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Steven V. Vasiloff
,
Edward A. Brandes
,
Robert P. Davies-Jones
, and
Peter S. Ray

Abstract

Nearly 2½ hours of dual-Doppler radar data with high temporal and spatial resolution are used to examine the evolution and morphology of a thunderstorm that evolved from a complex of small cells into a supercell storm. Individual storm cells and updrafts moved east-northeastward, nearly with the mean wind, while the storm complex, which encompassed the individual cells, propagated toward the south–southeast. Cells were first detected at middle levels (5–10 km) on the storm's right flank and dissipated on the left flank. Generally, the storm contained three cells—a forming cell, a mature cell, and a dissipating cell; life stages were apparently dictated by the source of updraft air. During the growth stage, cell inflow had a southerly component. As the cell moved through the storm complex, it started ingesting stable air from the north and soon dissipated.

A storm-environment feedback mechanism of updraft–downdraft interactions, in conjunction with increasing environmental vertical wind shear and buoyancy, is deemed responsible for an increase in the size and intensity of successive cells and updrafts. With time, a large region of background updraft, containing the updrafts of individual cells, formed on the storm's right flank. Unlike the individual cells, which moved nearly parallel to the mean wind and low-level shear vector, the region of background updraft moved to the right of the mean wind and low-level shear vector. It is believed that the formation and rightward motion of the background updraft region led to strong rotation on the storm's right flank. The larger cell and updraft size, with the same center-to-center spacing as at earlier times, made individual cell identification difficult, resulting in a nearly steady-state reflectivity structure.

The data support a growing consensus that a continuum of storm types, rather than a dichotomy, exists.

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Thomas T. Warner
,
Edward A. Brandes
,
Juanzhen Sun
,
David N. Yates
, and
Cynthia K. Mueller

Abstract

Operational prediction of flash floods caused by convective rainfall in mountainous areas requires accurate estimates or predictions of the rainfall distribution in space and time. The details of the spatial distribution are especially critical in complex terrain because the watersheds generally are small in size, and position errors in the placement of the rainfall can distribute the rain over the wrong watershed. In addition to the need for good rainfall estimates, accurate flood prediction requires a surface-hydrologic model that is capable of predicting stream or river discharge based on the rainfall-rate input data. In part 1 of this study, different techniques for the estimation and prediction of convective rainfall are applied to the Buffalo Creek, Colorado, flash flood of July 1996, during which over 75 mm of rain from a thunderstorm fell on the watershed in less than 1 h. The hydrologic impact of the rainfall was exacerbated by the fact that a considerable fraction of the watershed experienced a wildfire approximately two months prior to the rain event.

Precipitation estimates from the National Weather Service Weather Surveillance Radar-1988 Doppler and the National Center for Atmospheric Research S-band, dual-polarization radar, collocated east of Denver, Colorado, were compared. Very short range simulations from a convection-resolving dynamic model that was initialized variationally using the radar reflectivity and Doppler winds were compared with simulations from an automated algorithmic forecast system that also employs the radar data. The radar estimates of rain rate and the two forecasting systems that employ the radar data have degraded accuracy by virtue of the fact that they are applied in complex terrain. Nevertheless, the dynamic model and automated algorithms both produce simulations that could be useful operationally for input to surface-hydrologic models employed for flood warning. Part 2 of this study, reported in a companion paper, describes experiments in which these radar-based precipitation estimates and dynamic model–and automated algorithm–based precipitation simulations are used as input to a surface-hydrologic model for simulation of the stream discharge associated with the flood.

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Edward A. Brandes
,
Kyoko Ikeda
,
Guifu Zhang
,
Michael Schönhuber
, and
Roy M. Rasmussen

Abstract

Winter-storm hydrometeor distributions along the Front Range in eastern Colorado are studied with a ground-based two-dimensional video disdrometer. The instrument provides shape, size, and terminal velocity information for particles that are larger than about 0.4 mm. The dataset is used to determine the form of particle size distributions (PSDs) and to search for useful interrelationships among the governing parameters of assumed distribution forms and environmental factors. Snowfalls are dominated by almost spherical aggregates having near-exponential or superexponential size distributions. Raindrop size distributions are more peaked than those for snow. A relation between bulk snow density and particle median volume diameter is derived. The data suggest that some adjustment may be needed in relationships found previously between temperature and the concentration and slope parameters of assumed exponential PSDs. A potentially useful relationship is found between the slope and shape terms of the gamma PSD model.

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David N. Yates
,
Fei Chen
,
Margaret A. LeMone
,
Russell Qualls
,
Steven P. Oncley
,
Robert L. Grossman
, and
Edward A. Brandes

Abstract

A multiscale dataset that includes atmospheric, surface, and subsurface observations obtained from an observation network covering a region that has a scale order comparable to mesoscale and general circulation models is described and analyzed. The dataset is half-hourly time series of forcing and flux response data developed from the one-month Cooperative Atmosphere–Surface Exchange Study (CASES-97) experiment, located in the Walnut Watershed near Wichita, Kansas. The horizontal complexity of this dataset was analyzed by looking at the sensible and latent heat flux response of station data from the three main land surface types of winter wheat, grass/pastureland, and bare soil/sparse vegetation. The variability in the heat flux response at and among the different sites points to the need for a spatially distributed, time-varying atmospheric-forcing dataset for use in land surface modeling experiments. Such a dataset at horizontal spacings of 1, 5, and 10 km was developed from the station data and other remotely sensed platforms, and its spatial heterogeneity was analyzed.

