, respectively. These distributions vary with MJO phase, with the largest sub-MCS rainfall fraction in suppressed phases (phases 5–7) and maximum MCS precipitation in active phases (phases 2 and 3). Similarly, convective–stratiform rainfall fractions also varied significantly with MJO phase, with the highest convective fractions (70%–80%) in suppressed phases and the largest stratiform fraction (40%–50%) in active phases. However, there are also discrepancies between the Revelle radar and TRMM PR. Revelle radar data indicated a mean convective rain fraction of 70% compared to 55% for TRMM PR. This difference is mainly due to the reduced resolution of the TRMM PR compared to the ship radar. There are also notable differences in the rainfall contributions as a function of convective intensity between the Revelle radar and TRMM PR. In addition, TRMM PR composites indicate linear MCS rainfall increases after MJO onset and produce similar rainfall contributions to nonlinear MCSs; however, the Revelle radar statistics show the clear dominance of nonlinear MCS rainfall.

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Weixin Xu and Steven A. Rutledge

Abstract

This study investigates the convective population and environmental conditions during three MJO events over the central Indian Ocean in late 2011 using measurements collected from the Research Vessel (R/V) Roger Revelle deployed in Dynamics of the MJO (DYNAMO). Radar-based rainfall estimates from the Revelle C-band radar are first placed in the context of larger-scale Tropical Rainfall Measuring Mission (TRMM) rainfall data to demonstrate that the reduced Revelle radar range captured the MJO convective evolution. Time series analysis and MJO phase-based composites of Revelle measurements both support the “recharge–discharge” MJO theory. Time series of echo-top heights indicate that convective deepening during the MJO onset occurs over a 12–16-day period. Composite statistics show evident recharging–discharging features in convection and the environment. Population of shallow/isolated convective cells, SST, CAPE, and the lower-tropospheric moisture increase (recharge) substantially approximately two to three phases prior to the MJO onset. Deep and intense convection and lightning peak in phase 1 when the sea surface temperature and CAPE are near maximum values. However, cells in this phase are not well organized and produce little stratiform rain, possibly owing to reduced shear and a relatively dry upper troposphere. The presence of deep convection leads the mid- to upper-tropospheric humidity by one to two phases, suggesting its role in moistening these levels. During the MJO onset (i.e., phase 2), the mid- to upper troposphere becomes very moist, and precipitation, radar echo-top heights, and the mesoscale extent of precipitation all increase and obtain peak values. Persistent heavy precipitation in these active periods helps reduce the SST and dry/stabilize (or discharge) the atmosphere.

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Weixin Xu and Steven A. Rutledge

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This study investigates the convective cloud population, precipitation microphysics, and lightning activity associated with the boreal summer intraseasonal oscillation (BSISO) over the South China Sea (SCS) and surrounding landmasses. SCS rainfall shows a marked 30–60-day intraseasonal variability. This variability is less evident over land. The population of mesoscale convective systems (MCSs) and the stratiform rain fraction over the SCS, Philippines, and Indochina increase remarkably after the onset of BSISO. Convection over the SCS during inactive periods exhibits a trimodal population including shallow cumulus, congestus, and deep convection, mirroring the situation over tropical open oceans. The shallow mode is absent over land. Shallow cumulus clouds rapidly transition to congestus clouds over the SCS under active BSISO conditions. Over land, deep convection and lightning lead total rainfall and MCSs by 2–3 BSISO phases, whereas they are somewhat in phase over the SCS. Although convective instability over the SCS is larger during active periods compared to inactive periods, variability in convective intensity and precipitation microphysics is minimal, with active periods showing only higher frequency of moderate ice scattering and 30-dBZ heights extending to −10°C. Over the Philippines and Indochina, inactive phases exhibit substantially stronger ice scattering signatures, robust mixed-phase microphysics, and higher lightning flash rates, possibly due to greater convective instability and a stronger convective diurnal cycle. Total rainfall, convective environments, and convective structures over Borneo are all out of phase with that over the Philippines and Indochina, while southern China shows little BSISO variability on convective intensity and lightning frequency.

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Steven A. Rutledge and Peterv Hobbs

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Previous field studies have indicated that warm-frontal rainbands form when ice particles from a “seeder” cloud grow as they fall through a lower-level “feeder” cloud. In this paper we present results from a parameterized numerical model of the growth processes that can lead to the enhancement of precipitation in a “seeder-feeder” type situation. The model is applied to two types of warm-frontal rainbands. In the first (Type 1 situation) the vertical air motions are typical of those associated with slow, widespread lifting in the vicinity of warm fronts. In the second (Type 2 situation) the vertical air motions are stronger, and more characteristic of the mesoscale.

The model simulations show that in the Type 1 situations the growth of the “seed” ice crystals within the feeder zone is due to vapor deposition. The feeder zone in this case is slightly sub-saturated with respect to water due to the presence of the seed crystals. In regions where the feeder zone is not “seeded” from aloft, snow crystals, originating in the feeder zone, grow by deposition and riming and produce a precipitation rate of ∼1 mm h−1, compared to ∼2 mm h−1 for the combined seeder-feeder cloud system. The presence of seed crystals allows for the efficient removal of condensation produced by the feeder cloud. In the Type 2 situation, the strong mesoscale ascent provides liquid water from which the seed crystals grow primarily by riming.

For both Type 1 and 2 situations the condensation rates, radar reflectivities and rainfall rates predicted by the model are in reasonable agreement with field observations.

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