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Alexander V. Ryzhkov
,
Dusan S. Zrnic
,
John C. Hubbert
,
V. N. Bringi
,
J. Vivekanandan
, and
Edward A. Brandes

Abstract

Preliminary analysis of all components of the polarimetric radar covariance matrix for precipitation measured with the NCAR S-band dual-polarization Doppler radar (S-Pol) and the Colorado State University–University of Chicago–Illinois State Water Survey (CSU–CHILL) radars is presented. Radar reflectivity at horizontal polarization Z h, differential reflectivity Z DR, linear depolarization ratio LDR, specific differential phase K DP, cross-correlation coefficient |ρ hv|, and two co-cross-polar correlation coefficients, ρ xh and ρ xv, have been measured and examined for two rain events: the 14 August 1998 case in Florida and the 8 August 1998 case in Colorado.

Examination of the coefficients ρ xh and ρ xv is the major focus of the study. It is shown that hydrometeors with different types of orientation can be better delineated if the coefficients ρ xh and ρ xv are used. Rough estimates of the raindrop mean canting angles and the rms width of the canting angle distribution are obtained from the co-cross-polar correlation coefficients in combination with other polarimetric variables.

Analysis of the two cases indicates that the raindrop net canting angles averaged over the propagation paths near the ground in typical convective cells do not exceed 2.5°. Nonetheless, the mean canting angles in individual radar resolution volumes in rain can be noticeably higher. Although the net canting angle for individual convective cells can deviate by a few degrees from zero, the average over a long propagation path along several cells is close to zero, likely because canting angles in different cells vary in sign.

The rms width of the canting angle distribution in rain is estimated to vary mainly between 5° and 15° with the median value slightly below 10°.

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Steven V. Vasiloff
,
Dong-Jun Seo
,
Kenneth W. Howard
,
Jian Zhang
,
David H. Kitzmiller
,
Mary G. Mullusky
,
Witold F. Krajewski
,
Edward A. Brandes
,
Robert M. Rabin
,
Daniel S. Berkowitz
,
Harold E. Brooks
,
John A. McGinley
,
Robert J. Kuligowski
, and
Barbara G. Brown

Accurate quantitative precipitation estimates (QPE) and very short term quantitative precipitation forecasts (VSTQPF) are critical to accurate monitoring and prediction of water-related hazards and water resources. While tremendous progress has been made in the last quarter-century in many areas of QPE and VSTQPF, significant gaps continue to exist in both knowledge and capabilities that are necessary to produce accurate high-resolution precipitation estimates at the national scale for a wide spectrum of users. Toward this goal, a national next-generation QPE and VSTQPF (Q2) workshop was held in Norman, Oklahoma, on 28–30 June 2005. Scientists, operational forecasters, water managers, and stakeholders from public and private sectors, including academia, presented and discussed a broad range of precipitation and forecasting topics and issues, and developed a list of science focus areas. To meet the nation's needs for the precipitation information effectively, the authors herein propose a community-wide integrated approach for precipitation information that fully capitalizes on recent advances in science and technology, and leverages the wide range of expertise and experience that exists in the research and operational communities. The concepts and recommendations from the workshop form the Q2 science plan and a suggested path to operations. Implementation of these concepts is expected to improve river forecasts and flood and flash flood watches and warnings, and to enhance various hydrologic and hydrometeorological services for a wide range of users and customers. In support of this initiative, the National Mosaic and Q2 (NMQ) system is being developed at the National Severe Storms Laboratory to serve as a community test bed for QPE and VSTQPF research and to facilitate the transition to operations of research applications. The NMQ system provides a real-time, around-the-clock data infusion and applications development and evaluation environment, and thus offers a community-wide platform for development and testing of advances in the focus areas.

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Margaret A. LeMone
,
Robert L. Grossman
,
Richard L. Coulter
,
Marvin L. Wesley
,
Gerard E. Klazura
,
Gregory S. PouIos
,
William Blumen
,
Julie K. Lundquist
,
Richard H. Cuenca
,
Shaun F. Kelly
,
Edward A. Brandes
,
Steven P. Oncley
,
Robert T. McMillen
, and
Bruce B. Hicks

This paper describes the development of the Cooperative Atmosphere Surface Exchange Study (CASES), its synergism with the development of the Atmosphere Boundary Layer Experiments (ABLE) and related efforts, CASES field programs, some early results, and future plans and opportunities. CASES is a grassroots multidisciplinary effort to study the interaction of the lower atmosphere with the land surface, the subsurface, and vegetation over timescales ranging from nearly instantaneous to years. CASES scientists developed a consensus that observations should be taken in a watershed between 50 and 100 km across; practical considerations led to an approach combining long-term data collection with episodic intensive field campaigns addressing specific objectives that should always include improvement of the design of the long-term instrumentation. In 1997, long-term measurements were initiated in the Walnut River Watershed east of Wichita, Kansas. Argonne National Laboratory started setting up the ABLE array. The first of the long-term hydrological enhancements was installed starting in May by the Hydrologic Science Team of Oregon State University. CASES-97, the first episodic field effort, was held during April–June to study the role of surface processes in the diurnal variation of the boundary layer, to test radar precipitation algorithms, and to define relevant scaling for precipitation and soil properties. The second episodic experiment, CASES-99, was conducted during October 1999, and focused on the stable boundary layer. Enhancements to both the atmospheric and hydrological arrays continue. The data from and information regarding both the long-term and episodic experiments are available on the World Wide Web. Scientists are invited to use the data and to consider the Walnut River Watershed for future field programs.

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